Design Concept

The design objective for combined PET/CT imaging, as a technically straightforward combination of two complementary imaging modalities, was to provide clinical CT and clinical PET imaging capability within a single, integrated scanner (Table 8.1). The short CT scan duration compared with a typical whole-body PET acquisition time essentially eliminates the requirement for simultaneous CT and PET acquisition. The integration of the two modalities within a single gantry is more straightforward when the simultaneous operation of the CT and PET imaging systems is not required. The PET and CT components are mounted on the same aluminium support with the CT on the front and the PET at the back, as shown schematically in Fig. 8.2. The entire assembly rotates at 30 rpm and is housed within a single gantry of dimensions 170 cm wide and 168 cm high. The patient port is 60 cm in diameter with an overall tunnel length of 110 cm and a 60 cm axial displacement between the center of the CT and the center of the PET imaging fields. A single patient bed is used for both modalities with an axial travel sufficient to cover 100 cm of combined CT and PET imaging. To simplify the prototype development, the acquisition and reconstruction paths are not integrated, with CT and PET scanning controlled from separate consoles, as shown schematically in Fig. 8.2. Once acquired and reconstructed, the CT images are transferred to the

Table 8.1. A comparison of advantages and disadvantages of software- and hardware-based image fusion.

Software fusion

Hardware fusion

Image retrieval from different archives

Images avilable from one device

Repeated patient positioning

Single patient positioning

Different scanner bed profiles

One bed for both scans

Uncontrolled internal organ movement between scans

Consecutive scans with little internal organ movement in between

Disease progression in time between exams

Scans acquired close in time

Limited registration accuracy

Improved registration accuracy

Less convenient for patient (two exams)

Single, integrated exam

Labour-intensive registration algorithms

No further image alignment required

PET computer to provide the attenuation correction factors for the PET emission data. Final PET reconstruction and CT and PET fused image display is performed on the PET computer console.

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