PHYSICS Flashcards
If a gamma camera with parallel-hole collimator is moved farther away from the patient, how are resolution and sensitivity affected?
Collimator blur increases with distance, and thus, resolution worsens. The number of
counts passing through the collimator does not change appreciably as distance increases.
For thallium-201 imaging, the photons detected in the 70-keV window are produced by which mechanism?
X-rays (not gamma)
Thallium-201 decays by electron capture to mercury-201. The mercury-201 is in an
excited state and releases x-rays with energies near 70 keV.
In a planar gated blood pool study, 121,344 net counts are measured in the left ventricle at end
diastole, and 53,311 net counts are measured at end systole. What is the LVEF? (Assume that
background count subtraction has already been performed.)
The difference between the net counts (121,344 53,311 = 68,033) indicates the activity ejected by the left ventricle. The ejection fraction is (68,033/121,344) = 56%.
The frequency of energy peaking for a gamma camera should be performed?
The gamma camera should be checked each day before use to ensure that the energy
window has not drifted and the energy spectrum is of the appropriated shape
The technetium-99m photopeak on a gamma scintillation camera is progressively increasing. The most likely cause for this photopeak shift is ?
High-voltage supply to the photomultiplier tube (PMT).
The PMT consists of a photocathode that is light sensitive and a series of metallic
plates (dynodes). A high voltage is applied to the plates, and when a light photon strikes the crystal, the photoelectron is absorbed by the photocathode and is multiplied by each dynode and a pulse is generated, which is proportional to the amount of energy of the isotope interacting with the crystal. The voltage to these plates has to be very stable, and a slight variation will greatly affect the pulse generated.
To check if all the PMTs are working properly on a gamma camera, which of the following tests should be performed?
Flood field uniformity
A uniform source of radioactive source (flood sheet) is placed on the camera detector
to check if all the PMTs are operational. If a PMT tube is malfunctioning, then a blank spot is observed on the flood scan. The large white area in the lower right corner shows the abnormality. The rest of the detector is uniform.
The interaction of a technetium-99m gamma photon with the scintillation camera is primarily by:
Photoelectric effect
In photoelectric absorption, the gamma ray transfers all its energy to an orbital
electron of the absorbing material and a photoelectron is released. Compton scatter occurs when the gamma photon transfers only part of its energy to an orbital electron. The scattered photon travels in a different direction and may further interact in the material.
In order for the technetium-99m peak to be acquired within a 20% window, the window setting should be:
Technetium-99m has a mean photopeak energy of 140 keV. In order to set a 20%
window centered on the photopeak, the window width is 140 × 0.2 = 28. This gives a range of 14 keV on either side of 140 keV; thus, the energy window limits are 126 to 154 keV.
The best method to check for patient motion on a cardiac single photon emission computed (SPECT) is to?
Motion during acquisition may give the following artifacts: misalignment of the apex, matching 180-degree areas of decreased intensity in the anterior and inferior walls, hotspots in the septum or lateral wall, and “blobs” or a ring of activity in the anterior wall or an apical nipple. However, seeing such artifacts on the long- and short-axis images does not confirm that there was motion. Review of the
raw projection images in a cine format allows visual detection of vertical or up-and-down motion.
In a cardiac SPECT study, the acquired planar projections are first reconstructed into which body plane?
Thirty-two to sixty-four planar projections are acquired 180 degrees around the
patient perpendicular to the long axis of the body, and the initial reconstruction is into the transaxial plane. From these transaxial images, the long axis of the heart is defined, and subsequently, the conventional vertical and horizontal long-axis and the short-axis images are reconstructed for analysis.
In a multigated acquisition (MUGA) study, the cardiac cycle is divided in 20 equal time frames,
and the patient heart rate is 60 beats per minute. Average time per frame is?
60 s/(heart rate × the number of frames)
60/(60 × 20) = 0.05
Dual-head camera detectors are mounted next to each other in a 90-degree orientation to the gantry for a cardiac study; this is primarily done because:
In a cardiac SPECT study, typically 64 projections are obtained over 180 degrees at
3-degree increments, and each projection usually takes about 20 seconds. A total of ~21 minutes will be required. However, if two detectors at 90-degree orientation are used, the time to acquire the 64 projection will be half (11 minutes).
The amount of energy deposited by the radiation per unit length in an absorber is known as:
LET is the amount of energy deposited per unit length of the absorber, and units are
expressed as keV/m. Electromagnetic and beta particles have low LET, since they lose little energy per interaction. Alpha particles are heavy particles and lose energy very rapidly and have high LET.
What attenuation artifacts are common in males and females?
Inferior wall attenuation is often present in male patients (diaphragmatic attenuation),
and anterior attenuation is often seen in female patients (breast attenuation). These artifacts often affect the specificity of diagnosing CAD by introducing false-positive tests. Normal systolic thickening on gated SPECT in a fixed defect on both rest and stress images represents an attenuation artifact rather than a myocardial scar, which is associated with reduced systolic thickening
