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Purpose Noise levels of brain SPECT images are highest in central regions, due to preferential attenuation of photons emitted from deep structures. To address this problem, the authors have designed a novel collimator for brain SPECT imaging that yields greatly increased sensitivity near the center of the brain without loss of resolution. This hybrid collimator consisted of ultrashort cone-beam holes in the central regions and slant-holes in the periphery (USCB). We evaluated this collimator for quantitative brain imaging tasks. Methods Owing to the uniqueness of the USCB collimation, the hole pattern required substantial variations in collimator parameters. To utilize the lead-casting technique, the authors designed two supporting plates to position about 37 000 hexagonal, slightly tapered pins. The holes in the supporting plates were modeled to yield the desired focal length, hole length, and septal thickness. To determine the properties of the manufactured collimator and to compute the system matrix, the authors prepared an array of point sources that covered the entire detector area. Each point source contained 32 μCi of Tc-99m at the first scan time. The array was imaged for 5 min at each of the 64 shifted locations to yield a 2-mm sampling distance, and hole parameters were calculated. The sensitivity was also measured using a point source placed along the central ray at several distances from the collimator face. High-count projection data from a five-compartment brain phantom were acquired with the three collimators on a dual-head SPECT/CT system. The authors calculated Cramer-Rao bounds on the precision of estimates of striatal and background activity concentration. In order to assess the new collimation system to detect changes in striatal activity, the authors evaluated the precision of measuring a 5% decrease in right putamen activity. The authors also reconstructed images of projection data obtained by summing data from the individual phantom compartments. Results The sensitivity of the novel cone-beam collimator varied with distance from the detector face; it was higher than that of the fan-beam collimator by factors ranging from 2.7 to 162. Examination of the projections of the point sources revealed that only a few holes were distorted or partially blocked, indicating that the intensive manual fabrication process was very successful. Better reconstructed phantom images were obtained from the USCB+FAN collimator pair than from either LEHR or FAN collimation. For the left caudate, located near the center of the brain, the detected counts were 9.8 (8.3) times higher for UCSB compared with LEHR (FAN), averaged over 60 views. The task-specific SNR for detecting a 5% decrease in putamen uptake was 7.4 for USCB and 3.2 for LEHR. Conclusions The authors have designed and manufactured a novel collimator for brain SPECT imaging. The sensitivity is much higher than that of a fan-beam collimator. Because of differences between the manufactured collimator and its design, reconstruction of the data requires a measured system matrix. The authors have demonstrated the potential of USCB collimation for improved precision in estimating striatal uptake. The novel collimator may be useful for early detection of Parkinson's disease, and for monitoring therapy response and disease progression.

New collimator manufacturing technologies, such as photoetching, electrical discharge machining, and stereolithography, expand the range of possible cone-beam collimator configurations. For example, it might now be possible for brain SPECT to make a short-focusing cone-beam collimator with tapered holes that increase in size with distance from the collimator surface; conventional lead-casting techniques produce holes of constant size and, consequently, varying septal thicknesses. Moreover, the changes in hole shape and loss of close packing due to focusing leads to thicker septa in the collimator periphery, especially for shorter focal lengths. We investigated the potential advantages of new cone-beam collimator manufacturing processes, and proposed a new design for very short focal-length collimators for brain SPECT imaging. We compared three cone-beam collimators, a conventional collimator manufactured using casting techniques (CC), a novel collimator with uniform hole sizes on the collimator surface and constant hole size through the collimator thickness (FC), and a novel collimator with uniform hole sizes and tapered holes (TC). We determined the resolution of each collimator analytically for focal lengths ranging from 20–50 cm, and adjusted the entrance hole sizes of FC and TC to equalize resolution of all collimators. Sensitivity was calculated at several locations by Monte Carlo simulation. Sensitivity was higher at all points for TC and FC than for CC, and higher for TC than for FC. The differences in sensitivity were larger for shorter focal lengths. For a point on the focal line at 10 cm in front of the collimator entrance surface, the sensitivity gain for TC compared to CC was 7% and 45% for focal lengths of 50 and 20 cm, respectively. The sensitivity gain for a 20-cm focal length, compared to CC, averaged over all locations, was 44% for TC and 23% for FC. We have shown that the new collimator designs made possible by new manufacturing techniques will provide gains in sensitivity that depend strongly on focal length; the sensitivity gains are large for the very short focal lengths that, as we have previously shown, are beneficial for brain imaging.

Sensitivity near the center of the brain is critical for some clinical applications of brain SPECT, especially for those applications which require dynamic imaging. We have designed and built a novel collimator for the CeraSPECT dedicated brain SPECT system which uses variable focusing to increase the sensitivity for central regions of its field of view. The sensitivity at the center of the field of view is increased by a factor of two compared to the standard three-segment parallel-hole collimator; this sensitivity gain is achieved without compromising spatial resolution

We assessed improvements in performance in detection and estimation tasks due to a novel brain single photon computed tomography collimator. Data were acquired on the CeraSPECT{sup TM} scanner using both new and standard collimators. The new variable focusing collimator SensOgrade{sup TM} samples the projections unequally, with central regions more heavily represented, to compensate for attenuation of counts from central brain structures. Furthermore, it utilizes more of the cylindrical crystal surface. Two phantom studies were performed. The first phantom was a 21-cm-diameter cylindrical background containing nine spheres ranging from 0.5 to 5 cm{sup 3} in volume. {sup 99m}Tc sphere to background activity ratio was 10:1. Twenty-nine 10-min datasets were acquired with each collimator. The second phantom was the Radiology Support Devices (Long Beach, CA) striatal phantom with striatal-background ratios of 10:1 on the left and 5:1 on the right. Twenty-nine 4-min datasets were acquired with each collimator. Perfusion imaging using {sup 99m}Tc-HMPAO was also performed in three healthy volunteers using both collimators under identical simulations. Projections were reconstructed by filtered backprojection with an unwindowed ramp filter. The nonprewhitening matched filter signal-to-noise ratio (NPW-SNR) was computed as a surrogate for human performance in detecting spherical lesions. Sphere activity concentration, radius, and locationmore » coordinates were simultaneously estimated by fitting images to an assumed model using an iterative nonlinear algorithm. Resolution recovery was implicit in the estimation procedure, as the point spread function was incorporated into the model. NPW-SNR for sphere detection was 1.5 to 2 times greater with the new collimator; for the striatal phantom the improvement in SNR was 54%. The SNR for estimating sphere activity concentration improved by 46 to 89 % for spheres located more than 5 cm from the phantom center. Images acquired with the standard collimator were too noisy in the central regions to allow estimation of sphere activity. In {sup 99m}Tc-HMPAO human studies, SNR was improved by 21 to 41 % in the cortex, 66% in the basal ganglia, and 74% in the thalamus. The new collimator leads to substantially improved detection and estimation performance throughout the brain. The higher sensitivity will be particularly important for dynamic imaging.« less

We describe a new approach to the problem of collimator optimization in nuclear medicine; our methodology is illustrated for the challenging case of gallium-67 imaging. Collimator-design methods based on empirical rules, such as specification of an allowable level of single-septal penetration (SSP) at a fixed energy, are especially inappropriate for radionuclides characterized by an abundance of high-energy contaminant photons that scatter in the patient, collimator, and/or detector before detection within one of a few photopeak energy windows. Lead X-rays produced in the collimator are an additional source of contamination. We designed optimal collimation for /sup 67/Ga based on relevant clinical imaging tasks and a realistic simulation of photon transport in a phantom, collimator, and detector. Collimator designs were compared on the basis of performance in lesion detection, as predicted by a three-channel Hotelling observer (CHO), as well as in tumor and background activity estimation (EST), quantified by task-specific signal-to-noise ratios (SNRs). The optimal values of collimator lead content were 22.0 and 23.8 g/cm/sup 2/, respectively, for CHO and EST, while the optimal geometric resolution values were 1.8 and 1.6 cm full-width at half-maximum (FWHM), respectively, at a distance of 23.5 cm. The resolution of a commercially available medium-energy low-penetration collimator (MELP) is 1.9 cm FWHM at this distance. The optimal values for SSP at 300 keV were 7.3% and 5.8% based on CHO and EST, respectively, compared to 5.2% for the MELP collimator. Compared with the commercial MELP collimator, the /sup 67/Ga collimator optimized for tumor detection or activity estimation tasks provided improved geometric spatial resolution with reduced geometric efficiency and, surprisingly, allowed an increased level of single-septal penetration.

Single-photon emission-computed tomography (SPECT) imaging of deep brain structures is compromised by loss of photons due to attenuation. We have previously shown that a centrally peaked collimator sensitivity function can compensate for this phenomenon, increasing sensitivity over most of the brain. For dual-head instruments, parallel-hole collimators cannot provide variable sensitivity without simultaneously degrading spatial resolution near the center of the brain; this suggests the use of converging collimators. We have designed collimator pairs for dual-head SPECT systems to increase sensitivity, particularly in the center of the brain, and compared the new collimation approach to existing approaches on the basis of performance in estimating activity concentration of small structures at various locations in the brain. The collimator pairs we evaluated included a cone-beam collimator, for increased sensitivity, and a fan-beam collimator, for data sufficiency. We calculated projections of an ellipsoidal uniform background, with 0.9-cm-radius spherical lesions at several locations in the background. From these, we determined ideal signal-to-noise ratios (SNRCRB) for estimation of activity concentration within the spheres, based on the Cramer-Rao lower bound on variance. We also reconstructed, by an ordered-subset expectation-maximization (OS-EM) procedure, images of this phantom, as well as of the Zubal brain phantom, to allow visual assessment and to ensure that they were free of artifacts. The best of the collimator pairs evaluated comprised a cone-beam collimator with 20 cm focal length, for which the focal point is inside the brain, and a fan-beam collimator with 40 cm focal length. This pair yielded increased SNRCRB, compared to the parallel-parallel pair, throughout the imaging volume. The factor by which SNRCRB increased ranged from 1.1 at the most axially extreme location to 3.5 at the center. The gains in SNRCRB were relatively robust to mismatches between the center of the brain and the center of the imaging volume. Artifact-free reconstructions of simulated data acquired using this pair were obtained. Combining fan-beam and short-focusing cone-beam collimation should greatly improve dual-head brain SPECT imaging, especially for centrally located structures.

Compton scatter, lead X-rays, and high-energy contamination are major factors affecting image quality in Ga-67 imaging. Scattered photons detected in one photopeak window include photons exiting the patient at energies within the photopeak, as well as higher energy photons which have interacted in the collimator and crystal and lost energy. Furthermore, lead X-rays can be detected in the main energy photopeak (93 keV). We have previously developed two energy-based methods, based on artificial neural networks (ANN) and on a generalized spectral (GS) approach to compensate for scatter, high-energy contamination, and lead X-rays in Ga-67 imaging. For comparison, we considered also the projections that would be acquired in the clinic using the optimal energy windows (WIN) we have reported previously for tumor detection and estimation tasks for the 93, 185, and 300 keV photopeaks. The aim of the present study is to evaluate under realistic conditions the impact of these phenomena and their compensation on tumor detection and estimation tasks in Ga-67 imaging. ANN and GS were compared on the basis of performance of a three-channel Hotelling observer (CHO), in detecting the presence of a spherical tumor of unknown size embedded in an anatomic background as well as on the basis of estimation of tumor activity. Projection datasets of spherical tumors ranging from 2 to 6 cm in diameter, located at several sites in an anthropomorphic torso phantom, were simulated using a Monte Carlo program that modeled all photon interactions in the patient as well as in the collimator and the detector for all decays between 91 and 888 keV. One hundred realistic noise realizations were generated from each very-low-noise simulated projection dataset. The presence of scatter degraded both CHO signal-to-noise ratio (SNR) and estimation accuracy. On average, the presence of scatter led to a 12% reduction in CHO SNR. Correcting for scatter further diminished CHO SNR but to a lesser extent with ANN (5% reduction) than with GS (12%). Both scatter corrections improved performance in activity estimation. ANN yielded better precision (1.8% relative standard deviation) than did GS (4%) but greater average bias (5.1% with ANN, 3.6% with GS).

Simultaneous dual isotope imaging (/sup 99m/Tc//sup 123/I) has potential clinical applications but has not been implemented in the clinic. The aim of this work was to design an artificial neural network (ANN) for crosstalk and scatter correction using a smaller number of energy windows (8) than we had previously proposed (26) to allow implementation on some clinical cameras, and to validate our approach using realistic Monte Carlo simulations and anthropomorphic brain phantom acquisitions. Monte Carlo simulations of dual isotope SPECT studies of a digital brain phantom and physical acquisitions of the striatal brain phantom were used to validate our approach. Corrected projections were reconstructed using an iterative ordered subsets expectation maximization (OSEM) algorithm that modeled nonuniform attenuation and variable collimator response in the projector/backprojector. Results: In Monte Carlo simulations, ANN26 and ANN8 yielded similarly accurate quantitation of /sup 123/I activity (bias

Describes a method to estimate unattenuated projection images directly from Ga-67 SPECT data acquired over 360/spl deg/ at three photopeak energies. The algorithm compensates to third order for the effects of nonuniform attenuation, thereby yielding unattenuated projections with

The advantages of simultaneous acquisition of Tc-99m and Tl-201 myocardial perfusion SPECT images can be fully realized only if the effects of the Tc-99m agent can be accurately removed from the Tl-201 image. The authors and others have previously reported simultaneous dual-isotope techniques for cardiac studies which make use of a third energy-window to estimate the Tc-99m scatter to be subtracted from the Tl-201 window. The authors have recently demonstrated, however, using a Monte Carlo program which simulates all details of the photon transport, that lead X-rays produced in the collimator may also contribute significantly to contamination in the Tl-201 window. The spatial distribution of the Tc-99m scattered photons differs from that of the lead X-rays. Therefore, the authors modified their correction technique so that, at each projection angle, the contaminant image to be subtracted from the image in the Tl-201 window was estimated as a linear combination of a scatter-window (90-110 keV) image, blurred by a 2D Gaussian filter, and the Tc-99m photopeak image, blurred by a different Gaussian filter. For simulated data which included 'liver' activity and non-uniform 'lung' attenuation, the improved dual-window subtraction technique provided a more accurate estimate of the true Tl-201 image, with less image noise, than did the single-window correction. >

A Bayesian estimator has been developed as a paradigm for human observer performance in detecting lesions of unknown size in a uniform noisy background. The Bayesian observer used knowledge of the range of possible lesion sizes as a prior; its predictions agreed well with the results of a six-observer perceptual study. The average human response to changes in collimator resolution, as measured by the detectability index, dA, was tracked by the Bayesian detector's signal-to-noise ratio (SNR) somewhat better than by two other estimation models based, respectively, on lesser and greater degrees of lesion size uncertainty. As the range of possible lesion sizes increased, the Bayesian detector's SNR decreased and the optimal collimator resolution shifted towards better resolution. An analytic approximation for the variance of lesion activity estimates (which included the same prior) was shown to predict the variance of the Bayesian estimator over a wide range of collimator resolution values. Because the bias of the Bayesian estimator was small (1%), the analytic variance estimate permitted a rapid and convenient prediction of the Bayesian detection SNR. This calculation was then used to optimize the geometric parameters of a two-layer tungsten collimator being constructed from crossed grids for a new imaging detector. A Monte Carlo program was first run to estimate all contributions to the radial point-spread function for collimators of differing tungsten contents and spatial resolution values, imaging 140-keV photons emitted from the center of a 15-cm-diameter, water-filled attenuator. The optimal collimator design for detecting lesions with unknown diameters in the range 2.5-7.5 mm yielded a system resolution of approximately 8.5-mm FWHM, a geometric collimator efficiency of 1.21 x 10(-4), and a single-septum penetration probability of 1%.

Obtaining the best possible task performance using reconstructed SPECT images requires optimization of both the collimator and reconstruction parameters. The goal of this study is to determine how to perform this optimization, namely whether the collimator parameters can be optimized solely from projection data, or whether reconstruction parameters should also be considered. In order to answer this question, and to determine the optimal collimation, a digital phantom representing a human torso with 16 mm diameter hot lesions (activity ratio 8:1) was generated and used to simulate clinical SPECT studies with parallel-hole collimation. Two approaches to optimizing the SPECT system were then compared in a lesion quantification task: sequential optimization, where collimation was optimized on projection data using the Cramer–Rao bound, and joint optimization, which simultaneously optimized collimator and reconstruction parameters. For every condition, quantification performance in reconstructed images was evaluated using the root-mean-squared-error of 400 estimates of lesion activity. Compared to the joint-optimization approach, the sequential-optimization approach favoured a poorer resolution collimator, which, under some conditions, resulted in sub-optimal estimation performance. This implies that inclusion of the reconstruction parameters in the optimization procedure is important in obtaining the best possible task performance; in this study, this was achieved with a collimator resolution similar to that of a general-purpose (LEGP) collimator. This collimator was found to outperform the more commonly used high-resolution (LEHR) collimator, in agreement with other task-based studies, using both quantification and detection tasks.

Purpose: To determine the effect of liquid sample volume and geometry on counting efficiency in a gamma well-counter, and to assess the relative contributions of sample geometry and self-attenuation. Gamma wellcounters are standard equipment in clinical and preclinical studies, for measuring patient blood radioactivity and quantifying animal tissue uptake for tracer development and other purposes. Accurate measurements are crucial. Methods: Count rates were measured for aqueous solutions of 99m- Tc at four liquid volume values in a 1-cm-diam tube and at six volume values in a 2.2-cm-diam vial. Total activity was constant for all volumes, and data were corrected for decay. Count rates from a point source in air, supported by a filter paper, were measured at seven heights between 1.3 and 5.7 cm from the bottom of a tube. Results: Sample volume effects were larger for the tube than for the vial. For the tube, count efficiency relative to a 1-cc volume ranged from 1.05 at 0.05 cc to 0.84 at 3 cc. For the vial, relative count efficiency ranged from 1.02 at 0.05 cc to 0.87 at 15 cc. For the point source, count efficiency relative to 1.3 cm from the tube bottom ranged from 0.98 at 1.8more » cm to 0.34 at 5.7 cm. The relative efficiency of a 3-cc liquid sample in a tube compared to a 1-cc sample is 0.84; the average relative efficiency for the solid sample in air between heights in the tube corresponding to the surfaces of those volumes (1.3 and 4.8 cm) is 0.81, implying that the major contribution to efficiency loss is geometry, rather than attenuation. Conclusion: Volume-dependent correction factors should be used for accurate quantitation radioactive of liquid samples. Solid samples should be positioned at the bottom of the tube for maximum count efficiency.« less

Simultaneous 99mTc/123I SPECT allows assessment of two functions under identical physiological conditions. Separation of these isotopes is difficult, however, because their energies are close. Most correction methods do not fully model either physical effects or patient-specific activity and attenuation distributions, and often reject scattered photons, rather than using the information they carry. We have developed a fast Monte Carlo (MC) simulation-based multiple-isotope and multiple-energy joint ordered-subset expectation maximization (JOSEM) iterative reconstruction algorithm, MC-JOSEM, that simultaneously corrects for scatter and crosstalk. We evaluated MC-JOSEM for simultaneous brain perfusion (99mTc-HMPAO) and neurotransmission (123I-altropane) SPECT. MC simulations of 99mTc and 123I studies were generated separately and then combined to mimic simultaneous 99mTc/123I SPECT. All details of photon transport through the brain, the collimator, and detector, including Compton and coherent scatter, septal penetration and backscatter from components behind the crystal, were modeled. We reconstructed images from simultaneous dual-isotope projections in two ways. First, we reconstructed the photopeak-energy-window projections (with an asymmetric energy window for 123I) using the standard ordered subsets expectation maximization algorithm (AW-OSEM). Second, we reconstructed the 3-energy-window projections using a MC based iterative reconstruction that we have developed (MC-JOSEM). After fifteen iterations of reconstruction of 99mTc images, root mean square errors (RMSE) of activity estimates in several brain structures from MC-JOSEM were reduced by 79-94% compared to AW-OSEM in the thalamus, striata, white matter, and gray matter. Similarly, RMSE of activity estimates from 123I images reconstructed by fifteen iterations of MC-JOSEM were reduced by 30-58% compared to AW-OSEM in the striata and background.

Simultaneous dual isotope imaging (Tc-99m/I-123) has potential clinical applications but has not been implemented in the clinic. The aim of this work was to optimize the design of an artificial neural network (ANN) for cross-talk and scatter correction using a smaller number of energy windows (8) than we had previously proposed (26) to allow implementation on some clinical cameras, and to validate our approach using realistic Monte Carlo simulations and anthropomorphic brain phantom acquisitions. Monte Carlo simulations of dual isotope SPECT studies of a digital brain phantom and physical acquisitions of the striatal brain phantom were used to validate our approach. Corrected projections were reconstructed using an iterative OSEM algorithm that modeled non-uniform attenuation and variable collimator response in the projector/backprojector. In simulated I-123 images, ANN26 and ANN8 yielded similarly accurate (bias

Tumor avidity for Ga-67 is known to be correlated with the histopathologic stage of disease, yet quantitative imaging has remained elusive because Ga-67 images are seriously degraded by several factors. We have previously optimized and evaluated on projection images a general spectral (GS) approach to compensate for patient scatter and Pb x-rays from the collimator. We also recently developed a joint, multienergy iterative algorithm (OSEM-J) to compensate for nonuniform attenuation and for distance- and energy-dependent collimator resolution. In the work reported here, we evaluated the overall activity quantitation performance achieved by combining GS with OSEM-J, and compared this to the (almost ideal) performance obtained when reconstructing primary unscattered photons, as well as to that obtained with another proposed iterative approach (OSEM-S) in which the sum of the two lowest photopeak windows was reconstructed using a single, energy-weighted attenuation map. We simulated 96 projections of an anthropomorphic phantom containing seven 2-cm-diameter tumors with Ga-67 concentration values 4 to 8 times that of the local background. Sixteen noise realizations containing lesions and 16 without lesions were generated, and least-squares estimates of tumor activity were calculated for each reconstruction method. The mean bias of tumor estimates for primary+OSEM-J, GS+OSEM-J, and GS+OSEM-S was, respectively, 1.2%, 1.6%, and 8.0% The average precision of the corresponding estimates was 15.0%, 19.1%, and 24.8% These results suggest that use of a spectral-based estimate of contaminant photons within a joint multiple-energy iterative reconstruction leads to nearly optimal performance in quantitation of Ga-67 tumor activity.

We investigated the effects of resolution recovery and SPECT system sensitivity on performance in estimating binding potential (BP) from dynamic brain SPECT data obtained using I-123-altropane, a dopamine transporter imaging agent. BP is estimated by Fischman's approach, whereby a gamma variate function is fitted to the difference between striatal and occipital time-activity curves (TAC). The TAC were obtained using an approach published by Huesman (1984), who estimated the activity concentration in a ROI directly from the projection dataset without reconstructing the image. We modified this method by incorporating resolution recovery, using a Metz filter. We simulated dynamic projection datasets of a simple striatal phantom and determined the accuracy and precision of estimation of striatal activity concentration and binding potential, both for current system sensitivity and for the higher sensitivity of a new collimator, presently being manufactured. This collimator is expected to increase sensitivity at the center of the brain by a factor of 3, without degrading resolution. The parameter, P, of the Metz filter, which controls the extent of resolution recovery, was varied from 1 to 10/sup 5/. For estimation of striatal activity concentration, increasing the value of P over this range reduced bias and gradually increased variance. For estimation of BP, however, increasing the value of P beyond 2 (for current sensitivity) and beyond 10 (for increased sensitivity) dramatically increased variance. For estimation of striatal activity, there was a broad minimum in RMSE of /spl sim/12% for P between 7 and 100 at current sensitivity, and /spl sim/10% for P between 10 and 300 for improved sensitivity. For estimation of binding potential, the minimum RMSE was 32% (P=2) for current sensitivity, and 17% (p=7) for improved sensitivity. The differences in the effects of resolution recovery on estimation of binding potential and striatal activity concentration are due to the nonlinear nature of the former task.

Gallium-67 plays an important role in the field of nuclear medicine, in particular for evaluation and staging of lymphoma patients. However, optimal collimation has not yet been designed specifically for this radionuclide. Instead, most nuclear medicine clinics use generic, medium-energy (ME) collimators which may be suboptimal for Ga-67. Collimator-design methods based on empirical rules, such as specification of an allowable level of single-septal penetration (SSP) at a fixed energy, are especially inappropriate for Ga-67, which is characterized by an abundance of high-energy contaminant photons that scatter in the patient, collimator, and/or detector, and ire then detected within one of the three energy windows (93, 185, 300 keV) normally used for imaging. Furthermore, lead X-rays produced in the collimator represent another significant source of contamination in the 93-keV window. We have optimized the design of collimation tailored for Ga-67 imaging, based on relevant clinical imaging tasks and a realistic simulation of photon transport in a phantom, collimator, and detector. Collimator designs were compared on the basis of performance in lesion defection, as predicted by a three-channel Hotelling observer (CHO), as well as in tumor and background activity estimation (EST), by computing task-specific signal-to-noise ratios (SNR). For each collimator design and task, SNR values were averaged over five spherical lesion sizes (1-3 cm diam.) at each of five locations within a 45-cm-diameter low-activity background cylinder containing a 10-cm-diameter spherical 'organ' with a higher activity concentration. The optimal values of collimator lead content were 22.0 and 23.8 g/cm/sup 2/, respectively, for CHO and EST, while the optimal geometric resolution values at 23.5 cm were 1.8 and 1.6 cm FWHM, respectively. The resolution of a commercially available ME low-penetration collimator is 1.9 cm at this distance. The CHO, EST, and commercial collimators would allow, respectively, 7.3%, 5.8%, and 5.2% SSP at 300 keV.

A Bayesian estimator has been implemented to serve as a paradigm for human observer performance in detecting lesions of unknown size and activity in a uniform noisy background of unknown amplitude. The Bayesian observer used knowledge of the possible range of lesion sizes as a prior. The results of a six-observer ROC study were well predicted by the performance of the Bayesian estimator. A calculation of the Cramer-Rao bound on the variance of lesion activity estimates (which included the same prior) was then used to optimize the geometric parameters of a two-layer tungsten collimator being constructed from crossed grids for a new imaging detector. A Monte Carlo program was used to estimate all contributions to the radial point-spread function for collimators of differing tungsten contents and spatial resolutions, imaging 140-keV photons emitted from the center of a 15-cm-diameter, water-filled cylinder. The optimal collimator design for detecting small lesions with unknown diameters in the range 2.5-7.5 mm provided a geometric collimator efficiency of 1.21*10/sup -4/ and a single-septum penetration probability of 1%. >

The first step in all intrinsic attenuation-correction algorithms is multiplication of each measured projection by a function which compensates for photon attenuation between a line through the center of rotation, parallel to the detector, and the (convex) external object contour nearest the detector. A nonuniform collimator sensitivity profile which accomplishes this compensation physically rather than computationally would reduce the noise in the projections and, consequently, in the reconstructed images. The authors have compared the variance of reconstructed images of cylindrical phantoms of homogeneous activity concentration and attenuation coefficient for three collimator sensitivity profiles using both the original intrinsic attenuation-correction technique of Tretiak-Metz (1980) and Gullberg-Budinger (1981) and the variant developed by Tanaka (1984). The collimator sensitivity profiles the authors considered were the standard profile, with uniform sensitivity along the projection, and two nonuniform sensitivity profiles which were peaked at the center of the projection. The nonuniform collimator sensitivity profiles led to reduced variances throughout most of the image for both attenuation correction algorithms. These reductions in variance would be expected to lead to improved performance in quantitative imaging tasks. >

Purpose: We have designed a novel collimator for brain SPECT imaging that yields greatly increased sensitivity near the center of the brain without loss of resolution. The collimator was manufactured and initial evaluation has been completed. Methods: The collimator was time-consuming and challenging to build. Because our desired hole pattern required substantial variations in hole angle, we designed two supporting plates to securely position about 34,000 hexagonal, slightly tapered, 75-mm long steel pins. The holes in the plates were modeled to yield the desired focal length, hole length and septal thickness. Molten lead was poured in between the plates, and all pins were removed after cooling. The sensitivity gain compared to a fan-beam collimator was measured using a point source placed along the central ray at several distances from the collimator face. Visual inspection of the holes was not possible as the collimator was sealed so it could be safely mounted on a SPECT system. Therefore, we prepared a 2D array of 768, ∼48μCi Tc-99m point sources, separated by 1.6 cm. The array was imaged for 10 minutes at 4 shifted locations to reduce sampling distance to 8 mm. Results: The sensitivity of the novel cone-beam collimator varied with distancemore » from the detector face; it was higher than that of the fan-beam collimator by factors ranging from 3 to 176. Examination of the projections of the 4×768 point sources revealed that fewer than 2% of the holes were fully or partially blocked, which indicates that the intensive manual fabrication process was very successful. Conclusion: We have designed and manufactured a novel collimator for brain SPECT imaging. As expected, the sensitivity is much higher than that of a fan-beam collimator. Because of differences between the manufactured collimator and its design, reconstruction of the data will require a measured system function.« less

Purpose: The authors have previously reported the advantages of high-sensitivity single-photon emission computed tomography (SPECT) systems for imaging structures located deep inside the brain. DaTscan (Isoflupane I-123) is a dopamine transporter (DaT) imaging agent that has shown potential for early detection of Parkinson disease (PD), as well as for monitoring progression of the disease. Realizing the full potential of DaTscan requires efficient estimation of striatal uptake from SPECT images. They have evaluated two SPECT systems, a conventional dual-head gamma camera with low-energy high-resolution collimators (conventional) and a dedicated high-sensitivity multidetector cardiac imaging system (dedicated) for imaging tasks related to PD. Methods: Cramer–Rao bounds (CRB) on precision of estimates of striatal and background activity concentrations were calculated from high-count, separate acquisitions of the compartments (right striata, left striata, background) of a striatal phantom. CRB on striatal and background activity concentration were calculated from essentially noise-free projection datasets, synthesized by scaling and summing the compartment projection datasets, for a range of total detected counts. They also calculated variances of estimates of specific-to-nonspecific binding ratios (BR) and asymmetry indices from these values using propagation of error analysis, as well as the precision of measuring changes in BR on the order of the average annual decline in early PD. Results: Under typical clinical conditions, the conventional camera detected 2 M counts while the dedicated camera detected 12 M counts. Assuming a normal BR of 5, the standard deviation of BR estimates was 0.042 and 0.021 for the conventional and dedicated system, respectively. For an 8% decrease to BR = 4.6, the signal-to-noise ratio were 6.8 (conventional) and 13.3 (dedicated); for a 5% decrease, they were 4.2 (conventional) and 8.3 (dedicated). Conclusions: This implies that PD can be detected earlier with the dedicated system than with the conventional system; therefore, earlier identification of PD progression should be possible with the high-sensitivity dedicated SPECT camera.