Robert N. Beck

The geometric transfer function component for scintillation camera collimators with straight parallel holes.

A theoretical approach has been developed that allows the geometric transfer function component for conventional scintillation camera collimators to be predicted in closed form. If transfer function analysis is to be useful in describing imaging system performance, the image of a point source must not depend on source position in a plane parallel to the detection plane. This shift invariance can be achieved by analysis of system response in terms of an effective point spread function, defined as the normalised image of a point source that would be obtained if the camera collimator were uniformly translated (but not rotated) during image formation. The geometric component of the corresponding effective transfer function is shown to be expressed simply by the absolute square of the two-dimensional Fourier transform of a collimator hole aperture, with the spatial frequency plane scaled by a factor which depends on collimator length, source-to-collimator distance, and collimator-to-detection plane distance. Closed form algebraic expressions of the geometric transfer function have been obtained for all four common hold shapes (circular, hexagonal, square and triangular). Monte Carlo simulations and experimental measurements have shown these theoretical expressions to be highly accurate.

Fluorescent Thyroid Scanning: A New Method of Imaging the Thyroid

A method has been developed for imaging the thyroid gland through the use of K-shell fluorescence. This work was stimulated by the pioneer efforts of Jacobson (4) and Roy et al. (7), whose studies demonstrated the feasibility of radiologic detection of in vivo thyroidal iodine. Although it is well known that the thyroid gland selectively traps and incorporates significant amounts of iodine, it is not generally appreciated that the iodine content of the average thyroid gland is only 0.04 per cent by weight (6). Nevertheless, this quantity is sufficient to act as the target in the system to be described. The technic for imaging the thyroid gland which is presented in this communication incorporates the basic scheme of an x-ray fluorescent spectrometer. The equipment is a modification of the TMC Model 331 photon spectrometer used in conjunction with a dysprosium-159 radiation source. The dysprosium is placed in the flanges of a collimator, and a lithium-drifted silicon crystal (30 mm2 with 3 mm depleted regi...

Design considerations for a single tube gamma camera

It is pointed out that many potentially practical applications for nuclear medicine diagnostic procedures are difficult or impossible to perform because of the physical size of existing mobile gamma cameras. With the commercial availability of position sensitive photomultiplier tubes (PSPMTs) the development of an instrument ideally suited to these applications now appears to be practical. The major components necessary to construct such a camera have been tested, and a circuit configuration designed to provide a high level of clinical performance in such a device is proposed. Spatial resolution and linearity data from a developmental prototype are provided. Measurements have shown that by using conventional sum and difference analog circuits only, a 76-mm/sup 2/ PSPMT has good linearity and therefore uniformity of sensitivity over only the central 60% of its absolute field of view. It is believed that this high-quality imaging area can be increased to virtually the full field of view by implementing a digital geometric correction scheme using a position vector lookup table. >

A comparison of optimum detector spatial resolution in nuclear imaging based on statistical theory and on observer performance

An expression for the expected image of a spherical tumour in a uniform background was derived in terms of background thickness and concentration of radioactivity, the tumour size, depth and uptake ratio, the gamma-ray energy and the detector response function. Three models of human observer performance for tumour detection were developed from different signal-to-noise ratio measures based on the statistical theory of detection. The optimum detector spatial resolution predicted by each model was then compared to that obtained from an observer performance study in which the subjects viewed computer-simulated scintigrams. The predictions from two of these models seem to be consistent with the results of the observer performance study. Model II involves a comparison of the counts integrated over the tumour region with the counts integrated over a background region of the same area. Model III compares the count density estimates of signal-plus-background and background obtained from application of non-uniform weighting functions to the image data.

Clinical myocardial imaging with nitrogen-13 ammonia

Myocardial infarcts may be clearly imaged using intravenous nitrogen-13 as carrier-free ammonia in doses of 10–30 mCi. This positron emitter is well imaged with the Nuclear Chicago HP Anger Camera with heavy collimation. The rapid blood disappearance of the agent gives good image contrast, and the short half-life and high isotope dosage give high-count density images with little radiation absorbed dose (5 mrad∕mCi total body).

Retrospective fusion of radiographic and MR data for localization of subdural electrodes

Prior to epilepsy surgery, subdural electrodes are often implanted and monitored for a few days to identify the focus of abnormal electrical activity. During the implantation and subsequent brain resection, there may be uncertainty about the exact location of the electrodes with respect to features of brain anatomy such as specific gyral convolutions or lesions. In experiments with a phantom and patients, implanted electrodes were imaged with multiplanar skull radiographs (or CT scans). After retrospective registration with pre-implantation MR data, the electrodes were mapped from these studies onto an MR-derived three-dimensional brain model. The resulting multimodality displays showed the relationship of the electrodes to brain anatomy. In one patient the position of each electrode with respect to a metabolic lesion was also displayed by mapping preimplantation PET data onto the same brain model. This new display of electrode positions may strengthen the interpretation of subdural electrical recordings and thereby reduce uncertainty in planning the resection of epileptic tissue.

Optimum detector spatial resolution for discriminating between tumour uptake distributions in scintigraphy

The optimum detector spatial resolution has been determined for a scintigraphic decision task in which the observer must discriminate between two different distributions of radioactivity in tumours. The two kinds of tumour used are: (i) a solid sphere of increased uptake relative to background, and (ii) a thin spherical shell with high uptake in the shell and no radioactivity within the shell. Both tumours are embedded at the same depth within a cylinder of tissue-equivalent material containing a uniform distribution of radioactivity. On the basis of statistical decision theory, the optimum detector spatial resolution for discriminating between the two tumour activity distributions is predicted. The result of an observer performance experiment substantially agreed with the theoretical prediction, though some discrepancy was found, apparently due to a decrease in observer efficiency at poorer spatial resolution. The experimental result suggests that the optimum FWHM of detector spatial response for the discrimination task considered is about 65% of the tumour radius.

Aspects of Imaging and Counting in Nuclear Medicine Using Scintillation and Semiconductor Detectors

In the interest of minimizing patient irradiation, while maximizing diagnostic image quality, it is desirable to utilize all of the radiation emerging from the patient. For optimum utilization, it appears to be necessary to allow photons with different energies to contribute to the image with different weights. Optimum weights have been determined for the most common case of noise-limited images, where it is assumed that the weights should be chosen so as to maximize the signal-to-noise ratio. For less noisy images, sharpening may be achieved by assigning negative weights to scattered photons; that is, by scatter subtraction. In general, image formation with multiple weighted channels provides a greater degree of flexibility than is possible with a single channel.

Three-dimensional imaging utilizing energy discrimination. I

An algorithm is proposed for three-dimensional image reconstruction in nuclear medicine which uses scattered radiation rather than multiple projected images to determine the source depth within the body. Images taken from numerous energy windows are combined to construct the source distribution in the body. The gamma-ray camera is not moved during the imaging process. Experiments with both /sup 99m/Tc and /sup 67/Ga demonstrate that two channels of depth information can be extracted from the low-energy images produced by scattered radiation. By combining this technique with standard SPECT (single-photon emission computed tomography) reconstruction using multiple projections, the authors anticipate much improved spatial resolution in the overall three-dimensional reconstruction. >

Analysis of recorded image noise in nuclear medicine

The concepts of autocovariance function and Wiener spectrum have been applied to describe the recorded image noise in nuclear medicine. They were derived as functions of the expected detected count density and the detector and exposure point spread functions. It was shown that the detector system affects only the noise magnitude, whereas the recorder system affects both noise magnitude and texture. In experimental studies, the autocovariance function and Wiener spectrum of recorded image noise were measured by one-dimensional time-series analysis. Due to the non-linearity of the recording film, the best agreement between the theoretical predictions and the experimental results is found when the detected count density is sufficiently high and the size of the exposure spot is sufficiently large for the density fluctuations in the recorded noise image to be relatively low.