bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluoroscopic performance tests using a portable computer/frame grabber: Wiener spectra measurements
Authors: Goldman Lee W.
Published: March 2003

Currently, routine tests of fluoroscopic image quality in common use are highly subjective. As part of an effort to develop more quantitative routine tests of fluoroscopic image quality, a method was developed to quickly and easily measure Wiener spectra (WS) of TV‐viewed fluoroscopic systems that considers both spatial and temporal noise correlations. A PC‐mounted frame grabber captures images at the TV frame rate to form a three‐dimensional (3‐D) array of pixels. Scans of a “two‐dimensional slit” are then synthesized from which a one‐dimensional central section of a 3‐D WS is calculated. To avoid errors due to coarse (8‐bit) quantization, a video amplifier is used to expand a portion of the signal to the full digitizer range. A reference signal (2 mm of aluminum) is then used to normalize image contrast. Ensemble averages of 250 spectra were obtained in ∼1 min, including all processing. Results are presented to demonstrate reproducibility, sensitivity, and behavior of the WS. The eventual goal of this work is to use this method in conjunction with measurements of an MTF to calculate fundamental descriptors of image quality, such as SNR and NEQ.
Design of parallel plate ion chambers for buildup measurements in megavoltage photon beams
Authors: Rawlinson J. Alan , Arlen Dianne , Newcombe Dalton
Published: March 2003

Dose measurements in the buildup region of megavoltage photon beams are most commonly made using parallel plate ion chambers having fixed electrode separation. Fixed‐separation chambers generally do not read correctly under such beam conditions because of the contribution to the chamber signal of electrons from the side walls. In this work it is shown that the side wall error can be very large and published correction formulas are not accurate for all beam conditions and chamber geometries. The principal focus of this study has been to determine the design features of a fixed‐separation chamber that has negligible side wall error. The approach has been to study, in beams of 60Co, 6 MV, and 18 MV, the response of a specially built ion chamber in which several chamber parameters could be independently varied. The study has shown that the side wall error is primarily dependent on the ratio of the electrode separation to the wall diameter as well as on the wall density and wall angle. Based on these findings the design of a fixed‐separation chamber is described which reads to within about 1% of the correct dose. Guidelines are also provided for assessing the suitability of current commercial fixed‐separation ion chambers for buildup measurements.
Nuclear magnetic relaxation characterization of irradiated Fricke solution
Authors: Podgorsak Matthew B. , Schreiner L. John
Published: March 2003

The spin‐lattice relaxation rate of irradiated Fricke solution was studied as a function of the absorbed dose D. The R1 increases linearly with dose up to D∼400 Gy after which the response saturates. A model describing the R1 of a solution of either ferrous (Fe2+) or ferric (Fe3+) ions is presented; it is based on the spin relaxation of protons on water molecules in the bulk and protons on water molecules in the coordination shells of the ions with fast exchange occurring between the two water environments. All inherent relaxation parameters of the different proton groups are determined empirically at NMR frequencies of 9 and 25 MHz. An extension of the model is made to describe the spin‐lattice relaxation behavior of irradiated Fricke solution. Good agreement between model predictions and experimental results is observed. The model relates the spin‐lattice relaxation rate of a Fricke dosimeter to the chemical yield of ferric ion, thus potentially creating an absolute NMR dosimetry technique. Various practical aspects of the NMR‐Fricke system, such as the optimal initial ferrous concentration and the NMR frequency dependence of the sensitivity, are described.
An interactive beam‐weight optimization tool for three‐dimensional radiotherapy treatment planning
Authors: Starkschall George , Eifel Patricia J.
Published: March 2003

A computer software tool has been developed to aid the treatment planner in selecting beam weights for three‐dimensional radiotherapy treatment planning. The program consists of a feasibility search algorithm embedded in an interactive, user‐friendly driving program. The feasibility search algorithm is based on the iterative relaxation algorithm of Cimmino [La Ricerca Scientifica, Vol. I, pp. 326–333 (1938)] as applied to the radiotherapy inverse problem by Altschuler et al. [Med. Phys. 13, 590 (1986)]. Relative importances of structures based upon clinical considerations can be incorporated into the algorithm. In order to speed convergence, the relaxation parameter is made to vary, with its value based upon a measure of deviation from feasibility. The interactive driving program is designed so that the treatment planner can make reasonable judgments regarding the acceptability of a plan in the event that the dose constraints yield no feasible solution. An example of the use of this program applied to a problem in three‐dimensional radiotherapy treatment planning is illustrated.
Recommendations on performance characteristics of diagnostic exposure meters: Report of AAPM Diagnostic X‐Ray Imaging Task Group No. 6
Authors: Wagner L. K. , Fontenla Doracy P. , Kimme‐Smith Carolyn , Rothenberg Lawrence N. , Shepard Jeff , Boone John M.
Published: March 2003

Task Group 6 of the Diagnostic X‐Ray Imaging Committee of the American Association of Physicists in Medicine (AAPM) was appointed to develop performance standards for diagnostic x‐ray exposure meters. The recommendations as approved by the Diagnostic X‐Ray Imaging Committee and the Science Council of the AAPM are delineated in this report and provide specifications on meter precision, calibration accuracy, calibration reference points, linearity, energy dependence, exposure rate dependence, leakage, amplification gain settings, directional dependence, the stem effect, constancy checks, and calibration intervals. The report summarizes recommendations for meters used in mammography, general purpose radiography including special procedures, computed tomography, and radiation safety surveys for x‐ray radiography.
Two thermal methods to measure the energy fluence of a brief exposure of diagnostic x rays
Authors: Carvalho A. A. , Mascarenhas S. , dePaula M. H. , Cameron J. R.
Published: March 2003

This paper describes two simple thermal methods for measuring the energy fluence in J/cm2 from a diagnostic x‐ray exposure. Both detectors absorb essentially 100% of the radiation and give a signal that is directly proportional to the energy fluence of the x‐ray beam. One detector measures the thermal effect when a pulse of x rays is totally absorbed in the pyroelectric detector of lead–zirconium–titanate (PZT). The other detector measures the expansion of a gas surrounding a lead disk detector in a photoacoustic chamber. The increased pressure of the gas is transmitted through a 1‐mm duct to a sensitive microphone. Both detectors have previously been used to measure the energy fluence rate of continuous x‐ray beams in the same energy region using a chopped beam and a lock‐in amplifier. Measurement of the energy fluence of a pulse of radiation eliminates the need for the beam chopper and lock‐in amplifier and results in a simple, rugged, and inexpensive dosimeter. Either method can be combined with the area of the beam to give an estimate of the imparted energy to the patient from a diagnostic x‐ray exposure.
The advantages of absorbed‐dose calibration factors
Authors: Rogers D. W. O.
Published: March 2003

A formalism for clinical external beam dosimetry based on use of ion chamber absorbed‐dose calibration factors is outlined in the context and notation of the AAPM TG‐21 protocol. It is shown that basing clinical dosimetry on absorbed‐dose calibration factors ND leads to considerable simplification and reduced uncertainty in dose measurement. In keeping with a protocol which is used in Germany, a quantity kQ is defined which relates an absorbed‐dose calibration factor in a beam of quality Q0 to that in a beam of quality Q. For 38 cylindrical ion chambers, two sets of values are presented for ND/NX and Ngas/ND and for kQ for photon beams with beam quality specified by the ratio. One set is based on TG‐21's protocol to allow the new formalism to be used while maintaining equivalence to the TG‐21 protocol. To demonstrate the magnitude of the overall error in the TG‐21 protocol, the other set uses corrected versions of the TG‐21 equations and the more consistent physical data of the IAEA Code of Practice. Comparisons are made to procedures based on air‐kerma or exposure calibration factors and it is shown that accuracy and simplicity are gained by avoiding the determination of Ngas from NX. It is also shown that the kQ approach simplifies the use of plastic phantoms in photon beams since kQ values change by less than 0.6% compared to those in water although an overall correction factor of 0.973 is needed to go from absorbed dose in water calibration factors to those in PMMA or polystyrene. Values of kQ calculated using the IAEA Code of Practice are presented but are shown to be anomalous because of the way the effective point of measurement changes for 60Co beams. In photon beams the major difference between the IAEA Code of Practice and the corrected AAPM TG‐21 protocol is shown to be the Prepl correction factor. Calculated kQ curves and three parameter equations for them are presented for each wall material and are shown to represent accurately the kQ curve for all ion chambers in this study with a wall of that specified material and a thickness less than 0.25 g/cm2. Values of kQ can be measured using the primary standards for absorbed dose in photon beams.
Neutron fluence and kerma spectra of a p(66)/Be(40) clinical source
Authors: Jones D. T. L. , Symons J. E. , Fulcher T. J. , Brooks F. D. , Nchodu M. R. , Allie M. S. , Buffler A. , Oliver M. J.
Published: March 2003

High‐resolution neutron fluence spectra have been measured in the National Accelerator Centre's p(66)/Be(40) neutron therapy beam by the pulsed‐beam time‐of‐flight method. ICRU muscle kerma spectra have been derived from the fluence spectra. Spectral changes resulting from different irradiation conditions have been quantified in terms of the average neutron energy and the fractional low‐energy (<16 MeV) contribution. The changes observed with different thicknesses of polyethylene filtration are consistent with changes in quality parameters determined in biological and microdosimetric experiments. The dosimetry parameters and (Wgas)N calculated for the measured spectra agree with the values recommended in the neutron dosimetry protocol. The shapes of the present fluence spectra differ from previous measurements of p(>40)/Be spectra. In particular, they differ significantly from the spectrum measured by recoil techniques in an identical neutron therapy unit at the Clatterbridge Hospital, UK. The reasons for the difference are not known.
Radiation spectra for Auger‐electron emitting radionuclides: Report No. 2 of AAPM Nuclear Medicine Task Group No. 6
Authors: Howell Roger W.
Published: March 2003

Radiation spectra for radionuclides currently provided by the MIRD Committee and ICRP do not include the very low‐energy N‐ and O‐shell Auger electrons. These electrons, emitted in large numbers by radionuclides decaying by electron capture and/or internal conversion, are important for determining the absorbed dose in microscopic volumes. Accordingly, the present AAPM Report employs Monte Carlo computational methods to obtain a self‐consistent set of complete radiation spectra for a variety of radionuclides including 55Fe, 67Ga,99mTc, 111In,113mIn, 115mIn, 123I,125I, 193mPt, 195mPt,201Tl, and 203Pb. Although the conventional spectra provided by MIRD and ICRP are adequate for most dosimetry calculations, the Auger electron spectra provided in this report are recommended for calculating the dose to target volumes <1 μm in diameter.
A MRI gradient waveform model for automated sequence calibration
Authors: Barker Bruce R. , Archer Branch T. , Erdman William A. , Peshock Ronald M.
Published: March 2003

In order to set up magnetic resonance imaging (MRI) procedures of arbitrary voxel dimensions, slice orientation, and sequence timing in a reasonable time, some form of automatic gradient pulse calibration is required. One such method, involving simulation of gradient waveforms, is presented. Waveforms are modeled based on measurements of the step response. The model used divides each transition into three time regions: a “start” region in the first 0.3 ms, a “slew” region, and a “tail” region representing decay of the eddy current compensation error. In the “slew” region, the time derivative of the gradient, G′(t), is expressed as a function of G(t). The first two regions are nonlinear with respect to demand. The mean error in the simulated gradient is generally less than 0.04 mT m−1 in spin echo sequences. Image signal/noise ratios resulting from sequences calibrated using the model are within 5% of those of empirically calibrated sequences.
Localized beta dosimetry of 131I ‐labeled antibodies in follicular lymphoma
Authors: Hui T. Edmond , Fisher Darrell R. , Press Oliver W. , Eary Janet F. , Weinstein John N. , Badger Christopher C. , Bernstein Irwin D.
Published: March 2003

The purpose of this study is to assess the multicellular dosimetry of 131I ‐labeled antibody in follicular lymphoma based on histological measurements on human tumor biopsy tissue. Photomicrographs of lymph node specimens were analyzed by first‐order treatment to determine the mean values and statistical variations of the radii of follicles (260±90 μm), interfollicular distances (740±160 μm), and the number density of follicles [60±18 in a volume of (2×1480 μm)3]. Based on these measurements, two geometrical models were developed for localized beta dosimetry. The first, a regular cubic lattice model, assumes no variation in follicular radius of follicles and interfollicular distance. The second, a randomized distribution model, is a more complicated but more realistic representation of observed histological specimens. In this model, Monte Carlo methods were used to reconstruct the spatial distribution of follicles by simulating the distribution of the radii of follicles, interfollicular distances, and the number density of follicles. Dose calculations were performed using Berger's point kernels for absorbed‐dose distribution for beta particles in water, assuming the 131I ‐labeled antibodies as point sources. It was assumed that the activity concentration of the labeled antibody within the follicles was ten times the activity concentration in the interfollicular spaces. The spatial distribution of localized dose was calculated for a tumor having an average dose of 40 Gy. The localized dose was found to be highly nonuniform, ranging from 20 to 90 Gy, and varying by a factor of about 2 from the average tumor dose. Most of the tissue (70%–80% by volume) was found to receive a lower dose than the average tumor dose. No significant difference in localized dose distribution was found between the regular cubic lattice model and the randomized distribution model, suggesting that sometimes a simple geometrical model may be adequate as a starting point for dosimetry of more complex situations. Finally, the localized dose distribution is discussed in terms of its usefulness for extracting information such as the mean dose, the spatial variation of dose, and the fraction of tissue receiving various absorbed doses. An approach for applying localized dosimetry to improve the estimated cell‐killing efficiency is suggested.
Radial dose distribution from 192Ir seeds at distances far from the source
Authors: Thomason C.
Published: March 2003

Most current tissue attenuation and scattering factors for 192Ir are valid only to distances of 15 cm or less. Monte Carlo calculations using the EGS4 computer code were used to calculate tissue attenuation and scattering factors for 192Ir seed sources with both stainless steel encapsulation and with Pt encapsulation out to a distance of 32 cm from the sources. The data clearly show that polynomial tissue attenuation and scattering factors cannot be extrapolated beyond their range. Encapsulation specific factors are presented here that are valid at large distances from the source.
Automated determination of patient setup errors in radiation therapy using spherical radio‐opaque markers
Authors: Lam Kwok Leung , Haken Randall K. Ten , McShan Daniel L. , Thornton Allan F.
Published: March 2003

Patient positioning accuracy can be quantified by the three‐dimensional (3‐D) translations and rotations required to transform the patient back to the desired position. Results of the current study show that the translations and rotations could be obtained from two projection images obtained radiographically on a linear accelerator when spherical radio‐opaque markers were implanted inside or affixed to the surface of a skull phantom. In the study, films used to record the images were converted into digital gray scale images using a laser film digitizer. The marker images were located automatically by the computer using image processing techniques. By combining information from both projections, the 3‐D locations of the markers were determined to submillimeter precision. Pairs of films were also analyzed for the phantom displaced from its original location by known amounts. The accuracy of the computed translations and rotations required for realignment of the phantom were found to be better than 1 mm and 0.3°, respectively; comparable to the accuracy of the readout system of the equipment used. The general methodology could be coupled with an electronic portal imaging device for use in computer aided or automated correction of patient position in radiotherapy.
Computer‐assisted positioning of radiotherapy patients using implanted radiopaque fiducials
Authors: Gall Kenneth P. , Verhey Lynn J. , Wagner Miles
Published: March 2003

For certain external beam radiotherapy procedures, precise alignment of patients with the treatment beam is essential for good treatment outcome. A method has been developed for quickly achieving precise patient alignment with the aid of stereoscopically located fiducial markers. The alignment algorithm is developed from standard rigid body mechanics using closed form solutions, obviating the need for iterative fitting methods. The technique is implemented with a digitizing tablet and plane film radiographs. The accuracy of alignment with this method in phantom studies is better than 1 mm and 1 deg relative to CT data. The repeatability of positioning is 0.5 mm (standard deviation) and 0.36 deg (standard deviation).
Optimization of the dose delivery in a few field techniques using radiobiological objective functions
Authors: Södertröm Svante , Brahme Anders
Published: March 2003

A method for finding optimal primary fluence profiles for multiple field external beam radiation therapy techniques has been developed using a radiobiologically based objective function that quantifies the probability of achieving complication‐free tumor control, P+. The objective function P+ has the valuable property of giving the highest possible dose to the tumor without causing severe damage to normal tissues at risk. This radiobiologically based objective function selects suitable dose levels but also takes into account the dose homogeneity in the target volume to the extent that it is not causing an excessive risk of local recurrence or damage to surrounding normal tissues. The biological parameters used can either be patient specific, as determined by a predictive assay on biopsy specimens, or taken from a library of radiobiological parameter values characteristic for different tissue types of a reference patient. In its present form the method can be used to determine the optimum incident photon fluence profiles for each beam. The method has been used to investigate for a given target volume a large number of combinations of beam entry directions to find the best beam orientations with respect to the probability of achieving complication‐free tumor control. It is demonstrated that when nonuniform dose delivery is available it is unsuitable to combine parallel opposed beams in two‐beam techniques and to a lesser extent also to use perpendicular beams. In two‐beam techniques the best angle between the beams is generally in the 100°–120° range. The major symmetry characteristics of the P+ phase space for two‐beam techniques are also identified. The method can easily be extended from two to three dimensions and noncoplanar geometry, but it is presented here in its two‐dimensional form for clarity and speed of calculation.
A mathematical basis for selection of wedge angle and orientation
Authors: Sherouse George W.
Published: March 2003

The treatment plan optimization criterion that dose be homogeneous over an irradiated volume is equivalent to the criterion that the magnitude of the dose gradient be zero throughout that volume. If the dose gradient due to an individual beam is represented by a vector, the dose gradient due to an ensemble of beams is given by the weighted vector sum of the constituent beams' individual gradients. Given a fixed ensemble of beams, the two ways in which the total dose gradient can be modified are (1) by changes in relative beam weights and/or (2) by changes in the direction and/or magnitude of the dose gradient of one or more of the individual beams. Conventional wedges provide a simple mechanism for altering the dose gradient of a single beam. This paper describes a mathematical basis for the selection of wedge angles, wedge orientations, and relative beam weights, with the goal of producing a field of zero gradient over the volume of beam intersection. The approach is based on 3‐dimensional vector analysis of dose gradients, and is valuable not only for its formalism, but also for the conceptual basis it provides for discussing and solving general wedge selection problems.
Equivalent squares of irregular photon fields
Authors: Tatcher Morris , Bjärngard Bengt E.
Published: March 2003

The tables of equivalent fields published by the British Journal of Radiology (BJR) are intended for calculation of depth‐dose functions in rectangular photon fields. We have investigated the validity of the equivalent‐field concept for fields of arbitrary shape over a range of photon energies, field sizes and depths. We show that the empirical scatter‐radius function (Day function) used to generate the equivalent‐field tables is a good approximation to the average over energy of normalized scatter‐air ratios extracted from BJR beam data for depths up to 10 cm. However, this function tends to diverge from the data as depth increases. Accuracy can be improved by making the Day function depend on depth. Equivalent squares, determined by sector integration of the original or modified Day functions, are suitable for megavoltage photon‐beam dose calculations at central‐axis and off‐axis points in irregular as well as rectangular fields.
An evaluation of the EGS4 and CYLTRAN Monte Carlo codes with regard to boundary beta‐ray dosimetry by comparison with experimental beta‐ray dose backscatter factors
Authors: Nunes Josane , Prestwich William V. , Kwok Cheuk S.
Published: March 2003

Beta‐ray dose backscatter factors or dose ratios at planar soft‐tissue boundaries were calculated using the EGS4/PRESTA and CYLTRAN (version 2.1) Monte Carlo codes and these data were compared with experimental results. Since the beta‐ray source was 32P, this work addressed the transport of, and energy deposition by, electrons less energetic than 2 MeV. In particular, the simulations targeted the codes' performances with regard to the transport of low energy electrons across material boundaries and the backscattering of low energy electrons. In general, backscatter factors calculated at 7.25 mg/cm2 from several soft‐tissue interfaces agreed with experimental values to within about five percent. CYLTRAN was also used to calculate the variation of backscatter factor with distance from aluminum/soft‐tissue and air/soft‐tissue interfaces and was found to reproduce the shapes of experimental backscatter factor depth profiles.
An artificial neural network for estimating scatter exposures in portable chest radiography
Authors: Lo Joseph Y. , Floyd Carey E. , Baker Jay A. , Ravin Carl E.
Published: March 2003

An adaptive linear element (Adaline) was developed to estimate the two‐dimensional scatter exposure distribution in digital portable chest radiographs (DPCXR). DPCXRs and quantitative scatter exposure measurements at 64 locations throughout the chest were acquired for ten radiographically normal patients. The Adaline is an artificial neural network which has only a single node and linear thresholding. The Adaline was trained using DPCXR‐scatter measurement pairs from five patients. The spatially invariant network would take a portion of the image as its input and estimate the scatter content as output. The trained network was applied to the other five images, and errors were evaluated between estimated and measured scatter values. Performance was compared against a convolution scatter estimation algorithm. The network was evaluated as a function of network size, initial values, and duration of training. Network performance was evaluated qualitatively by the correlation of network weights to physical models, and quantitatively by training and evaluation errors. Using DPCXRs as input, the network learned to describe known scatter exposures accurately (7% error) and estimate scatter in new images (<8% error) slightly better than convolution methods. Regardless of size and initial shape, all networks adapted into radial exponentials with magnitude of 0.75, perhaps implying an ideal point spread function and average scatter fraction, respectively. To implement scatter compensation, the two‐dimensional scatter distribution estimated by the neural network is subtracted from the original DPCXR.
Computer‐aided diagnosis: Detection and characterization of hyperparathyroidism in digital hand radiographs
Authors: Chang Chair‐Li , Chan Heang‐Ping , Niklason Loren T. , Cobby Mark , Crabbe Jeffery , Adler Ronald S.
Published: March 2003

An automated method is developed for the detection and staging of skeletal changes due to hyperparathyroidism on digitized hand radiographs. Subperiosteal bony resorption, particularly along the radial margins of the middle and proximal phalanges, is among the earliest manifestations of secondary hyperparathyroidism. In order to quantify the severity of bone resorption in these regions, the computer method analyzes the roughness of the phalangeal margins, as projected on the radiograph. The regions of interest, which contain the phalanges, are obtained from the digitized hand radiographs by an image preprocessor. The radial margin of each phalanx is detected by a model‐guided boundary‐tracking scheme. The roughness of these boundaries is then quantified by the mean‐square variation and the first moment of the power spectrum. A receiver operating characteristic (ROC) study comparing the computer detection of hyperparathyroidism with the diagnosis by three experienced skeletal radiologists was performed by evaluating 84 hand images from 22 patients. Our present computer method can achieve a true‐positive rate of 94% and a true‐negative rate of 92%. Such a computer‐aided diagnosis system may assist radiologists in their assessment of primary and secondary hyperparathyroidism, since it is both accurate and not subject to either intra‐ or interobserver variations.
A novel algorithm for the edge detection and edge enhancement of medical images
Authors: Crooks I. , Fallone B. G.
Published: March 2003

A novel algorithm, histogram shifting (HS) is presented, which performs edge detection or edge enhancement with the choice of two parameters. The histogram of a region surrounding each pixel is found and translated toward the origin, resulting in the new pixel value. Images from a variety of medical imaging modalities were processed with HS to perform detection and enhancement of edges. Comparison with results obtained from conventional edge detection (e.g., Sobel) and with conventional edge‐enhancement algorithms is discussed. HS appears to perform the edge‐detection operation without introducing “double‐edge” effects often obtained with conventional edge‐detection algorithms. HS also appears to perform edge enhancement without introducing extensive noise artifacts, which may be noticeable with many edge‐enhancement algorithms.
Constraints of the multiple‐scattering theory of Molière in Monte Carlo simulations of the transport of charged particles
Authors: Andreo Pedro , Medin Joakim , Bielajew Alex F.
Published: March 2003

The breakdown of Molière's multiple‐scattering theory for short pathlengths occurring during Monte Carlo simulations with charged particles is demonstrated. It has been found that in certain conditions where the theory is assumed to be valid, significant distortions of the angular distribution occur that make the sampling of the polar angle questionable in numerous steps of Monte Carlo simulations. The limits of the theory have been investigated, both using a large number of terms in the Molière's series and using steps of Molière's theory where 1/B expansions are not involved. At B=4.5 the commonly accepted 3‐term series expansion yields differences up to ±6% compared with the evaluation of the complete Molière angular distribution, and up to 7 terms in the series are needed in order to achieve agreement within ±2%. One percent agreement requires B=10. Numerical values of the full distribution are given in terms of Molière's parameters B and reduced angle ϑ. By using the general dependence of the distribution results are valid for both electron and proton Monte Carlo simulations in any material.
Automatic generation of beam apertures
Authors: Brewster Linda , Mageras G. S. , Mohan Radhe
Published: March 2003

In order to specify arbitrarily shaped beam apertures for three‐dimensional radiation treatment planning, aperture contours (or outlines) are often manually drawn using a beam's eye view display of the target volume and nearby normal structures. This can be a very time consuming process, and can be impractical for multileaf collimation and computer‐aided optimization of a large number of fields. A method has been developed that allows automatic generation of aperture shapes that outline the target volume and may spare neighboring structures whenever desired. Margins of user‐specified sizes (positive or negative) around the target and normal structures are also incorporated. For a chosen beam orientation, a 3D surface of each anatomic structure of interest is formed and projected onto a plane at the beam's isocenter. The outlines of each projected object are detected by an edge following algorithm, and margins are added. The outlines of normal structures are combined with that of the target volume to obtain the final aperture shape. This is done by overlaying filled versions of the outlines in such a way that regions of the target overlapped by normal structures are cut away, leaving only the target volume region to be irradiated. The remaining target volume outline is again detected to produce an aperture contour. Normal structures may split the aperture into several pieces, so this method detects any number of disjoint aperture contours. The results of the algorithm are illustrated with apertures generated for nasopharynx and prostate tumors, including sparing of normal tissues.
Measurement of absolute blood iodine concentration during digital subtraction ventriculography
Authors: Fuerst Thomas P. , Van Lysel Michael S.
Published: March 2003

A system to measure the absolute iodine concentration (mg iodine/milliliter blood) in the left ventricle (LV) during digital subtraction ventriculography has been developed. The technique uses a catheter to draw blood from the LV through a detection cell. This occurs as the iodine bolus passes through the heart. The cell determines iodine concentration by measuring x‐ray attenuation as the iodinated blood passes between a low‐power x‐ray tube and a diode detector. In vitro and in vivo testing of the system was conducted. The system response was linear (r=0.99) with respect to iodine concentration. This response was independent of hematocrit. Dispersion of contrast medium in the catheter caused distortion of the shape of the time–concentration curve. The impulse response of the system was measured and found to be independent of hematocrit. A correction algorithm based on Wiener filter deconvolution was developed. In vitro testing using simulated time–concentration curves demonstrated that the rms error in the iodine measurement after dispersion correction did not exceed 4% over the region of the time–concentration curve extending from the peak to the point on the tail where the signal fell to 50% of its peak value. Cardiac output was measured from the time–concentration curve via the indicator‐dilution method in an animal model. This cardiac output measurement (COI) agreed closely with cardiac output measured simultaneously with an aortic flow probe (COP), namely, COI=1.02 COP−0.03 L/min, r=0.95, SEE=10%, p<0.001. The application of the blood iodine concentration measurement to the densitometric measurement of absolute LV volume from digital subtraction angiography (DSA) images is discussed.
Comparison of ROC and AFC methods in a visual detection task
Authors: Herrmann Clemens , Buhr Egbert , Hoeschen Dietmar , Fan Su‐Yue
Published: March 2003

The visual observer performance for detecting low‐contrast patterns by employing both the receiver operating characteristic (ROC) and the 9‐alternative forced choice (9‐AFC) method is investigated. For each observer and each experimental technique, the results were reproducible with respect to a repeated observation. When the results of different observers for each method are compared, a strong variation of values is found. From the ROC data obtained, the expected percentages of correct detection in a 9‐AFC experiment is calculated and compared with the corresponding values of the actual 9‐AFC experiment. For some observers, the latter values were significantly higher than the computed ones, whereas for others the results showed good agreement with predictions from theory.
Erratum: “Demonstration of megavoltage and diagnostic x‐ray imaging with hydrogenated amorphous silicon arrays” [Med. Phys. 19, 1455–1466 (1992)]
Authors: Antonuk Larry E. , Boudry John , Huang Weidong , McShan Daniel L. , Morton Edward J. , Yorkston John , Longo Michael J. , Street Robert A.
Published: March 2003

The images appearing in Fig. 12(a) and (b) were interchanged. The correctly labeled Fig. 12(a) and (b) along with the captions appear here.
A sensitive noninvasive analysis of Pt in external tissues. Followup of Pt deposition following cisplatin treatment
Authors: Gorodetsky Raphael , Vexler Akiva , Mou Xiaode , Kaufman Bella , Loewenthal Elia
Published: March 2003

Noninvasive analysis of heavy elements in external tissues by diagnostic‐x‐ray spectrometry (DXS) is presented. Pt can be detected accurately with sensitivity below 1 μg/g wet weight of tissue. In the present paper the possibility to monitor Pt accumulation and clearance in the external tissues of cancer patients treated with cisplatin [Cis‐diamminedichloro‐platinum (II)‐cDDP] chemotherapy is reported. The DXS method is based on x‐ray fluorescence analysis. Heavy elements in the small skin area of interest are analyzed by their excitation with a monochromatic soft x‐ray beam of 14.6 KeV. Spectral L lines of heavy metals such as Pt are detected with minimum interference by other elements in the tissues. Skin Pt levels up to about 6 μg/g were observed following several courses of cDDP treatment. The Pt seemed to be homogeneously distributed in different skin areas with similar levels in the dermis and epidermis. The rate of clearance of Pt from the skin (50% in about 30 days) was slower by three orders of magnitude than its clearance from plasma. Further studies may use DXS to establish the accurate kinetics of Pt deposition and clearance in tissues of cDDP treated patients, as well as the exact relation between tissue Pt levels and the development of the drug related late complications.
Effects of x‐ray tube‐voltage ripple and current mode on patient dose
Authors: Kubota Hideaki , Ozaki Yoshiaki , Matsumoto Masao , Kanamori Hitoshi
Published: March 2003

This paper proposes a radiographic sensitivity attenuation curve for screen‐films systems. This curve expresses the relative inverse value of mAs value needed to give the same photographic density. This curve decreases more slowly with increasing object thickness than the exposure attenuation curve generally used hitherto. By using the radiographic sensitivity attenuation curve and the exposure attenuation curve, the voltage‐ripple dependence of mAs values and patient dose for the same photographic density was obtained. As predicted theoretically in a previous paper [Med. Phys. 18, 921 (1991)], it was verified quantitatively the reverse order that larger ripple values resulted in lower patient dose and lower mAs value for the unsaturating current mode at large ripple. This reverse order clearly appeared in two‐pulse units. These phenomena were experimentally verified. As a result it was found that in radiographing with a two‐pulse unit, larger mA value results in lower patient dose.
Mechanisms of signal loss in magnetic resonance imaging of stenoses
Authors: Gatenby J. C. , McCauley T. R. , Gore J. C.
Published: March 2003

Some of the factors affecting the signal losses that occur in magnetic resonance images at a stenosis or other region of complex flow have been evaluated. The important determinants of dephasing within a volume element are the net gradient moments, which can be kept small even at long echo times. For compact gradient wave forms, the echo time by itself is unimportant and does not affect signal losses. Reducing the fraction of echo sampled is an alternate method to velocity compensation for reducing gradient moment dephasing that keeps higher moments small. The effects of reducing the fraction of echo sampled on the signal losses in flow distal to a stenosis have been measured experimentally. Another source of signal loss at a stenosis is the variation of the mean phase within a volume element that occurs for flow that varies from one phase encoded view to another. Changes in flow behavior between acquisitions lead to signal displacement in the image. These view to view changes have also been quantified.
Long term study of random noise and signal uniformity in spin‐echo brain imaging
Authors: Chandra Ramesh , Rusinek Henry
Published: March 2003

Random noise and MR signal uniformity were analyzed for a period of 9 months using two head coils on a 1.5 T commercial imager. Signal response from a cylindrical phantom filled with a 0.1 mmol/L CuSO4 solution was incorporated in a correction scheme and the effect of correction on uniformity and accuracy of tissue volume determination was measured. There was little change in random noise (CV<5%) and image uniformity (CV<15%) over the time of the study. The uniformity in a 130 cm2 region was 4% for a double saddle coil and 5.3% for a mirror coil. After correction, uniformity was improved to 1.1% for the double saddle coil and 1.2% for the mirror coil. In a 40 cm2 central region the uniformity was approximately three times better than in the 130 cm2 region. A second phantom, mimicking the relaxation times of CSF and the brain, consisted of saline encapsulated in vials of volume ranging from 35.6 ml to 249.6 ml and placed in a 0.1 mmol/L solution of MnCl2. Uniformity correction reduced the average error in volume measurement from 8.6% to 6.1%.
A theoretical analysis using ratios of the major elements measured by neutron activation analysis to derive total body water, protein, and fat
Authors: Sutcliffe J. F. , Smith A. H. , Barker M. C. J. , Smith M. A.
Published: March 2003

A theoretical analysis based upon Reference Man is used to derive the water, protein, and fat components of soft tissue using the ratios of carbon/oxygen and nitrogen/hydrogen determined by in vivo neutron activation analysis and measurement of body mineral content by dual energy x‐ray absorptiometry. The standard deviations in the estimates of water, protein, and fat are 0.58, 0.59, and 0.53 kg, respectively, if the reproducibility of measurement of the carbon/oxygen ratio is 2.5% and of the nitrogen/hydrogen ratio is 5%, the accuracy of measurement of bone mineral is 4%, and estimate of glycogen is 0.5 kg. New neutron activation technology may permit sufficiently precise measurements.
Implementation of the ETAR method for 3D inhomogeneity correction using FFT
Authors: Yu Cedric X. , Wong John W.
Published: March 2003

The equivalent tissue‐air‐ratio (ETAR) method employs three‐dimensional (3D) CT pixel information to approximate scatter dose contribution for inhomogeneity correction. In general, the calculation provides better agreement with measurements than the one‐dimensional (1D) methods typically used in commercial treatment planning systems. In its original implementation, the 3D formulation of the ETAR method is modified empirically as a 2D calculation in order to reduce computation time. The modification compromises the use of the method in several treatment geometries. An examination of the ETAR formulation shows that the calculation can be expressed as a convolution and thus can be performed in 3D using fast Fourier transform (FFT) techniques. The algorithm has been implemented as a 3D FFT convolution. Making use of the symmetric properties of the FFT, the new approach shows significant savings in computation time without excessive memory requirement. Despite its fundamental limitations when applied to regions of electronic disequilibrium, the ETAR method offers a practical solution to improving current dose calculation in 3D treatment planning, particularly when the more advanced scatter ray‐tracing dose calculation algorithms remain impractical for clinical use. Recent work to extend the method to approximate electron transport is also encouraging.
Pressure wave generated by the passage of a heavy charged particle in water
Authors: Sun Y. Y. , Nath Ravinder
Published: March 2003

Energy deposition around the trajectories of ionizing particles with linear energy transfer (LET) of 4, 40, and 400 keV/μm in water and subsequent diffusion of deposited heat is calculated using computational fluid dynamics. Immediately after the deposition of energy by the charged particle, the temperature and pressure in the vicinity of the particle track both increase dramatically, leading to the formation of a thermal spike and a pressure wave. Initially, the region of heat deposition is primarily localized to a region called the “thermal core,” which has dimensions of 0.3, 1, and 3 nm for particles with LETs of 4, 40, and 400 keV/μm, respectively. Instantaneous peak temperatures within the thermal core were 800 °C–2000 °C and peak pressures were about 25 000 atm. This sudden deposition of heat in a localized region leads to a very strong shock wave around the particle trajectory, which is shown to last for a duration of 10−9–10−8 s. Even at distances beyond 10 nm away from the particle trajectory, pressures above 100 atm could exist for a duration of up to 10−11 s. This local and transient environment, created by the passage of a charged particle in a medium, may lead to new mechanisms of radiation action leading to cell damage, as well as to the development of new radiation detectors.
Application of fast simulated annealing to optimization of conformal radiation treatments
Authors: Mageras G. S. , Mohan R.
Published: March 2003

Applications of simulated annealing to the optimization of radiation treatment plans, in which a set of beam weights are iteratively adjusted so as to minimize a cost function, have been motivated by its potential for finding the global or near‐global minimum among multiple minima. However, the method has been found to be slow, requiring several tens of thousands of iterations to optimize 50 to 100 variables. A technique to improve the efficiency for finding a solution is reported, which is generally applicable to the optimization of continuous variables. In previous applications of simulated annealing to treatment planning optimization, only one or two weights are varied each iteration. This approach is to change all weights simultaneously, using random changes that are initially large to coarsely sample the cost function, then are reduced with iteration to probe finer structure. The performance of different methods are compared in optimizing a plan for treatment of the prostate, in which the search space consists of 54 noncoplanar beams and the cost function is based on tumor control and normal tissue complication probabilities. The proposed method yields solutions with similar values of the cost function in only a fraction of the iterations compared either to a fixed single weight adjustment technique, or to a method which combines the Nelder and Mead downhill simplex with simulated annealing.
Verification of radiotherapy treatments: Computerized analysis of the size and shape of radiation fields
Authors: Leszczynski Konrad W. , Shalev Shlomo , Gluhchev Georgi
Published: March 2003

An automated technique has been developed for the verification of treatment field size and shape in external beam radiation therapy. Portal images from film or digital on‐line imaging system are analyzed, and basic parameters are derived to describe the field size and shape from the contour points on the field boundary. The initial set of parameters included length of the perimeter, area, aspect ratio, and orientation angle. The parameters found for the actual field in the portal image are compared against those calculated for the prescribed field and any discrepancies indicated to the operator. The accuracy of the field parametrization scheme has been tested on a number of on‐line portal images with varying fields. The relative error did not exceed a few percent in perimeter and area or 2° in the angle, which should be sufficiently low for the detection of major errors in field shaping.
X‐ray surface dose measurements using TLD extrapolation
Authors: Kron Tomas , Elliot Andrew , Wong Tony , Showell Greg , Clubb Bryce , Metcalfe Peter
Published: March 2003

Surface dose measurements in therapeutic x‐ray beams are of importance in determining the dose to the skin of patients undergoing radiotherapy. Measurements were performed in the 6‐MV beam of a medical linear accelerator with LiF thermoluminescence dosimeters (TLD) using a solid water phantom. TLD chips (surface area 3.17×3.17 cm2) of three different thicknesses (0.230, 0.099, and 0.038 g/cm2) were used to extrapolate dose readings to an infinitesimally thin layer of LiF. This surface dose was measured for field sizes ranging from 1×1 cm2 to 40×40 cm2. The surface dose relative to maximum dose was found to be 10.0% for a field size of 5×5 cm2, 16.3% for 10×10 cm2, and 26.9% for 20×20 cm2. Using a 6‐mm Perspex block tray in the beam increased the surface dose in these fields to 10.7%, 17.7%, and 34.2% respectively. Due to the small size of the TLD chips, TLD extrapolation is applicable also for intracavity and exit dose determinations. The technique used for in vivo dosimetry could provide clinicians information about the build up of dose up to 1‐mm depth in addition to an extrapolated surface dose measurement.