Physics and Radiobiology of Nuclear Medicine

Next, the four pulses (A, B, C, and D) are summed up to give a Zpulse,
which is then checked by the PHA if its amplitude is within the energy
window set for the 511-keV photons. If it is outside the window, it is
rejected; otherwise, it is accepted for storage.
The last step in data acquisition is the storage of the data in the com-
puter. Unlike conventional planar imaging where individual events are
stored in a (X,Y) matrix, the coincidence events in PET imaging are stored
in the form of a sinogram. Consider an annihilation event occurring at the

• position in Figure 13.6A. The coincidence event is detected along the LOR
  indicated by the arrow between the two detectors. It is not known where
  along the line of travel of the two photons the event occurred, because they
  are accepted within the set time window (say, 12 ns) and their exact times
  of arrival are not compared. The only information we have is the positions
  of the two detectors in the ring that registered the event, that is, the loca-
  tion of the LOR is established by the (X,Y) positioning of the two detec-
  tors. Many coincidence events arise from different locations along the LOR
  and all are detected by the same detector pair and stored in the same pixel,
  as described below.
  For data storage in sinograms, each LOR is defined by the distance (r)
  of the LOR from the center of the scan field (i.e., the center of the gantry)

192 13. Positron Emission Tomography

Y

AB

X

r

r

position

sinogram

angle

Projection

Fig. 13.6. PET data acquisition in the form of a sinogram.(A)Each LOR datum is
plotted in (r,f) coordinates.(B)Data for all rand fvalues are plotted to yield the
sinogram indicated by the shaded area (only a part is shown). (Reprinted with the
permission of the Cleveland Clinic Foundation.)