IMAGE PROTOCOL Flashcards
In the simultaneous acquisition dual-isotope protocol, when are the resting thallium-201 images acquired relative to the stress technetium-99m?
For the simultaneous acquisition protocol, the resting thallium-201 and stress technetium-99m perfusion images are acquired at the same time. This protocol simplifies acquisition and relies on the use of multiple energy windows to acquire the lower-energy thallium-201 at the same time as the higher-energy technetium-99m. A third window is used to correct for the down scatter of the higher-energy technetium-99m into the lower thallium-201 window. The patient is first injected with thallium-201, usually at the normal dose of 3 to 4 millicuries, and then, stress is performed using a lower dose, 9 to 15 millicuries, of technetium-99m. This allows a single acquisition and saves total protocol time
What is the most typical image artifact due to patient motion during single photon emission computed tomography (SPECT) acquisition?
Patient motion in the vertical, horizontal, or rotational planes causes decreased counts in walls that are contralateral or 180 degrees opposite to each other and usually do not follow coronary artery territory distribution. At the edges of the defects, tails or streaming effects may be noted and are responsible for the “hurricane” appearance, which gives this artifact a name. Motion correction may be used to compensate movement, but it can only correct in the vertical plane. Horizontal and rotational motions are much more difficult to detect on the raw projection images and cannot be corrected.
What causes ramp filter artifact?
The ramp filter artifact is seen during filtered back projection using a ramp filter and is caused by overlapping or adjacent liver or bowel activity and causes a loss of counts in the adjacent myocardial walls. The use of iterative reconstruction minimizes this artifact but does not make it go away completely. Waiting longer to get liver clearance or administration of small amounts of liquids to enhance peristalsis and move gastrointestinal activity may allow repeat imaging to improve image quality.
What causes ring artifacts?
Ring artifacts are seen with nonuniformity across the camera head, which may be due to problems with any of the following: photomultiplier tubes, damaged collimator, camera electronics, or sodium iodide crystal. Daily floods are critical to detecting problems at any of these levels.
What is the minimum peak pixel count in the left ventricular (LV) myocardium on a planar technetium-99m SPECT projection that is recommended to obtain best-quality interpretable images?
The greater the number of counts in the myocardium, the higher the quality of the images and the more accurate the interpretation. A minimum of 200 counts/pixel using technetium-99m and 100 counts/pixel using thallium-201 in the myocardium on an anterior projection is recommended. Fewer counts will introduce artifacts due to poor performance of the reconstruction filters, which are usually fixed regardless of the total counts acquired. The total number of counts is influenced by the following: body habitus or body mass index, level of exercise or pharmacologic stress, administered dose of radiopharmaceutical, acquisition time, energy window, and the type of collimator. Dose infiltration at the intravenous insertion site and residual activity in the injection syringe may lower the total dose received by the patient. This means that the delivered dose into the patient is less than the measured dose from the dose calibrator.
Which of the parameters will increase the total counts acquired on a SPECT technetium-99m study?
Continuous acquisition allows the gamma camera to acquire counts while it is rotating from one position to the next and increases total counts by eliminating the dead time during camera motion associated with conventional step-and-shoot acquisition. Although there may be image blurring due to the motion, the higher counts and application of appropriate filtering can eliminate the deleterious effects.
Which parameters will decrease the total counts acquired on a SPECT technetium-99m study?
High-resolution collimation decreases the total counts relative to the use of a general all-purpose collimator. Narrowing the energy window will lower the total counts. ECG gating has minimal effect on counts if separate channels are used to acquire the perfusion data, without applying arrhythmia rejection, and a second channel that acquires the gated information for functional analysis. If only a single channel is used to simultaneously acquire the perfusion and gated information, and arrhythmia rejection is applied, rejected beats will lower the total number of counts in the perfusion images due to beat rejection. This will cause deterioration in the perfusion images, and it is recommended that under these conditions, a very large acceptance window be used to avoid losing counts or prolonging the acquisition if a minimum number of beats or total counts in the myocardium are required.
