Investigation of causes of geometric distortion in 180 degrees and 360 degrees angular sampling in SPECT.

To investigate geometric distortion when 180 degrees or 360 degrees angular sampling techniques are used in single photon emission computed tomography (SPECT), a study of point sources imaged at different positions in a water filled cylindrical phantom, and reconstructed using filtered back projection, was conducted. A simulation study, based upon a serial model of the system point spread function (PSF), was used to investigate the contributions of attenuation, spatial resolution and scatter on distortion of the reconstructed PSFs. To study the geometric distortion in transverse (x-y plane), coronal (x-z plane), and sagittal (y-z plane) sections, the ratios of the full widths at half maximum (FWHM) and full widths at tenth maximum (FWTM) in the x/y, x/z, and y/z directions were calculated for the real and simulated PSFs. These results showed that, in an attenuating medium, there is more distortion of point sources into ovals for 180 degrees than for 360 degrees sampling. The simulation study indicated that the primary cause of geometrical distortion in SPECT studies, is the inconsistency of projections due to variable attenuation and spatial resolution. The impact of scatter on geometric distortion was small as measured by the ratios of FWHMs and FWTMs for PSFs. Attenuation correction applied to acquired PSFs significantly reduced geometric distortion in both 180 degrees and 360 degrees studies. To investigate distortion in extended objects, an Iowa heart phantom was placed inside an Alderson body phantom and 201Tl heart SPECT studies acquired. The phantom images confirmed the conclusion that in transverse sections of 360 degrees studies with arithmetic averaging of opposite views, geometric distortion is reduced compared to 180 degrees. The coronal and sagittal sections were equally distorted in both, the 180 degrees and 360 degrees studies, and the 180 degrees studies yielded better contrast.

An image-dependent Metz filter for nuclear medicine images.

To provide optimal image quality, digital filters should account for both the count level and the object imaged. That is, they should be image-dependent. By using the constraint equation of constrained least-squares (CLS) restoration to determine one parameter of the Metz filter, a filter which adapts to the image has been developed. This filter has been named the Constrained Least-Squares Metz filter. The filter makes use of a regression relation to convert the Metz filter parameter determined using the CLS criterion to the value which would minimize the normalized mean square error (NMSE). The regression relation and the parameters which specify the general form of the Metz filter were determined using images of the Alderson liver and spleen phantoms. The designed filter was tested for its ability to adapt to other objects with images from each of three different test objects. When the values of the Metz filter parameters for these images determined by the CLS-Metz filter were compared by a regression analysis to those which minimized the NMSE for each image, a correlation coefficient of 0.98, a slope of 0.95, and a zero intercept of 0.1 were obtained. With clinical images, the CLS-Metz filter has been shown to provide consistently good image quality with images as diverse as heart perfusion images and bone studies.

Digital restoration of indium-111 and iodine-123 SPECT images with optimized Metz filters.

A number of radiopharmaceuticals of great current clinical interest for imaging are labeled with radionuclides that emit medium- to high-energy photons either as their primary radiation, or in low abundance in addition to their primary radiation. The imaging characteristics of these radionuclides result in gamma camera image quality that is inferior to that of 99mTc images. Thus, in this investigation 111In and 123I contaminated with approximately 4% 124I were chosen to test the hypothesis that a dramatic improvement in planar and SPECT images may be obtainable with digital image restoration. The count-dependent Metz filter is shown to be able to deconvolve the rapid drop at low spatial frequencies in the imaging system modulation transfer function (MTF) resulting from the acceptance of septal penetration and scatter in the camera window. Use of the Metz filter was found to result in improved spatial resolution as measured by both the full width at half maximum and full width at tenth maximum for both planar and SPECT studies. Two-dimensional, prereconstruction filtering with optimized Metz filters was also determined to improve image contrast, while decreasing the noise level for SPECT studies. A dramatic improvement in image quality was observed with the clinical application of this filter to SPECT imaging.

Two-dimensional filtering of SPECT images using the Metz and Wiener filters.

Two-dimensional filtering, both before and after reconstruction, has been applied to the processing of single photon emission computerized tomographic (SPECT) images. The filters investigated were the count-dependent Metz filter and Wiener filter, both of which automatically adapt to the image being processed. Using a SPECT phantom, with images reconstructed with these filters rather than the ramp, we observed a statistically significant increase (p less than 0.05) in the image contrast for solid Plexiglas spheres, and significant decrease (p less than 0.05) in the percent fractional standard deviation of counts in a region of uniform activity. The adaptability of these filters is demonstrated by a comparison of SPECT acquisitions of the phantom at two different count levels. An example of their application to clinical studies is presented. We conclude that two-dimensional digital image restoration with these techniques can produce a significant increase in SPECT image quality, with a small cost in processing time when these techniques are implemented on an array processor.

Interactive visual optimization of SPECT prereconstruction filtering.

A number of factors must be considered when forming a digital filter to two-dimensionally filter single photon emission computed tomographic (SPECT) acquisition images. In an effort to provide subjectively optimal filtering, a program has been developed which provides "real-time" visual feedback. This allows a user to select from among a family of Metz filters tailored for the imaging conditions (i.e., formed to deconvolve scatter, septal penetration, and combined collimator and intrinsic spatial resolution losses). Also, a guideline for assisting the user in selecting from among the possible Metz filters has been formulated. This guideline is based upon knowledge of the probability distribution of the noise power spectrum, and consists of choosing the filter which has a value of 1.0 when the one-dimensional compression of the image power spectrum equals the 90% confidence limit for noise fluctuations. The program starts by filtering a planar reference image with the Metz filter computed for the radionuclide, collimator, magnification, and count-level of the image. This filter is displayed beside the image where it is overlayed on a plot of the logarithm of the one-dimensional compression of the image power spectrum. The user is then allowed to vary the filter parameters through movement of a joystick. By doing the filtering using an array processor, a new filtered image is formed and displayed less than a second after movement of the joystick. Visual feedback from the series of filtered images thus produced as well as the plots of the filter overlayed on the estimated blurred object power spectrum are used to obtain a visually "optimal" filter. The filter can be adapted to the visual preferences of the individual reader, and serves as a useful teaching tool on the effects of filtering.

Attenuation compensation in 99mTc SPECT brain imaging: a comparison of the use of attenuation maps derived from transmission versus emission data in normal scans.

UNLABELLED: Brain SPECT imaging using 99mTc lipophilic tracers such as hexamethyl propyleneamine oxime (HMPAO) attempts to estimate cerebral, cerebellar and subcortical perfusion by assessing the relative amount of tracer uptake among these regions. Most commonly, comparison is made with cerebellar activity. Because the assessment of relative tracer uptake may be rendered inaccurate by photon attenuation by the nonuniform attenuation properties of the head, brain SPECT reconstructions have been compared using attenuation correction (AC) with various methods for estimating the attenuation map. METHODS: Patients underwent 99mTc-HMPAO brain SPECT with transmission line source AC hardware. In addition to the emission dataset, emission downscatter and transmission datasets were acquired. Iterative reconstructions using three different attenuation maps were investigated. These included: (a) that obtained from transmission imaging, (b) that obtained from segmentation of a reconstruction from a lower energy Compton scatter window and (c) a slice-independent, uniform, elliptical attenuation map. No AC was also compared. RESULTS: Count profiles in patients having brain perfusion SPECT scans showed a significant difference in region count estimates in the brain depending on whether AC is used as well as on the attenuation map used. Scatter-based AC is able to provide external contour detection and attenuation compensation based on that contour, whereas transmission-based AC provides external contour detection as well as internal, nonuniform attenuation estimation and AC. If one considers transmission AC to be the clinical "gold standard," non-attenuation-corrected as well as fixed-ellipsoid, uniform attenuation-corrected studies provided unreliable regional estimates of tracer activity. CONCLUSION: This study shows the significant difference in clinical brain SPECT count profiles depending on how and whether there is compensation for attenuation. Based on prior studies validating the improved quantitative accuracy of SPECT using transmission-based AC, this study suggests that clinical 99mTc brain perfusion SPECT would benefit from and, in situations demanding rigorous quantitative assessment, requires transmission-based AC. Estimating attenuation maps with scatter-based methods was the next most accurate (clinical) method tested and can be used if and when transmission imaging cannot be used.

Bremsstrahlung imaging using the gamma camera: factors affecting attenuation.

Quantitative imaging of bremsstrahlung from pure beta emitters is proposed as a means for in vivo management of antibody therapy. The method involves the use of high-energy collimation, an empirically selected broad photon energy window to enhance detector sensitivity, and a Wiener restoration filter to compensate for system blur. The measured and filtered data were obtained for an idealized scattering medium and isolated spherical sources. An effective linear attenuation coefficient of about 0.13 cm-1 was determined from the raw image data of 32P. A coefficient of 0.14 cm-1 was determined after the images were restored using the Wiener filter. The measured attenuation was not significantly dependent on the size of the region of interest or the size of the source. Its variation was within the experimental error of measurement (+/- 5%). The measured sensitivity (6 x 10(-6) cps/Bq) was sufficient for imaging therapy doses of 32P or 90Y.

Theory and cardiac applications of electrical impedance measurements.

The methodology of the two-electrode, four-electrode, and guard-ring techniques is presented following a brief history of impedance plethysmography. The theoretical basis for predicting the sampling fields for conductivity and volume changes is presented. Theoretical and experimental studies of the sampling field associated with various electrode arrays are reviewed. With this background, the use of impedance plethysmography for cardiac monitoring and diagnosis is reviewed. The basic methodology is presented and models used to interpret the signal are reviewed. Theoretical and experimental studies of what is sampled are summarized. The accuracy of impedance stroke volume estimates is evaluated by surveying the results of human studies and examining critical animal studies. The usefulness of impedance cardiography for ventricular performance evaluation is also reviewed. Additional uses for cardiopulmonary diagnosis are briefly presented.

Variation of the count-dependent Metz filter with imaging system modulation transfer function.

A systematic investigation was conducted of how a number of parameters which alter the system modulation transfer function (MTF) influence the count-dependent Metz filter. Since restoration filters are most effective at those frequencies where the object power spectrum dominates that of the noise, it was observed that parameters which significantly degrade the MTF at low spatial frequencies strongly influence the formation of the Metz filter. Thus the radionuclide imaged and the depth of the source in a scattering medium had the most influence. This is because they alter the relative amount of scattered radiation being imaged. For low-energy photon emitters, the collimator employed and the distance from the collimator were found to have less of an influence but still to be significant. These cause alterations in the MTF which are more gradual, and hence are most pronounced at mid to high spatial frequencies. As long as adequate spatial sampling is employed, the Metz filter was determined to be independent of the exact size of the sampling bin width, to a first approximation. For planar and single photon emission computed tomographic (SPECT) imaging, it is shown that two-dimensional filtering with the Metz filter optimized for the imaging conditions is able to deconvolve scatter and other causes of spatial resolution loss while diminishing noise, all in a balanced manner.

Activity quantitation in SPECT: a study of prereconstruction Metz filtering and use of the scatter degradation factor.

A study of activity quantitation with prereconstruction Metz filtering and use of the scatter degradation factor (SDF) to numerically correct for scatter was conducted. The ratio of the count rate per unit activity for source locations within a 30 x 23-cm water-filled tub phantom to the count rate per unit activity for Tc-99m point sources of known activity imaged in air was used to judge the accuracy of activity determination. The investigation was conducted for certain locations within the tub when it was uniformly filled with Tc-99m activity, and for the same locations at the center of 5, 4, 3, and 2-cm diam, hot spheres imaged in a cold background. The source locations were the center, and one-fourth, one-half, and three-fourths the major axis. Various methods of combining the conjugate views for use with prereconstruction attenuation correction (arithmetic and geometric mean), and extent to which the Metz filter followed the inverse filter before rolling off to suppress noise were investigated. Without Metz filtering, attenuation correction was performed using a transmission curve that included buildup. With Metz filtering, the good-geometry attenuation coefficient was used and the combined views were scaled by the SDF calculated for the average body thickness. Depending on the size of the sphere and the extent to which the inverse filter was followed, Metz filtering combined with use of the SDF improved the accuracy of activity quantitation.

A Wiener filter for nuclear medicine images.

To improve the quality of digital nuclear medicine images, we have developed a new implementation of the Wiener restoration filter. The Wiener filter uses as its optimality criterion the minimization of the mean-square error between the undistorted image of the object and the filtered image. In order to form this filter, the object and noise power spectrums are needed. The noise power spectrum for the count-dependent Poisson noise of nuclear medicine images is shown to have a constant average magnitude equal to the total count in the image. The object power spectrum is taken to be the image power spectrum minus the total count, except in the noise dominated region of the image power spectrum where a least-squares-fitted exponential is used. Processing time is kept to a clinically acceptable time frame through use of an array processor. Pronounced noise suppression and detail enhancement are noted with use of this filter with clinical images.

Modifying constrained least-squares restoration for application to single photon emission computed tomography projection images.

Image restoration methods have been shown to increase the contrast of nuclear medicine images by decreasing the effects of scatter and septal penetration. Image restoration can also reduce the high-frequency noise in the image. This study applies constrained least-squares (CLS) restoration to the projection images of single photon emission computed tomography (SPECT). In a previous study, it was noted that CLS restoration has the potential advantage of automatically adapting to the blurred object. This potential is confirmed using planar images. CLS restoration is then modified to improve its performance when applied to SPECT projection image sets. The modification was necessary because the Poisson noise in low count SPECT images causes considerable variation in the CLS filter. On phantom studies, count-dependent Metz restoration was slightly better than the modified CLS restoration method, according to measures of contrast and noise. However, CLS restoration was generally judged as yielding the best results when applied to clinical studies, apparently because of its ability to adapt to the image being restored.

Constrained least-squares restoration of nuclear medicine images: selecting the coarseness function.

Image restoration using the constrained least-squares (CLS) method theoretically adapts to the image being processed. In addition, it only requires knowing the modulation transfer function of the imaging system when applied to nuclear medicine images. Prompted by these observations, a systematic evaluation of the effects of the form of the "coarseness function" [C(f)] used by the CLS method has been conducted. Nine C(f)'s are evaluated using an observer preference and a normalized mean-squared error (NMSE) criterion. This evaluation is conducted for three modulation transfer functions and a wide range of count levels. The results of the subjective studies support using the form of C(f) which has been most widely employed in previous studies, i.e., the form designed to minimize the energy in the second derivative of the restored image. A different form of C(f) is generally found to be optimal by the mean-squared error criterion. The CLS method is then compared to: (1) no processing, (2) count-dependent smoothing, and (3) count-dependent Metz restoration. When evaluated using objective measurements of error and contrast, the CLS method is found to be slightly inferior to the best method, Metz restoration. However, CLS restoration is found to be equal to or better than the other methods when judged by the results of observer preference studies.

The effect of intrinsic attenuation correction methods on the stationarity of the 3-D modulation transfer function of SPECT.

The application of stationary restoration techniques to SPECT images assumes that the modulation transfer function (MTF) of the imaging system is shift invariant. It was hypothesized that using intrinsic attenuation correction (i.e., methods which explicitly invert the exponential radon transform) would yield a three-dimensional (3-D) MTF which varies less with position within the transverse slices than the combined conjugate view two-dimensional (2-D) MTF varies with depth. Thus the assumption of shift invariance would become less of an approximation for 3-D post- than for 2-D pre-reconstruction restoration filtering. SPECT acquisitions were obtained from point sources located at various positions in three differently shaped, water-filled phantoms. The data were reconstructed with intrinsic attenuation correction, and 3-D MTFs were calculated. Four different intrinsic attenuation correction methods were compared: (1) exponentially weighted backprojection, (2) a modified exponentially weighted backprojection as described by Tanaka et al. [Phys. Med. Biol. 29, 1489-1500 (1984)], (3) a Fourier domain technique as described by Bellini et al. [IEEE Trans. ASSP 27, 213-218 (1979)], and (4) the circular harmonic transform (CHT) method as described by Hawkins et al. [IEEE Trans. Med. Imag. 7, 135-148 (1988)]. The dependence of the 3-D MTF obtained with these methods, on point source location within an attenuator, and on shape of the attenuator, was studied. These 3-D MTFs were compared to: (1) those MTFs obtained with no attenuation correction, and (2) the depth dependence of the arithmetic mean combined conjugate view 2-D MTFs.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmission imaging of large attenuators using a slant hole collimator on a three-headed SPECT system.

By combining conjugate views, truncation-free attenuation profiles of patients can be obtained by using slant hole collimators on three-headed SPECT systems. The alterations in reconstruction algorithms necessary for use with slant hole collimators and potential image artifacts are discussed. Based on an evaluation of the size of objects that can be imaged without truncation and the size of the overlap region in the conjugate views, a 15 degrees slant angle was determined to be optimal. Studies with a 30 degrees slant hole collimator verified the ability of slant hole transmission imaging to provide accurate, truncation-free attenuation maps of a 56 cm lateral width phantom. The center of rotation was determined to be dependent on the slant angle and radius of rotation of the slant collimator. These studies also demonstrated that the spatial resolution in the transaxial plane of the attenuation maps depends on radius of rotation of the slant hole collimator, but does not depend on the radius of rotation of an uncollimated transmission source. A multiline transmission source was investigated for use with estimating the attenuation map in Tc-99m labeled sestamibi perfusion imaging.

Relative importance of the error sources in Wiener restoration of scintigrams.

Through simulation studies, the relative importance of three error sources in Wiener filtering as applied to scintigrams is quantified. The importance of these error sources has been quantified using the percentage changed in squared error (compared to that of an image restored using an ideal Wiener filter) which is caused by estimating one of three factors in the Wiener filter. Estimating the noise power spectrum using the total image count produced to appreciable change in the squared error (less than 1%). Estimating the power spectrum of the true image from that of the degraded image produced small to moderate increases in the squared error (4-139%). In scintigraphic imaging, the modular transfer function (MTF) is dependent on source depth; hence, this study underscores the importance of using methods which reduce the depth dependence of the effective MTF prior to applying restoration filters. A novel method of estimating the power spectrum of the true image from that of the degraded images is also described and evaluated. Wiener restoration filters based on this spectral estimation method are found to be competitive with the image-dependent Metz restoration filter.

Characterization of the modulation transfer function of discrete filtered backprojection.

A mathematical expression for the modulation transfer function (MTF) of image reconstruction by discrete filtered backprojection (DFBP) is derived. A simulation study is used to investigate the dependence of the MTF of DFBP on: (1) the number of projection views; (2) the type of ramp filter used; (3) the interpolation method used during backprojection; and (4) the position of the object. These results were compared to MTFs calculated from point-source single-photon-emission computed tomographic (SPECT) acquisitions in air. The experimentally obtained MTFs contained much of the same structure as the MTFs of DFBP obtained through simulation. It is shown that the discretization of the filtered backprojection process can cause the tomographic transfer function to be anisotropic and nonstationary. However, through proper selection of the methods used in reconstruction, a nearly isotropic and stationary MTF can be obtained.

Noniterative compensation for the distance-dependent detector response and photon attenuation in SPECT imaging.

A filtering approach is described, which accurately compensates for the 2D distance-dependent detector response, as well as for photon attenuation in a uniform attenuating medium. The filtering method is based on the frequency distance principle (FDP) which states that points in the object at a specific source-to-detector distance provide the most significant contribution to specified frequency regions in the discrete Fourier transform (DFT) of the sinogram. By modeling the detector point spread function as a 2D Gaussian function whose width is dependent on the source-to-detector distance, a spatially variant inverse filter can be computed and applied to the 3D DFT of the set of all sinogram slices. To minimize noise amplification the inverse filter is rolled off at high frequencies by using a previously published Wiener filter strategy. Attenuation compensation is performed with Bellini's method. It was observed that the tomographic point response, after distance-dependent filtering with the FDP, was approximately isotropic and varied substantially less with position than that obtained with other correction methods. Furthermore, it was shown that processing with this filtering technique provides reconstructions with minimal degradation in image fidelity.

Exploring a chronic care model in a regional healthcare context.

The episodic acute care model of service delivery that consumes the majority of healthcare funding is inadequate to meet the needs of a society experiencing an increasing burden of chronic illness. This article describes, in a regional Canadian healthcare context, the application and preliminary evaluation of a model of chronic care delivery developed in a managed care environment in the United States. The strategic goals of this model are to empower clients by providing a redesigned service delivery environment that supports self-management; support care providers by developing clinical information systems and decision supports; and align community resources, policies and organization of care to support an informed, activated client and a proactive practice team.