Medical Imaging Detection Research Articles

SPECT electronic collimation resolution enhancement using chi-square minimization

An electronic collimation technique is developed which utilizes the chi-square goodness-of-fit measure to filter scattered gammas incident upon a medical imaging detector. In this data mining technique, Compton kinematic expressions are used as the chi-square fitting templates for measured energy-deposition data involving multiple-interaction scatter sequences. Fit optimization is conducted using the Davidon variable metric minimization algorithm to simultaneously determine the best-fit gamma scatter angles and their associated uncertainties, with the uncertainty associated with the first scatter angle corresponding to the angular resolution precision for the source. The methodology requires no knowledge of materials and geometry. This pattern recognition application enhances the ability to select those gammas that will provide the best resolution for input to reconstruction software. Illustrative computational results are presented for a conceptual truncated-ellipsoid polystyrene position-sensitive fibre head-detector Monte Carlo model using a triple Compton scatter gamma sequence assessment for a Tc point source. A filtration rate of 94.3% is obtained, resulting in an estimated sensitivity approximately three orders of magnitude greater than a high-resolution mechanically collimated device. The technique improves the nominal single-scatter angular resolution by up to approximately 24 per cent as compared with the conventional analytic electronic collimation measure.

Image quality evaluation of YVO 4:Eu phosphor screens for use in x-ray medical imaging detectors

YVO 4:Eu is a phosphor material emitting red light (621 nm) and providing excellent spectral compatibility with red sensitive optical detectors. However, it has never been used in medical X-ray imaging. In this study the imaging performance of six laboratory prepared YVO 4:Eu test screens with coating weights from 30 to 140 mg/cm 2 was examined by evaluating the modulation transfer function (MTF), the X-ray luminescence efficiency (XLE), and the detective quantum efficiency (DQE). DQE was evaluated by employing a method based on XLE and emission spectra measurements. The 30 mg/cm 2 screen had a significantly higher MTF than thicker screens while its zero frequency DQE peak value was 0.27 at 30 kVp. Results obtained indicate that YVO 4:Eu may be of value for use in X-ray imaging detectors.

Position sensitive detectors for medical imaging

The application of position sensitive radiation detectors in medical imaging is fundamental to improvements in image quality and quantification that underwrites the search for better diagnosis and treatment. Developments in particle physics, nuclear physics and astronomy could have a profound impact on medical imaging by overcoming the limitations of existing technology. The advent of new scintillators and semi-conductors allied to the enormous advances in micro-electronics and microprocessing should lead to substantial improvements in autoradiography, digital radiography, positron emission tomography, single photon emission tomography and other imaging methods relevant to medicine. In this paper we discuss recent developments which may be relevant to improved medical imaging on a broad front.

Improved performance of GaAs radiation detectors with low ohmic contacts

Gallium arsenide (GaAs) offers an attractive choice for room temperature Xand /spl gamma/-ray detectors. However performance of SI LEC bulk GaAs detectors is at present limited by the short carrier lifetime and by the diode breakdown occurring as soon as the electric field reaches the back ohmic contact. We have shown that ohmic contacts based on ion implantation allowed us to go far beyond the bias voltage necessary to achieve a fully active detector. However the conventional thermal treatments (850/spl deg/C, 30 s) required to anneal the damage induced by ion implantation strongly reduces the charge carrier lifetime in the detector. Alenia S.p.A. has developed two improved processes (RA and RE) which avoid high temperature annealing and the consequent charge carrier lifetime reduction. With the new detectors, in pixel (200/spl times/200 /spl mu/m/sup 2/) configuration, a charge collection efficiency (cce) of 90% for 59.5 keV X-rays and a FWHM of 3.35 keV have been achieved at room temperature. These features, thickness, applied voltage, cce and FWHM are suitable for application of GaAs pixel detectors in medical imaging. Results obtained with /spl alpha/ particles and X-rays at different temperatures and in a wide range of applied bias in detectors made with standard, implanted and improved processes are presented, compared and discussed.

Data acquisition for a medical imaging MWPC detector

Multiwire proportional chambers, combined with drilled Pb converter stacks, are used as position sensitive gamma-ray detectors for medical imaging at Queen's University. This paper describes novel features of the address readout and data acquisition system. To obtain the interaction position, induced charges from wires in each cathode plane are combined using a three-level encoding scheme into 16 channels for amplification and discrimination, and then decoded within 150 ns using a lookup table in a 64 Kbyte EPROM. A custom interface card in an AT-class personal computer provides handshaking, rate buffering, and diagnostic capabilities for the detector data. Real-time software controls the data transfer and provides extensive monitor and control functions. The data are then transferred through an Ethernet link to a workstation for subsequent image analysis.

Applications of a-Si:H radiation detectors

Detection of various kinds of radiation (charged particles, X-ray and γ ray) with hydrogenated amorphous silicon (a-Si:H) layers is discussed. Some applications of a-Si:H radiation detectors in high energy physics, medical imaging, materials and life sciences are described in this paper.

Detection of Charged Particles in Thick Hydrogenated Amorphous Silicon Layers

We show our results in detecting particles of various linear energy transfer, including minimum ionizing electrons from a Sr-90 source with 5 to 12 micron thick n-i-p and p-i-n diodes. We measured W ( average energy to produce one electron-hole pair) using 17keV filtered xray pulses with a result W = 6.0 /+-/ 0.2eV. This is consistent with the expected value for a semiconductor with band gap of 1.7 to 1.9eV. With heavily ionizing particles such as 6 MeV alphas and 1 to 2 MeV protons, there was some loss of signal due to recombination in the particle track. The minimum ionizing electrons showed no sign of recombination. Applications to pixel and strip detectors for physics experiments and medical imaging will be discussed. 7 refs., 8 figs.