This document summarizes key concepts in computed tomography (CT) imaging. It discusses how CT uses x-rays to measure the attenuation of objects along different projection angles to reconstruct cross-sectional images. Specifically, it covers:
1) How monoenergetic and polychromatic x-ray sources are used to measure attenuation projections and the artifacts that can arise from beam hardening and scatter.
2) Different scanning methods like fan beam rotational and fixed detector ring configurations.
3) Emission CT techniques like SPECT and PET that use radioactive tracers.
4) Ultrasound CT and magnetic resonance imaging which use different physical phenomena for tissue imaging and data collection.
5) Artifacts like
This document discusses fluoroscopy, which uses x-rays and a fluoroscope to obtain real-time moving images of internal structures. It was invented in 1896 by Thomas Edison. Fluoroscopy allows visualization of anatomical structures and organ motion/function. The key components are an x-ray source, fluorescent screen, and image intensifier system coupled to a TV camera. This allows radiologists to view live images on a monitor. Various fluoroscopy systems exist for different applications like surgery or interventional radiology. The document also describes the components and functioning of image intensifiers, TV cameras, and digital fluoroscopy detectors that allow the conversion of x-ray images to visible light and electrical signals.
it includes generations and advancement in CT. In generations fifth generation CT is described in detail.
UFC detector, stellar detectors and gemstone detector is also described
straton x-ray tube, MRC, LIMAX and aquillion one xray tube
different techniques used in CT
dual energy CT is also described
This document provides information about image reconstruction in multi-detector computed tomography (MDCT). It begins with an overview of the basic principles of CT imaging, including image formation steps and reconstruction methods. It then describes the principles of helical CT scanning and how this enables volumetric data acquisition. Finally, it discusses image reconstruction techniques for MDCT, including interpolation methods needed to reconstruct images from the helical scan data. In particular, it notes that multi-detector arrays allow acquisition of multiple slices with each rotation, significantly increasing scan speed and coverage compared to earlier single-detector row CT.
This document provides information about dose reduction techniques in CT scanning. It discusses how CT scan technology has advanced but also leads to higher radiation doses compared to other modalities. Various techniques can help reduce dose like adjusting acquisition parameters such as tube current, voltage, and scan length. Equipment designs with features like iterative reconstruction and dual-layer detectors can also help lower dose. Selecting the appropriate scan protocol tailored to the clinical task is important to optimize image quality while keeping radiation exposure as low as reasonably achievable.
The Role of Computers in Medical PhysicsVictor Ekpo
The document discusses the various roles of computers in medical physics. It describes how computers are used for tasks like data conversion, database management, image display, processing and analysis. Computers aid in areas such as radiodiagnosis, radiotherapy treatment planning, dosimetry and various medical imaging modalities. They provide benefits like speed, automation, accuracy and ability to store and share large amounts of data. Overall, the integration of computers has greatly enhanced the field of medical physics.
Ct instrumentation and types of detector configurationSUJAN KARKI
CT scanners have evolved through several generations with advances in detector and x-ray tube technology. Seventh generation CT scanners use multiple detector arrays and cone-shaped x-ray beams. Key components include the gantry, high voltage generator, slip ring technology, data acquisition system, filters and collimators. Adaptive collimation helps reduce over ranging and over beaming in multi-detector CT.
This document discusses fluoroscopy, which uses x-rays and a fluoroscope to obtain real-time moving images of internal structures. It was invented in 1896 by Thomas Edison. Fluoroscopy allows visualization of anatomical structures and organ motion/function. The key components are an x-ray source, fluorescent screen, and image intensifier system coupled to a TV camera. This allows radiologists to view live images on a monitor. Various fluoroscopy systems exist for different applications like surgery or interventional radiology. The document also describes the components and functioning of image intensifiers, TV cameras, and digital fluoroscopy detectors that allow the conversion of x-ray images to visible light and electrical signals.
it includes generations and advancement in CT. In generations fifth generation CT is described in detail.
UFC detector, stellar detectors and gemstone detector is also described
straton x-ray tube, MRC, LIMAX and aquillion one xray tube
different techniques used in CT
dual energy CT is also described
This document provides information about image reconstruction in multi-detector computed tomography (MDCT). It begins with an overview of the basic principles of CT imaging, including image formation steps and reconstruction methods. It then describes the principles of helical CT scanning and how this enables volumetric data acquisition. Finally, it discusses image reconstruction techniques for MDCT, including interpolation methods needed to reconstruct images from the helical scan data. In particular, it notes that multi-detector arrays allow acquisition of multiple slices with each rotation, significantly increasing scan speed and coverage compared to earlier single-detector row CT.
This document provides information about dose reduction techniques in CT scanning. It discusses how CT scan technology has advanced but also leads to higher radiation doses compared to other modalities. Various techniques can help reduce dose like adjusting acquisition parameters such as tube current, voltage, and scan length. Equipment designs with features like iterative reconstruction and dual-layer detectors can also help lower dose. Selecting the appropriate scan protocol tailored to the clinical task is important to optimize image quality while keeping radiation exposure as low as reasonably achievable.
The Role of Computers in Medical PhysicsVictor Ekpo
The document discusses the various roles of computers in medical physics. It describes how computers are used for tasks like data conversion, database management, image display, processing and analysis. Computers aid in areas such as radiodiagnosis, radiotherapy treatment planning, dosimetry and various medical imaging modalities. They provide benefits like speed, automation, accuracy and ability to store and share large amounts of data. Overall, the integration of computers has greatly enhanced the field of medical physics.
Ct instrumentation and types of detector configurationSUJAN KARKI
CT scanners have evolved through several generations with advances in detector and x-ray tube technology. Seventh generation CT scanners use multiple detector arrays and cone-shaped x-ray beams. Key components include the gantry, high voltage generator, slip ring technology, data acquisition system, filters and collimators. Adaptive collimation helps reduce over ranging and over beaming in multi-detector CT.
Physical principle of Computed Tomography (Scanning principle & Data acquisit...ThejaTej6
Computed tomography (CT) uses x-rays and computer processing to produce cross-sectional images of the body. CT scanners are composed of an x-ray tube, detectors, and a rotating gantry. During a scan, the gantry rotates around the patient as they pass through, emitting a narrow x-ray beam and detecting the transmitted x-rays. The detected data is used to construct tomographic images, or slices, of the body. CT images provide more detailed information than conventional x-rays by measuring the attenuation of x-rays through tissues.
The document discusses various aspects of image display in CT scanning. It describes different types of display monitors used, such as CRT and LCD. It also discusses window width and window level settings, which are used to adjust the contrast and brightness of CT images by mapping CT number ranges to grayscale values. Region of interest tools allow measuring values within a selected area of an image. Distance measurements, annotations, reference images, and magnification can also be used to analyze CT scans.
The transducer is the most critical component of an ultrasound system. It acts as both a transmitter and receiver of ultrasound waves. There are many types of transducers ranging from single element to electronic multi-array probes with hundreds of elements. The main components of a typical ultrasound transducer are the physical housing, electrical connections, piezoelectric element, backing material, acoustic lens, and impedance matching layer. Electronic multi-array transducers use an array of piezoelectric crystals to form images by transmitting narrow ultrasound beams along adjacent paths through the patient.
Single photon emission computed tomography (spect)Syed Hammad .
brief but informative knowledge about what basically SPECT is and what is the phenomenon behind this machine ... easy to understand as well as presenting during lectures and in classes . share it
This document describes the key components of a CT scanner, including the gantry, x-ray tube, detector array, high voltage generator, and patient support couch. The gantry houses the x-ray tube, detector array, and other components and rotates around the patient. The x-ray tube produces x-rays, while the detector array detects the x-rays that pass through the patient and produces images. A high voltage generator supplies power to the x-ray tube. The patient lies on a support couch that positions them for imaging and must be made of material that does not interfere with the x-rays.
Helical and multislice CT techniques provide advantages over traditional slice-by-slice CT scanning. Helical CT, also known as spiral CT, involves continuously transporting the patient through the gantry while acquiring data during multiple 360 degree scans, allowing for increased speed, improved image continuity and less motion artifact. Multislice CT uses multiple parallel detectors to scan a greater volume of the patient per rotation, providing shorter acquisition times, improved z-axis resolution, and more information for radiologists. Both techniques rely on technological advances like slip-ring devices and interpolation algorithms to efficiently process the continuously acquired data into diagnostic images.
MRI machines use magnetic fields, radio waves, and computers to detect properties of living tissue. The first MRI image of the human body was obtained in 1977 and detected cancer tissue. MRI requires a magnet to align nuclear spins, radio waves to excite the spins, magnetic field gradients for spatial encoding, and a computer system to form images. Stronger magnetic fields allow for higher resolution images. Electromagnets, resistive magnets, superconducting magnets, and permanent magnets can be used to generate magnetic fields, with superconducting magnets allowing the highest field strengths. Radiofrequency coils transmit the excitation signal and receive the emitted signal used to form images.
Calculation of air-kerma strength and dose rate constant for new BEBIG 60Co H...Anwarul Islam
Calculation of air-kerma strength and dose rate constant for new BEBIG 60Co HDR brachytherapy source: an EGSnrc Monte Carlo study
M. Anwarul Islam, Medical Physicist
SQUARE Hospitals Ltd, Bangladesh
anwar.amch@yahoo.com
Lec2 Ali 5.Lecture 5 - CT Scan Data Acquisition System.pptxAli Ayaz
The document discusses CT scan data acquisition. It describes how data acquisition systems systematically collect information from the patient by measuring transmitted radiation with x-ray tubes and detectors. The x-ray beam scans around the patient in a slice, and the detected photon intensity is converted to electrical signals and then digital values sent to the reconstruction computer. Detectors must efficiently capture, absorb, and convert photons while having a fast response time, wide dynamic range, and stability. Photomultiplier tubes and photodiodes are common detector types. The data acquisition system includes amplifiers, logarithmic amplifiers to compress the dynamic range, and analog-to-digital converters to convert the signals to digital values for computer processing.
brief but informative knowledge about how CT works and what are its components ... easy to understand as well as presenting during lectures and in classes . share it
Beam restricted device and filter used in x raySushilPattar
This document discusses various beam restricting devices and filters used in radiography to reduce radiation exposure. It describes common beam restricting devices like diaphragms, cones, cylinders and collimators which are used to limit the size of the primary x-ray beam and reduce scatter radiation. It also discusses different types of filters like inherent, aluminum, compound and molybdenum filters which absorb low energy photons and improve image quality. Maintaining proper collimation and use of appropriate filters helps achieve the ALARA principle of keeping radiation exposure As Low As Reasonably Achievable.
The document discusses X-ray grids, which are devices used in X-ray machines to filter out scattered radiation and improve image quality. It describes how X-ray grids work by absorbing randomly scattered photons that would otherwise reduce contrast and clarity in X-ray images. The document provides details on the history of X-ray grids, their construction, types including parallel, focused and criss-cross grids, as well as factors like grid ratio and Bucky factor.
The document discusses the history and components of fluoroscopy systems. Early fluoroscopy required complete darkness as it relied on rod vision, exposing patients and radiologists to high radiation. Modern systems use an image intensifier to amplify images 500-8000x, allowing viewing on a TV screen using cone vision with less radiation exposure. The image intensifier converts x-rays to light through an input phosphor, then light to electrons via a photocathode. Electrostatic lenses accelerate electrons onto an output phosphor, reconverting them to brighter light for display. Cesium iodide replaced earlier phosphors for better x-ray absorption and resolution.
Computed tomography (CT) uses X-rays and computer processing to create cross-sectional images of the body. It was invented in 1967 by Godfrey Hounsfield and independently by Allan Cormack, who shared the 1979 Nobel Prize in Medicine. A CT scan captures multiple X-ray measurements around a body section to reconstruct detailed images. The main components are the gantry with X-ray tube and detectors, patient table, computer for image reconstruction, and monitor. Filtered back projection is the most common reconstruction algorithm, combining back projection with ramp filtering to reduce blurring in the images.
The document discusses computed tomography (CT) scanning. It begins by introducing CT and comparing it to conventional radiography. CT provides more accurate diagnostic information by reconstructing 3D structures from multiple 2D projections, unlike conventional radiography which produces 2D shadow images. The document then covers key aspects of CT scanning including the components involved in data acquisition, the reconstruction process, and parameters such as slice thickness and radiation dose. It also describes advances in CT technology over generations from narrow single detector scans to modern multi-detector scanners.
The document discusses computed tomography (CT) scanning. It begins by introducing CT and comparing it to conventional radiography. CT provides more accurate diagnostic information by reconstructing 3D structures from multiple 2D projections, unlike conventional radiography which produces 2D shadow images. The document then covers key aspects of CT scanning including the components involved in data acquisition, the reconstruction process, and parameters such as slice thickness and radiation dose. It also describes advances in CT technology over generations from narrow single detector scans to current multi-detector scanners.
Physical principle of Computed Tomography (Scanning principle & Data acquisit...ThejaTej6
Computed tomography (CT) uses x-rays and computer processing to produce cross-sectional images of the body. CT scanners are composed of an x-ray tube, detectors, and a rotating gantry. During a scan, the gantry rotates around the patient as they pass through, emitting a narrow x-ray beam and detecting the transmitted x-rays. The detected data is used to construct tomographic images, or slices, of the body. CT images provide more detailed information than conventional x-rays by measuring the attenuation of x-rays through tissues.
The document discusses various aspects of image display in CT scanning. It describes different types of display monitors used, such as CRT and LCD. It also discusses window width and window level settings, which are used to adjust the contrast and brightness of CT images by mapping CT number ranges to grayscale values. Region of interest tools allow measuring values within a selected area of an image. Distance measurements, annotations, reference images, and magnification can also be used to analyze CT scans.
The transducer is the most critical component of an ultrasound system. It acts as both a transmitter and receiver of ultrasound waves. There are many types of transducers ranging from single element to electronic multi-array probes with hundreds of elements. The main components of a typical ultrasound transducer are the physical housing, electrical connections, piezoelectric element, backing material, acoustic lens, and impedance matching layer. Electronic multi-array transducers use an array of piezoelectric crystals to form images by transmitting narrow ultrasound beams along adjacent paths through the patient.
Single photon emission computed tomography (spect)Syed Hammad .
brief but informative knowledge about what basically SPECT is and what is the phenomenon behind this machine ... easy to understand as well as presenting during lectures and in classes . share it
This document describes the key components of a CT scanner, including the gantry, x-ray tube, detector array, high voltage generator, and patient support couch. The gantry houses the x-ray tube, detector array, and other components and rotates around the patient. The x-ray tube produces x-rays, while the detector array detects the x-rays that pass through the patient and produces images. A high voltage generator supplies power to the x-ray tube. The patient lies on a support couch that positions them for imaging and must be made of material that does not interfere with the x-rays.
Helical and multislice CT techniques provide advantages over traditional slice-by-slice CT scanning. Helical CT, also known as spiral CT, involves continuously transporting the patient through the gantry while acquiring data during multiple 360 degree scans, allowing for increased speed, improved image continuity and less motion artifact. Multislice CT uses multiple parallel detectors to scan a greater volume of the patient per rotation, providing shorter acquisition times, improved z-axis resolution, and more information for radiologists. Both techniques rely on technological advances like slip-ring devices and interpolation algorithms to efficiently process the continuously acquired data into diagnostic images.
MRI machines use magnetic fields, radio waves, and computers to detect properties of living tissue. The first MRI image of the human body was obtained in 1977 and detected cancer tissue. MRI requires a magnet to align nuclear spins, radio waves to excite the spins, magnetic field gradients for spatial encoding, and a computer system to form images. Stronger magnetic fields allow for higher resolution images. Electromagnets, resistive magnets, superconducting magnets, and permanent magnets can be used to generate magnetic fields, with superconducting magnets allowing the highest field strengths. Radiofrequency coils transmit the excitation signal and receive the emitted signal used to form images.
Calculation of air-kerma strength and dose rate constant for new BEBIG 60Co H...Anwarul Islam
Calculation of air-kerma strength and dose rate constant for new BEBIG 60Co HDR brachytherapy source: an EGSnrc Monte Carlo study
M. Anwarul Islam, Medical Physicist
SQUARE Hospitals Ltd, Bangladesh
anwar.amch@yahoo.com
Lec2 Ali 5.Lecture 5 - CT Scan Data Acquisition System.pptxAli Ayaz
The document discusses CT scan data acquisition. It describes how data acquisition systems systematically collect information from the patient by measuring transmitted radiation with x-ray tubes and detectors. The x-ray beam scans around the patient in a slice, and the detected photon intensity is converted to electrical signals and then digital values sent to the reconstruction computer. Detectors must efficiently capture, absorb, and convert photons while having a fast response time, wide dynamic range, and stability. Photomultiplier tubes and photodiodes are common detector types. The data acquisition system includes amplifiers, logarithmic amplifiers to compress the dynamic range, and analog-to-digital converters to convert the signals to digital values for computer processing.
brief but informative knowledge about how CT works and what are its components ... easy to understand as well as presenting during lectures and in classes . share it
Beam restricted device and filter used in x raySushilPattar
This document discusses various beam restricting devices and filters used in radiography to reduce radiation exposure. It describes common beam restricting devices like diaphragms, cones, cylinders and collimators which are used to limit the size of the primary x-ray beam and reduce scatter radiation. It also discusses different types of filters like inherent, aluminum, compound and molybdenum filters which absorb low energy photons and improve image quality. Maintaining proper collimation and use of appropriate filters helps achieve the ALARA principle of keeping radiation exposure As Low As Reasonably Achievable.
The document discusses X-ray grids, which are devices used in X-ray machines to filter out scattered radiation and improve image quality. It describes how X-ray grids work by absorbing randomly scattered photons that would otherwise reduce contrast and clarity in X-ray images. The document provides details on the history of X-ray grids, their construction, types including parallel, focused and criss-cross grids, as well as factors like grid ratio and Bucky factor.
The document discusses the history and components of fluoroscopy systems. Early fluoroscopy required complete darkness as it relied on rod vision, exposing patients and radiologists to high radiation. Modern systems use an image intensifier to amplify images 500-8000x, allowing viewing on a TV screen using cone vision with less radiation exposure. The image intensifier converts x-rays to light through an input phosphor, then light to electrons via a photocathode. Electrostatic lenses accelerate electrons onto an output phosphor, reconverting them to brighter light for display. Cesium iodide replaced earlier phosphors for better x-ray absorption and resolution.
Computed tomography (CT) uses X-rays and computer processing to create cross-sectional images of the body. It was invented in 1967 by Godfrey Hounsfield and independently by Allan Cormack, who shared the 1979 Nobel Prize in Medicine. A CT scan captures multiple X-ray measurements around a body section to reconstruct detailed images. The main components are the gantry with X-ray tube and detectors, patient table, computer for image reconstruction, and monitor. Filtered back projection is the most common reconstruction algorithm, combining back projection with ramp filtering to reduce blurring in the images.
The document discusses computed tomography (CT) scanning. It begins by introducing CT and comparing it to conventional radiography. CT provides more accurate diagnostic information by reconstructing 3D structures from multiple 2D projections, unlike conventional radiography which produces 2D shadow images. The document then covers key aspects of CT scanning including the components involved in data acquisition, the reconstruction process, and parameters such as slice thickness and radiation dose. It also describes advances in CT technology over generations from narrow single detector scans to modern multi-detector scanners.
The document discusses computed tomography (CT) scanning. It begins by introducing CT and comparing it to conventional radiography. CT provides more accurate diagnostic information by reconstructing 3D structures from multiple 2D projections, unlike conventional radiography which produces 2D shadow images. The document then covers key aspects of CT scanning including the components involved in data acquisition, the reconstruction process, and parameters such as slice thickness and radiation dose. It also describes advances in CT technology over generations from narrow single detector scans to current multi-detector scanners.
Computed tomography (CT) uses computer-processed X-rays to create cross-sectional images of the body. CT works by rotating an X-ray tube and detectors around the patient, acquiring multiple transmission measurements at different angles to reconstruct a 3D image. Image reconstruction involves algorithms like back projection and filtered back projection that use the transmission data to calculate the attenuation coefficients of different tissues and generate tomographic images representing slices of the body. CT numbers, measured in Hounsfield units, provide standardized values related to tissue density and visibility.
The document provides information on various medical imaging techniques and image reconstruction algorithms. It discusses computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and single photon emission computed tomography (SPECT). It describes iterative reconstruction methods like maximum likelihood expectation maximization (ML-EM) and median root prior (MRP) that are able to reconstruct images with incomplete data sets and reduce noise, in contrast to traditional methods like filtered back projection that make idealistic assumptions. The document also provides details on the physics and methodology behind PET and SPECT imaging.
Computerized tomography (CT) was pioneered by Godfrey Hounsfield and Allan Cormack in the 1970s. CT uses X-rays and computer processing to create cross-sectional images of the body. The first CT scanners used a translate-rotate design, while later generations used multiple detectors and spiral scanning for faster, more detailed imaging. Image reconstruction uses back projection to convert attenuation measurements into pixel values and display slices. CT provides excellent anatomical detail and is widely used for diagnosing conditions of the brain, blood vessels, lungs and other organs.
The document provides an overview of computed tomography (CT) scans. It discusses the history and development of CT scans, how they work, their components and circuitry. Key points covered include that CT scans were invented in the 1970s, use X-rays to generate cross-sectional images of the body, and have advanced from early generation whole body scanners to current high resolution multi-slice machines. CT scans provide important medical imaging capabilities with minimal risks when used properly.
CT scan and MRI are the techniques for body imaging. Computed Tomography or Computerized Axial Tomography is commonly referred to as a CT scan.
C- computed (Use of computer) and T- tomography (Greek word “Tomos” means “slice” and “Grapho” means “ To write”
The first commercial CT scanner was invented by Sir Godfrey Hounsfield in United Kingdom.
It is a diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce images of the inside of the body. It shows detailed images of any part of the body including the bones, muscles, fat, organs and blood vessels.
CT scans may be performed to help diagnose tumors, investigate internal bleeding, or check for other internal injuries or damage.
Computed Tomography or Computerized Axial Tomography is commonly referred to as a CT scan.
C- computed (Use of computer) and T- tomography (Greek word “Tomos” means “slice” and “Grapho” means “ To write”
The first commercial CT scanner was invented by Sir Godfrey Hounsfield in United Kingdom.
It is a diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce images of the inside of the body. It shows detailed images of any part of the body including the bones, muscles, fat, organs and blood vessels.
CT scans may be performed to help diagnose tumors, investigate internal bleeding, or check for other internal injuries or damage. Computed Tomography or Computerized Axial Tomography is commonly referred to as a CT scan.
C- computed (Use of computer) and T- tomography (Greek word “Tomos” means “slice” and “Grapho” means “ To write”
The first commercial CT scanner was invented by Sir Godfrey Hounsfield in United Kingdom.
It is a diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce images of the inside of the body. It shows detailed images of any part of the body including the bones, muscles, fat, organs and blood vessels.
CT scans may be performed to help diagnose tumors, investigate internal bleeding, or check for other internal injuries or damage. MRI stands for Magentic Resonance Imaging which is a non-invasive medical imaging test that produces detailed images of almost every internal structure in the human body, including the organs, bones, muscles and blood vessels.
MRI scanners create images of the body using a large magnet and radio waves.
No ionizing radiation is produced during an MRI exam, unlike X-rays. These images give your physician important information in diagnosing your medical condition and planning a course of treatment.
Raymond Damadian, the inventor of the first magnetic resonance scanning machine performed the first full-body scan of a human being in 1977.
The Nobel Prize was awarded to the American chemist, Paul Lauterbur, and the British physicist, Peter Mansfield, for developing a method to represent the information gathered by a scanner as an image. This is fundamental for the way the technology is used today.
MDCT Principles and Applications- Avinesh ShresthaAvinesh Shrestha
Multidetector CT (MDCT) is one of the most commonly used imaging modality in the field of Radiology. Development and advancement in MDCT has made it's application as a major component in diagnosis and treatment planning of multitude of disease across the planet. This presentation briefly describes its basic principle and it's wide variety of application in medical imaging.
Basic principle of ct and ct generationsTarunGoyal66
This document provides information about the history and development of computed tomography (CT) scanning technology. It discusses the key innovations and generations of CT scanners, including:
- The first generation translate-rotate scanner with a single detector and pencil beam.
- Second generation scanners that used a fan beam and multiple detectors to reduce scan time.
- Third generation rotate-rotate scanners that eliminated translation by using a rotating x-ray tube and detector array.
- Fourth generation rotate-fixed scanners with a stationary detector ring and rotating x-ray tube.
It also covers the basic components and functioning of modern CT scanners, image reconstruction principles, and factors that influence image quality.
Basic principle of ct and ct generationsTarun Goyal
This document provides information about computed tomography (CT) scanning. It discusses:
- The basic principles of CT scanning, which involves using X-rays from multiple angles to reconstruct cross-sectional images of the body.
- Key parts of a CT machine including the X-ray tube, detectors, collimators, and gantry which houses these components and rotates around the patient.
- How CT images are formed based on measuring the attenuation of X-rays through tissue, and assigning numbers in Hounsfield units to produce grayscale images.
- Factors that influence image quality such as noise, resolution, and radiation dose considerations.
The document discusses several medical imaging techniques:
- X-rays use high energy electromagnetic waves emitted from accelerated electrons to generate images. CT scans take multiple X-ray images from different angles to construct 3D images.
- Ultrasound uses piezoelectric transducers to generate and receive ultrasonic waves, which are reflected differently by tissues. This is used in A-scans and B-scans.
- MRI uses strong magnetic fields and radio waves to excite hydrogen nuclei in the body, and detects their signals to construct images based on tissue density and fluid content.
CT scanners produce cross-sectional images of the body by using X-rays from different angles to reconstruct tomographic slices. CT scanners rotate an X-ray tube and detector array around the patient, measuring attenuation profiles along rays. These projection data are reconstructed using filtered back projection to produce images with improved contrast compared to radiography but some artifacts. Advances like spiral CT allow faster full-volume imaging but increase patient dose. CT provides anatomical detail but cannot replace modalities like MRI for all clinical needs.
The document summarizes the history and technology of computed tomography (CT) scanners. It describes how CT was developed in the 1970s by Godfrey Hounsfield and Alan Cormack, who were later awarded the Nobel Prize. It outlines the key innovations in each generation of CT scanners, from the first generation's pencil beam geometry to later generations' use of detector arrays and helical scanning, which reduced scan times. The document also discusses the components of a CT scanner, including the x-ray tube, detectors, and techniques for image reconstruction and calibration.
This document provides an overview of various medical imaging and treatment techniques. It discusses diagnostic techniques like X-rays, CT scans, PET scans, ultrasound, MRI, and endoscopy. It explains how each works, such as how X-rays are produced via interactions between electrons and a tungsten target, and how PET scans detect gamma ray pairs to construct 3D images. The document also includes a quiz testing knowledge of these different imaging modalities.
CT scanning provides cross-sectional images of the body which can be manipulated and reformatted in various planes. It uses X-rays combined with computer processing to generate 3D images of tissues and organs. CT scanning is more detailed than standard X-rays and can detect abnormalities such as tumors, bleeding, fractures and blockages. It has various medical applications for imaging organs like the brain, lungs, kidneys and blood vessels. While it provides advantages over other imaging methods, it also involves exposure to ionizing radiation.
This document provides an overview of nuclear medicine techniques for obtaining medical images. It discusses gammagraphy, SPECT, and PET imaging. It explains that nuclear medicine uses radioactive substances introduced into the body to generate functional images by detecting the radiation emitted. The document covers basic concepts such as radionuclides commonly used, how the imaging systems work to detect gamma rays and produce images, and differences compared to X-ray imaging. It also provides examples of diagnostic uses of nuclear medicine imaging.
LCU RDG 402 PRINCIPLES OF COMPUTED TOMOGRAPHY.pptxEmmanuelOluseyi1
This document provides an outline for a course on principles of computed tomography. It discusses key topics that will be covered, including image digitization, computed radiography, basic CT principles, and care of radiographic equipment. The objectives are for students to understand the principles of image digitization, computed radiography, CT scanning, and components of CT machines. It also explains some of the technical aspects of digital imaging, spatial resolution, CT scanning principles, CT equipment components like the gantry and x-ray tube, and characteristics of ideal x-ray detectors.
This document discusses non-invasive bilirubin measurement. It describes bilirubin and how it is measured, including techniques like transcutaneous bilirubinometry and optical imaging. Transcutaneous bilirubinometry works by directing light into the skin and measuring the returned light intensity. Several companies produce bilirubinometers that use this technique. While non-invasive bilirubin measurement is desirable to screen for jaundice, limitations include effects of phototherapy and skin characteristics on accuracy.
This document discusses the frequency response of operational amplifiers. It defines frequency response as a measure of the output spectrum of a system in response to an input stimulus over a range of frequencies. It describes how open-loop gain, frequency compensation, closed-loop gain, gain-bandwidth product, and slew rate characterize the non-ideal frequency response of op-amps. Frequency compensation modifies the gain and phase characteristics to increase bandwidth by adding resistance-capacitance networks. The gain-bandwidth product provides a measure of an op-amp's useful bandwidth.
The document summarizes important parameters that influence electrical susceptibility in the human body:
1. Threshold and let-go variability depends on factors like gender, with women having lower thresholds on average. Skin impedance also influences thresholds.
2. Let-go current is minimized at power-line frequencies of 50-60 Hz.
3. Longer shock durations require lower stimulating currents to cause ventricular fibrillation, according to the strength-duration relationship. Body weight also correlates with higher fibrillation currents.
This document discusses the use of artificial intelligence in breast imaging, specifically for the early detection of breast cancer. It provides background on common breast imaging techniques like mammography, tomosynthesis, ultrasound and MRI. It then discusses traditional CAD (computer-aided detection) systems and their limitations in detecting cancers. The document introduces artificial intelligence and how techniques like machine learning and deep learning can improve upon traditional CAD systems. It reviews several studies that have found AI-based systems can help radiologists achieve higher accuracy and reduce false-positive rates compared to unaided diagnosis. Finally, it mentions several companies developing AI solutions for applications in mammography, tomosynthesis and breast MRI.
Computed radiography uses image plates containing photostimulable phosphor to digitally capture x-ray images. The image plate is exposed in the cassette, retaining a latent image. This image is released and converted to light when scanned by a laser, and detected to generate a digital image file. Key advantages include reduced failed exposures, cassette-based mobility, and reusable image plates. Disadvantages include potentially lower resolution than film and longer image read-out times.
The document discusses ventilator breathing circuits and their sterilization process. Breathing circuits create an artificial atmosphere between the patient and ventilator. They are produced via plastic injection and extrusion methods using various plastics. Circuits are packaged in Tyvek and sterilized using ethylene oxide which is the preferred method as it is effective and heat is not suitable. The sterilization process involves conditioning, exposure to ethylene oxide gas, and aeration to remove residues over 12-14 hours. Biological and chemical indicators are used to validate the sterilization process was effective.
THE TREATMENT OF INFECTIONS ON TOOTH SURFACES AND IN ROOT CANALS WITH THREE DIFFERENT LASER MODALITIES: Photodynamic Therapy, Photothermal Therapy, Photoablation
The document discusses the importance of calibrating x-ray devices and describes an x-ray test device that can perform this calibration. It notes that calibration is important to ensure x-ray devices provide accurate results and do not expose patients or employees to excessive radiation. The x-ray test device can calibrate various devices like x-ray machines, mammography machines, and CT/PET scanners. It performs tests such as checking the kVp, exposure time, output stability, filtration, and light field compatibility. A video also demonstrates how the x-ray test device calibration procedure works.
The document discusses several definitions of motivation provided by different authors, including the idea that motivation is the force that drives behavior and directs it towards a goal. It also summarizes Maslow's hierarchy of needs and Herzberg's two-factor theory of motivation. The expectancy theory of motivation holds that effort leads to performance and performance leads to rewards. Motivated employees work more productively and seek improvement. An organization's reward system is designed to influence employee behavior and performance. Job satisfaction depends on both organizational factors like leadership and pay as well as personal factors like personality and career development.
Gay marriage and the rights of LGBTQ individuals have been a long struggle. The document discusses the history and definitions related to being gay as well as perspectives on gay marriage around the world. While some cultures are less accepting, many places in Europe, America and Australia have become more supportive with some countries legally recognizing gay marriage and providing protections. However, lack of acceptance and discrimination can negatively impact the mental and physical health of gay individuals. There is also debate around gay couples adopting children, though research suggests children of gay parents develop normally.
Basavarajeeyam is an important text for ayurvedic physician belonging to andhra pradehs. It is a popular compendium in various parts of our country as well as in andhra pradesh. The content of the text was presented in sanskrit and telugu language (Bilingual). One of the most famous book in ayurvedic pharmaceutics and therapeutics. This book contains 25 chapters called as prakaranas. Many rasaoushadis were explained, pioneer of dhatu druti, nadi pareeksha, mutra pareeksha etc. Belongs to the period of 15-16 century. New diseases like upadamsha, phiranga rogas are explained.
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Basavarajeeyam is a Sreshta Sangraha grantha (Compiled book ), written by Neelkanta kotturu Basavaraja Virachita. It contains 25 Prakaranas, First 24 Chapters related to Rogas& 25th to Rasadravyas.
Muktapishti is a traditional Ayurvedic preparation made from Shoditha Mukta (Purified Pearl), is believed to help regulate thyroid function and reduce symptoms of hyperthyroidism due to its cooling and balancing properties. Clinical evidence on its efficacy remains limited, necessitating further research to validate its therapeutic benefits.
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There is increasing confidence that cell therapies will soon play a role in the treatment of autoimmune disorders, but the extent of this impact remains to be seen. Early readouts on autologous CAR-Ts in lupus are encouraging, but manufacturing and cost limitations are likely to restrict access to highly refractory patients. Allogeneic CAR-Ts have the potential to broaden access to earlier lines of treatment due to their inherent cost benefits, however they will need to demonstrate comparable or improved efficacy to established modalities.
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We’re talking about Vedic Meditation, a form of meditation that has been around for at least 5,000 years. Back then, the people who lived in the Indus Valley, now known as India and Pakistan, practised meditation as a fundamental part of daily life. This knowledge that has given us yoga and Ayurveda, was known as Veda, hence the name Vedic. And though there are some written records, the practice has been passed down verbally from generation to generation.
2. Overview
Measurement of Projection Data
X-Ray Tomography
Emission Computed Tomography
Ultrasonic Computed Tomography
Magnetic Resonance Imaging
Alliasing Artifacts and Noise in CT Images
Alliasing Artifacts
Noise in Reconstructed Images
3. Measurement of Projection Data
• Computed X-Ray Tomography
X-ray Computed Tomography (CT) is a
nondestructive technique for visualizing interior
features within solid objects, and for obtaining
digital information on their 3-D geometries and
properties.
X-ray attenuation is based on Beer-Lambert law
as given below:
where, I0 and I(x) , are X-ray intensities before
and after the matter and , are attenuation
coefficients and dimensions of volumes along
the X-ray beam.
Figure. Schematics of imaging with computed
x-ray tomography
4. Measurement of Projection Data
For the range of photon energies most
commonly encountered for diagnostic
imaging (from 20 to 150 keV), the
mechanisms responsible for these two
contributions to attenuation are the
photoelectric and the Compton effects,
respectively.
Figure. (A) Mechanism of photoelectric effect, (B)
Mechanism of Compton effect
• Computed X-Ray Tomography
5. Measurement of Projection Data
• Monochromatic X-Ray Projections
Figure. A parallel beam of x-rays is shown propagating
through a cross section of the human body.
: Attenuation coefficient
Nin : the total number of photons that enter
the object
Nd : the total number of photons exiting
ds : an element of length and where the
integration is carried out along line AB shown in
the figure.
6. Measurement of Projection Data
• Measurement of Projection Data with Polychromatic Sources
Figure. an experimentally measured x-ray tube
spectrum taken for an anode voltage of 105 kvp.
In practice, the x-ray sources used for medical
imaging do not produce monoenergetic
photons.
Sin(E) represents the incident photon number density
(also called energy spectral density of the incident
photons). Sin(E) dE is the total number of incident
photons in the energy range E and E + dE. This
equation incorporates the fact that the linear
attenuation coefficient, µ ,at a point (x,y) is also a
function of energy.
7. Measurement of Projection Data
To reconstruct the internal structure of an object, instead of
using the measured attenuation coefficients that are
converted to Hounsfield unit (HU) scale. The HU values of
distilled water, air and bone are 0, -1000 and 1000 HU
respectively. The transformation formula as follows:
Figure. Attenuation of X-ray beam while passing through the
matter which includes various attenuation coefficients
8. • Polychromaticity Artifacts in X-Ray CT
Measurement of Projection Data
Beam hardening artifacts are most noticeable in
the CT images of the head, and involve two
different types of distortions (cupping, dark
bands and streaks).
Figure. This reconstruction shows the effect of polychromaticity
artifacts in a simulated skull. (a) shows the reconstructed image using
the spectrum while (b) is the center line of the reconstruction for
both the polychromatic and monochromatic cases.
9. Measurement of Projection Data
Figure. Beam hardening artifact on a CT image.
• Polychromaticity Artifacts in X-Ray CT
10. Measurement of Projection Data
• Scatter
X-ray scatter leads to another type of error in the
measurement of a projection. Recall that an x-ray beam
traveling through an object can be attenuated by
photoelectric absorption or by scattering. Photoelectric
absorption is energy dependent and leads to beam
hardening. On the other hand, attenuation by
scattering occurs because some of the original energy
in the beam is deflected onto a new path. The scatter
angle is random but generally more x-rays are scattered
in the forward direction.
Figure. The effect of scatter on two different
projections is shown here. For the projections
where the intensity of the primary beam is high
the scatter makes little difference, When the
intensity of the scattered beam is high
compared to the primary beam then large
(relative) errors are seen.
11. Measurement of Projection Data
• Different Methods for Scanning
There are two scan configurations that lead to rapid data collection.
These are
i) fan beam rotational type (usually called the rotate-rotate or the third
generation)
ii) fixed detector ring with a rotating source type (usually called the
rotate-fixed or the fourth generation).
12. Measurement of Projection Data
Figure. Fan beam rotational type CT. In a third-
generation fan beam x-ray tomography machine a
point source of x-rays and a detector array are rotated
continuously around the patient.
• Different Methods for Scanning
Main advantage of this generation is the
decrease of the amount of time to get image.
Disadvantage of it is detector elements
makes it very expensive and also it produces
image artifact known as ring artifact. The
artifacts are generated because of the large
number of detectors.
13. Measurement of Projection Data
• Different Methods for Scanning
Figure. In a fourth-generation scanner an x-ray source
rotates continuously around the patient. A stationary ring
of detectors completely surrounds the patient.
Fourth generation is designed to ease the
artifacts produced by third generation.
The detectors that rotates the gantri were
removed and placed in a stationary ring
around the patient.
This system uses fan shaped x-ray beams to
construct the image.
14. An important difference exists between the third- and the fourth-
generation configurations.
The data in a third-generation scanner are limited essentially in the
number of rays in each projection, although there is no limit on the
number of projections themselves; one can have only as many rays in
each projection as the number of detectors in the detector array.
On the other hand, the data collected in a fourth-generation scanner are
limited in the number of projections that may be generated, while there is
no limit on the number of rays in each projection.
Measurement of Projection Data
• Different Methods for Scanning
15. Measurement of Projection Data
• Emission Computed Tomography
In conventional x-ray tomography, physicians use the attenuation
coefficient of tissue to infer diagnostic information about the
patient. Emission CT, on the other hand, uses the decay of
radioactive isotopes to image the distribution of the isotope as a
function of time.
Emission CT is of two types: single photon emission CT and
positron emission CT. The word single in the former refers to the
product of the radioactive decay, a single photon, while in positron
emission CT the decay produces a single positron.
16. Measurement of Projection Data
• Single Photon Emission Tomography
Single-photon emission computed tomography
(SPECT, or less commonly, SPET) is a nuclear
medicine tomographic imaging technique
using gamma rays. It is very similar to
conventional nuclear medicine planar imaging
using a gamma camera (that is, scintigraphy)
but is able to provide true 3D information. This
information is typically presented as cross-
sectional slices through the patient, but can be
freely reformatted or manipulated as required.
Figure In single photon emission
tomography a distributed source of gamma-
rays is imaged using a collimated detector.
17. • Positron Emission Tomography
Measurement of Projection Data
Figure. In positron emission tomography the decay of a
positron/electron pair is detected by a pair of photons.
Since the photons are released in opposite directions it
is possible to determine which ray it came from and
measure a projection.
Positron emission tomography (PET) is
a nuclear medicine functional
imaging technique that is used to observe
metabolic processes in the body.
The system detects pairs of gamma
rays emitted indirectly by a positron-
emitting radionuclide (tracer), which is
introduced into the body on a biologically
active molecule.
19. It uses ultrasound waves as physical phenomenon for
imaging. It is mostly in use for soft tissue medical
imaging, especially breast imaging.
When diffraction effects can be ignored, ultrasound CT
is very similar to x-ray tomography.
In the first measurement step a defined ultrasound
wave is generated with typically Piezoelectric ultrasound
transducers, transmitted in direction of the
measurement object and received with other or the
same ultrasound transducers.
• Ultrasonic Computed Tomography
Measurement of Projection Data
Figure. Measurement procedure of a 3D USCT:
semi-spherical water filled measurement
container lined with ultrasound transducer
arrays in cylindrical housings (transducer
elements as green dots). Centrally placed a
simple object (red). Spherical wave emitted
(semi-transparent blue), all other transducers
gather data. Wave-front interacts with object
and re-emits a secondary wave (semi-
transparent purple). Repeated iteratively for all
transducers.
20. Measurement of Projection Data
Figure. In ultrasound refractive index tomography
the time it takes for an ultrasound pulse to travel
between points A and B is measured.
USCT : the attenuation the wave's sound pressure
experiences indicate on the object's attenuation coefficient,
the time-of-flight of the wave gives speed of sound
information, and the scattered wave indicates on the
echogenicity of the object (e.g. refraction index, surface
morphology, etc.).
Ultrasonic Refractive Index Tomography
Ultrasonic Attenuation Tomography
• Ultrasonic Computed Tomography
21. Measurement of Projection Data
• Magnetic Resonance Imaging
Magnetic resonance imaging is based on
the measurement of radio frequency
electromagnetic waves as a spinning
nucleus returns to its equilibrium state.
Two of the more interesting atoms for
MRI are hydrogen and phosphorus.
First, energy from an oscillating magnetic
field temporarily is applied to the patient
at the appropriate resonance frequency.
The excited hydrogen atoms emit a radio
frequency signal, which is measured by
a receiving coil.
Figure. Schematic of construction of a cylindrical superconducting
MR scanner.
23. Alliasing Artifacts and Noise in CT Images
• Aliasing Artifacts
They are also known as undersampling. The
number of projections used to reconstruct
a CT image is one of the determining
factors in image quality. Too large an
interval between projections
(undersampling) and reduced number of
projections can result in misregistration.
Figure. Sixteen reconstructions of an ellipse are shown for
different values of K, the number of projections, and N, the
number of rays in each projection. In each case the
reconstructions were windowed to emphasize the
distortions.
Figure. The projection of an object with sharp
discontinuities will have significant high frequency energy.
24. Alliasing Artifacts and Noise in CT Images
Figure. (a) Reconstruction of an ellipse with N
= 64 and K = 512. (b) Reconstruction from only
the aliased spectrum. Note that the streaks
exactly match those in (a). (c) Image obtained
by subtracting (b) from (a). This is the
reconstruction that would be obtained
provided the data for the N = 64 case were
truly bandlimited.
• Aliasing Artifacts
25. Alliasing Artifacts and Noise in CT Images
• Noise in Reconstructed Images
There are two types of noise to be considered.
The first, a continuously varying error due to electrical noise or roundoff errors, can be
modeled as a simple additive noise.
The second type of noise is best exemplified by shot noise in x-ray tomography
Figure. Streaking artifacts due to noise. (a) Original scan with noise
and (b) with adaptive noise reduction
Editor's Notes
the attenuation of x-rays through an object as in conventional x-ray
tomography, the decay of radioactive nucleoids in the body as in emission
tomography, or the refractive index variations as in ultrasonic tomography.
When X-ray photons passing through the matter, they lose their energy because of absorption and scattering interactions between X-ray beams and matter. The underlying principle of X-ray imaging is to the determination of how much X-ray photons are attenuated when they passing through the object. X-ray attenuation is based on Beer-Lambert law as given below:
Photoelectric absorption consists of an x-ray photon imparting all its energy to a tightly bound inner electron in an atom. The electron uses some of this acquired energy to overcome the binding energy within its shell, the rest appearing as the kinetic energy of the thus freed electron. The electron that is removed is then called a photoelectron. The incident photon is completely absorbed in the process. Hence it forms one of the reasons for attenuation of X-ray beam as it passes through the matter. The Compton scattering, on the other hand, consists of the interaction of the x-ray photon with either a free electron, or one that is only loosely bound in one of the outer shells of an atom. As a result of this interaction, the x-ray photon is deflected from its original direction of travel with some loss of energy, which is gained by the electron.
If all the photons possess the same energy we may now consider p to be a function of two space coordinates, x and y, and therefore denote the attenuation coefficient by ~(x, y). Let Ni” be
the total number of photons that enter the object (within the time interval of experimental measurement) through the beam from side A. And let Nd be the total number of photons exiting (within the same time interval) through the beam on side B. When the width, 7, of the beam is sufficiently small, reasoning similar to what was used for the one-dimensional case now leads to the following relationship between the numbers Nd and Ni” where ds is an element of length and where the integration is carried out along line AB shown in the figure. The left-hand side precisely constitutes a ray integral for a projection. Therefore, measurements like In (Nin/Nd) taken for different rays at different angles may be used to generate projection data for the function ~(x, y). We would like to reiterate that this is strictly true only under the assumption that the x-ray beam consists of monoenergetic photons. This assumption is necessary because the linear attenuation coefficient is, in general, a function of photon energy.
In discussing polychromatic x-ray photons one has to bear in mind that there are basically three different types of detectors [McC75]. The output of a detector may be proportional to the total number of photons incident on it, or it may be proportional to total photon energy, or it may respond to energy deposition per unit mass. Most counting-type detectors are of the first type,
most scintillation-type detectors are of the second type, and most ionization detectors are of the third type. In determining the output of a detector one must also take into account the dependence of detector sensitivity on photon energy. In this work we will assume for the sake of simplicity that the detector sensitivity is constant over the energy range of interest. In the energy ranges used for diagnostic examinations the linear attenuation coefficient for many tissues decreases with energy. For a propagating polychromatic x-ray beam this causes the low energy photons to be preferentially absorbed, so that the remaining beam becomes proportionately richer in high energy photons. In other words, the mean energy associated with the exit spectrum, S&E), is higher than that associated with the Incident spectrum, Sin(E). This phenomenon is called beam hardening.
An x-ray beam is composed of individual photons with different levels of energies. As the beam passes through an object, it becomes harder which means that its mean energy increases, because the lower energy photons are absorbed faster than the higher energy photons. Although the beam-hardening effect cannot be eliminated completely, beam filtering techniques, software methods such as using iterative algorithm for reconstruction of images and hardware methods have been developed. In addition, lowering the thickness of the section and increasing the mAs value are also useful in reducing beam hardening effects Two types of artifact can result from this effect: cupping artifacts and dark bands or streaks between dense objects in the image[22]. Cupping : In this artifact, when the x-rays pass through a homogeneous cylindrical structure, more hardening of x-rays is seen in the middle of structure according to edges because, x-rays pass through more material. As the beam becomes harder, the rate at which it is attenuated decreases, so the beam is more intense when it reaches the detectors than would be expected if it had not been hardened. It can be prevented by using thin slices for reconstruction with regularized algorithm and by increasing homogenity of X-ray beam[22,23]. Dark bands and streaks : In a heterogeneous section, black bands or streaks can be observed between two dense objects. This is because the attenuation value at the position where the tube of the same power displays an object is less than the attenuation value at the other position. Such artifacts can be seen in bone-soft tissue interfaces and in imaging with contrast material
. To reduce this artifact, filtration, calibration verification and various softwares are used [22,23].
The only way to prevent scatter from leading to projection errors is to build detectors that are perfectly collimated. X-ray scatter leads to artifacts in reconstruction because the effect changes
with each projection. While the intensity of scattered x-rays is approximately constant for different rotations of the object, the intensity of the primary beam (at the detector) is not. Once the x-rays have passed through the collimator the detector simply sums the two intensities. For rays through the object where the primary intensity is very small, the effect of scatter will be large, while for other rays when the primary beam is large, scattered x-rays will not lead to much error. This is shown in Fig. 4.7
both of these schemes use fan beam reconstruction concepts. While the reconstruction algorithms for a parallel beam machine are simpler, the time to scan with an x-ray source across an object and then rotate the entire source-detector arrangement for the next scan is usually too long. The time for scanning across the object can be reduced by using an array of sources, but only at
great cost. Thus almost all CT machines in production today use a fan beam configuration.
In a fan beam rotating detector (third-generation) scanner, if one detector is defective the same ray in every projection gets recorded incorrectly. Such correlated errors in all the projections form ring artifacts
In a fixed-detector and rotating-source scanner (fourth generation) a large number of detectors are mounted on a fixed ring as shown in Fig. 4.11. Inside this ring is an x-ray tube that continually rotates around the patient. During this rotation the output of the detector integrators facing the tube is sampled every few milliseconds. All such samples for any one detector constitute what
is known as a detector-vertexf an. (The fan beam data thus collected from a fourth-generation machine are similar to third-generation fan beam data.) Since the detectors are placed at fixed equiangular intervals around a ring, the data collected by sampling a detector are approximately equiangular, but not exactly so because the source and the detector rings must have different radii. Generally, interpolation is used to convert these data into a more precise equiangular fan for reconstruction using the algorithms
when one detector fails in a fixed detector ring type (fourthgeneration) scanner, it implies a loss or partial recording of one complete projection; when a large number of projections are measured, a loss of one projection usually does not noticeably degrade the quality of a reconstruction
’ (It is now known that for good-quality reconstructions the number of projections should be comparable to the number of rays in each projection.
These isotopes may be administered to the patient in the form of radiopharmaceuticals either by injection or by inhalation. Thus, for example, by administering a radioactive isotope by inhalation, emission CT can be used to trace the path of the isotope through the lungs and the rest of the body.
The technique requires delivery of a gamma-emitting radioisotope (a radionuclide) into the patient, normally through injection into the bloodstream. On occasion, the radioisotope is a simple soluble dissolved ion, such as an isotope of gallium(III).
A serious difficulty with tomographic imaging of a gamma-ray emitting source is caused by the attenuation that photons suffer during their travel from the emitting nuclei to the detector. 3 The extent of this attenuation depends upon both the photon energy and the nature of the tissue.
With positron emission tomography (PET), we want to determine the concentration and location of a positron emitting compound in a desired cross section of the human body. Perhaps the most remarkable feature of a positron emitter, at least from the standpoint of tomographic imaging, is the fact that an emitted positron can’t exist in nature for any length of time. it interacts with an electron and, as a result, their masses are annihilated, creating two photons of 5 11 keV each.
The technique is based on the detection of radioactivity emitted after a small amount of a radioactive tracer is injected into a peripheral vein. The tracer is administered as an intravenous injection usually labelled with oxygen-15, fluorine-18, carbon-11, or nitrogen-13. The total radioactive dose is similar to the dose used in computed tomography.
PET scans take 10-40 minutes to complete. They are painless, and, as for computed tomography, the patient is fully clothed.
• A common use for PET is to measure the rate of consumption of glucose in different parts of the body.
• Accumulation of the radiolabelled glucose analogue 18-fluorodeoxyglucose (FDG) allows measurement of the rate of consumption of glucose. One clinical use of this is to distinguish between benign and malignant tumours (malignant tumours metabolise glucose at a faster rate than benign tumours). Whole body scans are often performed to stage a cancer.
• Other applications of PET include looking at the blood flow and oxygen consumption in different parts of the brain—for example, in understanding strokes and dementia. Tracking chemical neurotransmitters (such as dopamine, in Parkinson's disease) can also be performed with this technique.
Ultrasound computer tomographs use ultrasound waves for creating images. In the first measurement step a defined ultrasound wave is generated with typically Piezoelectric ultrasound transducers, transmitted in direction of the measurement object and received with other or the same ultrasound transducers. While traversing and interacting with the object the ultrasound wave is changed by the object and carries now information about the object. After being recorded the information from the modulated waves can be extracted and used to create an image of the object in a second step
In both x ray and ultrasonic cases a transmitter illuminates the object and a line integral of the attenuation can be estimated by measuring the energy on the far side of the object. Ultrasound differs from x-rays because the propagation speed is much lower and thus it is possible to measure the exact pressure of the wave as a function of time. From the pressure waveform it is possible, for example, to measure not only the attenuation of the pressure field but also the delay in the signal induced by the object. From these two measurements it is possible to estimate the attenuation coefficient and the refractive index of the object. İn computerized tomography it is essential to know the path that a ray traverses from the source to the detector. In x-ray and emission tomography these paths are straight lines (within limits of the detector collimators), but this isn’t always the case for ultrasound tomography. When an ultrasonic beam propagates through tissue, it undergoes a deflection at every interface between tissues of different refractive indices.
Unlike X-ray or other physical properties which provide typically only one information, ultrasound provides multiple information of the object for imaging: the attenuation the wave's sound pressure experiences indicate on the object's attenuation coefficient, the time-of-flight of the wave gives speed of sound information, and the scattered wave indicates on the echogenicity of the object (e.g. refraction index, surface morphology, etc.).
Magnetic resonance imaging is based on the measurement of radio frequency electromagnetic waves as a spinning nucleus returns to its equilibrium state. Any nucleus with an odd number of particles (protons and neutrons) has a magnetic moment, and, when the atom is placed in a strong magnetic field, the moment of the nucleus tends to line up with the field. If
the atom is then excited by another magnetic field it emits a radio frequency signal as the nucleus returns to its equilibrium position. Since the frequency of the signal is dependent on not only the type of atom but also the magnetic fields present, the position and type of each nucleus can be detected by appropriate signal processing.
Two of the more interesting atoms for MRI are hydrogen and phosphorus. The hydrogen atom is found most often bound into a water molecule while phosphorus is an important link in the transfer of energy in biological systems. Both of these atoms have an odd number of nucleons and thus act like a spinning magnetic dipole when placed into a strong field.
In most medical applications, protons (hydrogen atoms) in tissues containing water molecules create a signal that is processed to form an image of the body. First, energy from an oscillating magnetic field temporarily is applied to the patient at the appropriate resonance frequency. The excited hydrogen atoms emit a radio frequency signal, which is measured by a receiving coil. The radio signal may be made to encode position information by varying the main magnetic field using gradient coils. As these coils are rapidly switched on and off they create the characteristic repetitive noise of an MRI scan. The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. Exogenous contrast agents may be given to the person to make the image clearer.[4]
They are also known as undersampling. The number of projections used to reconstruct a CT image is one of the determining factors in image quality. Too large an interval between projections (undersampling) and reduced number of projections can result in misregistration by the computer of information relating to sharp edges and small objects. This leads to an effect known as view aliasing, where fine stripes appear to be radiating from the edge of, but at a distance from, a dense structure. Stripes appearing close to the structure are more likely to be caused by undersampling within a projection, which is known as ray aliasing. These aliasing artifacts doesn’t effect image quaility significantly but it must be prevented when the resolution of fine details are important [3,22]. This artifact can be corrected by increasing scan time, and reducing pitch. If a partial scan was used, this artifact can also be corrected by rescanning in a complete arc[25].
Noise in computed tomography is an unwanted change in pixel values in an otherwise homogenous image. Often, noise is defined loosely as, the grainy appearance on cross-sectional imaging; more often than not, this is quantum mottle. Noise in CT is measured via the signal to noise ratio (SNR); comparing the level of desired signal (photons) to the level of background noise (pixels deviating from normal). The higher the ratio, the less noise is present on the image
There are two types of noise to be considered. The first, a continuously varying error due to electrical noise or roundoff errors, can be modeled as a simple additive noise. The reconstructed image can therefore be considered to be the sum of two images, the true image and that image resulting from the noise. The second type of noise is best exemplified by shot noise in x-ray tomography. In this case the magnitude of the possible error is a function of the number of x-ray photons that exit the object and the error analysis becomes more involved.