Using PET/CT imaging to characterize 18 F-fluorodeoxyglucose utilization in fish.

Fish are becoming an increasingly important research species as investigators seek alternatives to mammalian models. Combined positron emission tomography/computed tomography with ¹8F-fluorodeoxyglucose (FDG-PET/CT) is a powerful new technology that has been extensively applied for high-resolution imaging in mammals but not fish. CT scanning provides detailed anatomical three-dimensional imaging. PET scanning detects areas of cellular activity using radio-labelled molecular probes with specific uptake rates appropriate to the tissue involved. FDG-PET is used in oncology because tissues with high glucose uptake, such as neoplasms, are intensely radio-labelled. PET/CT combines the two technologies, so that images acquired from both devices are merged into one superimposed image, thus more precisely correlating metabolic activity with anatomical three-dimensional imaging. Our objective was to determine if fish can be viable replacement animals in cancer studies using this technique by analysing the similarities between fish and humans in glucose uptake in select organs across multiple fish species. Rapid, quantifiable glucose uptake was demonstrated, particularly in brain, kidneys and liver in all imaged fish species. Standard uptake values for glucose uptake in the major organ systems of fish were more similar to those of humans than mice or dogs, indicating that fish may serve as effective alternative animal models using this technology. Applications for this technique in fish may include oncogenesis and metabolism studies as well as screening for environmental carcinogenesis.

Statistical list-mode image reconstruction for the high resolution research tomograph.

We have investigated statistical list-mode reconstruction applicable to a depth-encoding high resolution research tomograph. An image non-negativity constraint has been employed in the reconstructions and is shown to effectively remove the overestimation bias introduced by the sinogram non-negativity constraint. We have furthermore implemented a convergent subsetized (CS) list-mode reconstruction algorithm, based on previous work (Hsiao et al 2002 Conf. Rec. SPIE Med. Imaging 4684 10-19; Hsiao et al 2002 Conf. Rec. IEEE Int. Symp. Biomed. Imaging 409-12) on convergent histogram OSEM reconstruction. We have demonstrated that the first step of the convergent algorithm is exactly equivalent (unlike the histogram-mode case) to the regular subsetized list-mode EM algorithm, while the second and final step takes the form of additive updates in image space. We have shown that in terms of contrast, noise as well as FWHM width behaviour, the CS algorithm is robust and does not result in limit cycles. A hybrid algorithm based on the ordinary and the convergent algorithms is also proposed, and is shown to combine the advantages of the two algorithms (i.e. it is able to reach a higher image quality in fewer iterations while maintaining the convergent behaviour), making the hybrid approach a good alternative to the ordinary subsetized list-mode EM algorithm.

Characterization of a single LSO crystal layer high resolution research tomograph.

The purpose of this study was to determine the performance of a single lutetium oxy-orthosilicate (LSO) crystal layer High Resolution Research Tomograph (HRRT) positron emission tomography (PET) scanner. The HRRT is a high resolution PET scanner designed for human brain and small animal imaging. The scanner consists of eight panel detectors, which have one layer of 2.1 x 2.1 x 7.5 mm thick LSO crystals. Several phantom studies were performed to determine scanner characteristics, such as resolution, scatter fraction, count rate and noise equivalent count rates (NECR). NECR curves were measured according to both NEMA NU2-1994 and NU2-2001 for three different energy windows, i.e. lower level discriminators (lld) of 350, 400 and 450 keV and an upper level discriminator (uld) of 650 keV. Accuracy of scatter and single photon attenuation corrections was evaluated according to NU2-1994. Data were acquired using a ring difference of 67 and a span of 9. Reconstructions were performed using FORE + 2D FBP or OSEM. Transaxial resolution varied from 2.7 to 2.9 mm FWHM between I and 10 cm off centre locations, and axial resolution varied from 3.2 to 4.4 mm FWHM. Scatter fractions (NU2-1994) equalled 0.31, 0.42 and 0.54 for lld of 450, 400 and 350 keV, respectively. NECR data were highest for an lid of 400 keV and showed a maximum of 46 kcps at 38 kBq cm(-3). Lower NECR values were observed according to NU2-2001, but were still optimal for an lld of 400 keV. After scatter and attenuation corrections, pixel values within water, air and teflon inserts of the NU2-1994 phantom were 14, 4 and 35% of the background activity, respectively. The single layer LSO HRRT scanner shows excellent spatial resolution, making it suitable for small animal studies. The low count rate performance, due to the small amount of LSO, prohibits studies of the human brain, but is sufficient for studies in small laboratory animals.

Use of an in-field-of-view shield to improve count rate performance of the single crystal layer high-resolution research tomograph PET scanner for small animal brain scans.

The count rate performance of the single LSO crystal layer high-resolution research tomograph (HRRT-S) PET scanner is limited by the processing speed of its electronics. Therefore, the feasibility of using an in-field-of-view (in-FOV) shield to improve the noise equivalent count rates (NECR) for small animal brain studies was investigated. The in-FOV shield consists of a lead tube of 12 cm length, 6 cm inner diameter and 9 mm wall thickness. It is large enough to shield the activity in the body of a rat or mouse. First, the effect of this shield on NECR was studied. Secondly, a number of experiments were performed to assess the effects of the shield on the accuracy of transmission scan data and, next, on reconstructed activity distribution in the brain. For activities below 150 MBq NECR improved only by 5-10%. For higher activities NECR maxima of 1.2E4 cps at 200 MBq and 2.2E4 cps at 370 MBq were found without and with shield, respectively. Listmode data taken without shield, however, were corrupted for activities above 75 MBq due to data overrun problems (time tag losses) of the electronics. When the shield was used data overrun was avoided up to activities of 150 MBq. For the unshielded part of the phantom, transmission scan data were the same with and without shield. The estimated scatter contribution was approximately 8.5% without and 5.5% with shield. Reconstructed emission data showed a difference up to 5% in the unshielded part of the phantom at 5 mm or more from the edge of the shielding. Of this 5% about 3% results from the difference in the uncorrected scatter contribution. In conclusion, an in-FOV shield can be used successfully in an HRRT PET scanner to improve NECR and accuracy of small animal brain studies. The latter is especially important when high activities are required for tracers with low brain uptake or when multiple animals are scanned simultaneously.

Effectiveness and safety of long-term antidepressant treatment in bipolar disorder.

OBJECTIVE: We sought to review research on use of antidepressants for long-term treatment of bipolar depression. METHOD: We conducted a computerized literature search of the MEDLINE, HealthStar, Current Contents, PsychInfo, and National Library of Medicine databases to identify studies involving antidepressant, anticonvulsant, or lithium use in bipolar disorder or manic-depressive illness published from 1966 through 2000. RESULTS: Only 7 blinded, controlled trials of long-term antidepressant treatment in bipolar disorders were found. The available information is not adequate to support the safety or effectiveness of long-term antidepressant treatment for bipolar depression, with or without mood-stabilizing cotherapy. CONCLUSION: Antidepressant treatment of bipolar depression is extraordinarily understudied. Controlled trials comparing specific antidepressants, particularly to compare mood-stabilizing agents given alone and combined with an antidepressant, are needed.