ABR ABR Imaging Physics and Equipment 2

Question 1

In MRI, what phenomenon causes signal loss at tissue interfaces with different magnetic susceptibilities?

Accepted Answer

Susceptibility-induced T2* dephasing

Answer

Differences in magnetic susceptibility at tissue interfaces cause local field inhomogeneities that lead to rapid T2* dephasing and signal loss on gradient-echo sequences.

Question 2

What is the purpose of the slice-select gradient in MRI pulse sequences?

Accepted Answer

Excites only the spins within a chosen slice by matching their resonant frequency

Answer

The slice-select gradient creates a linear variation in magnetic field strength so that only protons within a specific slice have the Larmor frequency matching the applied RF pulse.

Question 3

In fluoroscopy, what is the automatic brightness control (ABC) system designed to do?

Accepted Answer

Maintain constant image brightness by adjusting kVp and/or mA as patient thickness changes

Answer

The ABC system automatically adjusts the x-ray generator's kVp and mA to compensate for changes in patient attenuation, keeping the image receptor input dose (and thus brightness) constant.

Question 4

What is the primary advantage of a flat-panel detector over an image intensifier in fluoroscopy?

Accepted Answer

Larger field of view with less geometric distortion and better DQE uniformity

Answer

Flat-panel detectors provide a larger, geometrically undistorted field of view and more uniform DQE across the image compared to the pincushion distortion and veiling glare of image intensifiers.

Question 5

In nuclear medicine, what is the purpose of a parallel-hole collimator on a gamma camera?

Accepted Answer

To accept only photons traveling perpendicular to the detector, providing spatial localization

Answer

The parallel-hole collimator accepts only photons traveling roughly perpendicular to its face, rejecting angled photons to provide spatial information about the source distribution.

ABR - American Board of Radiology Medical Physics Exam Practice Test

ABR - American Board of Radiology Medical Physics Exam Practice Test

ABR ABR Imaging Physics and Equipment 2