Photopeak Window Research Articles

SPECT instrumentation: Performance, lesion detection, and recent innovations

The use of a gamma camera for single photon emission computed tomography (SPECT) imaging places greater demand on camera performance than does planar imaging, since camera specifications that are adequate for planar imaging may produce serious image artifacts in SPECT. Manufacturers have responded to the challenge with improvements in both hardware and software, but the user must pay careful attention to quality control procedures. Field nonuniformity is caused mainly by spatial distortion, which may vary during rotation; some cameras incorporate a means of automatically correcting for angular changes. The purpose of performing SPECT is to improve lesion detection, ie, to improve contrast, which is influenced by many factors. Attenuation corrections may be applied either before or after reconstruction. For Tc-99m an attenuation coefficient less than that for water, ie, either 0.11 or 0.12, would appear to give better results; the visual appearance is improved but the effect on contrast is minimal. Scattered photons are the major cause of loss of contrast in SPECT images. Recent developments in on-line energy corrections allow the use of narrower photopeak windows as well as asymmetric energy windows. Offpeak energy windows have demonstrated significant improvements in contrast; however, the increase in nonuniformity can cause artifacts which may limit their use. Simultaneous collection of separate images, one in the photopeak region and the other in the scatter region (92 to 125 keV) may enable a transaxial scatter image to be subtracted from the transaxial photopeak image. This may permit quantification of the true radioactivity distribution. New collimator designs introduced to improve SPECT resolution include cast collimators which can be made with more uniform hole construction than the lead-foil type; for brain imaging, long bore parallel hole and converging fan beam collimators as well as astigmatic collimators which converge in both planes with different lines of focus. Some of these improve both sensitivity and resolution compared to parallel hole collimators. Noncircular orbits have also been introduced in order to improve resolution, but their use presents many problems which require careful monitoring. Transaxial multicrystal systems have been developed over many years, but have not been widely used, primarily because of their inherent complexity, high costs, and limited applicability. Some of these drawbacks may be overcome by a new type of SPECT camera currently under development by several different groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Cold lesions: enhanced contrast using asymmetric photopeak windows.

Cold-lesion contrast resolution was measured as a function of photopeak window position, photon energy, and scatter configuration using a clinical scintillation camera capable of asymmetric photopeak imaging. Two radionuclide solutions, technetium-99m and thallium-201, were studied at five photopeak window positions: the symmetric setting and window settings that were 5%, 10%, 20%, and 30% asymmetrically high (asymmetry was defined here by the intrinsic percentage of count loss relative to the symmetric window setting). Low contrast was examined with small cold-sphere phantoms and large cold-cube phantoms to produce high- and low-scatter conditions, respectively. High contrast was examined with a Hine cylindrical step phantom. Lesion contrast improved markedly with small degrees of window asymmetry but showed less dramatic improvement when positions at the higher degrees of asymmetry were studied. For the high-scatter and low-contrast situations, the maximum improvement in lesion contrast tended to be inversely proportional to lesion size. The maximum improvement in contrast resolution ranged from 10% to 45%, depending on lesion size, scatter configuration, and the radionuclide solution used.

Applications of emission tomography in the nuclear industry

An emission tomography system has been used to carry out experiments to simulate the scanning of irradiated nuclear fuel. A phantom made of titanium containing a multi-energetic radionuclide, 182Ta, has been used in conjunction with a high resolution intrinsic germanium detector to study the effects of scattering and attenuation on the image obtained. Reconstructed images produced from data in two energy regions, one a “photopeak window” and the other a “scattering window” show the importance of these effects with multi-energetic spectra. 182Ta sources separated by 0.5 mm of titanium could be resolved.

Monte Carlo evaluation of Compton scatter subtraction in single photon emission computed tomography.

In single photon emission computed tomography (SPECT), Compton scattering produces a background that degrades the image quality and contributes erroneously to quantitative measurements. A clinically implementable compensation algorithm has previously been reported that subtracts a Compton scatter image, acquired in an energy window set below the energy of the photopeak, from the primary image acquired in the photopeak window. We present an evaluation and justification of the assumptions made in the previous empirical development of the subtraction algorithm. A Monte Carlo model of the SPECT system in which the Compton scattered vents may be followed independently of the nonscattered events was used to evaluate this subtraction technique. Simulation shows that the assumptions made in the experimental application of this algorithm were valid. Specifically (1) the "scatter" energy window used in the experiment (91-125 keV for imaging Tc-99m) contains only scattered events, (2) the shape of the line spread function (LSF) for the events in the scatter window is a reasonable approximation to the shape of the scatter in the photopeak window, and (3) the ratio of the number of scattered events in the photopeak window to the number of events in the scatter window is 0.57, close to the value of 0.5 derived heuristically. Thus, Monte Carlo simulation validates the basic assumptions underlying the empirical implementation of the scatter subtraction algorithm.

Tc-99m attenuation coefficients in water-filled phantoms determined with gamma cameras.

Quantitative imaging with gamma cameras requires compensation for attenuation of source photons. Some methods of compensation make use of a constant or average estimated attenuation coefficient mu. A value for mu of 0.15 cm-1 for 140.5-keV photons in water or tissue is commonly used. This value, however, neglects scattered photons which are detected within the energy window in gamma camera imaging. Values for mu of 0.12 cm-1 used in attenuation compensation of Tc-99m single-photon emission computed tomography scans of uniform cylindrical sources have been shown to give improved results compared with use of mu = 0.15 cm-1. In this study, gamma cameras and a multichannel pulse-height analyzer were used to determine effective values of mu for photons in water as a function of energy window. Two cylindrical water-filled phantoms, circular and elliptical, were used with a point source of Tc-99m at depths up to 18 cm. Energy data were integrated over the top half of the photopeak, and over 10%, 20%, and 30% windows centered on the photopeak. Attenuation curves were exponential for all photopeak windows with values of mu of 0.12 +/- 0.014 cm-1 for all windows up to 20% and 0.1 cm-1 for a 30% window. This study suggests that a value of mu of 0.11-0.12 cm-1 is, in fact, appropriate for use in attenuation compensations where an average is required.