W. Bencivelli

EVALUATION OF ELEMENTAL AND COMPOUND SEMICONDUCTORS FOR X-RAY DIGITAL RADIOGRAPHY

Abstract We present a comparative study of the performance of elemental and compound solid state crystals of possible use in X-raY digital radiography. The general purpose EGS4 code was used to simulate photon-electron transport in the energy range 20 to 60 keV. The efficiency and the energy resolution, as a function of X-ray energy, are calculated and correlated to the different physical characteristics of the crystals considered.

Custom breast phantom for an accurate tumor SNR analysis

The capability of the scintimammography to diagnose subcentimeters sized tumors was increased by the employment of a dedicated gamma camera. The introduction of small field of view camera, based on pixellated scintillation array and position sensitive photomultiplier, allowed to enhance the geometric spatial resolution and contrast of the images due to reduced collimator-tumor distance. The aim of this paper is to investigate the realistic possibility of T1a tumors detection (/spl sim/5 mm size) by comparing the signal-to-noise ratio (SNR) values obtained by different imagers. To this end, we have utilized a self-designed solid breast phantom with different sized hot spots (tumors). The phantom consists of seven disks with different thickness, molded from resin epoxy activated with Co/sup 57/ isotope. The overlapped disks represent a pendula breast with about 800 cc volume. Hot spots have not wall. One disk has holes to fit the hot spots representing the different sized lesions. The imagers utilized were: a standard Anger Camera and three different cameras based on scintillator array, CsI(Tl) or NaI(Tl), coupled to position sensitive photomultiplier with different technologies, to make detectors with field of view of 3 and 5 inch. The experimental results are supported by Monte Carlo simulation. It was highlighted how spatial resolution is a predominant element in tumor visibility and how background causes a reduction of the image contrast. All gamma cameras show close results at SNR values less than 10 and a full detectability of 8 mm tumor size. However, the results show the 5 mm tumor size is lower detection limit for all cameras.

First X-ray images with a double-sided microstrips silicon crystal. A novel detector for digital radiography?

In the framework of an experimental program to study the possibility of using a silicon crystal for X-ray detection in the diagnostic energy range (10-100 keV), the authors present the first experimental results obtained with a silicon detector (300 mu m thick, 1.4*1.4 cm2 surface) with microstrips deposited on each side: 25 mu m spacing on the junction side and 50 mu m spacing on the ohmic side: 25 mu m spacing on the junction side and 50 mu m spacing on the ohmic side; the read-out pitch is 100 mu m for both sides. Energy and spatial resolutions have been measured by means of collimated radioactive sources: 241Am (E gamma =60 keV) and 109Cd (E gamma =24 and 88 keV). A very good linearity in the response to the photon energy has been measured. Tantalum wires, i.e. high contrast objects, have been used as phantoms when exposing the detector to the 60 keV photons; the authors present the images thus obtained.

Some new results on semi-insulating GaAs detectors for low energy X-rays

Abstract GaAs is a semiconductor suitable for room temperature X-ray detection but hitherto has suffered from incomplete charge collection and is affected by noise. Recent GaAs detectors made with LEC material have shown improved charge collection efficiency and energy resolution. In this paper we describe the fabrication process and present the results obtained with 80 μm thick pad detectors.

Electrical characterization and detector performances of a LPE GaAs detector for X-ray digital radiography

Abstract The electrical characteristics of a LPE n-type GaAs crystal have been fully measured. The experimental results were used to build a model, including the parametrization of the electric field inside the crystal, and to study the features of such material as a detector in view of a possible application for digital radiography. The predictions of a Monte Carlo based on this model are in very good agreement with the measured performances of this crystal when used as a detector for X-rays in the diagnostic energy range.

Digital imaging in radiology: preliminary results obtained with a high spatial resolution 2D silicon detector

Double-sided microstrip silicon crystals are being tested as detectors for X-rays in the diagnostic energy range (10-100 keV) for digital radiology. An analog-to-digital-converter and CAMAC based acquisition system has been developed to study the imaging capabilities of a silicon microstrip detector with 100 and 200 mu m read-out pitch. The authors present the first images of submillimeter high-contrast phantoms obtained with an X-ray mammography tube operating at high flux density. A preliminary contrast transfer function study was performed; a low frequency contrast of about 0.97 for a high-contrast phantom and a decrease of contrast at a frequency of 5 1p/mm corresponding to the detector intrinsic spatial resolution (100 mu m) were measured. >

Use of the EGS4 Monte Carlo code to evaluate the response of HgI2 and CdTe detectors for photons in the diagnostic energy range

Abstract We present the results of the detailed simulation of the response of a HgI 2 crystal in terms of efficiency, energy and space resolutions versus photon energy in the diagnostic energy range 20–100 keV. Some configurations of CdTe crystals for positron emission tomography are also evaluated.

Characterization of the response of a double side mu -strip silicon detector to X-rays in the diagnostic energy range

The use of a double sided mu -strip silicon crystal for X-ray detection is being investigated. The detector is 300 mu m thick and the read-out pitch is 100 mu m for both sides. It operates in capacitance charge division mode by means of floating strips between read-out strips. The detector has been irradiated by /sup 241/Am and /sup 109/Cd sources. Different zones within the 100- mu m read-out pitch have been individually exposed. The following characteristics have been studied as a function of the impact point of the photon: (a) the charge collection mechanism; (b) the relative detection efficiency; (c) the energy resolution; and (d) the spatial resolution. The absolute efficiency of the detector has been measured at three energy values. >

USE OF EGS4 FOR THE EVALUATION OF THE PERFORMANCE OF A SILICON DETECTOR FOR X-RAY DIGITAL RADIOGRAPHY

Abstract The general-purpose EGS4 code has been used to evaluate the efficiency, the energy and the spatial resolution of a silicon crystal with double-side microstrips readout to detect X-rays in the diagnostic energy range (10–100 keV). A single crystal configuration, a sandwich arrangement with a high- Z converter foil in between two crystals and a multicrystal configuration have been investigated. The simulation with a low energy spectrum X-ray beam and a breast phantom has shown that this type of detector would be of valuable use in mammography applications: breast calcification with a diameter smaller than 500 μm can be easily identified.

Monte Carlo study and experimental measurements of breast tumor detectability with the YAP-PEM prototype

A prototype for positron emission mammography is under development within a collaboration of the Italian Universities of Pisa, Ferrara, Bologna and Roma. The device is composed of two stationary detection heads, each with an active area of 6 cm /spl times/ 6 cm, made of 30/spl times/30 YAP:Ce finger crystals of 2 mm /spl times/ 2 mm /spl times/ 30 mm. The EGSnrc Monte Carlo code has been used to perform a complete simulation of this camera. We have used a fast three-dimensional iterative algorithm (30 s per iteration on a PC-Pentium III 800 MHz processor) for image reconstruction. The performed study indicates that tumors of 5 mm diameter, i.e., 0.065 cm/sup 3/ volume, with 37 kBq/cm/sup 3/ (1 /spl mu/Ci/cm/sup 3/) specific activity embedded in a breast active phantom, are detectable in 10 minutes for a 10:1 tumor/background ratio with an 8.7 Signal-to-Noise Ratio value. Experimental measurements with the small animal tomograph YAP-PET have validated the Monte Carlo predictions.