The gantry dimensions of the Somatom Emotion closely match those of the ECAT EXACT HR+, facilitating the mechanical integration of the two units. In comparison to the original prototype design mechanical and thermal isolation is maintained between the two devices for operational and servicing reasons. A schematic of the gantry is shown in Fig. 8.9. The gantry is 188 cm high and 228 cm in width. The overall length is 158 cm, although with the front and rear contouring, the effective tunnel length is only 110 cm. The axial separation of the centres of the CT and PET fields-of-view is 80 cm. The actual 56 cm patient port of the HR+ is increased to 70 cm to match that of the CT by cutting back the side shielding of the PET. The resulting patient port diameter is 70 cm throughout the length of the tunnel, which is essential when positioning most patients from radiation therapy, and which minimizes claustrophobic effects despite the 110 cm tunnel length.
For servicing, the gantries can be separated by moving the PET backwards on rails by about 1 m; access to the rear of the Emotion CT and the front of the EXACT HR+ is then possible. No service procedures on either device have been significantly modified as a consequence of the particular integration into the PET/CT.
The problem of the increasing vertical bed deflection with increasing distance into the scanner encountered with the prototype was resolved by a complete redesign of the patient handling system (PHS). Support of the patient bed throughout the scan range is important to avoid an increasing vertical deflection of the pallet: the pallet deflects downwards with the patient load, a deflection which increases as the bed moves into the tunnel, adversely affecting the CT and PET image registration accuracy. In the prototype (Fig. 8.10a), the pallet is not supported beyond the cantilever point and an approximate correction for the increasing downward deflection is applied in software, based on the patient weight and the pallet position. In the new design, shown in Fig. 8.10b, a carbon fibre pallet is supported at one end by a pedestal that moves horizontally on floor-mounted rails driven by a linear motor. Since the cantilever point does not change, the vertical deflection is limited to a few millimetres once the patient is aligned on the bed, allowing sub-millimeter intrinsic registration accuracy to be achieved between the CT and the PET, independently of the patient weight. A total length, including the head holder, of 145 cm can be scanned with both CT and PET. A flat pallet option is available for use with the PHS when scanning patients undergoing PET/CT for radiation therapy treatment planning.
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