Abstract

Roger Fulton
Department of PET and Nuclear Medicine, Royal Prince Alfred Hospital, School of Physics, University of Sydney, Sydney, Australia

The combined modality positron emission tomography (PET) and computed tomography (CT) scanner is a fascinating example of the ability of seemingly esoteric physical and mathematical principles to produce very real benefits. PET/CT scanners provide extraordinarily detailed images depicting the body’s internal structure and function. An understanding of the physical principles involved is vital to proper interpretation of the images, to an understanding of the factors that contribute to image quality, and to making well-informed decisions when selecting equipment.

We concentrate here on the physics aspects of PET.

We discuss the basics of coincidence detection, event types, scanner designs, tomographic principles, scintillator properties, and methods of correcting for corrections for random events, sensitivity variations, attenuation, and scatter. This leads to an examination of the factors affecting spatial resolution, and discussion of the effect of depth of interaction, and the effect of activity outside the axial field of view in 3D acquisition. We conclude with a discussion of time-of-flight (ToF) PET which has recently appeared on a commercial PET/CT scanner. Time of flight PET uses the time difference between the detection of two coincidence photons to better locate the annihilation position of the emitted positron. This additional information, although not perfectly precise, can be incorporated in the reconstruction algorithm to obtain reconstructions with improved signal to noise ratio (SNR), or to obtain equivalent quality scans in less time. With a time resolution of 1.2ns (FWHM), SNR increases by 50% in 40cm patient. Time-of flight PET requires crystal detectors with good time resolution (e.g. LSO). It has the potential to provide a significant advance in image quality and is likely become a standard in the future.