bluuebunny/crossref_metadata_2025 huggingface dataset:
The direct determination of dose‐to‐water using a water calorimeter
Authors: Schulz R. J. , Wuu C. S. , Weinhous M. S.
Published: March 2003

A flexible, temperature‐regulated, water calorimeter has been constructed which consists of three nested cylinders. The innermost “core” is a 10×10 cm right cylinder made of glass, the contents of which are isolated from the environment. It has two Teflon‐washered glass valves for filling, and two thermistors are supported at the center by glass capillary tubes. Surrounding the core is a “jacket” that provides approximately 2 cm of air insulation between the core and the “shield.” The shield surrounds the jacket with a 2.5‐cm layer of temperature‐regulated water flowing at 5 l/min. The core is filled with highly purified water the gas content of which is established prior to filling. Convection currents, which may be induced by dose gradients or thermistor power dissipation, are eliminated by operating the calorimeter at 4 °C. Depending upon the power level of the thermistors, 15–200 μW, and the insulation provided by the glass capillary tubing, the temperature of the thermistors is higher than that of the surrounding water. To minimize potential errors caused by differences between calibration curves obtained at finite power levels, the zero‐power‐level calibration curve obtained by extrapolation is employed. Also the calorimeter response is corrected for the change in power level, and therefore thermistor temperature, that follows the resistance change caused by irradiation. The response of the calorimeter to 4‐MV x rays has been compared to that of an ionization chamber irradiated in an identical geometry. For nitrogen‐saturated water, the grand mean of the calorimeter‐to‐ion chamber (Cal/Ion) dose ratio for five experiments conducted over a period of six months is 1.006±0.001. Three experiments with oxygen‐saturated water in the same core yeild a Cal/Ion ratio of 0.991±0.001. These results are consistent with radiochemical models and refined experiments that used water saturated with various gases, and suggest that nitrogen‐saturated water calorimeters of the type described may be used to determine the dose‐to‐water without reference to radiation‐dependent parameters or recourse to corrections for thermal defects.
Advances in analytical techniques for neutron capture therapy: Thin layer chromatography matrix and track etch thin layer chromatography methods for boron‐10 analysis
Authors: Schremmer Jacalyn M. , Noonan Denise J.
Published: March 2003

A new track etch autoradiographic technique for quantitating boron‐10 containing compounds used for neutron capture therapy is described. Instead of applying solutions of Cs2B12H11SH and its oxidation products directly to solid‐state nuclear track detectors, diethylaminoethyl cellulose thin layer chromatography (TLC) plates are utilized as sample matrices. The plates are juxtaposed with Lexan polycarbonate detectors and irradiated in a beam of thermal neutrons. The detectors are then chemically etched, and the resultant tracks counted with an optoelectronic image analyzer. Sensitivity to boron‐10 in solution reaches the 1 pg/μl level, or 1 ppb. In heparinized blood samples, 100 pg boron‐10/μl are detectable. This TLC matrix method has the advantage that sample plates can be reanalyzed under different reactor conditions to optimize detector response to the boron‐10 carrier material. Track etch/TLC allows quantitation of the purity of boron neutron capture therapy compounds by utilizing the above method with TLC plates developed in solvent systems that resolve Cs2B12H11SH and its oxidative analogs. Detectors irradiated in juxtaposition to the thin layer chromatograms are chemically etched, and the tracks are counted in the sample lane from the origin of the plate to the solvent front. A graphic depiction of the number of tracks per field yields a quantitative analysis of compound purity.
Wobbler facility for biomedical experiments
Authors: Renner Timothy R. , Chu William T.
Published: March 2003

A system for spreading relativistic heavy ion beams into large uniform radiation fields has been developed. The charged particles passing through the system are deflected into azimuthally symmetric distributions which can then be superimposed to produce dose distributions as large as 30 cm in diameter with less than ±3.5% variation in uniformity.
Total attenuation coefficients and scattering phase functions of tissues and phantom materials at 633 nm
Authors: Flock Stephen T. , Wilson Brian C. , Patterson Michael S.
Published: March 2003

Measurements have been made of the total attenuation coefficient Σt and the scattering phase function, S(θ), of 632.8 nm of light for a number of animal model tissues, blood, and inert scattering and absorbing media. Polystryrene microspheres of known size and refractive index, for which Σt and S(θ) can be calculated by Mie theory, were used to test the experimental methods. The purpose of the study was to define typical ranges for these optical properties of tissues, as a contribution to the development of experimental and theoretical methods of light dosimetry in tissue, particularly related to photodynamic therapy of solid tumors. The results demonstrate that, for the representative tissues studied, the total attenuation coefficients are of the order of 10–100 mm−1, and that scattering is highly forward peaked, with average cosine of scatter in the range 0.6–0.97.
A stimulated echo artifact from slice interference in magnetic resonance imaging
Authors: Crawley A. P. , Henkelman R. M.
Published: March 2003

When multiple slices are imaged with a short time between slice acquisitions, a disturbing line artifact along the direction of frequency encoding is often seen across the center of the images. The artifact consists of alternating bright and dark pixel intensities. In this paper, we show that the artifact is due to slice interference, and is caused by stimulated echoes that are produced in the regions of overlap between slices. A theoretical analysis of the formation of these stimulated echoes leads to ways of reducing the artifact, which are verified experimentally. The artifact can be suppressed most conveniently by extending the duration of the read gradient beyond the sampling window.
Reduction of pulsed gradient settling time in the superconducting magnet of a magnetic resonance instrument
Authors: Jensen Dye J. , Brey William W. , Delayre Jean L. , Narayana Ponnada A.
Published: March 2003

The implementation of electronic compensation for eddy currents to reduce the gradient settling times on a superconducting magnet is described. Field plots inside the magnet indicate the importance of assessing the field changes at several positions in the magnet. The effect of an asymmetry between the gradient coil system and the cryostat is demonstrated. A modified compensation circuit is described to overcome this mechanical asymmetry.
Dosimetric aspects of a rotating beam splitter used in tangential field breast treatment
Authors: Jani Shirish K. , Pennington Edward C. , Vigliotti Antonio P. , Hussey David H.
Published: March 2003

A rotating beam splitter was designed and fabricated for use in treating tangential breast fields on an AECL Theratron‐80 cobalt teletherapy unit. Its dosimetric properties were studied using a 0.6‐cm3 Baldwin–Farmer ionization chamber with Keithley electrometer and a Scanditronix RFA‐3 three‐dimensional water phantom scanner with semiconductor detector. An aluminum plate, which held the semicircular rotating 5‐HVL (half‐value layer) lead block, extended to the phantom surface (80‐cm source–surface distance). The beam was blocked directly along the central axis and also at distances up to 7.5 mm off‐axis, corresponding to the projected extent of the 1.5‐cm‐diam source. The penumbra at the central ray and at each off‐axis point was measured at dmax and at 5‐cm depth in water. A reduction in the penumbra from 8 to about 2 mm for 20×20 cm2 beam was observed regardless of the off‐axis distance of the block. Isodose distributions obtained for various field sizes indicated that the percent depth doses of the split fields agree well with the equivalent squares of the irradiated field sizes. Output measurements in water and in air indicated that scatter from the aluminum plate more than compensates for the reduction in backscatter factor, due to the decrease in irradiated area when the beam splitter is used. Isodose curves in various planes were obtained at clinically useful rotational angles of the beam splitter. Computer generated isodose curves have been obtained that match the measured curves to be used in treatment planning.
Performance characteristics of a dual‐energy detector for digital scan projection radiography
Authors: Morgan Douglas R. , Sones Richard A. , Barnes Gary T.
Published: March 2003

An energy discriminating x‐ray detector has been developed for dual‐energy, scan projection digital radiography. The detector is comprised of a pair of x‐ray intensifying screen/linear photodiode arrays, aligned one behind the other. Energy discrimination is achieved by employing a low atomic number phosphor in the front screen and a high atomic number phosphor in the back screen. The x‐ray response, modulation transfer function, and defective quantum efficiency of the detector are reported along with the experimental methodology utilized for the measurements. Also presented is an analysis which indicates that in a typical patient's lung field, the detector can resolve the projected density (g/cm2) of a 3‐mm‐thick, 1‐cm2 area of bone to better than 1.5%.
Dosimetric study of total skin irradiation with a scanning beam electron accelerator
Authors: Sharma Subhash C. , Wilson David L.
Published: March 2003

The Therac 20 6‐MeV scanned electron beam may be used for partial or total skin therapy. The maximum field size at 1 m is 30×30 cm defined by a set of primary photon collimators in conjunction with secondary trimmers. We have studied electron beam profiles with and without trimmers at the nominal source–skin distance of 1 m versus extended distances of 3–5 m. We find that the trimmers limit the field size and add little to the beam uniformity at extended distances. Beam energy, dose distributions, and output factors at extended distances were measured for single and multiple field arrangements with and without trimmers. Beam parameters were measured after introducing a degrader that lowered the energy to 3.7 MeV.
Perturbation of the temperature distribution in microwave irradiated tissue due to the presence of metallic thermometers
Authors: Constable R. T. , Dunscombe P. , Tsoukatos A. , Malaker K.
Published: March 2003

To date, satisfactory thermal dosimetry during the clinical application of localized hyperthermia can only be achieved using invasive thermometry. However the presence of commonly used metallic thermometers, such as thermocouples, may lead to the distortion of the temperature field due to self‐heating of the probe under microwave irradiation. A computer simulation of the effect of this self‐heating on the steady‐state temperature distribution in plane‐microwave irradiated homogeneous tissue has been undertaken and the significance of the effect for clinical hyperthermia dosimetry is assessed. The results indicate that a distortion of the temperature field in the neighborhood of the thermometer by several degrees can occur under adverse conditions.
Practical aspects of transmission cord blocks in radiotherapy
Authors: Ezzell Gary A. , Orton Colin G. , Maughan Richard L. , Spicka James T. , Vitalis Tom J. , Herskovic Arnold M.
Published: March 2003

We present the advantages of using partial transmission cord blocks throughout treatment, as opposed to adding full‐thickness blocks near the end. Such blocks reduce the risk of block omission or mispositioning and require less total time for construction. We also present an argument for the existence of an optimal width for cord blocks used in mediastinal treatments. A figure of merit has been derived which quantifies the tradeoff between narrow blocks, which increase the variation in dose across the block shadow and the risk of positioning errors, and wide blocks, which may unnecessarily shield potential tumor sites. For 60Co, 4‐ and 10‐MV beams, the figure of merit peaks at block widths of 2.0–2.5 cm at the level of the cord. Effective transmission data for cord blocks constructed of cerrobend are given for those three beams. Quality assurance studies show that transmission through cerrobend blocks can be controlled to the required precision.
Using background subtraction for measuring the dose distribution of an electron pencil beam
Authors: McParland B. J.
Published: March 2003

The spread of a narrow electron beam with depth in a phantom has been measured in the past using either a magnetically focused beam or one that has been collimated. The former approach is usually impractical with therapy accelerators and the latter must contend with the bremsstrahlung photons and scattered electrons that are introduced into the beam. This technical report describes a simple technique in which data taken with a “closed collimator” are subtracted from those measured with the collimator “open.” Such a subtraction process removes much of the background contaminants.
An ultrasound window to perform scanned, focused ultrasound hyperthermia treatments of brain tumors
Authors: Tobias J. , Hynynen K. , Roemer R. , Guthkelch A. N. , Fleischer A. S. , Shively J.
Published: March 2003

A series of calculations and tests were performed on polyethylene, polystyrene, acrylic, and a commerically available polymethyl methacrylate cranioplastic to determine which material would best serve as an acoustical window in the skull. The purpose of the window is to transmit focused ultrasound to treat brain tumors with hyperthermia. Each material was evaluated based on its ability to transmit power and to protect the brain. The results revealed that, of the four materials tested, polyethylene transmitted the largest percentage of incident power and is the toughest and best suited material to protect the brain. Further physical tests showed that a polyethylene plate does not significantly distort the sound field. Finally, acute and chronic dog tests (supported by theoretical calculations) using the polyethylene as an acoustical window through the skull showed that it will not reach temperatures greater than the target hyperthermia temperature (42 °C) at the required intensity levels unless the tumor is near the skull and the skin surface is not cooled. Since polyethylene effectively transmits power without distorting the sound field or overheating in dog tests, it may now be used in Phase 1 clinical untrasound hyperthermia tests on human patients. However, the chronic animal studies indicated that the collagen which composes the dura thickens under the cranioplasty and absorbs increasingly greater amounts of power with time. A thick layer of organized fibrous tissue also formed on the external surface of the cranioplasty, filling in the cavity formed by the exterior surface of the cranioplasty and by the excision of the temporalis muscle. Since a similar phenomenon is likely to occur, at least between the cranioplasty and the dura mater, in human subjects, the region over the cranioplasty may have to be cooled to ensure that the human brain is not overheated.
Performance characteristics of a helical microwave interstitial antenna for local hyperthermia
Authors: Wu Andrew , Watson M. L. , Sternick E. S. , Bielawa R. J. , Carr K. L.
Published: March 2003

A helical microwave antenna has been designed to improve heat deposition by interstitial applicators used for clinical hyperthermia. Iso‐specific‐absorption‐rate (SAR) curves of the helical antenna as well as a conventional monopole antenna were measured and compared in both muscle and brain tissue phantoms. The heating pattern of the helical antenna is more uniform along the length of the antenna which has important implications for multiarray implant configurations.
Flow measurements with a high‐speed computed tomography scanner
Authors: Jaschke Werner , Gould Robert G. , Assimakopoulos Panayotis A. , Lipton Martin J.
Published: March 2003

A high‐speed computed tomography (CT) scanner with a scan time of 50 ms was used to measure flow in a phantom constructed to simulate both tissue and vessels. After a bolus injection of iodinated contrast medium, the phantom was scanned at a rate of up to 2 images/s. A gamma‐variate curve was fit to the time–density data obtained from the inlet and outlet, as well as from the tissue‐equivalent part of the phantom. Flow was then calculated using different curves and curve parameters according to the Stewart–Hamilton equation, the mean transit time, and a modification of the Sapirstein principle. Actual flow rates were assessed by timed sampling. The results demonstrated that high‐speed CT can measure flow accurately by all these methods. Application of high‐speed CT for flow measurements in experimental animals and patients is, therefore, promising. The limitations of each technique for clinical application are discussed.
High‐resolution digital x‐ray detector utilizing a discrete array of CdWO4 scintillators and a self‐scanned photodiode array
Authors: Fritz Steven L. , Cook Larry T.
Published: March 2003

A high‐resolution x‐ray detector array for x‐ray imaging is described. It consists of a discrete array of CdWO4 scintillators and a Reticon 1024S photodiode array with a fiber‐optic faceplate. The scintillator elements measure 0.099×3×4 mm and are bonded together with aluminum foil separators to minimize light crossover. Detective quantum efficiency (DQE) of individual detector channels was 0.38. Systematic variations between channels were substantial. With only a simple correction for between channel variations, DQE for the entire array was 0.04. Evaluation of the edge response function of the array showed that resolution is similar to a detector whose line spread function is Gaussian with a standard deviation of 0.172 mm.
Dosimetry of an endocavitory treatment system
Authors: Tobochnik Neal , Bramlet Roland , Sischy Ben
Published: March 2003

The dosimetric properties of an x‐ray applicator system for the treatment of rectal carcinoma by endocavitory contact therapy are discussed. Half‐value layer, percent depth dose, field flatness, and output dose rate were evaluated. Film was used to evaluate field flatness and ionization chambers were used to evaluate the other quantities. The methods of measurement and their results are discussed.
Static multileaf collimator for fast‐neutron therapy
Authors: Chu James C. H. , Bloch Peter
Published: March 2003

This paper describes a multileaf collimator, designed to obtain irregularly shaped neutron fields, that fits into an existing fixed field‐size collimator system. The shape of the desired portal field is obtained by appropriately setting the position of each leaf of the collimator. The collimator consists of 40 polyethylene leaves to produce fields of irregular shape of up to 400 cm2. The leaves are interlaced with grooves to prevent neutron leakage. The measured penumbra defined by the multileaf collimator is similar to that defined by the same length fixed field‐size water extended polyester (WEP) collimator. The multileaf collimator has a lower neutron transmission due to its higher hydrogen concentration.
Geometrical properties of a digital beam attenuator system
Authors: Hasegawa Bruce H. , Dobbins James T. , Naimuddin Shaikh , Peppler Walter W. , Mistretta Charles A.
Published: March 2003

A digital beam attenuator system has been developed to automatically generate patient‐specific compensating filters for chest radiography. An initial low‐dose test image is used to generate the attenuator, which is fabricated by overprinting multiple layers of a heavy‐metal material onto a nonattenuating substrate. The attenuator is subsequently inserted into the x‐ray beam for a final compensated radiograph. The effects of focal spot blurring and limited attenuator resolution result in the final compensated image containing only high‐spatial frequency information. The frequency response of the process is not strictly describable by a modulation transfer function, but an approximation of the frequencies remaining in the compensated image is obtained for low‐contrast conditions. It is found that a 4×4 blurring function on the original 64×64 test image is required for the attenuator to give appropriate compensated image appearance. A proposed attenuator printing scheme prints the attenuator in a 16×16 matrix, staggering successively printed layers to achieve the required 64×64 sampling with appropriate blurring. The resulting compensated image has good anatomical definition and contains a frequency response similar to that obtained by compensation techniques being investigated by Plewes and Sorenson.
Scatter‐glare correction using a convolution algorithm with variable weighting
Authors: Naimuddin Shaikh , Hasegawa Bruce , Mistretta Charles A.
Published: March 2003

Several investigators have approximated the scatter component of digital radiographic images by applying a constant weighting factor to a convolved version of the detected image. The scatter approximation is then subtracted from the detected image. When a constant weighting factor is used, the scatter in areas of low transmission is underestimated. We have extended this technique by allowing for a spatially variable weighting factor based on the local image intensity. This technique improves the scatter estimate and should provide better results for general videodensitometric applications.
Correction of scatter in computed tomography images of bone
Authors: Hangartner T. N.
Published: March 2003

A cylindrical aluminum/Plexiglas phantom representing trabecular bone surrounded by various amounts of cortical bone was constructed. Measurements of this phantom using a computed tomography scanner with a 125I photon source demonstrated errors of 0% to 28% in the density of trabecular bone. Two contributing factors are identified: scatter and exponential edge‐gradient effect. A simple first‐order correction is developed to correct for the scatter‐induced error. Relative to the exponential edge‐gradient effect, which contributes up to 3.4% error over the range of cortical thicknesses measured, the correction procedure reduces the scatter‐induced error to a level of −0.66% to +0.61%. The consistency of the optimized correction parameters with the physical model as well as the effect of scatter measured by the same phantom on a GE 8800 scanner are shown.
Optimization of readout rate of television cameras in pulsed digital radiography
Authors: Rowlands J. A.
Published: March 2003

Adaptation of existing fluoroscopic television rooms to permit radiographs to be recorded directly by the television system would be a very practical way to introduce digital radiography into general use if the image quality was adequate. The main limitation of current systems is spatial resolution. Although a digital system can change the spatial frequency response by application of spatial frequency enhancement, the amount of enhancement which can be usefully applied is a function both of the spatial frequency response of the whole system and image noise. Slow scan television systems have been proposed as a way to reduce noise; and hence, as a way to potentially improve limiting resolution. We examine all the consequences of changing readout scan rate, both theoretically and with measurements on a commercial camera with a lead oxide image tube. It is found that recent improvements in camera tube design, specifically the increase in current capability of the electron gun, have reduced the advantages theoretically available with slow scan. However, an order of magnitude increase in useful dynamic range is still possible.
Use of fast Fourier transforms in calculating dose distributions for irregularly shaped fields for three‐dimensional treatment planning
Authors: Mohan Radhe , Chui Chen‐Shou
Published: March 2003

In three‐dimensional radiation treatment planning, essentially all fields are irregular and compensated. Consequently, it is important to predict accurately dose for such fields to ensure adequate coverage of the target region and sparing of healthy tissues. Traditional approaches, namely, those involving scatter integration and extended source and those utilizing negatively weighted fields, are inaccurate, especially near the boundaries defined by blocks and collimators. In the method presented in this paper, dose distributions for arbitrarily shaped beams are calculated by two‐dimensional convolution of the relative primary photon fluence distributions and kernels representing the cross‐sectional profiles of a pencil beam at a series of depths. The pencil beam dose distributions are computed, once and for all, with the Monte Carlo method for photon energy spectrum for each treatment machine. The finite size of the source, which is important for cobalt machines, is also taken into account using convolution of the source with the relative primary fluence distribution. Convolutions are performed using fast Fourier transforms on an array processor. Results of calculations are in excellent agreement with measured data. While no data are presented for fields modified by compensators, the method of calculation should apply at least as well for such fields since the variations in fluence distribution for compensated fields are not as sharp as for points near the block boundaries.
Detection efficiency of a high‐pressure gas scintillation proportional chamber
Authors: Fahey Frederic H. , Zimmerman Robert E. , Judy Philip F. , Lanza Richard C.
Published: March 2003

The detection efficiency of a high‐pressure, gas scintillation proportional chamber (GSPC), designed for medical imaging in the 30–150 keV energy range, has been investigated through measurement and Monte Carlo simulation. Measurements were conducted on a GSPC containing 4 atm of pure xenon separated from a hexagonal array of seven ultraviolet‐sensitive photomultiplier tubes by 1.27‐cm‐thick fused‐silica windows. Experimental measurements of the photopeak efficiency, fluorescence escape efficiency, and the energy collection efficiency were obtained. Results were also obtained for different photon energies and different values of temporal resolution. The measurements were compared with the results obtained from a Monte Carlo simulation designed specifically for investigating the imaging of low‐energy photons (below 150 keV) with a gas‐filled detector. The simulation was used to estimate photopeak efficiency, fluorescence escape efficiency, photopeak‐to‐fluorescence escape peak ratio, quantum interaction efficiency, energy collection efficiency, and local energy collection efficiency. The photopeak efficiency of the GSPC relative to that of a 3‐in. (7.62‐cm)‐thick sodium iodide crystal was measured to be 0.284±0.001 at 60 keV and 0.057±0.001 at 140 keV. Of the 60‐keV photons incident upon the detector, 70%±4% interacted in the detector, with 28%±1% being in the photopeak, as estimated both by experimentation and through the simulation. The maximum energy collection efficiency was found to be 65% at 60 keV, with 46% being deposited within 0.2 cm of the initial photon interaction. The information gained from this study is being used to design an optimized detector for use in specialized nuclear medicine studies.
Broad beam and narrow beam attenuation in Lipowitz's metal
Authors: El‐Khatib Ellen E. , Podgorsak Ervin B. , Pla Conrado
Published: March 2003

Attenuation properties of Lipowitz's metal have been studied for narrow and broad beams of cobalt‐60 gamma rays and 4–10 MV x rays. The measured transmitted fraction for geometries used in radiotherapy depends on the field size and depth of measurement. Therefore a calculation of dose for partially attenuated beams based on narrow beam attenuation coefficients can cause large errors in dosimetry. Our simple calculation of transmitted fractions based on primary attenuation and scattered radiation agrees quite well with the measured data for therapeutic geometries. Also given is a table for linear, mass attenuation, and mass energy absorption coefficients of Lipowitz's metal in the photon energy range from 10 keV to 10 MeV.
Photon dose perturbations due to small inhomogeneities
Authors: Yu Cedric X. , Wong John W. , Purdy James A.
Published: March 2003

An apparatus capable of measuring small fractional changes in ionization current has been used to study the effect of small inhomogeneities on photon dose in water. Small ring‐shaped inhomogeneities were introduced into a water phantom and measurements have been made for 4‐, 6‐, and 18‐MV x‐rays. The results show (1) beyond the range of secondary electrons, the dose perturbation is basically a photon transport phenomenon which becomes less important as the beam energy increases; (2) within the range of secondary electrons, dose perturbation also involves electron transport, which has a strong dependence on atomic number and could result in a substantially large effect on dose deposition.
Characteristics of the 6‐MV photon beam produced by a dual energy linear accelerator
Authors: Ikoro Nnaemeka C. , Johnson Donald A. , Antich Peter P.
Published: March 2003

Clinical dosimetry data are presented for the lower‐energy x‐ray beam of a Varian Clinac 1800 linear accelerator. This beam has comparable characteristics to single energy linear accelerators with the same stated 6‐MV x‐ray energy. The nominal beam energy was found to be 5.3±0.3 MV on the central axis. Beam quality expressed in terms of half‐value layer in water was found to vary by less than 10% over the entire field. The surface doses are only slightly, but consistently, larger than those reported in the literature for other 6‐MV linacs. Dosimetric results presented include central axis percentage depth dose (% DD) and tissue–maximum ratio (TMR), surface and buildup doses, output factors, and inverse square law applicability. The flatness and symmetry characteristics are within the manufacturer's specifications for both large and small fields.
Therapy imaging: A signal‐to‐noise analysis of metal plate/film detectors
Authors: Munro P. , Rawlinson J. A. , Fenster A.
Published: March 2003

We have measured the modulation transfer functions [MTF ( f )'s] and the noise power spectra [NPS( f )] of therapy x‐ray detectors irradiated by 60Co, 6‐ and 18‐MV radiotherapy beams. Using these quantities, we have calculated the noise‐equivalent quanta [NEQ( f )] and the detective quantum efficiency [DQE( f )] to quantitate the limitations of therapy detectors. The detectors consisted of film or fluorescent screen–film combinations in contact with copper, lead, or tungsten metal plates. The resolution of the detectors was found to be comparable to fluorescent screen–film combinations used in diagnostic radiology, however, the signal‐to‐noise ratio [SNR( f )] of the detectors was limited due to film granularity. We conclude that improved images can be obtained by using alternative detector systems which have less noise or film granularity.
Determination of the kerma factors in tissue‐equivalent plastic, C, Mg, and Fe for 14.7‐MeV neutrons
Authors: Wuu C. S. , Milavickas L. R.
Published: March 2003

Microdosimetric measurements were made with tissue‐equivalent plastic, (TEP), C‐, Mg‐, and Fe‐walled proportional counters filled with propane‐based tissue equivalent (TE) gas and Ar gas and irradiated with 14.7‐MeV neutrons. A theoretical model was used for the analysis of energy deposition in spherical detectors. An effective average mass stopping‐power ratio and a W correction were calculated to convert the gas ionization to the kerma in the wall material. The neutron fluence at the position of microdosimetric measurements was determined with an associated particle chamber mounted with surface barrier detectors. The experimental measurements along with the calculated correction factors yielded kerma factors of 0.660×10−8 cGy cm2 for TEP, 0.219×10−8 cGy cm2 for C, 0.122×10−8 cGy cm2 for Mg, and 0.479×10−9 cGy cm2 for Fe. The estimated uncertainties are 8.0% for TEP, 10.5% for C, and 9.3% for Mg and Fe.
Properties of the 18‐MV photon beam from a dual energy linear accelerator
Authors: Johnson Donald A. , Ikoro Nnaemeka C. , Chang Cheng‐Hui , Scarbrough Edward C. , Antich Peter P.
Published: March 2003

The beam characteristics of the 18‐MV photon beam of a Varian Clinac® 1800 are presented. The clinically relevant parameters of central axis depth dose, tissue‐phantom ratios, peak‐scatter factors, and relative output factors are discussed and compared to 18‐MV beam data previously reported. The nominal beam energy was found to be 18.3±0.8 MV on the central axis. The beam symmetry and uniformity meet the manufacturer's specifications. The inverse‐square law is applicable within 1.4% over the clinically useful range of distances and field sizes. An empirical fit equation for the central axis depth dose is presented.
Monte Carlo simulation of a cobalt‐60 beam
Authors: Han K. , Ballon D. , Chui C. , Mohan R.
Published: March 2003

We have used the Stanford Electron Gamma Shower (EGS) Monte Carlo code to compute photon spectra from an AECL Theratron 780 cobalt‐60 unit. Particular attention has been paid to the careful modeling of the geometry and material construction of the cobalt‐60 source capsule, source housing, and collimator assembly. From our simulation, we conclude that the observed increase in output of the machine with increasing field size is caused by scattered photons from the primary definer and the adjustable collimator. We have also used the generated photon spectra as input to a pencil beam model to calculate the tissue–air ratios in water and compared it to a model which uses a monochromatic photon energy of 1.25 MeV.
A graphite transmission ionization chamber
Authors: Austerlitz C. , Sibata C. H. , de Almeida C. E.
Published: March 2003

A pancake‐type transmission chamber made of high‐purity graphite and open to the atmosphere has been designed and constructed at the Secondary Standard Dosimetry Laboratory (SSDL‐Rio de Janeiro). Tests performed on the chamber following the International Electrotechnical Commission recommendations indicate that its performance characteristics are comparable to those expected from a secondary standard ionization chamber.
Scatter estimation for a digital radiographic system using convolution filtering
Authors: Love L. Alan , Kruger Robert A.
Published: March 2003

The use of a convolution‐filtering method to estimate the scatter distribution in images acquired with a digital subtraction angiography (DSA) imaging system has been studied. Investigation of more than 175 convolution kernels applied to images of anthropomorphic head, chest, and pelvic phantoms using 15‐, 25‐, and 36‐cm fields of view (digitized onto a 512×512 pixel image matrix) showed that two‐dimensional exponential kernels with a full width at half maximum (FWHM) of 50–150 pixels best reproduced the scatter fields within these images with a root‐mean‐square percentage error from 4% to 8%. A two‐dimensional exponential kernel with a FWHM of 75 pixels in each dimension applied to ten different anatomic presentations and fields of view, resulted in an average root‐mean‐square percentage error of 6.6% for the ten cases studied. The method should be implementable using an array of small lead beam stops placed in the field of only a single mask image and the above described convolution kernel applied to both mask and postopacification images. The mask beam‐stop data are used to scale both mask and postopacification convolution‐filtered images. This scaled, convolution‐filtered image is then subtracted from the original image to produce a largely scatter‐corrected image.
Cardiac computed tomography using redundant‐ray prospective gating
Authors: Moore Stephen C. , Judy Philip F.
Published: March 2003

A fourth‐generation, computed tomographic (CT) scanner, equipped for prospectively gated cardiac imaging, was modified to control the scan data acquisition by using knowledge of the location of redundant rays in the sinogram. In conventional prospective gating, a computer monitors the electrocardiogram (ECG) and calculates when to initiate the next scan in a gated series in order to acquire all 360° of projection data for a desired phase of the cardiac cycle. However, in each scan of a series, every projection ray is measured twice (when the positions of the source and detector are reversed). Redundant‐ray prospective gating takes advantage of this information to improve the efficiency of data acquisition. Using a heart phantom “beating” at 90 min−1, images of all phases of the cardiac cycle with 100‐ms temporal resolution were obtained in four scans with redundant‐ray gating; whereas a four‐scan series with conventional prospective gating yielded worse images of 170‐ms resolution.
Generalized conservation equation for multicompartmental systems
Authors: Adler Ronald S.
Published: March 2003

A projection operator technique is used to derive an equation for local tissue tracer content Q(t), assuming linear multicompartmental kinetics for tracer utilization. The resulting equation has the form (d/dt) Q(t)=FCa(t) −ΛQ(t)dτ [ξ(τ)Ca(t−τ)−ψ(τ)Q(t−τ)], where F and Ca(t) denote local blood flow and concentration of tracer, respectively. Tissue complexity is contained within the new parameters Λ, ξ(t), and ψ(t), where the time dependent coefficients are expressed as sums of exponentials. Two simple applications are considered: tissue heterogeneity and internal trapping of tracer. The relationship to effective single compartmental analysis, as is used for local cerebral blow flow determination, is evaluated.
The variability of clinical thermoluminescent dosimetry systems: A multi‐institutional study
Authors: Amols H. I. , Weinhous M. S. , Reinstein L. E.
Published: March 2003

Thirty‐two radiotherapy centers in the USA and Canada cooperated in a study of the variability of clinical thermoluminescent dosimetry (TLD) systems. The primary purpose of the survey was to ascertain the accuracy of TLD for the determination of in vivo dose measurements. Each participating institution provided two TLD packets for irradiation on a Clinac 4, at a prearranged time. Two batch irradiations were made. Thirty‐two TLD packets, one from each institution, were uniformly irradiated to a dose of 22.35 cGy (known by us, but not by the participants). A second group of 32 packets were likewise irradiated to a dose of 179.0 cGy. Participants were told only that their TLD's would be irradiated to doses between 10 and 50 cGy, and 100 to 200 cGy. TLD's were then returned to the institutions of origin for readout, and the doses reported to us for analysis. Calibration factors, readout and annealing procedures, etc., were all established independently by each participant. Although these procedures varied widely between institutions, the mean values of the reported doses were within 5% and 3% of the expected values for the low and high doses, respectively. Standard deviations in the reported doses were 10% and 5%. Also of interest, however, is the finding that 22% (i.e, 14 out of 64) of the dose reportings were in error by more than 10%. The implications of these findings vis à vis radiotherapy are discussed.
The influence of beam parameters on percentage depth dose in electron arc therapy
Authors: Pla Marina , Pla Conrado , Podgorsak Ervin B.
Published: March 2003

The dependence of rotational or arc electron beam percentage depth doses on the depth of isocenter di and nominal beam field width w is presented. A characteristic angle β, which uniquely depends on w and di, is defined and the dependence of the radial percentage depth doses on angle β discussed. It is shown that the characteristic angle β concept can be used in clinical situations to predict the shape of the percentage depth dose curve when w and di are known, or, more importantly, it can be used to determine the appropriate w when di and the percentage depth dose characteristics are known.
An algorithm for three‐dimensional visualization of radiation therapy beams
Authors: Reynolds R. A. , Sontag M. R. , Chen L. S.
Published: March 2003

A computer algorithm to display radiation beams superimposed on three‐dimensional (3‐D) views of patient anatomy has been developed. It may be implemented as a postprocessing step to existing software for 3‐D presentation and display. The algorithm takes as input a shaded 3‐D view (reconstructed, for example, from computed tomography scans), together with the associated depth map, and generates as output an enhanced 3‐D view highlighting in color the visible points which lie within the projected beam outlines. The algorithm is independent of the method used to generate the 3‐D view (surface or volume rendering techniques may be used) and is independent of beam shape (beams may be modified with shielding blocks). It is not restricted to external surfaces and will correctly show radiation beams projected onto cut‐away views of internal organs. The method is illustrated by application to a tangential pair for breast malignancy, using 3‐D views generated with volume rendering software.
Interface dose perturbation as a measure of megavoltage photon beam energy
Authors: Das Indra J. , Khan Faiz M. , Gerbi Bruce J.
Published: March 2003

The description of the quality of a photon beam has usually been characterized by a single value such as the half‐value layer, the effective attenuation coefficient, the percent depth dose, and most recently by the ionization ratio (IR). Although the IR is simple and easy to measure, it lacks sensitivity at photon energies above 10 MV. This paper describes a method based on dose perturbation at an interface and defines the forward dose perturbation factor (FDPF) as a measure of beam quality. Comparisons between the two methods are given for photon energies ranging from 60Co to 24 MV. The results show that the FDPF method is more sensitive to spectral changes at photon energies above 10 MV than the IR.
Electron backscatter corrections for parallel‐plate chambers
Authors: Hunt M. A. , Kutcher G. J. , Buffa A.
Published: March 2003

The wall of an ionization chamber is commonly assumed to have a negligible effect on chamber response in electron beams. For cylindrical chambers with thin walls this assumption is valid. However, parallel‐plate chambers commonly possess large mechanical supports which may affect chamber response in a manner not accounted for in current dosimetry protocols. This is due to changes with energy in the relative backscattered electron fluence between chamber support and phantom materials. To investigate this effect, electron backscatter from low atomic number materials has been measured with electrons from 6 to 20 MeV. The effect of the diameter and thickness of the backscattering material has also been studied. Based on these data, Lucite and polystyrene chambers in water phantoms are expected to underrespond by 1% and 2% at 6 MeV. The expected underresponse decreases to 0.8% and 0.4% for polystyrene and Lucite at 12 MeV and is insignificant above 16 MeV. Two commercially available parallel‐plate chambers were compared with a cylindrical chamber in electron beams from 6 to 20 MeV. Using the 20‐MeV intercomparison, the expected chamber responses at the lower energies were calculated and compared with measurements. Both parallel‐plate chambers underresponded by approximately 1% at 6 MeV and 0.5% at 9 MeV which is qualitatively consistent with the electron backscatter data. Recommendations for minimizing electron backscatter effects through chamber design are discussed.
Calculation of dose in homogeneous phantoms for partially attenuated photon beams
Authors: El‐Khatib Ellen , Podgorsak Ervin B. , Pla Conrado
Published: March 2003

Measured and calculated dose distributions under attenuators, which are of smaller cross‐sectional dimensions than the radiation field, are presented. The study was performed on a 4‐MV linac at a source–surface distance of 120 cm on the beam central axis in a water phantom for several thicknesses and cross sections of lead attenuators. Dose correction factors, which are used to multiply the open beam data to get dose distributions under partial attenuators, depend strongly on attenuator parameters and on depths in phantom. A method to calculate dose correction factors for any combination of attenuator parameters and any phantom depth is presented. The calculated dose distributions under partial attenuators agree well with measured data, which indicates that the method can be applied in clinical situations.
Dosimetry models for radioimmunotherapy
Authors: Langmuir Virginia K. , Sutherland Robert M.
Published: March 2003

Tumor therapy using radiolabeled antibodies presents a challenging problem in absorbed dose determination. The purpose of this study is to evaluate the effect of tumor size on the absorbed dose distribution from beta‐emitters when the radiolabeled antibody is not uniformly distributed throughout the tumor. Two theoretical dosimetry models are constructed, one for nonvascularized micrometastases and the other for vascularized tumors. All calculations assume no penetration of radionuclide into the tumor. These are compared to an even distribution of radionuclide throughout the tumor. In micrometastases of 1‐mm diameter or less, emitters of low energy such as 131I give higher dose rates than emitters of higher energy because less energy is lost outside the target volume. However, even with 131I, a significant proportion of the energy is not absorbed in the tumor and, as a result, the concentration of radionuclide necessary for a therapeutic radiation dose becomes higher as the tumor diameter gets smaller. Because it may be impossible to achieve these concentrations in very small tumors (<0.5‐mm diameter), alpha‐emitters may be useful in combination with beta‐emitters for therapy of micrometastatic disease. In vascularized tumors, higher energy emitters such as 90Y yield higher doses because of overlapping dose distributions from multiple vascular sources. This also produces a more even dose distribution across a tumor, even when there is poor penetration of the radiolabeled antibody. Thus tumor size, antibody penetration, and tumor vascularity all influence the choice of radionuclide and, depending on the circumstances, alpha‐emitters, low‐energy beta‐emitters, high‐energy beta‐emitters, or some combination of the three may be most efficacious.
Calculation of the dose delivered to organs outside the radiation beams
Authors: Francois P. , Beurtheret C. , Dutreix A.
Published: March 2003

An organ dose calculation based on measurements is described which models the total dose delivered outside a beam from 10 to 50 cm from the field edge. The dose distribution is parametrized for each beam energy as a function of depth, distance from the edge, and field size and shape. The algorithm also takes into account the organ location relative to the beam. Comparisons with measurements in an Alderson phantom using thermoluminescent dosimetry are given for various typical treatments.
The influence of ionization chamber and phantom design on the measurement of lung dose in photon beams
Authors: Rice R. K. , Hansen J. L. , Chin L. M. , Mijnheer B. J. , Bjärngard B. E.
Published: March 2003

Lung dose correction factors, commonly defined as the ratios of ionization chamber readings in the heterogeneous and homogeneous phantoms, have been compared with those based on accurately determined doses. An analysis of stopping power values, Pwall values, and measurements in lunglike and waterlike materials showed that the wall material and thickness are not very critical in the determination of lung dose correction factors under conditions of electronic equilibrium. When lateral electronic equilibrium is not established due to the extended range of scattered electrons in the low density material, Prepl differs significantly from unity for ionization chambers with thick walls which do not match the lung material in density. An attempt has been made to characterize this effect as a function of photon energy, lung density, field size, and wall thickness.