INTRODUCTION Designed by Godfrey N. Hounsfield to overcome the visual representation challenges in radiography and conventional tomography by collimating the X-ray beam and transmitting it only through small cross-sections of the body
G.N.HOUNSFIELD ALLAN M. CORMACK In 1979, G.N. Hounsfield shared the Nobel Prize in Physiology & Medicine with Allan MacLeod Cormack, Physics Professor who developed solutions to mathematical problems involved in CT.
Important events Dr. Ledley introduced the whole body CT scanner (ACTA scanner) 1974 G.N. Hounsfield shared the Nobel Prize with Allan MacLeod Cormack 1979 First paper on CT presented to British Institute of Radiology by Hounsfield and Dr. Ambrose 1972 First clinically useful CT head scanner was installed at Atkinson-Morley Hospital (England) 1971 G.N. Hounsfield developed first clinically useful CT head scanner 1969 EVENTS YEAR
Computed tomography (CT) scan machines uses X-rays , a powerful form of electromagnetic energy .
CT combines X radiation and radiation detectors coupled with a computer to create cross sectional image of any part of the body.
Cross-sectional slices Think like looking into a loaf of bread by cutting it into thin slices and then viewing the slices individually.
The internal structure of an object can be reconstructed from multiple projections of the object.
CT scanning is a systematic collection and representation of projection data.
Comparison of CT with Conventional Radiography
Conventional radiography suffers from the collapsing of 3D structures onto a 2D image
CT gives accurate diagnostic information about the distribution of structures inside the body
This is the basic idea of computer aided tomography. In a CT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles. Comparison of CT with Conventional Radiography A conventional X-ray image is basically a shadow. Shadows give you an incomplete picture of an object's shape.
Comparison of CT with Conventional Radiography
Comparison of CT with Conventional Radiography Radiographic procedure is qualitative and not quantitative
Comparison of CT with Conventional Tomography
Limitations of Conventional tomography:
Image blurring persists
Degradation of contrast due to scatter radiation
Problems with Film/screen combination
Comparison of CT with Conventional Radiography and Tomography
Although spatial resolution is lower in CT, it has extremely good low contrast resolution, enabling the detection of very small changes in tissue type
0.247 RED BLOOD CELLS 0.230 PLASMA 0.227 CSF 0.222 WATER 0.194 FAT LINEAR ATTENUATION COEFFICIENT ( µ ) MATTER
Data gathering techniques have developed in stages termed Generations.
Scan time reduction is the predominant reason for introducing new configurations
Narrow pencil beam
Single detector per slice
Translate –Rotate movements of Tube- detector combination
Designed only for evaluation of brain
I gen. CT
The axis of rotation passed through the centre of the patient’s head
The total number of transmission measurements = No. of linear measurements X No. of rotatory steps
= 160 X 180 = 28,800
Matrix size: 80 x 80
Scan time: 5min
Grey levels: 8
Over night image reconstruction
I Generation CT Scanner
Head kept enclosed in a water bath
Two side-by-side detectors
A reference detector
Narrow fan beam (3 0 -10 0 )
Linear detector array(30)
Translate-Rotate movements of Tube-Detector combination
Fewer linear movements are needed as there are more detectors to gather the data.
Between linear movements, the gantry rotated 30 o
Only 6 times the linear movements got repeated
Rotate(tube)-Rotate(detectors) Translatory motion is completely eliminated
Pulsed wide fan beam(50 0 -55 0 )
Arc of detectors(600-900)
Detectors are perfectly aligned with the X-Ray tube
Both Xenon and scintillation crystal detectors can be used
Scan time< 5secs
III gen. CT scanners
Continuous wide fan beam(50 0 -55 0 )
Ring of detectors(> 2000)
X-ray tube rotates in a circle inside the detector ring
When the tube is at predescribed angles, the exposed detectors are read.
Scan time< 2 secs
III Vs IV gen. CT scanners
CT Data Acquisition Components
The scanning process begins with data acquisition.
Data Acquisition refers to a method by which the patient is systematically scanned by the X ray tube and detectors to collect enough information for image reconstruction.
More heat loading and heat dissipation capabilities
Small focal spot size (0.6mm) to improve spatial resolution
Compensation filter is being used
To absorb low energy x rays
To reduce patient dose
To provide a more uniform beam
To decrease scatter radiation
To reduce patient dose
To improve image quality
Collimator width determines the slice thickness
The detectors gather information by measuring the x-ray transmission through the patient.
Scintillation crystal detector
(Cadmium tungstate+ Si Photodiode)
Can be used in third and fourth generation scanners
Xenon gas ionisation chamber
Can be used in third generation scanners only
Scintillation crystal detector used in I & II gen. CT scanners
Scintillation crystal detector used in III and IV gen. CT scanners
Detector cross talk occurs when a photon strikes a detector, is partially absorbed and then enters the adjacent detector and is detected again.
Crosstalk produces two weak and signals coming from two different detectors.
Crosstalk is bad because it decreases resolution.
Crosstalk is minimized by using a crystal that is highly efficient in absorbing X-rays (high stopping power).
Xenon gas ionization chamber
Gas filled detector’s efficiency
Gas filled detectors are less efficient than solid state detectors.
The problem can be partially overcome by the following 3 ways.
By using Xenon (z=54), the heaviest of the inert gases
By compressing the Xenon 8 to 11 atmospheres to increase its density
By using a long chamber to increase the number of atoms along the
path of the beam.
Why are Xenon gas detectors not used in IV generation CT scanners?
Typical size of a chamber is 1-2 mm wide, 10mm high and 8-10cm deep
These 10mm long side plates are the reason why Xenon detectors are not in IV generation CT scanners.
Disadvantage of Xenon gas detector
Efficiency - 50 to 60%
This low efficiency is caused by two factors.
Low density of the absorbing material
Absorption of X-rays by the front window, which is needed to contain the high pressure gas
OTHER SCAN CONFIGURATIONS
Interest in faster scan times evolves from a desire to image moving structures such as the wall of the heart and contrast material in blood vessel and heart chambers and to overcome motion artifacts due to cardiac rhythm and patient breathing .
Dynamic Spatial Reconstructor(DSR)
Electron beam computed tomography
DYNAMIC SPATIAL RECONSTRUCTOR
28 X-ray tubes
X-ray tubes are aligned with 28 light amplifiers and TV cameras that are placed behind a single curved fluorescent screen
The gantry rotates about the patient at a rate of 50 RPM
Data for an image acquired in about 16 ms.
Reconstruct 250 C.S. images from each scan data
Disadvantages of DSR
Mechanical motion is not eliminated
Electron Beam Computed Tomography
Large Arcs of tungsten targets
17 slices per second
What is the radiation dose with EBT?
An advantage of the EBT scanner over conventional scanners is instead of exposing the entire circumference of the body to the X-ray beam, the EBT X-ray beam enters from the back. Thus, anterior structures such as the breast and thyroid are subjected to a lesser dose of radiation (17% of the entrance skin dose). EBT scanning is usually 1/5th to 1/10th the radiation exposure as Spiral CT scanning.
The Dynamic Spatial Reconstructor
The seminal scanner for dynamic volumetric imaging is the x-ray CT scanner known as the Dynamic Spatial Reconstructor (DSR) designed and installed at the Mayo Clinic.[1, 2] The DSR has the ability to obtain up to 240 contiguous 0.9mm thick sections in a short a time period as 1/60 second and to repeat this acquisition rate 60 times per second. In practice, the data rate is somewhat reduced from these numbers. The DSR consists of a gantry weighing approximately 17 US tons with a length of 20.5 feet and a diameter of 15 feet. 14 x-ray guns reside in a hemicylindrical configuration and aim at a juxtaposed hemicylindrical fluorescent screen. The images produced on the fluorescent screen by the firing of the x-ray guns are recorded by a bank of 14 television cameras which, until recently, were image isocons which sent analogue signals to be recorded on a bank of 8 video disc recorders. Since american television is comprised of 240 usable lines, and video rates are 60 per second, it is possible to reconstruct a cross sectional image of the body by digitizing each of the 240 television line for each of the 14 cameras and produce 240 cross sections representing 1/60 second resolution. Each stop action stack of slices in actuality represents approximately 0.1second which is the time in which all 14 x-ray guns are sequentially pulsed on. Because there were only 8 video disc recorders, every two television line were averaged to reduce the lines per camera to 120, and the data from two cameras were recorded interlaced on a single video channel. In addition, as the images are produced the gantry rotates at 15 rotations per minute. Thus, in 1/60 second, the gantry moves a degree and a half such that if the organ system of interest moves slow enough to allow for 2/60 second of scanning, it is possible to generate 28 angles of view to use in the reconstruction process. Up to a point, the more angles of view used in the reconstruction process, the better the images. Typically, at least 4/60 seconds of scanning are used to generate a good quality reconstruction. This can not only be accomplished by utilizing contiguous 1/60 second data sets, but it is possible to retrospectively gate the data together by selecting the same time point from within several physiologic cycles such as the cardiac or respiratory cycle. Physiologic signals are recorded in with the video signals to allow for this retrospective gating process. The designers of the DSR, to achieve the ability to obtain dynamic, volumetric image data sets made compromises in the image resolution such that grey scale resolution was sacrificed. To improve the image quality, the video imaging chains have been converted from image isocon cameras to charged coupled device (CCD) cameras.  Much larger lenses were ground by Old-Delf and tapered fiberoptics were pulled to take the images from the lens to a microchannel plate intensifier which then transmits the image to the CCD chips. The process of pulling the tapered fiberoptics introduces some twisting of the fiberoptics and therefor custom warping algorithms had to be developed for each camera. The images are digitized on each, camera, the images are unwarped, and the data is sent now to digital tape running at video rates. Although the DSR has remained a one of a kind system and represents a true tour de force, much of the current image manipulation and display associated with the massive data sets generated have served as the vanguard for data handling of images coming off of the currently commercially available scanners.
Dynamic Spatial Reconstructor
The Dynamic Spatial Reconstructor (DSR) is a high-temporal resolution, three-dimensional (3-D) X-ray scanning device based on computed tomography (CT) principles. It was designed for investigation of some problems inherent in current diagnostic imaging techniques, and to allow quantitative studies of cardiovascular structure and function. One of the research protocols in which DSR is currently used involves studying selected pediatric patients with complex congenital heart disease. Initial results show that 3-D dynamic images can be obtained from these patients with minimal invasiveness and that these images may provide useful diagnostic information.
Dynamic Spatial Reconstructor
Dynamic Spatial Reconstructor
The first three-dimensional volume scanning (simultaneous acquisition of multiple contiguous slices) CT scanner with high temporal resolution (scan repetition rate of up to 60 times/sec), called the dynamic spatial reconstructor (DSR), was presented in 1980  .
This machine allowed to examine the renovascular anatomy, detecting arterioles as small as 1 mm in diameter  and providing detailed dilution curves that showed the transit of contrast in the four zones of the renal cortex (superficial, middle, inner and juxtamedullary) and in two different areas (outer and inner) of the renal medulla. These dilution curves allowed a precise calculation of intrarenal RBF. Despite the great potential of the DSR, it was not extensively used because of its limited availability, and its high operating and maintenance costs. However, further studies on intrarenal hemodynamics were made possible when Imatron, a California company, marketed the first commercially available EBCT, which is described in the next section.
the acquisition of the same physical image or set of images in rapid succession such that time dependent changes (e.g. contrast enhancement, motion) can be studied. The term was originally used and most often refers to computed tomography CT scanning. Very rapid dynamic CT acquisitions have been accomplished by cine CT , which has scan times as short as 50 msec. Recently, the development of slip ring technology has resulted in sub-second scan times on conventional X-ray tube based CT scanners. With these faster scan times, many dynamic processes can be monitored. Ultimately, the number of images that can be obtained during a dynamic image study is limited by the heat loading of the X-ray tube.
There are a variety of applications for dynamic CT scanning. These include dual-phase abdominal studies after contrast administration, cardiac imaging studies, studies of perfusion using iodinated contrast media, and measurement of cerebral blood flow using administered xenon as a diffusible agent (see xenon CT scanning and perfusion measurements ). Sometimes, the term dynamic scanning is used for rapid scanning of a volume even if only a single time point is sampled. This is in some respects a misuse of the term since the images do not portray dynamic events. It derives from the increased scanning rates made available by the advent of dynamic scanning (see incremented dynamic scanning ).
Dynamic Spatial Reconstructor
The Dynamic Spatial Reconstructor (DSR) is an experimental apparatus that deserves
mention, being the only method besides RT3D ultrasound capable of real time 3D cardiac
imaging. It was constructed for research (and recently decommissioned) at the Mayo Clinic
and was too expensive for general clinical use. The DSR incorporated aspects of fluoroscopy
and CT, using multiple X-ray sources and fluoroscopic screens to gathered 3D data in real-time
(Robb 1983). During its many years of operation, the DSR gathered unique and valuable data
for research in cardiac dynamics and for validation of other methods of measurement.
Cardiovascular Computed Tomography (CVCT)
Also called as ultrafast CT / Electron beam CT (EBCT)
Motion of the parts of the machine is completely eliminated
Electron gun (320cm long, 130keV)
Focusing and deflecting coils
Four 180cm diameter tungsten target arcs
2 Rings of detectors
Electron beam scans the large target, X-rays are produced and collimated into a 2cm wide fan beam by a set of circular collimators
The X-ray beam passes through the patient and is detected by an array of luminescent crystals
Both the tungsten targets and the detector array cover an arc of 2100
One scan can be obtained in 50Mts
Without moving the patient, 8 contiguous tomographic images can be obtained.
Electron Beam Computerized Tomography (EBCT)
This instrument represents a novel concept in the use of x-ray to obtain fast tomographic scanning. In contrast to the DSR and conventional CT, EBCT has no mechanical parts (x-ray tubes and/or TV cameras) moving around the patients, resulting in lower heat production and enabling fast scanning. An electron beam, originating from an electron gun located behind the patient is magnetically deflected sequentially onto four tungsten target rings, producing eight fan beams (two from each target ring) of x-ray radiation that pass through the patient. Eight almost simultaneous renal tomographic sections can thereby be obtained, that are thicker (8 mm) than those produced by the DSR. Alternatively, consecutive 1.5, 3, or 6 mm thick tomographic slices can be obtained by using a single target ring and moving the patient table at pre-determined increments. Although its temporal resolution is lower than that offered by the DSR (50 or 100 msec/image), it is nonetheless sufficient to obtain adequate evaluation of renal function. Furthermore, because of the slightly longer scan duration and lower image noise compared to the DSR, its spatial resolution is superior  .
Jaschke, et al. [11, 12] were the first to demonstrate the potential of the EBCT in measuring RBF, and establish the basic principles for that calculation which correlated highly with measurements obtained with radioactive microspheres. Subsequent validation studies demonstrated the accuracy of EBCT-derived measurements of renal, cortical and medullary (compared to their in vitro) volumes  and perfusion (compared to electromagnetic flowmetry) within a wide range of RBF values  , as well as changes in blood flow distribution
The Imatron Electron Beam Tomography (EBT) Scanner
Imatron's Electron Beam Tomography (EBT) scanner combines advanced science and technology to create an innovative diagnostic imaging system. Dramatically different from conventional (mechanical) CT scanners, Imatron's patented electron beam technology and unique design offer diagnosticians scan times as fast as 50 and 100 milliseconds. In addition to routine cross sectional imaging of all body organs, EBT is able to evaluate physiology and blood flow and to perform any other examination where speed is essential. Utilizing proprietary EBT technology, a powerful electron beam is generated and then focused onto one of four tungsten target rings positioned beneath the patient. Each 210 degree sweep of the electron beam produces a continuous 30 degree fan beam of x-rays that pass through the patient to a stationary array of detectors which generates cross-sectional images.
Imatron's engineering department is an accomplished group of scientists and engineers working to create the world's most advanced computed tomography (CT) scanner. Together, they harness and shape a powerful (650 mA at 130 kV) electron beam, then guide it through a high-vacuum chamber and around multiple tungsten-coated target rings. These patented image acquisition techniques are at the heart of the Electron Beam Tomography (EBT) scanner, creating a scanner so fast that it can capture stop-action images of the human heart. The vast amounts of data captured in a single breath-hold are transferred at speeds of 200 Mbs to 1 Gbs over a DICOM network to our near real-time reconstruction systems. Sophisticated programming techniques enable us to create 2D and 3D images, including virtual angiograms (fly-throughs of the arteries of the heart) and virtual colonoscopies (fly-throughs of the colon).
Radiation dose from EBT scans compared to other sources of radiation
EBT Coronary Calcium Scan Non-invasive Coronary Angiogram EBT Low-dose Lung Scan EBT Abdominal/Pelvis Scan Background Sunshine Radiation in 1 year Cross country airplane trip Standard Chest X-ray Standard Abdominal X-ray Standard Spine X-ray series Standard Coronary Angiogram Standard Lower G.I. X-ray series Spiral CT Whole body scan
50 to 70 mrem 80 to 120 mrem 100 to 150 mrem 100 to 300 mrem 300 mrem 2 mrem 8 to 10 mrem 48 mrem 300 mrem 500 to 1000 mrem 600 mrem 600 to 1000 mrem 0.5 to 0.70 mSv 0.8 to 1.2 mSv 0.75 to 1.5 mSv 0.75 to 3.0 mSv 3.0 mSv 0.02 mSv 0.08 to 0.1 mSv 0.48 mSv 3.0 mSv 6.0 to 10.0 mSv 6.0 mSv 6.0 to 10.0 mSv Mrem and mSv exposure varies with body size. Recommended safety limits for radiation exposure is less than 5,000 mrem per year.
Electron Beam CT
The latest foray of technology in CT is the electron beam CT (EBCT) scanner. In EBCT an electron beam is electro-magnetically steered towards an array of tungsten X-ray anodes that are positioned circularly around the patient. The anode that was hit emits X-rays that are collimated and detected as in conventional CT. The use of an electron beam allows for very quick scanning because there are no moving parts. An entire scan can be completed in 50 to 100 milliseconds. This quick scan time makes this the only CT method which can scan the beating heart. At the present time, these machines are installed in only a few sites world-wide.
Ultrafast / Electron Beam CT Scan
What is an ultrafast/electron beam CT (computed tomography) scan?
In conventional x-rays, a beam of energy is aimed at the body part being studied. A plate behind the body part captures the variations of the energy beam after it passes through skin, bone, muscle, and other tissue. While much information can be obtained from a regular x-ray, specific detail about internal organs and other structures is not available. With computed tomography (also called CT or CAT scan), the x-ray beam moves in a circle around the body. This allows many different views of the same organ or structure, and provides much greater detail. The x-ray information is sent to a computer which interprets the x-ray data and displays it in two-dimensional form on a monitor. A new technology called ultrafast CT (also known as electron-beam tomography, or EBT) is now used, in some cases, to diagnose heart disease. Ultrafast CT can take multiple images of the heart within the time of a single heartbeat, thus, providing more detail about the heart's function and structures, and also greatly decreasing the amount of time required for a study.
A three-dimensional (3D) version of ultrafast CT may be used to assess the pulmonary arteries and veins in the lungs.
Ultrafast CT scan may also be used to evaluate selected heart defects after birth.
Electron beam computed tomography (CT)
Electron beam computed tomography (CT) scanning is a new test that can be used to detect calcium buildup in the lining of arteries.
Electron beam CT scanning is much faster than standard CT scanning. Electron beam CT scanning can produce an image in a fraction of a second and can take an accurate picture of an artery even while the heart is beating. Standard CT scanning is not fast enough to take pictures of a pumping heart. In standard CT, several pictures, or "slices," of the heart are taken from different angles. These pictures are then analyzed using a computer to create a three-dimensional view of the heart.
Why Is It Done?
This test is used to identify calcium buildup in heart arteries, which can be a risk factor for coronary artery disease (CAD). It may be used as a screening tool to detect hardening of the arteries in people who are at high risk of developing atherosclerosis .
Fat and calcium buildup may be seen in the arteries during an electron beam CT scan. Aggressive treatment for atherosclerosis may be appropriate.
If electron beam CT scanning does not show the presence of calcium buildup in the arteries, then the chances of having CAD are low. 1 Most people who have a negative angiography test result also have a negative electron beam CT scan test result. 1
A high calcium score on an electron beam CT scan indicates a greater risk of having cardiovascular problems within the next 2 to 5 years, especially when a person also has multiple risk factors for developing coronary artery disease. 1 However, the scan does have a fairly high rate of false-positive results.
Electron Beam CT
Advanced Body Scan of Newport is a physician owned facility that opened in June 2001 with a mission to provide the community with cutting edge diagnostic and screening medical imaging procedures. Electron Beam Tomography (EBT) is an innovative outgrowth of Computed Tomography (CT) technology specifically developed to acquire images fast enough to freeze the beating heart. This 'Ultrafast' cat scanner is open and non-claustrophobic, and the X-ray radiation used to acquire the body images is only about 20% of what you would receive from a conventional CT scanner. The full body EBT scan delivers not much more radiation than your total annual radiation exposure from natural causes.
Electron Beam CT
When you visit us at Advanced Body Scan of Newport, you can be assured that you will be made to feel welcome and nurtured from your initial greeting to your final medical consultation. You stay fully clothed for the exam which takes only a few minutes to complete. No shots or needles are involved. You simply lie down on your back, hold your breath for a few seconds while the images are taken, and within minutes, you can see the results yourself on the screen.
Advanced Body Scan of Newport offers discount scan packages to corporate customers. Our sales staff will be happy to answer any questions and customize any multiple scan package to meet your needs.
Only GE- Imatron ™ Electron Beam CT Scanners have FDA approval for Coronary Artery Calcium Scoring and CT Coronary Angiography.
Electron Beam CT
Heart Scan: Coronary Artery Calcium Score) the EBT Heart Scan is a breakthrough in Early Detection and Cardiac Risk Assessment. Coronary artery calcium is a marker for plaque in a blood vessel. These calcium deposits form years before the first symptoms of heart disease, such as chest pain or shortness of breath, appear.
Full Body Scan: The Body Scan includes the Heart Scan, the Lung Scan, a CT scan of your Abdomen and Pelvis and a Bone Scan, allowing us to detect liver tumors, gallstones, kidney stones, kidney tumors, aneurysms and abnormalities of the prostate or ovaries. Before you leave you will have had a personal consultation with our health professional.
Electron Beam CT Coronary Angiography (EBA): Approved by the FDA in November 1999, the Electron Beam CT scanner provides an opportunity to examine your coronary arteries without the risks of conventional angiography. Coronary angiography done on the Electron Beam CT Scanner takes only about half an hour and is reimbursed by insurance plans
Electron Beam CT
Lung Scan : Lung Cancer strikes 150,000 Americans each year. Despite treatment advances, five-year survival for lung cancer patients remains only 14%. Only CT scans can find lung cancer at an earlier and potentially more curable stage. All current or former smokers or those exposed to carcinogens such as asbestos could benefit from this test.
3D QCT Bone Scan: Bone density provides the critical information you need to identify osteoporosis, a debilitating disease that can lead to bone fractures. An essential exam for all post-menopausal women, 3D QCT Bone Densitometry is more accurate than conventional DEXA studies.
Virtual Colonoscopy: Colorectal Cancer is the second leading cause of cancer death. This test is a new technique for visualizing colon polyps that could become cancerous. This alternative to standard Colonoscopy does not require a hospital or outpatient stay, or anaesthesia. Utilizing air and advanced image rendering, a virtual voyage through your colon is generated from the scan images.
Integrating knowledge from Philips' leadership in cardiac imaging and monitoring systems, Brilliance CT is uniquely designed to adapt to your patients and overcome the many challenges that unpredictable heartbeat rhythms can present. Philips-patented Rate Responsive™ image acquisition technology adapts to your patients - rather than the other way around. The many intelligent technologies built into the Brilliance CT configurations not only improve departmental workflow, but the early detection of cardiac disease as well.
Features and Benefits
Redefining patient care through automatic adjustment to variations in patient heart rate and rhythm during the scan and subsequent image reconstruction
Least invasive, most reliable solution for assessment through accurate determination of the most stable cardiac phase for each heart region
Achieve temporal resolutions as low as 53ms to visualize coronary anatomy and determine stenosis as well as plaque constituents.
Expanding the Limits of Cardiovascular CT Delivering 16 0.5mm isotropic slices with 400msec rotation times, the Aquilion™ 16 CFX images the smallest vessels with confidence in the widest range of patients.
Temporal resolution as low as 400 msec for clear, detailed cardiac evaluation
Flexible scan speeds and SURECardio software ensure optimal imaging for variable and high heart rates
Industry’s longest couch supports whole-body CTA without patient repositioning
Ideal for use in cardiology, trauma, neurology and oncology
A suite of applications, called SURETechnologies, specifically designed to provide the highest productivity and best image quality at the lowest possible dose
a very fast computed tomography CT scanner which can acquire images from a single or small number of image locations in a small fraction of a second. The scanner, also named millisecond CT, ultrafast CT or electron beam CT, requires no mechanical motion to acquire the projection data. Instead of rotating an X ray tube around the object, the cine CT system sweeps an intense electron beam across a large, stationary anode target which surrounds the patient. X-rays are emitted from the point where the electrons strike the target. The X-rays transmitted through the object are measured by a stationary array of detectors. The time needed for collecting data for a single slice is 50-100 ms, although a longer scan time can be used to reduce image noise.
In some scanning modes, two detector rings allow the imaging of two sections in a single scan of the X-ray target. Also, multiple target rings, scanned sequentially, allow a volume to be imaged rapidly. The system is very well suited to dynamic studies or studies where motion artefacts might be a problem (e.g. cardiac applications), and for imaging of uncooperative patients (e.g. children).
Ideally, the X-ray target would completely surround the object, thereby allowing the source of X-rays to rotate through 360. In the ideal system, the detectors would also be on a complete ring. Mechanical constraints do not allow this arrangement. As a result, the source and detectors are not actually on the same plane, introducing complications to the reconstruction process and the possibility of artefacts. Another limitation in image quality results from the limit on the electron beam current. Although the current is high (~ 1 000 mA), the very short imaging time leads to a low mAs. Cine CT systems generally have higher noise level and lower spatial resolution than conventional CT systems, but are ideal when their high scanning speed is well suited to the clinical application. Among the possible uses are cardiac imaging with and without the use of contrast agents, lung imaging, and paediatric studies.
The cine CT system has no mechanical scanning motion. In this system both the X-ray detector and the X-ray tube anode are stationary. The anode, however, is a very large semicircular ring that forms an arc around the patient scan circle, and is part of a very large, non-conventional X-ray tube. The source of X-rays is moved around the same path as a fourth generation CT scanner by steering an electron beam around the X-ray anode. Because the electron beam can be moved very rapidly, this scanner can attain very rapid image acquisition rates. In the literature, this system has been referred to variably as fifth generation and sixth generation. It has also been described as a stationary-stationary scanner. The terms millisecond CT, ultrafast CT and electron beam CT have also been used, although the latter can be confusing since the term suggests that the patient is exposed to an electron beam.
Applications of CVCT
CardioVascular CT is an effective, non-invasive imaging technique for patients with congenital and acquired heart disease. Various forms of aortic disease - including aortic malpositions, dissections, and coronary aneurysms - are well suited for investigation by CV CT. The ability of CV CT to acquire volumetric axial images in a short breath-hold enables clinicians to build accurate 3D anatomical models of the entire heart and vascular structures. In addition, CV CT can be used to screen pericardial lesions, and accurately show the extent and location of intracardiac masses and tumors adjacent to the heart.
Formation of CT images
CT Sinogram The data acquired for one CT slice can be displayed before reconstruction. This type of display is called a Sinogram.
What are we measuring? The average linear attenuation coefficient ( µ ), between tube and detectors Attenuation coefficient reflects the degree to which the X-ray intensity is reduced by a material
PIXEL & VOXEL
Cross sectional layer of the body is represented as an image matrix.
Each square of the image matrix is called pixel(picture element) and it represents tiny block of tissue called voxel (volume element)
The linear attenuation coefficient ( ) of each pixel is determined by :
Composition of the voxel
Thickness of the voxel
Quality of the radiation beam
Linear attenuation coefficient µ
Algorithms for image reconstruction
An algorithm is a mathematical method for solving a problem
The linear attenuation coefficients of all the pixels in the image matrix should be determined by solving thousands of equations.
All algorithms attempt to solve the equations as rapidly as possible without compromising accuracy.
Process of generating an image from the raw data or set of unprocessed measurements made by the imaging system.
Image reconstruction algorithms
Analytic methods : 2D Fourier analysis Filtered Back Projection
Simple back projection method
The image profiles look like steps. The height of the steps is proportional to the amount of radiation that passed through the block. The steps are then assigned to a gray scale density that is proportional to their height. When the rays from the two projections are super imposed, or back projected, they produce a crude reproduction of the original object. In practice, many more projections would be added to improve image quality.
All points in the back projected image receive density contributions from neighboring structures.
The radio dense material severely attenuates the beam and produces localized spikes in the image profiles. Back projection of the rays from these spikes produces a star pattern. A great number of projections will obscure the star pattern, but the back ground density remains as noise to deteriorate the quality of the CT image.
Simple back projection method
Assumption (for eg. all points in the matrix have same value)
Comparison (with the measured values)
Correction (to bring the two into agreement)
Repetition (of the process until the assumed and measured values are the same or within acceptable limits)
All projections for the entire matrix are calculated at the beginning of the iteration, and all corrections are made simultaneously for each iteration.
Ray by Ray correction
One ray sum is calculated and corrected and these corrections are incorporated into future ray sums, with the process being repeated for every ray in each iteration.
Point – by – point correction
The calculations and corrections are made for all rays passing through one point, and these corrections are used in ensuring calculations, again with the process being repeated for every point.
Two – dimensional Fourier analysis:
Basis: Any function of time [f(t)] or space [f(x)] can be represented by the sum of various frequencies and amplitudes of sine and cosine waves.
2D FOURIER ANALYSIS
Filtered back projection
The image is filtered or modified to counter balance the effect of sudden density changes, which causes blurring (star – pattern). Those frequencies responsible fro blurring are eliminated to enhance more desirable frequencies.
The density of the projected rays is adjusted that is, the inside margins of dense areas are enhanced while the centers and immediately adjacent areas are repressed. The net effect is an image more closely resembling the original object.
FILTERED BACK PROJECTION
FILTERED BACK PROJECTION
Comparison of Mathematical methods
In terms of speed, Analytical methods are faster than iterative methods
But with incomplete data, iterative methods are faster than analytical methods. Analytical methods does time – consuming interpolations to fill in missing data, whereas iterative methods simply average adjacent points.
CT NUMBER & HOUNSFIELD UNIT The computer calculates a relationship between the linear attenuation coefficients of the pixel and water which is given as CT number. To image materials with higher than dense bone CT number larger than 1000 should be available CT numbers based on a magnification constant of 1000 are Hounsfield units
Relationship between CT numbers & Gray scale
Windowing is the process in which the gray level can be manipulated using the CT numbers to provide optimum demonstration of different structures seen on the image.
Window Width (WW) is the range of CT numbers for the gray scale & WW control alters the image contrast
Window Level (WL) is the centre of the gray-scale image & WL control alters the image density/CT number of the tissue to be displayed
Graphic illustration of the effect of different WW & WL settings on the CT image
Image Quality in CT
Image quality is the visibility of diagnostically
important structures in the CT image.
The factors that affect CT image quality are
Quantum mottle (noise)
Resolution : Spatial and contrast
The factors are all interrelated
Quantum mottle (Noise)
Quantum mottle is the statistical fluctuations of X-photons absorbed by the detector
The only way to decrease noise is to increase the number of photons absorbed by the detector.
The way to increase the number of photons absorbed is to increase x-ray dose to the patient.
Mottle becomes more visible as the accuracy of the reconstruction improves.
RESOLUTION i) Spatial resolution
Spatial resolution is the ability of the CT scanner to display separate images of two objects placed close together.
ii) Contrast resolution
Contrast resolution is the ability of the CT scanner to display an image of a relatively large (2 or 3mm) object that is only slightly different in density from its surroundings.
Even distribution of radiation dose to the tissues as exposures are from almost all angles.
No overlapping of scan fields takes place.
Exposure factors used are higher to improve spatial and contrast resolutions and to reduce noise.
Comparison of CT with Conventional Radiography
Artifacts are distortions or errors in the image that are unrelated to the object scanned .
Most common artifacts in CT are
Beam hardening artifacts
Partial volume averaging artifacts
EFFECTS OF ARTIFACTS
DETERIORATE IMAGE QUALITY
SUBJECT INFORMATION IS LOST
PATHOLOGICAL DETAILS ARE LOST
Cause : Patient movement
Appearance: Blurred / streaks / ghost images
reduction in scan time
Clear and concise instruction to the patient
proper patient immobilization
if needed,administration of sedatives/antiperistaltic drugs
Cause: Presence and movements of objects of very high density(contrast media, metallic implants,surgical clips)
Remove the offending object if possible. Use a smoothing algorithm. e.g. Standard algorithm.
Streaking occurs due to inconsistency in a small group of readings