摘要:

A model based on the EGS4 Monte Carlo system is developed for calculating dose rate to water from an embedded low‐energy brachytherapy source, given measurement data of dose rate to water within a water‐substitute solid phantom for a source of given strength. The EGS4‐based model is used to calculate point source dose rate distributions per unit source strength for water and for several species of solid phantoms. As a model for the dose rate distribution to an infinitesimal water inhomogeneity embedded in the solid phantom, the Monte Carlo is used to calculate dose rate to a thin spherical shell of water contained within the solid phantom at various distances centered on the source. Comparing this distribution to that calculated for a homogeneous water phantom, one can derive correction factors to determine dose rate to homogeneous water medium from measurements with water dose‐calibrated detectors within the solid phantom. These factors are calculated for polymethylmethacrylate (PMMA or acrylic), solid water (WT1), and RW‐1, a material optimized for low‐energy dosimetry, with photon spectra from Pd‐103 and from two commercial models of I‐125 seed used as input. For model 6711 I‐125 seeds at 1 cm in PMMA and WT1, the calculated ratios of dose rate to water to dose rate to water in the solid phantom are 0.893 and 1.038, respectively. Applying these factors to published, apparently divergent measurements of the specific dose rate constant based on measurements in PMMA and WT1 phantoms, the specific dose rate constant inferred for the model 6711 seed are found to be in excellent agreement: 0.878 cGy h−1U−1(1.115 cGy‐ cm2h−1mCi−1) from the mean of the WT1 measurements and 0.879 cGy h−1U−1from the PMMA measurements. Among the phantoms studied for Pd‐103 and I‐125, measurements in RW‐1 would most closely approximate those for water.

ISSN:0094-2405    

DOI:10.1118/1.597317

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A difficult, error‐prone, and time consuming problem in brachytherapy is the determination of the three‐dimensional (3‐D) positions of implanted seeds. To triangulate 3‐D seed positions, the corresponding seed images in radiographs taken at different parallax (gantry) angles must be identified (matched). With only two radiographs, there is an inherent problem of ambiguity in matching individual seed images. The use of the assignment algorithm cannot bypass this ambiguity. However, when seeds are implanted within ribbons, their positions in 3‐D space can be determined accurately and (almost always) without ambiguity. This follows because the matching of ribbon images is much less ambiguous than the matching of individual seed images. We describe a semiautomated procedure to match the ribbon images on two radiographs, determine the 3‐D loci of the ribbons, and locate the 3‐D position of each seed on each ribbon. The ribbon matching problem involves a double application of the assignment algorithm. After image matching, the 3‐D locus of each ribbon is found from the intersection of the ray sets from its radiographic images and the analysis of a 3‐D spanning tree through the derived ray set intersection points. The seed positions on a ribbon locus are determined by interpolation of equally spaced seed intervals along the 3‐D ribbon locus. Experiments with a phantom show that for two radiographs separated by any parallax between 20° and 90°, our technique provides a systematic error in 3‐D seed location of 3.0 mm, and a random error of less than 2 mm.

ISSN:0094-2405    

DOI:10.1118/1.597318

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dose response of high‐sensitivity GafChromic film to photons from125I seeds for doses up to 200 Gy was established. The optical densities were measured using two types of densitometers: (a) a Macbeth spot densitometer with broadband light spectrum, and (b) an LKB He‐Ne laser scanning microdensitometer with red light of wavelength 632.8 nm. The net optical density was found to be a power function of dose with exponents of 0.858 and 0.997, for the Macbeth and LKB densitometers, respectively. Film sensitivity with the LKB densitometer was about double of that with the Macbeth densitometer. The dose measurements were performed using the high‐sensitivity GafChromic films for125I model 6702 seed in solid water phantom. Each film was positioned parallel to the seed's long axis and centered at the seed's transverse axis. Films were exposed at various distances, ranging from contact to 3 cm from the seed center. The radiation dose delivered to the film center varied from 7 to 50 Gy, depending on the distance. The optical density at the film center was measured using both types of densitometers. Dose conversion was achieved with the established dose response curves for the respective densitometers. The dose values, along the seed's transverse axis obtained using both densitometers, were compared with each other, and also compared with published thermoluminescent dosimeter (TLD) data and Monte Carlo results. General agreement was found. It was concluded that the high‐sensitivity GafChromic film measurement is a feasible method for125I seed dosimetry in solid water phantom. Furthermore, in order to investigate the energy dependence of the dose response of the high‐sensitivity GafChromic film, the dose responses of such film for photon energies, 660 and 1250 keV, emitted by137Cs and60Co, respectively, for doses up to 200 Gy were studied. The dose response curves, obtained with the LKB laser densitometer, were essentially linear for both radionuclides. However, those curves obtained with the Macbeth densitometer were fitted to power functions of dose, with an exponent of 0.839 and 0.849 for137Cs and60Co, respectively. The relative sensitivities for125I and137Cs compared to that for60Co were found to be 0.56 and 0.93, respectively.

ISSN:0094-2405    

DOI:10.1118/1.597245

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597246

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An alternate method to measure the phantom scatter factor in small fields is provided for high energy photon beams. The measurement technique is based on the density scaling theorem described by O'Connor [Phys. Med. Biol.1, 352–369 (1957)]. The phantom scatter factor (Sp) is measured in balsa and cedar wood to give effective field sizes in 3×3 to 0.5×0.5 cm2water. The extrapolated zero area phantom scatter factor from the average data of balsa and cedar is 0.45. This indicates that the variation of output is largely due to a sharp decrease in the phantom scatter factor for small field sizes.

ISSN:0094-2405    

DOI:10.1118/1.597319

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A study was conducted to evaluate the feasibility of simultaneous dual radionuclide brain imaging with123I and99mTc using photopeak image subtraction techniques or offset photopeak image acquisition. The contribution of the photons from one radionuclide to a second radionuclide's photopeak energy window (crosstalk) was evaluated for SPECT and planar imaging of a brain phantom containing123I and99mTc for a range of activity levels and distribution properties approximating those in rCBF images of the adult human brain. Crosstalk was evaluated for 10% symmetrical energy windows centered on the123I and99mTc photopeaks and for 10% energy windows asymmetrically placed to the left and right of the center of the respective photopeaks. Major observations include: (1) in the centered photopeak windows,99mTc crosstalk in the123I window is 8.9% of the99mTc seen in the99mTc window and ranges from 37.5% to 75.0% of the123I in the123I window.123I crosstalk is 37.8% of the123I seen in the123I window and ranges from 4.4% to 8.9% of the99mTc seen in the99mTc window; (2) the spatial distribution of a radionuclide's crosstalk photons differs from that observed in the radionuclide's photopeak window; (3) a99mTc photopeak window offset to the left does not decrease123I crosstalk, and the percentage of99mTc scattered photons is significantly increased in the window. Offsetting the123I window to the right decreases99mTc crosstalk to 9.0% to 17.9% of the123I counts, but decreases123I sensitivity by 39.9%; and (4) offsetting both photopeak windows to the right decreases the99mTc scattered photons in the99mTc window, but increases123I crosstalk to 17.0% to 33.8% of the99mTc counts. The findings show that image quality, spatial resolution, and quantitative accuracy are degraded to unacceptable levels with the combinations of energy windows tested for dual radionuclide imaging of99mTc and123I. This indicates that dual radionuclide imaging must be thoroughly tested and validated before use in clinical studies.

ISSN:0094-2405    

DOI:10.1118/1.597320

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Most commercial SPECT scanners do not properly compensate for scattering and partial volume effects, which severely impacts the quantitation of clinical and research SPECT studies, especially in CNS dopamine studies. Hence, in this paper, a brain phantom containing a monkey skull was used to calibrate a Picker Prism 3000 SPECT camera for quantitative studies of123I and99mTc‐labeled radiopharmaceuticals. In the calibration study, two small vials representing basal ganglia were inserted into a skull phantom. A series of images were taken with different concentration ratios between the basal ganglia and the other brain compartments to simulate the dynamic imaging study of the dopamineD2 receptor in a monkey brain. Based on this calibration, a simple method is proposed for count correction in the region of interest (ROI). This technique is superior to using a single calibration factor from a cylindrical uniform activity phantom since it can compensate for both the partial volume effect and the dynamically changing effect of scattering. A significant difference between the true and measured values was observed in the correction factors (by a factor as much as 3). Furthermore, a cylindrical phantom, approximately the size of a monkey's head and containing the same basal ganglia inserts, was used to investigate the feasibility of using a cylindrical phantom with the proposed correction method. The results indicate that it is feasible to use a cylindrical phantom for the case where the activity inside the brain cavity and the activity in the surrounding tissue are similar.

ISSN:0094-2405    

DOI:10.1118/1.597321

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The physical characteristics of a portable nonimaging scintillation probe system for continuous ambulatory monitoring of the left ventricular function are described. The detector of the equilibrium radionuclide labeled blood pool is a single cesium iodide (CsI) crystal coupled to a silicium photodiode and interfaced to a microcomputer. The spatial properties of this small CsI crystal (1×1×1 cm3) were evaluated with various single‐hole collimators. Linearity was studied in nonattenuating medium. Saturation began at 3000 cps, count loss was 10% at 4000 cps, maximal count rate was 24 000 cps. In attenuating medium, isocount curve of 5% of the maximal count rate was 100 mm deep and 160 mm wide. The most appropriate tested lead collimator to record the global ejection fraction of the left ventricle was a disc‐shaped (thickness 5 mm, diameter 41 mm) single‐hole (proximal aperture 8 mm, distal aperture 18 mm) collimator. Sensitivity was similar to the sensitivity of a sodium iodide nuclear probe. The detection performance appeared comparable to other available detector systems. Our results indicate that such a CsI‐photodiode probe is a promising candidate for left ventricular function monitoring. The application to an ambulatory multicrystal detector system is presented and discussed.

ISSN:0094-2405    

DOI:10.1118/1.597171

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Contrast‐detail (CD) analysis was used to compare the low‐contrast detectability of computed radiography (CR) and screen‐film (SF) as applied to the task of adult chest radiography. A phantom was constructed and imaged using the same exposure factors throughout all experiments. Within‐observer variance, between‐observer variance, and image sample variance were calculated and used to estimate the standard error for each experiment. The results of these CD experiments agreed with the predictions of the Rose model. Observers performed equally well for low‐contrast target detection using CR and SF.

ISSN:0094-2405    

DOI:10.1118/1.597172

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fluorine‐19 nuclear magnetic resonance of perfluorinated blood substitute materials provides a method for determination of oxygen tension (pO2)invivo. Use of a double resonant19F/1H radio frequency coil allows convenient correlation between the high resolution anatomic presentation of proton images and the fluorine distribution. However, quantitative19F measurements require an RF coil with goodH1field homogeneity over the image volume and a high quality factor (Q) to minimize errors caused by the low signal‐to‐noise levels available ininvivoimaging and image nonuniformities introduced by the large chemical shift of fluorocarbons. The birdcage coil design provides a highQstructure with optimumH1field uniformity and fill factor. However, at low resonance frequencies, the inherently low inductance of the birdcage geometry requires the use of a large number of chip capacitors giving rise to unwieldy coil fabrication and increased cost. This communication describes a modification to the birdcage design that reduces the chip capacitor requirement by at least a factor of 4 for a given dimension, yet retains the essential characteristics of the birdcage design. The modified structure was tuned for double resonance at 5.7/6.0 MHz for19F/1H magnetic resonance imaging at 0.14 T. For a coil with a length to diameter ratio of 1.67, anH1uniformity of ±2% for the19F resonance was obtained over a cylindrical region with radius ∼0.6r(r=radius of coil) and length ∼1.8rwithin the coil.

ISSN:0094-2405    

DOI:10.1118/1.597173

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY