A SPECT Camera for Combined MRI and SPECT for Small Animals.

We describe an MR-compatible SPECT camera for small animals. The SPECT camera system can be inserted into the bore of a state-of-the-art MRI system and allows researchers to acquire tomographic images from a mouse in-vivo with the MRI and the SPECT acquiring simultaneously. The SPECT system provides functional information, while MRI provides anatomical information. Until today it was impossible to operate conventional SPECT inside the MRI because of mutual interference. The new SPECT technology is based on semiconductor radiation sensors (CZT, ASICs), and it fits into conventional high field MRI systems with a minimum 12-cm bore size. The SPECT camera has an MR-compatible multi-pinhole collimator for mice with a ø25-mm field-of-view. For the work reported here we assembled a prototype SPECT camera system and acquired SPECT and MRI data from radioactive sources and resolution phantoms using the camera outside and inside the MRI.

MicroCT with energy-resolved photon-counting detectors.

The goal of this paper was to investigate the benefits that could be realistically achieved on a microCT imaging system with an energy-resolved photon-counting x-ray detector. To this end, we built and evaluated a prototype microCT system based on such a detector. The detector is based on cadmium telluride (CdTe) radiation sensors and application-specific integrated circuit (ASIC) readouts. Each detector pixel can simultaneously count x-ray photons above six energy thresholds, providing the capability for energy-selective x-ray imaging. We tested the spectroscopic performance of the system using polychromatic x-ray radiation and various filtering materials with K-absorption edges. Tomographic images were then acquired of a cylindrical PMMA phantom containing holes filled with various materials. Results were also compared with those acquired using an intensity-integrating x-ray detector and single-energy (i.e. non-energy-selective) CT. This paper describes the functionality and performance of the system, and presents preliminary spectroscopic and tomographic results. The spectroscopic experiments showed that the energy-resolved photon-counting detector was capable of measuring energy spectra from polychromatic sources like a standard x-ray tube, and resolving absorption edges present in the energy range used for imaging. However, the spectral quality was degraded by spectral distortions resulting from degrading factors, including finite energy resolution and charge sharing. We developed a simple charge-sharing model to reproduce these distortions. The tomographic experiments showed that the availability of multiple energy thresholds in the photon-counting detector allowed us to simultaneously measure target-to-background contrasts in different energy ranges. Compared with single-energy CT with an integrating detector, this feature was especially useful to improve differentiation of materials with different attenuation coefficient energy dependences.

Collaboration using Internet for the development of case-based teaching files: report of the Computer and Instrumentation Council Internet Focus Group.

UNLABELLED: The Internet and particularly the World-Wide-Web is becoming a useful tool for the nuclear medicine community. METHODS: The Computer and Instrumentation Council of the Society of Nuclear Medicine convened an Internet Focus group to discuss collaboration using the Internet. The prototype application considered was development of case-based teaching files using the World-Wide-Web. Teaching file cases (clinical history, images, description of findings and discussion) on World-Wide-Web servers at different institutions are integrated using the Internet. The user can navigate from case to case using point-and-click hypertext linking. RESULTS: The initial experience with collaboration has been encouraging. An etiquette to help foster collaboration has been proposed. Development of quality control mechanisms and introduction of peer review were identified as issues needing further work. CONCLUSION: The World-Wide-Web offers great potential for new forms of collaboration. There is, however, a need to learn how to make best use of this new resource.

Effects of induced charge in the kinestatic charge detector.

The principle of the kinestatic charge detector (KCD) for digital radiography depends on the synchronization of the scan velocity of a parallel plate drift chamber with the cation drift velocity. Compared with line-beam scanners, this motion-compensated imaging technique makes better use of the x-ray tube output. A Frisch grid traditionally has been used within the KCD to minimize unwanted signal contributions from both cations and negative charge carriers during irradiation. In this work the charge induction process in a parallel plate geometry was investigated for the special case of the KCD. In the limit of infinite plates, the cathode charge density due to both cations and negative charge carriers increases quadratically in time for a kinestatically scanned narrow slit. In the KCD the cathode is segmented into an array of narrow electrodes, each aligned with the incident x-ray beam. Our conformal mapping computation determined that the shape of the induced charge signal depends critically on delta x/w, the ratio of electrode width to drift gap. Our conclusion introduces the possibility of eliminating the Frisch grid from the KCD design because the value of delta x/w required for transverse sampling in the KCD is sufficiently low as to allow "self-gridding" to take effect.

Engineering aspects of a kinestatic charge detector.

The engineering aspects of a nine-channel digital radiographic system developed for bioimaging research, based on high gas pressure ionography and kinestatic principles, are presented. The research imaging system uses a pulsed x-ray beam which allows one to study simultaneously the ionic signal characteristics at 10 different ionization sites along the drift axis. This research imaging detector system allows one to investigate methods to improve the detection and image quality parameters as part of the development of a large scale prototype medical imaging system.

Evaluation of video gray-scale display.

Setting up and maintaining video display monitors properly will help to reduce display variation and improve overall presentation of the radiological image. Display monitor gray-scale characteristics were examined using the SMPTE test pattern. This test pattern may be used as a standard for adjusting brightness and contrast. The controls should be adjusted to display the full dynamic range so that the 5% and 95% signal levels in the pattern are visible. Measured luminance on a laboratory workstation used for radiological perceptual experiments, and on the Siemens CT gray-scale monitor was determined to range from 0.17 to 76.0 nit, and 0.17 to 24.66 nit, respectively. These were compared with the range of approximately 17 to 514 nit for a typical film-viewbox combination. Characteristic curves were determined for both monitors, and CRT gammas were 3.34 and 2.48 for the perceptual workstation and CT console, respectively. The display gamma was determined from fitting luminance data to a log-log plot of luminance versus input gray level. The usefulness of the SMPTE test pattern for visual presentation as well as photometric measurement is demonstrated.

Use of a low ionization potential dopant in a kinestatic charge detector: experimental findings.

The broadening of the line spread function (LSF) in the drift direction with increasing drift distance in the kinestatic charge detector is substantially reduced when small amounts (less than 1%) of trimethylamine [(CH3)3 N] are added to the x-ray detection medium (krypton or xenon). The LSF of a mixture of Kr and 0.01% trimethylamine (TMA) was measured as a function of distance at 15, 25, and 35 atm absolute pressure. The full width at half-maximum (FWHM) of the LSF was reduced from about 1.0 mm to less than 0.5 mm at a drift distance of 4.0 mm for the three pressures. The LSF's of mixtures of xenon and TMA at concentrations ranging from 0.0004% to 0.4% in one run and 0.06% to 4.0% in a second run were measured at a constant pressure of 20 atm. The FWHM of the LSF was reduced from 0.6 to 0.4 mm at 4.0 mm for the xenon measurements. The optimum concentration of TMA in Xe was found to be in the neighborhood of 0.1%. The use of TMA reduced the drift distance-dependent LSF broadening to the level expected from ionic diffusion, space charge repulsion, and electric field nonuniformity, and it may be possible to reduce the 0.4-mm FWHM plateau through the use of an improved Frisch grid design. Observation of negative charge carriers showed that electron attachment increases with increasing TMA concentration, although this could be caused by impurities in the TMA. The implications of these results are discussed in terms of extending the maximum drift distance attainable in a kinestatic charge detector.

Space charge effects in a kinestatic charge detector.

The effects of space charge in a kinestatic charge detector (KCD) were examined using computer solutions to Poisson's equation. The KCD is a strip-beam parallel-plate drift chamber used for digital radiography. It was assumed that there is negligible electron attachment, i.e. there are no negative ions formed. The ionization rate per mA as a function of x-ray interaction depth was calculated for a detector filled with xenon at 25.3 x 10(5) Pa. Solution of Thomson's equations gave the positive ion density at the cathode, also as a function of depth. Water filtration values ranging from 0 to 30 cm were used in order to estimate the range of ion density values expected in a clinical KCD. The case of steady-state x-ray illumination was simulated for ionization rates less than the zero field limit (above which space charge changes the polarity of the electric field). Line spread responses were found for varying ionization rates to show the effect of space charge due to electric field distortion on the spatial resolution performance in the drift direction. The effect of imaging ideal edges with a KCD was calculated and the expected output signal was plotted for densities up to the zero field limit. Space charge dependence on the selection of KCD design and operating parameters is discussed. Because of the dependence of the KCD drift-direction spatial resolution on the uniformity of the electric field, space charge effects impose an upper limit on the detector entrance exposure and define the dynamic range of the device.