As mentioned earlier, we may put together separate images to form a single whole- body view, without borderlines. However, this method works only for cameras with a rectangular field of view.

There is another way to construct whole-body images: by moving the table continu­ously, synchronized with the slab of the image matrix to where the acquired counts are added. (If the field of view is not rectangular, we have to correct for the differences in the effective acquisition time of each pixel.)

12.4.4.4 Reconstruction of Spatial (3D) Distribution

We already mentioned “tomography” (which means “slice imaging”) in Section 12.4.3. The mathematical procedure that calculates the distribution of cross sections from raw projections is called reconstruction.

In SPECT, the gamma camera detector(s) is moved on a 180° or 360° arc around the patient’s body and acquires 30—128 projection images of the distribu­tion of the radiopharmaceutical (see Figure 12.7). The movement (either continuous or “step and shoot”) is computer controlled.

There are advantages and disadvantages when comparing SPECT to X-ray trans­mission tomography (CT).

(C)

ft ft ft

Figure 12.7 The principle of SPECT data acquisition. (A) The detector(s) rotate around the patient. (B) With a dual-head camera, each detector covers 180°. (C) Many projection images are acquired. (Four of the brain projections are shown.)

12.4.4.5 Advantages of SPECT over CT

• Like all gamma camera methods, SPECT allows functional rather than structural imaging.

• We obtain images of a wide zone at once (the field of view is typically 30—40 cm along the body), so many slices can be reconstructed at once—and then slices in any slanting direction as well.

12.4.4.6 Limitations of SPECT

• As with gamma camera imaging in general, spatial resolution is relatively bad.

• Images are noisy because of the statistical nature of radioactive decay.

• Radiation attenuation occurs inside the patient’s body, and its extent depends on the depth of the organ as well as the structure of its environment.

• Radiation originating from an organ may scatter in the surrounding tissues, and the scat­tered photons may also be detected, further degrading spatial resolution.