bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessing fluoroscopic contrast resolution: A practical and quantitative test tool
Authors: Wagner Anthony J. , Barnes Gary T. , Wu Xizeng
Published: March 2003

Fluoroscopic contrast resolution is commonly determined at a specified kVp by imaging a test object comprised of targets where contrast decreases gradually and sequentially. Threshold contrast or contrast resolution is the contrast of the lowest contrast target that can be perceived. This approach suffers from two problems. First, test object contrast is specified at a x‐ray tube voltage that is not always obtainable in practice. Second, the small change in contrast between adjacent targets contributes to observer variability making consistent and reproducible contrast threshold determinations difficult. Described is a contrast resolution test tool that eliminates or reduces these problems. The novel target arrangement allows one to quickly and easily specify the contrast resolution of a fluoroscopic imaging chain to a precision of ≂0.5%. Tables of target contrast versus x‐ray tube potential are developed that permit one to employ the test object for contrast resolution determination over the normal range of tube potentials encountered on clinical units.
A method for splitting digital value in radiological image compression
Authors: Lo Shih‐Chung B. , Shen Ellen L. , Mun Seong K. , Chen Ji
Published: March 2003

A new decomposition method using image splitting and gray‐level remapping has been proposed for image compression, particularly for images with high contrast resolution. The effects of this method are especially evident in this radiological image compression study. In these experiments, the impact of this decomposition method was tested on image compression by employing it with two coding techniques on a set of clinically used CT images and several laser film digitized chest radiographs. One of the compression techniques used as zonal full‐frame bit‐allocation in the discrete cosine transform (DCT) domain, which is an enhanced full‐frame DCT technique that has been proven to be an effective technique for radiological image compression. The other compression technique used was vector quantization with pruned tree‐structured encoding, which through recent research has also been found to produce a low mean‐square error and a high compression ratio. The parameters used in this study were mean‐square error and the bit rate required for the compressed file. In addition to these parameters, the differences between the original and reconstructed images were presented so that the specific artifacts generated by both techniques could be discerned through visual perception.
Computerized detection of masses in digital mammograms: Analysis of bilateral subtraction images
Authors: Yin Fang‐Fang , Giger Maryellen L. , Doi Kunio , Metz Charles E. , Vyborny Carl J. , Schmidt Robert A.
Published: March 2003

A computerized scheme is being developed for the detection of masses in digital mammograms. Based on the deviation from the normal architectural symmetry of the right and left breasts, a bilateral subtraction technique is used to enhance the conspicuity of possible masses. The scheme employs two pairs of conventional screen‐film mammograms (the right and left mediolateral oblique views and craniocaudal views), which are digitized by a TV camera/Gould digitizer. The right and left breast images in each pair are aligned manually during digitization. A nonlinear bilateral subtraction technique that involves linking multiple subtracted images has been investigated and compared to a simple linear subtraction method. Various feature‐extraction techniques are used to reduce false–positive detections resulting from the bilateral subtraction. The scheme has been evaluated using 46 pairs of clinical mammograms and was found to yield a 95% true–positive rate at an average of three false–positive detections per image. This preliminary study indicates that the scheme is potentially useful as an aid to radiologists in the interpretation of screening mammograms.
Image feature analysis and computer‐aided diagnosis in digital radiography: Automated delineation of posterior ribs in chest images
Authors: Sanada Shigeru , Doi Kunio , MacMahon Heber
Published: March 2003

In order to facilitate computerized quantitative analysis of digital chest radiographs, an automated method for accurate delineation of posterior ribs in frontal chest images is being developed. This method is based on an analysis of vertical profiles in the lung regions and a statistical analysis of edge gradients and their orientations in small selected regions‐of‐interest (ROIs). A shift‐variant function is fitted to vertical profiles to obtain initial estimates of locations of rib edges. Rib edges are then determined more accurately by analyzing cumulative edge gradients and their orientations in small ROIs that are located adjacent to the initially estimated edges. The present computerized method can achieve a good agreement between the detected and the actual rib structures for posterior ribs in 74% of 50 cases examined. This suggests that automated detection of posterior ribs by a computerized method is feasible, and may be useful for computer‐aided diagnostic schemes in the chest.
Improving fluoroscopic image quality with continuously variable zoom magnification
Authors: Rudin Stephen , Bednarek Daniel R. , Wong Roland
Published: March 2003

Coning down is commonly used during fluoroscopy to increase image contrast by reducing scatter. However, the resulting image fills only part of a video display whose resolution is limited by line rate and bandwidth. Optical or electron‐optical zooming can be used to magnify the collimated image so that it fills a larger fraction of the viewable area of the video frame to make more effective use of the available video‐display capacity. Modulation‐transfer functions (MTFs) were measured for various zoom factors achieved using a zoom lens and the image‐intensifier (II) electronic magnification mode. Significant and continuing improvement in total system MTF was observed up to zoom magnifications of greater than 3.3. For larger zoom factors, the resolution limit becomes dominated by the intrinsic resolving power of the II and by geometric unsharpness rather than by the line rate of the video system. When the MTF at infinite zoom factor, obtained by extrapolation, was divided into the measured MTFs, the resultant MTFZ's were shown to scale predictably with zoom factor. Only a slight improvement in MTF was obtained using the II's electronic magnification mode compared to the same magnification using a zoom lens. It is concluded that, if improved image quality is the motivation for the use of coning down in fluoroscopy, then zooming to use fully the available video frame is warranted.
Bone mineral densitometry with x‐ray and radionuclide sources: A theoretical comparison
Authors: Chakraborty Dev P. , Barnes G. T.
Published: March 2003

Two methods of dual‐photon absorptiometry (DPA) utilizing an x‐ray tube instead of a radionuclide source have recently been introduced. In one method kVp switching is employed and two transmitted intensities at each pixel are determined. In the other method, K‐edge filtration combined with a single kVp spectrum is used, but photons in two energy windows are counted. We present a theoretical analysis of the two methods, focusing on a figure of merit which is essentially the exposure efficiency (the precision for a given entrance exposure) and tube loading. We also compare their exposure efficiencies to theoretical limits that no DPA system can exceed. Our study indicates that the K‐edge‐filtered method is more exposure efficient by about a factor of 2. The switched‐kVp method requires less heat units per scan by about a factor of 3. A hybrid K‐edge switched‐kVp method is suggested which achieves the same exposure efficiency as the K‐edge‐filtered method at lesser tube loading. Our theoretical model is based on published x‐ray spectra and attenuation coefficients and is in good agreement with other simulation work. It is of interest that a point source of Gd‐153 would be even more exposure efficient, achieving about 90% of the theoretical limit. However, in practice, the Gd source is of finite size and limited strength, and consequently the radionuclide method cannot achieve as good a precision as either x‐ray method in similar scan times.
Coherent scattering and bone mineral measurement: The dependence of sensitivity on angle and energy
Authors: Ndlovu A. M. , Farrell T. J. , Webber C. E.
Published: March 2003

The sensitivity of a technique for the measurement of trabecular bone mineral concentration has been examined theoretically and experimentally. The technique is based on coherent gamma ray scattering and corrections for attenuation are obtained from transmitted photons rather than Compton scattered photons. For an incident photon energy of 60 keV, the minimum detectable bone mineral difference is practically independent of scattering angle while for an incident energy of 100 or 122 keV the scattering angle must be less than 70° to optimize the minimum detectable difference.
Image restoration in digital radiography using dual sensor Wiener filter
Authors: Lee Tae Soo , Min Byoung Goo
Published: March 2003

A dual sensor Wiener filter (DSWF) technique was used to improve the image quality of a scanning type digital radiographic system. In a digital radiographic system, image quality is evaluated by resolution and SNR (signal‐to‐noise ratio), which are two important parameters representing the objective performance of the system. In this method, when two images are acquired in the same region using two sensors with different characteristics of resolution and SNR, they are processed simultaneously using DSWF, which is the extended concept of a Wiener filter to two dimensions. DSWF uses the cross power spectrum between dual sensor outputs of the same chest radiographic image in the design of filter parameters. It has been implemented with fast algorithm using FFT (fast Fourier transform). The performance of the proposed method is compared with that of conventional methods (Wiener filter and parametric projection filter). In simulation studies, it is shown in 12 cases that this new method has SNR improvement of 1–2 dB better than conventional methods.
The role of nonlinear ultrasound propagation during hyperthermia treatments
Authors: Hynynen K.
Published: March 2003

In this study the feasibility of utilizing nonlinear ultrasound propagation to control the power deposition patterns during ultrasound hyperthermia treatments of malignant tumors was investigated. From the in vivo results it appears that the power deposition pattern and the temperature distributions can be significantly altered by increasing the temporal peak power of the source while reducing the duty cycle to maintain the same acoustic time average power. The maximum temperature gains obtained during hyperthermia sonication (between 1.5 and 2) were significantly less than one could obtain by varying the frequency and the other parameters of transducers and thus, the linear characteristics of the ultrasound fields should be used when hyperthermia systems are designed and treatments are planned. Then during the treatment nonlinear propagation can be used to provide on‐line control of the power deposition patterns. The transducer characteristics determined the magnitude of the temperature gain and the nonlinear propagation region could not be reached when sharply focused (F number=1) transducers were used at the frequencies between 0.25 to 1.68 MHz due to the high pressure amplitudes that exceeded the transient cavitation threshold.
Spherical harmonic expansion of cranial surfaces
Authors: Holupka Edward J. , Kooy Hanne M.
Published: March 2003

A three‐dimensional analytical technique for the representation of the cranial surface based on a spherical harmonic expansion is presented. The analytical surface representation is used in conjunction with a sample iterative ray–intersection algorithm to compute the dose calculation depth. The technique is compared to other conventional techniques and it is determined that the spherical harmonic approach of approximating the cranial surface decreases the depth calculation time by approximately a factor of 60.
Photon energy dependence of the sensitivity of radiochromic film and comparison with silver halide film and LiF TLDs used for brachytherapy dosimetry
Authors: Muench Philip James , Meigooni Ali S. , Nath Ravinder , McLaughlin William L.
Published: March 2003

There is a new radiochromic film, a highly uniform, thin (100‐μm) detector whose sensitive layer (6 μm thick) changes from colorless to blue by dye polymerization without processing, upon exposure to ionizing radiation. Because the dose gradients around brachytherapy sources are steep, the high spatial resolution offered by film dosimetry is an advantage over other detectors such as thermoluminescent dosimeters (TLDs). This compares the photon energy dependence of the sensitivities of GafChromic film, silver halide verification film (Kodak X‐Omat V Film), and lithium fluoride TLDs (Harshaw), over the photon energy range 28 keV to 1.7 MeV, which is of interest in brachytherapy. Sensitivity of the radiochromic film is observed to decrease by about 30% as effective photon energy decreases from 1710 keV (4‐MV x rays) to 28 keV (60‐kV x rays, 2‐mm Al filter). In contrast, the sensitivity of verification film increases by 980% and that of LiF TLDs increases by 41%. The variation of the sensitivity of radiochromic film with photon energy is considerably less than that for silver halide film and similar to that for LiF TLDs, but in the opposite direction. Radiochromic film, like LIF TLDs, does not exhibit the drastic sensitivity changes below 127 keV that silver halide film exhibits. Dose distribution in the immediate vicinity of a high activity (370 GBq) brachytherapy 192Ir source has been mapped using radiochromic film and is presented to illustrate the applicability of this new technology to brachytherapy dosimetry.
Comparison of measured and calculated dose rates in water near I‐125 and Ir‐192 seeds
Authors: Williamson Jeffrey F.
Published: March 2003

Recent theoretical and experimental work indicates that currently accepted 125I dosimetry data may overestimate dose in water at 1 cm by 10%–24%. Among the most comprehensive measurements are those of the NCI‐sponsored brachytherapy contract participants. Absolute dose rates in water calculated by the Monte Carlo method have been compared with the NCI dose measurements about 125I and 192Ir seeds embedded in solid‐water phantoms. The photon transport code allows realistic geometric simulation of the complex internal seed structure, the National Institute of Standards and Technology air‐kerma strength standardization geometry, and the dose measurement setup. When the appropriate measurement medium and geometry are assumed, agreement between theory and measurement is excellent, within 3% at 1 cm and averaging 3% at larger distances. However, the data do not support the water equivalence of solid water at 125I energies indicating that solid‐water measurements underestimate 125I specific dose‐rate constants in water by 4.3%. Because of its higher ratio of absorption to scatter, 125I dose distributions measured in solid water are less penetrating (by 35% at 10 cm) than those measured in liquid water. For model 6711, model 6702, and steel‐clad 192Ir seeds, Monte Carlo calculations yielded specific dose‐rate constants (assuming liquid water medium) of 0.877, 0.932, and 1.122 cGy cm2 h−1 per unit air‐kerma strength, respectively. For 125I, currently accepted values are 18% and 11% larger for the two seed models.
Calibration of ophthalmic applicators at NIST: A revised approach
Authors: Soares C. G.
Published: March 2003

A revised approach to the problem of measuring a surface‐absorbed dose from beta‐particle‐emitting ophthalmic applicators is presented. The technique chosen employs an extrapolation chamber equipped with a 4‐mm‐diam collecting electrode to make current measurements at air gaps from 0.08 to 0.20 mm at 0.02‐mm intervals. These data yield a linear relationship between current and air gap, the slope of which is used to determine average surface‐absorbed‐dose rate over the central area of the source. For additional information about the distribution of the activity over the source surface, autoradiographs using calibrated radiochromic dye films are analyzed to map the dose‐rate profile across the surface of the applicator. Experiments varying several parameters of the extrapolation chamber measurement, including collecting electrode area, voltage gradient, range of air gaps used, and entrance foil material, are described. Also treated are calibrations of, and a description of the use of, radiochromic dye films for source profiling. Experiments for determining correction factors for the extrapolation chamber measurements are described, and an assessment of the uncertainties associated with these measurements is given.
Thermodynamics of movable inductively heated seeds for the treatment of brain tumors
Authors: Molloy J. A. , Ritter R. C. , Broaddus W. C. , Grady M. S. , Howard M. A. , Quate E. G. , Gillies G. T.
Published: March 2003

A thermodynamic study is presented of temperature distributions created by an inductively heated 6‐mm‐diam Ni sphere imbedded in vivo and in vitro into porcine brain tissue. This study was performed in support of the development of a system that creates localized heat‐induced lesions in deep‐seated brain tumors. In this system, a magnetic “seed” will be remotely repositioned within the brain by an externally produced magnetic field. Convective effects of a hot moving seed will produce a different thermodynamic situation than that arising from an array of static implants. In this work, a study is presented of part of the expected change, in which a static sphere is heated to high temperature. Measurements were made of the temporal and spatial dependence of the temperature rise in the vicinity of the heated sphere, in vivo in four animals and in one that was euthanized immediately prior to experimentation. These results are used for parameter estimation with a theoretical model based on a point source solution to a form of the thermal diffusion equation, i.e., the “bioheat transfer equation.” With this model thermal distributions from a power source of arbitrary geometry can be found using appropriate integration methods, and the method has widespread applicability. Estimates of blood flow rates, tissue thermal conductivity, and seed power absorption were found using the parameter estimation algorithm. The estimated blood perfusion exhibits a step increase following the first heating in multiple heating experiments. Thermal conductivity estimated using data from the nonperfused (in vitro) animal is 0.6 W/m °C. Seed power absorption is estimated correspondingly to be 0.9 W, a result confirmed independently with calorimetry. Statistical uncertainty is established for the radial decrease of the tissue temperature rise created by this method. This result allows estimation of a cell death boundary uncertainty of 0.6 mm, caused by fluctuations in power delivered to the seed, uncertainty in the temperature probe placements, and thermal properties such as blood perfusion and tissue thermal conductivity.
A beam intensity monitor for the Loma Linda cancer therapy proton accelerator
Authors: Coutrakon G. , Miller Dan , Kross B. J. , Anderson D. F. , DeLuca P. , Siebers J.
Published: March 2003

A beam intensity monitor was tested in a 230‐MeV proton beam at the Loma Linda Proton Therapy Accelerator during its commissioning at Fermi National Accelerator Laboratory. The intensity monitor was designed to regulate the beam intensity extracted from the proton synchrotron. The proton beam is tunable between 70 and 250 MeV with an adjustable intensity between 1010 and 1011 protons per spill. A beam spill is typically 1 s long with a 2‐s repetition period. The intensity monitor must be radiation hard, expose minimum mass to the beam, and measure intensity to 1% in 1‐ms time intervals. To this end, a 5‐cm‐thick xenon gas scintillator optically coupled to a photomultiplier tube (PMT) was tested to measure its response to the proton beam. The gas cell was operated at 1.2 atm of pressure and has 12.7‐μm‐thick titanium entrance and exit foils. The total mass exposed to the beam is 0.14 g/cm2 and is dominated by the titanium windows. This mass corresponds to a range attenuation equal to 1.4 mm of water. The energy lost to the xenon gas is about 70 keV per proton. Each passing proton will produce approximately 2000 photons. With a detection efficiency on the order of 0.05% for this UV light, one would anticipate over 1010 photoelectrons per second. In a 1‐ms time bin there will be approximately 107 photoelectrons. This yields a resolution limited by systematics. For unregulated 0.4‐s proton spills, we observe a response bandwidth in excess of 104 Hz. While signal‐to‐noise and linearity were not easily measured, we estimate as few as 103 protons can be observed suggesting a dynamic range in excess of 105 is available.
Phase‐encode order and its effect on contrast and artifact in single‐shot RARE sequences
Authors: Mulkern R. V. , Melki P. S. , Jakab P. , Higuchi N. , Jolesz F. A.
Published: March 2003

Substantial manipulation of tissue contrast can be achieved by varying the order in which phase‐encode values are applied to individual echoes within a 128‐echo single‐shot rapid acquisition relaxation enhanced (RARE) sequence. Appropriate ordering can then permit imaging of short T2 species like muscle and white matter with single‐shot RARE. For sequential phase encoding with an arbitrary initial phase‐encode value, the timing of the zero phase (ZP) encoded echo is found to be analogous to the echo time (TE) of standard spin‐echo sequences. This is demonstrated qualitatively with human brain images and is verified quantitatively with NiCl2 phantoms by correlating the time constant for signal decay with ZP echo time, with transverse relaxation times T2, as obtained with a 128‐echo Carr–Purcell–Meiboom–Gill (CPMG) imaging sequence. Banding artifacts accompanying the discontinuous traverse through K space are experimentally demonstrated in a rectangular phantom and expressions are developed for determining the dependence of this artifact on the phase‐encode gradient increments and durations, the ZP echo number, echo spacing, and T2. Simulations based on the expressions are shown to be useful for characterizing the observed “banding” artifacts perpendicular to the phase‐encode direction and for predicting the extent of tissue–tissue overlap to be expected with the use of this ultrafast rf echo planar imaging method.
Turbulent flow effects on NMR imaging: Measurement of turbulent intensity
Authors: Gao Jia‐Hong , Gore John C.
Published: March 2003

A general expression has been derived for the NMR signal from a fluid in steady but turbulent flow in a magnetic field gradient. The precise dependences of the spin echo amplitude on both flow and experimental parameters are described in terms of the temporal autocorrelation function for velocity fluctuations in the fluid. The correlation function width determines whether a diffusion model of turbulent eddies is valid, and for physiological flow in large vessels in NMR imaging a more appropriate measure is the turbulent intensity of velocity fluctuations. The theoretical predictions have been verified in experiments using gradient echo images of flowing water. The theory also predicts that even echo rephasing phenomena may still occur in MR imaging of turbulent flows.
Theoretical description of the interaction between birdcage coils and lossy samples
Authors: Harpen Michael D.
Published: March 2003

An analysis is presented of the interaction between a long cylindrical imaging coil, commonly called a low‐pass birdcage coil, and a cylindrical sample. Predicted and measured values of coil Q are shown to compare well throughout the resonance spectrum of an experimental coil–sample system. The analysis may be used to calculate feed‐point impedances and is easily extended to high‐pass coils.
Performance testing of a dedicated magnetic resonance peripheral blood flow scanner
Authors: Kofler James M. , Morin Richard L. , Johnson C. Michael
Published: March 2003

This work presents a novel investigation of an NMR device to measure blood flow. Continuous‐wave magnetic resonance scanners are now available for noninvasive peripheral blood flow measurements. The appearance of dedicated peripheral blood flow scanners in the clinical setting requires analysis of system performance. No reports were found that explore methods or results of clinical MR blood flow scanner performance evaluation. This work was undertaken to identify the testing procedures and characteristics of a clinical, dedicated NMR blood flow scanner. The dependence of flow measurement on tubing size, material, offset from isocenter, and angulation was examined. In addition, the precision and accuracy of the scanner were investigated. The results demonstrate a strong dependence between the measured blood flow and both the tube size and flow rate. The tube offset results exhibited a region of weak dependence in the center of the bore, which becomes stronger near the edge. The measured flow rate was found to be relatively insensitive to tube angles less than 30° to 55°. The repeatability was typically better than 5% and the table positioning was found to be highly accurate.
A Kα dual energy x‐ray source for coronary angiography
Authors: Manning Herbert L. , Shefer Ruth E. , Klinkowstein Robert E. , Mistretta Charles A.
Published: March 2003

The use of characteristic‐line radiation from rare‐earth targets bombarded by high‐energy (up to 1 MeV) electron beams has been evaluated as an x‐ray source for dual energy K‐edge subtraction imaging of the human coronary arteries. Two characteristic‐line x‐ray sources, one using the split Kα1 and Kα2 lines of lanthanum excited by a high‐energy electron beam and the other using the Kα lines of barium and cerium, were studied. A Monte Carlo electron–photon simulation was used to calculate x‐ray spectra and energy deposition profiles from targets of these elements bombarded by electrons in the energy range 140 keV to 1 MeV. A general dual‐energy imaging model was developed that used these calculated source spectra to numerically investigate the dependence of the subtraction image signal‐to‐noise ratio on such factors as the ratio of K‐line to x‐ray continuum yield, continuum spectral shape, x‐ray filtering, and detector response. A signal averaging technique for enhancing the signal‐to‐noise ratio was also evaluated. The results of these calculations were used to identify an optimum electron beam, target, filter, and detector configuration. A compact electron accelerator capable of providing the required electron beam parameters was designed. Calculations indicate that under ideal conditions the optimized system would be capable of imaging 2 mg/cm2 of iodine contrast agent in 20 g/cm2 of tissue with a signal‐to‐noise ratio of 5, a detector pixel size of 0.25 mm2, and a total image acquisition time of 10 ms. These parameters are consistent with those needed to image the human coronary arteries after an intravenous injection of iodine contrast agent. These capabilities, along with the relatively modest hardware requirements of this system, make it attractive as an x‐ray source for dual energy transvenous coronary angiography.
Rotational kernels for conformal therapy
Authors: Desobry G. E. , Wells N. H. , Boyer A. L.
Published: March 2003

Conformal therapy treatment planning involves determining an irradiation strategy to deliver a dose distribution which is optimized for a given tumor volume. A technique proposed by Brahme restricts the search for the optimal treatment strategy to x‐ray distributions in which points within the target volume are irradiated uniformly under rotation of the beam source around the patient. The dose distribution in this case may be calculated as a convolution of an irradiation weighting distribution with an invariant kernel. A procedure is described for calculating three‐dimensional kernels to be used for clinical treatment planning with x rays produced by an electron accelerator. The convolution kernel is calculated as the sum of pencil beams irradiating the center of a cylindrical phantom uniformly from all angles. The shape of the kernel at points off the center of the phantom is investigated by means of numerical calculations which support the assumption that the kernel is invariant with respect to position within the phantom. The calculated kernels are verified by comparison with experimentally measured rotational arc dose distributions.
Bremsstrahlung review: An analysis of the Schiff spectrum
Authors: Desobry G. E. , Boyer A. L.
Published: March 2003

A concise approximate formula computed by Schiff for the intensity spectrum of bremsstrahlung photons has been a valuable starting point for many medical physics applications, including the Task Group 21 protocols. This paper provides a brief review of the literature related to determination of the bremsstrahlung spectrum and to the Schiff formula in particular. It describes the approximations Schiff made to obtain this formula, including the Born approximation, an exponential nuclear screening potential, the infinite‐mass nucleus approximation, and the “extreme relativistic” approximation. A derivation of a more exact formula that avoids the last of these approximations is presented. This provides a check on the accuracy of the Schiff spectrum for linear accelerator energies used clinically. Comparisons with the Schiff thin‐target result are presented. A thick‐target bremsstrahlung spectrum is calculated and compared with the forward spectrum obtained from Monte Carlo simulations of the x‐ray production in two linear accelerator treatment heads.
Induced effects in ionization chamber cables by photon and electron irradiation
Authors: Campos Letícia L. , Caldas Linda V. E.
Published: March 2003

Photon and electron irradiation effects in cables and connectors of ionization chambers were investigated under different experimental conditions by means of measurements of the chamber response to a 90Sr standard check source and by measurements of radiation‐induced leakage current. Correction factors for irradiation effects on cables and connectors were obtained as a function of the radiation energy, cable length, and absorbed dose dependences.
Count‐based monitoring of Anger‐camera spectra—local energy shifts due to rotation
Authors: Swailem Fayez M. , Koral Kenneth F. , Rogers W. Leslie
Published: March 2003

This study reports on a spectral monitoring method in which (1) a small source fixed to the camera is used, (2) a narrow, offset window is set on the side of the photopeak, and (3) variations in count rate are measured to assess energy shifts in the vicinity of the source. For one camera model, the count rate drops from 100% to 76% over a rotation of 180°, implying a local energy shift of 1.4 keV. Also looked for are local count‐rate variations with rotation for (1) wide‐symmetric, (2) 20%‐symmetric, and (3) 10%‐asymmetric windows. The last is in limited use to partially compensate for Compton scattering. The effects of background and time stability are assessed.
A modified power‐law formula for inhomogeneity corrections in beams of high‐energy x rays
Authors: Thomas Simon J.
Published: March 2003

The Batho power‐law formula is in common use in many treatment planning systems to correct for the presence of lungs and other inhomogeneities. While giving excellent agreement with measurement for Cobalt‐60 radiation, it tends to underestimate the lung correction required for higher energy x rays and is undefined for distances beyond an interface less than the buildup distance. This paper suggests a simple modification that greatly improves the agreement with measured data and gives a continuously defined function at all depths. Measurements have been made in a polystyrene and cork phantom to simulate the effects of lung; data are presented for beams of 8‐MV x rays, 16‐MV x rays, and Cobalt‐60 γ rays.
A mathematical expression for %DD accurate from Co‐60 to 24 MV
Authors: Gerbi Bruce J.
Published: March 2003

Obtaining accurate %DD values for routine treatment calculations is essential in radiation therapy. Many papers have presented expressions to calculate this or related parameters but these expressions often required that the parameters needed by the equation be determined for each individual treatment unit. This paper presents an expression that calculates %DD values with a mean‐square accuracy of ∼1.0% versus measured values. The expression is applicable to beam energies ranging from Co‐60 to 24 MV, field sizes from 4×4 to 40×40 cm2, and depths from 1 cm deeper than dmax to 30 cm. The only information required by this expression that is machine specific is the ionization ratio.
Ngas values of the Memorial parallel‐plate chambers determined in 60Co and high‐energy electron beams
Authors: Kubo Hideo
Published: March 2003

The cavity‐gas calibration factor Ngas for Memorial parallel‐plate chambers was determined against calibrated Farmer chambers using two different calibration methods as suggested in the TG 21 protocol. These methods are: (a) 60Co calibration at 5‐cm depth; and (b) high‐energy electron beam calibration. The Ngas values determined from the 60Co in‐phantom calibrations were approximately 1.9% higher than the values determined from the high‐energy electron beams.
Comparison of the penumbra between focused and nondivergent blocks—Implications for multileaf collimators
Authors: Biggs P. , Capalucci J. , Russell M.
Published: March 2003

The penumbra from a 10‐MV x‐ray beam has been measured using various field‐shaping blocks located in the normal blocking tray position and compared to the penumbra of the adjustable photon jaws of the machine. The results showed that the penumbra was substantially degraded by these blocks only for field sizes greater than 15×15 cm2 and at the depth of maximum dose, dmax. At depths of 11 cm or greater, there was a significant difference only for the same range of field sizes in the 80% to 50% dose region. There was no significant difference at these depths for either the 95%–50% or the 90%–50% dose regions. No significant difference was observed between the use of lead and tungsten blocks and also between lead blocks with a straight edge and those with either an 0.5‐mm or a 1‐mm step in the face. Since the width of the 95% dose relative to the 50% dose is of greatest interest clinically, straight‐edged blocks are as effective as divergent blocks in most situations. These results imply that the design and complexity of a multileaf collimator can be greatly simplified from a double focusing device to one that is single focusing in the plane orthogonal to the leaves.
Modeling of asymmetric compensator geometries
Authors: Robinson Donald M. , Scrimger John W.
Published: March 2003

Dose distributions arising from the use of near unit density retracted missing tissue compensators in symmetric geometries have been successfully modeled on the basis of primary and first‐order scattered radiation. This method of analysis has been extended to both low‐ and high‐density materials in asymmetric geometries. Good agreement is achieved between theory and experiment.
Investigation of the relative surface dose from Lipowitz‐metal tissue compensators for 24‐ and 6‐MV photon beams
Authors: Cardarelli Gene A. , Rao S. N. , Cail David
Published: March 2003

Our radiation therapy department has acquired a tissue compensator system for construction of patient‐specific Lipowitz‐metal tissue compensators. Since the arrival of this apparatus its use has been limited due to the observance of marked skin reactions located directly beneath the area of the compensator. Although there has been extensive literature on the dose distribution of these metal compensators, there is little data on the effects to the skin for energies greater than 10 MV. Determination of the relative surface dose from Lipowitz‐metal tissue compensators is herein investigated for 24‐ and 6‐MV x‐ray beams. Effects of field size, source‐to‐skin distance and thickness of compensator are evaluated as well as the effect of the Lexan tray which supports the compensator during treatment.
Activity distribution of a cobalt‐60 teletherapy source
Authors: Jaffray D. A. , Munro P. , Battista J. J. , Fenster A.
Published: March 2003

In the course of quantifying the effect of radiation source size on the spatial resolution of portal images, a concentric ring structure in the activity distribution of a Cobalt‐60 teletherapy source has been observed. The activity distribution was measured using a strip integral technique and confirmed independently by a contact radiograph of an identical but inactive source replica. These two techniques suggested that this concentric ring structure is due to the packing configuration of the small 60Co pellets that constitute the source. The source modulation transfer function (MTF) showed that this ring structure has a negligible influence on the spatial resolution of therapy images when compared to the effect of the large size of the 60Co source.
Imaging of radiation dose by visible color development in ferrous‐agarose‐xylenol orange gels
Authors: Appleby A. , Leghrouz A.
Published: March 2003

Imaging of radiation dose distributions in gel phantoms by magnetic resonance techniques has previously been reported. In this paper a method of producing gels in which the distributions of radiation dose can also be visualized as a color change is reported. The color developed depends qualitatively and quantitatively on the concentrations of solutes in the gel.
Possible detection of turbulent blood flow using multiparametric encoding gradients in MRI
Authors: Kwiat D. , Einav S. , Elad D.
Published: March 2003

Magnetic resonance imaging (MRI) has significant potential as a highly accurate noninvasive flow measurement technique. Presented here is an approach to the imaging of turbulence in the velocity profiles. Recent publications have presented multiparametric encoding gradient methods, which are based on a deterministic rather than a statistical approach to spin motion. Following these methods of building up a general gradient from a basic set of gradients, the effects of turbulence and undetermined moments of motion on the image are discussed. Image blurring due to the nondeterministic behavior of spins (e.g., diffusion, turbulence) is not removed by these techniques, and this fact may be useful in identifying regions of turbulent flow, which are of importance in the clincial observation of cardiac and vascular haemodynamics. Due to its random nature, the elimination of turbulent effects by the use of a deterministic method is bound to fail. On the other hand, regions of turbulent flow may be identified due to signal decrease from those regions, provided one is careful to remove all the causes of nonturbulent signal reduction present due to nonturbulent flow moments. In the multiparametric phase encoding method, gradient amplitude tends to increase with higher moments of motion. The temporal behavior of these gradients is discussed, and it is suggested that the increase in their duration will enhance the encoding of higher motion terms at the expense of imaging time.
Determination of dose enhancement in cortical bone substitute material for electron beams
Authors: Prasad Satish C.
Published: March 2003

Dose measurements in Witt liquid, which simulates cortical bone, have been compared with dose in water for 6‐, 9‐, 12‐, and 15‐MeV electron beams. Measurements were made using a Farmer ionization chamber. The results of the study show dose enhancement in Witt liquid of 5%, 7%, 4%, and 0.4% for 6‐, 9‐, 12‐, and 15 MeV electrons at shallow depths. The dose to a small mass of soft tissue in bone has also been estimated using ionization measurements. The results show a significantly higher dose in bone.
Localized invivo high‐resolution NMR imaging using gradient subencoding technique
Authors: Cho Z. H. , Jo J. M.
Published: March 2003

A new spatial localization technique for in vivo high‐resolution imaging is presented here. In contrast to other localization techniques that use a series of rf pulses to define a volume of interest, only one rf pulse is utilized in the proposed method for selection of a region to be imaged. Instead of rf pulses for region selection, subencoding gradient pulses are used for the localization together with a convolution process on each phase‐encoding gradient by a set of additional gradients (e.g., y direction). Then the 2‐D localization is completed by restricting the bandwidth in the readout direction (e.g., x direction). The latter is simply achieved by using a low‐pass filter in the receiver system. By applying this technique on a human body, localized in vivo high‐resolution images are obtained for the knee with much improved resolution. 100×100‐μm in‐plane (x,y plane) resolution images obtained from the human knee demonstrate that localized in vivo high‐resolution imaging for both human and animals is possible with an in‐plane resolution of below 100 μm.
The determination of the contrast necessary for the detection of small positive or negative contrasting areas in flood field images by employing ROCs
Authors: Geldenhuys E. M.
Published: March 2003

Flood field images are acquired and inspected visually during weekly quality control procedures for scintillation cameras. Nonuniformities are quantified by parameters such as integral and differential uniformity (IU and DU). This study is a first step toward understanding the relation between the standard quantitative parameters measured in regular quality control procedures and the performance of clinicians in the interpretation of studies conducted with the same scintillation camera. This study quantified the performance of observers to detect nonuniformities in an ideal case. Flood field images (64×64) were simulated on a computer. One contrasting area (2×2) was superimposed at random positions on each image. Positive and negative contrast values of 10%, 8%, 6%, 4%, 2%, and 0% were employed. The linearly scaled computer images were transferred to film. These films were evaluated by 11 observers to obtain receiver operating characteristic (ROC) curves. The areas (A) below the average ROCs for every contrast value were utilized as an indication of the detectability of the contrasting areas superimposed on the flood images. The results indicate areas with positive contrasts >6% and negative contrasts <−8% were detected with 95% probability (A≥0.95). These contrasts correspond to optical density differences of >0.051 and <−0.072, respectively (assuming a gamma, G, or gradient of 2.0 for the x‐ray film characteristic curve). The contrast values and optical density differences obtained may serve as guide values, beyond which action must be taken to correct the scintillation camera nonuniformity to ensure optimum imaging. These results can be utilized by other institutions to predict the threshold contrasts of their imaging systems if G can be measured or estimated for the systems.
The effects of different correction techniques on absolute volume determination with SPECT using a threshold edge detection method
Authors: Pretorius P. Hendrik , van Aswegen Andries , Herbst Charles P. , Lötter Mattheus G.
Published: March 2003

Quantitation of planar radionuclide images is hampered by structures containing radioactivity which overlie or underlie the organ of interest. The introduction of single photon emission computerized tomography (SPECT) overcame this problem to a large extent and enhanced the contrast of the images. Attenuation of photons, however, degrades the resultant SPECT images and correction methods for photon absorption and scatter were subsequently proposed. The different correction methods have variable effects on the reconstructed images. If threshold techniques are used to quantitate organ volume, i.e., combining pixels with the same percentage of the maximum pixel count in the volume, the selected threshold values which give the most accurate volume determination, will be affected by the specific correction method used. In this study, the effect of various SPECT image correction methods on threshold was investigated. A thorax phantom containing volumes ranging from 30 to 1200 ml was used. Threshold values varying from 45.6% (210 ml without any correction) to 23.7% (1200 ml with a combination of scatter subtraction and attenuation correction) were used to produce correct quantitation when different methods were investigated. A negative correlation was found between threshold and volume. This reduction in threshold was most prominent when scatter and attenuation correction were combined. This study shows that correction methods for attenuation of photons influence the threshold value for volume quantitation and the use of a constant threshold value could lead to underestimation of larger volumes.
Simultaneous reconstruction, segmentation, and edge enhancement of relatively piecewise continuous images with intensity‐level information
Authors: Liang Z. , Jaszczak R. , Coleman R. , Johnson V.
Published: March 2003

A multinomial image model is proposed which uses intensity‐level information for reconstruction of contiguous image regions. The intensity‐level information assumes that image intensities are relatively constant within contiguous regions over the image‐pixel array and that intensity levels of these regions are determined either empirically or theoretically by information criteria. These conditions may be valid, for example, for cardiac blood‐pool imaging, where the intensity levels (or radionuclide activities) of myocardium, blood‐pool, and background regions are distinct and the activities within each region of muscle, blood, or background are relatively uniform. To test the model, a mathematical phantom over a 64×64 array was constructed. The phantom had three contiguous regions. Each region had a different intensity level. Measurements from the phantom were simulated using an emission‐tomography geometry. Fifty projections were generated over 180°, with 64 equally spaced parallel rays per projection. Projection data were randomized to contain Poisson noise. Image reconstructions were performed using an iterative maximum a posteriori probability procedure. The contiguous regions corresponding to the three intensity levels were automatically segmented. Simultaneously, the edges of the regions were sharpened. Noise in the reconstructed images was significantly suppressed. Convergence of the iterative procedure to the phantom was observed. Compared with maximum likelihood and filtered‐backprojection approaches, the results obtained using the maximum a posteriori probability with the intensity‐level information demonstrated qualitative and quantitative improvement in localizing the regions of varying intensities.
Molybdenum target x‐ray spectra: A semiempirical model
Authors: Tucker Douglas M. , Barnes Gary T. , Wu Xizeng
Published: March 2003

A semiempirical model for generating molybdenum target x‐ray spectra is presented. The model is an extension of a previous model developed by the authors for tungsten and takes into account the depth of production for both bremsstrahlung and characteristic x‐ray photons. As in the previous work, the optimal model parameters were determined using nonlinear least‐squares fits to experimental data. Good agreement between the two was obtained. By varying target angle, off‐axis angle, and filtration in the model in accordance with the x‐ray tube and geometry of interest, results consistent with tabulated spectra for different conditions have been obtained.