CT Science 2024-25 Past Paper PDF

Computed Tomography (CT) Science LO U I S E H A D L E Y L. H A D L E Y @ H E RT S. AC. U K OCTOBER 2024 Module learning outcomes Knowledge and Understanding - Successful students will typically... 1. Discuss the principles, components and operation of computed tomography, magnetic resonance...

Computed Tomography (CT) Science LO U I S E H A D L E Y L. H A D L E Y @ H E RT S. AC. U K OCTOBER 2024 Module learning outcomes Knowledge and Understanding - Successful students will typically... 1. Discuss the principles, components and operation of computed tomography, magnetic resonance imaging, ultrasound, radionuclide imaging, positron emission tomography and single-photon emission computed tomography. 2. Discuss the principles, components and operation of fluoroscopy, mobile, mammography and equipment for dual-energy X-ray absorptiometry. 3. Discuss the principles of digital imaging, picture archiving and communication systems. 4. Discuss the need for quality assurance testing and maintenance of imaging equipment. Intellectual, Practical and Transferable Skills - Successful students will typically... 1. Be able to apply their knowledge of specialised diagnostic imaging equipment and modalities in order to produce high quality diagnostic images whilst minimising potential detriment to patients arising from the use of ionising and non-ionising radiations. 2 Contents 1. Components of a CT system 2. Principle of operation 3. Factors that affects image quality in CT 4. Radiation safety in CT 5. Practice questions at end of each section 3 1. Components of CT 4  CT equipment components Gantry Moving X-ray couch/table X-ray tube Filters Collimators – pre and post patient Detector array (Vosper, 2016) 5 Gantry and couch 6 Inside the gantry … 7  X-ray tube Rotates in a continuous 360-degree arc in the axial plane around the patient’s body (slip ring technology) Produces 1000 X-ray pulses per tube rotation Higher heat capacity compared to standard X-ray tube Enhanced heat dissipation: ◦ Larger and thicker anode ◦ High speed anode rotation ◦ Cooling oil ◦ Focal spot cooling algorithms (system adjusts mA) CT tube (Bushong, 2013, P 1032) 8  Filters Improves X-ray beam and reduces patient radiation dose Absorbs low energy X-ray photons Quality of X-ray photon made more uniform. Often “Bow tie filters” Where edges of patient are Image retrieved October 2nd 2-023 from thinner than central part e.g. https://www.radiologycafe.com/frcr-physics-notes/ct-imaging/ct- equipment/ abdomen 9  Collimators Pre-patient collimators Mounted on tube housing Reduces radiation dose – limits volume of tissue irradiated Pre-detector collimators- Restricts X-ray beam viewed by detector array Improves contrast - limits scatter Collimators in CT (Bushong, 2016, P 452) 10  Detector array Multiple individual scintillating detectors arranged in rows X-ray>light>electrical current via photodiode High X-ray detection efficiency -90% Nil gaps between detectors Rotates in continuous arc opposite the X-ray tube Receives the attenuation information from the patient and transmits the data to the computer (Holmes, 2021, p 192) Typical multidetector CT scanners (MDCT) have arrays of between 64 (each of 0.5-0.625mm width) and 320 rows 11  Describe the following components of the CT scanner and their function: ◦ Table/couch ◦ Pre detector collimator Practice ◦ Detector array Questions: Discuss the heat capacity requirement of the CT X-ray tube and how this is achieved 2. Principles of operation 13  Principl es of CT: How CT works (Holmes, 2021, p 176) 14  Principles of CT: How CT works Tube and detector array rotate 360⁰ around the patient X-ray Tube - Fan beam - pulsed 1000 times per rotation ◦ Beam passes through body - attenuated differing amounts Detector array - multiple individual detectors - measures photons exiting patient ◦ Sends small signal proportional to the radiation received ◦ Occurs at multiple angles (projections) around patient Data collected from multiple ‘projections’ processed by computer ◦ Calculates attenuation value for each volume of tissue (voxel) ◦ Each attenuation value converted into “Hounsfield Unit” and displayed as shade of grey on reconstructed image (Holmes, 2021, p 176) 15 (Bushong, 2016, P 456) 16  Hounsfield Scale (AKA CT numbers) Hounsfield scale derived from the relative sizes of the X-ray attenuation coefficients of tissues compared to water Extends from -1000 to +1000 expressed in Hounsfield units (HU) with water at 0 HU Tissue that attenuates more than water have positive values Tissues that attenuate less than water have negative values Term “CT number” often used – based on the Hounsfield scale but extended to +3000 (e.g. metal implants) HU for each pixel is converted to a grey scale of (Seeram, 2015; p 59) image intensities in CT 17 Hounsfiel d units – different tissue types 18 Hounsfield Units E A C H P I X E L I N T H E I M A G E H A S I T S O W N H U / C T N U M B E R. T H I S I S C O N V E RT E D I N T O GREYSCALE. 19 CT Image Display: Windowing Windowing is the process where the CT image greyscale component of an image is manipulated via the HU (CT numbers) The eye can only see 32 shades of grey – windowing determines which HU values this is applied across Window width Some examples: ◦ The total range of CT numbers displayed on the monitor brain W:80 L:40 ◦ Controls image contrast lungs W:1500 L:- Window level/centre 600 mediastinum W:350 L:50 ◦ Determines CT number to be in the midpoint of the window. Abdo soft tissues W:400 ◦ Controls image density L:50 Scanners will have pre-sets for commonly used WW/WL Vertebra bone W:1800 L:400 20 Mediastinum window W:350 L:50 Lung window W:1500 L:-600 Narrow (high) contrast Wide (low) contrast 21 Different modes of CT data acquisition Slice by slice – rarely used today, but sometimes used for lung sampling. Table is stationary when image taken then table moves to next position. Volume acquisition – most commonly used - helical movement – table continuously moves in Z-direction as the X-ray tube and detectors rotate around the patient. Volume data (3D data) means we can do all sorts of reconstructions … 22  Fly through MPR – multiplanar MIP – maximum Volume reformats intensity rendering projection Things you can do with volume data  Some CT Scan parameters Tube voltage - typically 120kVp Beam width – width incident to (range 80 to 140 kVp) patient –measured at isocentre Tube current (mA) – Slice width (acquired slice thickness) – usually acquired as thin slices Scan time (s) Slice reconstruction (displayed Pitch thickness, orientation etc.) Scan length (Z-axis coverage) Reconstruction algorithm (kernels) Scan FOV (SFOV) Display FOV (DFOV) Most factors will be preset according to the protocol you are using Scan length Scan length is the exposed region of patient Z-axis coverage Affects scan time (s) z Directly relates to patient dose Adapt scan length to the zone of interest  Scan field of view (SFOV) Fan beam width in xy-axis Typically, fixed 240 mm for head scans and y up to 450 mm for body scans (Holmes et al., 2021) Some scanners you can select different sizes (S, M, L) Can alter the radiation dose of patients and spatial resolution of image Patient should be in isocenter to ensure mA modulation works x 26 Display FOV versus scanning FOV Display FOV – is the reconstructed FOV ◦ Must be smaller than or equal to the scan FOV ◦ Affects spatial resolution but NOT radiation dose In image opposite – the SFOV is shown in green and the DFOV is shown in yellow A well explained video of this concept can be found here: https://www.youtube.com/watch?v=Bp-vPUDGrTU 27  CT pitch Relates to the speed of couch movement during helical scanning (𝑐𝑜𝑢𝑐h 𝑚𝑜𝑣𝑒𝑚𝑒𝑛𝑡 𝑤𝑖𝑡h𝑒𝑎𝑐h 360°) 𝑝𝑖𝑡𝑐h= (𝐵𝑒𝑎𝑚𝑤𝑖𝑑𝑡h) Affects radiation dose and image quality Questions: Will a pitch of 1.5 have better or worse image quality than a pitch of 1? Will a pitch of 1.5 have lower or higher radiation dose? Retrieved September 25th, 2018 from https://www.slideshare.net/VivekElangovan1/ctdi-42682868 28  Reconstruction algorithms (kernel) Complex mathematical process Also called convolution filter Different kernel developed for different anatomical areas: Standard kernel – smoothed image for soft tissues (e.g. brain algorithm; soft tissue algorithm) Sharper kernel –sharper image with higher spatial resolution – use edge enhancement to sharpen image (e.g. bone algorithm or lung algorithm) Gaillard F, CT window and algorithm effects. Case study, Radiopaedia.org (Accessed on 13 Oct 2024) https://doi.org/10.53347/rID-55748 29  1. Explain how windowing affects how we visualise the CT image? 2. State which tissue is best imaged as WW:1500 WL:- 600 and explain why? 3. State TWO examples of post processed image reconstructions that can be achieved through Practice volume scanning questions: 4. Define the term “pitch”. How does this affect radiation dose and image quality? 5. Explain the difference between scan FOV and scan length? How do these affect image quality and dose (if at all?) 30 Time for a break 31 3: Factors that affect image quality in CT SPATIAL RESOLUTION CONTRAST RESOLUTION NOISE AND SNR ARTEFACTS 32  Spatial Resolution (SR) What is spatial resolution? Ability of an imaging modality to differentiate two adjacent structures as being distinct from one another (i.e. to see a small object) Line pairs per mm What affects spatial resolution in CT? Here are a few: o Detector size o Pixel size – FOV and matrix (xy axis SR) o Slice thickness and voxel depth (z axis SR) o Kernal – edge enhancement increases SR (lung, bone) Image retrieved October 3rd 2023 from https://www.radiologycafe.com/frcr-physics-notes/ct- o Pitch and no of projections imaging/ct-image-quality/ 33  Pixel size: affected by (Seerham , 2016) FOV and matrix Field of view (FOV ) 𝐏𝐢𝐱𝐞𝐥 𝐬𝐢𝐳𝐞 = 𝑚𝑎𝑡𝑟𝑖𝑥 (no of pixels ) Each pixel/voxel has its own grey scale Typical matrix for CT is 512 x 512, but can get 1024 x 1024 34 Edge enhancement kernels and spatial resolution (Seeram, 2016 p 215) 35  Contrast resolution (CR) What is contrast resolution in CT? The ability to distinguish one soft tissue from another without regard for size or shape is called contrast resolution (Bushong, 2016). Ability of the scanner to differentiate small differences in attenuation between closely spaced objects. Being able to differentiate between two tissues with similar attenuation is a crucial aspect of a CT scanner. Biggest threat to contrast resolution is NOISE! Contrast resolution can be evaluated on a low contrast test tool Image at bottom has more noise, & reduced contrast resolution (Seerem, 2016 p 2019) 36  Noise What is noise? Noise is defined as the standard deviation of signal in a homogeneous phantom. When a water phantom is imaged, each pixel should show a HU of 0 – but this is not the case. Random fluctuation in the image signal Gives grainy appearance. Higher mAs will generally result in lower noise Noisy image – note grainy appearance (Baker et al., 2012) 37  Signal to noise ratio (SNR) What is SNR? Measure of true signal (photons i.e. reflecting actual anatomy), to noise (e.g. random quantum mottle). More photons = higher signal Higher the ratio, the less noise affects the image In CT, SNR is mainly affected by: o mAs o Voxel size (Slice thickness and pixel size) 0.6mm slice 6mm slice o Patient size (Alshipli & Kabir, 2017)  CT artifacts Common in CT – may obscure or mimic pathology Categories of CT artifacts  Patient-based artifacts – motion- voluntary and involuntary movements, implants.  Scanner-based artifacts - due to faults in the scan function  Physics-based artifacts - relating to X-ray attenuation and other processes The appearances can include streaks, shadows, rings and distortion 39 Motion artifact (patient-based artifact example) Causes: patient, cardiac, respiratory and bowel motion Appearance: Blurring, double images, and long-range streaks (Boas & Fleischmann, 2012) Solutions: ◦Faster scans ◦Patient –comfort, immobilization or sedation ◦Respiratory -breath hold techniques ◦Cardiac motion – ECG gating (Boas & Fleischmann, 2012) 40  Ring artifact (Scanner based artifact example) Causes: mis-calibrated or defective detector element Appearance: rings centred on the centre of rotation. Solution: ◦ Recalibration of the detector ◦ Replacement of detector (Barrett & Keat, 2004) (Boas & Fleischmann, 2012) 41  Photon Starvation- physics based artifact Cause: ◦ Insufficient photons reach detector (Poisson noise) ◦ Occurs in high attenuation areas e.g. behind metal implants (metal artefact); between shoulders Appearance: ◦ Thin bright and dark streaks in direction (Boas & Fleischmann, 2012, p 3) of greatest attenuation (one form of streak artefact) ◦ Grainy appearance Solutions: ◦ Automatic mA modulation ◦ Iterative reconstruction techniques ◦ Adaptive filtration 42  Beam hardening artifact(Physics based artifact example) Cause: ◦ polychromatic x-ray beam passes through a very dense target (e.g. bone or iodinated contrast) resulting in selective attenuation of lower energy photons (like filtration) ◦ Only higher energy photons contribute to beam (beam is “hardened”) Appearance: ◦ Streak artifact – dark bands between dense objects ◦ Cupping artifact – CT values in centre of image appear lower (periphery appears brighter) Solution: ◦ Beam filtration ◦ Iterative reconstruction Beam hardening artifact – seen ◦ Metal artifact reduction software (MARS) helps if in posterior fossa (Popilock et al., metal cause 2008) 43  1. Define spatial resolution in CT 2. Which of the following will improve spatial resolution in CT (select all that apply): a) Increased FOV b) Increased matrix c) Increased pitch d) Thicker slices Practice questions: 3. Define contrast resolution in CT 4. Which of the following will IMPROVE contrast resolution in CT (select all that apply): a) Increased mA b) Increased voxel sizes c) Increased pitch d) Larger patients 44 A B C Practice question: Identify the artifacts shown in images A-C, and state what course of action needs to be taken to remedy the artifact 4: Radiation safety in CT 46 Radiation dose metrics in CT Measures of radiation output: Effective dose (E): ◦ Physical effect of total dose on patient CT dose index CTDIvol by the sensitivity of imaged area to ◦ Standardised measure of scanner radiation output ◦ Measure of uniform whole body ◦ Measures radiation per slice of tissue equivalent dose –mSv from head and body reference ◦ DLP x conversion factor on region of phantoms body scanned Dose Length Product (DLP) ◦ Total X-ray output for entire exam ◦ = CTDIvol x scanned length in cm – measured in mGy*cm Both CTDIvol and DLP can be used to compare CT scanner output – i.e. not a measure of the patient’s absorbed or effective dose 47 How do radiation doses in CT compare to other examinations? From COMARE report (16th), 2014, p 20 48 Some factors affecting radiation dose Tube current Doubling mA = double radiation dose Rotation time Doubling rotation time = doubles radiation dose Pitch Doubling pitch = halves radiation dose kVp Dose is approximately ∝ kVp2 i.e. doubling the kVp will increase the dose by a factor of 4 (approximately). Scan length Increasing Z coverage will increase radiation dose 49  DRLs in CT CT diagnostic reference levels (DRLs) set nationally and locally Use CTDIvol and DLP values – act as benchmarks CT protocols should be developed in line with these –high quality examination at lowest possible radiation dose To note: UK DRLs tend to be lower than other from UKHSA-RCE-1: doses from computed countries tomography (CT) exams in the UK (UK Health DRLs for CT have reduced significantly from Security agency, 2019) 2011-2019 – typically 20-30% - why?  Radiation dose and risk Radiation doses in CT are considered high dose ◦ Stochastic effects – lifetime cancer risks ◦ Deterministic effects – unlikely at diagnostic levels Risks depend on radiation dose, organ involved and age (COMARE, 2016) Risk dose benefit should be considered, and scan protocols should be optimised From COMARE report (16th), 2014 From COMARE report (16th), 2014, p 22 52 Relationshi p between radiation dose and image quality Images now optimised to accept some noise in image in UK to keep doses ALARA  Reducing radiation dose in CT – manufacturer solutions Radiation dose much lower in modern CT than old scanners Manufacturer CT dose reduction tools: can reduce dose by as much as 70% while producing high quality images (Cloke, 2011) Examples: ◦Iterative reconstruction (more recently - deep learning image reconstruction) ◦Automatic mA modulation ◦Bowtie filters 54  Iterative reconstruction Image reconstruction algorithm Reduces noise in image compared to traditional filtered back projection As noise is reduced, less mAs is required Images are higher quality and less radiation dose Filtered back projection versus iterative Also reduces artifacts reconstruction (Fan, et al., 2014) Most modern scanners will use this or newer “deep learning image reconstruction”  CT dose reduction technology – mA modulation Also called automatic tube current automation (ACTM) or automatic dose modulation Similar to automatic exposure control (AEC) in X-ray Setting varies mA in X, Y and z axis Limits dose where less radiation needed whilst maintaining image quality To optimise dose modulation the patient Effect on Radiation Dose and Image Quality of the Computed should be in the isocentre of the Tomography Tube Current Modulation (Khedr, et al., 2020). scanner  Justification: in line with local protocols ◦ Is there correct clinical information to justify the use of CT? Should alternatives be considered? ◦ Is the specific protocol justified? Note – use of multiphase scans will increase radiation dose What can ◦ Is the coverage correct for the patient's clinical history ? the Optimization radiographer ◦ Get it right first time! ◦ Prepare patient –no artefacts, keep still, breath holds etc. do to keep ◦ If contrast study – check adequate cannula size, and doses flushes well. Check pump is set correctly. ◦ Careful technique - Ensure the patient is correctly ALARA in positioned in the isocenter – this optimizes the use of mA modulation CT? (3 ◦ Use the correct protocol for the examination – these will be optimized ! principles ◦ Where possible/appropriate – move arms out of way if from chest/abdomen imaging ◦ Reduce scan volume/ Z-axis coverage to only cover the AOI IRMER…) Limitation: ◦ Check correct patient; inclusive pregnancy policy ◦ Correct use of protocol should be in line with DRLs - this should be recorded so if patterns of concern these can be 57 identified.  Scanning in the isocentre! Patient mis-centring is common in CT examinations Off centring can significantly affect dose (between reduction -36% to increase of 91%) (Al-Hayak et al., 2022) Dose modulation works less effectively if not centred correctly ◦ If placed too high – increase radiation dose ◦ Placed too low – reduced image quality Some modern scanners have automated patient positioning tools (e.g. 3D cameras) which improve centring (Image from Whitley et al., 2020, p27)  1. Explain the difference between CT dose index (CTDI vol) and the effective dose 2. Explain ONE manufacturer-based CT dose reduction tool that should be used to keep radiation doses as low as reasonably Practice practicable (ALARP) and maintain image quality questions 3. Discuss TWO ways the radiographer can ensure the patient’s radiation dose is kept as low as reasonably achievable whilst maintaining image quality 4. Identify the radiation risks associated with CT scanning of the abdomen Summary 1. Components of a CT system – gantry, couch, tube, filters, collimators (pre patient, pre detector), detector array 2. Principle of operation – how CT works, slice by slice vs volume acquisition, Hounsfield scale and windowing, scan parameters 3. Factors that affects image quality in CT – spatial resolution, contrast resolution, noise, signal to noise 4. Radiation safety in CT – dose metrics; radiation risks; factors affecting radiation dose (incl. manufacturer and radiographer methods) Reading to support this lecture Core – easy reading: Holmes, K., Elkington, M. and Harris, P. (2021) Clark's Essential Physics in Imaging for Radiographers. 2nd edn. Milton: Taylor & Francis Group. Further reading – more detailed: Bushong, S. C. (2016;). Radiologic science for technologists: Physics, biology, and protection (11th;11; ed.). St Louis: Elsevier Health Sciences Whitley, S. A., Dodgeon, J., Meadows, A., Cullingworth, J., Holmes, K., Jackson, M., Hoadley, G., & Kulshrestha, R. (2020). Clark's Procedures in Diagnostic Imaging: A System-Based Approach. Taylor & Francis Group. Any questions???  References Al-Hayek, Y., Zheng, X., Hayre, C., & Spuur, K. (2022). The influence of patient positioning on radiation dose in CT imaging: A narrative review. Journal of Medical Imaging and Radiation Sciences. Alshipli, M., & Kabir, N. A. (2017, May). Effect of slice thickness on image noise and diagnostic content of single- source-dual energy computed tomography. In Journal of Physics: Conference Series (Vol. 851, No. 1, p. 012005). IOP Publishing. Baker, M. E., Dong, F., Primak, A., Obuchowski, N. A., Einstein, D., Gandhi, N.,... & Vachani, N. (2012). Contrast- to-noise ratio and low-contrast object resolution on full-and low-dose MDCT: SAFIRE versus filtered back projection in a low-contrast object phantom and in the liver. American Journal of Roentgenology, 199(1), 8-18. Boas, F. E., & Fleischmann, D. (2012). CT artifacts: causes and reduction techniques. Imaging Med, 4(2), 229- 240. Bushong, S. C. (2017;). Radiologic science for technologists: Physics, biology, and protection (11th;11; ed.). St Louis: Elsevier Health Sciences. Cloke, P., Graham, D. T., & Vosper, M. (2011). Principles and applications of radiological physics (6th ed.). Edinburgh: Churchill Livingstone Elsevier. Committee on Medical Aspects of Radiation in the Environment (COMARE) 16 th Report. Patient radiation dose issues resulting from the use of CT in the UK. Public Health England. ISBN 978-0-85951-756-0 Fan, J., Yue, M., & Melnyk, R. (2014). Benefits of ASiR-V reconstruction for reducing patient radiation dose and preserving diagnostic quality in CT exams. White paper, GE Healthcare.  References Fursevich, Dzmitry & LiMarzi, Gary & O'Dell, Matthew & Hernandez, Manuel & Sensakovic, William. (2016). Bariatric CT Imaging: Challenges and Solutions. Radiographics : a review publication of the Radiological Society of North America, Inc. 36. 150198. 10.1148/rg.2016150198. Khedr, Yasser & A.Ali, Magdi & Kandil, Bothaina & M. El-Safwany, Mohamed. (2020). Effect on Radiation Dose and Image Quality of the Computed Tomography Tube Current Modulation. Egyptian Journal of Physics. 10.21608/ejphysics.2020.33536.1046. Popilock, R., Sandrasagaren, K., Harris, L., & Kaser, K. A. (2008). CT artifact recognition for the nuclear technologist. Journal of nuclear medicine technology, 36(2), 79-81. Seeram, E. (2016). Computed tomography: physical principles, clinical applications, and quality control (Fourth). Elsevier. UK Health Security agency (2019). UKHSA-RCE-1: doses from computed tomography (CT) exams in the UK. Retrieved Jan 30th 2023 from UK Health Security agency (2022). National Diagnostic Reference Levels (NDRLs) from 13 October 2022. Retrieved Jan 30th 2023 from https://www.gov.uk/government/publications/diagnostic-radiology-national-diagnostic-reference-levels-ndrls/ndrl # national-drls-for-computed-tomography Whitley, S. A., Dodgeon, J., Meadows, A., Cullingworth, J., Holmes, K., Jackson, M., Hoadley, G., & Kulshrestha, R. (2020). Clark's Procedures in Diagnostic Imaging: A System-Based Approach. Taylor & Francis Group.