Calibration drift in a laboratory high purity germanium detector spectrometry system.

For unknown radionuclide identification, it is important that a high purity germanium (HPGe) spectrometry system be calibrated correctly for energy. The energy calibration of an HPGe spectrometry system will drift over time due to a variety of factors including the ambient temperature, the line voltage applied to the system, variation in the electronics, and other possible influences. In order to better understand the nature of this energy calibration drift, calibration spectra were collected over a period of several months from a laboratory HPGe spectrometry system. System parameters, including detector voltage, amplifier gain, and preamplifier gain, were not deliberately modified during the course of the experiment. The system was calibrated routinely over the 90 days, and the results of the calibrations were compared in order to assess the drift in the energy calibration of the detector over time. The analysis of a 36% high purity germanium system demonstrated the energy calibration drifted an average of 0.014 keV d(-1) to 0.041 keV d(-1) depending upon energy. At 1,332 keV, one day after calibration, it was shown that up to half of the total error in energy calibration was as a result of calibration drift.

The sunlight OSL response of a commercially available alpha-Al2O3:C personnel dosimetry material.

Carbon-doped, anion-defective aluminium oxide has become a widely used and effective medium for personnel dosimetry applications using optically stimulated luminescence (OSL) techniques. Though the commercial products currently using this material employ light-tight packaging to prevent light-induced effects on the OSL signal, the material could be employed in environments where package integrity cannot be assured. This paper reports on the results of an experiment performed to quantify the effects of sunlight exposure on alpha-Al2O3:C. Samples of commercially available Luxel material were exposed to carefully recorded levels of sunlight both before and after irradiations to determine the nature and magnitude of both activation and fading phenomena in this material. The results confirm that both fading and activation processes are seen in this material and indicate that the material reaches an equilibrium dose level in response to prolonged sunlight exposure equivalent to a dose of approximately 15 mGy under the experimental test conditions.

A simple radon chamber for educational use.

Radon chambers are typically able to maintain a constant, known concentration of radon by pumping a constant activity concentration of radon gas into the chamber. A radon chamber has been designed by placing a radon source inside the chamber volume and allowing radon to build up inside the chamber. Because the radon concentration is not constant, calculations have been made to determine the integrated equivalent constant radon concentration in MBqhm for up to 4 d. The chamber, made by placing a radium dial inside an incubator, has interior dimensions of 87.6 cm x 55.9 cm x 51.4 cm for a total volume of 0.25 m. The chamber can produce an integrated equivalent constant radon activity concentration level of 0.013 MBq h m over the initial 24 h, 0.043 MBq h m over the initial 48 h, 0.078 MBq h m over the initial 72 h, and 0.118 MBq h m over the initial 96 h. The chamber can also demonstrate, for educational purposes, the kinetics of the build-up of a radioactive gas in an enclosed environment as well as the kinetics of washout and leakage.

The in vivo autoradiographic measurement of regional cerebral blood flow using stable xenon and computerized tomography: the effect of tissue heterogeneity and computerized tomography noise.

We have studied the effect of brain tissue (gray matter-white matter) heterogeneity and computerized tomography (CT) noise on the accuracy of xenon CT measurements of regional cerebral blood flow (rCBF) based upon "autoradiographic" and multiple-scan washin protocols. The results of our mathematical analysis indicate that both protocols are associated with a variety of measurement errors that lead to unacceptable and, to a large extent, unpredictable uncertainties in calculated values of rCBF. Brain tissue heterogeneity and high volumetric flow rates may--even in the absence of movement artifact and CT noise--lead to measurement errors in excess of 20%. Moreover, CT noise is additive in regard to these errors, and constitutes the most confounding variable of all.

Long-term performance of a multiplanar positron emission tomograph.

A protocol for the daily quality assurance (QA) of a multiplanar positron emission tomographic (PET) system was developed. This was implemented on a daily basis for the PC 4600 Neuro-PET, a multiplanar PET system designed for quantitative brain imaging. Sensitivity data collected as part of the protocol are presented for a 22-mo time period. These data show the need to periodically monitor instrument performance if meaningful quantitation is to be achieved. The methods presented have direct application to any quantitative multiplanar emission tomographic imaging program.

Radiation absorbed dose estimates for positron emission tomography (PET): K-38, Rb-81, Rb-82, and Cs-130.

Radiation absorbed dose estimates were made for K-38, Rb-81, Rb-82, and Cs-130, potential radionuclides for myocardial and brain studies with positron emission tomography (PET). Biodistribution data for K-42, Rb-86, and Cs-137 in rats were used, together with the most recent radiation absorbed dose computational techniques and data. Renal radiation absorbed doses for K-38, Rb-81, Rb-82, and Cs-130 were estimated to be 56, 120, 19, and 150 mrad, respectively, per mCi administered. Corresponding radiation absorbed doses in the heart were 42, 54, 13, and 68 mrad/mCi. Radiation absorbed doses in the brain of 1.3, 3.5, 0.25, and 3.7 mrad/mCi were estimated for these four radionuclides.

C-11 dimethyloxazolidinedione (DMO): biodistribution, radiation absorbed dose, and potential for PET measurement of regional brain pH: concise communication.

An improved radiochemical synthesis for C-11 dimethyloxazolidinedione (C-11 DMO) makes this agent attractive for the measurement of regional brain tissue pH (rpH) using positron emission tomography (PET). Toward this end, biodistribution data for C-14 DMO in rats at various times after an intravenous bolus injection are reported, together with estimates of radiation absorbed dose for C-11 DMO in man. An error analysis of C-11 DMO PET measurement of rpH indicates that rpH can be determined to within +/- 0.1 pH unit for pH greater than 6.5 with a 20-mCi injected bolus of C-11 DMO, a 30- 45-min equilibration time, and a 15-min PET imaging period.

F-18-labeled 3-deoxy-3-fluoro-D-glucose for the study of regional metabolism in the brain and heart.

Glucose is the major physiological substrate of the brain and an important physiological substrate for the myocardium. [19F]fluoro-3-deoxy-glucose [3-FDG(F-18)] was studied to determine whether it is a suitable tracer for evaluating the metabolic function of the brain and myocardium. 3-FDG(F-18) was rapidly accumulated in the mouse myocardium (10-12% injected dose/g) and remained constant up to 120 min. Blood, liver, and lung activities exhibited a rapid accumulation of activity (4% injected dose/g) at 1 min, followed by elimination of activity up to 30 min (2% injected dose/g), and then remaining unchanged for a period of 120 min. The arterial blood curve in the dog was fit best by three exponential components (T 1/2 = 0.52 min, 2.75 min, and 142.8 min). Transverse-section images were obtained of the dog's brain and myocardium. From sequential two-dimensional images, a clearance half-time of 26.88 min was determined for the canine brain. Radiation doses for man were calculated from tissue distribution data for mice.

Artifacts, anatomical and physiological variants, and unrelated diseases that might cause false-positive whole-body 131-I scans in patients with thyroid cancer.

The whole body 131-I scan remains an important component in the postoperative treatment of patients with well-differentiated thyroid cancer. Because normal thyroid tissue remnants and residual or metastatic foci of well-differentiated thyroid cancer have the unique ability to concentrate, organify, and store 131-I, the whole body scan provides a depiction of those tissues that can be ablated with therapeutic doses of 131-I. Over time, it has become obvious that the whole body scan may also reveal foci of 131-i uptake owing to a wide variety of other causes. We provide a detailed pathophysiological classification of the artifacts, anatomic and physiological variants, and nonthyroidal diseases that may give rise to false-positive whole body scans in postoperative patients with thyroid cancer. These include ectopic foci of normal thyroid tissue; nonthyroidal physiological sites (eg, choroid plexus, salivary glands, gastric mucosa, urinary tract); contamination by physiological sections; ectopic gastric mucosa; other gastrointestinal abnormalities; urinary tract abnormalities; mammary abnormalities; serous cavities and cysts; inflammation and infection; nonthyroidal neoplasms; and currently unexplained causes. This article also provides a detailed review of the widely scattered English language literature in which these phenomena were originally described.

Subjective evaluations of mammographic accreditation phantom images by three observer groups.

RATIONALE AND OBJECTIVES: Mammography providers are under increasing pressure to become certified by the American College of Radiology (ACR). Successful accreditation is contingent on passing a phantom image quality test. This study was undertaken to evaluate viewer performance with the phantom image evaluation process and to determine any observer group differences. METHODS: A series of standard phantom images were viewed by 30 medical physicists, 30 diagnostic radiologists, and 30 inexperienced observers. From the responses, object detection rates and passing rates according to the ACR criteria were established. These responses were analyzed with standard nonparametric tests to assess the degree of variability, correlation, and agreement among different observer groups. RESULTS: Median passing scores were similar for the radiologists and untrained readers, but the medical physicists appear to differ from the other two groups. There was not sufficient evidence to indicate that special training among physicists made a significant difference in median passing scores or mass detection rates. However, such training appeared to significantly affect the detection of microcalcification and fibril test patterns among the physicists' subgroups. Agreement among observer groups was high for all groups, but tended to be lower for overall passing rates than for any of the individual test objects. Agreement among physicists was affected by their subspecialty, presumably caused by their levels of specific training for these visual tasks. CONCLUSIONS: The authors conclude that choosing medical physicists to evaluate mammographic phantom films appears to be a good choice among potential observer groups, and that special training for reading these images affects their detection abilities and consistency. However, because passing rates did not appear to be affected by special training and given the current rapid degree of change in this area, more testing of medical physicists is desirable to examine these effects over time, and to study the effect of developing standards for training.

Optimization of xenon-enhanced CT studies: beam energy, enhancement, root mean square deviation, and repeatability.

The effects of varying beam energy on the computed tomographic (CT) enhancement-to-noise (S:N) ratio were studied experimentally with the DeltaScan 2020 and GE 8800 CT scanners and a 20-cm-diameter cyclindrical Plexiglas phantom containing 11 50 ml syringes filled with varying amounts of xenon and iodine. Enhancements of 54.2, 36.7, and 31.7 Hounsfield units (H)/mg l/ml were measured with the DeltaScan 2020 at 70, 100, and 120 kVp, respectively, with corresponding root mean square deviations (RMSDs) of 12, 7, and 5 H for 400 mAs scans. For the GE 8800, enhancements of 48.3, 37.6, and 32.7 H/mg l/ml were measured at 80, 100, and 120 kVp with RMSDs of 13, 8, and 7 H for 9.6 sec 320 mA scans (3.3 msec pulse). RMSD was independent of enhancement over the range of iodine concentrations studied (0-1.5 mg l/ml) and was only a weak function of region-of-interest (ROI) size. For repeated scans with the DeltaScan 2020, measurements in 17 X 17 pixel regions were reproducible to within 0.8 H for all techniques and a drift in calibration of less than 6% was observed after 8 months of clinical use. For both the DeltaScan 2020 and the GE 8800, at the milliamperage studied, lower-energy techniques offered no advantage over 120 kVp technique for xenon CT measurements of regional cerebral blood flow, which are feasible using either of these scanners.

Effects of axial spatial resolution and sampling on object detectability and contrast for multiplanar position emission tomography.

A multiplanar positron emission tomography (PET) system is simulated using Gaussian curves to model the axial point spread functions (PSFs) of the planes to study the effects of resolution and sampling. Poor spatial resolution or insufficient sampling may cause deleterious data losses or artifacts in the reconstructed image. For a multiplanar PET system with an axial full width at half-maximum (FWHM) of 6 mm and a 12 to 13 mm ring separation, a ripple in sensitivity of 9% is observed. A 1 mm object placed at the central direct plane results in detection of 59% of the signal in that plane. The theoretical observed contrast of a 3 mm object positioned at the center plane is 25% of the true contrast and decreases to 24% when the object is positioned between the central direct and cross planes. A PET system with an axial FWHM of 12 mm and a ring separation of 5-6 mm has a uniform sensitivity. A 1 mm object placed at the central direct plane detects 14% of the object signal in that plane. The theoretical observed contrast for a 3 mm object is 13% of true contrast when the object is positioned between the central direct and cross planes. It should be noted that all dimensions refer to the z direction through the center of the gantry in the simulated multiplanar system. The uniform sensitivity due to wider axial FWHMs decreases the amount of data loss for inter-ring gaps; however, the blurring associated with wider FWHMs decreases observed contrast.(ABSTRACT TRUNCATED AT 250 WORDS)

Positional radiotherapy beam dosimetry using a laser heated thermoluminescent plate.

A 4-W, laser-based system for the positional readout of thermoluminescent (TL) plates was utilized to obtain dose profiles for a 60Co radiotherapy beam. Experimentally obtained glow curves for LiF resulting from the rapid heating rates characteristic of laser heating agreed well with results predicted using a first-order kinetic model. Excellent system linearity was obtained for sensitivity to 60Co. The useful dynamic range for this tissue-equivalent system could extend over 8 orders of magnitude. A minimum detectable signal of 1.2 R was estimated for the presently configured system, whose spatial resolution is limited by the laser beam diameter, 1.7 mm. Depth dose profiles for the radiotherapy beam obtained using the prototype laser TL system with a specially designed composite 2.2 cm x 3.3 cm x 0.254 mm LiF plate agreed to within 14% of ionization chamber measured doses.

The effects of CT drift on xenon/CT measurement of regional cerebral blood flow.

A systematic increase in computed tomography (CT) number of approximately 0.13 Hounsfield unit per scan (HU/scan) was observed when serial DeltaScan 2020 CT scans of a uniform water phantom were equally spaced at 0.5, 1.0, or 2.0 min and a shaped aluminum beam-hardening filter was employed. Much smaller drifts (less than 0.06 HU/scan) were observed with flat aluminum or shaped beryllium oxide filters. This machine drift, which was not associated with a rise in water phantom temperature and did not consistently correlate with estimated x-ray tube heat, could result in a significant overestimation of regional cerebral blood flow (rCBF) for a xenon/CT rCBF protocol involving 5-7 sequential scans obtained at 1-min interscan intervals.

Automated analysis of the American College of Radiology mammographic accreditation phantom images.

A significant metric in federal mammography quality standards is the phantom image quality assessment. The present work seeks to demonstrate that automated image analyses for American College of Radiology (ACR) mammographic accreditation phantom (MAP) images may be performed by a computer with objectivity, once a human acceptance level has been established. Twelve MAP images were generated with different x-ray techniques and digitized. Nineteen medical physicists in diagnostic roles (five of which were specially trained in mammography) viewed the original film images under similar conditions and provided individual scores for each test object (fibrils, microcalcifications, and nodules). Fourier domain template matching, used for low-level processing, combined with derivative filters, for intermediate-level processing, provided translation and rotation-independent localization of the test objects in the MAP images. The visibility classification decision was modeled by a Bayesian classifer using threshold contrast. The 50% visibility contrast threshold established by the trained observers' responses were: fibrils 1.010, microcalcifications 1.156, and nodules 1.016. Using these values as an estimate of human observer performance and given the automated localization of test objects, six images were graded with the computer algorithm. In all but one instance, the algorithm scored the images the same as the diagnostic physicists. In the case where it did not, the margin of disagreement was 10% due to the fact that the human scoring did not allow for half-visible fibrils (agreement occurred for the other test objects). The implication from this is that an operator-independent, machine-based scoring of MAP images is feasible and could be used as a tool to help eliminate the effect of observer variability within the current system, given proper, consistent digitization is performed.

A new approach to film dosimetry for high energy photon beams: lateral scatter filtering.

A method of film dosimetry for high energy photon beams is proposed which reduces the required film calibration exposures to a set of films obtained for a small radiation field size and shallow depth (6 cm x 6 cm at 5 cm depth). It involves modification of a compression type polystyrene film phantom to include thin lead foils parallel to the vertical film plane at approximately 1 cm from both sides of the film emulsion. The foils act as high atomic number filters which remove low energy Compton scatter photons that otherwise would cause the film sensitivity to change with field size and depth. The proposed method is best described as "lateral scatter filtering." To validate the proposed method, central axis depth doses and isodose curves for a 4 MV photon beam were determined from films exposed within the modified phantom and the results compared with ionization chamber measurements. When no lateral filtering was used, for field sizes of 6 cm x 6 cm and 25 cm x 25 cm, this comparison demonstrated up to a 65% difference between film and ionization chamber central axis depth dose measurements. When using the lateral scatter filtering technique, less than a 4% difference was observed for these field sizes.

Simulated annealing image reconstruction method for a pinhole aperture single photon emission computed tomograph (SPECT).

A series of computer experiments was performed to determine the relative performance of simulated annealing, quenched annealing, and a least-squares iterative technique for image reconstruction for single photon emission computed tomography (SPECT). The simulated SPECT geometry was of the pinhole aperture type, with 32 pinholes and 128 or 512 detectors. To test the robustness of the reconstruction techniques upon arbitrary geometries, a 360-detector geometry with a random pixel-detector-factor matrix was tested. Eight computer-simulated, 10-cm-diameter planar phantoms were used with 1961 2-mm(2) reconstruction bins and a range of 3000 to 50,000,000 detected photon counts. Reconstruction quality was measured by a normalized, squared error picture distance measure. Over a wide range of noise, the simulated annealing method had slightly better reconstruction quality than the iterative method, although requiring greater reconstruction time. Quenched annealing was faster than simulated annealing, with comparable reconstruction quality. Methods of efficiently controlling the simulated annealing algorithm are presented.

Implementation of digital stereo imaging for analysis of metaphyses and joints in skeletal collections.

The surface structure of the growing portion of bones, called the metaphysis, contains clues about the locomotor characteristics of various species. Present methods of capturing this anthropologically interesting surface are time-consuming and subject to human error. The research implements a digital stereo imaging technique for bone metaphyses and joints in skeletal collections. The corresponding points in two images collected from different angles are determined using an area-based correlation matching method. The depths of matched points are computed from the difference in location of the points in the two images. The paper presents a practical implementation of computer vision for anthropology using an 80286-based personal computer, a camera and a video digitiser. The stereo matching algorithm, a practical implementation of classical stereo imaging methods, takes less than 1 min and produces reasonable representations of mammal bones. The accuracy of the depth measurements ranged from 0.7 to 12 per cent for 45-150 cm object-camera distances. False matches occurred in approximately 6 per cent of the total matched points.

Preliminary experiences with 222Rn gas in Arizona homes.

Results of a survey of 222Rn gas using four-day charcoal canister tests in 759 Arizona homes are reported. Although the study was not random with respect to population or land area, it was useful in identifying areas at risk and locating several homes having elevated indoor 222Rn air concentrations. Approximately 18% of the homes tested exceeded 150 Bq m-3 (4 pCi L-1), with 7% exceeding 300 Bq m-3 (8 pCi L-1). Several Arizona cities had larger fractions of homes exceeding 150 Bq m-3 (4 pCi L-1), such as Carefree and Cave Creek (23%), Paradise Valley (30%), Payson (33%), and Prescott (31%). The Granite Dells and Groom Creek areas of Prescott had in excess of 40-60% of the houses tested exceeding 150 Bq m-3 (4 pCi L-1). Elevated 222Rn concentrations were measured for a variety of home types having different construction materials. Private well water was identified as a potentially significant source of 222Rn gas in Prescott homes, with water from one well testing over 3.5 MBq m-3 (94,000 pCi L-1). A 222Rn concentration in air exceeding 410,000 Bq m-3 (11,000 pCi L-1) was measured using a four-day charcoal canister test in a house in Prescott which had a well opening into a living space. Additional measurements in this 150-m3 dwelling revealed a strikingly heterogeneous 222Rn concentration. The excessive 222Rn level in the dwelling was reduced to less than 190 Bq m-3 (5.2 pCi L-1) by sealing the well head with caulking and providing passive ventilation through a pipe.

Performance of a well counter and a dose calibrator for quantitative positron emission tomography.

Quantitative nuclear medicine techniques such as positron emission tomography (PET) require precise and accurate knowledge of both the amount of radioactivity administered to a patient and the radioactivity concentration in blood as a function of time. In addition, the uncertainties in such measurements must be known so that the accuracy and precision of quantitative in vivo metabolic measurements may be estimated. In order to characterize and minimize the measurement errors for PET, well counter and dose calibrator performances were studied over long and short time periods using positron-emitting isotopes. To ensure accurate quantitation with well counters, the use of correction factors for sample volume and high count rate effects is essential. Only small drifts in well counter sensitivity were observed during a 24-h period, but longer-term drifts and poor performance emphasized the need for continuing quality control procedures. Similar behavior was observed for dose calibrators. Dose calibrator, rather than well counter, data should be used for PET imaging instrument calibration. The methods presented have direct application to any quantitative nuclear medicine program.