Prospects for efficient detectors for fast neutron imaging.

A physical model describing in detail the process of fast neutron imaging in luminescent screens is presented. The detection quantum efficiency, luminosity and inherent spatial resolution of the screen were calculated using this model. Properties of transparent and disperse screens were compared. Two imaging systems were suggested to improve the detection efficiency and spatial resolution. A stack consisting of alternating neutron converters and image plates can help in obtaining both high spatial resolution and efficiency. A system containing a screen of special form and a diaphragm can be of use especially in the case of the fan beam.     

Optimized collimators for scintillation cameras.

The use of commerically available tantalum or lead tubes permits convenient fabrication of collimators optimized for specific imaging studies.     

Method for efficient fast spin echo Dixon imaging.

In order to satisfy the Carr-Purcell-Meiboom-Gill (CPMG) condition, echo shift as dictated in fast-spin-echo (FSE)-based Dixon imaging was previously achieved by applying a time shift to the readout gradient and the data acquisition window. Accordingly, interecho spacing is increased, which entails increased image blurring and, in multislice imaging, a significant reduction in the slice coverage for a given imaging time. In this work, a new method is developed by which the echo shift is induced by "sandwiching" in time the readout gradient with a pair of small gradients of equal area and of opposite polarity. While data with non-zero phase shifts between water and fat signals are collected as fractional echoes, no increase in echo spacing is necessary with the modified acquisition strategy, and increased time efficiency is therefore achieved. In order to generate separate water-only and fat-only images in data processing, a set of low-resolution images are first reconstructed from the central symmetric portion (either 128 x 128 or 64 x 64) of the acquired multipoint Dixon data. High-resolution images using all the acquired data, including some partial Fourier-reconstructed images, are then phase demodulated using the phase errors determined from the low-resolution images. The feasibility of the technique is demonstrated using a water and fat phantom as well as in clinical patient imaging.     

An improved method for estimating the heart-to-mediastinum ratio from cardiac sympathetic nerve imaging with low-energy high-resolution collimators

Background

Septal penetration causes underestimation of the heart-to-mediastinum (H/M) ratio in cardiac ¹²³I-metaiodobenzylguanidine (MIBG) imaging with a low-energy high-resolution (LEHR) collimator. We aimed to improve the method of estimating the H/M ratios using the LEHR collimator.

Methods and Results

4 hours after ¹²³I-MIBG injection, 40 patients were imaged successively with the medium-energy (ME) and LEHR collimators using gamma cameras having 3/8-inch crystals. Severe underestimation of the H/M ratios was observed with the LEHR collimator when compared to the ME collimator. Narrowing the energy window width did not reduce the underestimation. Application of ¹²³I-dual-window (IDW) correction using a narrow or wide subwindow reduced the underestimation substantially but not entirely. The H/M ratios estimated from the LEHR images with or without IDW correction were corrected based on their correlations with the ratios estimated from the ME images. This empiric correction removed systematic underestimation, and residual errors were reduced when the H/M ratios after IDW correction were converted using the empiric equation. The conversion equation was successfully applied to the correction of the H/M ratios determined in another 40 patients using a 5/8-inch crystal.

Conclusions

In estimating the H/M ratios using an LEHR collimator, empiric correction combined with IDW correction improves concordance with ME-based values in comparison with empiric correction alone.     

Imaging characteristics of gamma-ray collimators.

Imaging characteristics of 43 commercial gamma-ray collimators for rectillinear scanners are measured. The modulation transfer functions, plane sensitivity, full width at half maximum, depth-of-field, and focal length for each collimator are reported. The products of all five companies examined appear to represent the current state of the art.     

Design and evaluation of two multi-pinhole collimators for brain SPECT

Objective

SPECT is a powerful tool for diagnosing or staging brain diseases such as Alzheimer’s disease (AD) and Parkinson’s disease (PD) but is limited by its inferior resolution and sensitivity. At the same time, pinhole SPECT provides superior resolution and detection efficiency trade-off as compared to the conventional parallel-hole collimator for imaging small field-of-view (FOV), which fits for the case of brain imaging. In this study, we propose to develop and evaluate two multi-pinhole (MPH) collimator designs to improve the imaging of cerebral blood flow and striatum.

Methods

We set the target resolutions to be 12 and 8 mm, respectively, and the FOV at 200 mm which is large enough to cover the whole brain. The constraints for system optimization include maximum and minimum detector-to-center-of-FOV (CFOV) distances of 344 and 294 mm, respectively, and minimal radius-of-rotation (ROR) of 135 mm to accommodate patients’ shoulder. According to the targeted FOV, resolutions, and constraints, we determined the pinhole number, ROR, focal length, aperture acceptance angle, and aperture diameter which maximized the system sensitivity. We then assessed the imaging performance of the proposed MPH and standard low-energy high-resolution (LEHR) collimators using analytical simulations of a digital NCAT brain phantom with ^99mTc-HMPAO/^99mTc-TRODAT-1 distributions; Monte Carlo simulations of a hot-rod phantom; and a Defrise phantom using GATE v6.1. Projections were generated over 360° and reconstructed using the 3D MPH/LEHR OS-EM methods with up to 720 updates. The normalized mean square error (NMSE) was calculated over the cerebral and striatal regions extracted from the reconstructed images for ^99mTc-HMPAO and ^99mTc-TRODAT-1 simulations, respectively, and average normalized standard deviation (NSD) based on 20 noise realizations was assessed on selected uniform 3D regions as the noise index. Visual assessment and image profiles were applied to the results of Monte Carlo simulations.

Results

The optimized design parameters of the MPH collimators were 9 pinholes with 4.7 and 2.8 mm pinhole diameter, 73° acceptance angle, 127 mm focal length, 167 mm ROR for 12 mm and 8 mm target resolution, respectively. According to the optimization results, the detection efficiencies of the proposed collimators were 270 and 40% more as compared to LEHR. The Monte Carlo simulations showed that 7.9 and 6.4 mm rods can be discriminated for the MPH collimators with target resolutions of 12 and 8 mm, respectively. The eight 12 mm-thick discs of the Defrise phantom can also be resolved clearly in the axial plane as demonstrated by the image profiles generated with the MPH collimators.

Conclusion

The two collimator designs provide superior image quality as compared to the conventional LEHR, and shows potential to improve current brain SPECT imaging based on a conventional SPECT scanner.

     

Simple, fast preparation of gallium-68-labelled human serum albumin microspheres.

Following a study of the main factors involved in the 68-Ga labelling of human serum albumin microspheres (H.S.A.M.), especially methods of production and preparation of active solution and conditions of radioelement fixation on the protein support, the practical details of a fast technique (60 min) based on the process described by Hnatowich are presented. This method gives high labelling yields (93 +/- 3%), and after washing of the microspheres leads to a radiopharmaceutical product almost without free 68Ga (less than 2%). The spheres ready for use carry a total radioactivity corresponding to about 35%, including decay, of the activity originally recovered in the generator eluate and to more than 98% of that, found in the final suspension. The labelled product is sterile, non-pyrogenic and non-toxic. When it is injected in animals by left ventrical catheterization the uptake rates in the heart, lungs, spleen, left kidney and right kidney are similar to those observed with reference 85Sr-labelled carbonized microspheres. This radiopharmaceutical, easy to prepare and having excellent biological and nuclear properties, seems ideally suited for the scanning of organs by position emission tomoscintigraphy.     

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. METHODS: 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. RESULTS: 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). CONCLUSION: 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.     

The design of the detector and collimators for a hybrid scanner

The hybrid scanner is a scanning device in which a long crystal with two or more photomultiplier (PM) tubes acts as a gamma camera along the crystal axis; the device acts as a linear scanner in a direction perpendicular to the crystal axis. A detailed analysis of the intrinsic resolution and uniformity is given for a two-PM-tube hybrid scanner (with one PM tube at each end) and the expressions derived should help the designer to choose the best crystal system. Collimation theory is discussed for the general hybrid scanner. Expressions and graphs are given to help in the design of a collimator with the best balance between the conflicting requirements of resolution, sensitivity, depth independence and freedom from artifacts (collimator holes may be seen in the image at high energy). Examples of practical collimators are given for energies of 80-140,364,511,662 and 840 keV.     

Dosimetric assessment of nonperfectly abutted fields using asymmetric collimators.

The abutment of adjacent fields has been facilitated through the use of asymmetric collimators. Conceptually, the abutment yields a perfectly uniform dose distribution across the junction, provided the asymmetric jaw is set precisely at the beam central axis. However, the asymmetric jaw has an associated tolerance, which can cause the abutment to be misaligned. This study examined the dose distribution at the junction of nonperfectly abutted fields. The abutment of fields was carried out using an asymmetric collimation of 5 x 10 cm, with an asymmetric jaw positioned at the beam central axis. A film was initially exposed using this field with the collimator set at 90 degrees. The collimator was then rotated 180 degrees and the same film was exposed for the second time to create the field abutment. Positioning the asymmetric jaw with respect to the beam central axis set the amount of gap and overlap between the abutted fields. The dose distribution was measured for asymmetric jaw positioning of -2, -1, 0, + 1, and +2 mm from the beam central axis. In addition, the dose distribution was also computed mathematically by summing the 2 dose profiles with defined gap or overlap. A field mismatch of +/-1 mm would result in a dose nonuniformity of 17%, and a +/-2 mm mismatch would produce a 35% dose nonuniformity.     

准直器验收测试方法研究

目的单晶体γ相机测试标准中平面系统空间分辨率和灵敏度不能有效反映准直器的质量问题，需寻找更好的方法进行准直器验收测试。方法通过测试２０只平行孔准直器平面系统均匀性，并辅以测试断层均匀性和直接观察准直器内部结构，对准直器进行验收。结果平面系统均匀性是检测准直器的有效指标。对于系统泛源图像出现较明显非均匀区域，或准直器积分均匀性＞３％的准直器为验收不合格。结论系统均匀性测试为准直器验收中最佳方法。     

A standardized multifield irradiation technique for breast tumours using asymmetrical collimators and beam angulation.

Combined angulation and asymmetrical collimator matching techniques are used for field alignment in breast irradiation. A local regional irradiation technique has been developed that allows a uniform dose distribution to the entire target volume. The localization is characterized by simple measurement of a few patient co-ordinates. The equipment settings to produce a theoretical exact field alignment are calculated by a special computer program. These settings guide the simulation. We have designed the method such that the technique is standardized for all patients receiving breast and lymph node irradiation. This allows an easy, accurate and fast patient throughput at the simulator and accelerators.     

The evaluation and calibration of fan-beam collimators

The aims of this study were (a) to determine the true focal length of a fan-beam collimator and (b) to calibrate image size (mm/pixel) for each collimator to permit inter-comparison of image data acquired on different gamma camera systems. A total of six fan-beam collimators on three dual-head gamma camera systems were evaluated using a set of four cobalt-57 point source markers. The markers were arranged in a line in the transverse plane with a known separation between them. Tomographic images were obtained at three radii of rotation. From reconstructed transaxial images the distance between markers was measured in pixels and used to determine pixel size in mm/pixel. The system value for the focal length of the collimator was modified by up to +/-100 mm and transaxial images were again reconstructed. To standardize pixel size between systems, the apparent radius of rotation during a single-photon emission tomography (SPET) acquisition was modified by changes to the effective collimator thickness. SPET images of a 3D brain phantom were acquired on each system and reconstructed using both the original and the modified values of collimator focal length and thickness. Co-registration and subtraction of the reconstructed transaxial images was used to evaluate the effects of changes in collimator parameters. Pixel size in the reconstructed image was found to be a function of both the radius of rotation and the focal length. At the correct focal length, pixel size was essentially independent of the radius of rotation. For all six collimators, true focal length differed from the original focal length by up to 26 mm. These differences in focal length resulted in up to 6% variation in pixel size between systems. Pixel size between the three systems was standardized by altering the value for collimator thickness. Subtraction of the co-registered SPET images of the 3D brain phantom was significantly improved after optimization of collimator parameters, with a 35%-50% reduction in the standard deviation of residual counts in the subtraction images. In conclusion, we have described a simple method for measurement of the focal length of a fan-beam collimator. This is an important parameter on multidetector systems for optimum image quality and where accurate co-registration of SPET to SPET and SPET to MRI studies is required.