Development of video frame store and distortion correction facilities for an external-beam radiotherapy treatment simulator.
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Calibration↗
Biomedical subjects
W Swindell
Publications and source records attributed to W Swindell.
The temperature fields caused by acoustic standing waves in biological tissues.
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Humans↗
An endocavitary irradiator: two modifications for improved utility.
Two modifications to an endocavitary treatment cone are described, i) Viewing optics are incorporated which allow the treatment area to be observed at all times. ii) The cone is coupled to the tube housing by means of an electromagnetic clamp. These modifications make it easier to define and maintain a given treatment field during the setup procedure. The field can be verified after the X ray tube is in place, and if desired it can be monitored continuously during the application of therapy.
Brachytherapy↗
Breast tissue classification using diagnostic ultrasound and pattern recognition techniques: I. Methods of pattern recognition.
This paper discusses the application of statistical pattern recognition techniques to problems in diagnostic ultrasound. Using our own system as an example, we describe the concepts and specific methods that we have applied to a problem involving the computer-aided classification of breast tissue in vivo. Topics include feature generation, feature selection and classification, as well as a method which estimates the probability of error on classifying future data. An accompanying paper applies these methods to the classification of backscattered RF signals from normal and diseased breast tissue.
Breast Diseases↗
Breast tissue classification using diagnostic ultrasound and pattern recognition techniques: II. Experimental results.
The methods of statistical pattern recognition have been applied to the problem of in vivo ultrasonic characterization of breast disease in humans. Backscattered A-mode signals obtained from a commercial pulse imaging system were used to generate a large set of potentially useful features. Using statistical tests, a small subset of discriminatory features was selected to design a Bayes decision rule for each of two tissue classification schemes: malignant disease vs. benign disease, and malignant disease vs. (benign disease + normal tissue). Classification results obtained by the rotation method included sensitivities of 88 percent and 76 percent for the two schemes, based on data obtained from 32 women. These results are encouraging, though a definitive statement concerning the extrapolation of these numbers to the general population should only be made after obtaining results with a large data base.
Breast Diseases↗
A low-cost computed tomographic scanner.
The authors describe a film-recorded computed tomographic (CT) scanner that uses optical processing methods for reconstruction of a sectional image. Results are comparable to commercial CT methods using the same patient dose. Consequently, clinically satisfactory CT imaging can be performed at a substantially reduced cost.
Animals↗
A theoretical study of nonlinear effects with focused ultrasound in tissues: an "acoustic bragg peak".
This is a theoretical study of the absorbed power density that arises when a focused ultrasound beam is absorbed in a tissue-like medium. The specific application is selective tissue heating for cancer hyperthermia. Results are expressed in terms of an enhancement ratio which describes the fractional gain in absorbed power density when nonlinear effects are taken into account relative to the absorbed power density that results when only linear effects are considered. Starting from a standard configuration in which the transducer has a (Gaussian) diameter of 0.12 m, a radius of curvature of 0.16 m and an operating frequency of 1 MHz a numerical sensitivity analysis is performed in which transducer and tissue parameters are varied one at a time. Enhancement ratios are invariably greater than unity, often being in the range of 1.5-2 for regions near the focus of the beam. The prediction is that nonlinear effects will probably be useful in selectively increasing the temperature rise in the focal region of a focused applicator.
Absorption↗
CT determination of renal and hepatic microvascular volumes in experimental acute renal failure.
Alterations in renal blood flow are considered to play an important role in the pathogenesis of acute renal failure. Most techniques designed to assess organ blood flow and microcirculatory disturbances are relatively invasive and cumbersome. This study describes a noninvasive method for the determination of an organ's fractional vascular volume (FVV)-the fraction of an organ occupied by blood vessel lumen. It utilizes computed tomographic (CT) scanning and a contrast agent, perfluoroctylbromide (PFOB), which remains intravascular and is not excreted by the kidney. Acute renal failure (ARF) was induced in rats by intraperitoneal injection of glycerol (5 g/kg). CT scans of kidneys, liver, and heart were performed prior to and following intravenous administration of PFOB. FVV of kidney and liver were calculated prior to induction of ARF and at selected time periods following ARF (20-50 minutes and 60-120 minutes). FVV of the kidney decreased significantly 20-50 minutes following ARF and had returned to control values at 60-120 minutes. Renal histologic abnormalities were more severe at the later time period. Thus, early alterations in blood flow precede pathologic abnormalities in the kidney following glycerol-induced ARF. Determination of an organ's fractional vascular volume is a simple noninvasive technique which provides useful information on the microcirculation during the course of experimental disease.
Acute Kidney Injury↗
A 4-MV CT scanner for radiation therapy: the prototype system.
A Varian 4-MV isocentric therapy accelerator has been modified to perform also as a CT scanner. The goal is to provide a computed tomography capability for use in radiotherapy at low cost. We envision three principal uses for the system. These are (i) to provide two and three-dimensional maps of electron density distribution for CT assisted therapy planning; (ii) to aid in patient set up by providing sectional views of the treatment volume and high contrast scout-mode verification images; and (iii) to provide a means for periodically checking the patients anatomical conformation against that which was used to generate the original therapy plan. The machine was modified by mounting an array of detectors on a frame that is bolted to the counterweight end of the gantry in such a manner as to define a "third generation" CT scanner geometry. Details of the detectors and data acquisition electronics are provided. We present results of phantom studies which demonstrate that this prototype system has a spatial resolution of 4 mm and the ability to discriminate electron density differences of less than 1%. We also show that the system is exactly linear over a wide range of electron densities corresponding to those found in body tissues.
Particle Accelerators↗
A 4-MV CT scanner for radiation therapy; spectral properties of the therapy beam.
A Clinac-4 therapy accelerator has been modified to function also as a computed tomography (CT) scanner. We report on measurements made to calibrate the beam. 104 Bismuth-germanate silicon-photodiode detectors were placed in a circular arc of radius 180 cm, centered on the source, in the arrangement of a third-generation CT scanner. With the beam collimated to a fan section and just filling the detectors, the signal S(chi), obtained with thickness chi of plastic phantom, closely followed the relationship S(chi)/S(0) = exp [- (alpha chi + beta chi 2)], where coefficients alpha and beta varied systematically over the extent of the beam. This result agrees with results of similar work by others. It may be explained in terms of the spectral changes that are known to be an inherent property of such beams. Expressions relating alpha and beta to the source spectral density are derived; also estimates of the source spectral density are calculated using the method of Laplace inversion. It is shown that scattered radiation arising from the fan beam within the scattering medium does not contribute significantly to the values of alpha and beta.
Mathematics↗
Computed tomography with a linear accelerator with radiotherapy applications.
An earlier paper [Simpson et al., Med. Phys. 9, 574 (1982)] described a computed tomography (CT) scanner that was constructed by adding a detector array to a 4-MV isocentric linear accelerator. Since the previous article, the detector array has been improved and we now demonstrate better than 3-mm spatial resolution and better than 1% relative electron density discrimination. A series of pictures from volunteer patients is included. Normal anatomy is visualized with bone, muscle, fat, and air being clearly delineated.
Adult↗
The lens coupling efficiency in megavoltage imaging.
In TV-based megavoltage imaging systems it is useful to be able to estimate the geometrical efficiency g2 of the camera lens. It is shown that the appropriate expression is g2 = (16n2)-1 X [F(1 + 1/m)]-2, where n is the refractive index of the scintillator, F is the F-number of the lens and m is the optical magnification. This expression yields estimates for g2 that are 5 to 10 times smaller than the estimates that have appeared previously in the literature.
Radiographic Image Enhancement↗
A linear array, scintillation crystal-photodiode detector for megavoltage imaging.
An imaging device has been developed to acquire images during external photon-beam radiotherapy treatments. It consists of a linear array of 128 zinc tungstate (ZnWO4) scintillation crystals each of which is individually optically coupled to a photodiode and associated electronics. The image is formed by scanning the linear array across the radiation field using a stepping motor under the control of a microcomputer. Image archive, display, and analysis are performed using a microVAX II computer. Results from a general theoretical analysis are presented before a detailed description of the particular detector construction. The mechanical design of the detector is such that the detector is automatically positioned to within a millimeter relative to the treatment source. This simplifies procedures for analyzing setup variations when comparing a treatment image to any other treatment, or planning, images. Image acquisition takes under 4 s with a contrast resolution of better than 1% at a spatial resolution of 2.5 mm in the object plane. The primary dose used to form these images is 0.55 cGy although the dose received by the patient will be closer to 25 cGy due to the linear scanning geometry and 3.8-s scan time that is used.
Humans↗
The design of megavoltage projection imaging systems: some theoretical aspects.
This study investigates factors associated with the imaging of a patient using a high-energy radiotherapy treatment beam. Both single-stage (e.g., solid-state detector) and two-stage (e.g., scintillation screen plus TV) systems are considered. First an expression is derived that relates dose at the buildup depth in the object to the structure of the object, the scatter-to-primary signal-variance ratio and the differential-signal-to-noise ratio in the image. Second the number of bits required to digitize the image is derived. Third the effect of scattered radiation is investigated for photon counting, photopeak, and Compton detector types. Fourth the effect of noise in the detection process is considered. Finally, the relationship between x-ray source size, detector aperture, and image magnification is derived. The optimum magnification for given source size and detector aperture is discussed in terms of the system transfer function. The study indicates that at a primary beam energy of 2 MeV, a dose of 10(-3) cGy is required to detect reliably the presence of a bone section of area 10 x 10 mm and thickness 4 mm in 250 mm of soft tissue. For this example, it is also estimated that a digitization accuracy of 10 bits is required. The calculations indicate that for a Compton detector, the scatter-to-primary signal-variance ratio drops from a value of around 30% at the exit surface of the object to 5% at a distance of 80 cm from the object with a consequent small reduction in the dose required to form the image.
Equipment Design↗
A scanned, focused, multiple transducer ultrasonic system for localized hyperthermia treatments.
A commercial diagnostic ultrasound scanner (Octoson) was modified for performing hyperthermia treatments. The temperature elevations were induced in tissues by four large, focused ultrasonic transducers whose common focal zone was scanned along a computer controlled path as determined from B-scan images. The system is described and the results of preliminary tests demonstrating some of its capabilities are given. Extensive tests with canine thighs and kidneys were performed. The blood flow to the kidneys was controllable, and thus tumours having different blood perfusion rates could be simulated. The results showed that the system is capable of inducing a local temperature maximum deep in tissues (up to 10 cm was tested) and that tissues with high perfusion rates could be heated.
Animals↗
A dose distribution evaluation utilizing megavoltage CT imaging system.
An isodose distribution (collision kinetic energy distribution) was acquired using a megavoltage CT imaging system. Direct comparison of dose distributions has become important for verification in innovative treatment techniques. Three-dimensional implementation of this system is considered highly feasible.
Humans↗