High-energy Collimators Research Articles

Quantification of iodine 131 for therapeutic follow-up of thyroid cancer patients

Introduction The uptake of iodine 131 used in the treatment of differentiated thyroid cancers, is a powerful medical indicator. Recent technological developments, as hybrid SPECT/CT machines, dramatically improve the potential in terms of quantification. Regarding these new opportunities, the aim of this work is to evaluate their performances in order to provide helpful medical tool. Material and methods This study has been implemented on a SPECT/CT device (NM/CT 670 General Electric Healthcare) equipped with 5/8” crystal and high-energy collimator, linked to dedicated software for quantification (Dosimetry Toolkit). Two test objects were used (Jaszczak - Data & Spectrum Lawrence Livermore realistic phantom) to calibrate the acquisition and reconstruction processes and to validate the results. Five refillable spheres (1–2–4–8 & 16 ml) and one polymer object-mimicking thyroid have been used to reproduce clinical situations. Results Taking into account that 3D SPECT data have to be segmented, the threshold value must be first determined. Using the data carried out with refillable spheres positioned in the Jaszczak phantom, a value of 0.4 has been determined. Under these conditions the sensitivity coefficient was equal to 12 cps MBq-1 s-1. The maximum difference registered between expected and measured uptake was 25%, when the Jaszczak phantom with spheres was considered. Neither the parameters of reconstruction algorithm (number of subsets with OSEM) nor the position of the radioactive structures inside the phantom seem to have a more significant influence. A single acquisition using the anthropomorphic Livermore phantom highlighted 15% error for a realistic simulation of a thyroid remnant. Conclusion As accuracy ranged around ±20% is usually sufficient to allow monitoring of thyroid cancer, the results obtained in this study seem consistent with medical requirements. This methodology will now be applied to the pulmonary region in the case of isolated and/or spread uptake.

Beam–Materials Interactions

Challenging applications related to high-intensity beam–materials interactions are described along with consequences of controlled and uncontrolled impacts of intense beams on components of medium- and high-energy accelerators, beamlines, target stations, beam collimators and absorbers, detectors, shielding, and the environment. Requirements on simulation code capabilities for such applications are derived. The principal classes of deleterious effects in materials under irradiation are described with real-life examples, modeling techniques, and analyses of uncertainties in simulations.

VECTor: A Preclinical Imaging System for Simultaneous Submillimeter SPECT and PET

Today, PET and SPECT tracers cannot be imaged simultaneously at high resolutions but require 2 separate imaging systems. This paper introduces a Versatile Emission Computed Tomography system (VECTor) for radionuclides that enables simultaneous submillimeter imaging of single-photon and positron-emitting radiolabeled molecules. γ-photons produced both by electron-positron annihilation and by single-photon emitters are projected onto the same detectors by means of a novel cylindric high-energy collimator containing 162 narrow pinholes that are grouped in clusters. This collimator is placed in an existing SPECT system (U-SPECT-II) with 3 large-field-of-view γ-detectors. From the acquired projections, PET and SPECT images are obtained using statistical image reconstruction that corrects for energy-dependent system blurring. For PET tracers, the tomographic resolution obtained with a Jaszczak hot rod phantom was less than 0.8 mm, and 0.5-mm resolution images of SPECT tracers were acquired simultaneously. SPECT images were barely degraded by the simultaneous presence of a PET tracer, even when the activity concentration of the PET tracer exceeded that of the SPECT tracer by up to a factor of 2.5. Furthermore, we simultaneously acquired fully registered 3- and 4-dimensional multiple functional images from living mice that, in the past, could be obtained only sequentially. High-resolution complementary information about tissue function contained in SPECT and PET tracer distributions can now be obtained simultaneously using a fully integrated imaging device. These combined unique capabilities pave the way for new perspectives in imaging the biologic systems of rodents.

Efficacy of high-energy collimator for sentinel node lymphoscintigraphy of early breast cancer patients

IntroductionLymphoscintigraphy is an important part of sentinel node mapping in breast cancer patients. Sometimes star shaped artefacts due to septal penetration can be problematic during imaging. In the current study, we evaluated the possibility of high energy (HE) collimators use for lymphoscintigraphy.Patients and methodsTwenty patients with early breast carcinoma were included. Thirty minutes after radiotracer injection (99mTc-antimony sulphide colloid), anterior and lateral images were acquired using a dual head gamma camera equipped with a parallel hole low energy high resolution (LEHR) collimator on one head and HE collimator on another head. All images were reviewed by two nuclear medicine specialists regarding detectability and number of axillary sentinel nodes and presence of star artefact.ResultsAll images taken by LEHR collimators showed star artefact of the injection site. No image taken by HE collimator showed this effect. In two patients the sentinel node was visible only by HE collimator. Tumour location in both of these patients was in the upper lateral quadrant and both had history of excisional biopsy. In two patients additional sentinel node was visible adjacent to the first one only on the LEHR images.ConclusionsHE collimators can be used for sentinel lymph node mapping and lymphoscintigraphy of the breast cancer patients. This collimator can almost eliminate star-shaped artefacts due to septal penetration which can be advantageous in some cases. However, to separate two adjacent sentinel nodes from each other LEHR collimators perform better.

Letter to the Editor Bilateral radioiodine uptake by the non-lactating breast of a single nulliparous woman: a and literature review

Letter to the Editor Bilateral radioiodine uptake by the non-lactating breast of a single nulliparous woman: a and literature review

Quantitative Evaluation of Scintillation Camera Imaging Characteristics of Isotopes Used in Liver Radioembolization

BackgroundScintillation camera imaging is used for treatment planning and post-treatment dosimetry in liver radioembolization (RE). In yttrium-90 (90Y) RE, scintigraphic images of technetium-99m (99mTc) are used for treatment planning, while 90Y Bremsstrahlung images are used for post-treatment dosimetry. In holmium-166 (166Ho) RE, scintigraphic images of 166Ho can be used for both treatment planning and post-treatment dosimetry. The aim of this study is to quantitatively evaluate and compare the imaging characteristics of these three isotopes, in order that imaging protocols can be optimized and RE studies with varying isotopes can be compared.Methodology/Principal FindingsPhantom experiments were performed in line with NEMA guidelines to assess the spatial resolution, sensitivity, count rate linearity, and contrast recovery of 99mTc, 90Y and 166Ho. In addition, Monte Carlo simulations were performed to obtain detailed information about the history of detected photons. The results showed that the use of a broad energy window and the high-energy collimator gave optimal combination of sensitivity, spatial resolution, and primary photon fraction for 90Y Bremsstrahlung imaging, although differences with the medium-energy collimator were small. For 166Ho, the high-energy collimator also slightly outperformed the medium-energy collimator. In comparison with 99mTc, the image quality of both 90Y and 166Ho is degraded by a lower spatial resolution, a lower sensitivity, and larger scatter and collimator penetration fractions.Conclusions/SignificanceThe quantitative evaluation of the scintillation camera characteristics presented in this study helps to optimize acquisition parameters and supports future analysis of clinical comparisons between RE studies.

GATE based Monte Carlo simulation of planar scintigraphy to estimate the nodular dose in radioiodine therapy for autonomous thyroid adenoma

The recommended target dose in radioiodine therapy of solitary hyperfunctioning thyroid nodules is 300-400 Gy and therefore higher than in other radiotherapies. This is due to the fact that an unknown, yet significant portion of the activity is stored in extranodular areas but is neglected in the calculatory dosimetry. We investigate the feasibility of determining the ratio of nodular and extranodular activity concentrations (uptakes) from post-therapeutically acquired planar scintigrams with Monte Carlo simulations in GATE. The geometry of a gamma camera with a high energy collimator was emulated in GATE (Version 5). A geometrical thyroid-neck phantom (GP) and the ICRP reference voxel phantoms “Adult Female” (AF, 16 ml thyroid) and “Adult Male” (AM, 19 ml thyroid) were used as source regions. Nodules of 1 ml and 3 ml volume were placed in the phantoms. For each phantom and each nodule 200 scintigraphic acquisitions were simulated. Uptake ratios of nodule and rest of thyroid ranging from 1 to 20 could be created by summation. Quantitative image analysis was performed by investigating the number of simulated counts in regions of interest (ROIs). ROIs were created by perpendicular projection of the phantom onto the camera plane to avoid a user dependant bias. The ratio of count densities in ROIs over the nodule and over the contralateral lobe, which should be least affected by nodular activity, was taken to be the best available measure for the uptake ratios. However, the predefined uptake ratios are underestimated by these count density ratios: For an uptake ratio of 20 the count ratios range from 4.5 (AF, 1 ml nodule) to 15.3 (AM, 3 ml nodule). Furthermore, the contralateral ROI is more strongly affected by nodular activity than expected: For an uptake ratio of 20 between nodule and rest of thyroid up to 29% of total counts in the ROI over the contralateral lobe are caused by decays in the nodule (AF 3 ml). In the case of the 1 ml nodules this effect is smaller: 9-11% (AF) respectively 7-8% (AM). For each phantom, the dependency of count density ratios upon uptake ratios can be modeled well by both linear and quadratic regression (quadratic: r 2 > 0.99), yielding sets of parameters which in reverse allow the computation of uptake ratios (and thus dose) from count density ratios. A single regression model obtained by fitting the data of all simulations simultaneously did not provide satisfactory results except for GP, while underestimating the true uptake ratios in AF and overestimating them in AM. The scintigraphic count density ratios depend upon the uptake ratios between nodule and rest of thyroid, upon their volumes, and their respective position in a non-trivial way. Further investigations are required to derive a comprehensive rule to calculate the uptake or dose ratios based on post-therapeutic scintigraphy.

NEMA NU1-2001 performance tests of four Philips Brightview cameras

Abstract The Brightview is the latest generation dual-head SPECT and SPECT-CT camera from Philips. It is equipped with two large field of view γ-ray digital Anger detectors and with an X-ray flat panel detector. All primary and some secondary NEMA NU1-2001 performance tests were conducted on four (two SPECT and two SPECT-CT) recently installed Brightview systems mainly with 99mTc and low energy high resolution (LEHR) collimators. Intrinsic uniformity for four other isotopes (67Ga, 123I, 131I, 111In) and system uniformity with medium (ME) and high (HE) energy collimators were also measured. The measured parameters were almost identical for all the tested systems and the vast majority of measurements were within the manufacturer’s specifications. The only exception was some of the uniformity indexes of two of the heads but a new calibration solved that particular problem. System alignment could not be processed using the NEMA performance tests due to a file writing problem when performing 360° rotation for each head. However, the Philips COR test was conducted and found to be satisfactory.

Implementation of angular response function modeling in SPECT simulations with GATE

Among Monte Carlo simulation codes in medical imaging, the GATE simulation platform is widely used today given its flexibility and accuracy, despite long run times, which in SPECT simulations are mostly spent in tracking photons through the collimators. In this work, a tabulated model of the collimator/detector response was implemented within the GATE framework to significantly reduce the simulation times in SPECT. This implementation uses the angular response function (ARF) model. The performance of the implemented ARF approach has been compared to standard SPECT GATE simulations in terms of the ARF tables' accuracy, overall SPECT system performance and run times. Considering the simulation of the Siemens Symbia T SPECT system using high-energy collimators, differences of less than 1% were measured between the ARF-based and the standard GATE-based simulations, while considering the same noise level in the projections, acceleration factors of up to 180 were obtained when simulating a planar 364 keV source seen with the same SPECT system. The ARF-based and the standard GATE simulation results also agreed very well when considering a four-head SPECT simulation of a realistic Jaszczak phantom filled with iodine-131, with a resulting acceleration factor of 100. In conclusion, the implementation of an ARF-based model of collimator/detector response for SPECT simulations within GATE significantly reduces the simulation run times without compromising accuracy.

Acquisition and Processing Methods for a Bedside Cardiac SPECT Imaging System

We have developed a compact and mobile gamma imaging system that is designed for both PET and SPECT acquisitions within a bedside environment, such as an intensive care unit, surgical suite, or emergency room. This paper describes the SPECT performance evaluation of the system for cardiac imaging, including both Tc-99m and F-18 radionuclides. For cardiac F-18 imaging in a bedside environment, the 511 keV SPECT approach using high energy collimation and anterior 180 degree acquisition has the advantages of not requiring a detector to be positioned beneath the bed and greatly reducing attenuation effects. In this study, we evaluated SPECT reconstructed image quality using an anthropomorphic dynamic cardiac phantom filled with Tc-99m and F-18 separately. The specific issues that were addressed in this study included the following: 1) mitigating septal penetration effects with the 511 keV photons using de-convolution filtering, 2) reducing truncation effects due to the relatively small field-of-view of the camera using angle-dependent center-of-rotation shifts, and 3) investigating the effects of reduced angular sampling and angular range. We found that reducing angular sampling to 28 projections through 135 degrees with Tc-99m and 25 projections through 180 degrees with F-18 showed all features of the myocardium.

シールド装着状態のシリンジ製剤を用いたエネルギースペクトル収集法の検討

I previously reported on a spectrum sampling method with shielded syringes before use, although the report included only data obtained using technetium-99m. In this study, we sampled the energy spectrum in a similar manner using thallium-201, iodine-123, and gallium-67. In spectrum sampling, a radioisotopic source in a cylindrical shield is located midway between two opposed gamma-camera detectors equipped with collimators. An unshielded syringe before use emits excessive radiation and makes count rates too high to obtain accurate photopeak values. With a shielded syringe, we can sample the spectrum of radiation leaked from the needle side of the syringe and the unshielded part of its plunger side. Consequently, the detectors are exposed to lower-dose gamma rays and probably offer count rates appropriate to measure accurate photopeak values. The study results show the general validity of spectrum sampling and photopeak acquisition in our method. However, a syringe should be located accurately perpendicular to each detector; otherwise, gamma rays did not reach the detectors in some cases, resulting in measurement failures. In addition, when low-energy collimators are used for sampling from (123)I sources, photopeak values depend on penetration. More accurate measurements require the use of high-energy collimators.

Comparison of Medium- and High-Energy Collimators for 131I-Tositumomab Dosimetry

Residence time measurements obtained by serial whole-body conjugate-view imaging are commonly used in patient-specific dosimetry for radioimmunotherapy applications. In order to determine the effect of collimator selection on residence time measurements for (131)I, the accuracies of (131)I half-life measurements obtained with multiple gamma-camera and collimator combinations were investigated. Serial anterior and posterior whole-body images were acquired over a period of 15 d with 4 different gamma-cameras and medium- or high-energy collimators. Background-corrected geometric mean counts from the images were fitted to a monoexponential curve to determine the half-life of (131)I obtained with the different gamma-camera and collimator combinations. An average half-life of 8.15 d (SD, 0.07 d) was obtained with all gamma-camera and collimator combinations. A half-life of 8.12 d (SD, 0.11 d) was obtained with the high-energy collimators, and a half-life of 8.18 d (SD, 0.04 d) was obtained with the medium-energy collimators. These values are all very close to the (131)I physical half-life of 8.02 d and were not found to be statistically significantly different (P = 0.44). Similar results were obtained for the half-life obtained with single-head gamma-camera configurations (mean half-life, 8.15 d; SD, 0.12 d). The therapeutic (131)I-tositumomab dose resulting from the differences in the measured half-life ranged from 2.58 to 2.6 GBq (69.8-70.4 mCi). There is no significant difference in (131)I half-life and residence time measurements obtained with medium- or high-energy collimators in dual-head or single-head imaging configurations.

Recovery of total I-131 activity within focal volumes using SPECT and 3D OSEM

We experimentally investigated the SPECT recovery of I-131 activity in multiple spheres located simultaneously at different locations within a cylindrical phantom that had an elliptical cross section. The sphere volumes ranged from 209 cc down to 4.2 cc. A Prism 3000 camera and two types of parallel-hexagonal-hole collimation were employed: high energy (HE) and ultra high energy (UHE). Using appropriately-different 3D models of the point source response function for the two types of collimation, approximately the same recovery of activity could be achieved with either collimation by 3D OSEM reconstruction. The recovery coefficient was greater with no background activity in the phantom by 0.10, on average, compared to that with background. In the HE collimation case, the activity recovery was considerably better for all volumes using 3D OSEM reconstruction than it had been in the past using 1D SAGE reconstruction. Recovery-coefficient-based correction in a simulated patient case involving spherical tumours moderately improved the activity estimates (average error reduced from 14% to 9% for UHE collimation, and from 15% to 11% for HE collimation). For a test case with HE collimation, increasing the projection-image sampling density while decreasing the image voxel size increased the recovery coefficient by 0.075 on average, and, if used in a full set of calibration measurements of recovery coefficient versus volume, might lead to further improvement in accuracy for the patient case.

A design strategy for suppression of hole-pattern artifacts in high-energy collimators

Multihole collimators are the most important piece of instrumentation in determining the tradeoff between spatial resolution and noise in nuclear medicine images. Recently, we found that the collimator-to-detector gap greatly influences the hole-pattern artifacts. In addition, we also showed that specific values of the gap exist which minimize the artifacts: here we call this phenomenon "penumbral masking" (PM). In this paper, we study the matter in greater detail and we take PM into account in the problematic context of designing high-energy collimators by, basically, substituting the conventional hole-array pattern constraint with a mathematical expression of the PM effect. With this approach we found that a solution to the problem of maximizing sensitivity for a given spatial resolution always exists while, without the PM concept, in the case of lead (Pb), a solution exists below 300 KeV, a suboptimal solution can be found within the 300-320 KeV range and no solution at all exists beyond 320 KeV. Thus, one is able to minimize the hole-pattern artifacts while designing high -energy collimators even if a somewhat lower sensitivity has to be accepted: for example, with lead, at 511 KeV, sensitivity is reduced by 20% compared to what it would be without the introduction of PM. When high energies are involved, even with our approach the weight constraint is a major problem, but this does not affect the capability of reducing the hole-pattern artifacts. However, even when the theoretical solution may not be feasible, this approach clarifies the design of high-energy collimators.

Comparison of 3-D OSEM versus 1-D SAGE for focal total-activity quantification in I-131 SPECT with HE collimation

A three-dimensional (3-D) ordered-subset expectation maximization (OSEM) reconstruction algorithm used fits to the average behavior of the I-131 point-source-response function at different depths to model the detector-collimator response of a SPECT camera. The fitting function was a Gaussian, plus a single exponential to account for septal-penetration of the high-energy (HE) collimation. Results from that unregularized 3-D OSEM were compared to those with regularized one-dimensional (1-D) space-alternating generalized expectation maximization (SAGE). The latter used a simple strip integral. Each reconstruction algorithm employed attenuation correction and scatter compensation. The SPECT camera was calibrated by imaging a 200 cm/sup 3/ sphere containing a known I-131 activity. This sphere was located inside an elliptical cylinder. The water in the cylinder contained one of four I-131 activity concentrations. The ratio of background-over-target activity concentration b was used to characterize the different situations. The known activity and the reconstructed counts within the sphere at each b value yielded a counts-to-activity conversion factor, CF. After reconstruction with 3-D OSEM, CF varied much less with b than after reconstruction with 1-D SAGE. A test of activity quantification was carried out with 20 realizations of an anthropomorphic phantom with zero background activity. This phantom simulated a 100 cm/sup 3/ spherical tumor centrally located inferior to the lungs. The mean bias in the estimate of "tumor" activity was -7.3% with 3-D- OSEM-based quantification and -4.3% with 1-D-SAGE-based quantification. Recovery-coefficient-based correction was applied with both algorithms. With UHE collimation, the previously-published mean bias in the estimate of the "tumor" activity was -5.2% after adding a recovery-coefficient-based correction. For that collimation, the previously-published mean bias with 1-D-SAGE-based quantification was -10.7%. We conclude that 1) two-component fitting allowed 3-D OSEM to do a good job of compensating for the septal penetration of the HE collimation, 2) the procedure for activity quantification with 3-D OSEM was simpler than that with 1-D SAGE, and 3) activity accuracy for a test phantom had a similar quality for four situations, using 3-D OSEM-based activity quantification and 1-D-SAGE-based activity quantification and combining each with HE and UHE collimation.

Determining total I-131 activity within a VoI using SPECT, a UHE collimator, OSEM, and a constant conversion factor

Accurate determination of activity within a volume of interest is needed during radiopharmaceutical therapies. Single-photon emission computed tomography(SPECT) is employed but requires a method to convert counts to activity. We use a phantom-based conversion; that is, we image an elliptical cylinder containing a sphere that has a known amount of 131-I activity inside. The regularized space alternating generalized expectation (SAGE) algorithm employing a strip-integral detector-response model was employed for reconstruction in previous patient evaluations. With that algorithm and a high-energy collimator, the estimates for sphere activity varied with changes in: 1) the level of uniform background activity in the cylinder; 2) the image resolution due to different values of the radius of rotation R; and 3) the volume of the sphere. When one used those to convert reconstructed counts within a patient tumor into an activity estimate, the resultant value may have been in error because of patient-phantom mismatch. As a potential remedy, in this paper, we use an ordered subsets expectation maximization (OSEM) algorithm with a 3-D depth-dependent detector-response model and an ultra-high-energy collimator. Results after 100 OSEM iterations and using a maximum counts registration show the estimates for sphere activity: 1) have a dependence on the level of background activity with a slope whose absolute magnitude is typically only 0.37 times that with SAGE; 2) are independent of R; and 3) are independent of sphere volume down to and including a sphere volume of 20 cm/sup 3/. We conclude that using a global-average conversion factor to relate counts to activity and no volume-based correction might be reasonable with OSEM. For a test of that conclusion, target activity is estimated for an anthropomorphic phantom containing a 100 cm/sup 3/ spherical tumor centrally located inferior to the lungs. With OSEM-based quantification, using: 1) a global-average conversion factor and 2) no volume-based correction, mean bias in the simulated-tumor activity estimate over 20 realizations is -7.37% (relative standard deviation =5.93%). With SAGE-based quantification using: 1) the conversion factor corresponding to the experimental estimate of background and 2) volume-based correction, the mean bias is -10.7% (relative standard deviation =2.37%). The mean bias is smaller in a statistically significant way and relative standard deviation is not more than a factor of 2.5 bigger with OSEM compared to SAGE. In addition, with OSEM, a patient image apparently shows more highly resolved features, and the activity estimates for two tumors are increased by an average of 10%, relative to results with SAGE.

P11. Evaluation of a prototype dynamic line phantom attachment for the assessment of medium and high energy collimators

P11. Evaluation of a prototype dynamic line phantom attachment for the assessment of medium and high energy collimators

Comparison of 99mTc-sestamibi-18F-fluorodeoxyglucose dual isotope simultaneous acquisition and rest-stress 99mTc-sestamibi single photon emission computed tomography for the assessment of myocardial viability.

Dual isotope simultaneous acquisition single photon emission computed tomography (DISA SPECT) offers the advantage of obtaining information on myocardial perfusion using Tc-sestamibi ( Tc-MIBI) and metabolism using F-fluorodeoxyglucose ( F-FDG) in a single study. The prerequisite is that the Tc-MIBI images are not degraded by scattered 511 keV photons or poor count statistics due to the lower efficiency of the extra high energy (EHE) collimator. Therefore, we compared the registered Tc-MIBI uptake and image quality of DISA and single isotope acquisition. Furthermore, we investigated whether DISA yields additional information for the assessment of myocardial viability in comparison with rest-stress Tc-MIBI. Nineteen patients with known coronary artery disease and irreversible perfusion defects on previous rest-stress MIBI test studies were investigated. After oral glucose loading and simultaneous injection of 600 MBq of Tc-MIBI and 185 MBq of F-FDG at rest, DISA was performed using energy windows of 140 (+/-15%), 170 (+/-20%) and 511 keV (+/-15%). Planar 140 keV images were corrected for scatter by subtraction using the 170 keV window. The single and dual isotope Tc-MIBI images were both displayed in a polar map with 128 segments normalized to maximum counts. F-FDG and Tc-MIBI images were visually scored for a perfusion-metabolism mismatch pattern using nine regions per heart. There was an excellent correlation (r =0.93, P<0.0001) between the Tc-MIBI uptake detected in the single and dual isotope acquisition. The average difference between the dual and single isotope Tc-MIBI uptake was -1.2% (not significantly different from zero) and the coefficient of variation of the difference was 8.7%. Of the 79 regions with irreversible perfusion defects on previous rest-stress Tc-MIBI, six regions in five patients showed a perfusion-metabolism mismatch pattern. We conclude that DISA does not affect the quality of the Tc-MIBI images. Furthermore, F-FDG- Tc-MIBI DISA may show viability in a small but significant (7.6%, P<0.0034) number of regions with irreversible perfusion defects on rest-stress Tc-MIBI.

Study of high-energy multihole collimators

Recently, a general theoretical description of multihole collimators has been proposed by one of the authors. In this work, the theory was used to study the artifacts, which are caused by the hole-array pattern of high-energy collimators. The relevant outcomes of the study were that the response to a sheet source does not depend on its distance from the collimator, whereas the response depends on the gap between the collimator and the image plane that is located in the scintillation crystal; the dependence of hole-array artifacts on this gap was assessed and the existence of optimal values for which they are minimized was found. A large field of view camera, equipped with a high-energy general purpose collimator for /sup 131/I with hexagonal holes placed in an hexagonal lattice, was used. A series of images of a flood source placed on the external face of the collimator was acquired. Each image was acquired with a different value of the crystal-collimator distance. The analysis of the images clearly showed the existence of optimal crystal-collimator distances to reduce the hole-array artifacts. These distances have been found in agreement with the values predicted by the theory. This work may be useful for the design of new collimators, for the optimal arrangement of camera-collimator system and it may also foster the study of some artifacts compensation methods.

The role of hybrid cameras in oncology

The rapid advances in imaging technologies are a challenge for nuclear medicine physicians, radiologists, and clinicians who must integrate these technologies for optimal patient care and outcome at minimal cost. Multiple indications for functional imaging using F-18-fluorodeoxyglucose (FDG) are now well accepted in the field of oncology, including differentiation of benign from malignant lesions, staging malignant lesions, detection of malignant recurrence, and monitoring therapy. The use of FDG imaging was first shown using dedicated positron emission tomography (PET) with multiple full rings of bismuth germanate detectors. Most manufacturers now have available hybrid gamma cameras capable of imaging conventional single-photon emitters, as well as positron emitters such as FDG. This new technology was developed to make FDG imaging more widely accessible, first using single photon emission computed tomography (SPECT) with high-energy collimators, and then using dualhead coincidence (DHC) detection with multihead gamma cameras that improved spatial resolution. Most hybrid gamma cameras are now equipped with thicker NaI(TI) crystals to improve sensitivity. Technical developments are still evolving with correction for attenuation and new iterative reconstruction algorithms to improve the quality of the images. Users need to be familiar with the rapid developments of the technology as well as its limitations. Currently, one model of hybrid gamma camera is equipped with an integrated x-ray transmission system for attenuation correction, anatomic mapping, and image fusion. This powerful tool has promising clinical applications including intensity-modulated radiation therapy.