Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Pairs of digitalposition signalsX 3All pixels of ------.2image are y1set to zero. ------.01First ------.interaction3------.2Second ------.2------.interaction0 0 0 00 0 0 00 0 0 00 0 0 00 1 0 00 0 0 00 0 0 00 0 0 00 1 0 00 0 1 00 0 0 00 0 0 0Thirdinteraction3------.0 2 0 00 0 1 00 0 0 00 0 0 0FIGURE 21-21. Acquisition of an image in frame mode. This figure illustrates the incorporationof position data from the first three interactions into an image.ating cardiac mechanical performance, in cardiac blood pool studies and, sometimes,in myocardial perfusion studies. Gated acquisition requires a physiologicmonitor that provides a trigger pulse to the computer at the beginning of each cycleof the process being studied. In gated cardiac studies, an electrocardiogram (ECG)monitor provides a trigger pulse to the computer whenever the monitor detects aQRS complex.Figure 21-22 depicts the acquisition of a gated cardiac image sequence. First,space for the desired number of images (usually 16 to 24) is reserved in the computer'smemory. Next, several cardiac cycles are timed and the average time percycle is determined. The time per cycle is divided by the number of images in thesequence to obtain the time T per image. Then the acquisition begins. When thefirst trigger pulse is received, the acquisition interface sends the data to the firstimage in the sequence for a time T. Then it is stored in the second image in thesequence for a time T, after which it is stored in the third image for a time T. Thisprocess proceeds until the next trigger pulse is received. Then the process beginsanew, with the data being added to that in the first image for a time T, then thesecond image for a time T, etc. This process is continued until a preset time interval,typically 10 minutes, has elapsed, enabling sufficient counts to be collectedfor the image sequence to form a statistically valid depiction of an average cardiaccycle.