Absolute Quantitation of SPECT Studies.

Methods for absolute quantitation of SPECT images provide an estimate of the activity uptakes in various organs and tissues in units of (M)Bq or (m)Ci. However, because tomographic SPECT images generally are hampered by several physical and camera-specific effects, accurate and precise compensation methods are required. The most important effects are (1) photon attenuation in the patient resulting in a reduction of expected acquired count; (2) the contribution of events from photons, scattered in the phantom and the collimator but accepted by the energy window because of a poor energy resolution of the NaI(Tl) crystal; and (3) the effect of the collimator response function that degrades the image quality because of the relatively poor spatial resolution. In addition, camera-specific effects, such as dead time and pulse pile-up, are discussed. These effects can reduce the accuracy and precision in the activity estimate. In addition to these compensation methods, a careful and consistent calibration is needed to translate count in the image that corresponds to a location of the patent to activity (or activity concentration). This review summarizes the required compensation methods and the means by which they are implemented in an iterative reconstruction approach and discusses some applications and areas where quantitative SPECT might be important for the future.