Iterative reconstruction of SiPM light response functions in a square-shaped compact gamma camera.

Compact gamma cameras with a square-shaped monolithic scintillator crystal and an array of silicon photomultipliers (SiPMs) are actively being developed for applications in areas such as small animal imaging, cancer diagnostics and radiotracer guided surgery. Statistical methods of position reconstruction, which are potentially superior to the traditional centroid method, require accurate knowledge of the spatial response of each photomultiplier. Using both Monte Carlo simulations and experimental data obtained with a camera prototype, we show that the spatial response of all photomultipliers (light response functions) can be parameterized with axially symmetric functions obtained iteratively from flood field irradiation data. The study was performed with a camera prototype equipped with a 30 × 30 × 2 mm3 LYSO crystal and an 8 × 8 array of SiPMs for 140 keV gamma rays. The simulations demonstrate that the images, reconstructed with the maximum likelihood method using the response obtained with the iterative approach, exhibit only minor distortions: the average difference between the reconstructed and the true positions in X and Y directions does not exceed 0.2 mm in the central area of 22 × 22 mm2 and 0.4 mm at the periphery of the camera. A similar level of image distortions is shown experimentally with the camera prototype.

Iterative reconstruction of detector response of an Anger gamma camera.

Statistical event reconstruction techniques can give better results for gamma cameras than the traditional centroid method. However, implementation of such techniques requires detailed knowledge of the photomultiplier tube light-response functions. Here we describe an iterative method which allows one to obtain the response functions from flood irradiation data without imposing strict requirements on the spatial uniformity of the event distribution. A successful application of the method for medical gamma cameras is demonstrated using both simulated and experimental data. An implementation of the iterative reconstruction technique capable of operating in real time is presented. We show that this technique can also be used for monitoring photomultiplier gain variations.

Limits on the spin-dependent WIMP-nucleon cross sections from the first science run of the ZEPLIN-III experiment.

We present new experimental constraints on the WIMP-nucleon spin-dependent elastic cross sections using data from the first science run of ZEPLIN-III, a two-phase xenon experiment searching for galactic dark matter weakly interacting massive particles based at the Boulby mine. Analysis of approximately 450 kg x days fiducial exposure allow us to place a 90%-confidence upper limit on the pure WIMP-neutron cross section of sigma(n)=1.9x10(-2) pb at 55 GeV/c(2) WIMP mass. Recent calculations of the nuclear spin structure based on the Bonn charge-dependent nucleon-nucleon potential were used for the odd-neutron isotopes 129Xe and 131Xe. These indicate that the sensitivity of xenon targets to the spin-dependent WIMP-proton interaction could be much lower than implied by previous calculations, whereas the WIMP-neutron sensitivity is impaired only by a factor of approximately 2.

[Evaluation of ultrasonography effectiveness in prostate cancer detection].

The database of examining 624 patients aged 49-85 were analyzed to define the diagnostic effectiveness of gray-scale, color and power Doppler transrectal ultrasonography (TRUS). Among them there are 473 (75.8%) cancer patients and 151 (24.2%) benign prostate diseases patients. All the patients underwent a transrectal ultrasound examination with multifocal biopsy. The gray-scale, color and power Doppler transrectal ultrasound examination has the diagnostic accuracy of 75,3%. At this the effectiveness of the method increases with the Gleason sum growing.