Description of a prototype emission-transmission computed tomography imaging system.

We have developed a prototype imaging system that can perform simultaneous x-ray transmission CT and SPECT phantom studies. This system employs a 23-element high-purity-germanium detector array. The detector array is coupled to a collimator with septa angled toward the focal spot of an x-ray tube. During image acquisition, the x-ray fan beam and the detector array move synchronously along an arc pivoted at the x-ray source. Multiple projections are obtained by rotating the object, which is mounted at the center of rotation of the system. The detector array and electronics can count up to 10(6) cps/element with sufficient energy-resolution to discriminate between x-rays at 100-120 kVp and gamma rays from 99mTc. We have used this device to acquire x-ray CT and SPECT images of a three-dimensional Hoffman brain phantom. The emission and transmission images may be superimposed in order to localize the emission image on the transmission map.

Systematic bias in basis material decomposition applied to quantitative dual-energy x-ray imaging.

Basis material decomposition represents dual-energy x-ray attenuation measurements in terms of the attenuation coefficients or thickness of two standard materials which, when combined, produce attenuation equivalent to the object being measured. In tomographic imaging, the reconstructed attenuation coefficient is calculated in terms of the attenuation coefficients of the basis materials, while in projection imaging, the thicknesses of two materials can be specified in terms of the basis materials. This analysis shows that basis material decomposition is exact in a dual-monoenergetic system, but for broad spectra, x-ray beam hardening introduces a bias into quantitative measurements. The error is small enough that it can be ignored when dual-energy imaging is used primarily to enhance the contrast of one material over another. The magnitude of the error in quantitative measurements depends on the details of the specific application including the energy of the x-ray beam, and the composition and thickness of the materials included in the object. The magnitude of the error for dual-energy bone densitometry has been analyzed using a first-order propagation of error analysis and the calculations verified by computer simulation. This analysis shows that the magnitude of the systematic error can be as high as 3% for 1 g/cm2 of bone mineral when aluminum and acrylic basis materials are used for the calibration. This systematic error is eliminated when the basis materials are the same as the materials that are being quantified (i.e., bone mineral and water).

A prototype high-purity germanium detector system with fast photon-counting circuitry for medical imaging.

A data-acquisition system designed for x-ray medical imaging utilizes a segmented high-purity germanium (HPGe) detector array with 2-mm wide and 6-mm thick elements. The detectors are contained within a liquid-nitrogen cryostat designed to minimize heat losses. The 50-ns pulse-shaping time of the preamplifier electronics is selected as the shortest time constant compatible with the 50-ns charge collection time of the detector. This provides the detection system with the fastest count-rate capabilities and immunity from microphonics, with moderate energy resolution performance. A theoretical analysis of the preamplifier electronics shows that its noise performance is limited primarily by its input capacitance, and is independent of detector leakage current up to approximately 100 nA. The system experimentally demonstrates count rates exceeding 1 million counts per second per element with an energy resolution of 7 keV for the 60-keV gamma ray photon from 241Am. The results demonstrate the performance of a data acquisition system utilizing HPGe detector systems which would be suitable for dual-energy imaging as well as systems offering simultaneous x-ray transmission and radionuclide emission imaging.

Contrast and dose with Mo-Mo, Mo-Rh, and Rh-Rh target-filter combinations in mammography.

PURPOSE: To study the effect of three mammography target-filter combinations on contrast and dose. MATERIALS AND METHODS: With screen-film sensitometry, the contrast of a calcification target embedded in simulated breast tissue was measured for three target-filter combinations--molybdenum-molybdenum (Mo-Mo), molybdenum rhodium (Mo-Rh), and rhodium-rhodium (Rh-Rh)--as a function of x-ray tube potential, breast thickness, and breast composition. The corresponding average glandular tissue doses were also determined. RESULTS: Contrast and dose decreased with increasing kilovolt peak with all three target-filter combinations. Contrast was highest for Mo-Mo and lowest for Rh-Rh for images exposed with a low kilovoltage (< 29 kVp). For thick or radiographically dense phantoms, the contrast produced with Mo-Mo was less than or equal to that produced by the other two x-ray spectra when a higher kilovoltage (> or = 29 kVp) was selected. Average glandular dose was greatest for Mo-Mo and lowest for Rh-Rh for all phantom thicknesses, breast compositions, and tube potentials studied. CONCLUSIONS: For the thick or dense breast, the alternative target-filter selections can achieve contrast comparable to or better than that obtainable with Mo-Mo while using a smaller dose.

Computed radiography: user-programmable features and capabilities.

Digital or computed radiography (CR) using photostimulable storage phosphor plate technology is becoming increasingly popular in certain clinical applications, such as bedside radiography, where it possesses clear advantages over conventional screen-film imaging. The majority of CR systems in clinical use have been manufactured by Fuji Medical Systems USA, Inc (Stamford, CT) and provide a surprising degree of flexibility. Fuji CR units are delivered with preset menus, hardcopy format, and image-processing parameters for each examination. Of practical importance is that users may change the exam menu and printed film format as well as the image-processing parameters for each examination. There is, however, a lack of documentation describing these features and how they are programmed. This paper addresses these issues. Examples are given on how to change: 1) the printed film format, 2) the contrast and gray-scale processing, 3) spatial frequency enhancement, and 4) the appearance of the operator interface menus.

Normalized average glandular dose in molybdenum target-rhodium filter and rhodium target-rhodium filter mammography.

PURPOSE: To determine the normalized glandular dose (DgN) for molybdenum target-rhodium filter (Mo-Rh) and rhodium target-rhodium filter (Rh-Rh) mammography and compare the average glandular doses (Dg) that resulted with a conventional molybdenum target-molybdenum filter (Mo-Mo) source assembly. MATERIALS AND METHODS: X-ray spectra models for Mo-Rh and Rh-Rh were developed and used to calculate DgN values for these target-filter combinations as a function of x-ray tube potential, half-value layer, and breast thickness for three breast compositions. For the average glandular dose comparisons, 50/50 phantoms were imaged for the three target-filter source assemblies at three tube potentials. RESULTS: For the same parameters, DgN values for Mo-Rh and Rh-Rh were higher than for Mo-Mo. At the same voltage, the exposures required to image breast phantoms are substantially lower, and as a result, Dgs are also less with Mo-Rh and Rh-Rh than with Mo-Mo. CONCLUSION: DgN values presented permit practical evaluations of average glandular doses for Mo-Rh and Rh-Rh mammography. At a given potential, dose savings are realized with Mo-Rh and Rh-Rh source assemblies.

Gastrointestinal imaging: a systems analysis comparing digital and conventional techniques.

OBJECTIVE: The purpose of this study was to compare digital and conventional methods of gastrointestinal imaging based on the cost of image storage and estimated overall costs, radiation exposure to the patient, and duration of the examination. MATERIALS AND METHODS: Our study sample consisted of 128 patients who underwent conventional gastrointestinal studies (64 double-contrast upper gastrointestinal examinations and 64 double-contrast barium enemas) and 139 patients who underwent digital gastrointestinal studies (66 double-contrast upper gastrointestinal examinations and 73 double-contrast barium enemas). The number of images and films for each study was recorded, and the mean cost of image storage and the estimated overall costs for digital versus conventional studies were calculated. Both the duration of fluoroscopy and the time from start to completion of the study were obtained from our radiology information system. From these data, we calculated mean radiation exposure to the patient and the duration of the examination. Finally, referring physicians completed a questionnaire about their level of satisfaction with paper prints generated from digital gastrointestinal studies. RESULTS: When digital studies were compared with conventional studies, the mean cost of image storage decreased by 45% and the estimated overall 10-year costs decreased by 8%. The mean number of spot images increased by 8% for upper gastrointestinal examinations and by 25% for barium enema examinations, whereas the mean duration of fluoroscopy decreased by 4% and by 10%, respectively. As a result, radiation exposure to patients increased by only 2%, a difference that did not approach statistical significance. Finally, the mean duration of examinations decreased by 24% for upper gastrointestinal examinations and by 33% for barium enemas. Approximately 85% of the physicians who completed the questionnaires indicated that they reviewed the paper prints generated from digital studies and that they would like to continue receiving them. CONCLUSION: Digital gastrointestinal imaging systems are associated with higher initial costs than conventional systems, but the long-term costs of these digital imaging systems are slightly less because of the lower cost of image storage, and radiation exposure to patients is comparable. The shorter duration of digital examinations is a potential benefit of this technology, allowing improved patient throughput. Finally, referring physicians have a high level of satisfaction with paper prints generated from digital imaging.