Direct conversion of x-ray energy into electrical charge has been extensively developed into imaging products in the past few years. Applications include general radiography, mammography, x-ray crystallography, portal imaging, and non-destructive testing. Direct methods avoid intermediate conversion of x-rays into light prior to generating a measurable electrical charge. This eliminates light scattering effects on image sharpness, allowing detectors to be designed to the limit of the theoretical modulation transfer function for a discrete-pixel sensor. Working exposure range can be customized by adjusting bias and thickness of sensor layers in coordination with readout-electronics specifications. Mature amorphous selenium technology and recent progress on high-quality Thin-Film Transistor (TFT) arrays for computer displays have allowed development of practical large-area high-resolution flat-panel x-ray imaging systems. A variety of design optimizations enable direct-conversion technology to satisfy a wide range of applications.

At RMD we have fabricated structured CsI(Tl) screens tailored for macromolecular x-ray crystallography applications. Diffraction patterns typically consist of several closely spaced Bragg peaks of varying sizes and intensities, and the detection of such features requires screens with high light output, high resolution, and excellent x-ray absorption. Properties of these screens, for example, light output or spatial resolution, were tailored by post deposition treatments to suit the specific needs of the application. Specifically, we have produced up to 45 micrometers thick CsI(Tl) screens with excellent resolution over the spatial frequency range of 0 to 20 lp/mm and very low noise. Imaging characteristics of these screens along with the commercial Gd₂O₂S (GOS) have been measured using a CCD detector with a fiberoptic taper. Performance of these screens in terms of point spread function (PSF(f)), light output, noise power spectrum (NPS(f)), and the modulation transfer function (MTF(f)) was measured. It is observed that the intrinsic properties of the structured CsI(Tl) screens are heavily influenced by the substrate on which the films are deposited and on the post deposition coatings, thus providing a latitude for modifying the screen properties to match the needs of the application.

New results for polycrystalline HgI2 detectors are reported here. Due to its decent electrical properties and high stopping power for X-rays and gamma rays, HgI₂ is a good candidate for many medical imaging applications. HgI₂ were deposited by a hot wall Physical Vapor Deposition (PVD) method, and the electrical properties of the films, including X-ray response and dark current data are reported. Results of imaging capabilities and spatial resolution obtained by polycrystalline HgI₂ deposited onto a 2'x2' TFT imaging array on an amorphous silicon substrate are also given. These tests were carried out at Xerox-PARC Research Center.

Mercuric iodide (HgI₂) polycrystalline films are being developed as a new detector technology for digital x-ray imaging. Films have been grown with areas up to 80 cm² (4' diameter) and thickness of 20-250 micrometers using sublimation. The growth techniques used can be easily extended to produce much larger film areas (>10'x10'). Thickness of the grown layers and size of the grains can be regulated over a wide range by adjusting the growth parameters. The films were characterized with respect to their electrical properties and in response to ionizing radiation. Leakage current as low as 40 pA/cm² at the operating bias voltage of ~50 V has been observed. High sensitivity and excellent linearity in the response to x-rays was measured. Signals from these HgI₂ polycrystalline detectors, in response to ionizing radiation, compare favorably to the best published results for all high Z polycrystalline films grown elsewhere, including TlBr, PbI₂ and HgI₂. The low dark current, good sensitivity, and linearity of the response to x-rays put HgI₂ polycrystalline semiconductor detectors in position as a leading candidate material for use in digital x-ray imaging systems. Our future efforts will concentrate on optimization of film growth techniques specifically for deposition on a-Si:H flat panel readout arrays.

This article is divided into two parts: the first is an opinion, the second is a description. The opinion is that diagnostic medical imaging is not a detection problem. The description is of a specific medical image-processing program. Why the opinion? If medical imaging were a detection problem, then image processing would unimportant. However, image processing is crucial. We illustrate this fact using three examples ultrasound, magnetic resonance imaging and, most poignantly, computed radiography. Although the examples are anecdotal they are illustrative. The description is of the image-processing program ImprocRAD written by one of the authors (Dallas). First we will discuss the motivation for creating yet another image processing program including system characterization which is an area of expertise of one of the authors (Roehrig). We will then look at the structure of the program and finally, to the point, the specific application: mammographic diagnostic reading. We will mention rapid display of mammogram image sets and then discuss processing. In that context, we describe a real-time image-processing tool we term the MammoGlass.

A low-noise CCD coupled to a structured CsI(Tl) scintillator via a fiber optic plate was operated as X-ray detector for digital mammography. The imaging capabilities of the device were measured in terms both of spatial resolution (MTF) and of noise properties (DQE, NPS). The detector was characterized using a standard mammographic tube with and without the coupling to a pair of anti-scatter projective grids. Breast phantom images were collected and compared to a MonteCarlo simulation of the apparatus. Contrast enhancement was achieved by using the anti-scatter grids.

Semitransparent HOPG monochromators are being investigated with an emphasis on their potential applications in X-ray imaging systems. HOPG plates and films, 5-100micrometers thick and up to 14 cm² in area, were tested. Scanning techniques were used to measure the profiles of local reflectivity and transparency at different monochromator thicknesses. Mosaic spread, (Delta) (omega) , down to 3.6' was recorded. HOPG film areas with average (Delta) (omega) =5' and 6' were equivalent to 4.5 and 10 cm² respectively. The echelon monochromator arrangement produced a sharp increase in reflectivity leading to the possibility of primary beam sweeping. Results to date are very promising for future applications in fields such as medical diagnostics and surgical treatments monitored using digital X-ray cameras.

The cerebellum of the weanling piglet (Yorkshire) was used as a surrogate for the radiosensitive human infant cerebellum in a Swiss-led program of experimental microbeam radiation therapy (MRT) at the ESRF. Five weanlings in a 47 day old litter of seven, and eight weanlings in a 40 day old litter of eleven were irradiated in November, 1999 and June, 2000, respectively. A 1.5 cm-wide x 1.5 xm-high array of equally space approximately equals 20-30 micrometers wide, upright microbeams spaced at 210 micrometers intervals was propagated horizontally, left to right, through the cerebella of the prone, anesthetized piglets. Skin-entrance intra-microbeam peak adsorbed doses were uniform, either 150, 300, 425, or 600 gray (Gy). Peak and inter-microbeam (valley) absorbed doses in the cerebellum were computed with the PSI version of the Monte Carlo code GEANT and benchmarked using Gafchromic and radiochromic film microdosimetry. For approximately equals 66 weeks [first litter; until euthanasia], or approximately equals 57 weeks [second litter; until July 30, 2001] after irradiation, the littermates were developmentally, behaviorally, neurologically and radiologically normal as observed and tested by experienced farmers and veterinary scientists unaware of which piglets were irradiated or sham-irradiated. Morever, MRT implemented at the ESRF with a similar array of microbeams and a uniform skin-entrance peak dose of 625 Gy, followed by immunoprophylaxis, was shown to be palliative or curative in young adult rats bearing intracerebral gliosarcomas. These observations give further credence to MRT's potential as an adjunct therapy for brain tumors in infancy, when seamless therapeutic irradiation of the brain is hazardous.

Sentinel lymph node biopsy has been shown to be highly accurate for detecting metastatic diseases, such as melanoma and breast cancer. Gamma probes that measure only the relative presence of radioactivity are commonly used to identify sentinel lymph nodes. We have developed a small semiconductor gamma camera (SSGC) that allows the size, shape, and location of the target tissues to be visualized. The purpose of this study is to characterize the performance of the SSGC for radioguided surgery of metastatic lesions and for diagnosing other diseases amenable to the smaller- format associated with this prototype imaging system. Methods & Design: The detector head was comprised of a 32 x 32 Cadmium Telluride semiconductor array and application- specific integrated circuit (ASIC) with a tungsten collimator. The entire assembly was encased in a lead housing measuring 152 mm x 166 mm x 65 mm. The effective visual field was 44.8 mm x 44.8 mm. Two spherical 5 mm diameter Tc-99m radioactive sources having activities of 0.15 MBq and 100 MBq were used to simulate sentinel lymph nodes and injection site. The relative detectability of these foci was compared using the new detector and a conventional scintillation camera. Use of the prototype was also explored on patients in a variety of clinical applications. Results: the SSGC provided better spatial resolution on phantom studies than the conventional gamma camera control. Both foci could be visualized clearly by the SSGC using a 15 second acquisition time, whereas they could not be readily identified using the conventional system under comparable conditions. Preliminary clinical tests of the SSGC were found to be successful in imaging diseases in a variety of tissues including salivary and thyroid glands, temporomandibular joints, and sentinel lymph nodes. Conclusion: The SSGC has significant potential for use in diagnosing diseases and for facilitating subsequent radioguided surgery. (This project was supported by a Grant- in-aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan).

Coronary angiography is unable to define the status of the atheroma, and only measures the luminal dimensions of the blood vessel, without providing information about plaque content. Up to 70% of heart attacks are caused by minimally obstructive vulnerable plaques, which are too small to be detected adequately by angiography. We have developed an intravascular imaging detector to identify vulnerable coronary artery plaques. The detector works by sensing beta or conversion electron radiotracer emissions from plaque-binding radiotracers. The device overcomes the technical constraints of size, sensitivity and conformance to the intravascular environment. The detector at the distal end of the catheter uses six 7mm long by 0.5mm diameter scintillation fibers coupled to 1.5m long plastic fibers. The fibers are offset from each other longitudinally by 6mm and arranged spirally around a guide wire in the catheter. At the proximal end of the catheter the optical fibers are coupled to an interface box with a snap on connector. The interface box contains a position sensitive photomultiplier tube (PSPMT) to decode the individual fibers. The whole detector assembly fits into an 8-French (2.7 mm in diameter) catheter. The PSPMT image is further decoded with software to give a linear image, the total instantaneous count rate and an audio output whose tone corresponds to the count rate. The device was tested with F-18 and Tl-204 sources. Spectrometric response, spatial resolution, sensitivity and beta to background ratio were measured. System resolution is 6 mm and the sensitivity is >500 cps / micrometers Ci when the source is 1 mm from the detector. The beta to background ratio was 11.2 for F-18 measured on a single fiber. The current device will lead to a system allowing imaging of labeled vulnerable plaque in coronary arteries. This type of signature is expected to enable targeted and cost effective therapies to prevent acute coronary artery diseases such as: unstable angina, acute myocardial infarction, and sudden cardiac death.

A CCD imaging system has been developed for detecting and imaging the beta/X-ray emissions from radiolabelled samples, principally for use in autoradiography. By using a novel frame-by-frame acquisition method, quantitative images of ¹⁴C-deoxyglucose distribution in a mouse brain have been produced with a spatial resolution of ~35micrometers under cooled conditions. The energy resolution of the cooled device has been measured using X-rays from ²⁴¹Am, and found to be 0.5keV at 17.5 keV. We describe the problems associated with using a CCD at room temperature for radiation detection and imaging. To address these we have developed a simple histogram shift for dark current fixed pattern noise. We have also developed an image restoration method based on simulated annealing using CCD-specific models for the noise and the data. Applying both techniques to images of 20micrometers ¹⁸F labeled fibers obtained at room temperature yields FWHM measurements of ~85micrometers and ~39micrometers respectively.

The performance of solid-state photodetectors is limited by noise due to their capacitance and leakage current. A new type of photodetector is being investigated, which contains a micro-avalanche multiplication gain stage incorporated into the small anode structure of a silicon drift photodetector (SDP). This technology is expected to result in improved performance over large area avalanche photodiodes (APD's) because of the very small region of multiplication in the new A+SDP versus multiplication over the entire active area for APD's. APD reliability has generally deteriorated as a function of the size of the devices being manufactured. The A+SDP will be markedly better than PIN diodes because of both the low capacitance and the avalanche multiplication. The device also promises to be better than standard large area Silicon Drift Photodetectors (SDP's) by mitigating the remaining noise due to the leakage current that dominates the performance of these devices at room temperature. Large area SDP's require cooling to well below 0 degree(s)C to obtain satisfactory leakage current-related noise. Physical device simulation tools were used to model the dopant concentrations, E-field magnitude and potential distributions. A+SDP's could have practical application in scintillation detectors for gamma ray spectroscopy as well as PMT replacements in nuclear medicine.

We report on the count-rate performance of the unique PETRRA positron camera at activities up to 60MBq. The camera consists of two large area detectors, each comprising a tiled array of 10mm thick BaF₂ scintillation crystals interfaced to a multi-step avalanche chamber filled with 4.2mbar of pure TMAE vapor. Preliminary results demonstrate coincident count rates of over 80kcps for a cylindrical (20x20cm³) phantom with 50MBq of F-18 in the field-of-view using a 20ns coincidence time window. Each component of the readout cycle has been characterized in terms of dead-time loss. The camera's dead-time related count loss is well-described by a paralysable model with a dead-time of ~500ns. Other sources of count rate loss are also discussed.

We describe a new high-resolution neutron-imaging detector. A neutron scintillator is imaged onto a photon-counting 2D imaging detector, with most of the scintillation used for event discrimination in this patented detector. Detection time and location are recorded, so neutron energy (time-of-flight) is measured for accelerator-based sources. We are currently fabricating a 25cmx25cm detector with 512x512 pixels and 100 ns time resolution. Results are presented for scintillator characterization measurements and imaging characteristics measured at IPNS, where the detector will reside.

A novel method for detecting materials concealed in fully loaded cargo containers and trucks using pulsed neutrons has been developed. This method, called Pulsed Fast Neutron Analysis (PFNA), uses a collimated beam of nanosecond pulsed, mono-energetic neutrons to stimulate gamma rays emitted from the elements in the cargo materials through inelastic scattering. The collimation and precise timing of the gamma rays allows the position of the interactions to be determined. By sweeping the beam over the inspected object as it is conveyed, a full 3D image of the elemental contents is composed. Unique elemental signatures of specific materials are then used to identify the contraband. This technique has been incorporated into a cargo and truck inspection system capable of scanning full-sized cargo containers. Algorithms have been developed to detect concealed drugs, explosives and hazardous materials. The location of the detected materials is displayed on a unique, workstation-based, user interface. Pull-down menus allow the user to select the elements of interest and the resulting display indicates the location and spatial extent of the material in three-dimensions.

Compton scattering is a potential tool for the determination of bone mineral content or tissue density for dose planning purposes, and requires knowledge of the energy distribution of the X-rays through biological materials of medical interest in the X-ray and (gamma) -ray region. The energy distribution is utilized in a number of ways in diagnostic radiology, for example, in determining primary photon spectra, electron densities in separate volumes, and in tomography and imaging. The choice of the X-ray energy is more related to X-ray absorption, where as that of the scattering angle is more related to geometry. The evaluation of all the contributions are mandatory in Compton profile measurements and is important in X-ray imaging systems in order to achieve good results. In view of this, Compton profile cross-sections for few biological materials are estimated at nineteen K(alpha) X-ray energies and 60 keV (Am-241) photons. Energy broadening, geometrical broadening from 1 to 180 degree(s), FWHM of J(P_z) and FWHM of Compton energy broadening has been evaluated at various incident photon energies. These values are estimated around the centroid of the Compton profile with an energy interval of 0.1 keV and 1.0 keV for 60 keV photons. The interaction cross sections for the above materials are estimated using fractions-by-weight of the constituent elements. Input data for these tables are purely theoretical.

A plane monochromatic electric field incident upon a metallic interface is analyzed by way of the space-time Wigner distribution. An exact calculation is made and we discuss the behavior of the Wigner distribution at the boundary and inside the metal.

We report a non-silver, two-layered semiconductor-thermoplastic structure for visible and X-ray data recording. Photothermoplastic materials (PTPM) consist the chalcogenide glassy semiconductors and polymeric thermoplastic materials. Our researches were aimed to find out a possibility of using the PTPM as recording media for imaging including X-rays.

The characteristics of a new quasi-x-ray laser generator and its application to polycapillary radiography are described. The generator employs a high-voltage power supply, a low- impedance coaxial transmission line, a high-voltage condenser with a capacity of about 200 nF, a turbo-molecular pump, a thyristor pulse generator as a trigger device, and a new plasma flash x-ray tube. The high-voltage main condenser is charged up to 60 kV by the power supply, and the electric charges in the condenser are discharged to the tube after triggering the cathode electrode. The flash x- rays are then produced. The x-ray tube is of a demountable triode that is connected to the turbo molecular pump with a pressure of approximately 1 mPa. As the electron flows from the cathode electrode are roughly converged to the copper target by the electric field in the tube, the plasma x-ray source, which consists of metal ions and electrons, forms by the target evaporating. Both the tube voltage and current displayed damped oscillations, and their peak values increased according to increase in the charging voltage. In the present work, the peak tube voltage was almost equal to the initial charging voltage of the main condenser, and the peak current was about 25 kA with a charging voltage of 60 kV. When the charging voltage was increased, the linear plasma x-ray source formed, and the characteristic x-ray intensities of K-series lines increased. In the radiogrpahy achieved with a computed radiography system, we employed a polycapilary plate with a hole diameter of 20 micrometers and a thickness of 1 mm. The image resolution was primarily determined by the resolution of the CR system and had a value of about 100micrometers .

In radiology, the X-ray film is placed between two screens. The development of radiological intensifying screens has reduced the radiation dose in patients about a factor of 10 in the last decades. The sensitivity of X-ray films is enlarged due to the fluorescent light from these screens. In addition, the primary radiation can be scattered coherently (Rayleigh scattering) and incoherently (Compton scattering) which will degrade the image resolution. Scattered radiation produced in Gd₂O₂Si:Tb intensifying screens was simulated by using a Monte Carlo radiation transport code - the EGS4 (Electron - Gamma Shower), which is a general purpose package for Monte Carlo simulation of the coupled transport of electrons and photons. The magnitude of scattered radiation striking the film is typically quantified using the scatter to primary radiation (SPR). The angular distribution of the intensity of the scattered radiation (sum of both the scattering effects) was simulated, showing that the ratio of scatter- to-primary radiation incident on the X-ray film is about 8.39% and 7.73% for the front and back screen, respectively, over the range from 0 to (pi) rad.

The tentative experiment for production low photon energy characteristic x-rays using a capillary is described. The capillary of this flash x-ray tube was improved in order to increase the x-ray intensity and to generate high-intensity characteristic x-rays by forming the linear plasma x-ray source. The generator consists of a high-voltage power supply, a polarity-inversion ignitron pulse generator, a turbo-molecular pump, and a radiation tube with a capillary. A high-voltage condenser of 0.2(mu) F in the pulse generator is charged up to 20 kV by the power supply, and the electric charges in the condenser are discharged to the capillary in the tube after closing the ignitron. In the present work, the pump evacuates air from the tube with a pressure of about 1 mPa, and the aluminum anode and cathode electrodes are employed to produce characteristic x-rays. The diameter and the length of the capillary are 2.0 and 29 mm, respectively, and both the cathode voltage and the discharge current displayed almost the damped oscillations. The peak values of the voltage and current increased when the charging voltage was increase, and their maximum values were -9.2 kV and 4.6 kA, respectively. The x-ray durations detected by a 1.6 micrometers aluminum filter were less than 10microsecond(s) , and we observe the intensity of aluminum characteristic x-rays.

Experimental results of transmission photon radiography of bulk materials using the laser-Compton photon beam in the energy range of 2-20 MeV are given. The purpose of this work is to demonstrate the effectiveness and to survey a potential need and a technical limit of the present method for industrial application, such as nondestructive test of bulk materials. Several radiographs of metals, ceramics, and concrete were measured with the present method. Position resolution of the system was measured with using 10 MeV photon beam and slit. It was less than 1 mm.

The interest in the use of CdZnTe room-temperature, solid-state detectors for Nuclear Medicine applications continues to grow. Efforts are underway at several companies and institutes to develop CdZnTe detector systems to compete with existing scintillator-based Large Field of View (LFOV) gamma cameras. eV PRODUCTS is focusing on the development of Small Field of View (SFOV) gamma cameras using monolithic CdZnTe detector arrays coupled to custom designed, low noise analog read-out electronics. Electronic noise in the ASIC's has been minimized, and is typically less than 100 e^- rms (0pF). The detector readout system has the capability to perform automatic energy calibration, gain setting and discrimator level setting, along with a comprehensive self-diagnostic routine. The complete integrated unit includes a bias voltage generator, counters and communication control. A first prototype 256-channel device has been developed and constructed, with pixel dimensions of 1.8 x 1.8 mm², on a 34 x 34 x 5mm3 monolithic CdZnTe detector. Results obtained from this system will be presented, showing both the energy resolution and uniformity data for several arrays. The energy resolution has been demonstrated to be 3-5% FWHM 140 keV, with photopeak efficiencies of 70-80%.