When imaging clinically with the PET/CT, a typical acquisition protocol begins with a 260 MBq injection of FDG, followed by a 60 min uptake period. The patient is then positioned in the scanner with the first transverse section to be imaged aligned with the CT field-of-view. An initial scout scan (topogram) is performed to determine the appropriate axial range for the study. The maximum axial extent of a single spiral scan depends on the defined slice-width and pitch. The total axial length to be scanned is subdivided into contiguous, overlapping, 15 cm segments. For the Somatom AR.SP, the spiral scan of each segment typically takes about 40 s, and the 25 kW X-ray tube may sometimes require cooling between segments (Fig. 8.3). Patients are instructed to breathe in a shallow manner during the CT scan. The time for the complete whole-body CT scan is about 5 min. Once the spiral CT covering the required axial length is completed, the patient bed is moved automatically to the start position of the multi-bed PET acquisition, and the PET scan is initiated. An emission scan time of 6-10 min per bed position is selected depending on the number of bed positions, resulting in a total PET scan duration of 45-50 min (Fig. 8.3). An axial overlap of 4 cm is used between bed positions. The CT images are used for attenuation correction as will be described below, and the corrected emission data are reconstructed using Fourier rebinning and attenuation-weighted ordered-subset EM . In this implementation reconstruction takes over one hour to complete.
From July 1998 to July 2001, over 300 patients with a wide variety of different cancers were scanned on the prototype PET/CT [24, 25]. The main indications, most suited to anatomical and functional imaging, are head and neck cancer, and abdominal and pelvic disease, particularly ovarian and cervical cancer. Combined PET/CT in the head and neck is important because normal uptake of FDG in muscles and glands makes interpretation of the studies especially difficult. PET
applications in the abdomen and pelvis are complicated by benign, non-specific uptake in the stomach, intestines and bowel that may be difficult to distinguish from malignant disease. Combined imaging for clinical routine allows accurate localization of lesions, the distinction of normal FDG uptake from pathology, and the assessment of response to therapy (Fig. 8.4). Additionally, the use of registered CT and PET images was envisaged for efficient radiation therapy planning, traditionally based on CT alone .
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