bluuebunny/crossref_metadata_2025 huggingface dataset:
NMR relaxation behavior in living and ischemically damaged tissue
Authors: Barroilhet L. E. , Moran P. R.
Published: August 2003

A study by pulsed NMR techniques in living liver tissue has led to the discovery that the observed longitudinal relaxation decay behavior is strongly multicomponent. After death of the experimental animal, the relaxation decay curves evolve toward a single‐component behavior. These changes can also be observed within a few minutes after the liver is excised and placed in a test tube, and they involve a high degree of quantitative and qualitative regularity and reproducibility. An excellent description of all observed NMR behavior is obtained from a dynamic two‐compartment model. Rapidly relaxing volumes exchange water molecules with slowly relaxing volumes; associating only an increasing water molecule exchange rate with increasing ischemia accounts in quantitative detail for all observed changes. The exchange‐rate values and their variation with tissue deterioration are in good agreement with that estimated for intra‐ to extracellular water exchange as limited by cell‐membrane osmotic permeabilities. Possible applications of these results in different biomedical areas are discussed.
Spectroscopy of diagnostic x rays by a Compton‐scatter method
Authors: Yaffe M. , Taylor K. W. , Johns H. E.
Published: August 2003

A method is described for measuring the energy spectra of diagnostic x rays using a high‐resolution intrinsic germanium spectrometer. The method is applicable over the wide range of x‐ray exposure rates normally used in radiography. Reduction of the high x‐ray beam intensity to a level acceptable to the germanium spectrometer is achieved by measuring the spectrum of photons scattered through 90° by a lucite disc. From this measured spectrum the primary spectrum is obtained by calculation. Some typical x‐ray spectra are presented.
Biomedical probe using a fiber‐optic coupled scintillator
Authors: Swinth K. L. , Ewins J. H.
Published: August 2003

A high‐sensitivity biomedical radiation probe which employs a fiber‐optic coupled NaI(Tl) scintillator as a detector is described. It was developed for in vivo counting of low‐energy 239Pu photons from material located in the tracheobronchial lymph nodes. This probe is 20 times as sensitive as a solid‐state probe (avalanche diode) previously developed for this application. Tests with 99mTc show a sensitivity more than 90 times greater than biomedical probes using DcTe of GaAs; however, the improved sensitivity is largely due to an increased sensitive volume. Probes were evaluated in animals and phantoms for detection of 239Pu and for location of lung tumors labeled with 111In.
Characterization of aberrations in image‐intensified fluoroscopy
Authors: Casperson Lee W. , Spiegler Peter , Grollman J. H.
Published: August 2003

A procedure is developed for characterizing and measuring the distortion and intensity variations which are present in x‐ray image‐intensifier systems. These effects often seriously limit the usefulness of such systems, and the results here make possible an evaluation and comparison of competing intensifier instruments.
In vivo quantitation of lesion radioactivity using external counting methods
Authors: Thomas Stephen R. , Maxon Harry R. , Kereiakes James G.
Published: August 2003

Quantitative external counting methods for the in vivo determination of the activity uniformly distributed in an area of interest (lesion) have been developed. The theoretical work has been extended to include the effects of regions of differing attenuation coefficient and of non‐target organ activity in the tissue surrounding the lesion. A clinical quantitative procedure utilizing a “calibrated” gamma camera and associated computer system is described. Studies including a bone substitute material in a water phantom confirmed the basic theoretical work.
Calibration methods for measuring splenic sequestration by external scanning
Authors: Clarke Laurence P. , Maugham Elaine Z. , Laughlin John S. , Knapper William H. , Mayer Klaus
Published: August 2003

Conventional methods for measuring splenic sequestration of labeled cells rely on stationary probe counting over the liver and spleen. A quantitative spleen‐scanning method is proposed as a means of performing a more accurate diagnosis of the degree of splenic sequestration. Improved methods for calibrating a rectilinear scanner employing “constant resolution” collimators, designed for the in vivo measurement, are reported. Clinical evaluation of this method was performed by scanning several patients scheduled for splenectomy. Good correlation between in vivo and in vitro measurements of splenic radioactivity for 29 patients was obtained. A least‐squares fit to the data yielded a coefficient of determination r2=0.93.
Liquid xenon scintillators for imaging of positron emitters
Authors: Lavoie Louis
Published: August 2003

The current understanding of xenon scintillation physics is summarized and keyed to the use of xenon as a γ‐ray detector in medical radioisotope imaging systems. Liquid xenon has a short scintillation pulse (∼10−8 sec) and high γ‐ray absorption and scintillation efficiencies. The fast pulse may facilitate imaging in vivo distributions of hot positron sources and allow recovery of additional spatial information by time‐of‐flight techniques. We begin by describing our own study of the feasibility of making a practical positron scanning system, and consider the problems of scintillation decay time, linearity, efficiency, purity, and electric‐field amplitifcation. The prospects for a practical instrument are considered.
Verification timer for radiation therapy machines
Authors: Parrino P. A.
Published: August 2003

Many cobalt and x‐ray therapy machines use mechanical timers to control the given dose. There is no other timer on the machine to verify that the correct time was set or that the timer did not malfunction. This article describes a digital timer that has been installed on the MIR therapy machines as a lapsed‐time indicator.
Spectral considerations for absorption‐edge fluoroscopy
Authors: Kelcz F. , Mistretta C. A. , Riederer S. J.
Published: August 2003

In our previous reports on absorption‐edge fluoroscopy, it was not possible to relate fully the subtleties involved in the selection of spectral parameters. This paper is intended as an overview of this important aspect of the technique. It is shown that, by using the 1‐kVp, 2‐filter technique, it is possible to image certain elements (e.g., iodine and xenon) in the presence of tissue variations of ±2 cm about the thickness at which perfect tissue cancellation takes place. Use of logarithmic signal processing extends this range, but bone thickness variations may not be accommodated because only two x‐ray energies are involved in the imaging process. Use of a 3‐kVp, 3‐filter technique with logarithmic signal processing is shown to solve this general problem. Computer simulations show that 1‐mg/cm2 iodine may be imaged in the presence of 10 cm or more tissue variations and 2000‐mg/cm2 bone variations.
Photoactivation ratios for specification of high‐energy x‐ray quality: Part II, 4–15‐MV x rays
Authors: Schulz R. J. , Nath Ravinder
Published: August 2003

A sensitive and relatively simple method for obtaining an index of the spectral quality of x rays in the 4–15‐MV range is presented. The method is based upon the simultaneous production of 115mIn by inelastic scattering of x rays by an indium foil and the production of 116mIn by neutron capture, where the neutrons are obtained from the photodisintegration of deuterium. Since the cross sections for the production of 115mIn and 116mIn have markedly different energy response functions, the ratio of radioactivities is very sensitive to the incident x‐ray spectrum. Using a detector consisting of an indium foil sandwiched between two plastic bottles of D2O, photoactivation ratios (PAR) have been measured for 4‐, 6‐, and 10‐MV x rays. It is shown that the PAR method offers a sensitive and practical means for quality control of x‐ray spectra and comparison of high‐energy accelerators.
ESR spectra of quasirandomly oriented centers: Application to radiation damage centers in bone
Authors: Panepucci Horacio , Farach Horacio A.
Published: August 2003

The angular dependence of the ESR line shape from polycrystalline samples due to incomplete randomness in the orientation of the crystallites is calculated for centers with axial symmetry. The effect of a preferential axis of orientation is considered and the results are applied to radiation produce centers in bone.
Correction for spectral artifacts in cross‐sectional reconstruction from x rays
Authors: McDavid William D. , Waggener Robert G. , Payne William H. , Dennis Michael J.
Published: August 2003

A monoenergetic response correction is described which, along with adequate filtration, may be used to remove the spectral shift artifact encountered in three‐dimensional reconstruction from x rays. Reconstructions were carried out by means of a convolution algorithm for simulated data using this method. These are compared with reconstructions obtained using fixed‐length water‐bath scans as a remedy for the special artifact. These studies suggest that the spectral artifact can be successfully eliminated from computerized cross‐sectional scans without resorting to the use of the water bath while, at the same time, improving quantum statistics and/or permitting operation at a lower tube current.
Modulation transfer function of the EMI CT head scanner
Authors: Bischof C. J. , Ehrhardt J. C.
Published: August 2003

The two‐dimensional modulation transfer function [MTF(νx,νy)] has been determined for the EMI CT head scanner by measuring the point spread function (PSF) in different locations in the field of view. This PSF was obtained by scanning a fine wire supported perpendicular to the tomographic slice. Based on these MTFs, the resolving power of the EMI scanner was found to be 3.1 line pairs/cm. Our results also verify the symmetry of the system response and the uniformity of the system resolution.
Practical dosimetric considerations of a 10‐MV photon beam
Authors: Van Dyk J.
Published: August 2003

An overall practical dosimetric study of a 10‐MV photon beam produced by a Varian Clinac‐18 linear accelerator is presented. In particular measurements were made to provide data which could be utilized in computerized dosage calculations using the concept of dividing the radiation beam into primary and scatter components. From the measured percentage depth doses, tissue–phantom ratios are calculated. Special consideration is given to the derivation and measurement of zero‐area tissue–phantom ratios such that scatter–phantom ratios could be obtained.. The computer techniques were tested under a number of specified conditions by comparing the calculated results to the measured data. The variation of dose with field size and distance is considered and attenuation data for shielding, wedge and compensating materials is provided.
Use of a dynamic rangeshifter for modifying the depth‐dose distributions of negative pions
Authors: Amols H. I. , Liska D. J. , Halbig J.
Published: August 2003

In order to use negative pions for the treatment of large deep‐seated tumors in radiotherapy, it is necessary to produce depth‐dose distributions tailored to specific shapes. We present here a method of beam shaping which utilizes a fluid‐filled piston having a programmable, computer‐controlled, time‐dependent thickness. The fluid alters the residual range of the pions such that predetermined depth‐dose distributions can be obtained. Changing from one distribution to another can be accomplished simply and rapidly without access to the treatment room. Depth‐dose distributions which are flat over a range in depth up to 10 cm have been produced. Distributions tailored to produce flat “effective dose” versus depth have also been obtained.
Use of electron filters to improve the buildup characteristics of large fields from Cobalt‐60 beams
Authors: Leung Philip M. K. , Johns H. E.
Published: August 2003

It has been shown that for Cobalt‐60 units capable of producing large fields (up to 40×40 cm2) at 80 cm, the dose distribution in the buildup region is quite different from that observed with small fields. First, the maximum dose occurs at 1–2 mm instead of 5 mm; and second, the maximum dose in some extreme cases is 15%–20% higher than the dose at 5 mm. Thus, the dose is decreased by more than 15%–20% by 3 mm of tissue, suggesting severe electron contamination. In this report, it is shown that this contamination can be best removed by a filter of thickness 0.4 g/cm2 of medium atomic number, placed some 25 cm from the source inside the collimator. For existing units, positioning the filter inside the collimator is not practical. Acceptable results, however, can be obtained by placing the filter just below the collimator.
Flattening of proton dose distributions for large‐field radiotherapy
Authors: Koehler A. M. , Schneider R. J. , Sisterson J. M.
Published: August 2003

Methods for obtaining flattened charged‐particle dose distributions over large areas are described. The system being used at Harvard for proton radiotherapy is discussed in detail. It is an extension of usual multiple‐scattering techniques to include blocking out some of the central peak, followed by rescattering to fill in the profile, resulting in flat distributions up to 30 cm in diameter. The unusually long source‐to‐skin distance (4.6‐m SSD) plus the small lateral spread (2 mm rms) from multiple scattering in the patient results in little divergence of the beam throughout the treatment volume.
Characteristics of Clinac‐18 wedged fields for 10‐MV x rays
Authors: Tolbert Donald D. , Lane Richard G. , Paliwal Bhudatt R. , Frost Stephanie W. , Chin K. Barbara , Dercks Judith
Published: August 2003

The characteristics of wedged fields which affect their clinical use have been examined for the 10‐MV x‐ray beam from the Clinac‐18. The methods used for obtaining and analyzing the wedged‐field data are discussed. These characteristics have been examined in terms of (a) the wedge angle, (b) the variation of the wedge angle with field size, (c) the variation in the angle through which isodose curves between the approximate depths of 5 and 15 cm are turned relative to the central axis as a function of depth, and (d) the variation of wedge central‐axis transmission factor with field size. Analysis of the data pertinent to these categories is presented for the 15‐, 30‐, 45‐, and 60‐deg wedges. In addition, the effect of the 60‐deg wedge upon the position of maximum dose on the central axis was determined.
Fast neutrons from a 25‐MeV betatron
Authors: Fox John G. , McAllister John D.
Published: August 2003

Since published results for the fast‐neutron dose per x‐ray rad from high‐energy therapy installations have differed by as much as a factor of 300, we have measured the neutron production from our 25‐MeV betatron. Fast‐neutron activation of aluminum foils was the method used. The effect of photoneutron production in the detectors, which has affected some past work, has been eliminated. A major source of neutrons in the treatment field was the platinum target. The neutron spectra used in the calculations of fluence were obtained by interpolation between published energies and between neighboring elements. Fluences per rad of x rays without a phantom were largely independent of field size and energy in the range 18–23 MeV. At 100 cm SSD and 23 MeV a large beam flattener contributed 15% of the neutrons, the remainder coming equally from the target and background. A phantom increased the neutron fluence/rad of x rays by 0%–10% depending on the field size. At 23 MeV we estimated the neutron dose to a patient to be 2.2×10−4 rad per rad of x rays inside the treatment field and 3×10−5 at 20 cm outside the field. The uncertainty in these figures is believed to be ±50%. In the electron beam the neutron dose per rad was about 50 times smaller than in the x‐ray beam. Estimates were made of neutron fluences at other energies and target thicknesses. We discuss our results in comparison with those of others.
Dosimetric considerations of symmetric and asymmetric 60Co teletherapy split fields
Authors: Wrede Don E. , Givens Stephen B.
Published: August 2003

Split fields are commonly used for a variety of treatments for which local shielding is desired; a single field is split into two fields by means of lead blocks placed in the beam. The condition for assuming independence of the two fields is investigated by using the area over perimeter calculational method and comparing this to experimental depth‐dose data measured in a water phantom using a Shonka ionization chamber. Two principal variations affecting dose distributions are examined: block width and position of the block expressed as the fractional perpendicular distance between the central ray of the overall field and the center of the block. An intercomparison between calculation and experimental data shows that the right and left sides of the split field behave as independent fields giving TAR values in agreement with experimental data to within ±1%. If the fields are asymmetric and if the larger field is less than 70% of the total area including the blocked part, then the dosimetry should be based on average TARs, off‐center ratios, and backscatter factors for the two fields. If the larger area is greater than 70% of the total area, then the treatment time can be calculated from the larger field only.
Energy dependence of correction factors for the Victoreen Model 666 Portable Radiographic Health Survey System
Authors: Morgan Tommie J. , Brateman Libby , Dirkse John
Published: August 2003

The energy dependence for each of three models of Victoreen 666 ionization chambers is characterized through an examination of the correction factors determined over the diagnostic x‐ray range (0.4–4.9 mm Al HVL) for many samples of each model. The results of an analysis of variance performed for each chamber model on the correction factors are reported as mean correction factors for the x‐ray beams evaluated, with a relative standard error of less than 0.6% in all cases. Energy‐dependence curves for the three chamber models are given, and their use is described.
Energy dependence of correction factors for some Victoreen Model 570 Condenser R‐Meter medium‐energy chambers
Authors: Morgan Tommie J. , Brateman Libby , Dirkse John
Published: August 2003

The energy dependence of each of six models of medium‐energy Victoreen 570 Condenser R‐Meter chambers was determined by measurement of the exposure correction factors over the diagnostic x‐ray beam‐quality range (0.4–4.1 mm Al HVL) for several samples of each model. The results of an analysis of variance performed by model on the correction factors are reported as mean correction factors for the x‐ray beams evaluated, with a relative standard error less than 0.8% in all cases. Energy‐dependence curves for the six chamber models are given; their use is described.
Neutron spectral measurements in an intense photon field associated with a high‐energy x‐ray radiotherapy machine
Authors: Holeman G. R. , Price K. W. , Friedman L. F. , Nath Ravinder
Published: August 2003

High‐energy x‐ray radiotherapy machines in the supermegavoltage region generate complex neutron energy spectra which make an exact evaluation of neutron shielding difficult. Fast neutrons resulting from photonuclear reactions in the x‐ray target and collimators undergo successive collisions in the surrounding materials and are moderated by varying amounts. In order to examine the neutron radiation exposures quantitatively, the neutron energy spectra have been measured inside and outside the treatment room of a Sagittaire medical linear accelerator (25‐MV x rays) located at Yale–New Haven Hospital. The measurements were made using a Bonner spectrometer consisting of 2‐, 3‐, 5‐, 8‐, 10‐ and 12‐in.‐diameter polyethylene spheres with 6Li and 7Li thermoluminescent dosimeter (TLD) chips at the centers, in addition to bare and cadmium‐covered chips. The individual TLD chips were calibrated for neutron and photon response. The spectrometer was calibrated using a known PuBe spectrum. Spectrometer measurements were made at Yale Electron Accelerator Laboratory and results compared with a neutron time‐of‐flight spectrometer and an activation technique. The agreement between the results from these independent methods is found to be good, except for the measurements in the direct photon beam. Quality factors have been inferred for the neutron fields inside and outside the treatment room. Values of the inferred quality factors fall primarily between 4 and 8, depending on location.
Radiation dose to the lungs from ventilation studies with 133Xe
Authors: Watson Evelyn E. , Cloutier Roger J. , Howard Barbara Y.
Published: August 2003

The radiation dose to the lungs from 133Xe in lung air is directly proportional to the time integral of 133Xe concentration; i.e., the cumulated concentration in the lungs. Using kinetic models developed to fit clinical observations, we have studied the effect of retention on cumulated concentration in lung air. The models studied were (1) equal exponential washin and washout rate constants, (2) unequal exponential washin and washout rate constants, and (3) single‐compartment washin and two‐compartment washout. Our results show that the radiation dose varies greatly with the model chosen. A simplified method for calculating the average dose to the lungs from activity in lung air is presented. Although we have applied this method only to studies where xenon is rebreathed at constant volume and then washed out, the technique can be adapted to other protocols.
Arc technique for total‐body irradiation by a 42‐MV betatron
Authors: Engler M. J. , Feldman M. I. , Spira J.
Published: August 2003

An arc technique for total‐body irradiation (TBI) has been developed using a Siemens 42‐MV betatron. Dose is administered with the isocenter approximately one‐third the distance from source to patient. A modified inverse‐square function for a beam arcing isocentrically was derived and confirmed experimentally. Relationships between stationary and arcing beam equilibrium and depth doses have been established theoretically and experimentally. Depth doses for TBI by 42‐MV beams are compared with those for 60Co and 4‐MV beams. The simplicity of setup and short treatment times are comparable to setup and treatment times of 60Co and linear accelerator TBI techniques.
Computational analysis and dosimetric evaluation of a commercial irregular‐fields computer program
Authors: Hallberg Jerome R. , Ibbott Geoffrey S. , Carson Paul L. , Hendee William R. , Aymar Michael A.
Published: August 2003

The proper evaluation of the accuracy of a new computer program for radiation‐therapy dosimetry requires consideration of both the mathematical algorithm used in the program and the performance required in clinical applications. As an example, our evaluation of the irregular‐fields dosimetry program currently marketed by SHM for their Rad‐8 system is described. The evaluation begins with an explanation of the mathematical computation method, with emphasis on the points where the calculation differs from previous methods. Next, the procedure for setting up the beam data file is discussed. Finally, a step‐by‐step procedure is described in which calculated doses are compared with measured doses, using a Varian Clinac‐4 with lead flattening filter, and the limits within which a ±5% accuracy is attainable are estimated. Some sources of error and areas for possible improvements are mentioned.
Physical characterization of neutron beams produced by protons and deuterons of various energies bombarding beryllium and lithium targets of several thicknesses
Authors: Amols H. I. , Dicello J. F. , Awschalom M. , Coulson L. , Johnsen S. W. , Theus R. B.
Published: August 2003

Protons of 35 and 65 MeV and deuterons of 35 MeV were used to bombard beryllium and lithium targets of various thicknesses. Four types of experiments were conducted in order to characterize the neutron fields. They were (1) central axis depth‐dose measurements in a water phantom, (2) dose buildup at small depths in tissue‐equivalent plastic, (3) microdosimetric measurements and LET distributions, and (4) neutron yields and energy spectra at an angle of 0 deg. The results generally show that (a) the central axis depth doses for the 35 and 65 MeV particles roughly approximate those of 60Co and 4‐MeV bremsstrahlung photons, respectively, (b) the neutron dose buildups are more rapid than those of the above‐mentioned photon sources, (c) the microdosimetric spectra show differences which are consistent with the measured neutron energy spectra, and (d) p–Li compared to p–Be neutron spectra have larger high‐energy particle flux for similar target and beam configurations.
Computer simulation study of Siemens star x‐ray image artifacts
Authors: Amtey Sharad R. , Tyson Robert K.
Published: August 2003

In the Siemens star image, exact determination of the first disappearance frequency, which is used to measure the focal spot size, is difficult since the disappearance band has a finite width and the image also has other artifacts. The origin of these artifacts and their appearance was studied by Siemens star image simulation on a digital computer. The simulated images were manipulated by using many different modulation and phase transfer functions. It is shown that the bending of spokes is not related to zero contrast; exact triplet splitting can occur only at the disappearance frequency, and therefore splitting is a valuable indicator of that frequency.
Selective iodine imaging using K‐edge energies in computerized x‐ray tomography
Authors: Riederer S. J. , Mistretta C. A.
Published: August 2003

Iodine is commonly used as a contrast material in computerized x‐ray tomography. In some cases the determination of the iodine distribution in the image may be prevented by the presence of bone or tissue variations within the tomographic slice. This paper describes a method for quantitative selective imaging of the iodine concentration in the slice. The method employs scans using three heavily filtered x‐ray beams, two having mean energies which straddle the iodine K edge (33 keV) and another at a slightly higher energy. The results are independent of tissue and bone over a broad range of projection path lengths. It is shown that, for separation of iodine from one other material, a two‐beam K‐edge approach requires less integral dose than a two‐beam technique at conventional CT energies for slice diameters up to 30 cm. For selective iodine imaging in the presence of more than one other material, the three‐spectrum K‐edge technique is a necessity. Exposure requirements and beam‐hardening corrections are discussed in detail and a computer‐simulated CT image generated by the proposed scheme is presented.
Comparison of moving‐strip therapy using a Cobalt‐60 teletherapy unit, a Varian 4‐MV linear accelerator, and a Varian 10‐MV linear accelerator
Authors: Leavitt Dennis D. , Stryker John A. , Campbell Dale W. , Strockbine Melvin F. , Wright Ann
Published: August 2003

Cobalt‐60 γ rays and 4‐ and 10‐MV x rays are compared for moving‐strip therapy in terms of the dose uniformity and the given dose needed to deliver a prescribed tumor dose. Dose distributions in phantoms of 14–32‐cm thickness were calculated for each therapy unit. Individual given doses which would deliver the most uniform dose along the midline of the treatment volume were determined by computer and were verified experimentally by thermoluminescent dosimetry. Using computer optimization techniques, the midplane dose uniformity is improved significantly for the three therapy units considered.
X‐ray photographic method for determination of stable iodine in tissues
Authors: Puumalainen P. , Suominen P.
Published: August 2003

A new x‐ray photographic technique for imaging of small concentrations of iodine in tissues is presented. The method is based on the use of two x‐ray beams of slightly different energies, which are obtained from the same primary source by appropriate use of critical absorbers. The beam energies are chosen so that the more energetic one is high enough to excite K x rays of the element under study while the less energetic beam is just below the required excitation energy. Two x‐ray pictures are taken using the beams consecutively; thereafter, a difference picture is made by subtracting photographically the lower energy beam picture from that of higher energy. The difference picture then displays the distribution of the element to be imaged. This method is sensitive to iodine concentrations less than 100 μg/mm2.
Field characteristics of geometric unsharpness due to the x‐ray tube focal spot
Authors: Doi Kunio
Published: August 2003

The basic theory of field characteristics is discussed. It is shown that the point spread function and optical transfer function (OTF) of geometric unsharpness at any arbitrary field position can be derived from those at the central beam position. At all locations in the radiation field, the OTF in a direction parallel to the x‐ray tube cathode–target axis does not depend on the field position X, which is perpendicular to the to the tube axis, whereas the OTF in a perpendicular direction does not depend on the field position Y, which is parallel to the tube axis. The theoretical result is used to demonstrate the field characteristic of the uniform, square focal spot.
X‐ray‐transmission computed tomography
Authors: McCullough Edwin C. , Payne J. Thomas
Published: August 2003

The immediate goal of clinically based x‐ray‐transmission computed tomography (CT) is to provide a measurement of the x‐ray linear attenuation coefficient in cross section with the ultimate goal of impacting on patient managerment and care. To do this with the accuracy needed for clinical goals requires the careful integration of x‐ray physics, detector technology, and mathematical reconstruction theory. Performance evaluation and quality assurance are necessary adjuncts to a CT scanning program. A number of investigative studies are under way.
Postmortem transition in the dynamic mechanical properties of bone
Authors: Fitzgerald Edwin R.
Published: August 2003

Measurements of dynamic shear compliance versus dead time at room temperatures are reported for a sample of rabbit bone and several samples of human bone, including an extracted wisdom tooth. In all cases an abrupt, second‐order transition is observed in one or both components of the audio‐frequency complex shear compliance, , at about 5 hours after death; this is considered to be an indication of the life‐to‐death transition in bones.
Sample mass determination using Compton‐ and total scattered excitation radiation for energy‐dispersive x‐ray fluorescent analysis of trace elements in soft tissue specimens
Authors: Cox Hollace L. , Ong Poen S.
Published: August 2003

Compton profiles and total scattered intensities have been measured to determine the total sample mass, analyzed by an x‐ray probe energy‐dispersive analyzer. Under photon excitation, fluorescent x rays are emitted from the trace elements in a biological matrix. From incident radiation, the number of photons which are Compton and elastically scattered by low‐atomic‐number elements is directly proportional to the total specimen mass. Tissue specimen masses have been measured from Compton intensities using Zr Kα and Ma Kα excitation x rays and mass calibration standards based upon carbon scattering. This procedure has been extended to include lower‐energy excitation radiation, such as Cu, where the resolution of an energy‐dispersion system requires that the total scattered intensity be used to determine the sample mass. Trace element weight‐fraction concentrations are determined from this scheme with precisions of 1% in 2%, relying only upon information contained in the energy‐dispersive x‐ray spectrum. By adjusting for the difference between tissue and carbon scattering, the accuracy of the elemental weight‐fraction concentrations is brought to within l0% of elemental concentrations measured by atomic absorption spectroscopy, for samples weighing up to 25 mg. In the case of heavier samples, absorption corrections are necessary to achieve this accuracy.
Temporal response of microdensitometers
Authors: Villafana Theodore
Published: August 2003

Microdensitometers have both spatial and temporal finite responses which may lead to degradation in images being analyzed. These responses may be quantitated in terms of spatial and temporal modulation transfer functions (MTFs). The temporal response of microdensitometers is studied here. Specifically, the technique of differentiating temporal step‐function responses to determine the temporal MTF is presented. Experimental results illustrating the theory are described using a Baird Atomic microdensitometer.
An instrument with digital readout for indirect determination of kVp
Authors: Chaney Edward L. , Hendee William R.
Published: August 2003

An electronic instrument with digital readout has been designed and constructed for indirect determination of the kVp applied to diagnostic x‐ray tubes. The signals from two detectors exposed simultaneously to a differentially filtered x‐ray beam are processed by analog computing circuitry to yield an output signal directly proportional to the applied peak kilovoltage. Theory and preliminary results are presented and discussed.
Energy dependence of correction factors for two models of Keithley diagnostic ion chambers
Authors: Morgan Tommie J. , Brateman Libby , Dirkse John
Published: August 2003

The energy dependence for each of two models of ionization chambers compatible with the Keithley model 35020 Digital Dosimeter, is determined by measurement of the exposure correction factors determined over the diagnostic x‐ray range (0.4–4.9 mm AL HVL) for several samples of each model. The results of an analysis of variance performed by model on the correction factors are reported as mean correction factors for the x‐ray beams evaluated, with a relative standard error of less than 1% for both cases. Energy‐dependence curves for the two chamber models are given, and their use is described.
An instrument for aligning patient‐positioning lasers
Authors: Horwitz Norman H. , Forsaith Ann L.
Published: August 2003

The accurate adjustment of patient‐positioning lasers is difficult and tedious. An optical instrument has been developed which enables rapid and extremely accurate adjustment of these. The instrument may be applied to several other tests of the alignment and adjustment of teletherapy and simulator machines.
Thermometry considerations in localized hyperthermia
Authors: Cetas T. C. , Connor W. G.
Published: August 2003

The introduction of local hyperthermia as a method of cancer therapy implies the necessity of quantitative measurements of the thermal dose. Our intention is to describe the nature of the problem, both physically and physiologically, with illustrations drawn from thermographic measurements in phantoms and in animals. The characteristics of a thermometry calibration facility are described. Some measurement problems associated with conventional thermometer probes are mentioned and several new thermometers which were developed for use in the electromagnetic fields are reviewed. We present some of the concepts that will guide the development of noninvasive thermometry. Systemic hyperthermia is not considered. We recommend that other reviews specifically directed toward localized hyperthermia be prepared on the methods of heating and on thermal physiological problems.
Theoretical approach to laser trabeculotomy
Authors: Goldschmidt Chmouel R. , Ticho Uriel
Published: August 2003

Theoretical aspects of laser characteristics as well as the physics parameters involved in laser trabeculotomy in open‐angle glaucoma are put forward. Outflow hydrodynamics and perforation mechanism as well as the required optical matching system are evaluated in detail.
Thermal conductivity and diffusivity of neuroblastoma tumor cells
Authors: Ahuja Avtar S. , Prasad Kedar N. , Hendee William R. , Carson Paul L.
Published: August 2003

In the application of hyperthermia to cancer management, it would be useful to know the temperature/time profile of heated tissues, including the tumor and surrounding normal structures. To obtain this information, knowledge of thermal conductivity and diffusivity of the tissues is required. The thermal conductivity of neuroblastoma was determined by a transient technique to be 89% of the thermal conductivity of water at 25°, 37°, and 44°C. From the latter measurements, the thermal diffusivity of neuroblastoma cells was estimated as 93% of the thermal diffusivity for water. Further, in this study of neuroblastoma cells, the water content was measured as 87.4 g/100 ml of cells, a rather high value not atypical of tumor cells. From literature values of density, specific heat, and thermal conductivity, values for the thermal diffusivity of a variety of normal tissues were estimated. The thermal diffusivity values of normal tissues and neuroblastoma cells exhibit an excellent correlation with water content.
Utilization of the 16O(n,p) reaction for monitoring the output of 14 MeV neutron generators
Authors: Buchanan Ron J. , Beach Joseph L.
Published: August 2003

A system based upon the 16O(n,p)16N reaction has been developed for monitoring 14‐MeV neutron production. The system has proven to be quite applicable for monitoring the output of a disk‐shaped neutron source and computer analysis has shown it equally suitable for monitoring the ouput of cone‐shaped neutron sources from gas targets presently being considered for cancer therapy.
Wedge‐shaped dose distributions by computer‐controlled collimator motion
Authors: Kijewski Peter K. , Chin Lee M. , Bjärngard Bengt E.
Published: August 2003

We have recently installed a linear accelerator, modified to allow computer control of several machine parameters during irradiation of the patient. As an initial feasibility study of computer‐controlled radiation therapy, its application to produce wedge‐shaped dose distributions by moving the collimator jaws has been evaluated. The required collimator motions have been calculated with an iterative technique. When these routines were used during irradiations of phantoms containing radiographic film, a good correspondence between calculated and measured dose distributions was observed. It is concluded that computer‐controlled motion of the collimator jaws to shape the dose distribution is technically feasible. Additionally, this technique has the advantage that the wedge angle can be continuously adjusted and the isodose curves optimized for a particular depth and field size.
Evaluation of the Therac 6 linear accelerator for radiation therapy
Authors: Grant W. , Ames J. , Almond P. R.
Published: August 2003

The Therac 6 is new generation of low‐energy linear accelerator. It incorporates a PDP‐11/05 computer for beam control, treatment‐factor input, and beam shutdown in the event of failure of the system. The performance of the unit has not been hindered by computer or software malfunction, and the computer has provided an excellent means for preventive maintenance and repair. Dosimetry parameters are similar to other 6 MV x‐ray beams, and comparison to 60Co therapy beams shows that this unit may be more like 60Co units in penumbra and absence of off‐axis peaking than other low‐energy accelerators.
Image resolution of a microchannel plate x‐ray image intensifier
Authors: Gould Robert G. , Judy Philip F. , Bjärngard Bengt E.
Published: August 2003

The resolution has been evaluated for an experimental x‐ray image intensifier employing three microchannel plates (MCPs) as the photon absorber and electron multipliers. The line spread function (LSF) was measured and used for determination of the modulation transfer function (MTF). The MTF was found to be independent of the incident photon energy from 20 to 150 keV. An additional measurement using a lead‐bar test pattern showed that the resolution exceeded 7 line pairs (lp)/mm. The factors influencing the resolution capabilities of the intensifier are discussed. The resolution is limited primarily by the 53‐μm center‐to‐center separation of the channels of the MCPs.
A least‐squares technique for extracting neutron spectra from Bonner sphere data
Authors: Zaidins Clyde S. , Martin Jerome B. , Edwards Fredric Marc
Published: August 2003

The accurate assessment of neutron flux and dose‐rate levels in a medical environment is a topic of much current interest. In this paper, a least‐squares data‐analysis technique has been used for extracting neutron spectra and related information from Bonner sphere data. This technique, incorporated in the fortran iv code nfls is worthy of consideration as an alternative to the count‐rate ratios and iterative‐unfolding techniques used in the past. The analysis provides calculated total neutron flux density, dose‐equivalent rate, and average and median‐neutron‐energy information as well as a plot of integral neutron‐flux‐density spectra. The method allows the calculation of the statistical uncertainty of each of the above quantities, which has not always been possible with other analytical methods. Results of calibration and experimental data analysis are presented and compared to results of the iterative‐unfolding technique.
Computational inaccuracy of irregular field dosimetry
Authors: Padikal Thomas N. , Deye James A.
Published: August 2003

It is pointed out that the computer program that calculates the effect of the flattening filter on the dosimetry of the PC‐12 is incorrect. (AIP)
Determination of radiographic screen–film system characteristic curve and its gradient by use of a curve‐smoothing technique
Authors: Chan Heang‐Ping , Doi Kunio
Published: August 2003

One of the important factors affecting radiographic image quality is the film contrast of the recording system, which may be expressed by the gradient of its H &  D curve. Experimental determinations of H & D curves are subject to relatively small error, which can nevertheless cause large fluctuations and uncertainties in the calculated gradient. To reduce this fluctuation, we wrote a computer program for orthogonal‐polynomial (discrete Legendre polynomials) curve fitting by means of least‐squares estimation of parameters. This procedure facilitates curve smoothing and gradient computation following sensitometric measurements. The program was tested against a large number of measured characteristic curves. A brief description of the program is given, with examples which illustrate its applications.
Magnetic modification of the electron‐dose distribution in tissue and lung phantoms
Authors: Whitmire Daniel P. , Bernard Davy L. , Peterson Mary D.
Published: August 2003

Moderately strong transverse‐magnetic fields were used to modify conventional electron‐dose distributions in tissue‐ and lung‐equivalent phantoms. Magnetically modified symmetrical‐isodose contours and central‐axis depth‐dose curves were measured for central fields in the range of 9–18 kG, field gradients of ≈5 kG/cm, and accelerator energies of 10–45 MeV. To the extent that our experimental field strengths and gradients can be reproduced clinically, the measurements showed that magnetic distributions can be generated (in tissue) which are superior to conventional distributions for the treatment of tumors lying at depths ≲ 7 cm provided that the tumor cross‐section dimensions are equal to or greater than tumor depth. The surface dose in tissue is typically reduced by ≈40% compared to the conventional surface dose for treating the same tumor volume. For the lung phantom data, a significant reduction (≳50%) in the integrated central‐axis dose to healthy tissue was achieved for tumor depths of 10–14 cm. The possibility of reproducing our experimental magnetic fields and gradients inside a patient under realistic clinical conditions is discussed.
Elongation effects on the Therac 6 linear accelerator
Authors: Dawson Donald J.
Published: August 2003

The effects of field elongation on the central‐axis output at dmax and on the depth‐dose curves for the 6‐MV x rays from the Therac 6 linear accelerator are considered. These parameters are independent of the collimator angle for both square and elongated fields. The exchange of collimator pairs results in significant output differences but has a negligible effect on the depth‐dose curves. Both the central‐axis output at dmax and the depth‐dose curves agree satisfactorily with the predictions of the equivalent‐square technique. An alternative method of greater accuracy is also indicated for the prediction of central‐axis outputs at dmax for elongated fields.
Measured neutron and gamma spectra from californium‐252 in a tissue‐equivalent medium
Authors: Elson Howard R. , Stupar Terry A. , Shapiro Alvin , Kereiakes James G.
Published: August 2003

A method of experimentally obtaining both neutron and gamma‐ray spectra in a scattering medium is described. The method utilizes a liquid‐organic scintillator (NE‐213) coupled with a pulse‐shape discrimination circuit. This allows the separation of the neutron‐induced pulse‐height data from the gamma‐ray pulse‐height data. Using mathematical unfolding techniques, the two sets of pulse‐height data were transformed to obtain the neutron and gamma‐ray energy spectra. A small spherical detector was designed and constructed to reduce the errors incurred by attempting spectral measurements in a scattering medium. Demonstration of the utility of the system to obtain the neutron and gamma‐ray spectra in a scattering medium was performed by characterizing the neutron and gamma‐ray spectra at various sites about a 3.7‐μg (1.5 cm active length) californium‐252 source in a tissue‐equivalent medium.
Computer generated scatter dose distributions for 6‐MV radiotherapy photon beams
Authors: Bloch Peter , Wallace Robert
Published: August 2003

The time required for accurate computer dose distributions is of major concern in radiation therapy. Isodose distributions were obtained by calculating the primary and scatter radiation reaching a point. A description of the primary dose distribution, especially at the edge of the beam, requires the calculation of the dose on a finely spaced grid. However, the scatter dose distributions can be described on a much coarser grid. The primary and scatter dose distribution calculations were separated into two computer subroutines which permitted choosing the appropriate grid spacing for each dose component. A more rational partition of the computing time for the primary and scatter dose was obtained. In addition, the separation of the calculation of the scatter dose from the total dose allows one to examine the calculated scatter dose distribution as the radial and angular increments used in the Clarkson–Cunningham summation are varied. Accurate scatter dose distributions were generated with a tenfold time reduction by optimization of grid spacing and radial and angular increments.
On the constancy in composition of polystyrene and polymethylmethacrylate plastics
Authors: Schulz R. J. , Nath Ravinder
Published: August 2003

Variations in the atomic compostion, and mass and electron densities of polystyrene and polymethylmethacrylate (PMM) plastics were assessed from experimentally determined mass attenuation coefficients for 125I and 137Cs gamma rays. The means and standard deviations in the mass densities of 16 samples of PMM and 10 samples of polystyrene were found to be 1.174±1.4% and 1.042±0.6% g/cm3, respectively. Based upon transmission measurements on various solutions of ethyl alcohol in water, the standard deviations in the effective atomic numbers of PMM and polystyrene were determined to be 0.77% and 1.3%, respectively. Based upon experimentally determined mass attenuation coefficients for 137Cs, the standard deviations in electron density for PMM and polystyrene were 0.5% and 1.2%, respectively. Similar measurements on tap water and two grades of distilled water failed to detect any differences in atomic composition.