The trend of PET/CT scanners is perhaps best illustrated by a design in which a 16-slice CT scanner, the Sensation 16 (Siemens Medical Solutions, Forchheim, Germany) is combined with the recently-announced high-resolution, LSO PET scanner (CPS Innovations, Knoxville, TN). The new PET scanner has unique 13 x 13 LSO block detectors each 4 mm x 4 mm in cross-section (Fig. 8.17). The pico-3D read-out electronics, adapted to the speed and light output of LSO, is operated with a coincidence time window of 4.5 ns and a lower energy threshold of 425 keV. The significance of these high-resolution detectors is illustrated in Fig. 8.17 for a patient with squamous cell carcinoma of the right tonsil. Following treatment that included a right tonsillectomy, radical neck dissection and chemotherapy, the patient was restaged by scanning first on an ECAT EXACT (CPS Innovations, Knoxville, TN) with 6.4 mm x 6.4 mm BGO detectors, and then on
Figure 8.17. High-resolution PET and PET/CT imaging using LSO-based detectors (Hi-Rez). A 52 y/o male patient, 70 kg, diagnosed with squamous cell tonsillar cancer and a 4 cm positive node in the neck. The patient underwent pre-surgical chemotherapy, a right tonsillectomy and a right radical neck dissection for removal of the positive node and 45 additional nodes; all of the additional nodes had negative pathology. The patient suffered post-surgical infectious complications. A follow-up PET scan (Standard) acquired with arms down showed a diffuse band of activity in the right neck (arrow) seen on a coronal section. A PET/CT scan acquired with arms up and with the new high-resolution LSO-based detector blocks (Hi-Rez) clearly resolved this diffuse band of activity into individual, sub clinical lymph nodes (arrow).
the high resolution PET/CT scanner. A coronal section from the PET scan demonstrates a diffuse band of activity in the right neck. The corresponding PET section from the PET/CT scan (Fig. 8.17) resolves this diffuse band into individual nodes in the neck of the patient.
The recent introduction of the fast scintillators LSO and GSO as PET detectors has occurred at just the right moment for PET/CT where a reduction in the lengthy PET imaging time is essential to more closely match that of the CT. These tomographs are aimed primarily at high throughput with whole-body imaging times below 30 min. While it is unlikely that whole-body PET imaging times will be reduced to the 30-60 s that is required for CT scanning, a scan time less than 10 min is feasible with new high-performance LSO area detectors currently under development. Such a design will represent a breakthrough in cancer imaging, eliminating problems of patient movement and truncated CT field-of-view, and substantially reducing artefacts due to respiration. Throughput will increase significantly, as will patient comfort and convenience. New applications, such as dynamic whole-body scans and the use of short-lived radioisotopes (e.g., 11C with a 20 min half-life) will then be within reach.
Future developments in combined PET/CT scanners will be exciting, attaining a higher level of integration of anatomical and functional imaging performance than before. By fulfilling an important role, not only in the diagnosis and staging of cancer, but in designing and monitoring appropriate therapies, the combined PET/CT scanner will undoubtedly have a significant impact on patient care strategies, patient survival and quality of life.
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