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Using iodine-123 labelled radiotracers, the presence of 2.5% high-energy photons causes image deterioration due to increased scatter. To investigate the influence of these photons on image quality, we measured the spectrum of 123I with a medium-energy (ME), a low-energy all-purpose (LEAP) and a low-energy high-resolution (LEHR) collimator. Even in air, using low-energy collimators a high baseline activity was observed over the total energy detection range of the gamma camera. The 159-keV photopeak to scatter activity ratio fell from 5.9 for ME to 3.6 and 2.9 for LE collimators. Acquisition of images with LEHR collimators with energy windows set at 159 keV and 500 keV demonstrated that the 159-keV LEHR image is a combination of the ME image of the object and of the LEHR 500-keV image. Because of their important septal penetration and greater geometric detection efficiency compared with the 159-keV photons of 123I, the contribution of high-energy photons is dependent on the source-detector distance. For a small source placed in air, the scatter to photopeak activities varied from 17.4% at 80 cm to 37.8% at 5 cm distance from an LEHR collimator. Considering only the scatter problem, ME collimators are the best choice for 123I studies. When using LE collimators for high-resolution tomography with 123I-labelled compounds, scatter contribution from high-energy photons has to be corrected for quantitative analysis or when dual-isotope studies are performed, whether or not these studies are acquired simultaneously.

The serotonergic system has been implicated in emotional and cognitive functions since early work. In particular, an important role has been attributed to the 5-HT2A receptor in schizophrenia, depression, eating disorders, and anxiety. The aim of the study was to evaluate the involvement of the brain 5-HT2A receptor in dogs with severe anxiety disorder, using 123 I-5I-R91150 and SPECT. Methods: SPECT was performed with the 5-HT2A receptor-specific radioligand 123 I-5I-R91150 to determine the 5-HT2A receptor binding index (BI) in the brains of dogs. Sixteen dogs with pathologic anxiety problems were compared with 22 normal-behaving reference dogs. Results: Lower 5-HT2A receptor BI was found in the left (P 5 0.001) and right (P 5 0.002) frontal cortices in the group of dogs with anxiety disorders than in the reference group. Right (P 5 0.022) and left (P 5 0.048) temporocortical BIs were also significantly lower in the dogs with anxiety disorders. Finally, the BI was significantly lower in the right occipital cortex (P 5 0.038) of dogs with anxiety disorders than in the reference dogs. After correction for multiple comparisons (P , 0.0056), only the bilateral frontocortical lower BI remained significant. Conclusion: The findings in this study indicate that the 5-HT2A receptor is involved in the pathophysiology of anxiety disorders in dogs. The affected brain regions are in concordance with the brain regions involved in human anxiety disorders. The acquired data confirm the potential of using the dog as a natural model for investigation of the different mechanisms of anxiety disorders. In this regard, the use of dogs may contribute to the development of novel treatment approaches and new drugs for veterinary and human use.

A new radioisotopic method of determining the separate glomerular clearance of the kidneys is described. After injection of 99mTc-DTPA, the data from a scintillation camera are recorded by computer. The renal curves are corrected for extrarenal activity and the plasma curve is obtained from a precordial curve. At each instant the slope of the renal curve divided by the corresponding plasma concentration gives the separate clearance value, in the period extending from 80 to 180 s after injection of the tracer. The error and correction factors introduced are discussed. Normal values are established on 25 patients and correspond to inulin clearance values. Left kidney: 60 ml/min (SD=13). Right kidney: 57 ml/min (SD=12). The method is simple for the patient, takes only 20 min, is not dangerous, is particularly applicable to young children and can be repeated frequently in the same patient.

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In patients with a normal-sized heart, LVEF and volume estimates computed from different commercially available software packages for quantitative gated SPECT are well correlated. LVEF and volumes are only slightly sensitive to changes in matrix size. Smoothing, by contrast, is associated with significant changes in volumes but usually not in LVEF values. However, owing to the specific characteristics of each algorithm, software should not be interchanged for follow-up in an individual patient. In small hearts, on the other hand, both the used software and the matrix size or smoothing significantly influence the results of quantitative gated SPECT. LVEF values in the higher range are frequently observed with all the studied software except for SU-Segami. A larger matrix or a sharper filter could be suggested to enhance the accuracy of most commercial software, more particularly in patients with a small heart.