bluuebunny/crossref_metadata_2025 huggingface dataset:
Two‐orthogonal‐view method for quantification of rad dose to neck lesions in thyroid cancer therapy patients
Authors: Koral Kenneth F. , Adler Ronald S. , Carey James E. , Kline Robert C. , Beierwaltes William H.
Published: August 2003

A new technique is described for the determination of the dose in rad to metastatic lesions in the neck following administration of 131I for the treatment of thyroid carcinoma. The technique utilizes a high‐energy pinhole collimator mounted on an Anger camera which can be rotated to record lateral and anterior views from known positions relative to the patient. A new algorithm is derived for the determination of lesion location in three dimensions, utilizing the distance of closest approach between back‐projected rays from the two views. In addition, a new derivation is presented for the estimation of ellipsoid volume from two pinhole projections. The method makes use of a computer edge detection program to delineate the projected lesion boundaries. The output is either a mean estimate from one view or a more definitive upper bound derived from both views. The uptake in counts is determined from daily anterior images which are corrected for pinhole efficiency falloff with distance. This uptake is converted into microcuries by use of an in‐air calibration curve. An attenuation correction is applied using the calculated lesion depth and an assumed attenuation coefficient. The dose rate is calculated using standard assumptions and is integrated graphically over the first few days when the rate peaks and by utilizing a single‐decaying‐exponential fit to the data over the remaining days. Illustrative patient data are presented in which doses are calculated for three neck lesions after an administration of 175 mCi of 131I. These doses include a lower bound of 2400 rad and estimates of 5300 and 9500 rad.
A 4‐MV CT scanner for radiation therapy: The prototype system
Authors: Simpson R. G. , Chen C. T. , Grubbs E. A. , Swindell W.
Published: August 2003

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.
An overview of a camera‐based SPECT system
Authors: Greer Kim L. , Jaszczak Ronald J. , Coleman R. Edward
Published: August 2003

Camera‐based single photon emission computed tomography (SPECT) provides increased information about the organ scanned by eliminating superpositioned radionuclidic activity in the reconstructed images. Current literature contains little detail regarding the potential of and the technical aspects involved in using this emerging imaging modality. A description of the instrumentation supplements a general, but practical, background for nuclear medicine personnel. Some of the considerations for successfully utilizing a SPECT system are described, along with a discussion of the basis for these procedures. Pitfalls are mentioned and clinical images from our SPECT imaging system are included.
The influence of certain physical and physiological factors on the in vivo measurement of lung density
Authors: Coates G. , Webber C. E. , Kennett T. J.
Published: August 2003

The effects of multiple photon scattering and the assumption of infinitesimal beam geometry on the results obtained with a clinical lung densitometer were investigated. In phantom materials, it was possible to account almost completely for the multiple scatter contribution by measurement of transmission for the 170Tm source. A theoretical analysis based on a simplified model showed that the influence of finite geometry was small. Measurements in six normal volunteers showed that lung density is a function of the fractional air content of the lung. To relate density to fractional water content of the lung, measurements must be made at a fixed lung volume. The height in the lung at which measurements are made appears to be unimportant. The results of these experiments enable predictions to be made concerning the clinical usefulness of lung density measurements.
ATC‐4 waveguides may be field repairable
Authors: Nelson Stephen J.
Published: August 2003

Three accelerator tube failures have occurred on an ATC‐4 at the Medical University of South Carolina. On the third occasion, the accelerator tube was repaired without removal from the machine by the in‐house service group. This action saved the hospital thousands of dollars in replacement costs and treatment downtime.
3D thin lead sheet compensating system
Authors: Laursen J. F. , Andersen H. C. , Hansen H. P.
Published: August 2003

The compensating filter system presented consists of a simulator mounted camera and light beam device. The light beam device produces a flash light beam 3‐mm wide perpendicular to the x‐ray axis. The distance between flashes can be varied from 0.5 to 2.5 cm in steps of 0.25 mm. An exposure of the patient in treatment position subject to a series of light flashes will produce a three‐dimensional representation of the patient surface. This map is then properly enlarged. Lead sheets are cut and fixed to a Perspex plate which fits into a slot in the head of the accelerator. Regardless of energy, all compensators are made from 0.5‐mm‐thick lead sheets. To determine the proper distance between light flashes, measurements were performed in a water phantom for each x‐ray energy to establish the equivalence of increasing numbers of layers of 0.5‐mm lead. These measurements were compared to theoretical calculating using tissue maximum ratios, and finally checked against a wedged water phantom of 25° as well as against a head and neck treatment field for the Alderson phantom. This way, a beam flattening of ±2% compared to the standard isodose lines was achieved.
A practical method to measure the MTF of CT scanners
Authors: Droege Ronald T. , Morin Richard L.
Published: August 2003

A method has been devised which enables the modulation transfer function (MTF) of computerized tomographic (CT) scanners to be simply and rapidly determined. The method relies upon the measurement of the standard deviation of the pixel values within the image of cyclic bar patterns. The method is shown to produce results which are indistinguishable from those derived by less practical conventional methods.
Theoretical analysis of the two‐coil method for measuring fluid flow using nuclear magnetic resonance
Authors: Devine R. A. B. , Clarke L. P. , Vaughan S. , Serafini A.
Published: August 2003

A theoretical study of the application of pulsed nuclear magnetic resonance (NMR) to the measurement of fluid flow has been made. Calculations of detector signal line shapes, for a two‐coil NMR laboratory experiment, were performed using a computer to determine the appropriate choice of flow signature. Results of calculations are presented showing the sensitivity of the method to various experimental parameters. The position of the received peak signal, in time, was found to correlate directly with mean flow velocity. However, the shape of the received signal was observed to depend markedly upon the state of flow, i.e., laminar, turbulent (plug). Hence, accurate determinations of flow required a knowledge of flow type. The theoretical calculations provided a better understanding of the potential of NMR for measuring fluid or blood flow.
Neutron insensitive depolymerization of polyacrylamide for californium‐252 photon dosimetry
Authors: Moyers Michael F. , Beach J. Larry
Published: August 2003

A photon dosimeter based on degradation of polyacrylamide (PAA) in dilute aqueous solution has been found to be insensitive to neutrons from Cf‐252, independent of photon energy from 34 keVeff to 1.33 MeV, and useful over a dose range of 1 to 100 Gy. Because this dilute polymer solution is virtually 100% water, it is nearly tissue equivalent for photon absorption as well as neutron scatter and attenuation. Radiation response was determined by measuring decreases in viscosity three days after irradiation. The measured response is dose rate independent from 10 to over 300 Gy/h but increases rapidly at lower dose rates. Long‐term, radiation‐induced viscosity changes were also observed. The many advantages of this tissue equivalent system for photon/neutron dosimetry suggest the need for further studies of the observed over‐response at low dose rates.
The optimization of signal detectability in digital fluoroscopy
Authors: Zamenhof Robert G.
Published: August 2003

An optimization analysis of digital fluoroscopy has been carried out using (signal/noise)2/dose as a figure of merit. Three signal detection tasks have been considered: (i) small iodine‐containing blood vessels; (ii) regions of fat; and (iii) microcalcifications. The effects of x‐ray beam filtration and kilovoltage on the figure of merit have been parameterized and the associated x‐ray tube power loadings have been defined. Results are presented in a fashion that facilitates the choice of parameters (amount of aluminum filtration, kilovoltage, and x‐ray tube power loading) such that an acceptable balance can be achieved between signal detection sensitivity, patient dose, and x‐ray tube power loading.
Modification of the 50% maximum dose depth for 41‐MeV ( p+,Be) neutrons by use of filtration and/or transmission targets
Authors: Smathers J. B. , Graves R. G. , Earls L. , Otte V. A. , Almond P. R.
Published: August 2003

Several target configurations for the 41‐MeV ( p+,Be) reaction have been evaluated for the characteristics of the radiation field produced; depth dose, dose rate per μA. From analysis, it is concluded that to achieve the desired 13.2‐cm depth for 50% of maximum dose and acceptable dose rate at a target‐to‐skin distance (TSD) of 125–150 cm, the neutron spectra must be filtered to preferentially absorb the lower‐energy neutrons. Further increases in depth of 50% of maximum dose and a significant reduction in beryllium heating problems result if a partial transmission target is used with the terminal 30% of proton energy being deposited in a copper target backing.
Ultrasonically tissue‐mimicking liver including the frequency dependence of backscatter
Authors: Madsen Ernest L. , Zagzebski James A. , Insana Michael F. , Burke Thomas M. , Frank Gary
Published: August 2003

Tissue‐mimicking (TM) liver, previously reported by our laboratory, mimics the speed of sound, density, and attenuation coefficient (including frequency dependence) of liver; however, scatter properties are only qualitively simulated. Two new versions of TM liver are reported here which not only mimic liver with respect to speed of sound, density, and attenuation coefficient, but also with respect to backscatter coefficients, including the frequency dependence of the latter. Compositions, methods of production, and comparisons of ultrasonic properties with those found in the literature for human liver are presented.
Factors affecting the trapping performance of xenon holdup—filters in nuclear medicine applications
Authors: Bolmsjö M. S. , Persson B. R. R.
Published: August 2003

Activated charcoal traps are used in nuclear medicine departments to capture exhaled radioactive xenon gas. In the present study, the trapping performance of charcoal was investigated for different physical qualities in experimental and clinical situations. Various factors affecting the trapping capacity are identified, such as the charcoal mass, moisture and carbon‐dioxide concentrations in the sweep gas, gas‐flow rate through the trap, and temperature. Improper drying of the exhaled air prior to passage through the charcoal bed appears to be a critical factor, leading to a much earlier breakthrough of xenon. Commercial xenon traps intended for ambient temperature operation were found to be useful only for a limited number of patients due to an early breakthrough point. By refrigerating the trap, however, a much larger capacity was achieved. If correctly managed, xenon traps constitute a useful alternative for isolating radioactive xenon wastes in clinical and experimental applications.
Phosphorus activation neutron dosimetry and its application to an 18‐MV radiotherapy accelerator
Authors: Bading James R. , Zeitz Louis , Laughlin John S.
Published: August 2003

Neutron fluxes and dose rates in and near the 18‐MV x‐ray beam of a Therac‐20 accelerator were determined with measured activities from the nuclear reactions 31P(n,p)31Si (fast neutrons) and 31P(n,γ)32P (thermal neutrons), published cross sections, and neutron energy spectra from Monte Carlo calculations. Measurements were made in the patient plane in air and at a 10‐cm depth in a tissue‐similar phantom, and in a plane containing the x‐ray target. Orthophosphoric acid solution was identified as a suitable and convenient phosphorus dosimeter material. In the 31P activation method, fluxes and dose rates are determined as the product of measured saturation activity per 31P atom and a conversion factor, which depends on the shape of the assumed neutron spectrum. For fast neutrons, which deliver most of the dose, the accuracy error in the saturation activity determinations was shown to be ≲25%. An inconsistency resulting from neglect of the accelerator's adjustable collimator in the Monte Carlo calculations was demonstrated between the measured saturation activities and the theoretical neutron spectra. The maximum neutron dose equivalent rate observed was 5.9 mSv/Gy of x‐ray absorbed dose at the accelerator calibration point. Surface dose equivalent rates of the present study are less than those of fluxmeter and remmeter studies at sites outside Therac‐20 treatment fields by as much as factors of 2.4 and 2.8, respectively. The phantom study showed that at 18 MV internally produced neutrons have a negligible effect on the neutron field within the patient.
Characteristics of A‐150 plastic‐equivalent gas in A‐150 plastic ionization chambers for p(66)Be(49) neutrons
Authors: Awschalom M. , Rosenberg I. , Ten Haken R. K. , Pearson D. W. , Attix F. H. , DeLuca P. M.
Published: August 2003

The evaluation of a gas mixture having an atomic composition similar to that of A‐150 tissue‐equivalent (TE) plastic has been extended to a high‐energy neutron therapy beam. “A‐150” gas, air, and methane‐based TE gas were each flowed through A‐150 plastic‐walled ion chambers of different sizes and irradiated with p(66)Be(49) neutrons. A tentative value for W̄(A‐150) of 27.3±0.5 J C−1 was derived for this beam. The W̄ value of the A‐150 gas mixture is compared to those of methane‐based TE gas and of air for the p(66)Be(49) neutron beam as well as to corresponding values found in similar experiments using 14.8‐MeV monoenergetic neutrons.
Performance of automatic exposure controls in mammography
Authors: Karila Kauko T. K.
Published: August 2003

This study describes the phantoms and methods for performance evaluation of 23 phototimers used in mammography. The evaluation is primarily based on an analysis of the optical densities of image receptors (films). According to the results, the kVp compensation, the response capability, the saturation, and the density controls of the phototimers are very poor. The variation in optical film density exceeded ±0.3 optical density (OD) in 87% of the phototimers for the tube voltage range 25–40 kVp and for the 2–6‐cm‐thick phantoms. The operation of the density controls should be adjusted in 70% of the phototimers. Recommendations are presented.
A derivation of Batho's correction factor for heterogeneities
Authors: Lulu Bruce A. , Bjärngard Bengt E.
Published: August 2003

Batho's correction factor for dose in a heterogeneous, layered medium is derived from the tissue–air ratio method (TARM). The reason why the Batho factor is superior to the TARM factor at low energy is ascribed to the fact that it accounts for the distribution of the scatter‐generating matter along the centerline. The poor behavior of the Batho factor at high energies is explained as a consequence of the lack of electron equilibrium at appreciable depth below the surface.
Limitations of two‐dimensional treatment planning programs
Authors: Goitein Michael
Published: August 2003

Some limitations of conventional two‐dimensional computer programs used to plan radiation therapy are presented. These include: a lack of involvement in defining the clinical problem; deficiencies in the algorithms for computing dose; failure to compute dose throughout the volume of interest; an inability to handle treatments with noncoplanar beams; failure to provide estimates of error; lack of tools for comparing rival plans; inadequate definition of geometric coverage of anatomic structures by external beams; and failure to provide tools for specifying and verifying the accuracy of treatment delivery. Some suggestions are offered for improvements in some of these areas. In particular, subtraction of dose distributions is proposed as a tool for comparing plans, and displays which exhibit the results of error analyses are presented.
The dose contribution due to photonuclear reactions during radiotherapy
Authors: Allen P. D. , Chaudhri M. A.
Published: August 2003

The dose equivalent due to photonuclear reactions within a patient irradiated with 24‐MeV bremsstrahlung has been calculated to be about 1% of the photon dose. This is more than two times smaller than the previously accepted estimate.
A xenon ionization detector for scanned projection radiography: Theoretical considerations
Authors: Rutt Brian K. , Drost Dick J. , Fenster Aaron
Published: August 2003

Xenon ionization detectors have been used successfully in computed tomography (CT) scanners; however, the detector design used in CT scanners does not provide sufficient spatial resolution for scanned projection radiography. We have been investigating a new design of xenon detector with individual element widths of 0.5 mm. In this design, there are no metal septa separating individual elements. As a result, detection efficiency is better than the design with septa, and construction of an array with submillimeter element widths is simpler; however, crosstalk will now occur between elements. Theoretical calculations of efficiency and resolution for our septaless design of xenon detector are presented. Results of these calculations indicate that for a spectrum of 100 kVp, element dimensions of 0.5 mm×0.5 mm×10 cm, a front window of 0.5‐mm aluminum, and a xenon pressure of 20 atm, quantum efficiency will be greater than 95%, detective quantum efficiency (DQE) will be approximately 75%, and both energy and conversion efficiencies will be limited to 50% by K‐fluorescent escape. The calculations also predict that for the same design, the reduction in lesion contrast induced by crosstalk will be less than 10% for all typical spectra. These theoretical results have encouraged us to pursue the construction of a prototype septaless xenon detector for scanned projection radiography.
Correction of microwave‐induced thermistor sensor errors
Authors: Samaras George M. , Rosenbloom Samuel , Cheung Augustine Y.
Published: August 2003

Accurate and reliable thermometry is essential in the development of microwave‐induced hyperthermal cancer therapy. While temperature measurements in strong electromagnetic fields usually require special sensors, this does not hold true for interstitial radiator/sensor systems. Miniature thermistors (with metallic leads) bonded to invasive microwave applicators exhibit a sensor error linearly related to the radiator's transmitted power. This relationship permits thermistor sensor error correction and temperature measurements to within ±0.1 °C or better. The instrumental methods and empirical validation are presented.
Dosimetry of small radiation fields for 10‐MV x rays
Authors: Houdek Pavel V. , VanBuren John M. , Fayos Juan V.
Published: August 2003

Dosimetry for 10‐MV x rays has been extended to radiation fields smaller than 4×4 cm which may be suitable for radiation therapy of small lesions, e.g., intracranial tumors, benign or malignant. Special consideration in this study was given to (i) the variation of dose with field size (collimator and phantom scatter), (ii) the central axis percentage depth doses, and (iii) the moving‐beam therapy dose distribution. We conclude that simple dosimetric techniques can provide adequate physics background for stereotaxic radiosurgery with small radiation fields and high‐energy x rays.
A practical method to routinely monitor resolution in digital images
Authors: Droege Ronald T.
Published: August 2003

A quality assurance (QA) test has been developed as a practical means to routinely monitor digital image resolution. The method is based upon a parameter termed the image modulation M. When noise is present M cannot be directly measured. Instead, a biased estimate of M, designated M′, is measured. This report describes the application of the parameter M′ to QA monitoring of digital image resolution. A measurement of M′ is generally simpler than alternative measurements, and is shown to outperform visual evaluation in the clinical QA environment.
Microcomputer supplementation of a liquid scintillation spectrometer
Authors: Astrahan Melvin A. , Oldendorf William H.
Published: August 2003

Quench correction, calculation of disintegration rate, and other numerical processing of data from older liquid scintillation spectrometers may be greatly accelerated by replacing the teletypewriter printers of these instruments with an inexpensive microcomputer system. Correction of long‐term system drift due to environment and aging, reformatting of data output, plotting, and statistical analysis are among other possible applications. We present a simple method for interfacing a Packard 3390 Tri Carb with a Rockwell AIM‐65 microcomputer and dot matrix printer.
Effect of silicone gel breast prosthesis on electron and photon dose distributions
Authors: Krishnan Leela , St. George Franz J. , Mansfield Carl M. , Krishnan Engikolai C.
Published: August 2003

The effect of a silicone gel breast prosthesis on the absorbed dose distribution of 9–20 MeV electron beams and 1.25–15 MV photon beams was studied. Compared to water measurements, at depths smaller than the practical range of the electron beams, the central axis depth dose values below the prothesis were lower for all energies by as much as 3.5%. However, at depths near the practical range, the central axis depth dose values for the prosthesis were greater than that of water by as much as 33%. Since this occurs near the end of the electron range, the resultant difference may not be clinically significant. Results of the effect of breast prosthesis on photon depth dose distributions reveal that no clinically significant perturbation is produced by the breast prosthesis using Co‐60, 6‐ and 15‐MV radiations.
A 4‐MV CT scanner for radiation therapy; spectral properties of the therapy beam
Authors: Swindell William
Published: August 2003

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(x), obtained with thickness x of plastic phantom, closely followed the relationship S(x)/S(0)=exp [−(αx+βx2)], where coefficients α and β 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 α and β 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 α and β.
Normalization (division) versus subtraction of digitized images
Authors: Baily Norman A. , Rimkus Daniel , Nalcioglu Orhan
Published: August 2003

The propagation of noise in digitized images arrived at by algebraic manipulation of two images has been investigated. The algebraic manipulations investigated were logarithmic subtraction, linear subtraction, and division of the two images. The two images were identical except for the presence of a contrast‐adding component in one of them. The two extreme cases of video‐fluoroscope system behavior, gray level proportional to incident photon flux and gray level proportional to the product of thickness and linear attenuation coefficient, were investigated. The ratio of signal to noise was calculated for each of these cases and a figure of merit established for the ratio of signal to noise of the division process versus each of the subtraction processes for each of the two fluoroscopic system behaviors. In all cases, this figure of merit is greater than unity. Values of these figures of merit for iodine concentrations of 5, 20, and 300 mg/cm3 are given in the paper. The process of division appears to be advantageous as a signal‐to‐noise enhancer for all applications not requiring a quantitative relationship between image contrast and photon flux.
Bubble dissolution physics and the treatment of decompression sickness
Authors: Kunkle Thomas D. , Beckman Edward L.
Published: August 2003

The treatment of decompression sickness often involves both recompressing the victim and administering hyperbaric oxygen in the hope of more rapidly dissolving the bubbles which cause this malady. Although many hundreds of such treatments are conducted each year in the United States alone, the underlying physical principles governing the dissolution of such bubbles are not well understood and only empirically tested. In this paper, we present a mathematical theory of bubble dissolution that is verified by comparison with laboratory experiments. This theory suggests that the commonly employed treatment techniques would be only marginally effective, and that in many situations the bubbles that cause the disease cannot be adequately dissolved using existing techniques and facilities.
Physical aspects of a rotational total skin electron irradiation
Authors: Podgorsak E. B. , Pla C. , Pla M. , Lefebvre P. Y. , Heese R.
Published: August 2003

A technique for rotational total skin electron irradiation is presented in which the patient stands on a slowly rotating platform (SSD=285 cm) in a large uniform linear accelerator electron field (E0=3.5 MeV). The beam is scattered by the transmission ionization chamber and by a special lead/aluminum scattering filter, and then degraded by a sheet of Lucite. A Farmer chamber is used as a patient dose monitor and a method for absolute dose calibration is presented. The field is uniform to within ±5% for dimensions of 180×40 cm2. The surface dose for rotational therapy is equal to 45% of the maximum dose in a stationary beam. The rotating beam exhibits a dose maximum on the surface, falls to 80% at 0.5 cm and has an x‐ray contamination of ∼4%. The surface dose rate is about 25 cGy/min for the rotating beam. The rotational beam percentage depth dose distributions, calculated using stationary beam information, agree well with measured data. The stationary beam exhibits a dose maximum at 4 mm in tissue, a surface dose of 93%, 80% dose at a depth of 1 cm, a practical range of 1.75 cm, and an x‐ray contamination of 2.5%. The rotational total skin electron irradiation significantly reduces the patient treatment and setup time and solves the problem of beam matching, when compared to standard multiple‐beam techniques.
Temperature distribution measurements in two‐dimensional NMR imaging
Authors: Parker Dennis L. , Smith Vernon , Sheldon Philip , Crooks Lawrence E. , Fussell Lauren
Published: August 2003

This paper represents a preliminary study of the effects of regional temperature distribution in two‐dimensional nuclear magnetic resonance (NMR) T1 imaging. It is found, as expected, that variations in local temperature appear as variations in the corresponding T1 image. The potential use of NMR T1 imaging in temperature measurements is evaluated in the case of water and blood samples. Using containers where the temperature could be either known or directly controlled with reasonable accuracy, images are obtained with samples having at least two regions at different temperatures. As expected, T1 is found to vary linearly with 1/T over the range of 0 °C to about 40 °C for blood. The potential use of T1 imaging in hyperthermia applications is also discussed.
Experimental simulation evaluation of ECG‐gated heart scans with a small number of views
Authors: Joseph Peter M. , Whitley Joseph
Published: August 2003

Computed tomography (CT) scanning techniques designed to visualize the beating heart utilize the electrocardiogram (ECG) waveform to gate or select the phase of the cardiac cycle being imaged. Most such methods require that each slice to be imaged be scanned many times to obtain the usual number of projections in a single phase interval of the cardiac cycle. We are studying a new method in which only a single scan of 20 s should be required per imaged slice. The central problem in the proposed technique is to test the diagnostic utility of images reconstructed from a very limited number (about 25) of projections. An essential part of the proposed algorithm is the elimination of all material outside a circle chosen to contain only the heart and the structures immediately surrounding it. Experiments are reported here on scans of calf 's heart in an 11‐cm container of water, with most views deleted so as to simulate the results from this type of gated scan. The results show that if no high‐contrast objects are within the chosen circle, then image quality is satisfactory and could be clinically useful.
Errors in off‐axis treatment planning for a 4‐MeV machine
Authors: Lam Wing‐Chee , Lam Kam‐Shing
Published: August 2003

Computer treatment planning systems allow dose computation in planes parallel to the central one (off‐axis plans). The beam data may consist of, e.g., percentage depth doses along the central axis plus off‐axis ratios (OAR) at several depths. In some systems, the calculation of an off‐axis plan is based on the assumption that the OAR can be represented by a separable function: OAR(x, y)= f(x)⋅ g( y), where x and y are the symmetry axes perpendicular to the beam axis and the functions f and g are equal for a square open field. The errors of this assumption for a 4‐MeV machine were measured for open fields and wedged fields at five different depths. The measured dose was compared with that predicted by the above equation for points at 50%, 75%, and 88% of the half field width from the beam axis. Maximum deviation of more than 10% was observed with the probable error of the measurement being 1%.
Evaluation of the spatial resolution of a CT scanner by direct analysis of the edge response function
Authors: Bentzen S. M.
Published: August 2003

A new approach to the measurement of the spatial resolution of a computed tomography (CT) scanner system is presented. The method is based on a direct least‐squares fit of an analytical expression to a set of data obtained from a CT image of the interface between two materials. The implementation of the method in connection with the G. E. RTPLAN computer configuration is described. The method has been applied in determining the resolution of an EMI‐7070 scanner and it is shown that the assumption of uniformity of the system resolution across the CT image is fulfilled within the accuracy of the present method. The reproducibility of the method has been estimated from a series of spatial resolution determinations performed on ten images taken with identical scan parameters. The standard deviation of this series was 3.2%.
Characterization of tissue via coherent‐to‐Compton scattering ratio: Sensitivity considerations
Authors: Karellas Andrew , Leichter Isaac , Craven J. D. , Greenfield M. A.
Published: August 2003

It is known that the ratio (R) of the detected coherent and Compton scattered photons from bone can be used in order to determine its mineral density. This technique utilizes the dependence of the coherent scattering on the effective atomic number (Z̄) of the scattering medium. It is generally accepted that a small scatter angle is preferred in order to ensure adequate counting statistics by favoring the detection of more coherent photons. Moreover, it has been assumed that a change in the scatter angle does not affect the sensitivity of the measurement. Our theoretical calculations for 60‐keV photons and for the range of Z̄ that corresponds to trabecular bone, indicate that increasing the scatter angle results in a stronger power dependence of the measured ratio on Z̄. This implies that by increasing the scatter angle, smaller changes in the mineral density can be detected, thus improving the sensitivity of the measurement. This effect was investigated experimentally by using a collimated beam of 59.54‐keV photons from Am‐241 (44.4 GBq) and a collimated intrinsic germanium detector. Solutions of K2HPO4 with different concentrations were used in order to simulate trabecular bone. The scatter spectra were recorded for all solutions at six scatter angles between 37° and 98° and the value of R was computed for each spectrum. The sensitivity of the measurement, evaluated from these experiments increased, with the increase of the scatter angle.
The k‐trajectory formulation of the NMR imaging process with applications in analysis and synthesis of imaging methods
Authors: Twieg Donald B.
Published: August 2003

The fundamental operations of nuclear magnetic resonance (NMR) imaging can be formulated, for a large number of methods, as sampling the object distribution in the Fourier spatial‐frequency domain, followed by processing the digitized data (often simply by Fourier transformation) to produce a digital image. In these methods, which include reconstruction from projections, Fourier imaging, spin‐warp imaging, and echo‐planar imaging, controllable gradient fields determine the points in the spatial‐frequency domain which are sampled at any given time during the acquisition of data (the free induction decay, or FID). The detailed time dependence of the resulting trajectory of sample points (the k trajectory) determines the relative weight and accuracy with which image information at each spatial frequency is measured, establishing theoretical limitations on image quality achievable with a given imaging method. We demonstrate here that these considerations may be used to compare the theoretical capabilities of NMR imaging methods, and to derive new imaging methods with optimal theoretical imaging properties.
Calculational methods for estimating skin dose from electrons in Co‐60 gamma‐ray beams
Authors: Higgins P. D. , Sibata C. H. , Attix F. H. , Paliwal B. R.
Published: August 2003

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co‐60 γ‐ray beams. Either the Klein–Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source–surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.
A scanning system for chest radiography with regional exposure control: Practical implementation
Authors: Plewes D. B. , Vogelstein E.
Published: August 2003

An experimental scanning apparatus for chest radiography is described which offers good scatter rejection and regional manipulation of film exposure. The apparatus is capable of studying two scanning geometries that uses either a fan of radiation swept laterally over the film in 5 s or a spot of radiation scanned over two dimensions in a raster pattern in 8.8 s. The manipulation of tube output during the scan is achieved by pulse width modulation with film exposure monitored by a fluorescence detector placed behind the film cassette. Measurements of the scatter rejection properties of the system show that it is superior to that of a 10:1 grid when used with 120‐kVp radiation. Phantom images with both geometries demonstrate the capability of the system to improve uniformity of film exposure and soft tissue contrast throughout the mediastinal, diaphragmatic, and retrocardiac areas. The relative characteristics of the one‐ versus two‐dimension scan geometries are given. Clinical raster images formed with the 8.8‐s scan time show signs of motion unsharpness which becomes insignificant when the scan time is reduced to 4.5 s.
Ionization produced by electron beams beneath curved surfaces
Authors: Ritenour E. R. , Cacak R. K. , Hendee W. R.
Published: August 2003

Ionization produced by high‐energy (6–18 MeV) electrons beneath cylindrical surfaces has been measured and compared to ionization at corresponding locations under a flat surface. Results are reported as ionization ratios over a range of depths, off‐axis distances, radii of curvature, and electron energies. Phantoms with cylindrical entrance surfaces (radii 6–15 cm) were utilized to simulate patient contours. Measurements reveal that ionization ratios may depart from unity by as much as 50% and that a simple shifting of isodose curves may be insufficient to compensate for perturbation in electron dose distribution introduced by curved surfaces.
The application of CaSO4:Dy (TLD‐900) to diagnostic x‐ray exposures
Authors: Niroomand‐Rad Azam , DeWerd Larry A.
Published: August 2003

The properties of a new commercial thermoluminescent dosimeter, CaSO4:Dy without a LiF binder (TLD‐900) was studied for low‐exposure measurements in diagnostic radiology. The former TLD‐900 had a LiF binder (herein referred to as “TLD‐900/LiF”). The principle features of this dosimeter are its high sensitivity to low‐energy radiation and its relatively low fading which permits measurements down to less than 2.6×10−8 C kg (0.1 mR) with an accuracy better than 20%. The characteristics, annealing procedures, light sensitivity, energy response, reproducibility, and fading, of TLD‐900 are discussed and compared with TLD‐900/LiF. When the precautions presented in this paper are used, the dosimeters can be used for the measurement of x‐ray exposures.
Performance evaluation of a new quality control dose monitor for radiation therapy
Authors: Dixon Robert L. , Ekstrand K. E. , Wilenzick Raymond M. , Williams Kenneth D.
Published: August 2003

The performance of a new quality control dose monitor for radiation therapy with respect to precision, stability, temperature coefficient, and radiation damage has been extensively evaluated. The results of routine use at several centers are reported.
Variation of output with atmospheric pressure and ambient temperature for Therac‐20 linear accelerator
Authors: Sharma Subhash C. , Wilson David L. , Jose Baby
Published: August 2003

The Therac‐20 (a linear accelerator manufactured by the Atomic Energy of Canada, Ltd.) employs an unsealed monitor chamber to control the dose output. Daily fluctuations in machine output for both x rays and electron beams were observed to vary with ambient temperature and atmospheric pressure. These variations were not related to any other machine parameters. Variations as large as 3.5% were seen by monitoring 18‐MV x‐ray output over several months. We recommend that the manufacturers take steps to eliminate the atmospheric dependence of dose rate.
Sources of electron contamination for the Clinac‐35 25‐MV photon beam
Authors: Petti P. L. , Goodman M. S. , Sisterson J. M. , Biggs P. J. , Gabriel T. A. , Mohan R.
Published: August 2003

A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25‐MV photon beam generated by Varian's Clinac‐35. Three sources of contamination were examined: (a) the flattening filter and beam monitor chamber, (b) the fixed primary collimators downstream from the monitor chamber and the adjustable photon jaws, and (c) the air volume separating the treatment head from the observation point. Five source‐to‐surface distances (SSDs) were considered for a single field size, 28 cm in diameter at 80 cm SSD. It was found that for small SSDs (80–100 cm), the dominant sources of electron contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons. Thirteen percent of the remaining electrons originated in the downstream primary collimators and the photon jaws, and 17% were produced in air. At larger SSDs, the fraction of unwanted electrons originating in air increased. At 400 cm SSD, 61% of the contaminating electrons present in the beam were produced in air, 34% originated in the flattening filter and beam monitor chamber, and 5% were due to interactions in the fixed collimators downstream from the monitor chamber and the adjustable photon jaws. These calculated results are substantiated by recent experiments.
An equivalent squares template
Authors: Rosen Isaac I. , Lane Richard G. , Van Camp Harlan L.
Published: August 2003

A template for calculating equivalent squares of irregularly shaped fields is described. Calculations using the template are essentially scatter summations. Comparisons with a computer program showed good agreement in equivalent squares and tumor doses (within 0.8%). Comparisons with area–perimeter ratio methods of computing equivalent squares showed consistently better accuracy using the template. For highly irregular fields, the template calculation required 3–10 min for central axis computation, comparable to that required by the other manual methods.
A linear accelerator monitor unit totalizer
Authors: Pillar Dennis G. , Gillin Michael T. , Kline Robert W. , Grimm Daniel F.
Published: August 2003

An independent primary dose counter or totalizer has been developed for our linear accelerator. This counter is reset independently of the control console and thus will totalize the number of monitor units (MUs) delivered to the patient. This is especially useful for patients being treated at extended distances when it is necessary to reset the control console several times. The independent totalizer does not load down the control console dosimetry circuit and can be adapted to the digital logic used. The details of the system will be presented.
Absorption and noise in cesium iodide x‐ray image intensifiers
Authors: Rowlands J. A. , Taylor K. W.
Published: August 2003

The measured and theoretically predicted values of detective quantum efficiency (DQE) for a CsI x‐ray image intensifier are compared for nine monoenergetic beams of x rays. The agreement between measurement and theory of better than ±5% indicates that we have a sound understanding of the physical parameters controlling the DQE. It is shown that the fraction of K‐fluorescent x rays escaping from the input phosphor is independent of incident energy. The number of electrons released within the x‐ray image intensifier (XRII) by an incident x ray has been measured. The mechanism for energy broadening within the XRII is shown to be predominantly the limited number of electrons and not light absorption.
A Monte Carlo model for the absorption and flux distributions of light in tissue
Authors: Wilson B. C. , Adam G.
Published: August 2003

A Monte Carlo computer model has been developed to study the propagation of light in tissues. Light attenuation is assumed to result from absorption and isotropic scattering. The model has been used to predict the distribution of absorbed dose in homogeneous tissues of different absorption/scattering ratios, for illumination both by external light beams and via implanted optical fibers. The photon flux into optical fibers placed in the tissue as detectors has also been investigated. The results are interpreted in relation to the use of visible light irradiation for photo radiation therapy.
Scaling neutron absorbed dose distributions from one medium to another
Authors: Awschalom M. , Rosenberg I. , Ten Haken R. K.
Published: August 2003

Central axis depth dose (CADD) and off‐axis absorbed dose ratio (OAR) measurements were made in water, muscle and whole skeletal bone tissue‐equivalent (TE) solutions, mineral oil, and glycerin with a clinical neutron therapy beam. These measurements show that, for a given neutron beam quality and field size, there is a universal CADD distribution at infinity if the depth in the phantom is expressed in terms of appropriate scaling lengths. These are essentially the kerma‐weighted neutron mean free paths in the media. The method used in ICRU Report No. 26 to scale the CADD by the ratio of the densities is shown to give incorrect results. The OARs measured in different media at depths proportional to the respective mean free paths were also found to be independent of the media to a good approximation. Therefore, neutron beam CADDs and OARs may be measured in either TE solution (USA practice) or water (European practice), and having determined the respective scaling lengths, all measurements may be scaled from one medium to any other. It is recommended that for general treatment planning purposes, scaling be made to TE muscle with a density of 1.04 g cm−3, since this value represents muscle and other soft tissues better than TE solution of density 1.07 g cm−3. For such a transformation, relative measurements made in water are found to require very small corrections. Hence, it is further recommended that relative CADD and OAR measurements be performed in water because of its universality and convenience. Finally, a table of calculated scaling lengths is given for various neutron energy spectra and for various tissues and materials of practical importance in neutron dosimetry.
Measurements of the distribution of technetium‐99m sulfur colloid and technetium‐99m HEDP in normal humans
Authors: Grossman Laurence W. , Sodd Vincent J. , Nishiyama Hiroshi , Feller Paul A. , Hoops Richard G.
Published: August 2003

Technetium‐99m sulfur colloid and technetium‐99m hydroxyethylidene diphosphonate (HEDP) are two radiopharmaceuticals that have been widely used in nuclear medicine. Radiation dose estimates for both materials have been published in the literature and are found in the package inserts of the commercial kits. However, these estimates were made without the benefit of quantitative human organ uptake data. Estimates of absorbed radiation dose to the kidneys differ by nearly a factor of 6 between the two calculations for HEDP. The results of this study are in agreement with the lower previous estimate. Other than that one exception, agreement of the previous estimates with the measurements reported here is surprisingly good.