RANZCR – AIT 2016 (Answers)

April 2016

Section 1 Radiation Biology and Safety

Question 1 

1. The ICRP has developed a system of dose limits for the protection of individuals and populations. In 2011 the ICRP reduced the equivalent dose limit for occupational exposure that applies to the lens of the eye. 
   1. What is the new dose limit and why did the ICRP consider it necessary to reduce it? (1.5 marks) 
   2. What precautions might you take to minimise your eye exposure during a prolonged interventional procedure? (1.5 marks) 
2. In the context of the biological effects of ionizing radiation discuss: 
   1. What is meant by stochastic effects and tissue reactions? Relate your answer to the type of damage that may be caused at the cellular level and provide an example of each. (4 marks) 
   2. Somatic versus hereditary effects. (1 mark) 
   3. The absolute risk and relative risk models (2 marks) 

Question 2 

A surgical registrar is organizing a CT abdomen on a 65 year old patient with suspected diverticulitis and a possible abscess. During the consultation with you the registrar confesses they have minimal knowledge of the radiation exposure and safety implications for CT. They are calling on your expertise to provide enough knowledge to impress their consultant on the ward round the following day. The registrar asks you to explain: 

1. The anticipated typical organ and effective radiation doses from a CT abdomen. (3 marks) 
2. The effect of doing both pre- and post- contrast studies. (1 mark) 
3. What the possible risks are to the patient from radiation in the context of this scenario. (5 marks) 
4. The magnitude of the risk in the context of natural background radiation. (1 mark) 

Question 3 

1. Discuss possible foetal effects that might arise following irradiation if the foetal age is approximately 6 weeks. (4 marks) 
2. CT fluoroscopy is a convenient imaging modality for needle biopsy and drainage etc. It is accompanied by increased risks to the radiologist or registrar performing the procedure. What are these risks and what precautions might you take to minimise them? (6 marks) 

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

Question 1 

1. What is meant by X-ray beam filtration and what is the difference between the inherent and the added filtration of an X-ray tube? In particular, explain what the purpose of adding filtration is. (4 marks) 
2. Identify up to four features which contribute to the inherent filtration. (2 marks) 
3. The vertically-mounted chest X-ray stand (frequently called the ‘Bucky’) typically has three ionisation chambers located on the patient side of the image receptor. Explain the purpose of these ionisation chambers and discuss the advantages of using these devices, including the reason(s) for having more than a single ionisation chamber. (4 marks) 

Question 2 

1. At the atomic level, identify the major X-ray photon interaction that gives rise to scattered radiation in diagnostic radiography and describe the interaction mechanism. (2 marks) 
2. Explain why scattered radiation is generally of concern in radiology. (2 marks) 
3. An anti-scatter grid is often used in radiography to minimize scatter. Briefly describe the components of a grid and explain how it works. (4 marks) 
4. List four practical measures that can be used to reduce the effects of scatter in plain X-ray images. (2 marks) 

Question 3 

1. Geometric magnification is used to improve spatial resolution in Mammography. Explain how this magnification and increase in resolution is achieved. (3 marks) 
2. What is the typical geometric magnification provided with clinical mammography systems? (1 mark) 
3. Multi field size image intensification has been the traditional method of undertaking fluoroscopy. Explain the benefits of using a small field size (magnification mode) and indicate, with reasons, what the patient dose implications might be when choosing a small versus a large field size when the image intensifier is operated under automatic brightness control. Specifically, if you were to change from a field size of 30 cm diameter to one of 20 cm what would be the approximate change in dose to the patient if the kV was kept unchanged? (4 marks) 
4. A digital image receptor is typically composed of detector elements or ‘dels’ grouped to form an image matrix. Assuming the input beam quality and dose to the image receptor is unchanged discuss the impact on image quality resulting from the linear dimension of the del being halved. (2 marks) 

Section 3 – CT, MRI, US & Nuclear Medicine 

Question 1 

1. The patient dose from a helical CT scan is affected by a number of machine parameters that are controllable by the operator. List four (4) of these parameters and describe how they affect patient dose (assume that all other parameters are unchanged). In each case also briefly describe any impact of the parameter on image quality. Note that at least three (3) of the parameters described must have a direct effect on dose. (8 marks) 
2. Briefly describe the relationship between CT dose-length-product (DLP) and patient effective dose. (2 marks) 

Question 2 

1. Briefly describe the general method by which spatial localization of the signal is obtained in 2-dimensional multislice magnetic resonance imaging (MRI). (2 marks) 

Slice selection: a gradient is applied along an axis perpendicular to the plane on interest, resulting in a linear variation in precessional frequencies. A 90° RF pulse with a narrow beamwidth excites a select range of protons with precessional frequencies corresponding to the BW of the RF pulse.

Phase encoding: a gradient is applied along the y-axis of the selected slice, causing the protons to precess at incrementally different frequencies. When the gradient is turned off, the protons revert to their initial Larmor frequency but now with varying phases.

Frequency encoding: a gradient is applied along the x-axis of the selected slice, causing the process to precess at incrementally different frequencies along the x-axis.

2. Give a detailed description of the timing in the pulse sequence necessary to achieve spatial localization in 3 dimensions. (6 marks) 

A 90° RF excitation pulse is simultaneously applied with a slice selection gradient that is perpendicular to the plane of interest

A phase encoding gradient of short duration is then applied.

A 180° RF refocusses the pulse at TE / 2

A frequency encoding gradient is applied with the peak signal amplitude/echo at time to echo (TE) where the FID is recorded

At time to repeat (TR) the next 90° RF RF excitation pulse is applied again.

3. What factors determine the thickness of the slice in multislice MRI? Describe what is required to decrease the slice thickness. (2 marks) 

Slice thickness depends on the bandwidth of the 90° RF pulse and the slice selection gradient strength across the field of view.

Therefore, to decrease the slice thickness:
– a narrower bandwidth of 90° RF is required,
– or an increased gradient strength across the field of view, which results in a large range of frequencies

Question 3 

1. Real time ultrasound instruments make a number of basic assumptions in order to produce an image. State 3 of these assumptions. (3 marks) 
2. Lateral resolution in real time ultrasound imaging using a phased array transducer can be improved by focusing the ultrasound beam in the scan plane. 
   1. Briefly describe the construction of a phased array transducer and how focusing is achieved. (3 marks) 
   2. Describe how the depth of the focal zone can be adjusted. (2 marks) 
3. Lateral resolution can be improved over a range of depths by using multiple focal zones. If 2 focal zones are set and the line density and maximum depth of imaging are unchanged, explain what will happen to the frame rate and why. (2 marks) 

Question 4 

1. A gamma camera with a parallel hole collimator is used to image two small adjacent lesions in the liver with the camera surface placed as close as possible to the patient’s abdomen. Describe the effects of moving the camera away from the abdomen a distance of 20 cm on spatial resolution of the image and the total count rate of the two lesions. It is essential that you give reasons for your answers. (4 marks) 
2. Describe the differences between normal gamma camera imaging and SPECT imaging with a gamma camera. In particular describe briefly how a SPECT image is created. There is no need to describe the operation of a gamma camera. (5 marks) 
3. PET imaging uses somewhat different radionuclides to that used in conventional gamma camera imaging. Describe the main difference between them. (1 mark) 

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September 2016

Section 1 – Radiation Biology and Safety 

Question 1 

1. Starting with the definition of absorbed dose indicate how the estimated effective dose may be calculated and indicate in what context it may be regarded as a useful dose metric. (4 marks) 

The absorbed dose is the amount of energy deposited per unit mass in any target material, measured in Gy (equivalent to J/kg).

The relative biological effect of differing types of radiation is then taken into account, by multiplying the absorbed dose by a radiation weighting factor to produce the equivalent dose.

The sum of the equivalent doses of each organ is then multiplied by an organ-specific weighting factor to produce the effective dose.

Effective doses are a useful metric for calculating risks of stochastic effects of irradiation and for comparing doses from different modalities.

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

The effective dose is derived by models and simulations using anthropomorphic phantoms representing idealised anatomy in terms of size, shape and position of tissue. Therefore, it calculates radiation risk based on population averages and doesn’t account for individual risks such as age and gender.

3. In the context of biological effects of radiation describe what is meant by: 
   1. the Genetically Significant Dose (GSD) and (2 marks)

The GSD is the fraction of radiation exposure is absorbed by the gonads of a person who subsequently produces children.

It estimates the genetic significance of gonad radiation dose.

3. the latent period. (2 marks) 

The time interval between exposure and development of effects.

Question 2 

1. A female patient of childbearing age presents at a small medical clinic with clinical indications of renal colic. She undergoes three AP abdominal X-rays on the basis that she is not pregnant. She subsequently undergoes an ultrasound examination which reveals that she was 7 weeks pregnant at the time of the examination. The referring doctor wants to know what he can tell his patient about any possible concerns for the foetus. An entrance skin dose of 4 mGy is posted as the local diagnostic reference level for an abdominal X-ray in the clinic. Given that the patient was of average size, outline your response to this situation beginning with an approximate estimate of the foetal dose. (6 marks) 

The effective dose to the foetus is approximately the effective dose to the patient’s uterus, which would be included in the field of view in an abdominal x-ray.

If the ESD is 4mGy for a single projection, the patient has received a total of 12 mGy. The equivalent dose is 12 mSv.

The latest IRCP tissue weighting factor for the gonads is 0.08, hence the effective dose to the uterus is 0.96 mSv.

There are no deterministic effects of 0.96 mSv at 7 weeks of gestational age as

1. As a general rule, at what radiation dose limit to the foetus would the possibility of therapeutic abortion be considered? Discuss whether it is conceivable that this limit could ever be approached or even exceeded with routine diagnostic examinations before the pregnancy is actually identified. (4 marks) 

Question 3 

Discuss measures that may be taken to minimise staff exposure to secondary radiation during fluoroscopically guided interventions. Specifically, address the disadvantages of using an over table versus an under table X-ray tube from the perspective of radiation protection of the operator. (10 marks) 

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

Question 1 

1. Name two quantities with appropriate units used when plotting X-ray spectrum? (2 marks) 
2. Distinguish between the physical processes happening in the anode material that give rise to the Bremsstrahlung and Characteristic Radiation features of an X-ray spectrum. (2 marks) 
3. Compare the appearance of X-ray spectra of a typical clinical beam generated with a tungsten anode when the kV applied to the X-ray tube is changed from 60 kV to 80 kV. (2 marks) 
4. Explain what happens to the above spectrum when a 2.5 mm aluminium filter is added to the output of the X-ray tube. (2 marks) 
5. Explain the differences between two 70 kV clinical spectra of the same filtration, one generated with a single phase x-ray generator, the other generated with a medium frequency generator. (2 marks) 

Question 2 

1. Identify the three major interactions of diagnostic energy X-rays with matter. (1 mark) 
2. Describe what happens at the atomic level in each of these interactions. (3 marks) 
3. For tissue, how would you expect the probability of each of these interactions to change with increasing X-ray energy? (3 marks) 
4. What is meant by the K-edge and how does it arise? (1 mark) 
5. An angiogram acquired at 100 kV, 250 mA with tight beam collimation displays poor image contrast in the blood vessels of interest. What change in X-ray factors would you suggest to improve the situation? (2 marks) 

Question 3 

1. Describe the construction and operation of an indirect digital radiography (DR) image receptor. (3 marks) 
2. Different types of phosphors (composition and structure) can be used in such receptors. What two materials are generally used in these phosphors? (1 mark) 
3. How do these two materials differ in their structure and performance? (2 marks) 
4. What is meant by the modulation transfer function (MTF) of an image receptor? (2 marks) 
5. How do the phosphor structures alluded to in (b) and (c) above affect the MTF of the overall image receptor? (2 marks) 

Section 3 – CT, MRI, US & Nuclear Medicine 

Question 1 

1. Noise in a CT image is an important determinant of the image quality. Describe the main source of noise in the CT image. (2 marks) 
2. Also describe four (4) operator controlled factors that will directly affect the noise in a helical CT scanning image. For each factor detail what effect changing the factor will have on both the image noise and patient dose assuming all other factors remain unchanged. (8 marks) 

Question 2 

1. Contrast in a spin echo MRI image depends on three (3) parameters intrinsic to the tissue being imaged. Name these three parameters. (2 marks) 
2. Also explain what operator adjustable factors are required to cause each of them to predominate in a spin echo sequence, giving reasons for your selected factors. (6 marks) 
3. In the sequences where relaxation times give the most contrast, tissues with what values of the intrinsic parameters will appear brightest? (2 marks) 

Question 3 

1. In real time ultrasound imaging the Pulse Repetition Frequency (PRF) is a fundamental instrument factor in image formation. 
   1. Briefly explain the definition of PRF (1 mark) 
   2. State and explain the effect of increasing the PRF on the maximum imaging depth in tissue (2 marks) 
2. In duplex Doppler imaging pulsed Doppler is utilized to determine Doppler shifts within a user specified sample volume. 
   1. Briefly explain how pulsed ultrasound is used to determine Doppler shift. Include in your answer any implications around the PRF of the Doppler pulses and define the Nyquist frequency limit (3 marks) 
   2. What occurs in pulsed Doppler if the PRF is too low? (1 mark) 
3. The image provided shows an example of an artifact posterior to a fluid filled cyst. Name the artifact and explain the cause of this artifact. (3 marks) 

Question 4 

1. In diagnostic nuclear medicine imaging increasing the activity administered to the patient allows image acquisition time to be reduced. Discuss why this is true and any drawbacks associated with using this as a strategy to reduce acquisition time. (3 marks)
2. In a gamma camera an array of photomultiplier tubes (PMTs) are positioned at the back of the camera’s NaI crystal and a parallel hole collimator is normally fitted to the front of crystal facing the patient. Explain the role of the PMTs and the collimator in gamma camera image formation. (4 marks) 
3. In contrast to gamma cameras, PET cameras utilise a ring of detectors and do not require collimation. Explain the principles behind why PET cameras do not need collimators to form an image. (3 marks)