Respiratory Motion

Several PET/CT groups have described respiratory motion and the resulting discrepancy of the spatial information from CT and PET as a source of potential artefacts in corrected emission images after CT-based attenuation correction [58, 61, 62]. These artefacts become dominant when standard full-inspiration breath hold techniques are transferred directly from clinical CT to combined PET/CT examination protocols scanning without suitable adaptations (Fig. 8.6a). In the absence of routinely available respiratory gating options the anatomy of the patient captured during the CT scan must be matched to the PET images that are acquired over the course of multiple breathing cycles. Reasonable registration accuracy can be obtained, for example, with the spiral CT scan being acquired during shallow breathing [61,63,64]. Alternatively, a limited breath hold protocol can be adopted with either a 1- or a 2-row system, or when dealing with uncooperative patients. Patients are then required to hold their breath in expiration only for the time that the CT takes to cover the lower lung and liver, which is typically around 15 s [65].

Breath hold commands (in normal expiration, for example) can be combined with very fast CT scanning, and therefore may help reduce respiration mismatches over the entire whole-body examination range. With multi-row CT, such as in third generation PET/CT systems (Table 8.3), it is now possible to scan the entire chest at high resolution within a single breath hold. Nevertheless, when respiration commands are not tolerated well and significant respiration-induced artefacts are suspected [66], it is advisable to reconstruct the emission data without attenuation correction and to review the two sets of fused PET/CT images very carefully.

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