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Computed Tomography NOREEN MARWAT SENIOR SCIENTIST NORI, Islamabad. Introduction 4th decade of clinical use Invaluable diagnostic tool for many clinical applications Cancer diagnosis to trauma to Osteoporosis screening Introduction Computed tomography (CT) scanning, also called computerized axial tomography (CAT) scanning, is a medical imaging procedure that uses x-rays to show cross-sectional images of the body. Introduction A CT imaging system produces crosssectional images or "slices" of areas of the body, like the slices in a loaf of bread. These cross-sectional images are used for a variety of diagnostic and therapeutic purposes Axial Tomography Computed Axial Tomography A motorized table moves the patient through a circular opening in the CT imaging system. While the patient is inside the opening of the CT imaging system, an x-ray source and detector within the housing rotate around the patient. A single rotation takes about 1 second. The x-ray source produces a narrow, fan-shaped beam of x-rays that passes through a section of the patient's body. A detector opposite from the x-ray source records the xrays passing through the patient's body as a "snapshot" image. Many different "snapshots" (at many angles through the patient) are collected during one complete rotation. For each rotation of the x-ray source and detector, the image data are sent to a computer to reconstruct all of the individual "snapshots" into one or multiple crosssectional images (slices) of the internal organs and tissues. Invention Godfrey Newbold Hounsfield 1972 EMI Central Research Laboratories Allan McLeod Cormack 1963 Tufts University Nobel Prize in medicine in 1979. CT Scan machine is “the great legacy” of the Beatles Invention The first CT scanner EMI Mark 1, produced images with 80×80 pixel resolution. Each pair of slices required 4.5 minutes of scan time and 1.5 minutes of reconstruction time. Basic principles (cont.) Plain film imaging reduces the 3D patient anatomy to a 2D projection image Density at a given point on an image represents the x-ray attenuation properties within the patient along a line between the xray focal spot and the point on the detector corresponding to the point on the image Basic principles (cont.) With a conventional radiograph, information with respect to the dimension parallel to the x-ray beam is lost Limitation can be overcome, to some degree, by acquiring two images at an angle of 90 degrees to one another For objects that can be identified in both images, the two films provide location information Basic principles- Conventional Radiograph Basic principles- Conventional Radiograph Basic principle of operation When the abdomen is imaged with conventional radiographic techniques, the image is created directly on the film image receptor and is low in contrast principally because of scatter radiation. The image is also degraded because of superposition of all the anatomic structures in the abdomen. For better visualization of an abdominal structure, such as the kidneys, conventional tomography can be used. Basic principles Mathematical principles of CT was first developed by Radon in 1917. Radon’s treaties proved that an image of an unknown object could be produced if one had an infinite number of projections through the object. Basic principle Intensity profile/ Projection The intensity of radiation detected varies according to this attenuation pattern and forms an intensity profile, or projection Intensity profile/ Projection If this process is repeated many times, a large number of projections is generated. These projections are not displayed visually but are stored in digital form in the computer. The computer processing of these projections involves the effective superimposition of each projection to reconstruct an image of the anatomic structures in that slice. The tomographic is a picture of a slab of the patient’s anatomy. The 2D image corresponds to 3D section of patient. So that even with CT a 3D image is compressed into 2D image. 2D array of pixel in CT image corresponds to 3D voxels in the patient. Pixels and Voxels 1st Generation Data Collection Hounsfield’s CT Formulation Measurement Ni written as sum of attenuation of pixel along path Solve simultaneous equations from data at many positions and angles Experiments achieved 0.5% accuracy. Hounsfield’s Experimental CT Device 1st Generation Data Collection 1 Pencil Beam and 1 NaI detector 160 samples/traverse 1o increms over 180o 28,800 samples Solved simultaneous equations (Fortran) 1602 image matrix but reduced to 802 for practical clinical use EMI Mark 1 What are we measuring? The average linear attenuation coefficient “µ” between tube and detectors Attenuation coefficient reflects the degree to which x ray intensity is reduced by a material. Tomographic acquisition Single transmission measurement through the patient made by a single detector at a given moment in time is called a ray A series of rays that pass through the patient at the same orientation is called a projection or view Two projection geometries have been used in CT imaging: Parallel beam geometry with all rays in a projection parallel to one another Fan beam geometry, in which the rays at a given projection angle diverge Acquisition (cont.) Purpose of CT scanner hardware is to acquire a large number of transmission measurements through the patient at different positions Single CT image may involve approximately 800 rays taken at 1,000 different projection angles Before the acquisition of the next slice, the table that the patient lies on is moved slightly in the cranialcaudal direction (the “z-axis” of the scanner) Tomographic reconstruction Each ray acquired in CT is a transmission measurement through the patient along a line The unattenuated intensity of the x-ray beam is also measured during the scan by a reference detector I t I0e t ln( I 0 / I t ) t Image Reconstuction Image Reconstuction Backprojectio n Backprojection (con’t) Backprojection Back Projection Backprojection Backprojection is formed by smearing each view back through the image in the direction it was originally acquired. The final backprojected image is then taken as the sum of all the backprojected views backprojected image is very blurry Filtered Backprojection Filtered Backprojection Convolution Filtered Backprojection Filtered Backprojection CT Image Characteristic Image matrix: Original EMI format was 80 x 80 so there were 6400 cells of information called pixels. Today the format is 512 x 512 resulting in 262,144 pixels. The numerical number in each pixel is a CT number or Hounsfeld Number. CT Image Characteristic CT number or Hounsfeld Number represents the tissue volume in the pixel. Field of View (FOV)is the diameter of the reconstructed image. As the FOV increases, the size of the pixel increases. Voxel: is the square of the matrix times the thickness of the slice. Hounsfeld or CT Number The precise CT number is related to the attenuation of the tissue contained in the voxel. Bone = +1000 Muscle= +50 Lung= -200 Air = -1000 CT Numbers: Hounsfield Units Example 1: voxel contains water (up= uw): CT# = 1000 x (uw - uw)/ uw = 0 Example: voxel contains air (up≈ 0): CT# = 1000 x (0 - uw)/ uw = 1000 x (-1) = 1000 CT Numbers Generations of CT machines First generation Second generation Third generation Fourth generation Spiral CT Difference in all above generation is speed of scanning and movements of gantry. In first two generation there are 1) Linear & 2) Rotary movements In next two generation there is only one movement i.e. rotary movement. 1st generation: rotate/translate, pencil beam Only 2 x-ray detectors used (two different slices) Parallel ray geometry Translated linearly to acquire 160 rays across a 24 cm FOV Rotated slightly between translations to acquire 180 projections at 1-degree intervals About 4.5 minutes/scan with 1.5 minutes to reconstruct slice 1st generation (cont.) Large change in signal due to increased x-ray flux outside of head Solved by pressing patient’s head into a flexible membrane surrounded by a water bath NaI detector signal decayed slowly, affecting measurements made temporally too close together Pencil beam geometry allowed very efficient scatter reduction, best of all scanner generations First Generation CT Scanner Pencil Beam Translate-Rotate Design 180 one degree images or translations. One or two detectors. 5 minutes scan time 2nd generation: rotate/translate, narrow fan beam Incorporated linear array of 30 detectors More data acquired to improve image quality (600 rays x 540 views) Shortest scan time was 18 seconds/slice Narrow fan beam allows more scattered radiation to be detected 3rd generation: rotate/rotate, wide fan beam Number of detectors increased substantially (to more than 800 detectors) Angle of fan beam increased to cover entire patient Eliminated need for translational motion Mechanically joined x-ray tube and detector array rotate together Newer systems have scan times of ½ second Ring artifacts The rotate/rotate geometry of 3rd generation scanners leads to a situation in which each detector is responsible for the data corresponding to a ring in the image Drift in the signal levels of the detectors over time affects the t values that are backprojected to produce the CT image, causing ring artifacts 4th generation: rotate/stationary Designed to overcome the problem of ring artifacts Stationary ring of about 4,800 detectors 3rd vs. 4th generation 3rd generation fan beam geometry has the x-ray tube as the apex of the fan; 4th generation has the individual detector as the apex 5th generation: stationary/stationary Developed specifically for cardiac tomographic imaging No conventional x-ray tube; large arc of tungsten encircles patient and lies directly opposite to the detector ring Electron beam steered around the patient to strike the annular tungsten target Capable of 50-msec scan times; can produce fastframe-rate CT movies of the beating heart 6th generation: helical Helical CT scanners acquire data while the table is moving By avoiding the time required to translate the patient table, the total scan time required to image the patient can be much shorter Allows the use of less contrast agent and increases patient throughput In some instances the entire scan be done within a single breath-hold of the patient 7th generation: multiple detector array When using multiple detector arrays, the collimator spacing is wider and more of the x-rays that are produced by the tube are used in producing image data Opening up the collimator in a single array scanner increases the slice thickness, reducing spatial resolution in the slice thickness dimension With multiple detector array scanners, slice thickness is determined by detector size, not by the collimator