Clinical CT scans of the thorax and abdomen take a few minutes to acquire and can generally be performed with the patient's arms out of the field-of-view. However, in PET/CT imaging total examination time is defined primarily by the time for the emission scan. A typical whole-body PET scan with the prototype PET/CT could last for 1 h, and for scan times of this duration it is difficult for patients to keep their arms comfortably above their head, out of the field-of-view. More recently, with the introduction of full-ring PET components and faster PET detectors into combined PET/CT designs, total examination time for PET/CT is reduced to 30 min , or less. Despite the dramatic reduction in total imaging time some patients may still not tolerate having their their arms raised and supported for the duration of the PET/CT scan, and therefore CT and PET imaging must be performed with the arms down and close to the body. However, since the transverse field-of-view of the CT is 50 cm in diameter (45 cm in the prototype), a small angular range of projections around the anterior-posterior direction is, for many patients, truncated (Fig. 8.7). The artefacts caused by truncation affect not only the CT images but also the accuracy of the attenuation correction factors generated from the CT images. The effect is illustrated in Fig. 8.7 for a patient who was imaged on the prototype PET/CT with arms down. The transverse CT field-of-view is limited to 45 cm in diameter. As shown in Fig. 8.7a, truncation leads to ring artefacts around the arms that affect both the accuracy of the CT images and the attenuation correction factors. As a result the tracer distribution in the reconstructed and corrected emission images appears masked near the arms. Figure 8.7b shows a similar patient study from a second generation PET/CT system. Although the transverse CT field-of-view is increased to 50 cm truncation may still occur when imaging large patients. The theory for an effective correction of these truncation artefacts exists today [37, 38], and simplified correction schemes for application in the context of PET/CT imaging are currently being pursued (see Chapter 5).
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