FIGURE 7.5 Colon cancer patient (119 kg) imaged with fluorine-18-FDG illustrating improvement in lesion detectability with TOF compared to conventional (non-TOF) PET for the same number of detected events and the same number of iterations in the reconstruction algorithm. The data were reconstructed without TOF information (middle) and with TOF information (right) and compared to low-dose CT image (left) acquired immediately beforehand on combined PET/CT instrument. Note the TOF reconstruction shows higher uptake and better definition of the lesion (at arrow). SOURCE: Courtesy of Joel Karp, University of Pennsylvania.

7.3
CURRENT STATE OF THE FIELD AND EMERGING PRIORITIES

Emerging goals for nuclear medicine include early detection, which will require improvements in equipment sensitivity; the accurate quantification of biomarker uptake in disease for the evaluation of treatment response; and the quantification of radiotracer heterogeneity, which may be of potential utility for dose-painting applications of intensity-modulated radiotherapy. These limits may be tested in preclinical equipment such as microSPECT and microPET scanners, which operate at the cutting edge of the technology, with volumetric resolutions that are approximately 10-fold higher than their counterpart systems in the clinic. For example, the spatial resolution requirements to conduct meaningful preclinical research in mice are much more stringent than those required to conduct clinical studies in patients. To illustrate, a simple argument can be made that since the mouse brain volume is about 1/1,700th the volume of the human brain, we should scale the linear spatial resolution of a human scanner by a factor of 12 in each of the three dimensions to achieve a comparable resolution for mouse brain imaging. With current state-of-the-art technology for human brain imaging, 2.4-mm linear spatial resolution can be achieved with a dedicated brain scanner. To be able to achieve a similar linear spatial resolution in mice, the target is to reach a linear spatial resolution of 0.2 mm. To date, the best linear spatial resolution achieved for small-animal imaging in PET with a commercial instrument is 1.2 mm, which is still a factor of 6 too high (i.e., a factor of 216 in terms of volumetric resolution). Some university-based



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