Optimization of Coded Aperture Radioscintigraphy for Sentinel Lymph Node Mapping

Molecular Imaging and Biology · 2011-05-12 · 27 citations

Improved Dose Regimen in Pediatric PET

Journal of Nuclear Medicine · 2010-01-15 · 54 citations

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).

Slit-Slat and Multi-Slit-Slat Collimator Design and Experimentally Acquired Phantom Images From a Rotating Prototype

IEEE Transactions on Nuclear Science · 2010-02-01 · 14 citations

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.

Injected Dose in Pediatric PET

Journal of Nuclear Medicine · 2010-09-16

REPLY: We would like to thank Dr. Watson for his interest in and comments about our paper ( 1 ). The main concern raised is the rationale behind the choice of noise-equivalent count density (NECD) over noise-equivalent count (NEC). The choice was indeed guided by the logic inherent in equalizing

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

IEEE Transactions on Nuclear Science · 2010-02-01 · 3 citations

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.

Correction to “Analytic Derivation and Monte Carlo Validation of a Sensitivity Formula for Slit-Slit Collimation With Penetration” [Feb 10 135-143

IEEE Transactions on Nuclear Science · 2010-04-01

In the above titled paper (ibid., vol. 57, no. 1, pp.135-143, Feb. 10), the funding source was omitted. That information is presented here.

Comparison of Circular and Polygonal Planar Orbits for Pinhole and Slit-Slat SPECT

IEEE Transactions on Nuclear Science · 2009-06-01 · 2 citations

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.

Geant4 evaluation of the impact of spatial resolution improvement on the contrast recovery coefficient in a small-animal PET system with collimation

2009-10-01

Two limitations on the resolution of a reconstructed PET image are sampling and detector pixel size. Using collimation that partially blocks each crystal reduces the effective crystal size. Using different collimation positions increases sampling. In this study we determine the Contrast Recovery Coefficient (CRC) for a small-animal PET scanner with and without collimation in the transverse direction. We performed simulations of a single-slice small-animal PET system (205 mm diameter and 2 ? 2 ? 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> LYSO crystals). The septa forming the collimation were 1 ? 2 ? 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> tungsten pieces covering half of each crystal transaxially. Phantoms (25 mm diameter) with one cold and three hot lesions with diameter D (D = 0.5, 1, 2, 3 mm) were simulated with two S:B ratios (4:1, 6:1). CRC = (S/B-1)/(T-1) where S and B are mean lesion (hot or cold) and background count densities, and T is true uptake ratio. CRC was measured from reconstructions to quantify the impact of the resolution improvement. Results show collimation improves mean CRC compared to non-collimated PET. For 1 mm hot lesions (4:1), scanned for the same duration, the collimated mean hot lesion CRC values (STD) were 0.44 (0.03) (center) and 0.24 (0.02) (off-center), where STD is the standard deviation of measured CRCs of an ensemble of images. Non-collimated results were 0.31 (0.01) and 0.18 (0.01), respectively. Although the total number of coincidences for the same scan time is fewer by about a factor of 4 in the collimated system, the measured mean CRC is higher. The efficiency loss in collimated PET manifests itself as worse STD in measured CRC and noisier images. When the collimated PET scan time is increased, the STD of measured CRCs improves and reaches that of non-collimated PET. In certain imaging scenarios, it may be possible to scan longer with a collimated PET system to make up for the efficiency loss. In conclusion, our study shows the use of collimation has the potential to improve the quantification and detection capabilities of a small-animal PET system.

First Experimental Results from a Prototype Rotating Slit-Slat Collimator

2008-10-01 · 1 citations

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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−</sup> 1 (instead of h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> for pinhole (PIN) and either decreasing near the collimator for fan-beam (FB) or constant for parallel-beam (PB)), where h is the distance from a point to the slit plane; (ii) the transverse resolution is comparable to PIN, which is better than FB and PB for small objects; (iii) the axial resolution is worse than 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).

Verification of the sensitivity and resolution dependence on the incidence angle for slit–slat collimation

Physics in Medicine and Biology · 2008-01-23 · 30 citations

Single photon emission computed tomography (SPECT) can be performed with various collimator types, which have an inherent tradeoff between the properties of sensitivity, resolution, field of view and complete sampling. Slit-slat collimation, which has seen recent interest in the literature, combines a slit parallel to the axis of rotation of a gamma camera with a set of septa perpendicular to the slit. This collimator geometry exhibits properties that may enhance some SPECT imaging applications, specifically imaging of the breast, limbs and medium-sized animals. However, a complete description of its system response is critical for a comparison to other collimator types and for accurate reconstruction of projection data. Herein, experimental and Monte Carlo methods are used to determine the sensitivity and transaxial and axial resolutions as a function of the incidence angle theta, which is the angle formed by the line from the photon source to the center of the slit and the plane of the slit, to compare to theoretical expectations. Four configurations are investigated by varying the slit width, septal spacing and septal height. Monte Carlo sensitivity data not modeling penetration and scatter exhibit a sin(3)theta dependence. Experimental and Monte Carlo-derived sensitivity data modeling scatter and penetration are consistent with each other and have a sin(x)theta dependence, where x is greater than 3. Transaxial resolution data show a small dependence on theta, and axial resolution data are consistent with no angular dependence.