Analytic Derivation and Monte Carlo Validation of a Sensitivity Formula for Slit-Slit Collimation With Penetration.

A slit-slit collimator consists of two orthogonal slits and can be conceptualized as a generalized pinhole. Since the two slits are independent of each other, there can be independent axial and transaxial acceptance angles. A small axial acceptance angle may help mitigate axial blurring with circular orbits, allowing multiple copies axially. In addition, since the two slit planes can be placed at different distances with respect to the source, a better detector usage can be achieved, especially in the case of detectors and imaged objects with different aspect ratios. In this paper an analytical expression is derived for the sensitivity of slit-slit collimation including effective slit widths for photon penetration. An analytical expression for sensitivity is necessary in order to accurately model the system response. This expression could also be useful for comparing the slit-slit's sensitivity performance with others. When the effective slit width is used instead of the geometric slit width, the derived analytical expression accurately accounts for photon penetration of the aperture. The derived expression for the sensitivity was validated by Monte Carlo simulation for both geometric and penetrative cases.

Slit-Slat and Multislit-Slat Collimator Design and Experimentally Acquired Phantom Images from a Rotating Prototype.

We have previously found and validated expressions for slit-slat (SS) geometric efficiency and resolution. These expressions have suggested that SS may be a good choice for imaging mid-size objects or objects that are long axially since (i) the geometric efficiency increases near the slit as h(-1) (instead of h(-2) for pinhole (PIN) and either decreases near the collimator for fan-beam (FB) or remains constant for parallel-beam (PB)), where h is the distance from a point to the slit plane; (ii) the transverse resolution is comparable to that of PIN, which is better than that of FB and PB for small objects; (iii) the axial resolution is worse than that of PIN since there is no axial magnification; (iv) there is a large axial FOV, unlike PIN, which is likely to be useful when imaging mid-size or long objects; and (v) there is no need for 3D orbits (e.g., helical) since each slice is complete (like PB and FB).We have developed a rotating prototype SS collimator that is capable of single-slit or multi-slit acquisition of data. The focal length (FL) is shorter than that of a typical PIN since increasing the FL requires taller slats to maintain resolution; taller slats reduce geometric efficiency. A lead rectangular box was used to provide support and shielding around the slit-slat collimator. Lead slats, spaced with Rohacell foam, were mounted in an assembly with 3 mm pitch.We have performed preliminary characterization with point sources and acquired micro hot- and cold-rod phantoms and a Deluxe Jaszczak phantom. The projections have been reconstructed using an MLEM algorithm and show good resolution.Comparisons indicate that SS is more sensitive than PB and FB for the same resolution for smaller-diameter objects. The advantage of SS over PB and FB increases as the desired resolution improves. SS can also be used in configurations that yield projections that have non-isotropic resolution; it is possible for SS to achieve transverse resolutions that are unreachable by PB, since PB does not magnify, and by FB, since its magnification factor for small objects is much smaller than that of SS. Experimental results show that the resolution of the reconstructed phantoms is consistent with theoretical expectations.

Comparison of circular and polygonal planar orbits for pinhole and slit-slat SPECT.

Analytic formulas and Monte Carlo simulation are used to calculate and compare the sensitivity of circular and polygonal orbits at different points in the Field of View (FoV) for both pinhole and slit-slat collimation. Results show that for a given slit-slat collimator an N-sided polygonal orbit tangent to the FoV generally provides average sensitivity lower than the tightest circular orbit consistent with the same aperture angle, but with better spatial resolution that can be traded for sensitivity for a constant-resolution comparison. This generally results in a slight advantage for the polygonal orbit. However, this advantage depends on the clearance that must be allowed between the orbit and the FoV and decreases quickly, vanishing when even a few millimeters of space are left, which in practice is necessary to accommodate mechanical constraints. For a pinhole collimator the advantage for the tangent polygonal orbit is more consistent, but similar conclusions are reached again when clearance is considered. A direct comparison at constant resolution between slit-slat and pinhole collimation in a single transverse plane is shown to be possible with parameters typical of small-animal imaging applications only for detectors with excellent intrinsic resolution; in this case, pinhole collimation is shown to be more sensitive in magnifying geometries, but reduced axial FoV and increased axial blurring should also be considered for a more complete comparison.

On-axis sensitivity and resolution of a slit-slat collimator.

UNLABELLED: A slit-slat collimator combines a slit along the axis of rotation with a set of axial septa, offering both magnification in the transaxial direction and complete sampling with just a circular orbit. This collimator has a sensitivity that increases for points near the aperture slit. The literature treats this collimator as having the same sensitivity as a single-pinhole collimator, ignoring the effect of the axial septa. Herein, the sensitivity and resolution of this collimator are reevaluated. METHODS: Experimental and Monte Carlo methods are used to determine the sensitivity and resolution in both the transaxial and axial directions as a function of distance from the slit (h). Eight configurations are tested, varying the slit width, septal spacing, and septal height. RESULTS: Both the experimental and the Monte Carlo sensitivities agree reasonably with an analytic form that is the geometric mean of the pinhole and parallel-beam formulas, disagreeing with previous literature. Transaxial resolution is consistent with the pinhole-resolution formula. Axial resolution is consistent with the parallel-beam resolution formula. CONCLUSION: The sensitivity of this collimator is proportional to h(-1) and has resolution in the transaxial direction that is consistent with pinhole resolution and in the axial direction that is consistent with parallel-beam resolution.

Resolution-effective diameters for asymmetric-knife-edge pinhole collimators.

The effects of penetration are included in the formulas for the prediction of the resolution of pinhole collimators through the use of effective diameters. Expressions of the resolution-effective diameter for pinholes with a double-knife-edge (DKE) profile are available in the literature. In this paper the expressions applicable to asymmetric-knife-edge (AKE) profiles, which include the important case of the single-knife-edge (SKE), are presented. Results indicate that the simplest methods that are still accurate in the calculation of DKE effective diameters do not produce in general formulas with similar accuracy for AKE profiles, due to increased susceptibility to penetration. Especially at high energy (365 keV), for the SKE case more advanced formulas are necessary and were, therefore, derived.

Optimal number of pinholes in multi-pinhole SPECT for mouse brain imaging--a simulation study.

This study simulates a multi-pinhole single-photon emission computed tomography (SPECT) system using the Monte Carlo method, and investigates different multi-pinhole designs for quantitative mouse brain imaging. Prior approaches investigating multi-pinhole SPECT were not often optimal, as the number and geometrical arrangement of pinholes were usually chosen empirically. The present study seeks to optimize the number of pinholes for a given pinhole arrangement, and also for the specific application of quantitative neuroreceptor binding in the mouse brain. An analytical Monte Carlo simulation based method was used to generate the projection data for various count levels. A three-dimensional ordered-subsets expectation-maximization algorithm was developed and used to reconstruct the images, incorporating a realistic pinhole model for resolution recovery and noise reduction. Although artefacts arising from overlapping projections could be a major problem in multi-pinhole reconstruction, the cold-rod phantom study showed minimal loss of spatial resolution in multi-pinhole systems, compared to a single-pinhole system with the same pinhole diameter. A quantitative study of neuroreceptor binding sites using a mouse brain phantom and low activity (37 MBq) showed that the multi-pinhole system outperformed the single-pinhole system by maintaining the mean and lowering the variance in the measured uptake ratio. Multi-pinhole collimation can be used to reduce the injected dose and thereby reduce the radiation exposure to the animal. Results also suggest that the nine-pinhole configuration shown in this paper is a good choice for mouse brain imaging.

Sub-millimeter technetium-99m calibration sources.

PURPOSE: Small animal radioscintigraphic imaging systems aim to achieve sub-millimeter resolution. At the present time, sub-millimeter calibration sources that can be placed at will within an imaged volume are not readily available. We have developed a method for producing technetium-99m (Tc-99m) sources in less than 15 minutes with readily available reagents. PROCEDURES: Tc-99m pertechnetate [TcO(4)](-) was incubated with 45 microm to 106 microm diameter spherical anion exchange beads, washed, and mounted as desired for instrument calibration. RESULTS: The procedure yields spherical sources having between 6.8 microCi to 11.1 microCi of Tc-99m per source. This work shows that dual imaging of these sources using white light and radioscintigraphy permits measurement of system performance with high precision. CONCLUSION: Easily prepared, sub-millimeter Tc-99m spherical calibration sources are described, and it is demonstrated that such sources are useful for measuring the resolution and sensitivity of radioscintigraphic systems, such as those designed for small animal imaging.

Analytic determination of the resolution-equivalent effective diameter of a pinhole collimator.

To account for photon penetration, the formulas used to calculate the geometric resolution of a pinhole collimator use an equivalent diameter d(e) rather than the physical diameter of the aperture. The expression commonly used for d(e), however, was originally derived to account for penetration in sensitivity calculations. In this paper, we show that the concept of equivalent diameter is also applicable to resolution calculations, propose angular-dependent expressions for d(e) specific to resolution calculations, and discuss the limits of their applicability and how they compare to other expressions. Results show that for normal incidence Paix's expression for d(e) tends to overestimate the resolution-equivalent diameter for full-width-at-half-maximum resolution, whereas Anger's is a better approximation, but may produce underestimates for submillimeter resolution imagers, especially in the case of high-energy photons. For grazing incidence, both expressions may result in significant overestimates.

Optimization of a fully 3D single scatter simulation algorithm for 3D PET.

We describe a new implementation of a single scatter simulation (SSS) algorithm for the prediction and correction of scatter in 3D PET. In this implementation, out of field of view (FoV) scatter and activity, side shields and oblique tilts are explicitly modelled. Comparison of SSS predictions with Monte Carlo simulations and experimental data from uniform, line and cold-bar phantoms showed that the code is accurate for uniform as well as asymmetric objects and can model different energy resolution crystals and low level discriminator (LLD) settings. Absolute quantitation studies show that for most applications, the code provides a better scatter estimate than the tail-fitting scatter correction method currently in use at our institution. Several parameters such as the density of scatter points, the number of scatter distribution sampling points and the axial extent of the FoV were optimized to minimize execution time, with particular emphasis on patient studies. Development and optimization were carried out in the case of GSO-based scanners, which enjoy relatively good energy resolution. SSS estimates for scanners with lower energy resolution may result in different agreement, especially because of a higher fraction of multiple scatter events. The algorithm was applied to a brain phantom as well as to clinical whole-body studies. It proved robust in the case of large patients, where the scatter fraction increases. The execution time, inclusive of interpolation, is typically under 5 min for a whole-body study (axial FoV: 81 cm) of a 100 kg patient.

Supranormal renal function in unilateral hydronephrotic kidney can be avoided.

INTRODUCTION: There are reports and controversy in the literature of supranormal (defined as >55%) differential renal function (DRF) in the hydronephrotic kidney in children with unilateral hydronephrosis. It is not confirmed whether supranormal DRF is an artifact or a true finding. In patients in whom the relative renal function deteriorates, relief of obstruction becomes surgically necessary and if this artifact can be removed. Supranormal function in an obstructed kidney is confusing and there is no consensus on how to manage these patients. There is no agreement if this is a true entity or an artifact. We wanted to address this issue by reporting our experience. METHODS: We reviewed all the consecutive cases from August 2000 to October 2001 who were studied in our center with the diagnosis of unilateral renal obstruction for confirmation or evaluation of DRF. All patients had MAG-3 studies that were interpreted by experienced nuclear medicine physicians. The DRF were measured within the first to second minute of the MAG-3 injection. Regions of interest were drawn by the imaging software and the images were corrected for background counts by drawing regions of interest 2 pixels away from the edge of the renal cortex. RESULTS: Fifty-seven patients were confirmed to be obstructed unilaterally; 41 (72%) patients had obstructive lesions in the left kidney and 16 (28%) in the right kidney. There was no case of supranormal DRF in the obstructed kidneys in our study. CONCLUSION: The supranormal renal function, as noted in some reports in the literature, was not seen in any patients at our institution. We believe that this entity is an artifact and can be avoided by using MAG-3 and projecting regions of interest by computer software; we plan to start analyzing multiple algorithms in phantoms with different ROI selection for background analysis.

Optimization of coded aperture radioscintigraphy for sentinel lymph node mapping.

PURPOSE: Radioscintigraphic imaging during sentinel lymph node (SLN) mapping could potentially improve localization; however, parallel-hole collimators have certain limitations. In this study, we explored the use of coded aperture (CA) collimators. PROCEDURES: Equations were derived for the six major dependent variables of CA collimators (i.e., masks) as a function of the ten major independent variables, and an optimized mask was fabricated. After validation, dual-modality CA and near-infrared (NIR) fluorescence SLN mapping were performed in pigs. RESULTS: Mask optimization required the judicious balance of competing dependent variables, resulting in sensitivity of 0.35%, XY resolution of 2.0 mm, and Z resolution of 4.2 mm at an 11.5-cm field of view. The findings in pigs suggested that NIR fluorescence imaging and CA radioscintigraphy could be complementary, but present difficult technical challenges. CONCLUSIONS: This study lays the foundation for using CA collimation for SLN mapping, and also exposes several problems that require further investigation.

Improved dose regimen in pediatric PET.

UNLABELLED: PET image quality depends strongly on patient weight and habitus, decreasing for increasing weight and body mass index. Common adult injection rules prescribe either a dose proportional to weight or a fixed dose. In light patients, image quality may improve for decreasing weight more than by inverse proportion. If better quality than in average-adult studies does not justify the associated dose burden, attractive options are to reduce scan time, reduce dose, or any combination of the 2. The objective of this study was to determine quantitative injection rules for pediatric PET allowing clinical implementation of these trade-offs. METHODS: Literature methods combining phantom with clinical data were followed to derive patient-specific noise-equivalent count rate density (NECRD) curves as a function of injected dose. From these, it was possible to estimate retrospectively for each patient the scan time that would have been sufficient for the same NECRD as in a 70-kg reference adult; the reduced dose sufficient for constant NECRD and scan time; and a general relationship among scan time, dose, and NECRD. Correlation to the patient statistic giving highest correlation, which was found to be weight, provided rules applicable prospectively. Data from 73 patients (weight, 11.5-91.4 kg; mean, 45.4 kg) were acquired and analyzed. RESULTS: Following the clinical injection rule, which was proportional to weight, the NECRD increased linearly with decreasing weight. The expression exp[0.019 x (weight [kg] - 70)] for the time reduction possible with the current dose at constant NECRD correlated well with data (R(2) = 0.86). The dose (in MBq) necessary for constant NECRD that should be injected 60 min before imaging is predicted well by 14.8 x exp[0.046 x weight (kg)] (R(2) = 0.88) with the current scan time. A more complex expression to convert NECRD in whole or part to both dose and time savings was also derived. Comparison to common pediatric injection rules showed reasonable agreement with Clark's rule, albeit not at all weights. CONCLUSION: Results suggest that pediatric PET of constant image quality (in an NECRD sense) can be performed with time or dose savings, up to 50% for the lightest patients (10-20 kg).

Analytic derivation of the longitudinal component of the three-dimensional point-spread function in coded-aperture laminography.

Near-field coded-aperture data from a single view contain information useful for three-dimensional (3D) reconstruction. A common approach is to reconstruct the 3D image one plane at a time. An analytic expression is derived for the 3D point-spread function of coded-aperture laminography. Comparison with computer simulations and experiments for apertures with different size, pattern, and pattern family shows good agreement in all cases considered. The expression is discussed in the context of the completeness conditions for projection data and is applied to explain an example of nonlinear behavior inherent in 3D laminographic imaging.

Pinhole imaging of 131I-metaiodobenzylguanidine (131I-MIBG) in an animal model of neuroblastoma.

PURPOSE: To evaluate (131)I-MIBG scintigraphic localization of xenotransplanted and spontaneously arising neuroblastomas in murine models of high-risk neuroblastoma. METHODS: Neuroblastoma xenografts were created by inoculation of human neuroblastoma cell suspensions into the subcutaneous flanks of athymic nude mice. In addition, spontaneous paraspinal neuroblastomas were detected by direct palpation in MYCN transgenic mice. After measured tumor volumes exceeded 200 mm(3), each mouse received an intraperitoneal injection of 18 muCi/g (131)I-metaiodobenzylguanidine ((131)I-MIBG). Pinhole scintigraphy was performed to evaluate the MIBG biodistribution and to attempt to visualize the tumors. Each mouse was imaged on a gamma camera equipped with a 3-mm pinhole on one head and an HEGP collimator on the other. RESULTS: Images demonstrated absorption of radiolabeled MIBG and visualization of tumors. Analysis of the images allowed for quantification of relative MIBG uptake and for determination of linear and area measurements of the tumors. CONCLUSION: High-energy pinhole imaging effectively demonstrates uptake of radiolabeled MIBG by human neuroblastoma tumors in murine laboratory models. This technique allows for in vivo assessment of tumor burden. In the future, we plan to use this method to evaluate sensitivity for detecting metastatic spread as well as investigating the therapeutic efficacy of high-dose (131)I-MIBG in combination with radiosensitizing agents.

Coded aperture nuclear scintigraphy: a novel small animal imaging technique.

We introduce and demonstrate the utility of coded aperture (CA) nuclear scintigraphy for imaging small animals. CA imaging uses multiple pinholes in a carefully designed mask pattern, mounted on a conventional gamma camera. System performance was assessed using point sources and phantoms, while several animal experiments were performed to test the usefulness of the imaging system in vivo, with commonly used radiopharmaceuticals. The sensitivity of the CA system for 99mTc was 4.2 x 10(3) cps/Bq (9400 cpm/microCi), compared to 4.4 x 10(4) cps/Bq (990 cpm/microCi) for a conventional collimator system. The system resolution was 1.7 mm, as compared to 4-6 mm for the conventional imaging system (using a high-sensitivity low-energy collimator). Animal imaging demonstrated artifact-free imaging with superior resolution and image quality compared to conventional collimator images in several mouse and rat models. We conclude that: (a) CA imaging is a useful nuclear imaging technique for small animal imaging. The advantage in signal-to-noise can be traded to achieve higher resolution, decreased dose or reduced imaging time. (b) CA imaging works best for images where activity is concentrated in small volumes; a low count outline may be better demonstrated using conventional collimator imaging. Thus, CA imaging should be viewed as a technique to complement rather than replace traditional nuclear imaging methods. (c) CA hardware and software can be readily adapted to existing gamma cameras, making their implementation a relatively inexpensive retrofit to most systems.