Page 288 - Radiochemistry and nuclear chemistry

272 Radiochemistry and Nuclear Chemistry

II
II.

FIG. 9.15. Geometry in (i) positron emission tomography (PET) and (ii) gamma-camera.

The radionuclide injected into the patient should preferably decay by the emission of a
single -y, and the lower the E./the better becomes the resolution of the image. Since the
detectors are energy sensitive, the camera can be tuned to the primary unperturbed (Ch. 6)
-y's, i.e. the signals from the PMTs are accepted only if they are near the photo-peak
energy. The energy window of the detector has some width allowing some scattered
photons to contribute to the number of events registered from a single point radionuclide
source; this is suppressed by narrowing the energy window (Fig. 9.13 E). Figure 9.13 D
indicates, in the upper diagram (A), the actual events detected through the pin holes, and
how these can be sharpened into a peak if the collimator moves during the exposure time
(Fig. 9.13 B); this is of particular importance in SPECT-investigations. The gamma camera
is supported by hardware (Fig. 9.13.A) and extensive computer software.
One distinguishes between "scintigraphic" and "SPECT" investigations. Gamma cameras
are used in both investigations, but the camera is stationary in the former case, while it
usually moves in SPECT. About 95 % of all nuclear imaging investigations carried out by
the nuclear medicine department of a modem hospital is scintigraphy with stationary gamma
camera, usually referred to as "gamma camera (investigation)'. In about 2/3 of these
investigations a single picture is taken of the patient's organ: heart, kidney, liver, etc. This
yields an image of the organ's content of the radionuclide, i.e. of its function with respect
to the pharmaceutical administered. About 1/3 of the investigations are studies of the
dynamic behavior of an organ. Taking thousands of "pictures" (commonly 10 per second),
storing and treating them allows a direct study of the function, e.g. heart beat. Though the
resolution in such investigations is rarely better than 10 ram, they nevertheless provide
information not available by TCT.