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CT image acquisition
- 1. CT IMAGE ACQUISITION Kezia Tyagi CT IMAGE ACQUISITION DR. Pradeep Patil Prof. Department of Radio-diagnosis, DY Patil medical college, hospital & research institute
- 2. INTRODUCTION TO CT Invented in 1972 by Godfrey Hounsfield First CT scanner was installed between 1974- 1976 Overtime, many improvements have been made in speed, patient’s comfort and resolution. Tomographi c Slice Scintillati on detector Analysis Reconstruct ion
- 3. CT GENERATIONS First Generation Second Generation Multiple detectors (Around 30) are arranged in a row, translation and rotation movement, fan beam is used.
- 7. IMAGE ACQUISITION A method by which the patient is systematically scanned by the x-ray tube and detectors to collect information for image reconstruction
- 8. OVERVIEW OF PROCESS 1) Patient preparation 2) Patient positioning 3) X-Ray tube rotation 4) Data Acquisition 5) Image Reconstruction
- 10. COMPONENTS OF CT SCANNER 1) X-ray Tube Source of x-rays radiation source should supply a monochromatic x-ray beam Earlier CT scanners used radionuclides Earlier x-ray tube models used oil-cooled fixed anode, large spot tubes of about 120kV and 30 mA New fan beam units
- 11. COMPONENTS OF CT SCANNER 2) Detector Array •series of sensors that detect the x-rays •2 types: • Scintillation Crystals • Xenon gas ionization chambers
- 12. COMPONENTS OF CT SCANNER 3) Collimator •Used to reduce the size and shape of the beam as well as remove the scattered photons. •Types: • Pre-Patient • Post- Patient
- 13. COMPONENTS OF CT SCANNER 3) Gantry •large circular structure that houses the x-ray tube and detector array. •rotates around the patient during scan to capture x-ray projections from multiple 4) Patient table •long narrow bed that patient lies on during the scan •can be moved horizontally or vertically to position the area of interest within the view.
- 14. COMPONENTS OF CT SCANNER 5) Computer •“brain” of the CT scanner •receives data and uses algorithims for image reconstruction 6) Console •control center of the CT scanner
- 15. PROCESS OF IMAGE ACQUISITION x-ray generation detector array analog to digital conversion Data collection Data processing Image reconstructio n Image display
- 17. X-RAY GENERATION •x-rays produced when high energy electrons are accelerated and collide with metal target •X-ray beam is produced, detected and converted into electrical signals •x-ray tube consists of cathode and anode. •Collimation •Dosing
- 20. ANALOG- TO - DIGITAL CONVERSION •converts analog signals produced by the detector array into digital signals that can be processed by the computer system of the CT scanner. •Continous electrical signals varying in ampltitude •Data Acquisition System
- 21. DATA COLLECTION •Collects the digital signals produced by ADC and organizing them into matrix of data points. • As the x-ray beam beam passes through the patient’s body, it is absorbed to varying degrees by different tissues, producing a unique pattern of x-ray attenuation that is detected by the detector array. •Divided into three stages: • Pre-scan • Scan • Post Scan
- 22. In computed tomography a cross- sectional layer of the body is divided into many tiny blocks Each block is assigned a number proportional to the degree that the block attenuated the x-ray beam. Individual blocks are called voxel. Their composition and thickness, along with the quality of the beam, determine the degree of attenuation
- 25. TYPES OF CT SCANS •Helical •Multislice •Cone Beam •Dual Energy
- 28. DATA PROCESSING •Raw data is processed by the CT scanner’s computer system to produce a series of cross-sectional images. • has 3 substeps : • Image reconstruction • Image enhancement • Image analysis
- 31. THANK YOU
Editor's Notes
- In 1972 att the Annual Congress of the British Institute of Radiology, 1972, G.N. Hounsfield, a senior research scientist at EMI Limited announced the invention of a new imaging technique, which he called "computerized axial transverse scanning.“ a thin cross section of the head, a tomographic slice, was examined from multiple angles with a pencil-like x-ray beam. The transmitted radiation was counted by a scintillation detector, fed into a computer for analysis by a mathematical algorithm, and reconstructed as a tomographic image. The basic principle behind CT is that the internal structure of an object can be reconstructed from multiple projections of the object.
- The basic principle behind CT is that the internal structure of an object can be reconstructed from multiple projections of the object.
- 1) Before a CT scan, patient is required to remove any metallic object or jewelry which can interfere with the scan. In some cases, contrast might need to be infused prior to the scan. 2) Patient positioning- once the patient is prepared, they are positioned on the scanner bed. the area of the body being scanned is positioned at the center of the scanner. the patient is secured to the scanner bed to prevent movement during the scan. 3) X-ray tube rotation: Ct scanner consists of an x-ray tube and a detector array that rotate around the patient. X-ray tube emits a beam of x-ray that passes through the patient’s body and is detected by the detector array on the opposite side of the patient 4) data acquisition- as the x-ray tube and detector array rotate around the patient, multiple x-ray projections are acquired. the data from x-ray projections are recorded by the CT scanner and used to reconstruct an image of the area scanned 5) Image reconstruction- once the data is acquired it is processed by a computer to create a series of cross-sectional images of the body. These images can be viewed individually or compiled into 3D models. 6) Image interpretation- the final step in CT acquisition process is image interpretation. Radiologists or other medical professionals review these images.
- Tube emits x ray beam Beam is then shaped “ “ collimated Attenuated Transmitted xrays are detected and converted to electrical signal ADC converts electrical signal to digital data Digital data is then sent to computer
- x-ray tube- source of the x-rays used in a CT scan. small, high powered device that generates a beam of x-rays that pass through the patients body. Ideally, the radiation source for CT would supply a monochromatic x-ray beam (i.e., one made up of photons all having the same wavelength). With a monochromatic beam, image reconstruction is simpler and more accurate. Earlier ct scanners used radionuclides but radiation intensities were too low to be clinically useful. X-ray tubes are currently being used in all ct scanners. Earlier models of x-ray tubes used oil-cooled fixed anode, large spot tubes of about 120kv and 30mA. New fan beam units have a diagnostic type x-ray tube with a rotating anode and a much smaller focal spot, in some units down to 0.6 mm. These tubes have large heat loading and heat dissipation capabilities to withstand the very high heat loads generated when multiple slices are acquired in rapid sequence. Anode heat capacities as high as 3,500,000 heat units are available, with tube currents of over 600 mA possible.
- series of sensors that detect the x-rays as they pass through the patients body. located on oppposite side of the patient from the x-ray tube and rotates around the patient along with the x-ray tube. scintillation detectors- convert the x-ray into visible light photon. Uses scintillation crystal coupled to photomultiplier tube to convert light to electron Xenon gas ionization chambers- must have the following: 1. An anode and a cathode 2. A counting gas (inert gas) 3. A voltage between the anode and cathode 4. Walls that separate the detector from the rest of the world 5. A window for the radiation photon to enter the detector
- receives the data from the detector array and uses complex algorithms to reconstruct an image of the area being scanned.
- x-ray beam produced by the ct scanner passes through the patient’s body and the x-rays that pass through are detected and converted into electrical signals by the detector array. x-ray tube consists of cathode and anode. cathode emits a beam of electrons, which are accelerated towards the anode by a high voltage applied across the tube. When the electrons strike the anode, they produce x-rays that are emitted in a cone shaped beam towards the patient. x-ray tube is mounted on a gantry that rotates around the pt. beam of x-rays is collimated to produce a thin fan shaped beam that passes through the patients body. collimation is achieved using a series of lead plates or shutters that can be adjusted to control the width and thickness of the x-ray beam. to produce high quality images, it important to use an appropriate level of x-ray energy. this is controlled by adjusting the volatge applied to the x-ray tube. high voltage produces x-rays with higher energy level that penetrate deeper into the body. lower voltages are better suited got imaging superficial tissue. to reduce the radiation exposure, modern ct scanners use dose modulation which adjusts the x-ray beam’s intensity based on the patient’s size and shape.
- analog signals produced by the detector array are continuous electrical signals that vary in amplitude based on the x-ray intensity that strikes each detector element. .the ADC samples the analog signal at regular intervals and converts each sample into a corresponding digital value that represents the x-ray intensity at that point in time. After the ADC converts the analog signals into digital signals, the digital data is collected by the data acquisition system and organized into matrix of data points. Each data point represents a specific position in a patient’s body and a specific position angle of rotation of the x-ray tube. The digital data is then processed by the ct-scanner’s computer system to produce a series of cross sectional images of the body part being imaged. The quality of digital signal is determined by number of levels used by the ADC. it is imortant to minimize quanization error to produce accurate and high-quality images.
- Controlled by CT scanner’s software and hardware, which coordinate the movement of the x-ray tube and detector array around the patient’s body. Pre scan- patient is positioned correctly and the ct scanner is calibrated. Scan stage: involves actual data collection where x-ray tube and detector rotate around the patient to collect the data Post- scan data- Processing the data to produce images Amount of data can be substantial with each data point representing a small vol. of tissue. to reduce theb amount of data, modern ct canner uses technqiues such as cone-beam CT or helical CT which can collect data over a larger vol of tissue with each rotation.
- Helical- Use a rotating x-ray tube and detector array to acquire data as the pt is moved thru the scanner. results in a continuous spiral like data acquisition that can be used to create 3D images of the scanned area. Images produced are very detailed and can detect small abn. used for imaging organs in motion such as heart and lungs. Multislice- Have multiple rows of detectors that allow for acquisition of multiple slices of data simultaneously. results in faster scan time and higher image resolution commonly used in cardiac imaging. also useful in imaging other organ and structures such as brain, lungs and abdomen. Cone beam- use a cone shaped x-ray beam to acquire data in a single rotation around the patient. useful for detecting dental and ortho problems such as fractures, bone deformities Dual energy- use 2 different x-ray energies to acquire data whuch is used to differentiate between different types of tissue. commonly used in oncology and vascular imaging.
- Data processing involves substeps- Image reconstruction- computer system uses specialized algorithms to reconstruct the cross sectional images from the raw data. Image enhancement - process of manipulating the reconstructed image to improve its visual quality and highlight features of interest. Image analysis- process of interpreting the reconstructed and enhanced image.
- Filtration- process of removing noise from collected data to produce clearer image. most common filter used is ramp filter Backprojection- projecting the collected data back into a two-dimensional image plane. During back-projection, the collected data is used to determine the attenuation of the x-ray beam at each point in the image plane. Backprojection process involevs taking a weighted average of the collected data at each point in the image plane, with the weighting determined by the distance between x-ray source and detector Inerpolation- process of filling in missing data points in the image plane. . final imge is typically displayed in grayscal, with brighter regions indicating areas of higher x-ray attenuation and darker regions with lowsr attenuation End result is a 2d representation of the attenuation og the x-ray beam at each point in the image plane