Volume 24, Issue 1, January 1997
Index of content:
24(1997); http://dx.doi.org/10.1118/1.597917View Description Hide Description
A new special-purpose computed tomographic (CT)imaging system is described which produces images based on measurements of the low-angle x-ray diffraction properties of an object. Low-angle scatter in the diagnostic x-ray energy range is dominated by coherent scatter, and the system uses first-generation CT geometry to acquire a diffraction pattern for each pencil beam. The patterns are used to reconstruct a series of images which represent the coherent-scatter intensity at a series of scatter angles. To demonstrate the potential of coherent-scatter CT (CSCT), the scanner has been built and used to image a phantom consisting of a water-filled Lucite cylinder containing rods of polyethylene, Lucite, polycarbonate, and nylon. In this paper, the system is described and a sequence of CSCT images of this phantom is shown. Coherent-scatter cross sections of these materials are generated for each pixel from this sequence of images and compared with cross sections measured separately. The resulting excellent agreement shows that the angular-dependent coherent-scatter cross section can be accurately imaged in a tomographic slice through an object. These cross sections give material-specific information about the object. The long-term goal of this research is to make measurements of bone-mineral content for every pixel in a tomographic slice
24(1997); http://dx.doi.org/10.1118/1.597921View Description Hide Description
The latest American College of Radiology (ACR) MammographyQuality Control Manual contains a new method for evaluating focal spot performance, which this paper refers to as the “line-pair pattern test.” The ACR describes a variety of methods for performing this test, and does not advocate one method over another. The authors of this paper conducted an investigation to compare the optional ways for performing the test. Resolution measurements were obtained using a prototype line-pair resolution phantom imaged with a GE DMR mammography unit. Measurements were made with the line-pair pattern 4.5 cm above the breast support platforms in both conventional (contact) and magnification geometries. Both 4.5 cm of air and Lucite were tested as attenuators between the line-pair pattern and the breast support platform. Image receptors that were employed included film alone, screen-film, and screen-film that was not allowed to wait the recommended 15 min before exposure. kVp was varied as was the orientation of the line-pair pattern relative to the chest wall. For the air attenuator case, the screen degraded the measured resolution by 1–3 lp/mm when compared to the direct film. The Lucite attenuator reduced the resolution by an additional 1 lp/mm. Increasing kVp improved the resolution slightly for the conventional mode, but decreased it slightly for the magnification mode. Based upon the results of this study, recommendations are made for improving the test protocol. For a test of focal spot performance, one should use the no-attenuation with direct film detector setup. For a measure of the resolution of the entire imaging chain, one should use the Lucite attenuator with screen-film detector setup.
24(1997); http://dx.doi.org/10.1118/1.597941View Description Hide Description
The purpose of this study was to compare identifications of microcalcification clusters on mammograms by a computerized detection scheme and by human observers having their eye position recorded. Eighty digitized mammograms (half with a subtle microcalcification cluster) were analyzed by a computerized detection scheme and then were read from laser-printed films by six mammographers while eye position was recorded. The computer had 83% true positives with a false-positive rate of 0.5 per image. The true positives of the radiologists ranged from 78% to 90%, with false-positive rates ranging from 0.03 to 0.20. Locations of true and false positives identified by computer and by the human were compared. All but 5% of the true clusters were identified by either the computer, human, or by both. Here 10% of the clusters were detected by only the computer, and 11% were missed by the computer but detected by at least one radiologist. False positives were of three types: identified by computer only, by the human reader only, or by both. Eye-position data indicated significant differences in dwell time between both true-positive and false-positive locations reported by the radiologist versus the computer detections. A follow-up analysis indicated that microcalcification clusters and false positives were judged to have more identifiable characteristics of true calcifications and were associated with longer gaze durations than those with fewer microcalcification characteristics. In general, the computer was able to detect clusters judged to have few or no features that the radiologists were not able to detect. Comparison of computer versus human identification of microcalcification clusters may be useful for improving computerized detection schemes to serve as clinical aids to mammographers, and for understanding what image features lead to false-positive decisions for both the computer and the human reader.
Automated three-dimensional registration of magnetic resonance and positron emission tomography brain images by multiresolution optimization of voxel similarity measures24(1997); http://dx.doi.org/10.1118/1.598130View Description Hide Description
Approaches using measures of voxel intensity similarity are showing promise in fully automating magnetic resonance(MR) and positron emission tomography(PET)image registration in the head, without requiring extraction and identification of corresponding structures. In this paper a method of multiresolution optimization of these measures is described and five alternative measures are compared: cross correlation, minimization of corresponding PET intensity variation, moments of the distribution of values in the intensity feature space, entropy of the intensity feature space and mutual information. Their ability to recover registration is examined for ten clinically acquired image pairs with respect to the size of initial misregistration, the precision of the final result, and the accuracy assessed by visual inspection. The mutual information measure proved the most robust to initial starting estimate, successfully registering 98.8% of 900 trial misregistrations. Success is defined as providing a visually acceptable solution to a trained observer. A high resolution search ( mm step size) of 30 trial misregistrations showed that optimization using the mutual information measure provided solutions with 0.13 mm, 0.11 mm and 0.17 mm standard deviations in the three Cartesian axes of the translation vector and 0.2°, 0.3° and 0.2° standard deviations for rotations about the three axes. The algorithm takes between 4 and 8 minutes to run on a typical workstation, including visual inspection of the result.
X-ray imaging technique for in vitro tissue composition measurements using saline/iodine displacement: Technique optimization24(1997); http://dx.doi.org/10.1118/1.597916View Description Hide Description
An in vitroradiographic technique which uses saline/iodine displacement has been developed to study the thickness of bone-equivalent and soft-tissue-equivalent materials within atherosclerotic plaques in arterial specimens which have been cut open longitudinally and laid flat. Results concerning the optimization of the imaging parameters are presented and discussed. The technique consists of imaging arterial specimens under two different conditions: (1) when it is immersed in an isotonic saline solution, to estimate the calcium content, and (2) when it is immersed in a concentrated iodine solution, to estimate the total thickness of the specimen. Calibration step wedges made out of bone-mimicking and soft-tissue-mimicking materials are imaged simultaneously to generate calibration curves which are used to convert the radiographs into bone-equivalent and soft-tissue-equivalent thickness images. The optimal spectral parameters were determined to be 45 and 100 kV P for the saline and the iodine images, respectively, with a significant amount of added filtration for both images. Inherent systematic inaccuracies due to (1) the nonidealities due to linear attenuation coefficient mismatch between tissue and calibrationmaterials and (2) beam hardening due to heel effect are determined theoretically, and can be used to correct a set of bone-equivalent and the soft-tissue-equivalent images to within ±6 μm with an ideal, noise-free imaging system.
Empirical investigation of the signal performance of a high-resolution, indirect detection, active matrix flat-panel imager (AMFPI) for fluoroscopic and radiographic operation24(1997); http://dx.doi.org/10.1118/1.597918View Description Hide Description
Signal properties of the first large-area, high resolution, active matrix, flat-panel imager are reported. The imager is based on an array of 1536×1920 pixels with a pixel-to-pixel pitch of 127 μm. Each pixel consists of a discrete amorphous siliconn-i-pphotodiode coupled to an amorphous siliconthin-film transistor. The imager detects incident x rays indirectly by means of an intensifying screen placed over the array. External acquisition electronics send control signals to the array and process analog imaging signals from the pixels. Considerations for operation of the imager in both fluoroscopic and radiographic modes are detailed and empirical signal performance data are presented with an emphasis on exploring similarities and differences between the two modes. Measurements which characterize the performance of the imager were performed as a function of operational parameters in the absence or presence of illumination from a light-emitting diode or x rays. These measurements include characterization of the drift and magnitude of the pixel dark signal, the size of the pixel switching transient, the temporal behavior of pixel sampling and the implied maximum frame rate, the dependence of relative pixel efficiency and pixel response on photodiode reverse bias voltage and operational mode, the degree of linearity of pixel response, and the trapping and release of charge from metastable states in the photodiodes. In addition, x-ray sensitivity as a function of energy for a variety of phosphor screens for both fluoroscopic and radiographic operation is reported. Example images of a line-pair pattern and an anthropomorphic phantom in each mode are presented along with a radiographicimage of a human hand. General and specific improvements in imager design are described and anticipated developments are discussed. This represents the first systematic investigation of the operation and properties in both radiographic and fluoroscopic modes of an imager incorporating such an array.
Empirical and theoretical investigation of the noise performance of indirect detection, active matrix flat-panel imagers (AMFPIs) for diagnostic radiology24(1997); http://dx.doi.org/10.1118/1.597919View Description Hide Description
Noise properties of active matrix, flat-panel imagers under conditions relevant to diagnostic radiology are investigated. These studies focus on imagers based upon arrays with pixels incorporating a discrete photodiode coupled to a thin-film transistor, both fabricated from hydrogenated amorphous silicon. These optically sensitive arrays are operated with an overlying x-ray converter to allow indirect detection of incident x rays. External electronics, including gate driver circuits and preamplification circuits, are also required to operate the arrays. A theoretical model describing the signal and noise transfer properties of the imagers under conditions relevant to diagnostic radiography,fluoroscopy, and mammography is developed. This frequency-dependent model is based upon a cascaded systems analysis wherein the imager is conceptually divided into a series of stages having intrinsic gain and spreading properties. Predictions from the model are compared with x-ray sensitivity and noise measurements obtained from individual pixels from an imager with a pixel format of 1536×1920 pixels at a pixel pitch of 127 μm. The model is shown to be in excellent agreement with measurements obtained with diagnostic x rays using various phosphor screens. The model is used to explore the potential performance of existing and hypothetical imagers for application in radiography,fluoroscopy, and mammography as a function of exposure, additive noise, and fill factor. These theoretical predictions suggest that imagers of this general design incorporating a CsI:Tl intensifying screen can be optimized to provide detective quantum efficiency (DQE) superior to existing screen-film and storage phosphor systems for general radiography and mammography. For fluoroscopy, the model predicts that with further optimization of a-Si:H imagers, DQE performance approaching that of the best x-ray image intensifier systems may be possible. The results of this analysis suggest strategies for future improvements of this imaging technology.
24(1997); http://dx.doi.org/10.1118/1.597915View Description Hide Description
Results from beta testing of a commercially available three-dimensional electron pencil beam algorithm (CMS FOCUS, Computerized Medical Systems, Inc. St. Louis, MO) are reported. Straight on beams were evaluated at normal and extended distances, and obliquely incident beams at angles up to 40°. Shaped electron fields with small circular cutouts, and narrow elongated, centered and offcentered, rectangular field shapes were investigated. Slab inhomogeneities were studied for lung and bone equivalent material, and isodose distributions for small inhomogeneities of these materials were compared with film and TLD measurements. All tests reported here were performed with electrons of 6, 12, and 20 MeV from a Cl-1800 accelerator.
24(1997); http://dx.doi.org/10.1118/1.598063View Description Hide Description
Modern treatment planning systems for three-dimensional treatment planning provide three-dimensionally accurate dose distributions for each individual patient. These data open up new possibilities for more precise reporting and analysis of doses actually delivered to irradiated organs and volumes of interest. A new method of summarizing and reporting inhomogeneous dose distributions is reported here. The concept of equivalent uniform dose (EUD) assumes that any two dose distributions are equivalent if they cause the same radiobiological effect. In this paper the EUD concept for tumors is presented, for which the probability of local control is assumed to be determined by the expected number of surviving clonogens, according to Poisson statistics. The EUD can be calculated directly from the dose calculation points or, from the corresponding dose-volume distributions (histograms). The fraction of clonogens surviving a dose of 2 Gy (SF2) is chosen to be the primary operational parameter characterizing radiosensitivity of clonogens. The application of the EUD concept is demonstrated on a clinical dataset. The causes of flattening of the observed dose-response curves become apparent since the EUD concept reveals the finer structure of the analyzed group of patients in respect to the irradiated volumes and doses actually received. Extensions of the basic EUD concept to include nonuniform density of clonogens, dose per fraction effects, repopulation of clonogens, and inhomogeneity of patient population are discussed and compared with the basic formula.
24(1997); http://dx.doi.org/10.1118/1.597920View Description Hide Description
The Method of Moments is generalized to predict the dose deposited by a prescribed source of electrons in a homogeneous medium. The essence of this method is (i) to determine, directly from the linear Boltzmann equation, the exact mean fluence, mean spatial displacements, and mean-squared spatial displacements, as functions of energy; and (ii) to represent the fluence and dose distributions accurately using this information. Unlike the Fermi–Eyges theory, the Method of Moments is not limited to small-angle scattering and small angle of flight, nor does it require that all electrons at any specified depth z have one specified energy E(z). The sole approximation in the present application is that for each electron energy E, the scalar fluence is represented as a spatial Gaussian, whose moments agree with those of the linear Boltzmann solution. Numerical comparisons with Monte Carlo calculations show that the Method of Moments yields expressions for the depth-dose curve, radial dose profiles, and fluence that are significantly more accurate than those provided by the Fermi–Eyges theory.
24(1997); http://dx.doi.org/10.1118/1.597922View Description Hide Description
The characteristic angle-β concept provides a simple semiempirical method for determination of dose distributions in electron arc therapy. Initially, the method required a set of measured radial depth dose distributions for each electron beam energy used for arc therapy. In this paper, we report an extension of the angle-β concept that enables the determination of arc therapy depth doses for an arbitrary electron energy from the known set of depth dose data at a reference energy. Depth dose distributions of stationary and arc electron beams have been studied in the energy range from 9 to 18 MeV. The stationary electron beams used for electron arc therapy were collimated by photoncollimators only, no secondary collimation was used in our experiments. For stationary electron beams and for arc electron beams with a given characteristic angle β, the depths of dose maximum as well as the depths of a given percentage depth dose beyond the depth of dose maximum are linearly proportional to the mean incident electron energy. This simple geometrical and dosimetric relationship increases the potential usefulness of the angle-β concept in clinical electron arc therapy.
24(1997); http://dx.doi.org/10.1118/1.597913View Description Hide Description
Some linac manufacturers provide cones for defining circular electron fields. During commissioning of one of our new radiotherapy units, we noted that the output of these cones has a strong dependence on the diaphragm opening that precedes the cone. In particular, for the 8 MeV beam, the output of a 3 cm diameter cone increased by more than a factor of 2 as the diaphragm was opened from 5×5 to 20×20 cm. One concludes that the particular mechanical design results in an output factor dependency that is exceptional when compared to the data presented in standard texts such as Khan’s “The Physics of Radiation Therapy.” The importance of quality assurance and communications with the manufacturer is underscored by this example.
24(1997); http://dx.doi.org/10.1118/1.597914View Description Hide Description
The problem of calculating the neutron capture γ-ray dose rate due to thermal neutron capture in a boron or cadmium rectangular shield is considered. An example is given for shielding for a door at the exit of medical accelerator room maze in order to determine the optimum location of lead relative to the borated polyethylene.
24(1997); http://dx.doi.org/10.1118/1.598064View Description Hide Description
Mammographic screening is acknowledged as the best method for the reduction of breast cancer mortality. However, breasts containing a significant fraction of dense, fibroglandular tissue produce a range of exposure which exceeds the dynamic range of conventional mammography. Equalization radiography involves the modulation of the incident x-ray exposure distribution to compensate for variations in x-ray transmission within the patient, ensuring optimal contrast throughout the image. It has been shown that equalization radiograpy offers the potential for dose-efficient, improved lesion detection in the dense breast. However, current equalization geometries are not practical due to tube loading and scan duration inefficiencies. I propose a scan–rotate geometry for equalization radiography [rotary scanning equalization radiography (RSER)] in which the image receptor is exposed by repeated scans of a modulated fan beam, oriented at a variety of angles with respect to the patient. The superposition of exposure from appropriately modulated, rotated fan beams will produce an entrance exposure distribution that will efficiently equalize the film exposure. In this thesis, it is shown that less than half of the area of a dense breast is imaged conventionally with high contrast. The RSER geometry is described and its imaging characteristics are shown theoretically to be similar to current equalization geometries. Furthermore, RSER is resistant to exposure artifacts in typical mammographicimaging tasks. Numerical simulations which compared the imaging performance of RSER, current equalization geometries and conventional mammography show that RSER produces images with the same degree of equalization as current equalization geometries, with minimal tube loading, and only four scanning angles. Finally, an experimental prototype RSER system is described and characterized. Experimental images confirm the theoretical and numerical predictions. High contrast, artifact-free images of anthropomorphic breast phantoms can be obtained with minimal scan times and tube loading. The prototype system increases the fraction of the breast imaged with high contrast from 46% to 80%. These results indicate that RSER is an efficient, simple, and practical means of overcoming the latitude limitations of film screen mammography, and improving the detection of breast cancer.
24(1997); http://dx.doi.org/10.1118/1.598065View Description Hide Description
A Monte Carlo model has been developed, using the EGS4 code, to aid the design and optimization of a polarized source for in vivox-ray fluorescence(XRF) analysis of platinum (Pt). The modeled code is based on a 300 kV clinical x-ray therapy unit at Singleton Hospital. The code handles five individual components (x-ray tube, filter, polarizer, sample, and detector) which the user assembles as needed, thus allowing the facility for simulation of a wide variety of XRF systems. For each component, all the physical dimensions and materials involved are set by the user. The model includes polarized photon transport modification, K-edge fluorescent photon transport modification and K-edge sampling method for mixtures (a facility previously available only for simple compounds). It incorporates a variance reduction technique (forcing photon interaction) and uses random number generator called RANMAR. Also EGS4PICT (The EGS4 Shower Display System for PC) is built in to display a shower picture at every stage of simulation. The selected polarizermaterials tested are C, Al, Fe, Cu, Sn, and Pb. The material shape and thickness are investigated. The secondary beam collimator diameter, length, and material are also studied. Results are compared with analytical models and experiment.