PET systems: the value of added length

Abstract

The past decade has brought unbelievable advances in CT technology. These have been reflected in a progressive widening of the radiation beam in the axial direction (from “fan beam” to “cone beam” geometry) and in a corresponding increase in the number of detector rings. From the four detectors and 2 cm axial field of view (AFOV), acquired in half a second, that characterised the systems of a decade ago, we now have, with the present technology, 256/312 multi-row detectors that allow the acquisition of an 8/16 cm axial field in less than a third of a second. In addition, dual-source CT is offering a wealth of new possibilities. These new systems, whose realisation demanded the solution of complex mechanics, electronics and information technology problems, have opened up a completely new field of medical investigation: the four-dimensional study of the heart and the vascular system. The past few years have also seen evolutions in the field of PET/CT. Indeed, this combined modality has benefitted not only from improved CT technology, but also from gradual advances in the PET sphere—advances prompted by the introduction of new crystal detectors (such as LSO, GSO, LYSO), new iterative reconstruction algorithms, faster electronics, the “gating” technique (synchronisation of data acquisition with respiratory motion) and, especially, “time of flight” PET technology (TOF) [1–4]. Yet despite these significant improvements, the length of the detector cylinder, or length of the system’s sensitive volume (“field of view”), has remained essentially the same (15–18 cm). With this acquisition geometry, only a very small fraction of the radiation emitted at any given instant is acquired (geometric efficiency ≤0.2). Therefore, to obtain, for example, a “whole-body” study of about 80 cm in length (from the temples to the bladder of the patient), a series of six to eight static scans (“views”) need to be acquired. The adjacent single views, obtained with the movement of the bed, must, of course, also provide an appropriate region of overlap. Obviously, the acquisition time increases in proportion to the number of views. A “standard study” with the injection of 10 mCi of F-FDG takes about 3 min per view and, therefore, approximately 25 min per entire study. To this about a minute should be added for the CT scan. However, when used for wholebody studies in diagnostic oncology—this use accounts for approximately 90% of all PET scans—this procedure presents major limitations. The limited length of the sensitive volume not only imposes the need for multiple views, and thus prolongs the overall duration of the examination, it also (and this is particularly important) limits the maximum “line of response” (LOR) angle (with G. Borasi (*) : F. Fioroni Medical Physics Department, Santa Maria Nuova Hospital, V. le Risorgimento 80, 42100 Reggio Emilia, Italy e-mail: giovanni_borasi@libero.it