ct

• CT table

  Lightweight carbon, motorised platform that patient lies on, only supported at 1 end, DR tables dipped for patient comfort
• Filter

  Removes any low energy photons from passing through that would increase skin dose and not contribute to image, usually Tin
• Bow-tie Filter

  Filters beam so that high energy photons to thickest part of the body, allows more high energy photons to thickest part of the body, reduces skin dose by 23%
• Collimators

  Restricts the beam to the detector array and reduces scatter radiation, 2 types - pre patient and pre detector
• Rings


  Slip rings - circular conductive rings surround the detector tube, Data and power are transmitted to the gantry using 'brushes', allows the tube to move around the gantry while the table moves through, Allows much faster scans - 0.3 sec
• Scintillation detectors

  1. ray's interact with the crystals in the detector and are converted to light, The light is converted into an electrical signal by photodiodes, allows for a quick conversion of x-ray photons into signals, Stable
• Helical Scanning


  1. Images are acquired constantly while the patient table moves slowly through the gantry

  2. Faster scan times

  3. Possible to scan an entire tissue vol. in one breath hold

  4. Don't need to keep pausing the scan to move the patient/table

  5. Reduced movement artefacts - patient breathing, misregistration, movements

  6. Overall faster scan times
• Multi-slice CT


  Multiple detector rows in the Z-axis - multiple slices obtained per rotation, also termed 'cone beam CT', Able to acquire a whole vol. of tissue in 1 tube rotation
• Benefits of multi-slice CT

  • Speed of acquision - Sub -second rotation, reduced movement artefact, Don't need to keep pausing the scan and move the patient/table, Good for trauma, children or unresponsive patients, Longer scan vol. possible - Good for chest/lung imaging and CT angiography, Cover the whole body for trauma imaging, Can obtain images using very small detector elements - gives sub-millimeter scanning, Multi planar reformats, 3D reconstructions, High resolution CT, Data is obtained by each detector element - Can add the data from neighbouring elements together to form thicker slices, Post processing you can choose- thicker or thinner slices
• Thicker vs. thinner slices

  Better contrast vs. better spatial resolution, Reduced noise vs. reduced partial vol. artefact, Better for PACS vs. radiologists prefer Normal clinicals prefer vs. used for reconstructions
• CT Scanning indications

  • To assess: internal organs within the chest, abdomen and pelvis, Bone, Blood vessels, Brain
• Patient prep and contrast

  • Patient checklist, Pregnancy?, Contraindications to contrast agent?, Changed into gown, Depending on the scan: contrast is given Orally - to visualise the GI tract - water or gastrograffin, Intravenously - differentiate blood vessels and vasculature of organs - helps detect pathology
• Centring the patient in the isocenter

  Reduces dose, Bow-tie filter - reduces intensity variations across the detector - designed to reduce dose to periphery of patient - if patient not centred correctly the rules break down, Optimised image quality - reduces localised noise, Reduces artefacts - artefact reduction methods based on patient being in center of scanner
• Scout Scan

  Tomogram, Allows you to plan your slices and FOV, Check for any artefacts e.g. piercings, Allows the machine to calculate Ma modulation
• Ma modulation


  Varies the attenuation of the tube current along the patient, The amount of mAs required for optimum images - varies from the x and y plane, the scanner should calculate different projected mA values in these planes, if they are not different maximum mA levels may need to be adjusted if the image quality is to remain optimum, Projected mA values should be checked prior to the scan
• Parameters that can be amended

  • Scan mode - helical or sequential, slice thickness - does the info from the detector elements need to be put together, Pitch, FOV, Contrast agents
• Pitch

  Distance the table travels during 1 complete rotation of the x-ray tube, Pitch = table movement per rotation/ slice thickness
• Increasing pitch

  • decr. Scan time - faster scan, Decr. Scan quality, Less projections will be obtained of the tissue, May be necessary when patient cant stay still or struggling to hold breath
• Decreasing pitch

  • incr scan time - slower scan, Will allow additional projections to be obtained of the tissue, Useful when the tube output is limited
• FOV

  How much of the gantry window should be included in the image, FOV will vary depending on area of interest, Only 2 FOV you can modify: Scan FOV(sFOV) - actual area included in data acquisition, Display FOV(dFOV) - area seen on the CT images, Smaller dFOV useful as spatial resolution can be maximised
• IV contrast agents

  Used to visualise blood vessels and vasculature of organs, Contrast usually injected by pump injector, Injection will usually start at a set interval before the scan is activated, Iv contrast showing blood vessls - arterial phase - scan activated 20 secs after injection begins, Iv contrast showing vasculature of organs - portal venous phase - scan activated 70 seconds after injection begins
• Bolus Tracking

  Used when looking at arterial system, Region of interest placed over vessel of interest, Scan triggered once the HU in the ROI has reached a particular level
• Common CT artefacts

  • Beam hardening - can be reduced with iterative reconstruction, Motion, Ring artefact, Partial voluming artefact - 2 different tissues within 1 voxel - the computer averages the densities producing an end result which is not representative of either tissue, Common sites - brain and contrast filled pulmonary arteries, reduce slice thickness to reduce this and choose smaller voxel sizes
• Types of doses

  • CTDLvol - vol. CT dose index, Absorbed dose per unit mass of irradiated vol., Measured in mGy, calculated y Perspex phantom, Related to intensity of x-ray beam, ideally used to provide a comparison metric between scanners, DLP - dose length product, Calculated by - Ctvol x length of scanned vol. (cm), measured in mGy, More indicative of total irradiation to patient, Approx. related to radiation risk
• National DRLs
• Multi planar reconstruction


  Converts data from a certain plane into another plane
• Maximum intensity projections (MIPS)

  Computer visualisation method for 3D data, Can be used to display CT data sets, Max. intensity encouraged along each ray within volumetric data is projected onto the screen
• Volume rendering


  Type of data visualisation technique which creates a 3D representation of data
• Methods to minimise dose and incr. image quality

  • Optimisation of protocols, Limit scanning to only area of interest, Staff training, Daily QA and air calibration of scanner, Dose reduction software (mA modulation , IR), Use appropriate protocols (e.g. low dose, peads)
• Advantages of CT compared to general x-ray

  • Better spatial resolution in CT enabling the detection of very small changes in tissue type, No superimposition of structures in CT, x-ray superimposition makes it hard to determine detail, Especially in areas of similar densities
• Average linear coeffeficient


  What we are measuring in CT, Reflects the degree to which the x-ray intensity has been reduced by a material, we acquire attenuation coefficients at multiple angles ('projections') add them together to form average attenuation scores for tissues at each point to show varying densities of the tissue
• Data acquisition


  Data acquisition is obtained by the gantry - it acquires data and converts it into a digital signal, The computer knows how much radiation the tube emits and can calculate how much is subsequently received by the detector, It can calculate exactly how much radiation has been attenuated, Each tiny detector element creates a digital signal relative to how much the x-ray beam has been attenuated, The CT computer knows the exact position of the tube and the detector when each data point is obtained - using algorithms it maps the calibrated data by over lapping the various projections into an image matrix - results in a blurry image, The ct computer reconstructs the image using filtered back projection
• Convolution


  To prevent the blurriness when the computer maps the calibrated data by overlapping various projections onto an image matrix the data is filtered before reconstruction
• Filtered back projection/order

  Image reconstruction tool, Detectors receive x-rays, Detectors produce a signal according to how many/few x-rays have been attenuated, CT computer filters this data (convolution), Because the CT computer knows where the tube and detectors were when each projection was being obtained - it back projects the data onto an image matrix
• Iterative reconstruction


  Additional reconstruction step, Compares the back projected data to 'model' data produces images with reduced noise and improved resolution, Cycle can repeat until happy with image
• Advantages of IR

  • Reduces the amount of noise on images, Dose reduction (most use 40%), Artefact reduction - improve image quality
• Disadvantages of IR

  • Incr. reconstruction time, 'plastic' looking scans
• Hounsfield units

  each voxel is allocated a unit of density (HU on a grey scale) depending on its average attenuation value, As the scale is calibrated to water - HU's are quantitive - reproducible across different scanners
• Windowing


  When you sample the Hounsfield scale depending on what tissue we are interested in, Changing the level of the scale - changing the window level
• Window width


  Represents how many HU we are sampling, Contrast, Densities below the min. WW - black, Densities above the max. WW - white