RANZCR – AIT 2019 (Answers)

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March 2019

Section 1 – Radiation Biology and Safety

Question 1

1. In the context of the biological effects of ionising radiation, explain what is meant by deterministic effects (also known as tissue reactions). Relate your answer to the type of damage that may be caused at the cellular level and the dose-response relationship of such effects. (3 marks) 

Results from radiation-induced cell damage/death in a non-random occurrence.

Only occurs if a threshold dose has been exceeded. The dose-response relationship is such that increased severity occurs with increased dose and the absorbed dose is the more relevant quantity.

Involves a large number of cells, generally affecting skin. Effects often taken time to manifest and include; erythema, epilation, tissue necrosis, cataractogenesis

2. Following a lengthy neurointerventional procedure, the total air kerma incident on an area of the patient’s skin is estimated to be 4 Gy. State what tissue reactions the patient may experience, with approximate timeframes. (3 marks) 

Absorbed dose likely higher than total air kerma of 4 Gy.

4 Gy will produce erythema (which may be transient) and take 2 – 24 hours to manifest.

3. The neurointerventional lab is equipped with a state-of-the-art fluoroscopy unit with flat panel digital detector. It is used clinically under Automatic Exposure Rate Control. List 4 practical procedural measures that you might implement to ensure that the patient entrance skin dose is minimised, with a brief explanation for each measure. (4 marks) 

1. Avoid continuous screening when possible
Screening only when necessary is the easiest method of reducing entrance skin dose

2. Effective collimation to region of interest
This reduces the area of skin being irradiated and also improves contrast resolution by reducing scatter radiation

3. Use pulsed fluoroscopy with the lowest frame rate
Use the lowest frame rate to obtain images of acceptable quality

4. Avoid use of magnification
When magnifying anatomy, a smaller area of the input phosphor is used, resulting in a loss of minification gain. In ordered to achieve the same level of output brightness, the system increases the radiation output thus dose to the patient.

Question 2

1. A 20-year-old female patient is 6 weeks pregnant and has suspected pulmonary embolism. Her referring doctor is considering a CTPA scan. The foetal dose associated with this procedure has been estimated by the medical physicist as 0.05 mSv.
   1. Briefly describe the potential radiation risks to the foetus and how you would communicate these to the referring doctor. (2 marks)

At 6 weeks of gestation, the foetus is within the organogenesis stage, thus the potential deterministic risks would be organ malformation and growth retardation. However, this only occurs above the deterministic threshold of 100 mSv (well above the estimated foetal dose of 0.05 mSv)

Therefore the relevant risks are stochastic effects, the most important of which is carcinogenesis. For foetal doses greater than 10 mSv, the excess risk is 5-6% per Sv. The baseline risk of cancer is approximately 0.15%.

1. The medical physicist offers to estimate maternal organ doses. State, with reasons, which organ dose you consider most relevant for this patient. An estimate of organ dose is not required. (2 marks)

1. You recommend that a chest radiograph is performed first to rule out other causes of the symptoms. List 3 strategies you would employ to minimise the maternal and foetal radiation dose, with brief reasons. (3 marks) 

1. Effective collimation
– collimating only to the region of interest will reduce maternal dose and foetal radiation dose (by reducing internal scatter)

2. Automatic exposure control
– Using the automatic exposure terminating device ensures that ‘dose creep’ or intraoperative variation in exposure parameters does not lead to unnecessary radiation dose to achieve the same image quality

3. Proper positioning
– Eliminating air-gap ensures th

1. One of your radiology colleagues informs you that she is pregnant. 
   1. What is the regulatory dose limit to the foetus for a pregnant radiation worker? (1 mark) 

1 mSv

The foetus is treated as a member of the public.

3. Over the full term of the pregnancy, what dose reading would you allow on her personal radiation monitoring badge to ensure that the regulatory limit is met? Give a reason for your answer. (2 marks) 

Question 3

1. Working in a DSA suite, what personal protective equipment should you expect to be provided by your employer? (3 marks) 

Lead gown
Thyroid shield
Lead glasses

2. List the major components of a program you would initiate for the safe management of your X-ray protective gowns (often referred to as lead aprons). (2 marks) 

3. Briefly explain: 
   1. the concepts that are used to develop an effective dose (mSv) from an external source of air kerma (mGy) delivered from a diagnostic imaging procedure. (3 marks) 

2. Why is it inappropriate to use effective dose, E (mSv), as an individual risk estimate for a specific patient undergoing a diagnostic imaging procedure? (2 marks)

The effective dose is derived from models (e.g. Monte Carlo modelling) and simulations using anthropomorphic phantoms representing idealised anatomical forms in terms of size, shape and position of tissue. Similarly, the tissue weighting factors were developed for a population of both genders and wide range of ages. Therefore, the model calculates radiation risks to the reference person based on population averages, not individual risks which depends on factors such as age and sex.

Section 2 – Basic Physics & Technology including Mammography, Fluoroscopy & DSA 

Question 1

1. Briefly describe the impact of 3 factors that have a substantial effect on limiting spatial resolution in projection radiographic imaging. (3 marks) 

Spatial resolution can be affected by factors causing blur:

1. Focal spot size – large effective focal spot size results in an increased image penumbra, causing image blur

2. Geometric unsharpness – short SID or large OID results in an increased image penumbra, causing image blur

3. Motion blur artifact – patient movement, particularly during long exposures can cause image blur

2.  
   1. With regard to image noise, what is ‘quantum mottle’ (QM)?

Quantum mottle refers to the statistical variation in the intensity of photons incident on the imaging receptor and therefore results in variations in the signal detected. QM is the main component of image noise.

2.  
   2. What determines the amount QM present in an image?

The amount of QM depends on the amount of radiation absorbed by the detector on a per-pixel basis. This depends on the amount incident, absorbed and scattered through the patient.

2. What are the clinical uses for the exposure modes of acquisition and fluoroscopy as used in angiographic procedures? For an average sized patient, what is the approximate ratio of the dose rates between the two modes? (3 marks) 
3. A referrer mentions that the x-ray images from your facility are diagnostically adequate but noisier than those from another service provider. Under what conditions could you justify the quality of these images from your facility. (2 marks) 

Question 2

1. Identify and describe the principal atomic interaction responsible for the scatter x-ray component production during a diagnostic x-ray examination. (2 marks) 

Compton scatter.

Occurs due to the interaction of the x-ray photon with free electrons or loosely bound valence shell (outer shell) electrons. The resultant incident photon is scattered (changes direction) and imparts energy to the electron (recoil electron). The scattered photon will have a different wavelength (observed phenomenon) and thus different energy (E = hc/λ). Energy and momentum are conserved in this process. 

2. Define what is meant by the term Contrast to Noise Ratio (CNR) in the context of digital image quality. Describe how this concept can be applied in a clinical context. (2 marks) 

The CNR is a metric for describing the signal amplitude relative to the ambient noise in an image.

In digital image quality, the CNR is useful when testing simple objects that generate homogenous signals (i.e. the grayscale value of the object is constant). Due to the ability to post-process digital images, the CNR describes contrast potential in the image rather than just the contrast itself.

3. Identify 3 factors that can be altered during x-ray image acquisition that can favourably improve the Contrast to Noise Ratio in a planar digital radiograph. With each identified factor please also supply the reason why the CNR is improved. (6 marks) 

Use of an anti-scatter grid – reduces image noise by reducing scattered radiation from the patient before it reaches the imaging detector

Use of contrast media – where applicable (e.g. gastrointestinal imaging), the use of contrast media improves signal in the region of interest by increasing beam attenuation via the photoelectric effect.

Use of tissue compression – where applicable (e.g. mammography), tissue compression; reduces the thickness of the irradiated tissue and therefore the degree of scatter/noise,

Question 3

1. Explain the intent behind the operation of an Automatic Exposure Control (AEC) system used with a Digital Radiography system. A description of the technology is not required. (2 marks) 

The intent of the AEC system is to achieve a satisfactory signal-to-noise value at the lowest possible exposure to the patient, in keeping with the principle of ALARA and the IRCP principle of optimisation.

It also assists in ensuring intra-operative exposures are consistent and reduces the chance of dose creep. Dose creep occurs in digital radiography due to the large dynamic range (re-scaling of pixel values).

2. A wall mounted vertical Bucky typically has three ionisation chambers located on the patient side of the image receptor. What is the position of the grid in relation to the ionisation chambers and the image receptor? In the context of Chest AP imaging explain why there are 3 chambers, and how these can be used effectively. (3 marks) 

In general radiography, the grid is placed in front of the ionisation chambers, such that the x-ray beam passes firstly through the patient, then the grid, ionization chamber and finally the imaging receptor.

In chest radiography specifically, the left and right chambers are positioned over the lung fields (the region of interest) to ensure signal obtained from the region is the determinant for the cut-off. The central chamber is not useful as the overlying vertebral column and mediastinum would likely overexpose the lung fields.

3. What is meant by the term ‘modulation transfer function’ (MTF) for an image receptor? (2 marks)

The modulation transfer function (MTF) is a measure of spatial resolution that describes the percentage of output signal contrast from an imaging system to the signal contrast input into the system as a function of spatial frequency.

Due to various sources of blur in the imaging chain, the output signal contrast is always reduced compared to the input signal contrast. As spatial frequency, which is inversely related to object size, increases, MTF decreases. The limiting resolution of an imaging system is often given as the spatial frequency
at which the MTF reaches 10%.

3. The figure shows hypothetical MTF curves for two mammographic systems: one screen film and the other digital. Which system represents the digital system? Justify your choice. Discuss the reasons why the image quality of this system is judged superior to the other. (3 marks)

Section 3 – CT, MRI, US & Nuclear Medicine

A CT exam is performed where the exposure factors are 120 kV, 150 mAs, a pitch of 1:1. Automatic current modulation is not employed. The reconstruction is performed using filtered back projection with a bone filter, and the reconstructed slice width is 1 mm. The signal to noise ratio (SNR) of the resultant images is too low. The patient is rescanned with the mAs increased to 300 mAs, and all other factors unchanged. 

Question 1

1. Explain why this will increase the SNR. (2 marks) 

2. Describe the effect on effective dose, and why. (2 marks) 
   1. List two ways that the initial data set could have been reconstructed to improve SNR without having to rescan the patient. 
   2. For each, briefly explain why the SNR is improved. (4 marks) 
3. A multislice CT scanner has 32 detector rings each 0.625 mm in length along the z (long) axis. If a single tube rotation resulted in 16 slices, and all of the detector rings were exposed, what is the 
   1. nominal beam width (ignore overbeaming)? (1 mark) 
   2. acquired slice width? (1 mark) 

Question 2 

1. From the perspective of radiation protection of the patient, what is main advantage of MRI over CT? (1 mark) 
2. With regard to the chemical shift artefact in MRI, briefly describe 
   1. Its appearance (1 mark) 

Occurs due to mismapping of fat and water pixels, resulting in artifactual white or dark bands, usually at the interface of the two substances.

2. With regard to the chemical shift artefact in MRI, briefly describe 
   2. Its cause (1 mark) 

Chemical shift is due to the differences between resonance frequencies of fat and water. It occurs in the frequency-encode direction where a shift in the detected anatomy occurs because fat resonates at a slightly lower frequency than water.

3. For each patient device, list one reason why it may contraindicate an MRI: 
   1. Cardiac pacemaker (1 mark) 
   2. Aneurysm clip (1 mark) 
4. Aside from the slice section gradient, two other types of gradient fields are applied during MRI image acquisition to encode spatial information. The se are applied to encode what rotational information for the precessing net magnetic moment in a voxel? (2 marks) 
5. Briefly describe the key difference between the inversion recovery sequence and the spin-echo sequence. (1 mark)

A 180 degree radiofrequency inversion pulse is applied prior to the 90 degree readout pulse.

Question 3

1. Diagnostic ultrasound imaging is mostly performed using sound with frequencies in the range from around 2 MHz to around 15 MHz. General abdominal ultrasound imaging generally uses probes with a frequency in the range 2 MHz – 5 MHz. However, for imaging superficial structures higher frequency probes, in the range of 5 MHz – 15 MHz would normally be used. Explain why probes having the frequencies in ranges noted above are used in these two types of imaging applications. (4 marks) 

2. The image below shows a colour Doppler scan and the colour map on the left shows that blue is the ‘toward’ colour and red is the ‘away’ colour. The scan is of the common femoral artery (in red) and vein (in blue). The artery shows a patch of blue (arrow). Assuming flow in the vessel is not turbulent, what does the blue in the artery indicate and explain why this has occurred. (3 marks) 
3. In general, pulsed Doppler has the greatest potential for inducing thermal bioeffects in tissues. 
   1. Give two reasons why this is true. (2 marks) 
   2. Modern diagnostic ultrasound equipment provides on screen feedback to the operator via two on screen numerical parameters that indicate the risk of bioeffects. State one of these parameters. (1 mark) 

Question 4 

1. Briefly describe the physical principles of image acquisition in PET imaging. (Note: do not describe methods of PET radioisotope production). (5 marks) 
2. A colleague suggests increasing the activity used for FDG PET scans for a 70kg patient by 20% from 300 MBq to 360 MBq as a means to reduce imaging time and increase throughput. Comment on any risks to patients and staff that this would impose. (3 marks) 
3. For PET imaging there is a fundamental physical factor related to the positron emission energy that limits the ultimate spatial resolution achievable by a PET camera. Ga-68 is a position emitting isotope that is now widely used in addition to F-18 for PET imaging. The maximum energy of positrons for Ga-68 1.92 MeV compared to 0.65 MeV for F-18. Explain why positron energy affects the spatial resolution achievable in a PET image and as a result make a conclusion about whether Ga-68 or F-18 images will have the best achievable spatial resolution. (2 marks)

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September 2019

Section 1 – Radiation Biology and Safety

Question 1 

1. The dosimetric quantity kerma-area product (KAP, also known as DAP) can be used as the DRL quantity for general x-ray examinations. Define this quantity with units and identify two reason why this quantity is used in x-ray patient dosimetry (4 marks) 
2. What does the mean glandular dose (MGD) to the breast indicate in the context of mammography screening? What is the maximum allowable MGD delivered to an ACR phantom for use in screening in Australia and New Zealand? (3 marks) 
3. The dosimetric quantity cumulative air kerma (CAK) is used in interventional radiology. At what spatial point in relation to the equipment is it measured at? What practical indication does it give to the clinician and what is a limitation of this quantity? (3 marks) 

Question 2 

One of the tenets of the International Commission on Radiological Protection (ICRP) system of radiation protection in the medical use of ionising radiation is that of Justification. There are 3 levels of justification, two of these relate directly to common radiological practice 

1. Discuss the principle of justification in medical imaging. (2 marks) 

Justification is a principle of radiation protection

2. Give an example of a radiological examination that utilises specific procedure justification (level 2) and discuss the implications of this on clinical practice. (3 marks) 

Level 2 ‘generic’ justification refers to particular radiological exams performed for patients with a given clinical condition.

Example: CT pulmonary angiogram performed for investigation of pulmonary embolus.

3. Explain what is meant by individual patient justification (level 3). Name a tool that is useful in this type of justification. (2 marks) 

Level 3 ‘individual’ justification means that the particular imaging procedure should be judged to do more good than harm to the indvidual patient.

4. You are performing an abdominal angiographic investigation using a C arm unit. Name three operational practices that you can use to minimize the radiation dose to both yourself and the patient and explain how each practice reduces the dose. (3 marks) 

1.

Question 3 

When considering the risk of cancer induction as a result of radiation exposure, the linear no-threshold (LNT) model relationship is usually applied. 

1. Briefly describe the LNT model and its implications for a busy CT service. (3 marks) 
2. What is the approximate accepted risk of cancer mortality for a 10 year old child who undergoes a complex CT procedure and receives 10 mSv of effective dose? How does this risk compare to that of an adult with the same received effective dose? (2 marks) 
3. When considering the risk of leukemia as a result of radiation exposure sometimes a different relationship model is utilized. What is the name of this model and how does it differ from the LNT model? (3 marks) 
4. Apart from cancer induction, identify two other broad radiation detriment effects that can occasionally be associated with interventional radiology. Why are these effects rare? 

Section 2 – Basic Physics & Technology including Mammography, Fluoroscopy & DSA

Question 1

1. Identify and describe the principal radiation interaction in tissue responsible for x-ray absorption during a diagnostic x-ray examination (3 marks) 

Compton effect

This occurs due to interaction of the photon with free or loosely bound valence shell electrons. The incident photon imparts energy to the electron, ionising the atom, then scatters in a different direction with less energy.

2. For a PA Chest X-ray examination, what kVp would you select and what is the rationale for your choice? (2 marks) 

3. Refer to the diagram:
   1. Name the discontinuity labelled A and briefly explain why it arises (2 marks) 
   2. What element would you expect to match the data shown in the graph? Provide one medical imaging application for this element and explain how you would achieve the optimum beam quality for this application (3 marks) 

Question 2 

1. Explain how the design and operation of an automatic exposure control (AEC) system used for mammography differs from an AEC system used for general radiography (3 marks) 
2.  
   1. Briefly describe the relationship between image signal and image receptor dose for film-screen mammography and digital (DR) mammography. Your answer should include a definition of image signal for each image receptor (3 marks) 
   2. Explain why the dynamic range of digital mammography is considered superior to that of film-screen mammography (2 marks) 
   3. State what image quality metric you would use to ensure that the performance of a digital mammography system is optimised and give reasons for your choice (2 marks) 

Question 3

Modern fluoroscopic imaging systems may use either an image intensification (II) chain or a flat panel detector (FPD). Within this context answer the following: 

1. Describe why minification gain occurs within the fluoroscopy II imaging chain (2 marks) 

Minification gain is a component contributing to the brightness of the image. It occurs due to the concentration of photons from a large input screen to a smaller output screen, within the image intensifier.

2. Describe the composition of a Flat Panel Detector used for fluoroscopic imaging (2 marks) 

Flat-panel detectors are composed of thin-film transistor (TFT) arrays of individual detector elements called dexels.

Direct systems first use a semiconductor layer (amorphous selenium) between two electrodes to create electron hole-pairs when x-ray photon interacts with the layer. The TFT layer then works by applying a

2. Name and describe two factors that specifically influence the system spatial resolution in a FPD (4 marks) 
3. Describe how these two factors directly relate to the maximum resolution of the FPD (2 marks)

Section 3 – CT, MRI, US & Nuclear Medicine

Question 1 

1. Either in words or by equation, describe how the CT number (or Hounsfield Unit) of a voxel is calculated. (2 marks) 

The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature is defined as zero Hounsfield units, while the radiodensity of air is defined as −1000 HU. In a voxel with average linear attenuation coefficient μ, the corresponding HU value is therefore given by:

HU = 1000 × ((μ – μ_water) / μ_water – μ_air)

2. Suppose a multislice CT scanner has 100 detector rings each 1 mm in length along the z (long) axis. If all rings are fully exposed and 50 slices are acquired in a single rotation: 
   1. What is the imaged slice width and the nominal x-ray beam width? (1 marks) 

Slice width: 2mm
Nominal x-ray beam width: 100mm

3. In practice, the actual beam width will be slightly longer than the nominal beam width. Explain the purpose of this. (2 marks) 

Due to beam geometry, the actual width is wider due to penumbra. The penumbra does not get used for imaging purposes and contributes to patient dose.

Overranging is the increase in dose-length product due to the additional rotations at the beginning and
at the end of a spiral scan required for data interpolation to reconstruct the first and the last slice of the imaged body region.

Overbeaming is the excess dose per rotation that results if focal spot penumbra falls outside the
active detector area and is not used for imaging purposes.

4. To best visualise subtle differences in soft tissue, state whether you should use a relatively thick or thin reconstructed slice width and give reasons for your answer. (3 marks) 

Thick slices

Thick slices will improve the signal to noise ratio, providing better contrast resolution. However, spatial resolution will be degraded

4. Explain the cause of the cupping artifact whereby CT numbers at the centre of the image are erroneously decreased. (2 marks) 

Cupping artifact occurs when x-rays passing through the central portion of a uniform cylindrical phantom are hardened more than those passing peripherally, resulting in an attenuation profile with a characteristic cupped shape.

Example: CT of the Head

Question 2 

1. For a standard spin-echo pulse sequence performed with a 3T scanner: With regard to the chemical shift artefact in MRI, briefly describe:
   1. define the terms TR and TE. (1 mark) 
   2. Give approximate numerical values in ms required for TR and TE to produce a T1 weighted image of brain tissue. (NB: partial marks may be awarded for a qualitative description of the relative lengths of TR and TE e.g.short). (2 marks)
2. An artifact is present in two of the three figures, A, B and C. Identify which 2 figures, and name the artefact’ (3 marks). Explain how an inversion recovery sequence could be used to suppress the signal from fat. (3 marks) 

Figure unavailable.

Inverse recovery sequences are basically a spin-echo sequence that is preceded by a 180° RF pulse. They are performed to selectively nullify signals from certain tissues (e.g. fat).

The initial 180° RF pulse inverts the longitudinal magnetization (M_z), to its negative value, -M_z. Tissues regain Mz at different longitudinal (T1) relaxation rates determined by their T1 relaxation times. The spin-echo 90° pulse is then applied at the exact time when M_z of the tissue to be suppressed reaches the null point.

This time is called the time to inversion (TI), and is simply the duration between the 180° RF inveison pulse and the 90° RF readout pulse.

2. Explain why a quenching event presents a safety hazard. (1 mark)

During a quenching event, if there are leaks of helium in the ventilation system, oxygen may be displaced out of the room.

Question 3 

1. Lateral spatial resolution in real time ultrasound imaging can be improved at a particular depth by focusing the beam in the scan plane at that depth.
   1. Define lateral spatial resolution and discuss why the above statement is true. (3 marks) 

Lateral resolution is defined as the ability of the system to distinguish two points in the direction perpendicular to the direction of the ultrasound beam.

Lateral resolution is affected by the beam width and depth of imaging, as wider beams typically diverge further in the far-field, decreasing its line density.

Focussing narrows the beam width at the depth of interest.

2. Explain how beam focusing is achieved for a linear array transducer. (NB. You do NOT need to describe beam steering) (2 marks) 

Focussing produces a narrower beam width to compensate for divergence that occurs at depth. This is achieved by delaying the emission of ultrasound waves by individual piezoelectric crystal elements. The outermost elements emit ultrasound waves first then the inner elements are progressively delayed to form a curved wavefront.

2. When using real time ultrasound imaging, explain why a lower pulse repetition frequency must be used to image to a maximum depth of 10 cm than when imaging to a maximum depth of 5 cm. (2 marks) 

In order to achieve
This is the ability of the system to display events occurring at different times as separate images. It is measured in frames per second. It is reduced by:
Greater number of focal zones
Having doppler on
Deeper object (echo takes longer to reach object and return
Large sector width (more space to scan)
Each pulse of a transmitter contains a transmit (during which the ultrasound wave is produced) and a receive (during which the transducer “listens” for the returning echo) phase. The pulse repetition frequency (PRF) is the number of pulses of ultrasound sent out by the transducer per second. It depends on the velocity of sound and the depth of the tissue being imaged – the deeper the tissue, the longer the transducer has to wait for the echoes to come back i.e. lower PRF.

2. When performing an abdominal ultrasound imaging examination one element of the screen display is TIB: 0.8. 
   1. Discuss what this parameter means and what safety implications, if any, it has for the procedure at the value displayed. (2 marks) 

TIB stands for Thermal Index for Bone, and expresses the potential for rise in temperature at the ultrasound beam’s focal point.

The greatest significance when performing an abdominal ultrasound is if the patient is a pregnant female. Restrictions on duration of scanning may then occur based on the thermal index.

For non-obstetric examinations

2. If TIB was 3.0 would the safety implications change and if so actions would you take? (1 mark) 

Question 4 

1. When performing SPECT imaging with a gamma camera a parallel hole collimator is usually fitted to the camera. What is the primary purpose of the collimator. (1 mark) 

For spatial mapping of gamma photons exiting the patient’s body.

2. The face of the collimator is normally positioned as close as possible to the patient. What is the reason for this? (1 mark) 

Spatial resolution is dependent upon source (i.e. patient) to collimator distance.

3. In PET imaging no collimator is necessary, explain why this is true. (2 marks) 

In PET imaging, two gamma rays emitted from a positron-electron annihilation event travel in opposite directions to one another are detected by a ring of detectors. A line of response is then calculated from photons detected in coincidence.

If the measured line of response places the annihilation event along an artefactual projection, the coincidence is rejected. This is how collimation is achieved in PET imaging.

4. Explain what implications the fact that SPECT imaging requires a physical collimator when PET imaging does not has on the relative sensitivity of PET and SPECT systems? (2 marks) 

PET has much higher sensitivity (2 – 3 orders of magnitude) than SPECT due to the ability to detect a higher percentage of emitted events. This is because in SPECT and planar imaging, coincidences are confined to a single slice, which can only be achieved using physical coollimators to reject photons that are not within a small angular range, otherwise the angle of incidence will not be known.

In PET, due to the nature of the emission of opposite travelling photons in the same event, collimators can be entirely removed. Coincidences from a much great volume of tissue is accepted. This increases the total count rate due to the more coincidences being allowed to reach the detector. It is useful when there is relatively little scatter/administered radiation such as in the brain or paediatric imaging.

4. When performing whole body PET/CT the mAs selected for the CT component of the scan is normally significantly lower than would be used for a diagnostic scan. Explain why this is acceptable and what benefits there are from using a low mAs CT. (2 Marks)

CT is only used for anatomical structural correlation with functional imaging (PET)
Low mAs lowers patient dose.