The CT data acquired in combined PET/CT studies provide a fast and essentially noiseless source for the correction of photon attenuation in PET emission data. To this end, the CT values relating to attenuation of photons in the range of 40-140 keV must be transformed into linear attenuation coefficients at the PET energy of 511 keV. As attenuation depends on photon energy and the absorbing material, an accurate theoretical relation cannot be devised. The transformation implemented in the Discovery LS PET/CT scanner (GE Medical Systems, Milwaukee, Wis.) uses a bilinear function based on the attenuation of water and cortical bone at the CT and PET energies. The purpose of this study was to compare this transformation with experimental CT values and corresponding PET attenuation coefficients. In 14 patients, quantitative PET attenuation maps were calculated from germanium-68 transmission scans, and resolution-matched CT images were generated. A total of 114 volumes of interest were defined and the average PET attenuation coefficients and CT values measured. From the CT values the predicted PET attenuation coefficients were calculated using the bilinear transformation. When the transformation was based on the narrow-beam attenuation coefficient of water at 511 keV (0.096 cm(-1)), the predicted attenuation coefficients were higher in soft tissue than the measured values. This bias was reduced by replacing 0.096 cm(-1) in the transformation by the linear attenuation coefficient of 0.093 cm(-1) obtained from germanium-68 transmission scans. An analysis of the corrected emission activities shows that the resulting transformation is essentially equivalent to the transmission-based attenuation correction for human tissue. For non-human material, however, it may assign inaccurate attenuation coefficients which will also affect the correction in neighbouring tissue.
PET/CT performed with nonenhanced CT is more sensitive and specific than is contrast-enhanced CT for evaluation of lymph node and organ involvement, especially regarding exclusion of disease, in patients with Hodgkin disease and high-grade non-Hodgkin lymphoma.
The authors describe the initial application for tumor staging with an in-line system with a positron emission tomographic (PET) scanner and a multi-detector row helical computed tomographic (CT) scanner combined in one machine. Fifty-three patients underwent imaging with four CT tube currents and PET emission and transmission data acquisition. Stepwise analysis of coregistered images revealed a significant (P <.05, McNemar test) improvement in lesion classification between PET images alone and coregistered images from the PET-CT examination.
Institutional review board approval and written informed consent were obtained. Patients with newly diagnosed prostate cancer and patients suspected of having recurrent prostate cancer were prospectively evaluated with fluorine 18 fluorocholine (FCH) combined in-line positron emission tomography (PET) and computed tomography (CT). In 19 patients (mean age, 67 years +/- 8; range, 57-85 years), standardized uptake values of FCH in 17 different tissues were determined by using volumes of interest. In nine patients evaluated at initial staging, histologic findings of the resected prostate were compared to FCH uptake. Only small variations of physiologic tracer accumulation were measured in all organs but the kidneys. Differentiation of benign hyperplasia from cancerous prostate lesions was not possible with FCH PET/CT. However, in patients with recurrent prostate cancer, FCH PET/CT is a promising imaging modality for detecting local recurrence and lymph node metastases.
This study was designed to explore the feasibility of PET using [11C](R)-PK11195 as an in vivo marker of activated microglia/brain macrophages for the assessment of neuroinflammation in Rasmussen's encephalitis (RE). [11C](R)-PK11195 PET was carried out in four normal subjects, two patients with histologically confirmed RE, and three patients with clinically stable hippocampal sclerosis and low seizure frequency. Binding potential maps showing specific binding of [11C](R)-PK11195 were generated for each subject. Regional binding potential values were calculated for anatomically defined regions of interest after coregistration to and spatial transformation into the subjects' own MRI. In one patient with RE who underwent hemispherectomy, the resected, paraffin-embedded brain tissue was stained with an antibody (CR3/43) that labels activated human microglia. Whereas specific binding of [11C](R)-PK11195 in clinically stable hippocampal sclerosis was similar to that in normal brain, patients with RE showed a focal and diffuse increase in binding throughout the affected hemisphere. In RE, [11C](R)-PK11195 PET can reveal in vivo the characteristic, unilateral pattern known from postmortem neuropathologic study. PET imaging of activated microglia/brain macrophages offers a tool for investigation of a range of brain diseases where neuroinflammation is a component and in which conventional MRI does not unequivocally indicate an inflammatory tissue reaction. [11C](R)-PK11195 PET may help in the choice of appropriate biopsy sites and, further, may allow assessment of the efficacy of antiinflammatory disease-modifying treatment.
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