Fat necrosis mimicking B-cell lymphoma: a PET/CT and FDG study.

F-18 fluoro-2-deoxyglucose positron emission tomography combined with computed tomography (FDG and PET/CT) is increasingly becoming the standard in staging and restaging patients with a range of malignancies including B-cell lymphoma. However, there are well-known pitfalls in PET/CT with FDG imaging, which comprise infection, inflammation, physiological variants, and benign pathologic conditions. Fat necrosis is the result of death of adipose tissue from disease, injury, or pathologic conditions. We describe a case of false positive PET/CT and FDG scan in a patient with fat necrosis mimicking B-cell lymphoma after 6 cycles of rituximab with cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) treatment. In interpreting PET/CT and FDG images with inconsistency in lesion response, fat necrosis should be considered in the differential diagnosis.

Sensitivity and daily quality control of a mobile PET/CT scanner operating in 3-dimensional mode.

UNLABELLED: This study investigated the stability of the sensitivity of a mobile PET/CT scanner and tested a phantom experiment to improve on the daily quality control recommendations of the manufacturer. Unlike in-house scanners, mobile PET/CT devices are subjected to a harsher, continuously changing environment that can alter their performance. The parameter of sensitivity was investigated because it reflects directly on standardized uptake value, a key factor in cancer evaluation. METHODS: A (68)Ge phantom of known activity concentration was scanned 6 times a month for 11 consecutive months using a mobile PET/CT scanner that operates in 3-dimensional mode only. The scans were acquired as 2 contiguous bed positions, with raw data obtained and reconstructed using parameters identical to those used for oncology patients, including CT-extracted attenuation coefficients and decay, scatter, geometry, and randoms corrections. After visual inspection of all reconstructed images, identical regions of interest were drawn on each image to obtain the activity concentration of individual slices. The original activity concentration was then decay-corrected to the scanning day, and the percentage sensitivity of the slice was calculated and graphed. The daily average sensitivity of the scanner, over 11 consecutive months, was also obtained and used to evaluate the stability of sensitivity. RESULTS: Our particular scanner showed a daily average sensitivity ranging from -8.6% to 6.5% except for one instance, when the sensitivity dropped by an unacceptable degree, 34.8%. CONCLUSION: Our 11-mo follow-up of a mobile PET/CT scanner demonstrated that its sensitivity remained within acceptable clinical limits except for one instance, when the scanner had to be serviced before patients could be imaged. To enhance our confidence in the uniformity of sensitivity across slices, we added a phantom scan to the daily quality control recommendations of the manufacturer.

Comparative analysis of striatal FDOPA uptake in Parkinson's disease: ratio method versus graphical approach.

UNLABELLED: Striatal-to-occipital ratio (SOR) and influx constant K(i)(occ) are commonly used as analytic parameters in L-3,4-dihydroxy-6-(18)F-fluorophenylalanine (FDOPA) PET studies. Both have been shown to be useful in discriminating Parkinson's disease (PD) patients from healthy subjects. We evaluated the relative performance of SOR and influx constant (K(i)(occ)) in the clinical assessment of nigrostriatal dopaminergic function in PD. METHODS: Twenty-one parkinsonian patients (Hoehn and Yahr scale I-IV; mean age +/- SD, 56 +/- 9.2 y) and 11 healthy subjects (mean age, 60 +/- 16 y) underwent 3-dimensional dynamic FDOPA scanning from 0 to 100 min. After spatial realignment, PET images at each frame were integrated by summing 4 central striatal slices, and time-activity curves (TACs) were generated after placing a standard set of elliptic regions of interest over striatal and occipital structures. SOR and K(i)(occ) values for each subject were then computed from TACs at different times using an input function from the occipital cortex. RESULTS: Both SOR and K(i)(occ) showed significant bilateral decreases in striatal dopamine uptake in the PD group compared with the control group. SOR values estimated for 10-min frames between 65 and 95 min are statistically equivalent in group discrimination. In addition, SOR values in the caudate and putamen correlated strongly with K(i)(occ), especially toward the end of the scanning epoch. Both parameters correlated significantly and comparably with Unified Parkinson's Disease Rating Scale motor scores. CONCLUSION: These results suggest that SOR determined from a single 10-min scan at 95 min is as accurate as K(i)(occ) in separating PD patients from healthy subjects and in predicting clinical measures of disease severity.

Quantitative Brain PET. Comparison of 2D and 3D Acquisitions on the GE Advance Scanner.

PURPOSE: Recent developments in the design of positron emission tomography (PET) scanners have made three-dimensional (3D) data acquisition attractive because of significantly higher sensitivity compared to the conventional 2D mode (with lead/tungsten septa extended). However, the increased count rate in 3D mode comes at the cost of increased scatter, randoms, and dead time. Several schemes to correct for these effects have been proposed and validated in phantom studies. In this study, we evaluated the overall improvement afforded by 3D imaging in quantitative human brain PET studies carried out at our institution.METHODS: Subjects were studied using sequential/interleaved 2D and 3D data acquisition with a GE Advance scanner. We calculated regional and global cerebral glucose metabolism with [(18)F]flourodeoxyglucose (FDG) and estimated rate constants for striatal [(18)F]fluorodopa (FDOPA) uptake.RESULTS: FDG: Global mean glucose metabolic rates were in almost complete agreement (within 1%) between the two modes whereas the regional differences ranged from -7.7% to +9% for all cortical structures. However, for small regions (<2 cm(2)) like caudate nuclei, the maximum difference was 14.7%. FDOPA: A significant improvement in image quality was evident in 3D mode and there was complete agreement between the estimated parameters in the two scanning modes for the same noise equivalent counts: Striatal-to-occipital ratio (SOR) and striatal FDOPA uptake (K(i)(FD)) had mean differences of less than 2% and 5%, respectively.CONCLUSIONS: 3D FDG studies can be done with either half the injected dose or half the scan duration to a comparable 2D study. 3D PET imaging has distinct advantages over 2D in the quantitative fluorodopa studies.