Volume 21, Issue 10, October 1994
Index of content:
21(1994); http://dx.doi.org/10.1118/1.597262View Description Hide Description
The detection of cancer in the radiographically dense breast is problematic, since the breast will produce a range of exposure that exceeds the useful dynamic range of high contrast film‐screen combinations. It has been shown previously that mammographic scanning equalization radiography (MSER) can be used to overcome the latitude limitations of film‐screen mammography. However, the tube loading of MSER is orders of magnitude greater than conventional mammography. A new rotary geometry for equalization radiography is proposed, in which the image receptor is exposed by repeated scans of a modulated slot beam, oriented at a variety of scanning angles with respect to the object. The superposition of the exposure from appropriately modulated, rotated slot beams produces an entrance exposure that will effectively equalize the film exposure. The principle advantages of this geometry is its simplicity and reduced tube loading. To determine the effectiveness and feasibility of RSER the effect of conventional, MSER, and RSER have been numerically simulated on the appearance of clinical mammograms, the relative heat loading, and the fraction of the breast imaged with high contrast are calculated. It is found that RSER produces images that are free of artefacts, and exhibit a similar degree of equalization, as found in MSER images. RSER accomplishes this with only four scanning angles, and a beam that is approximately 4 cm wide. The resulting tube loading is only three times greater than that found in conventional imaging. Numerical simulations indicate that RSER is a simple, feasible means of overcoming the latitude limitations of film‐screen mammography.
21(1994); http://dx.doi.org/10.1118/1.597263View Description Hide Description
Fractalanalysis has recently been suggested [Med. Phys. 20, 1611–1619 (1993)] as a means to characterize the structure of cancellous bone by measuring the fractal dimension using a box counting algorithm. This work re‐examines the possible fractal nature of such structures on nuclear magnetic resonance(NMR)images of cancellous bone by estimating the trabecular boundary length as a function of box size under various experimental conditions. On high‐resolution images (pixel sizes on the order of 50 μm) and signal‐to‐noise ratios of 30, the trabecular boundary turns out to be a smooth surface relative to the achievable resolution and is thus nonfractal. The fractal dimension of the trabecular structure is undefined and can vary significantly as a function of image signal‐to‐noise ratio. The present work further indicates the ‘‘apparent’’ fractal dimension obtained by box counting to be a reflection of marrow pore size. In conclusion, the results indicate that, at the currently achievable resolution, the box counting algorithm is not suitable for fractalanalysis on images of cancellous bone and that the fractal appearance of the trabecular network reported previously is artifactual.
An analytical edge spread function model for computer fitting and subsequent calculation of the LSF and MTF21(1994); http://dx.doi.org/10.1118/1.597264View Description Hide Description
The previous work of Yin, Giger, and Doi [Med. Phys. 17, 962–966 (1990)] demonstrated that using a computerized fit of an analytic line spread function to experimentally measured data is very useful for determining the presampling modulation transfer function of an imaging system. In this report, the work of Yin et al. is extended to include an analytic expression for the edge spread function (ESF). By fitting experimentally determined edge spread function data to the analytical expression, the normalized line spread function (LSF) and the normalized modulation transfer function(MTF) can be easily calculated from four ESF fit coefficients. The extension from the line spread function to the edge spread function should be valuable in cases where slit measurements are impractical, for example, in very high resolution imaging systems where the required slit dimensions become impractically small, or in measurements of the transfer properties of scattered radiation or off‐focus radiation, where large area exposures are necessary.
21(1994); http://dx.doi.org/10.1118/1.597411View Description Hide Description
This study evaluated the relative roles of physical and perceptual factors in flattening the contrast‐detail (CD) curve on liver CT scans. To estimate the role of physical factors, the theoretical CD curve for a calculated theoretical observer (i.e., a nonprewhitening matched filter) was predicted using the measured noise power spectrum and measured modulation transfer function of the CT system. Another theoretical CD curve was also produced from the output of the same calculated observer after taking the human visual response function (VRF) into account. Perceptual factors were evaluated by analyzing human observers’ replicated ratings of the visibility of details superimposed on liver CT scans. The CD curve for the calculated theoretical observer was below the CD curve actually measured for nine human observers and showed no flattening. With the VRF included, flattening of the theoretical CD curves was only produced by fixed image viewing distances of less than 30 cm, a reading style not employed by the human observers. Correlated ROC analysis of observers’ replicated ratings indicated that while random, intraobserver variation was present, the magnitude of this so‐called observer noise was insufficient to explain the flattening of CD curves. Use of narrow display windows did not eliminate this flattening effect. The main reason for human observers’ inefficient detection of large, low contrast liver lesions appears to be a consistent misuse of the image information.
21(1994); http://dx.doi.org/10.1118/1.597265View Description Hide Description
The Rotoscan is a computed tomographyscanner that combines the advantages of variable geometric resolution and adjustable size of measurement diameter of translate‐rotate scanners with the improved speed of rotate‐only scanners. Because of the small number of only 26 detectors used for this scanner, a special data collection scheme of multiple rotations with interleaved detector positions was employed. In order to avoid angular data interpolation after reordering of the projections from the fan‐ to a parallel‐beam geometry, the detectors were incrementally moved at a right angle to the centerline of the fan rather than rotated about the source. The measurement time of 40 s for one cross‐section is comparable to that of second‐generation systems. However, for longer measurement diameters, the measurement time for second‐generation systems increases, whereas that of the Rotoscan remains constant.
A simulation of emission and transmission noise propagation in cardiac SPECT imaging with nonuniform attenuation correction21(1994); http://dx.doi.org/10.1118/1.597266View Description Hide Description
Transmission computed tomography provides information needed for nonuniform attenuation correction of cardiacsingle photon emission computed tomography(SPECT). Nonuniform attenuation correction is accomplished using an iterative ML‐EM algorithm and a projection‐backprojection operation that incorporates attenuation factors measured from the reconstructed transmission map. The precision and accuracy of the attenuation corrected emission reconstruction is a function of emission and transmission statistics. This paper presents an error propagation analysis that uses a mathematical cardiac chest phantom to simulate various combinations of total emission counts C̄ and transmission flux Ī0 under ideal imaging conditions (without geometric response distortion and without scatter). The spatial average, spatial variance, and accuracy measures for a 4×4 pixel region in the heart are tabulated after 30 iterations of the ML‐EM algorithm. The confidence intervals for these measures were determined from 1000 realizations of reconstructions from projections randomly generated with the same transmission and emission statistics. It can be shown empirically from the simulation results that the spatial %rms uncertainty for the simulated cardiac region has a simple expression: %rms2=K 1/C̄+K 2/Ī2 0+B 2 where K 1 and K 2 are least‐square estimates based on the simulation results, and B is the measuredspatial %rms uncertainty for the simulation at infinite statistics. For a transmission incident flux of 1500 events per projection bin of 0.712 cm and typical clinical emission events totaling 1×105, the spatial %rms uncertainty is approximately 14%. At clinical transmission and emission statistics, the statistical noise in the simulated attenuation‐corrected reconstructions are dominated by the emission statistics.
A new dual‐isotope convolution cross‐talk correction method: A TL‐201/TC‐99m SPECT cardiac phantom study21(1994); http://dx.doi.org/10.1118/1.597234View Description Hide Description
Simultaneous dual‐isotope SPECTimaging provides a clear advantage in situations where two concurrent metabolic, anatomic, or background measurements are desired. It obviates the need for two separate imaging sessions, reduces patient motion problems, and provides exact image registration between images. However, a potential limitation of dual‐isotope SPECTimaging is contribution of scattered and primary photons from one radionuclide into the second radionuclide’s photopeak energy window, referred to here as cross‐talk. Cross‐talk in both photopeak energy windows can significantly degrade image quality, resolution, and quantitation to an unacceptable level. Simple cross‐talk correction method used in dual‐radionuclide in vitro counting, even applied on a pixel‐by‐pixel basis, does not account for the differences in spatial distribution of the photopeak and cross‐talk photons. Here a new convolution cross‐talk correction method is presented. The convolution filters are derived from point response functions (PRFs) for Tc‐99m and Tl‐201 point sources. Three separate acquisitions were performed, each with two 20% wide energy windows, one centered at 140 keV and another at 70 keV. The first acquisition was done with Tc‐99m solution only, the second with Tl‐201 solution only, and the third with a mixture of Tc‐99m and Tl‐201. The nonuniform RH‐2 thorax–heart phantom was used to test a new correction technique. The main difficulty and limitation of the convolution correction approach is caused by the variation in PRF as a function of depth. Thus, average PRF should be used in the creation of an approximative filter. The results of this study show that simple cross‐talk correction provides Tl‐201 images of poor quality. However, the proposed method, based on convolution technique, improves the quality of the simultaneous Tl‐201/Tc‐99m SPECTimaging.
21(1994); http://dx.doi.org/10.1118/1.597412View Description Hide Description
Functional single photon emission computed tomography(SPECT)images of brain activation are based on a comparison of base line and activation images. The correctness of the functional images depends, among other factors, on the accurate spatial registration (alignment) of the base line and activation image data. The relationship between the registration errors and the errors of the resulting functional images is studied. It is shown that misregistration errors as small as a shift by pixel or rotation by 1° result in 5%–10% errors of the pixel values of functional SPECTimages of regional blood flow (the ratio and the relative difference images).
21(1994); http://dx.doi.org/10.1118/1.597211View Description Hide Description
Energy straggling along electron trajectories has been incorporated into a numerical algorithm for electron beam dose calculations. Landau’s theory is used to predict, at any point in the absorber, the broadening of the primary electron energy spectrum due to energy loss straggling. Numerical calculations have been performed for electron beams with energies of 10–30 MeV incident upon water in order to determine the variation of dose with depth and variation of energy spectra with pathlength. These calculations are compared with the results of Monte Carlo simulations performed with the EGS4 code. The inclusion of energy loss straggling in the numerical calculations leads to predictions of energy spectra and dose deposition that are in good agreement with modified Monte Carlo simulations in which bremsstrahlung is ignored and the energy given to knock‐on electrons is deposited at the site of their creation. Less satisfactory agreement was achieved when these calculations were compared to full Monte Carlo simulations that included the bremsstrahlung events and transported the knock‐on electrons. It is concluded that bremsstrahlung energy loss must also be included into this algorithm, if an acceptable dose computation accuracy is to be achieved for clinical applications.
21(1994); http://dx.doi.org/10.1118/1.597260View Description Hide Description
Conventional linear accelerators have four field‐defining jaws or collimators. Usually, one set of the two opposing jaws moves concurrently to define the field width and the other set defines the field length. The resultant square or rectangular field will have the field centerline coincide with the collimator axis. However, some modern linacs have independent collimators or jaws that can be set asymmetrically. In this case, one of the two opposing jaws can be closed down independently of the other one to define an asymmetric field of smaller dimension. The field center now does not coincide with the collimator axis. Asymmetric collimators have found many clinical applications, but have complicated the dosimetry for physicists.Data acquisition and treatment planning implementations are tedious and complicated. An algorithm has been developed to correct for the reduced dose in the smaller asymmetric field. The approach used is similar in principle to the Day’s equivalent field calculation. The difference in dose between an asymmetric and a symmetric radiation field is accounted for by a correction factor that is a function of the asymmetric and symmetric field sizes, off axis distance, and depth of measurement. The correction method presented here applies only to the closing down of one independent jaw. Beam profiles for asymmetric fields are measured for both the 6 and 10 MV photon beams. For a 6 MV photon beam, the calculated doses at the asymmetric field center at depths of 2, 10, and 25 cm are within 0.2% of the measured doses for a 20×20 cm2 field blocked to a 5×20, 10×20, and 15×20 cm2 asymmetric field. The agreement between the calculated and measured doses for the 10 MV photon beam is, on average, within 0.5%. The calculated beam profiles for asymmetric fields are also found to be in good agreement with measurements for both the 6 and 10 MV photon beams. The calculation method has been implemented into an in‐house developed treatment planning system. Examples of isodose distributions for the 6 MV asymmetric fields show good agreement with the measured data.
21(1994); http://dx.doi.org/10.1118/1.597261View Description Hide Description
LiF: Mg,Cu, P is a TL material presenting unique dosimetric features. The TL sensitivity of this material was studied as a function of the annealing temperature and of the repeated cycles of annealing–irradiation–readout. A fading study was carried out over a period of 40 days with the purpose of checking the stability of the stored dosimetric information as a function of different annealing temperatures. A detailed statistical analysis of sets of data, obtained from repeated measurements on a group of ten dosimeters, is presented.
Mixed field dosimetry of epithermal neutron beams for boron neutron capture therapy at the MITR‐II research reactor21(1994); http://dx.doi.org/10.1118/1.597267View Description Hide Description
During the past several years, there has been growing interest in BoronNeutron Capture Therapy (BNCT) using epithermal neutron beams. The dosimetry of these beams is challenging. The incident beam is comprised mostly of epithermal neutrons, but there is some contamination from photons and fast neutrons. Within the patient, the neutron spectrum changes rapidly as the incident epithermal neutronsscatter and thermalize, and a photon field is generated from neutron capture in hydrogen. In this paper, a method to determine the doses from thermal and fast neutrons,photons, and the B‐10(n,α)Li‐7 reaction is presented. The photon and fast neutron doses are measured with ionization chambers, in realistic phantoms, using the dual chamber technique. The thermal neutron flux is measured with gold foils using the cadmium difference technique; the thermal neutron and B‐10 doses are determined by the kerma factor method. Representative results are presented for a unilateral irradiation of the head. Sources of error in the method as applied to BNCT dosimetry, and the uncertainties in the calculated doses are discussed.
Design of a high‐flux epithermal neutron beam using 235U fission plates at the Brookhaven Medical Research Reactor21(1994); http://dx.doi.org/10.1118/1.597268View Description Hide Description
Beams of epithermal neutrons are being used in the development of boron neutron capture therapy for cancer. This report describes a design study in which 235U fission plates and moderators are used to produce an epithermal neutron beam with higher intensity and better quality than the beam currently in use at the Brookhaven Medical Research Reactor (BMRR). Monte Carlo calculations are used to predict the neutron and gamma fluxes and absorbed doses produced by the proposed design. Neutron flux measurements at the present epithermal treatment facility (ETF) were made to verify and compare with the computed results where feasible. The calculations indicate that an epithermal neutron beam produced by a fission‐plate converter could have an epithermal neutron intensity of 1.2×1010 n/cm2.s and a fast neutron dose per epithermal neutron of 2.8×10−11 cGy.cm2/nepi plus being forward directed. This beam would be built into the beam shutter of the ETF at the BMRR. The feasibility of remodeling the facility is discussed.
21(1994); http://dx.doi.org/10.1118/1.597269View Description Hide Description
Accelerator‐based intense epithermal neutron sources for Neutron Capture Therapy (NCT) have been considered as an alternative to nuclear reactors. Lithium (Li) has generally received the widest attention for this application, since the threshold energy is low and neutron yield is high. Because of the poor thermal and chemical properties of Li and the need for heat removal in the target, the design of Li targets has been quite difficult. Beryllium (Be) has been thought of as an alternative target because of its good thermal and chemical properties and reasonable neutron yield. However, in order to have a neutron yield comparable to that of a thick Li target bombarded with 2.5 MeV protons, the proton energy required for a thick Be target must be approaching 4 MeV. Consequently, the neutrons emitted are more energetic. In addition, a significant amount of high‐energy gamma rays, which is undesirable, will occur when Be is bombarded with low‐energy protons. Regardless of the more energetic neutrons and additional gamma rays, in this paper it is shown that it is possible to develop a high‐quality and high‐intensity epithermal neutron beam based on a thick Be target for NCT treatment. For a fixed proton current, the optimal Be‐target‐based beam (with 4‐MeV protons) can produce a neutron beam, with both quality and intensity slightly better than those produced by the optimal Li‐target‐based beam (with 2.5‐MeV protons). The single‐session NCT treatment time for the optimal Be‐target‐based beam is estimated to be 88 min for a proton current of 50 mA. The major advantage of a Be target over a Li target is the simplicity in design, construction, operation, and maintenance.