2Image Reconstruction II Generating an image from the acquired data involves determiningthe linear attenuation coefficients of the individual voxels.A mathematical algorithm takes the projection data andreconstructs the cross‑sectional CT image.There are three common ways to carry this out:1: Backprojection2: Iterative3: Fourier methods
3BackprojectionThe field of view is simulated as a 2-D array within the computer.512 x 512The attenuation for each voxel of a scan is1: projected back along the direction of the beam2: the attenuation is added to the value in each array elementthe beam passes through.At the end of the procedure a rough image is held in the array elements.
4Filtered Backprojection No commercial CT scanners use simple back projectionThe process produces artifacts where the attenuation changes rapidly.Most modern scanners use filtered back projection algorithmswhich have filter functions applied to reduce these effects.Different filters may be used, offering tradeoffsbetween spatial resolution and noise.Some filters permit reconstruction of fine detail but with increasednoise in the image such as in bone algorithms.Algorithms such as those for soft tissue provide some smoothing,which decreases image noise but also decreases spatial resolution.The choice of the best filter to use with the reconstruction algorithmdepends on the clinical task.
7Image display Reconstructed images are viewed on CRT monitors or printed onto film using a laser printer.Each pixel is normally represented by 12 bits, or 4096 grey levelsThe normal grey levels available on a display are 256Window width and window level are used to optimise theappearance of CT images by determining the contrast andbrightness levels assigned to the CT image data .Window level, or centre, is the CT number or HU value to bedisplayed as the median intensity in the image.The window level is normally chosen close to the average HUvalue of the tissue of interest (e.g., 10 to 50 HU for soft tissue).Window width is the range of CT numbers displayed around theselected centre. and it determines the contrast.A narrow window width provides higher contrast than a widewindow width.
8Image Quality Image quality may be characterised in terms of contrast noisespatial resolutionImage quality involves tradeoffs between these three factorsand patient radiation dose.Artifacts encountered during CT scanning can degrade image quality.
9 Contrast The difference in the HU values between tissues. Usually increases as kVp decreasesbut is not affected by mA or scan times.CT contrast may be artificially increased by adding a contrast mediumsuch as iodine.Image noise may prevent detection of low‑contrast objects suchas tumours with a density close to the adjacent tissue.The displayed image contrast is primarily determined by the CTwindow width and window level settings.
10Noise CT noise is determined primarily by the number of photons used to make an image (quantum mottle).Quantum mottle decreases as the number of photons increases.The typical noise with a modern CT system is approximately5 HUCT noise is generallyreduced by increasing the· kVp,· mA· scan time· voxel size(i.e., by decreasing matrix size, increasing FOV,or increasing section thickness).
11ResolutionTypical resolution in CT scanning ranges from 0.7 to 1.5 lp/mm.If the CT Field Of View [FOV] is dand the matrix size is M,then the pixel size is d /M.For a typical head scan with an FOV of 25 cmA matrix of 512 pixelsThe pixel size is 0.5 mm.Factors that may also improve CT spatial resolution· smaller focal spots· smaller detectors· more projections.· smaller FOV· larger matrix size.Resolution perpendicular to the section is dependent on slice thickness
12RadiationThe dose profile in a CT scanner is not uniform along the patient axisand may vary within any irradiated section.Typical maximum doses for a single section are40 mGy (4 rads) for a head scan20 mGy (2 rads) for a body scan.
13RadiationIn head scans, the surface to centre ratio is approximately 1:1.In body scans, the surface to centre ratio is approximately 2:1.
14RadiationBecause of scattered x‑rays, the CT section dose profile is notperfectly square but has tails that extend beyond the section edges.Tissues beyond the section are thus exposed to radiation.When contiguous sections are scanned, the cumulative radiation dosein a section may be as high as twice the radiation dose associatedwith a single section.
15Artifacts Partial volume artefact CT scanners may have artifacts in the reconstructed images.Partial volume artefact is the result of averaging the linearattenuation coefficient in a voxel that is not uniform in composition.Partial volume artefact increases with pixel size and section thickness.
16Artifacts Streak Artifacts Random or unpredictable motion (e.g., if the patient sneezes)produces streak artifacts in the direction of motion.In high‑density structures, such as metal implants, the detectormay record no transmission.The algorithm generates streaks adjacent to the high‑densities.
17Artifacts Beam Hardening Artifacts Beam hardening artifacts or "cup" artifacts are caused by thepolychromatic nature of the x‑ray beam (beam hardening).As the lower energy photons are preferentially absorbed,the beam becomes more penetratingand results in lower computed values of the attenuation coefficient (HU).Beam hardening artifacts are most marked at high‑contrast interfacessuch as between dense bone in the skull and the brain
18Artifacts Ring artifacts Ring artifacts may arise in third‑generation systems ifa single detector is faulty or miscalibrated.Artifacts caused by equipment defects are rare on modern CT systems