Most of the modern PET tomographs are capable of acquiring data in two different modes: two-dimensional (planar) acquisition with septa in-place and three-dimensional (volumetric) acquisition with septa retracted, exposing the detectors to oblique and transaxial annihilation photon pairs. Both modes of configuration for data acquisition are shown in Fig. 2.5. In two-dimensional imaging, each ring of detectors is separated by septa made of lead or tungsten. The main aim is to keep the scatter and random coincidence event rates low so as to minimize the cross-talk between rings. However, in doing so, the sensitivity of the scanner is drastically reduced. Three-dimensional acquisition can be used to improve the sensitivity by removing the interplane septa, thus allowing coincidences that happened within all rings of detector to be detected. Although the sensitivity of the scanner is increased, higher fraction of scattered and random coincidences and substantial dead time are more apparent.
In a tomograph, each detector pair records the sum of radioactivity along a given line (i.e. the line integral or projection) through the body. The data recorded by many millions of detector pairs in a given ring surrounding the body is stored in a two-dimensional (projection) matrix called sinogram, as shown in Fig. 2.6(B) and Fig. 2.6(A) shows how data is acquired in two-dimensional mode. Each point in the sinogram represents the sum of all events detected with
Figure 2.6: Schematic diagram showing how projection data is acquired (A) and stored in the sinogram (B) for two-dimensional PET imaging.
a particular pair of detectors, and each row represents the projected activity of parallel detector pairs at a given angle relative to the detector ring. In other words, if p represents the sinogram and p(r, 0) represents the value recorded at the (r, 0) position of p, then p(r, 0) represents the total number of photon emissions occurring along a particular line joining two detectors at a distance r from the center of the tomograph, viewed at an angle 0 with respect to the y-axis (or the x-axis, depending on how the coordinate system is chosen) of the tomograph. However, the sinogram provides only little information about the radiopharmaceutical distribution in the body. The projection data in the sinogram has to be reconstructed to yield an interpretable tomographic image.
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