bluuebunny/crossref_metadata_2025 huggingface dataset:
Medical pattern recognition pitfalls in a clinical setting: Renal cell carcinoma survival prediction
Authors: Kimme‐Smith Carolyn , Cochran Sachiko T. , Roe Denise J.
Published: March 2003

Three studies to predict renal cell carcinoma patient survival from tumor textures found in digitized early venous phase arteriograms were successful individually. However, when the three methods were compared, they were not consistent and no single method was clinically useful. The first study predicted 5 yr survival of 37 patients with 87% accuracy. The second study added 29 patients to the data base; the poor survival of the 21 patients who died within 5 yr of diagnosis was predicted with 80.9% accuracy. When 27 of these cases were redigitized with a laser scanner, average survival prediction accuracy was 78%. In these studies, digitization hardware, radiographic technique, normalization methods, window selection, and contrast medium distribution all contributed to differences in the statistics separating poor from good patient survival.
Absorbed dose comparison among commercial ionization chambers in polystyrene and acrylic phantoms
Authors: Kubo Hideo , Cheng Pocheng
Published: March 2003

Using the AAPM Task Group 21 protocol, photon and electron doses were compared in polystyrene and acrylic phantoms. The ionization chambers used were a Farmer graphite chamber, a PTW acrylic chamber, a homemade polystyrene chamber, and an Exradin air‐equivalent chamber, all being of cylindrical type. A Memorial parallel‐plate polystyrene chamber was also included. 60Co, 4‐, 6‐, and 18‐MV x rays as well as 9‐ and 20‐MeV electron beams were investigated.
Resonator coils for magnetic resonance imaging at 6 MHz
Authors: Samaratunga Ranasinghage C. , Busse Lawrence J. , Pratt Ronald G. , Thomas Stephen R.
Published: March 2003

The Alderman and Grant design of the slotted tube resonator has been modified with a different lumped capacitor distribution to obviate the need for a balanced electrical feed at low frequencies (6 MHz). A saddle coil resonator and a strip line resonator have been tuned with similar lumped capacitor distributions to obtain electrically balanced resonance.
Scatter‐glare corrections in quantitative dual‐energy fluoroscopy
Authors: Molloi Sabee Y. , Mistretta Charles A.
Published: March 2003

Previous attempts to use time subtraction intravenous digital subtraction angiography for ventricular imaging have been hampered by artifacts due to cardiac and respiratory motion. We have previously reported a motion‐immune dual‐energy technique in which kVp is switched between 60 and 120, at 300–500 mA, 30 times/s. In order to quantitate parameters such as ejection fraction and left ventricular volume, it is necessary to correct for scatter and veiling glare (SVG), which are the major sources of nonlinearities in videodensitometric digital subtraction angiography (DSA). In this report, a convolution filtering method has been investigated to estimate SVG in DSA images. In the first step, a grey level transformation of the detected image is utilized to get an estimated SVG image. In the second step this image is convolved to produce an image with appropriate spatial frequency content. Estimates of SVG in several Humanoid chest phantom images were obtained using Gaussian convolution kernels with a full width at half‐maximum (FWHM) of 51–125 pixels. The root‐mean‐square (rms) percentage error of these estimates was obtained by comparison with direct SVG measurement. A convolution kernel with a FWHM of 75 pixels in each dimension applied to 16 Humanoid phantom images with various projections, thicknesses, and beam energies resulted in an average rms percentage error of 9.7% in the SVG estimate, for the 16 cases studied. The SVG estimation consisting of grey scale‐to‐SVG fraction lookup table (LUT) is made based on previous measurements. The x‐ray settings required for each patient are utilized to alter the LUT in order to account for patient thickness variations. The technique is easy to implement and can be performed within a few seconds, without any additional patient exposure.
Nuclear magnetic resonance microscopy with 4‐μm resolution: Theoretical study and experimental results
Authors: Cho Z. H. , Ahn C. B. , Juh S. C. , Lee H. K. , Jacobs R. E. , Lee S. , Yi J. H. , Jo J. M.
Published: March 2003

Nuclear magnetic resonance (NMR) microscopy with 4‐μm resolution, a step closer to the 1‐μm resolution with which in vivo cellular imaging would be possible is described. An analysis of the ultimate resolution and voxel size dependent signal‐to‐noise ratio (SNR) in NMR microscopy is presented and experimentally verified. For microscopic scale objects (<1‐mm diameter), the SNR based on the geometrical scale factor (s) is found to be proportional to sn where n<2, rather than n=3 as previously supposed. This comes about because of a drastic reduction in sample noise coupled with a significant sensitivity gain realized in small diameter radiofrequency coils. A new pulse sequence which reduces both diffusion dependent resolution degradation and signal attenuation is presented. The selection of optimal bandwidth and acquisition time for maximal SNR is discussed. Experimental results obtained on both a 2.0‐T whole‐body system and a 7.0‐T small bore system adapted for microscopy indicate the potentials of 4‐μm resolution microscopy with the existing magnets.
Dependence of noise on array width and depth in digital radiography
Authors: Shroy Robert E.
Published: March 2003

Measurements are reported for the image noise levels in digital radiographs using 2562, 2562‐zoom, 5122, and 10242 storage arrays. The noise was essentially the same with 2562‐zoom or 5122 arrays, while it was 25% lower for a 2562 array, and only 15% larger for a 10242 array than for a 5122 array. Compatibility of these results with different models is evaluated. Also the number of bits/pixel was varied to determine the effect on noise; noise before digitization ranged from 4.3 to 0.13 gray levels. The noise level was found to be increased only if the noise was less than approximately one‐half of a gray level.
Inhalation regional cerebral blood flow: The use of tidal CO2 data to find radionuclide activity associated with exhaled alveolar gas
Authors: Allison Jerry D. , Kingsbury Theodore B. , Sullivan Humbert G. , Goode Jamie J.
Published: March 2003

When calculating cerebral blood flow by the inhalation regional cerebral blood flow technique, radionuclide activity associated with exhaled alveolar gas is used to represent the arterial input function for each brain region. In this study, tidal CO2 data are used to identify respiratory gas samples that contain alveolar gas. Traditional methods identify alveolar gas samples by searching for maxima and minima in the raw air curve. The raw air curve is determined by sequentially counting radionuclide activity in respiratory gases sampled at the mouth. Traditional methods sometimes erroneously identify and use maxima or minima that do not represent alveolar gas. The use of CO2 data is advantageous since the range of CO2 during exhalation can identify those exhalations that approach the functional reserve capacity and hence represent alveolar gas. The arterial input function is represented by counting intervals from the raw air curve which coincide with exhalation of alveolar gas as identified by CO2 data. This approach for representing the arterial input function is fully automatic, accurate, and reproducible.
The role of humidity and other correction factors in the AAPM TG‐21 dosimetry protocol
Authors: Rogers D. W. O. , Ross C. K.
Published: March 2003

A detailed derivation is presented of the formulas required to determine Ngas and Dmed in the AAPM TG‐21 dosimetry protocol. This protocol specifies how to determine the absorbed dose in an electron or photon beam when using exposure or absorbed dose calibrated ion chambers. It is shown that the expression given in TG‐21's recent letter of clarification is incorrect. Accounting for humidity correctly increases, by 0.4%, all absorbed dose determinations using an exposure calibrated ion chamber. Taking into account other correction factors in the equation for exposure could also have varying, but significant effects (possibly over 1%). These are the stem scatter correction, the axial nonuniformity correction and the electrode correction for electrodes made of different materials from the wall. Attention is drawn to differences in the definitions of the exposure and absorbed dose calibration factors Nx and ND, respectively, as supplied by the NBS and the NRCC.
Analysis of tissue‐maximum ratio/scatter‐maximum ratio model relative to the prediction of tissue‐maximum ratio in asymmetrically collimated fields
Authors: Loshek David D.
Published: March 2003

The tissue‐maximum ratio/scatter‐maximum ratio model has been examined as an empirical model capable of predicting the variation in tissue‐maximum ratio observed in fields shifted off‐axis by independent collimators. This model has been assessed for its sensitivity to the necessary extrapolation of percentage depth dose data and phantom‐scatter factors to zero field size and to large field sizes. This model was found to yield good results for both 6‐ and 24‐MV x rays and to be relatively insensitive to the assumptions made regarding unmeasurable beam parameters.
Source continuity and boundary discontinuity considerations in Bayesian image processing
Authors: Liang Z. , Hart H.
Published: March 2003

This paper extends the Bayesian image processing (BIP) formalism by considering the effect of simple source continuity and boundary discontinuity and a priori information in estimating an optimal source distribution from observed data. The a priori source information is formulated in terms of probability density functions of source element strengths and spatial correlations. The estimation is carried out iteratively by a BIP algorithm derived by applying the expectation maximization technique to the a priori source probability density functions and assuming the data obey Poisson statistics. The suppression of boundary oscillations and enhancement of overall image are demonstrated for computer generated ideal and Poisson randomized data.
Artifacts due to residual magnetization in three‐dimensional magnetic resonance imaging
Authors: Wood Michael L. , Runge Val M.
Published: March 2003

An artifact is identified in magnetic resonance images produced by the three‐dimensional FLASH technique, which features a short repetition time TR. The artifact is caused by differential spoiling of transverse magnetization by the phase‐encoding gradients. The image intensity in different slices becomes altered, especially for short TR and large flip angle, which are conditions for achieving strong T1‐weighted contrast. The effectiveness of spoiler gradients and rephasing gradients in suppressing the artifact is evaluated experimentally in images of a uniform phantom. Spoiler gradients that are incremented in amplitude cause even more slices to deviate in intensity, and are therefore less effective than in two‐dimensional techniques. Rephasing gradients make the slices uniformly intense, but also enhance the intensity of tissues that have longer T2. The further addition of constant spoiler gradients has reduced this intensity increase by one‐half and allowed for an intensity difference between white matter and gray matter comparable to without a rephasing gradient.
Improved detectability in low signal‐to‐noise ratio magnetic resonance images by means of a phase‐corrected real reconstruction
Authors: Bernstein M. A. , Thomasson David M. , Perman William H.
Published: March 2003

We show that for magnetic resonance (MR) images with signal‐to‐noise ratio (SNR)<2 it is advantageous to use a phase‐corrected real reconstruction, rather than the more usual magnitude reconstruction. We discuss the results of the phase correction algorithm used to experimentally verify the result. We supplement the existing literature by presenting closed form expressions (in an MR context) for the probability distribution and first moments of the signal resulting from a magnitude reconstruction.
A comparison using tissue electrical properties and temperature rise to determine relative absorption of microwave power in malignant tissue
Authors: Joines William T. , Shrivastav Shashi , Jirtle Randy L.
Published: March 2003

This paper compares two methods for determining the radio frequency absorbed power in tissue: from the measured electrical properties of the tissue, and from the induced temperature rise per unit time. In previous research, we measured the radio frequency electrical properties of muscle, mammary gland, and malignant mammary tissue (SMT‐2A mammary adenocarcinoma) in female W/Fu isogeneic rats. From those measurements we calculated for each tissue the power absorption versus frequency, and formed the ratio of malignant‐to‐normal power absorption. This ratio exhibited a peak within the 150 to 400 MHz range, indicating a selective absorption of power in this type of malignant tissue over that of the normal host tissue. In the present study, by an entirely different method, we have directly tested the results of our earlier research. We filled a 20‐cm‐long section of rigid coaxial line (ordinarily air filled) with either normal (beef muscle or fat) or malignant (SMT‐2A) tissue, and measured the temperature increase versus time at the irradiated tissue surface for the same absorbed power in each tissue type. We made the measurements from 50 to 915 MHz, and found that the initial temperature increase per second per watt absorbed (dT/dt/Pa) was greater in malignant tissue than in muscle or fat at each frequency tested, with the greatest differences occurring below 450 MHz. Power absorption based on the measured values of dT/dt/Pa was again greatest for the malignant tissue (SMT‐2A mammary adenocarcinoma) within the 150 to 400 MHz range.
Quantitative measurement of small‐angle gamma ray scattering from water, nylon, and Lucite
Authors: Bradley D. A. , Dance D. R. , Evans S. H. , Jones C. H.
Published: March 2003

The small‐angle (2°–10°) total‐differential‐scattering cross sections of water, methyl methacrylate, and nylon 6 have been measured at a photon energy of 59.54 keV. An annular target geometry has been used with an acceptance of ±0.5° and an experimental uncertainty of ∼4% achieved. The experimental methods and data corrections necessary to obtain this accuracy are discussed in detail. The results demonstrate clearly the contribution of intermolecular effects to the scattering process and allow critical comparison with calculations using molecular form factors for water obtained from interpolated x‐ray diffractometer data; agreement is obtained within the experimental uncertainties. They also support earlier Monte Carlo predictions of scattering in water at angles of the order of 2° which are in disagreement with certain published experimental data.
Noise correlations in images acquired simultaneously with a dual‐energy sandwich detector
Authors: Sones Richard A. , Barnes Gary T.
Published: March 2003

An energy discriminating detector for dual‐energy radiography can be configured as a two‐layer sandwich, where the mean energy of photons detected by the two layers differs. To characterize the quantum noise of such a detector, the noise covariance between the two layers must be known in addition to the noise variance in each layer. A theory is presented which permits the calculation of the noise covariance, and it is found to be negligibly small. Experimental results, based on measurements with a Nal sandwich detector and an isotope gamma ray source, are reported and shown to confirm the theory. The quantum noise in each layer is independent and Poisson.
The accuracy of single‐seed dose superposition for I‐125 implants
Authors: Burns G. S. , Raeside D. E.
Published: March 2003

The Monte Carlo method was used to study perturbations of single I‐125 seed dose distributions created by the presence of one or three neighboring seeds for the case of seeds immersed in a water phantom. Perturbation factors were determined within the geometric shadow of neighboring seeds for two‐seed designs, four‐seed spacings, and several choices of dose point. The results were compared to dose estimates obtained by the simple superposition of single‐seed data for one‐ and two‐plane implants. Some significant differences were found.
Dose anisotropy around an Au‐198 seed source
Authors: Jani Shirish K. , Pennington Edward C. , Knosp Boyd M.
Published: March 2003

For interstitial implants with radioactive Au‐198 seeds, the dosimetric calculations usually ignore the finite source size and employ a point source approximation, resulting in a computed isotropic dose distribution. However, the measured radiation fluence from Au‐198 seeds is reported to be anisotropic, suggesting some amount of dose anisotropy in tissue. We have measured this dose anisotropy around Au‐198 seeds (2.5×0.8 mm2) using Kodak X‐OMAT XV‐2 films placed in contact with individual seeds in a phantom. Autoradiographs obtained for various exposure times were digitized and studied with an image analysis computer network. The network's overall spatial resolution was about 0.01 cm. The optical isodensity contours around the seed were obtained. The useful range of optical density (OD) for the system was found to be 1 to 2 OD units. Within this range, the shape of an optical isodensity contour would be identical to an isodose contour. Proximal to the source, the contours were elliptical in shape, elongated along the seed axis. However, further away, the elongation was in the direction normal to the seed axis. This was in agreement with the reported data on the radiation fluence around Au‐198 seeds. It was concluded that measurable dose anisotropy existed around Au‐198 seeds. However, it was too small to be of any clinical significance.
Effective atomic numbers of biological materials in the energy region 1 to 50 MeV for photons, electrons, and He ions
Authors: Parthasaradhi K. , Rao B. Mallikarjuna , Prasad S. Guru
Published: March 2003

A study of effective atomic numbers for biological materials such as, bone, muscle, spleen, liver, mucin, and water has been carried out in the energy region 1 to 50 MeV for photons, electrons, and He ions. It is noticed that the effective atomic number for photons and electrons increases with energy, and remains, more or less the same, for He ions.
Film dosimetry of small elongated electron beams for treatment planning
Authors: Niroomand‐Rad Azam
Published: March 2003

The characteristics of 5, 7, 10, 12, 15, and 18 Mev electron beams for small elongated fields of dimensions L×W (where L=1, 2, 3, 4, 5, and 10 cm; and W=1, 2, 3, 4, 5, and 10 cm) have been studied. Film dosimetry and parallel‐plate ion chamber measurements have been used to obtain various dose parameters. Selective results of a series of systematic measurements for central axis depth dose data, uniformity index, field flatness, and relative output factors of small elongated electron beams are reported. The square‐root method is employed to predict the beam data of small elongated electron fields from corresponding small square electron fields using film dosimetry. The single parameter area/perimeter radio A/P is used to characterize the relative output factors of elongated electron beams. It is our conclusion that for clinical treatment planning square‐root method may be applied with caution in determining the beam characteristics of small elongated electron fields from film dosimetry. The calculated and estimated relative output factors from square‐root method and A/P ratio are in good agreement and show agreement to within 1% with the measured film values.
Electron dose rate and photon contamination in electron arc therapy
Authors: Pla Marina , Podgorsak Ervin B. , Pla Conrado
Published: March 2003

The electron dose rate at the depth of dose maximum dmax and the photon contamination are discussed as a function of several parameters of the rotational electron beam. A pseudoarc technique with an angular increment of 10° and a constant number of monitor units per each stationary electron field was used in our experiments. The electron dose rate is defined as the electron dose at a given point in phantom divided by the number of monitor units given for any one stationary electron beam. For a given depth of isocenter di the electron dose rates at dmax are linearly dependent on the nominal field width w, while for a given w the dose rates are inversely proportional to di. The dose rates for rotational electron beams with different di are related through the inverse square law provided that the two beams have (di,w) combinations which give the same characteristic angle β. The photon dose at the isocenter depends on the arc angle α, field width w, and isocenter depth di. For constant w and di the photon dose at isocenter is proportional to α, for constant α and w it is proportional to di, and for constant α and di it is inversely proportional to w. The w and di dependence implies that for the same α the photon dose at the isocenter is inversely proportional to the electron dose rate at dmax.
Electron dose calculation using multiple‐scattering theory: Second‐order multiple‐scattering theory
Authors: Jette David , Bielajew Alex
Published: March 2003

This article is part of a series on the calculation of electron dose using multiple‐scattering theory. It presents systematically the second‐order multiple‐scattering theory which is a generalization of the (first‐order) Fermi–Eyges theory, outlining its derivation and giving explicit formulas for its defining functions. The predictions of the Fermi–Eyges theory and of the second‐order theory are compared with modified Monte Carlo calculations, demonstrating the increased accuracy of the latter multiple‐scattering theory. We derive and compare broad‐beam angular distributions for the two theories, and note the effect of large‐angle scattering upon dose profiles. Finally, we present the second‐order theory in Fourier‐transformed space, which is appropriate to a high‐speed dose‐calculation algorithm using the fast Fourier transform (FFT) technique.
Low‐energy imaging with high‐energy bremsstrahlung beams
Authors: Galbraith Duncan M.
Published: March 2003

A method is described for portal imaging with low‐energy (≲150 keV) photons from a radiotherapy accelerator operating in a diagnostic mode. The low‐energy photons are produced in the bremsstrahlung process, but are normally filtered out by thick high atomic number (Z) target materials. This absorption can be reduced by choosing a low Z target with the minimum thickness required to stop the electrons in the target. If, in addition, the operating energy is kept low (∼5 MeV) and the flattening filter is removed, low‐energy photon images can be produced from the broad spectrum of photon energies by using standard diagnostic radiology high‐Z fluorescent screen/film systems that strongly absorb at low but not high photon energies. Motion artifacts can then be avoided since only a small dose is required for such a procedure.
Eddy current disruption: Effect on nuclear magnetic resonance coil impedance and power loss
Authors: Harpen Michael D.
Published: March 2003

We present a theoretical development and experimental verification of a description of power loss and sample resistance for a lossy sample in a nuclear magnetic resonance radio frequency coil for a sample geometry where the eddy current streamlines are disrupted from their usually assumed circular paths. Specifically treated is the case of a lossy hemisphere. The problem is solved for two orientations; with the induction parallel and perpendicular to the flat surface of the hemisphere. Results of this analysis as well as those for the full sphere as presented by Hoult and Lauterbur are compared with observation for a variety of sample conductivities and orientation.
A two‐fluid model for hematocrit distribution in microvascular networks
Authors: Hokkanen J. E. I.
Published: March 2003

A new theoretical network model for evaluating discharge hematocrits, explicitly based on plasma skimming at branches, is introduced. The particular network geometry chosen simulates bat wing microvasculature. Blood in vessels is approximated to be a two fluid with red cell suspension as the core and a plasma layer surrounding it. The plasma layer width depends on hematocrit, which leads to nonlinear hydrodynamic equations solved by iteration. Discharge hematocrit distributions are calculated by a computer for five generations of vessels. Dispersion of hematocrit values was found to be correlated to plasma skimming at branches. Contrary to previous suggestions, plasma skimming did not result in lowered mean hematocrit towards the capillaries. Network structure was found to be an important factor affecting the hematocrits. Mean discharge hematocrit remained steady against changes in vessel dimensions, capillary resistances, red cell concentration in plasma layer, and shape of the separation surface defining the streamlines entering the side branch. This high stable mean hematocrit is based on symmetry of the model network. Enhanced asymmetry tended to lower the hematocrit.
Spatial light distribution in tumors: Phantom measurements
Authors: Lenz P.
Published: March 2003

The optical characteristics of 39 excised human tumors are determined. Then liquid phantoms, representative of these tumors, are used to measure the space irradiance in a plane containing the optical axis for two irradiation geometries (external and interstitial) and two wavelengths (514 and 630 nm); a rotating miniature probe allows a spatial resolution of 0.1 mm. It is found that, on the optical axis, the space irradiance at the surface of a phantom is higher than the irradiance applied externally and exhibits a maximum prior to reaching an exponential decrease. A zone where green light is expected to provide better photochemical efficiency than red light is determined and the space irradiances obtained with external and interstitial irradiation are compared quantitatively. The clinical usefulness of the results is limited by the fact that individual samples belonging to the same type of tumor differ considerably in their optical characteristics, at least unless a more detailed classification (based on a greater number of tumor samples) is available.
Compensation for beam intensity fluctuation in determination of Pion, the ion‐recombination correction factor for ionization chambers, by the two‐voltage technique
Authors: Hayakawa Yoshinori , Loch Cinthia P. , Tada JunIchiro , Inada Tetsuo
Published: March 2003

We have developed a method of compensation for fluctuations in beam intensity that may occur during measurement of Pion, the ion‐recombination correction factor, by the two‐voltage technique. The method requires signals proportional to beam intensity during measurement. We used a parallel‐plate ionization chamber, whose Pion was known, and a vacuum chamber to obtain signals that were proportional to the beam intensity. Experiments were conducted using pulsed proton beam providing doses that ranged from 0.16 to 0.01 cGy/pulse. The value of Pion of a thimble ionization chamber was measured. With these measurements, the validity of the method which we proposed for pulsed beam was verified experimentally.
Backscatter dose perturbation at high atomic number interfaces in megavoltage photon beams
Authors: Das Indra J. , Kahn Faiz M.
Published: March 2003

Most computer algorithms used clinically for photon beam treatment planning are unable to predict the effect of electron backscattering on dose distribution from high atomic number materials. It has been observed that there is a significant dose enhancement at such an interface. We define the dose enhancement in terms of backscatter dose factor (BSDF), which depends on the energy of the photon beam, thickness and width of the inhomogeneity, distance from the interface, and the atomic number of the inhomogeneity. For all energies studied, the dose fall‐off is initially very rapid and disappears beyond a few millimeters upstream from the interface. Empirically derived equations are presented for dose calculation at the interfaces of various media, including bone and soft tissue, for photon energies in the range of Co‐60 gamma rays to 24 MV x rays.
Preliminary validation of the opposing view method for quantitative gamma camera imaging
Authors: Eary Janet F. , Appelbaum Frederick L. , Durack Larry , Brown Paul
Published: March 2003

We perform gamma camera imaging to generate data for estimation of internal radiation dose in our radioimmunotherapy candidates. Because of the inability of single photon emission computed tomography (SPECT) to account for patient attenuation variability without serious error, quantitative planar imaging was performed to estimate the radioactive content of normal organs. We undertook the following studies to further validate this method. A realistic fillable human phantom was used to determine the accuracy of I‐131 filled organ estimation. A transmission scan of the unfilled phantom was produced with an I‐131 filled flat flood source. Anterior and posterior planar images of the filled organs were acquired with region of interest determination of the activity in the organ. Correction by the attenuation factor and a camera calibration factor yielded the MBq in the organ. The procedure was also performed in a simple phantom. Three dogs were imaged and sacrificed to validate the technique in vivo. A high degree of accuracy in estimation of organ radioactive content was found to be present using the phantom and dog models. Use of this method requires further validation but provides a solid basis for estimation of internal radiation dosimetry in radioimmunotherapy planning.
Probability of causation for cancers potentially induced by ionizing radiation
Authors: Wagner Louis K. , Brodsky Allen , Jose Donald E. , Rao Gopala , Webster Edward W.
Published: March 2003

Prompted by increasing litigation claiming cancer induction from exposure to radiation from nuclear weapons tests, the United States Congress enacted an amendment to the Orphan Drug Act (Public Law 97‐414, January 4, 1983), through which the National Institutes of Health was directed to and did produce Probability of Causation (PC) Tables. This review defines PC and discusses the data, limitations, and uses of the concept.
Comparison between continuous and discrete sources in the Paris system of implants
Authors: Paul J. M. , Philip P. C. , Brandenburg R. W. , Koch R. F.
Published: March 2003

Since the Paris system of interstitial brachytherapy is based on iridium wires, the consequences resulting from the substitution of discrete point sources in afterloading ribbons have been analyzed. The results are presented in this article for single ribbons, single plane implants of thicknesses 0.5 and 1.0 cm, and biplanar implants of both equal and unequal areas of sizes most frequently used clinically in the thickness range 1.5–3.0 cm. These results indicate changes in the quality of implants using wires and point sources up to the maximum spacing of 1.5 cm. The quality of the implants is judged on the basis of a parameter, the uniformity index, which takes into account both the size of the treatment volume for a given target volume and the homogeneity within the treatment volume as determined from the integral dose. Other implant parameters such as total activity, treatment volume, total volume receiving greater than twice the stated dose rate, average dose rate, and variation of basal dose rate, etc., are also provided to indicate quality of the implants. The results with the customary seed spacing of 1 cm are compatible with those using wires and are found to have improved quality in many cases.
Maximum likelihood analysis of free‐response receiver operating characteristic (FROC) data
Authors: Chakraborty Dev P.
Published: March 2003

Receiver operating characteristic (ROC) methodology is widely used in evaluating medical imaging modalities. While appropriate in some cases, it has several drawbacks when the detection task, e.g., nodule detection, involves localizing the abnormality. Free‐response receiver operating characteristic (FROC) methodology offers a more natural framework to describe observer performance in such studies and has other advantages. Due to the lack of a statistical analysis procedure comparable to the maximum likelihood procedure (rocfit program) available for ROC studies, the FROC method has not gained widespread acceptance. This work presents and solves a two parameter model for the statistical analysis of FROC data. The model assumes that the probability density of the signal stimuli is normally distributed, as is the probability density for producing one or more false positives per image. A program (frocfit) is described for estimating the parameters and their uncertainties from experimental data. An index of performance is proposed to quantify observer performance in FROC experiments. Application of this methodology to several FROC data sets produced good to excellent fits.
Optical dosimetry for interstitial photodynamic therapy
Authors: Arnfield M. R. , Tulip J. , Chetner M. , McPhee M. S.
Published: March 2003

An approach to photodynamic treatment of tumors is the interstitial implantation of fiber optic light sources. Dosimetry is critical in identifying regions of low light intensity in the tumor which may prevent tumor cure. We describe a numerical technique for calculating light distributions within tumors, from multiple fiber optic sources. The method was tested using four translucent plastic needles, which were placed in a 0.94×0.94 cm grid pattern within excised Dunning R3327‐AT rat prostate tumors. A cylindrical diffusing fiber tip, illuminated by 630 nm dye laser light was placed within one needle and a miniature light detector was placed within another. The average penetration depth in the tumor region between the two needles was calculated from the optical power measured by the detector, using a modified diffusion theory. Repeating the procedure for each pair of needles revealed significant variations in penetration depth within individual tumors. Average values of penetration depth, absorption coefficient, scattering coefficient, and mean scattering cosine were 0.282 cm, 0.469 cm−1, 250 cm−1 and 0.964, respectively. Calculated light distributions from four cylindrical sources in tumors gave reasonable agreement with direct light measurements using fiber optic probes.
Dose enhancement in buildup region by lead, aluminum, and lucite absorbers for 15 MVp photon beam
Authors: Ciesielski B. , Reinstein L. E. , Wielopolski L. , Meek A.
Published: March 2003

Changes in dose distributions in buildup region resulting from the presence of lead, aluminum, and lucite absorbers above the surface of a polystyrene phantom were evaluated. The surface dose, as a function of the absorber thickness, is presented as well as the influence of the air gap between the lead absorber and the phantom surface. It has been found that the surface dose does not depend on absorber thickness for absorbers thicker than the range of secondary electrons in the absorber material (after corrections for the attenuation of the primary beam in the absorber). Similarly, the depth dose curves in the phantom were elevated only at depths lower than the range of secondary electrons in the phantom. The applicability of the presented data in clinical radiotherapy is discussed.
Long term variation in beam symmetry as a function of gantry angle for a computer‐controlled linear accelerator
Authors: Loyd M. D. , Lane R. G. , Laxton J. , Chow C. H. , Rosen I. I.
Published: March 2003

Testing computer‐controlled linear accelerators for patient safety and proper patient dose delivery requires that certain beam characteristics be monitored over an extended period of time. Computer‐controlled conformal radiation therapy using asymmetric collimator jaw settings necessitates stable symmetric treatment beams. Long term beam symmetry measurements have been performed on a Philips SL20 dual energy computer‐controlled linear accelerator. Symmetry in both the radial and transverse axis of each x‐ray beam was monitored for eight gantry positions. These measurements were undertaken to determine the effectiveness of the SL20 beam steering system during dose delivery of 50 monitor units (MU) per field. Evaluation of the data shows that careful beam steering setup procedures result in x‐ray beams in which fluctuations in symmetry as a function of gantry angle are within ±1.5%. Day to day instabilities produce a total overall variation in beam symmetry on the order of ±2.0%. Results suggest the measurement of symmetry as a function of gantry position become a routine quality assurance procedure for this accelerator.
Calibration frequency as determined by analysis of machine stability
Authors: Schultheiss T. E. , Boyer A. L. , Horton J. L. , Gastorf R. J.
Published: March 2003

Simple statistical analysis is applied to the evaluation of the output measurements of equipment used in radiotherapy. The calibration frequency is calculated based on the stability of the equipment and the performance parameters required by the quality control criteria.
A method of calculating the output factors of arbitrarily shaped electron fields
Authors: McParland Brian J.
Published: March 2003

It has been well demonstrated in the past that the dose output of an electron beam is highly dependent upon the design of the collimation system, the beam energy, and the field dimensions. Various approaches have been taken to calculate the output factor of an arbitrary rectangular beam. These methods, however, cannot be readily extended to accurately calculate the output factor of an irregular field. In this work, the output factor of an arbitrarily shaped electron field is calculated as the product of three terms: a weighting factor which is explicitly derived from a discrete electron pencil beam model, the known output factor of the regular electron field defined by an applicator or a set of trimmers from which the irregular field is formed, and a machine‐dependent term which models the scatter contribution from the shielding. For the machine and fields of clinical interest discussed here, it was found that this latter term could be neglected; this may not be true in general. Results from this calculation are compared to measured data.
Radiation probe for indirect evaluation of the high‐voltage waveform of a Mo anode mammography unit
Authors: Gambaccini M. , Marziani M. , Rimondi O. , Indovina P. L.
Published: March 2003

A radiation probe was designed for indirectly determining the high voltage of a Mo anode mammography unit. The real time processing of the probe outputs yields exposure time, voltage waveform, kVp, and ripple in the range 24–40 kVp useful for screen–film mammography. The probe, connected to a portable computer, will be employed in a survey of radiation dose and image quality in mammography as part of an ongoing program in Italy.
A review of image reconstruction techniques for electrical impedance tomography
Authors: Barber D. C.
Published: March 2003

There has recently been an increasing interest in the possibility of producing images of electrical impedance within the human body. When an electric current is applied to the body of a voltage distribution is developed across the body surface. This distribution is in part dependent on the internal impedance distribution within the body and its is possible to estimate this distribution from a suitable set of voltage measurements. Because of the nonlinear relationship between the impedance distribution and the voltage distribution at the surface of the body, the reconstruction problem is much more difficult than for other tomographic imaging techniques, but a significant amount of progress has been made, and it is now possible to produce tomographic images of in vivo distributions of impedance, albeit with low spatial resolution. Future developments should improve image quality.
Absolute diameter measurements of coronary arteries based on the first zero crossing of the Fourier spectrum
Authors: Weber David M.
Published: March 2003

Current edge detection methods used to determine coronary artery dimensions from digital (DSA) images suffer a strong dependence on the system's modulation transfer function (MTF). The videodensitometric algorithms are less sensitive to MTF blurring, yet still result in an overestimation of the vessel size of 10% to 25% for blurring aperture sizes of 50% to 80% of the vessel diameter. We propose a new algorithm to measure the absolute diameter of a vessel which has a lower sensitivity to the system MTF for blurring aperture sizes up to 80% of the vessel diameter. A consequence of the similarity theorem of Fourier transform pairs is that the “width” of the Fourier transform, as characterized by the first zero crossing in frequency space, is inversely proportional to the width of the vessel profile. For an ideal (unblurred) vessel image, the width of the vessel profile is equal to the diameter of the vessel. For a blurred image this is not true. In frequency space, however, the transform of the blurred profile is simply the product of the transformed ideal profile and the system MTF. Thus, if the blurring aperture of the system is below some critical value, the first zero of the unblurred profile will still dominate the transform of the blurred profile. For vessels of circular cross section and a rectangular blurring aperture, this critical aperture size is approximately 80% of the vessel diameter. A more detailed explanation of the theory and calculations involved in this measurement, along with measurements of computer simulated and phantom vessels is presented.
Comparison of dual and single exposure techniques in dual‐energy chest radiography
Authors: Ho Jung‐Tsuoe , Kruger Robert A. , Sorenson James A.
Published: March 2003

Conventional chest radiography is the most effective tool for lung cancer detection and diagnosis; nevertheless, a high percentage of lung cancer tumors are missed because of the overlap of lung nodule image contrast with bone image contrast in a chest radiograph. Two different energy subtraction strategies, dual exposure and single exposure techniques, were studied for decomposing a radiograph into bone‐free and soft tissue‐free images to address this problem. For comparing the efficiency of these two techniques in lung nodule detection, the performances of the techniques were evaluated on the basis of residual tissue contrast, energy separation, and signal‐to‐noise ratio. The evaluation was based on both computer simulation and experimental verification. The dual exposure technique was found to be better than the single exposure technique because of its higher signal‐to‐noise ratio and greater residual tissue contrast. However, x‐ray tube loading and patient motion are problems.
Analysis of capacitive coupling and associated loss for a solenoidal magnetic resonance imaging radio‐frequency coil
Authors: Harpen Michael D.
Published: March 2003

Presented is an analysis of the capacitive coupling between a magnetic resonance imaging radio‐frequency (rf) coil and a dielectric sample. Calculated is the coil self‐capacitance and power loss in the medium. The capacitively coupled power loss is compared with the more familiar inductively coupled eddy current loss and the predicted dependence of self‐capacitance on coil and sample size is studied experimentally in a series of single‐turn slotted solenoids of various sizes.
The geometry of avoiding beam intersections and blocking tray collisions
Authors: Yorke Ellen D.
Published: March 2003

The intersection of a therapy x‐ray beam with steel rails beneath or along the side of the patient support couch should be avoided. Collisions of the blocking tray mount with any portion of the couch are also prohibited. Equations are derived to determine the range of gantry angles which must be avoided for specified patient setups. These equations were incorporated into a brief computer program. The forbidden angular ranges for a representative set of field widths and isocenter positions are graphically presented for quick clinical reference.
Measurements of backscattered radiation from Therac‐20 collimator and trimmer jaws into beam monitor chamber
Authors: Kubo Hideo , Lo Koon Kai
Published: March 2003

The field size dependent photon output is known to be influenced by the existence of backscattered radiation (BSR) generated in the collimator or trimmer jaws of a linear accelerator. This paper describes the results of measurements made to study the existence of such backscatter by simulating the geometry of the treatment head of a Therac‐20 linear accelerator. The machine's monitor chamber, flattening filter, and collimator jaws were simulated by another real monitor chamber, a 1‐cm thick lead sheet and 2.5‐cm thick low‐melting‐point alloy divergent blocks, respectively. BSR from the simulated collimator jaws (SCJ) was measured with the simulated monitor chamber (SMC) as a function of the openings of the SCJ and as a function of distance between SMC and SCJ. The present results demonstrate the presence of BSR in an 18‐MV photon beam from a Therac‐20 linear accelerator.
Telescopic measurements of backscattered radiation from secondary collimator jaws to a beam monitor chamber using a pair of slits
Authors: Kubo Hideo
Published: March 2003

A contribution to field‐size dependent output by backscattered radiation (BSR) from secondary collimator jaws to a beam monitor chamber of a linear accelerator was measured with a Farmer ionization chamber, positioned 200 cm from the source behind a low‐melting‐point alloy slab with a 10‐cm wide slit. Another slit was positioned against the collimator jaws. Both slits were in the form of a 6.3‐mm‐diam hole in the middle and were aligned to the source. The use of a pair of slits was intended to eliminate any influence on the ion chamber readings due to field‐size dependent charge contribution from the flattening filter and collimator jaw forward scattering. In addition, the setup permits to observe the degree of field‐size dependence on BSR. Charge measurements from the Therac‐20 18‐MV x rays showed a 7.5% field‐size dependence on BSR whereas 6‐ and 18‐MV x rays from Varian Clinac‐1800 showed <2% dependence on BSR. The telescopic method was found to be easy to use and permitted direct determination of BSR contributions.
Reply to comments of Olch and Kagan
Authors: Saw Cheng B. , Suntharalingam Nagalingam
Published: March 2003

Olch and Kagan, with their comments on our paper, continue to assert that treating the entire target volume to the prescribed dose should be the aim even if additional normal tissue is treated in the process. It is our contention, with proper input from our clinician colleagues, that the selection of a dose rate to deliver the prescribed dose should result in the delivery of highly homogeneous dose throughout the target volume, adequately cover the target volume, and minimize irradiation of tissues surrounding the target volume. We do feel strongly that the consideration of all three factors is important for the best management of a patient's treatment.
Investigation of basic imaging properties in digital radiography. 13. Effect of simple structured noise on the detectability of simulated stenotic lesions
Authors: Ohara Ken , Doi Kunio , Metz Charles E. , Giger Maryellen Lissak
Published: March 2003

We investigated the effects of structured background noise on the detectability of stenotic lesions. Digital subtraction angiographic (DSA) images of stenotic blood vessels were simulated and superimposed onto uniform noise samples. Eighteen‐alternative forced choice (18‐AFC) experiments were employed to determine the detectability of the stenotic lesion in the structured‐noise background of a blood vessel. In this study, the dependence of detectability on lesion size, vessel size, and incident x‐ray exposure was examined. Our results indicate that the presence of structured noise in an image will reduce the detectability of a lesion. However, the relative performance of an observer when the lesion size and incident exposure were varied was the same with and without the presence of the structured background. Thus, conclusions obtained previously with regard to changes in the detectability of a lesion in the presence of uniform background noise can be applied directly to conditions in which simple structured anatomic background is present.