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Pinhole magnification imaging is an important technique for practitioners of pediatric nuclear medicine. This article reviews basic principles of pinhole magnification imaging and ways for optimizing image acquisition with this technique. Applications to skeletal scintigraphy, scrotal scintigraphy and renal cortical scintigraphy are discussed and illustrated.

Tantalum-178 is a short-liver radionuclide (T 1/2 = 9.3 min.) and emits primarily 56- to 64-keV characteristic x-rays. We have determined the imaging characteristics with this radionuclide and a large-field-of-view Anger camera. With a pinhole collimator, good spatial resolution is possible with Ta-178, although the image quality is superior with Tc-99m under comparable conditions. Spatial resolution with parallel-hole or converging collimators was much less satisfactory with Ta-178 because of septal penetration by high-energy photons. Pulmonary perfusion and liver-spleen images of excellent quality were obtained in the rabbit using the pinhole collimator and Ta-178-labeled agents.

A 30 degrees slant-hole collimator was used during radionuclide ventriculography of the cardiac blood pool to improve imaging of the heart in both the modified left anterior oblique (MLAO) and right anterior oblique (RAO) views. In the MLAO view, with the holes slanted caudally, good separation between the left atrium and left ventricle was achieved, and the septum was displayed without foreshortening. In the RAO view with the collimator flat against the chest there was better resolution of the cardiac apex. The results of ejection fraction and wall motion analysis in these patients correlated well with contrast ventriculography (r=0.94). Combination of the slant-hole collimator, in vivo red blood cell labeling with stannous pyrophosphate, simultaneous collection of all phases of the cardiac cycle, and cine mode display, provide a practical system for the noninvasive measurement of left ventricular performance parameters.

The detection efficiency of a high-pressure, gas scintillation proportional chamber (GSPC), designed for medical imaging in the 30-150 keV energy range, has been investigated through measurement and Monte Carlo simulation. Measurements were conducted on a GSPC containing 4 atm of pure xenon separated from a hexagonal array of seven ultraviolet-sensitive photomultiplier tubes by 1.27-cm-thick fused-silica windows. Experimental measurements of the photopeak efficiency, fluorescence escape efficiency, and the energy collection efficiency were obtained. Results were also obtained for different photon energies and different values of temporal resolution. The measurements were compared with the results obtained from a Monte Carlo simulation designed specifically for investigating the imaging of low-energy photons (below 150 keV) with a gas-filled detector. The simulation was used to estimate photopeak efficiency, fluorescence escape efficiency, photopeak-to-fluorescence escape peak ratio, quantum interaction efficiency, energy collection efficiency, and local energy collection efficiency. The photopeak efficiency of the GSPC relative to that of a 3-in. (7.62-cm)-thick sodium iodide crystal was measured to be 0.284 +/- 0.001 at 60 keV and 0.057 +/- 0.001 at 140 keV. Of the 60-keV photons incident upon the detector, 70% +/- 4% interacted in the detector, with 28% +/- 1% being in the photopeak, as estimated both by experimentation and through the simulation. The maximum energy collection efficiency was found to be 65% at 60 keV, with 46% being deposited within 0.2 cm of the initial photon interaction. The information gained from this study is being used to design an optimized detector for use in specialized nuclear medicine studies.

Blood flow can be measured by monitoring the count rate of Krypton-81m after its parent, Rubidium-81 (a potassium analogue), has been deposited in the tissue. The steady-state Kr-81m count rate reflects both production by decay of Rb-81 and washout due to blood flow. Its use is theoretically superior to that of Xenon-133 for quantification of blood flow (cc/min per 100 gm) since: (1) multiple flow measurements can be obtained from a single arterial injection, (2) flow-dependent changes in the count rate of Kr-81m provide a steady-state measure of specific flow, and (3) errors due to uptake in fat are eliminated. The count rate of Kr-81m was measured as a function of flow in a dynamic phantom with a NaI crystal, suing both pure cyclotron-produced Rb-81 and commercially available samples with as much as 25% contamination from Rb-82m. The phantom was calibrated by measuring the flow-rate constants with Xe-133. No significant difference was found between the flow-rate constant measured with three pure samples. The ratio of the zero-flow Kr-81m count rate to the rate observed in the presence of flow showed excellent correlation with calibrated flow over a range of rate constant from 0 to 0.02 sec (-1). This study suggests that regional specific flow can be measured in vivo with currently available Nal detecting systems after the intra-arterial injection of Rb-81.