Single-photon emission computed tomography (SPECT) imaging is based on the emission of single photons (γ-rays or x-rays) produced in the decay of certain radionuclides. The most commonly used radionuclide for SPECT imaging is technetium-99m (^99mTc). The γ-ray emitted from ^99mTc has an energy of 140 keV. A summary of some radionuclides used for SPECT is given in Table 4.1.

The data used to form SPECT images are acquired with a gamma camera. The detection of radiation by a gamma camera is depicted in Figure 4.1. A photon is emitted from a radiotracer molecule in a random direction and leaves the body. The gamma camera includes a lead collimator with multiple parallel holes.

These holes restrict the acceptance of photons so that only radiation emitted (roughly) perpendicular to the face of the gamma camera will be detected; the rest of the protons are stopped by the lead. The photons that make it through the collimator holes eventually hit the detector material, which is a sheet of thallium-doped sodium iodide [NaI(Tl)]. The vast majority of systems use ³/₈-inch (9.5-mm) thick NaI(Tl), although thicker crystals are available and are more appropriate for energies greater than 140 keV. The radiation penetrates the crystal to a depth that depends on the energy of the radiation. The vast majority of 140-keV photons will stop within ³/₈ of an inch, which justifies this thickness for 140 keV and lower energies. When a photon stops, some of its energy is released in the form of visible light. This light is measured by photomultipliers on the back of the crystal. The energy of the incident particle is accurately estimated from the sum of all the light energy measured in the photomultipliers. In addition, the position where the photon hit the detector is determined to 3.5- to 4.5-mm accuracy by an electronically implemented algorithm that compares the relative amount of light measured in each photomultiplier. The result for each incident photon that stops in the detector is a set of three numbers: two give the position of the photon in the scintillator (x and y), and the third gives its energy E.

As detected events accumulate, a projection view of the radiotracer in the body develops. The longer the acquisition, the more precisely this image represents the actual projection of the radiotracer distribution. SPECT imaging requires the acquisition of a series of these projections at successive angles around the body. To achieve this, the camera must be mounted on a gantry on which it can orbit the body. A complete data set requires an orbit of 180 degrees, and many acquisitions use 360 degrees of angular coverage. A typical angular increment is 3 degrees, which results in 60 total projections for a 180-degree orbit. Projection views and reconstructed slices from a cardiac SPECT study are shown in Figure 4.2. In this case, the field of view of the camera in the axial direction is 40 cm, much larger than necessary to image the heart.

Compared with positron emission tomography (PET), SPECT acquisition is a less efficient way of acquiring data, because only the photons leaving the body (roughly) perpendicular to the detector (about 1/1000, depending on the specific collimator design) will pass through the collimator and be detected. With PET, any photon pairs leaving the body in the same plane as the detector ring will be detected. Another disadvantage of SPECT is that only a single projection view is acquired at a time, whereas in PET all angles are acquired simultaneously.

Most single-photon emitters used with SPECT are longer lived (more than 6 hours; see Table 4.1) than the conventional PET radionuclides (less than 2 hours). The implications are great for producing and delivering radiotracers, because half-lives less than 2 hours require either on-site production or careful delivery planning, whereas longer-lived tracers allow more flexibility in production, delivery, and use.

SPECT images are degraded by several physical factors (1) that ultimately limit SPECT image quality and accuracy. Collimator blur, statistical noise, scatter, and attenuation each have specific negative effects.