Technical and practical limitations of PET and SPET compared with other imaging modalities

PET and SPET excel in the measurement of neurochemical parameters in vivo at very low (subnanomolar) concentration. (1..,2> Such sensitivity cannot be matched by other in vivo methods such as proton magnetic resonance spectroscopy (millimolar range). However, radiation dosimetry limits the number of scans that subjects may receive. Full quantitation can often be achieved with PET, unlike SPET. However, for imaging blood flow change, functional magnetic resonance imaging ( MRI) now offers the possibility of repeated measures (without radiation exposure) that far exceed that possible with PET- and SPET-based methods of flow mapping. However, full quantitation of blood flow is not yet readily achievable with functional MRI without injection of contrast agents. One disadvantage of functional MRI over PET, for some subjects, is the noisy claustrophobic environment of the scanner, but generally subjects and paradigms studied with PET flow mapping can be readily investigated with functional MRI (see Chapter 2.3..i8), although all test materials in the vicinity of the scanner have to be non-magnetic.

Structural MRI scanning is often used in conjunction with PET activation and ligand binding techniques. The high-resolution anatomical information contained in MRI images can be used to precisely define areas of activation or radiotracer binding observed in PET studies from single subjects. (2)

PET and SPET, and even functional MRI, have relatively poor temporal resolution (seconds) compared with electrophysiological methods such as EEG, event-related potentials, and magnetoencephalography (milliseconds), but these methods in turn suffer from poor spatial resolution. Attempts to integrate information from these different modalities are now a major focus of methodological research in many imaging centres.

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