bluuebunny/crossref_metadata_2025 huggingface dataset:
A prototype high‐purity germanium detector system with fast photon‐counting circuitry for medical imaging
Authors: Hasegawa Bruce H. , Stebler Benno , Rutt Brian K. , Martinez Alberto , Gingold Eric L. , Barker Craig S. , Faulkner Kenneth G. , Cann Christopher E. , Boyd Douglas P.
Published: March 2003

A data‐acquisition system designed for x‐ray medical imaging utilizes a segmented high‐purity germanium (HPGe) detector array with 2‐mm wide and 6‐mm thick elements. The detectors are contained within a liquid‐nitrogen cryostat designed to minimize heat losses. The 50‐ns pulse‐shaping time of the preamplifier electronics is selected as the shortest time constant compatible with the 50‐ns charge collection time of the detector. This provides the detection system with the fastest count‐rate capabilities and immunity from microphonics, with moderate energy resolution performance. A theoretical analysis of the preamplifier electronics shows that its noise performance is limited primarily by its input capacitance, and is independent of detector leakage current up to approximately 100 nA. The system experimentally demonstrates count rates exceeding 1 million counts per second per element with an energy resolution of 7 keV for the 60‐keV gamma ray photon from 241Am. The results demonstrate the performance of a data acquisition system utilizing HPGe detector systems which would be suitable for dual‐energy imaging as well as systems offering simultaneous x‐ray transmission and radionuclide emission imaging. Key words: high‐purity germanium detectors, data acquisition electronics, x‐ray radiography, radionuclide imaging
Physical performance characteristics of spiral CT scanning
Authors: Kalender Willi A. , Polacin Arkadiusz
Published: March 2003

CT scanning in spiral geometry is achieved by continuously transporting the patient through the gantry in synchrony with continuous data acquisition over a multitude of 360‐deg scans. Data for reconstruction of images in planar geometry are estimated from the spiral data by interpolation. The influence of spiral scanning on image quality is investigated. Most of the standard physical performance parameters, e.g., spatial resolution, image uniformity, and contrast, are not affected; results differ for pixel noise and slice sensitivity profiles. For linear interpolation, pixel noise is expected to be reduced by a factor of 0.82; reduction factors of 0.81 to 0.83 were measured. Slice sensitivity profiles are changed as a function of table feed d, measured in millimeters per 360‐deg scan; they are smoothed as the original profile is convolved with the object motion function. The motion function is derived for linear interpolation that constitutes a triangle with a base line width of 2d and a maximal height equal to 1/d. Calculations of both the full width at half‐maximum and the shape of the profiles were in good agreement with experimental results. The effect of the widened profiles, in particular of their extended tail ends, on image quality is demonstrated in phantom measurements.
Calibration of Mg2SiO4(Tb) thermoluminescent dosimeters for use in determining diagnostic x‐ray doses to Adult Health Study participants
Authors: Kato Kazuo , Antoku Shigetoshi , Sawada Shozo , Russell Walter J.
Published: March 2003

Characteristics of Mg2SiO4(Tb) thermoluminescent dosimeters (TLD) were ascertained preparatory to measuring doses from diagnostic x‐ray examinations received by Adult Health Study participants. These detectors are small, relatively sensitive to low‐dose x rays, and are appropriate for precise dosimetry. Extensive calibration is necessary for precisely determining doses according to their thermoluminescent intensities. Their sensitivities were investigated by dose, according to x‐ray tube voltage, and by exposure direction, to obtain directional dependence. Dosimeter sensitivity lessened due to the fading effect and diminution of the planchet. However, these adverse effects can be avoided by storing the dosimeters at least 1.5 h and by using fresh silver‐plated planchets. Thus the TLDs, for which sensitivities were determined in this study, will be useful in subsequent diagnostic x‐ray dosimetry.
The use of importance sampling techniques to improve the efficiency of photon tracking in emission tomography simulations
Authors: Haynor David R. , Harrison Robert L. , Lewellen Thomas K.
Published: March 2003

Monte Carlo simulations are widely used to study the transmission and scattering of γ rays. Use of this method for simulations of emission tomographs suffers from geometric inefficiency resulting from the low solid angle of acceptance of most tomograph designs. We have applied several importance sampling techniques—stratification, forced detection, and weight control through Russian roulette and splitting—to increase the computational efficiency of the Monte Carlo method 10‐ to 300‐fold. A description of these techniques, their validation, and sample performance results are given. Application of importance sampling methods makes it practical to study photon scattering in heterogeneous attenuators on workstations and minicomputers.
Elements of microdosimetry
Authors: Rossi Harald H. , Zaider Marco
Published: March 2003

This is an introductory tutorial to the field of microdosimetry. Following a brief historical outline, we review the physics of experimental and theoretical microdosimetry and the application of microdosimetry to radiation biology within the so‐called “site” approximation. Modern interpretations of microdosimetry that make use of methods developed in integral geometry are described under the heading “structural microdosimetry;” this also illustrates the conceptual basis of dual radiation action. A discussion concerning some future prospects of this field concludes the review.
A prototype beam delivery system for the proton medical accelerator at Loma Linda
Authors: Coutrakon G. , Bauman M. , Lesyna D. , Miller D. , Nusbaum J. , Slater J. , Johanning J. , Miranda J. , DeLuca P. M. , Siebers J. , Ludewigt B.
Published: March 2003

A variable energy proton accelerator was commissioned at Fermi National Accelerator Laboratory for use in cancer treatment at the Loma Linda University Medical Center. The advantages of precise dose localization by proton therapy, while sparing nearby healthy tissue, are well documented [R. R. Wilson, Radiology 47, 487 (1946); M. Wagner, Med. Phys. 9, 749 (1982); M. Goitein and F. Chen, Med. Phys. 10, 831 (1983)]. One of the components of the proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume in the patient. To this end, a prototype beam delivery system was tested during the accelerator's commissioning period. The beam delivery system consisted of a beam spreading device to produce a large, uniform field, a range modulator to generate a spread out Bragg peak (SOBP), and various beam detectors to measure intensity, beam centering, and dose distributions. The beam delivery system provided a uniform proton dose distribution in a cylindrical volume of 20‐cm‐diam area and 9‐cm depth. The dose variations throughout the target volume were found to be less then ±5%. Modifications in the range modulator should reduce this considerably. The central axis dose rate in the region of the SOBP was found to be 0.4 cGy/spill with an incident beam intensity of 6.7×109 protons/spill. With an accelerator repetition rate of 30 spills/min and expected intensity of 2.5×1010 protons/spills for patient treatment, this system can provide 50 cGy/min for a 20‐cm‐diam field and 9‐cm range modulation. The distal edge of the spread out Bragg peak was observed at 27.5‐cm depth with an incident proton energy of 235 MeV. The dose at the distal edge falls from 90% to 10% of peak value in 7 mm. The 90%–10% penumbras as small as 6 mm were measured at 3‐cm depth without range shifter material in the beam and as large as 15 mm when measured at 25‐cm depth with 10 cm of range shifter placed in the beam. The unmodulated Bragg curve had a peak to entrance dose ratio of 3, while the peak to entrance dose ratio for the 9‐cm spread out Bragg peak was observed to be only 1.3.
Time resolved properties of acoustic pulses generated in water and in soft tissue by pulsed proton beam irradiation—A possibility of doses distribution monitoring in proton radiation therapy
Authors: Tada JunIchiro , Hayakawa Yoshinori , Hosono Katsuhisa , Inada Tetsuo
Published: March 2003

Time‐resolved acoustic pulses generated in water and in soft tissue by pulsed proton beam irradiation were observed. The spatial resolution of depth dose distribution in the clinically applied beam intensity is estimated about 3 mm by means of TOF measurement. The dependence of the acoustic signal intensity on the temperature of medium was examined. Proportionality of acoustic pulse intensity to absorbed dose per pulse was confirmed as well. These results suggest the possibility of clinical application to monitor dose distribution in the patient's body during irradiation of pulsed proton beam.
The effect of patient motion on dose uncertainty in charged particle irradiation for lesions encircling the brain stem or spinal cord
Authors: Daftari Inder , Petti Paula L. , Collier John M. , Castro Joseph R. , Pitluck Samuel
Published: March 2003

A specialized charged‐particle radiotherapy technique developed at Lawrence Berkeley Laboratory (LBL) is applied to patients with lesions abutting or surrounding the spinal cord or brain stem. This technique divides the target into two parts, one partially surrounding the critical structure (brain stem or spinal cord) and a second excluding the critical structure and abutting the first portion of the target. Compensators are used to conform the dose distribution to the distal surface of the target. This technique represents a novel approach in treating unresectable or residual tumors surrounding the spinal cord or brain stem. Since the placement of the patient with respect to beam‐shaping devices is critical for divided‐target treatments, a method for calculating dose distributions reflecting random patient motion is proposed, and the effects of random patient motion are studied for two divided‐target patient examples. Dose‐volume histograms and a normal‐tissue complication probability model are used in this analysis. For the patients considered in this study, the normal‐tissue‐complication probability model predicts that random patient motion less than or equal to 0.2 cm is tolerable in terms of spinal cord complications.
Transfer function measurement and analysis for a magnetic resonance imager
Authors: Mohapatra Shashadhar M. , Turley Jeff D. , Prince John R. , Blechinger Joseph C. , Wilson Don A.
Published: March 2003

The transfer function characteristics of a 1.5T imager have been determined. An edge response function (ERF) was obtained from a water/Plexiglas interface at various pixel widths ranging from 0.312 to 1.0 mm. An SE pulse sequence was used with a 5‐mm transaxial slice. The ERF was smoothed, differentiated, and Fourier transformed to obtain MTF curves. The LSF was analyzed for skewness and kurtosis. The area under the MTF amplitude curves and the equivalent bandpass were calculated. All ERFs, LSFs, and MTFs were well behaved. The resulting LSF was Gaussian. All calculated MTFs had cutoff frequencies slightly less than the theoretical Nyquist limit. The MTF calculated from the theoretical Gaussian LSF is slightly superior to that calculated from experimental data and provides an upper limit to the MTF. Spatial resolution in our MR imager is dominated by the pixel size via the Nyquist sampling theorem. System performance is slightly less than theoretically predicted, possibly due to image processing algorithms during the reconstruction process.
Accurate characterization of image intensifier distortion
Authors: Rudin Stephen , Bednarek Daniel R. , Wong Roland
Published: March 2003

Image intensifier distortion due to photocathode curvature and electron optics is shown to be approximated by a simple two parameter odd‐power polynomial. The accuracy of this fit was found to be far better than that of two other one parameter characterizations of distortion when applied to experimental data from four different model image intensifiers ranging in diameter from 9 to 14 in. The standard errors of the two parameter fits were less than 0.1 mm or 0.03% of the field of the IIs and were within the estimated measurement error.
Survey of tissue–air ratio data: A review of the literature for diagnostic radiology
Authors: Hale John
Published: March 2003

I have compared the four sets of tissue–air ratio (TAR) data in the literature as a basis for recommending data to include in the revision of the Medical Physics Data Book, NBS Handbook 138 [Edited by T. N. Padikal and S. P. Fivozinshy (U.S. Department of Commerce, Washington, DC, 1982)].
Heating patterns of microwave applicators in inhomogeneous arm and thigh phantoms
Authors: Chou C. K. , McDougall J. A. , Chan K. W. , Luk K. H.
Published: March 2003

Heating rate (HR) patterns in cylindrical structures were studied with inhomogeneous limb phantoms. These phantoms, arm and thigh models consisting of fat, bone, and muscle material, were heated with Clini‐Therm L, M, and MS applicators at 915 MHz. The thigh model is 18 cm in diameter with 2.65‐cm‐thick fat on the outside and a 4‐cm‐diam bone in the center. The arm model is 9 cm in diameter with 1.35‐cm‐thick fat and 2‐cm‐diam bone. All models are 29 cm long with phantom muscle in the space between fat and bone and were heated with their long axes parallel or perpendicular to the E field. HR patterns in the transverse and longitudinal planes were obtained thermographically. A large water bolus, with the water channels parallel to the E field, was used in every case. In the thigh model, maximum heating was in the muscle for both L and M applicators when the E field was parallel to the long axis. When it was perpendicular, the maximum heating occurred in the fat layer. However, the peak HR in the fat remained about the same for both E field orientations. For the small applicator, heating was mostly in the fat, and the two field orientations did not cause much difference in the heating pattern. In the arm model, the maximum heating of all three applicators occurred mostly in the muscle for both E‐field orientations. However, the maximum HR was reduced by a factor of 2 to 3 when the E field was perpendicular rather than parallel to the long axis. These results indicate that placing the applicators with the E field parallel to the long axis of cylindrical structures can optimize muscle heating, minimize required power, and reduce stray radiation.
Corner transmission in several linear accelerator photon beams
Authors: Prasad Satish C. , Bassano Daniel A.
Published: March 2003

Maximum x‐ray field sizes on many linear accelerators are obtained only with truncated corners. Transmissions through the corners of such fields has been measured utilizing film and ion chamber dosimetry for a number of accelerators. Transmissions are found to be significantly larger than for the movable jaws.
Evaluation of an iron‐filtered epithermal neutron beam for neutron‐capture therapy
Authors: Musolino S. V. , McGinley P. H. , Greenwood R. C. , Kliauga P. , Fairchild R. G.
Published: March 2003

An epithermal neutron filter using iron, aluminum, and sulfur was evaluated to determine if the therapeutic performance could be improved with respect to aluminum–sulfur‐based filters. An empirically optimized filter was developed that delivered a 93% pure beam of 24‐keV epithermal neutrons. It was expected that a thick filter using iron with a density thickness >200 g/cm2 would eliminate the excess gamma contamination found in Al–S filters. This research showed that prompt gamma production from neutron interactions in iron was the dominant dose component. Dosimetric parameters of the beam were determined from the measurement of absorbed dose in air, thermal neutron flux in a head phantom, neutron and gamma spectroscopy, and microdosimetry.
Dosimetric investigations in the dose buildup using bolus techniques for fast neutron therapy
Authors: Hess Axel , Schmidt Rainer , Thom Michael
Published: March 2003

A moulded bolus is commonly used to compensate for contour irregularities of patients treated with fast neutrons. The bolus material should have the same attenuation characteristics as tissue, should be easily mouldable and sufficiently solid for multiple applications. For fast neutron beams, the mixture of equal parts of beeswax and paraffin is a suitable bolus material. As in photon beams the bolus, when placed directly on the patient skin, removes completely the skin sparing effect. However, in neutron beams the dose buildup can mostly be recovered by lining the bolus surface in contact with the patient with a thin layer of lead. This paper presents dosimetric investigations in the dose buildup of a collimated 14‐MeV DT‐neutron beam underneath the surface of a solid phantom behind a 1.0‐cm layer of bolus material lined with 0.03 cm of lead. It is shown that most of the dose buildup of the bolus‐free neutron beam can be recovered.
Calibration of 192Ir high‐dose‐rate afterloading systems
Authors: Goetsch Steven J. , Attix F. H. , Pearson D. W. , Thomadsen B. R.
Published: March 2003

A method is described for calibration of 192Ir high‐dose‐rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma‐ray spectrum of iridium‐192, an interpolation procedure is employed, using calibrations above (137Cs, 662 keV) and below (250 kVcp, 146‐keV x rays) the exposure‐weighted average 192Ir energy of 397 keV. The same total wall+cap thickness must be used for both calibrations, and for the 192Ir measurements. A wall+cap thickness of 0.3 g/cm2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source–chamber distance and room scatter during the source calibration in inverse‐square‐law geometry. A new well‐type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated 192Ir source, and offers a simple means for verifying the decay rate and for calibrating 192Ir replacement sources.
Superposition on a multicomputer system
Authors: Murray David C. , Hoban Peter W. , Round W. Howell , Graham Ian D. , Metcalfe Peter E.
Published: March 2003

Superposition (convolution using a noninvariant kernel) has been shown to be a highly promising technique for use in calculating dose distributions in radiotherapy treatment planning. However, one major difficulty that currently prevents use in routine planning is the computational effort required to perform the calculation in three dimensions. To help solve this problem the superposition technique has been implemented on a parallel processor multicomputer in order to examine the performance characteristics of such a system. Up to eight elements have been connected in a pipeline (linear array), and tree networks of three and seven processors have also been constructed (using INMOS T800 transputers). The significant results obtained with these networks are: (1) Both topologies provide near‐linear speedup with increasing processor number (8 processors provide 7.81 times the computing power of a single processor when using an optimal communication packet size); (2) increasing communication packet size from 1 voxel to an optimum of approximately 40 voxels significantly reduces communication overhead per processor. Overhead per processor for a 7‐element linear array is 6.9% when using 1‐voxel packets, but only 1.8% when using 40‐voxel packets; (3) the topology of the network has some effect on communication overhead: Arranging 7 processors in a 1–2–4 binary tree reduces overhead to 80.1% of that encountered using a 7‐element linear array (with packet size of 1 voxel).
3‐D dose‐volume compensation using nonlinear least‐squares regression technique
Authors: Weeks K. J. , Sontag M. R.
Published: March 2003

A method for external beam dose‐volume optimization is presented. The Gauss–Marquardt nonlinear least‐squares regression technique is applied to compensator design and determination. The dose distribution (uniform or otherwise) desired throughout a volume is specified. Compensators optimized to produce the necessary variation of beam intensity across the surface of each beam are simultaneously determined for all the beams. Solutions for homogeneous dose, homogeneous target dose, and restricted dose to exterior target volume structures, and inhomogeneous target dose cases are presented. Dependence of the results on the number of parameters as well as the role of degree of desirability weighting is explained and illustrated via examples. Discussion of the significance and limitations of this optimization method is also presented.
Design of beam‐modulating devices for charged‐particle therapy
Authors: Petti P. L. , Lyman J. T. , Renner T. R. , Castro J. R. , Collier J. M. , Daftari I. K. , Ludewigt B. A.
Published: March 2003

The computer modeling program used to design beam‐modulating devices for charged‐particle therapy at Lawrence Berkeley Laboratory has been improved to allow a more realistic description of the beam. The original program used a single calculated Bragg peak to design the spread Bragg peak. The range of this curve was shifted so that Bragg curves of varying ranges could be superimposed. The new version of the program allows several measured Bragg curves with different ranges to be used as input, and interpolates between them to obtain the required data for the superposition calculation. The experimental configuration for measuring these input curves simulated therapy conditions. Seven beam‐modulating propellers with spread Bragg‐peak widths ranging from 2.2 to 14.4 cm were designed and constructed for a 215‐MeV/u helium beam using this new design program. Depth‐dose distributions produced by these new propellers were in good agreement with predicted distributions, and these propellers are currently being used clinically.
Radiosurgery with unflattened 6‐MV photon beams
Authors: O'Brien P. F. , Gillies B. A. , Schwartz M. , Young C. , Davey P.
Published: March 2003

One of the major drawbacks to doing stereotactic radiosurgery with a linear accelerator is the long time required to deliver the target dose. Single fractions of 25 Gy delivered at the isocenter and at depth in the skull may require beam times in excess of 15 min for a typical linear accelerator with a maximum dose rate of 250 cGy/min in tissue. In an effort to decrease the treatment time for this technique, the flattening filter has been removed from an AECL Therac‐6 linear accelerator and the characteristics of the resulting beam have been measured. Flatness is acceptable for the field sizes used with this technique and the dose rate is increased by a factor of 2.75.
Low‐contrast focal lesion detectability phantom for 1H MR imaging
Authors: Madsen Ernest L. , Blechinger Joseph C. , Frank Gary R.
Published: March 2003

A phantom made from tissue‐mimicking materials is reported for testing 1H MRI systems regarding their ability to detect small low‐contrast focal lesions and to delineate the boundaries of larger lesions. Two sets of seven spherical simulated lesions with diameters of 2, 3, 4, 5, 6.3, 7.9, and 9.5 mm have T1 and T2 values somewhat higher than the corresponding values in the surrounding simulated normal tissue. Relaxometer determinations of T1 and T2 for the simulated normal tissue yielded 955 and 106 ms, respectively, at 22 °C and 1 T. The corresponding values in set ♯1 of the simulated lesions were 1017 and 175 ms and in set ♯2 were 1002 and 127 ms. These T1 and T2 values are similar to those for brain and brain lesions but are too high to represent soft tissue such as liver and muscle. The centers of the 14 lesions are coplanar, and eight alignment devices surround them facilitating superposition of the scan plane and the plane containing the centers of the lesions. Illustrative images made with a 1.5‐T system are shown. All simulated lesions are detectable in T2‐weighted head coil images. Corresponding T1‐weighted images are also shown, the 2‐ and 3‐mm lesions of set ♯2 and the 2‐mm lesion of set ♯1 not being detectable. The phantoms could be useful for performance or acceptance testing and perhaps for quality assurance testing.
A linear array, scintillation crystal–photodiode detector for megavoltage imaging
Authors: Morton E. J. , Swindell W. , Lewis D. G. , Evans P. M.
Published: March 2003

An imaging device has been developed to acquire images during external photon‐beam radiotherapy treatments. It consists of a linear array of 128 zinc tungstate (ZnWO4) scintillation crystals each of which is individually optically coupled to a photodiode and associated electronics. The image is formed by scanning the linear array across the radiation field using a stepping motor under the control of a microcomputer. Image archive, display, and analysis are performed using a microVAX II computer. Results from a general theoretical analysis are presented before a detailed description of the particular detector construction. The mechanical design of the detector is such that the detector is automatically positioned to within a millimeter relative to the treatment source. This simplifies procedures for analyzing setup variations when comparing a treatment image to any other treatment, or planning, images. Image acquisition takes under 4 s with a contrast resolution of better than 1% at a spatial resolution of 2.5 mm in the object plane. The primary dose used to form these images is 0.55 cGy although the dose received by the patient will be closer to 25 cGy due to the linear scanning geometry and 3.8‐s scan time that is used.
Physical aspects of a liquid‐filled ionization chamber with pulsed polarizing voltage
Authors: van Herk M.
Published: March 2003

The charge collection in a liquid‐filled matrix ionization chamber system has been investigated. This system, which is used for megavoltage radiography, is scanned electronically by switching the polarizing voltage. In such a system the charge collection cannot be described by the classical ionization chamber theory since this is only valid for a fixed polarizing voltage. A model is presented for an ideal plane‐parallel ionization chamber with switched polarizing voltage that gives a good qualitative description of the physical phenomena in our nonideal system. The new model predicts the amount of charge collected and the signal‐to‐noise ratio as a function of several parameters such as electrode distance, polarizing voltage, radiation intensity, and some liquid characteristics. An important result is that the quantum noise contribution can be made quite small. This situation occurs in liquids with a low ion mobility and long ion lifetimes by a charge‐integrating effect in the liquid. Experiments were performed to test various aspects of the model. A reasonable agreement is found between theoretical and experimental results. The possible use of ultrahigh‐mobility liquids is also discussed.
Potential and limitations of invariant kernel conformal therapy
Authors: Boyer A. L. , Desobry G. E. , Wells N. H.
Published: March 2003

A treatment planning methodology was developed to investigate the invariant kernel form of conformal therapy proposed by Brahme. Three‐dimensional dose distributions were calculated by convolving a rotationally symmetric, invariant kernel with weighting distributions. Fourier transform convolution techniques implemented on an array processor were used to achieve high calculation speeds, thereby allowing iterative techniques in the spatial and frequency domains for computing dose distributions that asymptotically approach a desired dose distribution. To use rotationally symmetric kernels, the generality of the solution is traded for a fast, deterministic, inverse planning approach. The limitations imposed on the dose distributions by this loss of generality are characterized and tentative conclusions are drawn about the potentials and limits of clinical application of this form of the methodology. Further developments of the concept are suggested.
Dose perturbations at interfaces in photon beams: Annihilation radiation
Authors: Werner Barry L.
Published: March 2003

A model is presented for estimating the contribution of annihilation radiation to the dose perturbation at interfaces between high and low atomic number materials. The contribution is small, but not negligible relative to the total interface dose perturbation. The maximum contribution occurs for photon beams of about 8 MeV in energy. For an 8‐MeV beam passing first through lead, then through polystyrene, the annihilation radiation contribution to the interface dose perturbation is about 8%, at a copper/polystyrene interface, the contribution is about 7%, and at an aluminum/polystyrene interface, the contribution is about 3%.
Angular distribution of bremsstrahlung from 15‐MeV electrons incident on thick targets of Be, Al, and Pb
Authors: Faddegon Bruce A. , Ross C. K. , Rogers D. W. O.
Published: March 2003

Bremsstrahlung spectra from thick cylindrical targets of Be, Al, and Pb have been measured at angles of 0°, 1°, 2°, 4°, 10°, 30°, 60°, and 90° relative to the beam axis for electrons of 15‐MeV incident energy. The spectra are absolute (photons per incident electron) and have a 145‐keV lower‐energy cutoff. The target thicknesses were nominally 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20‐cm‐diam by 25‐cm‐long cylindrical NaI detector. Measured spectra were corrected for pile‐up, background, detector response, detector efficiency, attenuation in materials between the target and detector and collimator effects. Spectra were also calculated using the EGS4 Monte Carlo system for simulating the radiation transport. There was excellent agreement between the measured and calculated spectral shapes. The measured yield of photons per incident electron was 9% and 7% greater than the calculated yield for Be and Al, respectively, and 2% less for Pb, all with an uncertainty of ±5%. There was no significant angular variation in the ratio of the measured and calculated yields. The angular distributions of bremsstrahlung calculated using available analytical theories dropped off more quickly with angle than the measured distributions. The predictions of the theories would be improved by including target‐scattered photons.
Depth dose characteristics of 24‐MV x‐ray beams at extended SSD
Authors: Jani Shirish K. , Pennington Edward C.
Published: March 2003

Depth dose characteristics of 24‐MV beams from a medical linear accelerator at various SSDs have been measured. The dose buildup, percent depth dose (PDD), and output were measured using ionization chambers and a diode detector under full scatter. Surface dose ranged from 8% to 51% depending upon the collimator setting at 100‐cm SSD. It decreased by 10%–15% as distance was increased to 200 cm, but remained unchanged beyond this distance. The dmax migrated rapidly toward the surface with increasing field size at 100‐cm SSD. At extended SSD, the dmax occurred at greater depths and shifted a little toward the shallow depths with increasing field size. Large field PDDs measured at extended SSD agreed well with those predicted from 100‐cm data. The output followed the 1/(distance)2 relationship quite well. The extended SSD beam data are clinically useful when treating hemibody or performing total body irradiation (TBI) procedures.
Quantification of coronary arterial calcium by dual energy digital subtraction fluoroscopy
Authors: Molloi Sabee , Detrano Robert , Ersahin Atila , Roeck Werner , Morcos Charle
Published: March 2003

Clinical studies of the heart with fluoroscopy have shown that fluoroscopic visualization of calcium in the coronary arteries is strongly associated with coronary artery disease. However, fluoroscopic detection is limited by its low sensitivity, which is partly due to the interfering background tissue structures and image quantum noise. Moreover, quantification of the absolute amount of calcium in an arterial segment has not been possible. A real‐time dual‐energy subtraction technique has been investigated as a possible solution to the above problem. In this energy subtraction technique, the kVp and filtration are switched at 30 Hz. In order to assess the potential utility of this videodensitometric technique to quantitate coronary artery calcium, arterial phantoms and excised segments of diseased human arteries were imaged. The low‐ and high‐energy images were corrected for scatter and veiling glare before subtraction. Calcium measurements were made using the tissue‐suppressed energy‐subtracted images. The estimated calcium phosphate and ashed weights of the calcified arterial segments (N=20) were highly correlated (slope=1.04, Intercept =−0.33 mg, r=0.92).
Analysis of sample power loss in MRI gradient fields
Authors: Harpen Michael D.
Published: March 2003

A derivation of the power dissipation in a conducting spherical sample in a time‐dependent gradient magnetic field is presented and the predicted values are compared with those observed in an experimental apparatus consisting of a four‐element saddle coil and a Maxwell coil pair. The derived expressions for power dissipation are shown to agree with values obtained experimentally via Q measurements of the loaded coils. It is also shown the large rapidly fluctuating gradient fields encountered in echo planar imaging produce relatively small levels of absorbed power.
A numerical investigation of the dependence of NMR signal from pulsatile blood flow in CINE pulse sequences
Authors: Gao Jia‐Hong , Gore John C.
Published: March 2003

The Bloch equations have been solved using numerical techniques for a uniform fluid undergoing periodic pulsatile flow in an NMR imaging experiment. The magnetization and NMR signal have been calculated for experimental parameters appropriate for a CINE sequence (TR=40 ms, TE=14 ms) applied to the study of pulsatile aortic or other arterial flows. The flow velocity profile is obtained by Fourier superposition of different harmonics and it is shown that the steady‐state NMR signal has reduced high‐frequency components. There is also a time delay between peak signal intensity and flow because the backflow effects that can be as much as 100 ms. The apparent pulsatility depends on the NMR sequence parameters. Some limitations of the phase contrast flow‐imaging method are also discussed for nonuniform flow. Key words: magnetic resonance imaging, Bloch equations, pulsatile flow
Use of storage phosphor imaging plates in portal imaging and high‐energy radiography: The intensifying effect of metallic screens on the sensitivity
Authors: Barnea G. , Navon E. , Ginzburg A. , Politch J. , Roehrig H. , Dick C. E. , Placious R. C.
Published: March 2003

The sensitivity of storage phosphor imaging plates (SPIP) at megavolt photon energies (60Co, 6‐, 10‐, and 18‐MV radiotherapy beams) is studied both experimentally and by Monte Carlo radiation transport calculations. In addition, the same techniques are used to investigate the intensifying effect of metal screens on the sensitivity of the SPIP. The results provide evidence that the sensitivity of the SPIPs is proportional to the absorbed energy in the phosphor layer per cGy. The spectral sensitivity is calculated for photon energies between 10 keV and 20 MeV for various SPIP‐screen combinations.
Clinical implementation of AAPM Task Group 32 recommendations on brachytherapy source strength specification
Authors: Williamson Jeffrey F. , Nath Ravinder
Published: March 2003

Historically the strength of sealed brachytherapy sources has been described by many physical quantities, including true activity, apparent activity, and equivalent mass of radium. Recently, the AAPM Task Group 32 recommended that these quantities be replaced by a single quantity, air‐kerma strength, with units of μGy m2 h−1. A set of equations has been developed for unambiguously converting source strength estimates and renormalizing published dose‐rate tables, which assume traditional quantities and units, into forms consistent with air‐kerma strength. For commonly used brachytherapy sources, multiplicative conversion factors for each source‐strength formalism and set of units are given. To convert equivalent mass of radium to air‐kerma strength requires a single multiplicative factor, 7.23 μGy m2 h−1/mgRaEq, applicable to all sources. Based upon a review of vendor source specification practices, the factors for converting source strength of 198Au, 103Pd, and 125I seeds from apparent mCi to air‐kerma strength are 2.06, 1.29, and 1.27 μGy m2 h−1/mCi(apparent), respectively. These factors are independent of source geometry but depend on the nominal exposure rate constant value selected by the vendor. Conversion factors applicable to mass of radium or true activity depend upon both source geometry and radionuclide identity. Because many of these conversion factors depend upon vendor choices of physical constants and exposure rate constants, readers are cautioned to carefully review vendor source strength specification practices before adopting these values clinically. Finally, the relationships between the various source strength quantities and absorbed dose rate in the medium surrounding the source are elucidated.
A new method for characterizing beta‐ray ophthalmic applicator sources
Authors: Sayeg J. A. , Gregory R. C.
Published: March 2003

A technique is described which enables one to obtain detailed dose characteristics of 90Sr beta‐ray ophthalmic applicators. A radiochromic radiation detector which is a solid‐state solution of hexahydroxyethyl pararosaniline cyanide in a nylon polymer (i.e., thin foil), has been used to determine the surface dose rate and dose distribution of these sources. The detectors are rugged, easily handled, have an equivalent response (optical density per unit absorbed dose) to photons and electrons, and produce high‐resolution images. They have been found useful for this application due to the high surface dose rates [0.10–1.0 Gy (H2O)/s] and their low sensitivity (approximately 104 Gy for an optical density of 1.0). The foils have been evaluated on a He–Ne scanning laser densitometer with a resolution of 0.3 μm. Comparison with NIST (formerly NBS) extrapolation ionization chamber measurements indicates surface dose‐rate agreement within 6%. Spectral dosimetric characteristics are presented and discussed.
Polarity effect for various ionization chambers with multiple irradiation conditions in electron beams
Authors: Aget Hélène , Rosenwald Jean‐Claude
Published: March 2003

The effect of reversing the voltage polarity applied to an ionization chamber has been investigated in electron beams for several types of chambers and several irradiation conditions. It has been found that differences in readings can be significant for cylindrical chambers (about 10%) as well as for plane parallel chambers (20%). The effect is larger for large field sizes than small ones. It generally includes an appreciable stem and cable effect. Differences in readings with both polarities are related to the energy distribution of the electron beam and are greater for lower electron energies than higher. Polarity effect and charge deposit within the chamber wall material appear to be closely connected. This charge deposit, expressed as a proportion of the total collected charge, can be directly derived from double polarities measurements. Careful investigation of the effect should be made to avoid significant error (over 5%) in the determination of the absorbed dose. Key words: electrons, polarity effect, ionization chambers
Compensators for three‐dimensional treatment planning
Authors: Mageras G. S. , Mohan Radhe , Burman C. , Barest Glenn D. , Kutcher Gerald J.
Published: March 2003

Presented here is a method of designing compensators for a single beam or one or more pairs of beams, not necessarily parallel opposed. The objective is to produce a flat distribution in a plane that may be perpendicular to the central ray or may be an arbitrarily oriented plane, for example, a plane that bisects the hinge angle between two beams. The method takes into account not only surface irregularities but also tissue inhomogeneities, hinge angles between beams, distance from the source, and even “horns” in the beam. The design process employs convolution of Monte Carlo generated pencil beams with photon fluence distributions, appropriately modified for the presence of beam modifiers (blocks and compensators), to compute dose in a flat homogeneous phantom. Corrections for inhomogeneities and surface curvature are applied by using computerized tomography information to determine the effective path length through tissue. Multiple interations are used to arrive at a compensator that properly incorporates changes in radiation transport, and therefore dose distribution, resulting from the presence of beam‐shaping devices. In each iteration it is assumed that the required reduction in dose at a point can be achieved by reducing the fluence along the ray joining the source to computation point proportionately. The compensator design is represented as a finely spaced matrix of thickness values which is entered into a prorammable milling maching for fabrication. Dose measurements in phantom exposed to 6‐MV x rays with and without compensation are presented.
Critical examination of the uniformity requirements for single‐photon emission computed tomography
Authors: O'Connor M. K. , Vermeersch C.
Published: March 2003

It is generally recognized that single‐photon emission computed tomography (SPECT) imposes very stringent requirements on gamma camera uniformity to prevent the occurrence of ring artifacts. The purpose of this study was to examine the relationship between nonuniformities in the planar data and the magnitude of the consequential ring artifacts in the transaxial data, and how the perception of these artifacts is influenced by factors such as reconstruction matrix size, reconstruction filter, and image noise. The study indicates that the relationship between ring artifact magnitude and image noise is essentially independent of the acquisition or reconstruction matrix sizes, but is strongly dependent upon the type of smoothing filter applied during the reconstruction process. Furthermore, the degree to which a ring artifact can be perceived above image noise is dependent on the size and location of the nonuniformity in the planar data, with small nonuniformities (1–2 pixels wide) close to the center of rotation being less perceptible than those further out (8–20 pixels). Small defects or nonuniformities close to the center of rotation are thought to cause the greatest potential corruption to tomographic data. The study indicates that such may not be the case. Hence the uniformity requirements for SPECT may be less demanding than was previously thought.
The nature and subtlety of abnormal findings in chest radiographs
Authors: MacMahon Heber , Montner Steven M. , Doi Kunio , Liu Katherine J. M.
Published: March 2003

All detected abnormal findings were recorded for 1085 consecutive chest x‐ray examinations. Each finding was classified by descriptive criteria and graded for subtlety. Accuracy of interpretation was evaluated in a randomized sample of 100 cases using follow‐up or multiple readers. Seventy percent of standard examinations and 93% of bedside examinations revealed abnormal findings. Pulmonary infiltrates were the most commonly detected abnormality, being present in 55% of abnormal cases. Noncalcified pulmonary nodules and pneumothoraces were each present in approximately 5% of abnormal cases. The most commonly encountered subtle findings were due to intravenous catheters, pulmonary infiltrates, pneumothoraces, rib lesions, and pulmonary nodules in descending order of frequency. It is concluded that it is reasonable to use selected examples of these findings in observer tests when evaluating new imaging modalities such as digital radiography.
Use of a low ionization potential dopant in a kinestatic charge detector: Experimental findings
Authors: Wagenaar Douglas J. , DiBianca Frank A. , Tenney Charles R. , Reed Mark S. C. , Vance Joseph E.
Published: March 2003

The broadening of the line spread function (LSF) in the drift direction with increasing drift distance in the kinestatic charge detector is substantially reduced when small amounts (less than 1%) of trimethylamine [(CH3)3 N] are added to the x‐ray detection medium (krypton or xenon). The LSF of a mixture of Kr and 0.01% trimethylamine (TMA) was measured as a function of distance at 15, 25, and 35 atm absolute pressure. The full width at half‐maximum (FWHM) of the LSF was reduced from about 1.0 mm to less than 0.5 mm at a drift distance of 4.0 mm for the three pressures. The LSF's of mixtures of xenon and TMA at concentrations ranging from 0.0004% to 0.4% in one run and 0.06% to 4.0% in a second run were measured at a constant pressure of 20 atm. The FWHM of the LSF was reduced from 0.6 to 0.4 mm at 4.0 mm for the xenon measurements. The optimum concentration of TMA in Xe was found to be in the neighborhood of 0.1%. The use of TMA reduced the drift distance‐dependent LSF broadening to the level expected from ionic diffusion, space charge repulsion, and electric field nonuniformity, and it may be possible to reduce the 0.4‐mm FWHM plateau through the use of an improved Frisch grid design. Observation of negative charge carriers showed that electron attachment increases with increasing TMA concentration, although this could be caused by impurities in the TMA. The implications of these results are discussed in terms of extending the maximum drift distance attainable in a kinestatic charge detector.
Analysis and correction of imperfections in the image intensifier–TV–digitizer imaging chain
Authors: Boone John M. , Seibert J. Anthony , Barrett William A. , Blood Eric A.
Published: March 2003

Image intensifier–television–video digitizer (IITVD) systems are commonly used for digital planar image acquisition in radiology. However, the well‐known distortions inherent in these systems limit their utility in research and in some clinical applications where quantitatively correct images are required. Software correction techniques have been implemented which restore both the spatial and grey scale quantitative integrity, allowing IITVD systems to be used as analytical research tools. Previously reported and novel correction techniques were used to reduce veiling glare, pincushion distortion, dc bias, and residual shading effects. The results indicate that excellent quantitative integrity can be achieved when these straightforward artifact reduction techniques are employed.