bluuebunny/crossref_metadata_2025 huggingface dataset:
On the potential use of stimulated positron emission (SPE) in the detection and monitoring of some bone diseases
Authors: Benjamin Menashe , Macovski Albert
Published: August 2003

The feasibility of a new method for in vivo regional measurement of the loss of bone tissue in osteoporosis and bone mineral in osteomalacia has been studied analytically. A cross section in the patient's body containing the bone is excited by high energy x rays (hν>1.022 MeV) to produce positron–electron pairs. The positron distribution is imaged by detecting the resulting annihilations using two detectors placed on opposite sides of the irradiated slice and focused to receive annihilations produced inside the bone. Since the cross section for pair production is proportional to the square of the atomic number of the absorber, any pathological processes involving demineralization or loss of bone tissue are expected to strongly affect the number of counts received from the region imaged. This technique is unique among other radiological procedures for bone measurements in its direct three‐dimensional imaging capability. It has no fundamental limit on precision, and higher precision is achieved by resorting to higher dose or greater bony volume irradiated. In addition, the measurement can be confined to an anatomically and physiologically appropriate bone site, thus maximizing the signal and minimizing interference from surrounding tissues. Using radiological models for osteoporosis and osteomalacia, it is shown that a high sensitivity to small changes (∼5%) in density or amount of mineral of cortical bone can be obtained with a relatively low radiation dose (<0.5 rad). With the same radiation dose, the method is less sensitive to changes in vertebral bone, due to the lower mineral fraction in this type of bone. The dependence of sensitivity on incident x‐ray energy is studied, and it is shown that the optimum energy, from a point of view of maximizing sensitivity while minimizing the loss of spatial resolution due to positron range lies at between 2 and 3 MeV. Finally, the application of the technique in the detection and monitoring of bone cancer and tumors metastatic to bone is suggested.
X‐ray spectra vs attenuation data: A theoretical analysis
Authors: Baird L. C.
Published: August 2003

Laplace transform theory is used to investigate the relationship between x‐ray spectra and the corresponding attenuation curves. It is shown that in principle, each is sufficient to determine the other; but in practice, experimental considerations lead one to require conformity with some preconceived mathematical model.
Photon activation—15O decay studies of tumor blood flow
Authors: Ten Haken Randall K. , Nussbaum Gilbert H. , Emami Bahman , Hughes Walter L.
Published: August 2003

A direct, noninvasive method for measuring absolute values of specific capillary blood flow in living tissue is described. The method is based on the photon activation, in situ, of tissue elements and the measurement of the subsequent decay of the positron activity induced, employing coincidence detection of the photon pairs produced in positron annihilation. Analysis of the time‐dependent coincidence spectrum reveals the contribution to the total signal from the decay of 15O, from which the specific capillary blood flow in the imaged, activated volume is ultimately determined. By virtue of its introduction of the radioisotope of interest (15O) directly and uniformly into the tissue volume under investigation, the method described permits both the nonperfused and well perfused fractions of an activated volume to be estimated and hence, the average specific blood flow within imaged tumor volumes to be computed. The model employed to describe and analyze the data is discussed in detail. Results of application of the technique to measurement of specific blood flow in rhabdomyosarcoma tumors grown in WAG/Rij rats are presented and discussed. The method is shown to be reliable and well suited to studies designed to determined the effects of various agents, such as heat, radiation and drugs, on tumor blood flow.
Fast minimum variance estimator for limited angle CT image reconstruction
Authors: Buonocore Michael H. , Brody William R. , Macovski Albert
Published: August 2003

Many applications of diagnostic cross sectional imaging require that images be reconstructed from a limited number of projections (limited angle). Convolution back projection has been unsuitable in these applications. Methods for reconstruction based on stochastic estimation theory, such as the minimum variance estimator, use a discrete linear measurement model and are suitable for limited angle reconstruction. Unfortunately, the computational requirements of these methods have precluded their use. In this paper, starting from the general minimum variance estimator , a computationally efficient (fast) estimator is derived for limited angle reconstruction by choosing and in the simplest way consistent with the geometric considerations of data acquisition. Minimum variance has in the past been precluded from use by the large amount of computation required to compute . With the fast estimator, the computation is avoided because has a particular form that allows factorization of the matrix into a product of matrices, each of which is easily inverted. A demonstration of the estimator for the reconstruction of sharp peaks is provided. Image quality is similar to that obtained with other methods.
Lead shielding for electrons
Authors: Khan Faiz M. , Werner Barry L. , Deibel Firmin C.
Published: August 2003

Using a 13 MeV electron beam as an example, transmission curves for various thicknesses of lead were measured. The data indicated that the choice of shielding thickness depends greatly on the depth at which the measurements are made. The importance of this reference depth and criteria for shield design when shields of minimum thickness are required is discussed.
Application of fission track detectors to californium‐252 neutron dosimetry in tissue near the radiation source
Authors: Oswald Richard A. , Lanzl Lawrence H. , Rozenfeld Martin
Published: August 2003

Fission track detectors were applied to a unique problem in neutron dosimetry. Measurements of neutron doses were required at locations within a tumor of 1 cm diameter implanted on the back of a mouse and surrounded by a square array of four 252Cf medical sources. Measurements made in a tissue‐equivalent mouse phantom showed that the neutron dose rate to the center of the tumor was 2.18 rads μ g−1 h−1±8.4%. The spatial variation of neutron dose to the tumor ranged from 1.88 to 2.55 rads μ g−1 h−1. These measurements agree with calculated values of neutron dose to those locations in the phantom. Fission track detectors have been found to be a reliable tool for neutron dosimetry for geometries in which one wishes to know neutron dose values which may vary considerably over distances of 1 cm or less.
Scatter transmission through an ideal grid
Authors: Siddon Robert L.
Published: August 2003

An analytic expression is derived for the transmission of isotropic scattered radiation incident upon an ideal parallel grid. It is found that the expression for the transmission usually found in the literature overestimates the transmission by approximately a factor of two. Key words: ideal grid, scattered radiation
Preliminary evaluation of display format effects on perceptibility in a low contrast ultrasound test object
Authors: Sommer F. Graham , Orphanoudakis Stelios C. , Taylor Kenneth J. W.
Published: August 2003

A phantom consisting of a gel closely resembling human liver on B‐mode scanning, and containing a rounded zone of decreased echogenicity, was used to create a series of B‐mode ultrasonic images containing simulated “lesions” in random locations on the final recorded image. A homogeneous gel phantom was used to create a series of blank images. Images were recorded in three display formats: conventional white on black background, conventional black on white background, and black on a white background with luminance normalized to that of the conventional white on black image. Six subjects were presented with a randomized series of scans in the 3 display formats, and receiver operating characteristic analysis was used to assess the effect of display format on low‐contrast detectability. Detectability of low‐contrast “lesions” was lower in a high contrast image with black on white background compared to the medium contrast image in white on black format. Detectability with the black on white format having normalized luminance was generally intermediate between that observed for the two conventional formats.
Dose efficiency and the effects of resolution and noise on detail perceptibility in radiographic magnification
Authors: Wagner Louis K. , Cohen Gerald , Wong Wai‐Hoi , Amtey Sharad R.
Published: August 2003

The detail signal–to–noise ratio model of radiographic imaging is quantitatively analyzed in terms of its accuracy in describing observer threshold perceptibility of radiographic detail. The model is found to adequately describe the effects of magnification, scatter radiation, and system resolution on observer threshold perceptibility. However, it is shown that the model does not apply in screen/film radiography for very low contrasts and high scatter conditions due to insufficient optical density contrast. The dose‐to‐information conversion efficiency of a radiographic imaging system is defined and the effects of magnification, scatter, resolution, image processing, detector efficiency, grids, patient table support, field size, and geometry on the dose efficiency of the imaging system are investigated.
CT scanner comparison
Authors: Southon F. C.
Published: August 2003

A survey of six production CT scanners in clinical use has been carried out. The scanner types are the ACTA 0200, EMI 1007, EMI 5005, GE 7800, DELTA 2010, and the AS & E 500. Measurements were made of the noise, spatial resolution, edge enhancement, axial position sensitivity, artefact, and dose. Dose is expressed in terms of the total absorbed energy or the volume integral of the dose, as this is considered the factor most directly related to patient risk. A relationship between dose and picture quality parameters is developed and applied to the scanners for a wide range of settings. Particular emphasis has been placed on evaluating the different operating modes of each scanner. The results show a great variation between machines, especially in the relationship between resolution and noise, and the performance achieved for the dose delivered.
Calculated response and wall correction factors for ionization chambers exposed to 60Co gamma‐rays
Authors: Nath Ravinder , Schulz R. J.
Published: August 2003

The response and wall correction factors for various ionization chambers in a cobalt‐60 gamma‐ray field have been calculated using a Monte Carlo photon‐electron transport code. Among the chamber parameters studied are chamber wall material and its thickness, central electrode material and its dimensions, and the shape and size of the sensitive volume. The calculations show that the response and wall correction factors are sensitive to the shape and volume of the ionization chamber, but relatively independent of the choice of material for the chamber wall and electrode when these are compared on the basis of electron density. Data are presented for cylindrical, plane‐parallel, and spherical ionization chambers constructed from carbon, magnesium, aluminum, water, Lucite, polystyrene, and ICRU muscle, as well as for a number of commerically available ionization chambers.
Film dosimetry of megavoltage photon beams: A practical method of isodensity‐to‐isodose curve conversion
Authors: Williamson Jeffrey F. , Khan Faiz M. , Sharma Subhash C.
Published: August 2003

The central problems of photon beam film dosimetry are the dependence of film response upon photon energy, processing conditions, and film plane orientation. We have overcome these problems by accurately fitting the depth‐dependent sensitometric curve of Kodak XV‐2 film (exposed parallel to beam axis) to the equation OD(D,d) =ODs(1 − exp {−α0[1+β(d−dm)]D}) where OD(D,d) is the optical density for dose D at depth d. ODs, α0, and β are constants characteristic of the film and beam energy but are independent of field size. Only central axis depth dose data for a single field are required to determine their values. A computer program based upon this equation has been written which successfully generates single field isodose curves from film data for a variety of field sizes (including wedged fields) with an accuracy of ±3%. Data are presented for 60Co, and 4 and 10 MV x rays.
Characterization of a p(66)Be(49) neutron therapy beam I: Central axis depth dose and off‐axis ratios
Authors: Rosenberg I. , Awschalom M.
Published: August 2003

Results of measurements of central axis depth doses, off‐axis ratios, and wedge filter effects are presented for a p(66)Be(49) neutron beam. All measurements were made and are reported in tissue equivalent solution. Algorithms that reproduce the various measured characteristics of the beam are also discussed.
Exposure of rhesus monkeys to 20 000 G steady magnetic field: Effect on blood parameters
Authors: Battocletti Joseph H. , Salles‐Cunha Sergio , Halbach Richard E. , Nelson Jedd , Sances Anthony , Antonich Fred J.
Published: August 2003

Sixteen monkeys were used in a study to determine the effects, or noneffects, of exposure to a steady magnetic field of 2 T (20 000 G). Arterial and venous blood samples were taken before, immediately after, and several weeks following the test. Blood gas analyses were made of arterial samples, and hemotologic cell data and Technicon analyses were made of venous samples. Differential and absolute white blood cell counts of segmented neutrophils and lymphocytes were considerably changed during the test, even for monkeys which were not exposed to the magnetic field. These changes have also been observed in monkeys whose environment and daily routine are modified appreciably. Eight of the 16 monkeys were tested twice: first, in the magnet with the magnet turned on, and second, two months later, in the magnet, but with the magnet turned off. A large superconducting magnet (0.63 m i. d. and 1.85 m long warm‐air bore) was used, which accomodated two monkeys at one time. Paired‐t tests of 45 blood parameters showed no significant differences between the two tests. Exposure to a gradient field compared with exposure to a uniform magnetic field showed no significant differences.
Linac monitor end effect
Authors: Baird Leemon C.
Published: August 2003

It is pointed out that method used for accelerator calibration should be consistent with that used to calculate treatment planning dosage. (AIP)
A comparison of chemical and ionization dosimetry for high‐energy x‐ray and electron beams
Authors: Durocher J. J. G. , Boese H. , Cormack D. V. , Holloway A. F.
Published: August 2003

A comparison was made of ferrous sulfate (Fricke) and ionometric methods for determining the absorbed dose in a phantom irradiated with 4‐MV x‐rays, 25‐MV x‐rays, or electron beams having various incident energies between 10 and 32 MeV. Both chemical and ionization instruments were calibrated in a 60Co beam at a point in water where the absorbed dose had been previously determined. The chemical yield measurements were corrected for spatial variations in dose within the volume of the solution and used to obtain a value of the absorbed dose for each of the x‐ray and electron beams. The ratios of G‐values required for these determinations were taken from ICRU reports 14 (1) and 21.(11) Ionization instrument readings from three types of commercial ionization chambers were used to obtain alternate values of the absorbed dose for each radiation. Cλ and CE values used in determining these ionization values of dose were also taken from the above ICRU reports. For 4‐MV x‐rays the values of absorbed dose obtained from chemical measurements agreed to within 0.5% with values obtained from ionization measurements; for 25‐MV x‐rays the chemical values were about 1% higher than the ionization values; for the electron beams the chemical values were 1%‐4% below the ionization values. These discrepancies suggest an inconsistency among the recommended G, Cλ, and CE values similar to that which has been noted by other workers.
Evaluation of tissue‐air ratios and tissue‐maximum ratios with the photon energy‐fluence absorption dose model
Authors: Ahuja Sain D. , Stroup Steven L. , Bolin Marion G.
Published: August 2003

Values of tissue‐air ratio (TAR) and tissue‐maximum ratio (TMR) have been calculated with the photon energy‐fluence absorption dose model, at depths d>dn (depth of dose normalization) in a homogeneous water phantom, for Co‐60 γ‐ray, and 4, 6, 10, 14, and 45 MVp x‐ray beams of EQS (the side of the equivalent square) up to 20 cm. It has been found that by (i) slightly altering the formula for the depth modification factor so that it depends on the difference (d−dn) rather than d alone; and (ii) permitting E, the effective energy of the beam, to vary with EQS for some x‐ray beams, it is possible for the model to reproduce within 2.5% the measured data at depths d>dn, on TAR, TMR and PDD (the central axis percent depth dose), for any source to surface distance.
Application of the noise power spectrum to positron emission CT self‐absorption correction
Authors: Riederer Stephen J.
Published: August 2003

Two methods are compared in correcting for self‐absorption in positron emission CT scans, or equivalently, in determining the integral of attenuation along a strip in a cross section. These are CT reconstruction and direct measurement. It is shown that the former method is slightly more precise than the latter with the degree of improvement proportional to the number of angles used. Additionally, it is demonstrated that the CT approach is statistically very inefficient in its use of detected events in comparison to the direct approach. For the CT case, it is shown using the noise power spectrum that two‐thirds of the variance in the attenuation integral is from one projection alone, namely that along the direction of the strip. The remaining one‐third is primarily from adjacent projections. For the problem of determining the attenuation in a single strip within the slice, the CT approach is not recommended. However, if the problem is extended to a complete set of strips within the slice, as is desired in positron emission CT, the CT approach may require considerably less counting time than the direct approach for comparable precision.
An illustration of transfer function correction in classical tomography
Authors: Harding G.
Published: August 2003

Several tomographs of a head phantom are presented to illustrate the improvement in imaging characteristics of a tomographic system with corrected transfer function.
Speculation concerning average tissue–air ratio determinations for rotational 60Co radiation therapy treatment planning
Authors: Morin R. L. , Raeside D. E. , Wizenberg M. J.
Published: August 2003

The question of the number of radii which are necessary to accurately determine the average tissue–air ratio (TAR) to be used in 60Co rotational radiation therapy treatment planning was studied using an analytical expression for the TAR as a functon of depth and field size, and an elliptic, homogeneous patient cross section. It was found that for such a contour 12 radii determine the average TAR to within 0.01% of the value obtained using 360 radii.
Comparison of the perturbation correction in a parallel plate and a cylindrical ion chamber
Authors: Jones Douglas
Published: August 2003

The perturbation correction factors for a 0.03 cc parallel plate chamber and a 0.6 cc cylindrical ionization chamber have been compared. The current from these ion chambers was measured in electron beams of energy in the range 7 to 12 MeV and in a cobalt‐60 gamma ray beam. It is concluded that there is no difference between the perturbation correction factor for these two chambers. A significant polarity effect was observed for the parallel plate chamber, which was a function of energy and depth in the phantom. This work supports the ICRU recommendation for displacement correction.
Buildup region and skin‐dose measurements for the Therac 6 Linear Accelerator for radiation therapy
Authors: Tannous N. B. J. , Gagnon W. F. , Almond P. R.
Published: August 2003

Buildup and surface‐dose measurements were taken for the 6 MV photon beam from a Therac 6 linear accelerator manufactured by Atomic Energy of Canada Limited (AECL) with and without a lucite blocking tray in place. Further measurements were made with a copper filter designed to reduce secondary electrons emitted by photon interactions with the Lucite tray. The results are discussed in relation to skin‐sparing for radiation therapy patients. The measurements were made with a fixed volume PTW parallel‐plate ionization chamber and corrected to zero‐chamber volume. The results were found to be consistent with similar measurements taken with a variable volume extrapolation chamber.
Resolution in radiographic magnification
Authors: Wagner Louis K. , Cohen Gerald , Wong Wai‐Hoi , Amtey Sharad R.
Published: August 2003

Radiographic resolution and image sharpness are analyzed as a function of magnification for a few screen/film systems and a microfocal spot x‐ray tube. Resolution and sharpness are described in terms of both MTF and effective aperture. The analysis demonstrates advantages of magnification for any combination of radiographic screen/film system and x‐ray focal spot, and, in addition, allows one to compare different screen/film systems each used at different magnifications. Such an analysis is applicable to any radiographic system and may serve as a guide in selecting screen/film systems for use in magnification procedures.
Variations in the beam characteristics of the Varian Clinac‐4 (Pb)
Authors: Jones Douglas , Shumacher Douglas , Washington John , Hilko Robert
Published: August 2003

The Northwest Center for Radiological Physics has used a well‐defined protocol in the review of radiotherapy machines. The central axis depth dose and off‐axis factor of fifteen Clinac‐4's are reported. The range in depth dose was on the order of 3.5% and the range in off‐axis factor on the order of 8%. Similar measurements on a Theratron‐80 Cobalt‐60 teletherapy machine are reported to demonstrate the reproducibility of the measurement technique.
Image processing of conventional tomograms
Authors: Renner Wendel Dean , Luke Jon C.
Published: August 2003

Previous authors have suggested that high pass spatial filtering of axial transverse and circular tomograms would improve the perceived image. A preliminary study here shows that good signal‐to‐noise ratio is critical for the success of image processing for the contrasts obtainable in the clinical situation. The successful image processing of a small object on the Philips Polytome is demonstrated, as well as a clinical example. A similar trial with the Toshiba axial transverse tomography machine was not successful due to lower subject contrast with higher kVp techniques. A derivation of the optical transfer function of circular and axial transverse tomograms is given, with emphasis on those features that image processing can improve. To accomplish image processing, two methods employing incoherent light were investigated. Either method may be regarded as an analogue equivalent to the mathematical operations employed by digital computers in the reconstruction of CT images. First, a simple all‐purpose image processor is described that makes use of the Herschel effect. Second, a simple film copying technique that simulates a high pass spatial filter is introduced and used to improve tomographic images.
Dye‐sensitized photodynamic inactivation of cells
Authors: Pooler John P. , Valenzeno Dennis Paul
Published: August 2003

Living cells may be modified in diverse ways by the combined action of visible light and photosensitizing molecules. The effects appear most frequently as disruptions of subcellular structure, changes in surface membrane function or inhibition of mitotic ability. This review concentrates on the four most thoroughly studied cell types—yeast cells, nerve cells, erythrocytes, and cultured tumor cells. Research on these cells indicates that potency of sensitization depends at least as much on the factors affecting an association between sensitizer and cell prior to illumination as on photochemical properties. While sensitizers which permeate may lead to altered DNA, it appears that surface membrane modification occurs simultaneously and may be critical in the inactivation mechanism. There is much circumstantial evidence suggesting that excited singlet molecular oxygen acts as an intermediate between photoexcited sensitizer and target alteration. Proteins, lipids, and nucleic acids are all susceptible to photosensitized attack, but the correlation between cellular and molecular modification remains ill‐defined. The use of the photodynamic process as a therapeutic technique, particularly in the treatment of malignant tumors, holds great promise, but awaits further research to develop greater selectivity of action.
Multisegmented ion chamber for CT scanner dosimetry
Authors: Moore Marleen M. , Cacak Robert K. , Hendee William R.
Published: August 2003

A multisegmented, ionization chamber capable of determining dosimetric profiles from a CT scanner has been developed and tested. The chamber consists of a number of 2 mm wide electrically isolated segments from which ionization currents may be measured. Presented here are the performance characteristics of the chamber including energy response, dose linearity, and corrections for “cross talk” between segments. Sample dosimetric profiles are depicted for 3 and 6 mm nominal beam widths at two locations in a dosimetric phantom positioned in the x‐ray beam of a fourth generation CT scanner. The results agree well with the conventional method of obtaining dosimetry measurements with TLD chips.
Scattered radiation in chest radiography
Authors: Niklason Loren T. , Sorenson James A. , Nelson James A.
Published: August 2003

Luminance scatter fractions were measured for patients and phantoms in various regions of the chest film. Scatter fractions were found to be highly variable, being quite high in the regions of the chest with a large equivalent tissue thickness (e.g. mediastinum, chest wall, and subdiaphramatic areas). For example, approximately 90% of the radiation reaching the mediastinum region of the chest film was found to be scattered radiation when a grid was not used. Even with a 12:1 grid, less than 50% of the available primary beam contrast was being displayed in the mediastinum. In regions of the chest film with high scatter fractions, film latitude was also a problem, further reducing image contrast in these regions. Image contrast and scatter fractions in chest radiography were investigated for different scatter rejection techniques including air gaps, antiscatter grids, and slit radiography.
Mean energy in electron beams
Authors: Andreo Pedro , Brahme Anders
Published: August 2003

The mean energy of the energy spectrum is an essential parameter for the dosimetry of therapeutic electron beams. Frequently it is assumed that the mean energy of such beams remains constant across the beam and only its degradation with depth is considered. The present work analyzes the variation of the mean energy of primary electrons with depth and lateral position in an electron beam using the Monte Carlo method. Results are compared with relations commonly employed for determination of mean energy at a depth. For the variation of the mean electron energy with depth in broad beams, good agreement was found between Monte Carlo results and an analytic continuous slowing down expression, which takes the variation of radiation stopping power with depth into account. Due to the calculated lateral variation of the mean energy, the relative absorbed dose profile determined with an air ionization chamber in a clinical beam should differ by less than 1% from the measured ionization profile.
An algorithm for the reduction of metal clip artifacts in CT reconstructions
Authors: Glover Gary H. , Pelc Norbert J.
Published: August 2003

Implanted surgical metal clips often produce objectionable artifacts in CT reconstructions. The artifacts appear as streaks which emanate radially from the site of the clip. It is shown in this paper that these artifacts stem primarily from motion of the clip during the scan. An algorithm is described which reduces the intensity of these artifacts. The procedure attempts to remove the metal object entirely from the scan data by replacing the measured projection values of rays that passed through a neighborhood of the clip with calculated values consistent with an object whose density is an average of the surround. Examples are given for head and body scans as well as for computer simulations which show substantial reduction of the streak intensity. Key words: computed tomography, artifacts, reconstruction algorithms, surgical clip streaks, starburst artifacts
“Battered data”—Some clinical effects of the abuse of multiple regression methods: A second look at some observations on Ro07−0582 (Misonidazole)
Authors: Herbert Donald E.
Published: August 2003

The paper presents an argument for the wider use of simple and standard statistical methods in the design and evaluation of both laboratory and clinical experiments with toxic radiosensitizers. It also presents an argument against the frequent practice of superposing a curve estimated from laboratory observations upon scattergrams of clinical observations to thereby assist the reader to the (often fallacious) inference that the two disparate sets of observations have enough in common bo be graduated by the same curve. Such a practice achieves a substitution of reasoning by analogy for the more reliable statistical procedures of estimation and of testing a hypothesis.
Performance of a refrigerated charcoal trap for xenon‐133
Authors: Powell Malcolm , Grando Robert , Robeson William
Published: August 2003

The impulse response function of a charcoal trap to a bolus of xenon‐133 was determined as a function of the total number of hours run both at room temperature and at 25°C. The peak of the response function for a new trap at room temperature reached a value of 360 MPC at 11 h. After 150 h of operation, the impulse response function was determined at −25°C reaching a value of only 35 MPC at 25 h. The exhaust concentration of a trap in a busy nuclear medicine department using 150 mCi of xenon per week was measured and found to be 1600 MPC. The trap was placed in the freezer and kept there while it continued in use. Over a period of 3 weeks, the concentration of xenon in the exhaust of the trap dropped to a value of 13 MPC, or less than 1% of its value at room temperature. Key words: Xenon, charcoal trap, refrigation, trap
Cardiac phase: Amplitude analysis using macro programming
Authors: Logan K. William , Hickey Keith A.
Published: August 2003

The analysis of EKG gated radionuclide cardiac imaging data with Fourier amplitude and phase images is becoming a valuable clinical technique, demonstrating location, size, and severity of regional ventricular abnormalities. Not all commercially available nuclear medicine computer systems offer software for phase and amplitude analysis; however, many systems do have the capability of linear image arithmetic using simple macro commands which can easily be sequenced into stored macro‐strings or programs. Using simple but accurate series approximations for the Fourier operations, macro programs have been written for a Digital Equipment Corporation Gamma‐11 system to obtain phase and amplitude images from routine gated cardiac studies. In addition, dynamic cine‐mode presentation of the onset of mechanical systole is generated from the phase data, using only a second set of macro programs. This approach is easily adapted to different data acquisition protocols, and can be used on any system with macro commands for image arithmetic. Key words: Fourier analysis, cardiac cycle, gated blood pool imaging, amplitude image, phase image
An evaluation of a fluorescent screen–isocon camera system for x‐ray imaging in radiology
Authors: Nelson Robert S. , Barbaric Zoran L. , Gomes Antionette S. , Moler Charles L. , Deckard Michael E.
Published: August 2003

A large field format imaging system which utilizes a flat fluorescent screen rather than the traditional image intensifier as the primary x‐ray detector is discussed in this paper. A low light level image isocon camera is optically coupled to a rare‐earth screen. An overview of the isocon camera and its return beam mode of operation is included. Theoretical limitations to this approach to real time radiographic imaging are discussed in terms of system efficiency and signal‐to‐noise ratio (SNR) requirements. The viability of this concept for radiographic imaging is considered with respect to the image intensifier–camera unit. The merits of employing a CsI:Na screen as the primary detector and an isocon tube with an improved SNR are presented as potential areas for further investigation.
Comparison of dose determinations for 15‐MeV neutrons
Authors: McDonald J. C. , Barschall H. H. , Howell R. H.
Published: August 2003

Absorbed dose in A‐150 plastic irradiated by 15‐MeV neutrons was measured using three dosimetric methods: an A‐150 plastic calorimeter, A‐150 ionization chambers, and activation of gold foils. The dose determinations by the calorimeter and ionization chambers agreed to better than 1%, while the activation method yielded a 3.5% lower dose. The agreement between the methods is within the experimental uncertainties.
Microdosimetric measurements of radiation quality variations in homogeneous phantoms irradiated by fast neutron beams
Authors: Beach J. L. , Milavickas L. R.
Published: August 2003

The Dual Radiation Action Theory of Kellerer and Rossi (DRA), along with presently available microdosimetric techniques, is applied to the detrmination of radiation quality variation within tissue equivalent phantoms irradiated by collimated fast neutron beams. The neutron beams investigated were produced by the bombardment of 22.5 and 16 MeV d+ on beryllium and by the T(d,n)4He reaction (15‐MeV neutrons). Microdosimetric spectra were obtained at points of varying depth and lateral distance from the central axis within a tissue equivalent phantom, including points within the penumbra. From the microdosimetric spectra the parameter RQ, a first approximation to RBE derived from DRA theory, is calculated for each point. All RQ values are calculated for the same level of effect. For these three different beams the results show that the RQ values for the total radiation spectrum of neutron and gamma radiation remain fairly constant with depth and with lateral distance from the beam axis at 2 and 10 cm depths. The largest central axis variation in RQ is 8% for the d(16)+Be beam. The largest variation between a penumbra and an on‐axis RQ value is 4% at 2 cm depth in the d(22.5)+Be beam. The results for the d(22.5)+Be beam disagree with previously reported radiobiological results while the 15 MeV beam results are in good agreement.
Comparisons of electron beam dose measurements in water and polystyrene using various dosimeters
Authors: Kase Kenneth R. , Adler Gary J. , Bjärngard Bengt E.
Published: August 2003

A comparison has been made of central axis percent depth dose and absorbed dose in electron beams of 7.8 and 10.2 MeV, measured with devices of differing geometry and construction. Flat and cylindrical ionization chambers have been used as well as thin thermoluminescent dosimeters. The ionization chambers had walls of air equivalent or tissue equivalent plastic. Results indicate that central axis depth dose measurements are independent of measuring device. No significant difference was found among the various ionization chambers with air equivalent walls in the determination of absorbed dose. The dose determined by the tissue‐equivalent wall chamber was about 3% higher than the dose determined by the other ionization chambers. Dose maximum on the central axis in water is about 4% greater than when this same quantity is calculated from data measured in polystyrene.
Transient cavitation in tissues during ultrasonically induced hyperthermia
Authors: Sommer F. Graham , Pounds Douglas
Published: August 2003

An isospherical array of ultrasonic transducers designed for hyperthermia treatment of malignant tumors in humans was employed in studies of transient cavitation in tissues during ultrasonic heating. Both in vitro and in vivo studies revealed the presence of transient cavitation within tissues during ultrasonic heating, as indicated by the detection of the first half‐order subharmonic of insonifying ultrasound with an interrogating transducer. The applied power to the focal region of the array required to exceed cavitation thresholds in tissue was 75±8 W/cm2, a level considerably higher than that required to heat tissues to the temperature range used for cancer treatment (43–45 °C.).
The measurement of trabecular bone mineral density using coherent and Compton scattered photons in vitro
Authors: Ling Shih‐Shen , Rustgi S. , Karellas A. , Craven J. D. , Whiting J. S. , Greenfield M. A. , Stern R.
Published: August 2003

A photon scattering method for measuring the trabecular bone mineral density (TBMD) in vitro is described. This method involves the measurement of the ratio of coherent to Compton 90° scattered photons from Am–241 by using a narrow beam geometry with an intrinsic germanium detector. The feasibility of using smaller scattering angles for better counting efficiency and the associated problems in their application for in‐vivo measurements were investigated. Calibration of the system with fresh trabecular bone samples showed a linear relationship between the coherent to Compton ratio R of the detected counts and the TBMD (r = 0.94). The effect of the overlying soft tissue on the R ratio was significant while the effect of self‐attenuation by the trabecular bone itself and the cortical layer was negligible. It was found that the marrow fat content could alter the value of the R ratio. Our results show that for a 10% increase in the fat content in the interstices of the trabecular bone there is a 2.5% decrease in the R ratio. This technique together with soft tissue corrections will enable us to measure the TBMD of the calcaneum in vivo, assuming a small variation in the trabecular fat content. The estimated absorbed dose to the bone marrow is about 139 mrad.
Clinical prospects for liposomes
Authors: Yatvin M. B. , Lelkes P. I.
Published: August 2003

The use of liposomes has recently been the subject of considerable attention as a promising and versatile approach to drug delivery. Particularly intriguing is the possibility of targeting liposomes to specific areas of the body such as tumors or sites of inflammation or parasitic invasion for either local accumulation or release of associated drugs. This review focuses mainly on recent in vivo work having clinical potential. An extensive discussion of liposome preparation and entrapment of drugs for controlled release in vivo is also included. The stability of liposomes in biological fluids is a major problem. The mode of administration, either intraperitoneal, subcutaneous, local, oral, or respiratory, is closely related to the life of the liposomes in vivo. Following in vivo administration the lifetime of a liposome is critically dependent on its composition, size, and charge. Liposome toxicity appears to be minimal, but should be considered when administering liposomes to patients. Tissues such as the liver, spleen, and lungs, because of macrophage ingestion of liposomes, become potential sites of drug toxicity. The use of liposomes to deliver antiparasitic drugs in the treatment of malaria and leishmaniasis is promising; so is the use of surfactant‐carrying liposomes in the treatment of respiratory distress syndrome in premature babies. Recent cancer studies utilizing liposomes both in vivo and in vitro have shown promise. In tumor‐bearing animals a liposome drug delivery system has caused a regression, delayed tumor growth, and increased survival time. Although the clinical use of liposomes is only in its infancy, its potential in future therapy appears promising.
The use of nonhydrogenous wedges for therapeutic neutron beam shaping
Authors: Ten Haken Randall K. , Awschalom Miguel , Rosenberg Ivan
Published: August 2003

Field shaping wedges made of Teflon, aluminum, steel, and lead were designed for a p(66)Be(49) therapeutic neutron beam. Design considerations, expectations, and actual measured changes in dose profiles are presented.
A microcomputer‐based system for radiotherapy beam compensator design and patient contour plotting
Authors: Andrew John W. , Aldrich John E.
Published: August 2003

A microcomputer‐based system for radiotherapy beam compensator design has been constructed. A patient surface measuring device which projects a grid pattern on the patient is attached to the collimator of a therapy treatment simulator. A linear motion potentiometer, pivoted at one end on a ball joint attached to the collimator, is extended to touch each grid point in turn. The extension of the potentiometer is monitored by a microcomputer which calculates the coordinates of the patient's surface as each grid point is entered. A template for compensator construction is subsequently printed on the microcomputer printer. The patient surface profile along any grid line can also be plotted and used in conjunction with the compensator data to produce an isodose distribution for the compensated field.
Disintegrating belt jams cobalt‐60 source drive mechanism
Authors: Anderson D. W.
Published: August 2003

A Picker C9 teletherapy machine cobalt‐60 source failed to retract because fragments of a badly deteriorated drive belt had caught on the drive gear and jammed the pinion gear. (AIP)
Absolute measures of physical image quality: Measurement and application to radiographic magnification
Authors: Sandrik John M. , Wagner Robert F.
Published: August 2003

Two absolute measures of physical image quality—noise equivalent quanta (NEQ) and detective quantum efficiency (DQE)—are reviewed and calculated as functions of spatial frequency from measurements of the imaging characteristics of radiographic screen–film systems. NEQ is the effective number of information bearing quanta used to form an image, and DQE is the ratio of NEQ to the actual number of quanta used. NEQ and DQE are derived from the sensitometric characteristic, modulation transfer function (MTF), and noise power spectrum (NPS) of a screen–film system. Spatial frequency ( f) distributions of NEQ and DQE were determined for three calcium tungstate screens—Du Pont Cronex Detail, Par Speed, and Hi‐Plus—used with Kodak XRP film. The relative ranking of NEQ( f) of these three systems followed the ranking of MTF and the exposure required to produce a given film density (Detail>Par Speed>Hi Plus). The ranking of DQE( f) followed that of system speed—Hi Plus>Par Speed>Detail—at frequencies <2.5 cycles/mm, but was the same as the ranking of NEQ( f) at frequencies >5 cycles/mm. Noise equivalent apertures defined in terms of MTF were larger than analogous apertures defined in terms of NEQ for these screen–film systems, suggesting that, for a specific imaging task, these systems introduce more degradation in image quality due to loss of contrast than to loss of signal‐to‐noise ratio (SNR). Finally, the application of NEQ( f) and DQE( f) distributions to imaging system analysis is demonstrated by applying them to an optimization of magnification radiography with respect to image SNR.
The effect of missing backscatter on the dose distribution of a p(66)Be(49) neutron therapy beam
Authors: Awschalom M. , Rosenberg I. , Ten Haken R. K.
Published: August 2003

The dose reduction in phantom due to missing backscattering material was measured on and off the central axis of a p(66)Be(49) neutron beam. The doses at different depths in phantom for various field sizes are found to drop by 4%–10% relative to full backscatter conditions when backscattering material is removed. A simple algorithm that predicts the magnitude of this drop on the basis of equivalent square size of the beam is presented. The algorithm may be used to correct the dose (usually computed from data obtained with full backscatter) at all points near the exit side of a phantom or patient.
Design of a target system for producing clinically useful quantities of oxygen‐15 using an electron linear accelerator
Authors: Piltingsrud Harley V.
Published: August 2003

The nuclear medicine uses of short‐lived positron emitters, such as 15O and 11C, rely primarily on the cyclotron production of these radionuclides. This paper presents an alternate approach to production of 15O by the use of photonuclear reactions on 16O, using bremsstrahlung radiation produced by an electron linear accelerator. Methods are described which produce useable quantities of relatively high specific activity 15O (25.1 mCi/4 min at 27.9 Ci/g), using a reboiling H2O target, with an electron linear accelerator having an electron beam energy of 26 MeV and a beam current of 100 μA. Results of this target system are compared to those of a target system developed recently by other investigators.
Soft tissue image latitude (STIL) of radiographic imaging systems
Authors: Rao Gopala U.
Published: August 2003

A mathematical formulation for the calculation of image latitude in radiographic imaging systems is proposed. It involves the determination of a quantity called the soft tissue image latitude (STIL). The STIL is shown to be equal to (5.82/γ)(HVT)ST where γ is the slope of the characteristic curve for the film used and (HVT)ST is the half‐value thickness of the incident primary beam in soft tissue. Calculated values of STIL are presented for typical radiographic systems.
A clinical system for the in vivo measurement of lung density
Authors: Webber C. E. , Coates G.
Published: August 2003

A three‐source, three‐detector system for the Compton scattering measurement of lung density is described. The average density is measured in a volume of tissue which is similar in shape to a sphere of diameter 4.0 cm. While scattering measurements of lung density are precise, results in phantoms showed that density in normal man is only about 70% of the gravimetric density. This error is considered to be due predominantly to the influence of multiple photon scattering.
Temperature distributions and thermal response in humans. I. Simulations of various modes of whole‐body hyperthermia in normal subjects
Authors: Volpe Brett T. , Jain Rakesh K.
Published: August 2003

The necessity for a mathematical model of human heat transfer as an aid in determining the clinical effectiveness of whole‐body hyperthermia technique is brought out. A 45‐compartment lumped parameter model of the human thermal system is developed. The model accounts for changes in metabolism, blood flow, and other physiological mechanisms during hyperthermia. Simulations of the model's projected temperature distributions and responses to six clinical whole‐body techniques are investigated.