2 X-ray films:1: Reveal the total attenuation along the line of sight.The x-ray image can only show a projection of all of the organslying between the source of x-rays and the film.It does not provide any 3-D or depth information.2: Allow contrasts of the order of 2% to be seen.Details of heart anatomy or blood vessels cannot be seenWithout using a liquid contrast agent.3: Provide high resolution images
3 Computer Tomography:1: Provides better contrast between tissues with slightly varyingattenuation coeficients as well as depth information.2: Generates cross‑sectional images in the axial or transverseplane with a spatial resolution of about 1mm3: Maps the relative linear attenuation values of tissues withdensity resolution of about 1%CT x‑ray techniques use a high kVp (120 to 140) withheavy filtration and so can penetrate bone.The dominant interaction with tissue is due to Comptonscattering which depends on the electron density of the material.
4 ImagingThe image produced by the CT is a two dimensional slice throughthe transverse plane of the body.Field of view (FOV) is the diameter of the areabeing imaged (e.g., 25 cm for a head).CT pixel size is determined by dividing the FOVby the matrix size.The matrix size used for CT is generally 512 x 512 (0.5 k).For example, pixel sizes are 0.5 mm for a 2S‑cmdiameter FOV head scan (25 cm divided by 512)and 0.7 mm for a 35‑cm FOV body scan(35 cm divided by 512).
6 VoxelsThe section is divided intoVoxels (volume element)The Voxel volume is theproduct of the pixel areaand slice thicknessdepends on the spatialresolution of the CT.
7 The Relative Attenuation Coefficient (m) The attenuation of the tissue in each projection is normally convertedto Hounsfield units (HU) or CT numbers.HUx = x (m x ‑ mwater)/ m water· The 1000 in this equation determines the contrast scale.· By definitionHU value for water is 0, andHU value for air is ‑1000.· mx and mwater are dependent on photon energy (keV)therefore HU values depend on the kVp and filtration.· HU values generated by a CT scanner are approximate and· only valid for the effective kVp used to generate the image.
8 Hounsfield Unit for Representative Materials Material Density (e/cm3)Electron Density(e/cm3) x 1023Approx HU ValueAir< 0.01<.01‑1000Lung0.250.83‑300Fat0.923.07‑90Water1.003.33White matter.033.4230Gray matter.043.4340Muscle.063.4450Cortical bone1.85.591000+
9 An overview of Scanner Design Since the introduction of the first CT their design has improved to allow1. faster scans to decrease the effect of patient movement2. lower x-ray doses3. 3-D imaging using table motion and spital scanning4. improved x-ray tube reliability
10 First‑ generation scanners The following procedures occur:· An x‑ray beam is scanned across the patient· The measured intensities are used to produce x‑raytransmission values or projections.· The gantry is rotated by a few degreesThe tangential scan repeated.· CT images are derived bymathematical analysis ofmultiple projections.· 1970: EMI scanners useda pencil beam and sodium iodide(NaI) detectors that moved across the patient( and generated approximately 160 data points per projection
11 Second‑generation scanners translate‑rotate technologymultiple detectorsfan‑shaped beamlarger rotational incrementsfaster scanswith a single section beinggenerated in approximately 1 minute.
12 Modern Computed Tomography Scanners Third‑ and Fourth‑generation scanners· Third‑generation scannersrotating fan beam and detectors (rotate‑rotate system)
13 Fourth‑generation scanners · Rotating tube and fixed ring of detectors (up to 4800)the gantry (rotate‑fixed system)· Single section in l or 2 seconds.· Use of slip ring technologyVoltage and Datasupplied to the tubethrough contact ringsWith slip ring scanners, thex‑ray tube can rotatein one direction continuously.
14 Fifth‑generation scanners Use an electron gun that deflects and focuses electron beamalong a 210‑degree arc of a large diameter tungsten target ring· Multiple detector rings permit the simultaneousacquisition of multiple image sections.· No moving parts, images obtained 50 to 100 ms,· Used in cardiac imaging
15 Helical computed tomography Continuous motion of tableAchieving a continuous spiral of helical scan.This mode of scanning requires the gantry rotates continuously.· Slip ring CT scanners may be used in a helical (spiral) mode. · The x‑ray beam central ray entering the patient follows ahelical path during the CT scan.· · The relation between patient and tube motion is called pitchtable movement (mm ) / collimation width (mm).· Example5‑mm section thickness,patient may move 10 mm during the 1 second for 1 revolutionPitch would thus be equal to 2.·
17 X‑ray tubes and collimators CT tubes have focal spots of approximately 1 mm.Heat loading on CT x‑ray tubes is generally high,requiring high anode heat capacities.Modern x‑ray tube capacities of more than 2 megajoules.High performance CT x‑ray tubes may cost $50,000 or more.The beam is collimated as it exits the tube and againbefore it strikes the detectors.Collimation defines the section thickness and reduces scatter.Adjustable collimators allow section thickness to range betweenl and l0 mm.The heavy filtration used with CT scanners produces a beam withan aluminium half‑value layer (HVL) of approximately 10 mm.
21 Detectors and data acquisition 1: ScintillatorsScintillation crystals produce light when x‑ray photons are absorbed.Coupled to a light detector (photomultiplier tube or photo diode),Common material in solid‑state detectors is cadmium tungstenate(CdWO4), which is an efficient x‑ray detector.Large single crystals separated by tungsen collimator plates.Plates reduce the scattered radiation falling on the detector.
22 Detectors and data acquisition 1:Gas detectorsOlder Ct scanners use Xenongas ionisation detectorsIncident x‑ray photonsionise the gasproducing electron‑ion pairs.The current produced isproportional to the intensity of incident radiation.Gas detectors are usually maintained at a high pressure (25 atm)to increase x‑ray detection efficiency.Gas detectors are more stable than solid‑state detectors.