Richard Freifelder

Design evaluation of A-PET: A high sensitivity animal PET camera

In recent years it has been shown that PET is capable of obtaining in vivo metabolic images of small animals. These serve as models to study the development and progress of diseases within humans. Imaging small animals requires not only image resolution better than 2 mm, but also high sensitivity in order to image ligands with low specific activity or radiochemical yields. Toward achieving these goals, we have developed a discrete 2 /spl times/ 2 /spl times/ 10 mm/sup 3/ GSO Anger-logic detector for use in a high resolution, high sensitivity, and high count-rate animal PET scanner. This detector uses relatively large 19 mm diameter photomultiplier tubes (PMT), but nevertheless achieves good spatial and energy resolution. The scanner (A-PET) has a port diameter of 21 cm, transverse field-of-view of 12.8 cm, axial length of 11.6 cm, and operates in 3-D volume imaging mode. The absolute coincidence sensitivity is 1.3% for a point source. Due to the use of large PMTs in an Anger design, the encoding ratio (number of crystals/PMT) is high, which reduces the complexity and leads to a cost-effective scanner. Simulation results show that this scanner can achieve high NEC rates for small cylindrical phantoms due to its high sensitivity and low dead-time. Initial measurements show that our design goals for spatial resolution and sensitivity were realized in the prototype scanner.

Dedicated PET scanners for breast imaging.

We have used computer simulations to compare two designs for a PET scanner dedicated to breast imaging with a whole-body PET scanner. The new designs combine high spatial resolution, high sensitivity, and good energy resolution to detect small, low-contrast masses. The detectors are position sensitive NaI(Tl) scintillators. The first design is a ring scanner surrounding the breast and the second consists of two planar detectors placed on opposite sides of the breast. We have employed standard performance measures to compare the different designs: contrast, percentage standard deviation of the background, and signal-to-noise ratios of reconstructed images. The results of the simulations show that both of the proposed designs have better lesion detectability than a whole-body scanner. The results also show that contrast is higher in the ring breast system but that the noise is lower in the planar breast system. Overall, the ring system yields images with the best signal-to-noise ratios, although the planar system offers practical advantages for imaging the breast and axilla.

Optimizing the performance of a PET detector using discrete GSO crystals on a continuous lightguide

The authors are designing a new detector for PET using discrete 4/spl times/4/spl times/10 mm/sup 3/ GSO crystals on a continuous lightguide with 39 mm PMTs. The Light Response Function (LRF) of a detector is the amount of light received by a PMT as a function of the source position. It has to be controlled by a careful design of the lightguide in order to identify 4 mm crystals. The ideal LRF should be narrow with a linear variation over the PMT diameter. Simulations show that a 1.81 cm thick lightguide produces a narrow LRF with good crystal discrimination. However, the tails of this LRF are long. A further improvement can be achieved by using a 2.31 cm lightguide with 5 mm slots cut in its front surface. This results in a sharp edged, almost triangular, LRF. The slotted lightguide also minimizes the spatial dependence on varying depths of interaction of the gamma ray. The effect of varying slot depths was also investigated through the simulations. This was done while keeping the thickness of the lightguide continuous area constant. Experiments were performed and shown to be in general agreement with the simulations. The good spatial resolution and narrow LRF of such a detector will result in a high resolution PET scanner with good count rate capability.

Triple energy window scatter correction technique in PET

A practical triple energy window technique (TEW) is proposed, which is based on using the information in two lower energy windows and one single calibration, to estimate the scatter within the photopeak window. The technique is basically a conventional dual-window technique plus a modification factor, which can partially compensate object-distribution dependent scatters. The modification factor is a function of two lower scatter windows of both the calibration phantom and the actual object. In order to evaluate the technique, a Monte Carlo simulation program, which simulates the PENN-PET scanner geometry, was used. Different phantom activity distributions and phantom sizes were tested to simulate brain studies, including uniform and nonuniform distributions. The results indicate that the TEW technique works well for a wide range of activity distributions and object sizes. The comparisons between the TEW and dual window techniques show better quantitative accuracy for the TEW, especially for different phantom sizes. The technique is also applied to experimental data from a PENN-PET scanner to test its practicality.

Design and performance of the HEAD PENN-PET scanner

A new PET scanner for brain imaging (and animals) has been designed with very high sensitivity and spatial resolution. The design is an evolution of the PENN-PET scanner, which uses large position-sensitive NaI(Tl) detectors, with Anger-type positioning logic, and which allows 3D volume imaging, without septa. The new design is built with a single annular crystal coupled to 180 photomultiplier tubes, and uses local triggering electronics to subdivide the detector into small zones and to determine coincident events within the detector. The axial acceptance angle of /spl plusmn/27 deg, with a field-of-view of 25.6 cm, is larger than any currently operating PET scanner. Performance measurements are presented. >

Comparison of diffuse optical tomography of human breast with whole‐body and breast‐only positron emission tomography

We acquire and compare three-dimensional tomographic breast images of three females with suspicious masses using diffuse optical tomography (DOT) and positron emission tomography (PET). Co-registration of DOT and PET images was facilitated by a mutual information maximization algorithm. We also compared DOT and whole-body PET images of 14 patients with breast abnormalities. Positive correlations were found between total hemoglobin concentration and tissue scattering measured by DOT, and fluorodeoxyglucose (18F-FDG) uptake. In light of these observations, we suggest potential benefits of combining both PET and DOT for characterization of breast lesions.

First results of a dedicated breast PET imager, BPET, using NaI(Tl) curve plate detectors

We present the first imaging results from phantom measurements of a dedicated, breast-only positron emission imager, BPET, using NaI(Tl) Curve Plate detectors. The scanner uses 19 mm thick NaI(Tl) detectors in a split-ring design which surrounds the breast as the woman lies prone and the breast hangs down from the body. Because the detectors are close to the breast and the scanner detects photons that do not pass through the body, system sensitivity and spatial resolution are both optimized. The split ring design provides for flexibility for needle aspirations of masses or alternate viewing orientations. We have measured energy resolution, spatial resolution, scatter fraction, and system sensitivity. We have compared the BPET scanners performance to our clinical whole-body scanner using a breast phantom with hot spheres simulating lesions. The results show that for activity concentrations that correspond to clinical FDG doses, the dedicated scanner has better lesion detectability than the whole-body scanner for the 20 cm detector separation used.

The radiation response of cells from 9L gliosarcoma tumours is correlated with [F18]-EF5 uptake.

Purpose: Tumour hypoxia affects cancer biology and therapy-resistance in both animals and humans. The purpose of this study was to determine whether EF5 ([2-(2-nitro-1-H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide]) binding and/or radioactive drug uptake correlated with single-dose radiation response in 9L gliosarcoma tumours. Materials and methods: Twenty-two 9L tumours were grown in male Fischer rats. Rats were administered low specific activity 18F-EF5 and their tumours irradiated and assessed for cell survival and hypoxia. Hypoxia assays included EF5 binding measured by antibodies against bound-drug adducts and gamma counts of 18F-EF5 tumour uptake compared with uptake by normal muscle and blood. These assays were compared with cellular radiation response (in vivo to in vitro assay). In six cases, uptake of tumour versus muscle was also assayed using images from a PET (positron emission tomography) camera (PENN G-PET). Results: The intertumoural variation in radiation response of 9L tumour-cells was significantly correlated with uptake of 18F-labelled EF5 (i.e., including both bound and non-bound drug) using either tumour to muscle or tumour to blood gamma count ratios. In the tumours where imaging was performed, there was a significant correlation between the image analysis and gamma count analysis. Intertumoural variation in cellular radiation response of the same 22 tumours was also correlated with mean flow cytometry signal due to EF5 binding. Conclusion: To our knowledge, this is the first animal model/drug combination demonstrating a correlation of radioresponse for tumour-cells from individual tumours with drug metabolism using either immunohistochemical or non-invasive techniques.

Performance of a GSO brain PET camera

A high sensitivity, high resolution brain PET scanner has been developed. The scanner comprises 58 rows of 320 4/spl times/4/spl times/10 mm/sup 3/ gadolinium orthosilicate (GSO) crystals, coupled to a continuous light guide that is sampled by 288 39-mm photomultiplier tubes in a hexagonal grid. A distortion removal algorithm has been developed to remove position non-linearities and to identify individual crystal regions in a flood image with physical crystal locations on the scanner. A central profile through a sinogram of a point source in the center of the scanner shows that the spatial resolution (fwhm) is 3.5 mm (fwtm=8.4 mm). The Hoffman brain phantom has been successfully imaged to demonstrate the capability of the scanner.

Reducing edge effects and improving position resolution in position sensitive NaI(TI) detectors

Large two-dimensional position-sensitive NaI(Tl) crystals used in positron emission tomographs and elsewhere normally have gaps or inactive, unusable areas at the edges. Experiments aimed at reducing these edge effects were performed. Unencapsulated crystals were used to test the feasibility of optically coupling crystals together to decrease gap size. Other experiments tried to increase the sampling of the scintillation light at the edges in order to obtain better position sensitivity. Work was also performed to treat the edges to reduce unwanted reflections and increase the position sensitive area. Finally, experiments aimed at improving the position resolution throughout the crystal as well as at the edges were performed.<<ETX>>