Download Physics Radiology In-Training Test Questions for Diagnostic

Physics Radiology
In-Training Test Questions
for Diagnostic Radiology Residents
May, 2014
Sponsored by:
Commission on Education
Committee on Residency Training in Diagnostic Radiology
© 2014 by American College of Radiology. All rights reserved.
1891 Preston White Drive -- Reston, VA 20191-4326 -- 703/648-8900 -- www.acr.org
1. As the main magnetic field strength increases and maintains homogeneity, which one of
the following relaxation constants increases MOST?
A.
B.
C.
D.
T1
T2
T2*
Spin Density
Rationales:
A. Correct. Spin-lattice relaxation occurs when the molecular vibrational frequencies of the
tissues approximate the angular precessional frequencies of the protons. With increasing
precessional frequencies resulting from increasing magnetic field strength, there is less overlap
of the precessional/vibrational frequencies, thus requiring a longer time for the protons to
release their energy back to the lattice.
B. Incorrect. As T2 is little affected by variations in the static magnetic field.
C. Incorrect. As T2 is not significantly affected, since homogeneity is maintained.
D. Incorrect. Spin-density is unchanged by changes in the magnetic field
2. Which of the following increases MR chemical shift artifact for a given slice thickness?
A.
B.
C.
D.
Increasing RF receiver bandwidth
Decreasing readout gradient strength
Decreasing matrix size
Decreasing field strength (B0)
Rationales:
Chemical shift occurs as a result of the difference in the resonant frequency of protons
associated with very hydrogenous (fat) tissues compared to protons associated with water
molecules. Protons are localized by the variation of resonance frequencies under the influence
of an external magnetic gradient over a specific field of view. This results in a net range of
precessional frequencies across the field of view, which is sampled according to the matrix size.
If the gradient strength is low, that means that a small range of frequencies will be manifested
over the FOV, and for a given matrix size, the bandwidth across a single pixel will be similarly
small. Since the chemical shift results in frequency shifts of about 3 parts per million, for a 1 T
magnet, the frequency differences in fat versus water protons is about 1 T × 42.58 MHz / T × 3
× 10-6 = ~ 125 Hz. If the bandwidth across the pixel is small, then the signals due to fat can
potentially be mapped into adjacent (or even further away) pixels, and the anatomical
presentation will manifest “chemical shift” artifacts.
A. Incorrect. A broad bandwidth receiver setting will result in similarly larger bandwidth across
each pixel, reducing the effects of the chemical shift.
B. Correct. The lower readout gradient strength for a given FOV will result in a smaller range of
frequencies across each pixel, and therefore will more likely map signals due to fat in different
pixels than signals due to water.
C. Incorrect. A smaller matrix size mapped to a given FOV will have larger pixels, and therefore
will have larger bandwidth across each pixel. The larger bandwidth per pixel will reduce the
effects of chemical shift.
D. Incorrect. Lower main magnetic strength will cause the absolute chemical shift to also be
lower (actual chemical shift = relative chem. Shift (ppm) × B0 × γ.
3. A series of single-slice axial CT images are acquired with 5 mm slice thickness and 5
mm slice spacing. The study is repeated in a helical mode with a 5mm slice thickness,
7.5 mm table motion per rotation, with reconstructed images every 2 mm. What is the
ratio of the average patient dose in the helical study to that in the axial study?
A.
B.
C.
D.
0.4
0.67
1.5
2.5
RATIONALES:
A. Incorrect. See the explanation above
B. Correct. The pitch is 1.5 for the helical scan with 7.5 mm table motion per rotation and 5 mm
collimated slice. This is a gap-like technique and would result in a dose of 1/pitch or 1/(1.5) or
0.67 of that of a procedure with a pitch of one, or of an axial series of contiguous slices.
C. Incorrect. Higher pitch implies reduced dose
D. Incorrect. Reconstructed slice spacing does not affect dose.
References:
Physics of Radiology, 2nd edition. Anthony Brinton Wolbarst, Medical Physics Publishing,
Madison WI (2005), Chap 40, p418.
4. In digital radiography, the kV and mAs are selected to provide optimal ________ at the
lowest possible dose to the patient.
A.
B.
C.
D.
signal to noise ratio
optical density
spatial resolution
image contrast
RATIONALES:
A. Correct: Unlike screen-film detectors that have a fixed film contrast and typically require low
kV to ensure adequate subject contrast that is mapped to radiographic contrast by the film
characteristic curve, digital detectors have extremely wide exposure latitude (also known as
dynamic range). This permits flexibility in determining the best combination of kV and mAs to
achieve proper signal to noise ratio that subsequently can be adjusted (contrast and spatial
resolution enhancement) with image processing algorithms.
B. Incorrect: Optical density is a feature of the screen-film detector, and is the log of the opacity
of the film.
C. Incorrect: In most instances where there is sufficient exposure and within the range of typical
diagnostic techniques, the kV and mAs do not affect the spatial resolution characteristics of the
image. Spatial resolution is chiefly determined by the detector element area (dimensions) and
by the sampling pitch (distance between detector elements
D. Incorrect: Image contrast with a digital system is freely adjustable as long as there is
sufficient signal to noise ratio of the statistical information in the image. That is why the digital
detectors are often referred to as “SNR limited” while screen-film detectors are referred to as
“contrast limited.”
References:
Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging,
2nd Edition, Chapter 11, p. 308.
5. Concerning leakage radiation for a unit used for pediatric chest filming, what is the
maximum leakage radiation allowed at 1 meter from the x-ray tube source when the
system is operated at the maximum continuous allowable tube current and kilovoltage?
A.
B.
C.
D.
2 mR/week
10 mR/hour
100 mR/hour
No specified limits
RATIONALES:
A. Incorrect. This is maximum permissible dose limit outside an x-ray room.
B. Incorrect.
C. Correct. According to NCRP report no. 91, the allowed maximum limit for leakage radiation
is 100 mR/hr at 1 meter when the system is operated at maximum continuous mA and kVp.
D. Incorrect.
Reference:
The Essential Physics of Medical Imaging by Bushberg JT et. al., Second Edition, Chapter 23:
Radiation Protection.
6. Increased dynamic range in digital mammography as compared to screen-film
mammography results in which of the following?
A.
B.
C.
D.
Increased temporal resolution
Increased contrast resolution
Reduced spatial resolution
Radiation dose is decreased
Rationales:
A. Incorrect. Temporal resolution is not affected by increased dynamic range.
B. Correct. Different exposure levels mapping to a wider range allows for increased contrast in
a digital image.
C. Incorrect. increased dynamic range does not affect spatial resolution
D. Incorrect. increased dynamic range does not affect radiation dose
References:
Mahesh M, AAPM/RSNA Physics
Tutorial for Residents: Digital Mammography: An Overview, RadioGraphics 2004; 24:1747–
1760
7. An interventional fluoroscopy procedure results in a patient skin entrance dose of 2 Gy.
Which of the following radiation-induced skin injuries MOST LIKELY can occur?
A.
B.
C.
D.
Necrosis
Main erythema
Dry desquamation
Early transient erythema
Rationales:
A. Incorrect. Necrosis of the skin has dose threshold of >16 Gy.
B. Incorrect. The dose threshold for main erythema is 6 Gy
C. Incorrect. Desquamation occurs above 14 Gy.
D. Correct. This effect has a lower dose threshold of 2 Gy. It may begin within hours after
irradiation and peak at about 24 hour
References:
Balter S. Interventional Fluoroscopy: Physics, Technology and Safety, p.167.
8. Concerning the reverberation artifact in ultrasound, which one of the following is TRUE?
A.
B.
C.
D.
The echoes increase in intensity with distance.
Horizontally positioned linear echoes are superimposed on each other.
It occurs when the ultrasound signal reflects repeatedly between highly reflective
interfaces.
The presence of air creates side-lobe artifacts and not reverberation artifact.
RATIONALES:
A. Incorrect. In reverberation artifact echoes decrease in intensity with distance.
B. Incorrect. Reverberation artifact has horizontally positioned linear echoes that are spaced
equally.
C. Correct. The highly reflective interfaces causing the reverberation artifact are usually, but not
always, near the transducer. They allow identification of a specific type of reflector such as a
surgical clip.
D. Incorrect. Presence of air can be identified by the presence of reverberation artifact. Side
lobes are due to radial vibration of the transducer crystal and not the presence of air. Side lobes
arise from sound beams that are emitted from the side of the primary beam.
References:
1. Ryan K Lee. Grayscale Ultrasound artifacts. In :Dogra V, Rubens DJ(eds): Ultrasound
Secrets. 1st Edition. Philadelphia; Hanley and Belfus; 2004, Page 8-14.
2. Rumack CM, Wilson SR, Charboneau JW. Diagnostic Ultrasound. 3rd ed. Mosby Yearbook
Inc., St. Louis, MO. 2005, Page 19-22.
9. The American College of Radiology recommends that quality control on ultrasound
transducers be performed:
A.
B.
C.
D.
daily.
monthly.
quarterly.
annually.
Rationales:
A. Incorrect. Daily testing is not recommended by the ACR.
B. Incorrect. Monthly testing is not recommended by the ACR.
C. Correct. Ultrasound transducers need to be tested more frequently than annually.
D. Incorrect. This would not be frequent enough given the failure rate of transducer
components.
10. In which of the following x-ray interactions can an incident photon be back-scattered?
A.
B.
C.
D.
Rayleigh scattering
Coherent scattering
Compton interaction
Photoelectric effect
Rationales:
A. Incorrect. For Rayleigh scattering or coherent scattering, an x-ray photon of the same
energy and slightly different direction is emitted.
B. Incorrect. See above.
C. Correct. For a Compton interaction, an electron is emitted and the incident photon is
scattered at some angle with reduced energy. A 180 degree backscattered photon occurs
when the incident photon transfers the maximum amount of energy to the emitted electron.
D. Incorrect. In the photoelectric effect, all of the incident photon energy is transferred to an
electron, which is ejected from the atom.