bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of electronic portal imaging device for missing tissue compensator design and verification
Authors: Roback Donald M. , Gerbi Bruce J.
Published: March 2003

The purpose of this investigation is to determine if electronic portal imaging devices (EPIDs) can be used for the design and verification of compensating filters. In order to do this, we investigated the operating characteristics of a commercially available EPID and the variation in transmitted dose for various measurement situations. We performed four initial tests to determine the EPID response specific to compensator situations. The tests determined EPID response to variable patient SSDs, different gantry angles, positions of an inhomogeneity within a phantom, and the sensitivity variation of different parts of the imager. After these tests, we determined the attenuation functions relating EPID response to phantom thickness for various phantom materials. With these functions, we tested simple compensation situations to demonstrate that missing tissue compensators can both be designed and verified using EPIDs.
Extrafocal radiation: A unified approach to the prediction of beam penumbra and output factors for megavoltage x‐ray beams
Authors: Sharpe M. B. , Jaffray D. A. , Battista J. J. , Munro P.
Published: March 2003

An extrafocal source model has been developed to explain the dependence of head scatter and beam penumbra on field size. In this model, the x‐ray source of a medical linear accelerator is described by two components: a small but intense focal component; and a broadly distributed extrafocal component of low intensity. The extrafocal component is so large that it can be “eclipsed” by the field‐defining collimators. Extrafocal radiation was found to account for 12% of the energy fluence on the central axis of the 6 MV x‐ray beam from a Varian Clinac 2100c accelerator. Head scatter factors were calculated “in‐air” for symmetric, asymmetric, and half‐blocked fields. Calculations agreed with measured values to better than 0.5%, on average. However, head scatter factors for asymmetric fields were underestimated by 1.2% when one of the field dimensions was reduced to 4 cm (the minimum jaw setting that was tested). The extrafocal source model was combined with a convolution/superposition dose calculation algorithm to calculate dose‐per‐monitor‐unit calibration (output) factors and beam dose profiles in water. These dose calculations predict the degradation of the field edge as a function of field size, and calculate output factors to within 0.5%, on average. In the most extreme case of a 4 cm field width, output factors were underestimated by 2%. Dose profiles are predicted without the aid of an empirical fit to measured beam penumbra data. The extrafocal source model will be particularly useful for fields defined by independent jaw and multileaf collimation systems.
Comparative Ngas measurements for a parallel plate chamber in proton, electron, and 60Co beams
Authors: Wu Genevieve N. , Sandison George A. , Bloch Charles
Published: March 2003

The TG21 protocol introduced the Ngas calibration method for parallel plate chambers in high‐energy electron beams. This calibration method was performed for a Markus parallel plate chamber in proton and electron beams of various energies as well as a 60Co beam. For an individual chamber, the Ngas value in proton beams differs from the Ngas value in cobalt and electron beams by the ratio of (W ̄/e) for proton beams to that of a 60Co beam. While the replacement correction factor is essential for Markus chambers in low‐energy electron beams, the results of our measurements in proton beams showed that the Markus chamber does not need a replacement correction factor for therapeutic proton beams of energy 20–170 MeV. These results indicate that the 0.7‐mm guard ring of the Markus chamber is adequate to prevent the in‐scattering of secondary electrons produced by proton irradiation of the chamber wall or medium.
Dosimetric characterization of radiosurgical beams with a diamond detector
Authors: Rustgi Surendra N. , Frye Douglas M. D.
Published: March 2003

Dosimetric characteristics of small diameter 6 MV photon beams for a commercial radiosurgery system have been measured in a solid water phantom with a new diamond detector and compared with measurements using a p‐type Si photon diode, small volume cylindrical and parallel plate ionization chambers, and radiographic films. Tissue maximum ratios (TMR) and total scatter factors (Sc,p) were measured with the three ionometric (diamond, diode, ion chamber) detectors for 12 circular beams ranging in diameter from 12.5 to 40 mm. The TMR values obtained with the three ionometric detectors agreed well (±1%) for all cone sizes and depths investigated when the displacement of the sensitive volume of the detector from its front surface is taken into account. The Sc,p factors obtained with the ionometric detectors also agreed well (±1.2%) for field sizes greater than 20 mm in diameter. For smaller field sizes, the cylindrical and parallel plate chambers measure a smaller Sc,p factor, as a result of the steep dose gradients across their sensitive volumes. Cross‐beam profiles acquired with the diamond detector agree well with the measurements with the diode detector and radiographic film. A distortion in the measured profiles in terms of broadened penumbra is observed with a small volume cylindrical ionization chamber.
Genetic and geometric optimization of three‐dimensional radiation therapy treatment planning
Authors: Ezzell Gary A.
Published: March 2003

The thesis of this report is that potentially useful treatment beams can be chosen based on geometric heuristics and that a genetic algorithm (GA) can be constructed to find an optimal combination of beams based on a formal objective function. The paper describes the basic principles of a GA and the particular implementation developed. The code represents each plan in the population as two paired lists comprised of beam identifiers and relative weights. Reproduction operators, which mimic sexual reproduction with crossover, mutation, cloning, spontaneous generation, and death, manipulate the lists to grow optimal plans. The necessary gene pool is created by software modules which generate beams, distribute calculation points, obtain clinical constraints, add wedges, and calculate doses. The code has been tested on a set of artificial patients and on four clinical cases: prostate, pancreas, esophagus, and glomus. All demonstrated consistent results, indicating that the code is a reliable optimizer. Additional experiments compared the results for a full set of open beams to the geometrically selected set and the GA code with simulated annealing. Geometric selection of beam directions did not significantly compromise optimization quality. Compared to simulated annealing, the genetic algorithm was equally able to optimize the objective function, and calculations suggest it may be the faster method when the number of beams to be considered exceeds approximately 70.
Using the field edge correction (FEC) method to generate accurate POCRs and OCRs for asymmetric fields
Authors: Cadman Patrick
Published: March 2003

Dose planning programs originally intended for use with symmetric fields have been adapted for use with asymmetric fields. An accurate representation of the change in primary beam quality with off‐axis distance and depth is essential for accurate dose calculation and is usually represented in the computer as a primary radiation profile or primary off‐center ratio (POCR). The original field edge correction (FEC) method described by Cadman [Med. Phys. 22, 457 (1995)] to determine POCRs has been extended to allow accurate POCR values to be obtained to an off‐axis distance defined by the corners of the largest field, typically at an off‐axis distance of 28.3 cm. This technique requires only routine symmetric field measurements including beam profiles, TMRs, and collimator and phantom scatter factors. The POCRs obtained using the FEC technique were used to generate off‐center ratios (OCRs) using the boundary factor technique of Chui et al. [Med. Phys. 15, 92 (1988)]. Excellent agreement with measured values was obtained for cross‐beam OCRs using a 10×10‐cm2 field defined by a single set of asymmetric jaws with a field center offset of 15 cm and for diagonal OCRs using a 20×20‐cm2 field with each pair of jaws in a half‐blocked configuration.
Radiation damage of amorphous silicon, thin‐film, field‐effect transistors
Authors: Boudry J. M. , Antonuk L. E.
Published: March 2003

The effect of 60Co radiation on the noise and drain‐source current characteristics of hydrogenated amorphous silicon (a‐Si:H) field‐effect transistors (FETs) was examined as a function of dose to cumulative doses as high as ∼2×104 Gy. Following these measurements, room‐temperature and elevated‐temperature annealing of induced radiation damage was examined. The FETs examined are representative of those incorporated in a‐Si:H arrays under development for various x‐ray medical imaging applications. No significant effect upon the noise characteristics of the FETs was observed as a result of the radiation. The predominant drain‐source current effect with increasing dose was a shift of the transfer characteristic toward negative gate voltage and/or a decrease of the transfer characteristic subthreshold slope. This resulted in large increases in leakage current for gate voltages where the FETs were initially highly nonconducting. This leakage current increase was less pronounced for more negative gate voltages and was further diminished by maintaining the FETs at a more negative gate voltage during the irradiation. Following the radiation measurements, room‐temperature annealing resulted in a 10% to 50% reduction in the leakage current in the first day followed by a logarithmic decrease thereafter. Elevated‐temperature annealing for 2 h at 200 °C restored FET leakage current and threshold voltage properties to their preirradiation values. The irradiation effects are small for cumulative doses less than ∼100 Gy, which is larger than the clinical lifetime dose for an imaging detector for chest radiography or for fluoroscopy (with infrequent exposure to the direct beam). For significantly higher dose applications such as mammography, fluoroscopy (with frequent direct beam exposure), and radiotherapy imaging, the results suggest that periodic elevated‐temperature annealing or operation of the arrays at more negative gate voltages may be necessary.
Design of a laser scanner for a digital mammography system
Authors: Rowlands J. A. , Taylor J. E.
Published: March 2003

We have developed a digital readout system for radiographic images using a scanning laser beam. In this system, electrostatic charge images on amorphous selenium (a‐Se) plates are read out using photo‐induced discharge (PID). We discuss the design requirements of a laser scanner for the PID system and describe its construction from commercially available components. The principles demonstrated can be adapted to a variety of digital imaging systems.
Radionuclide photon dose kernels for internal emitter dosimetry
Authors: Furhang Eli E. , Sgouros George , Chui Chen‐Shou
Published: March 2003

Photon point dose kernels and absorbed fractions were generated in water for the full photon emission spectrum of each radionuclide of interest in nuclear medicine, by simulating the transport of particles using Monte Carlo. The kernels were then fitted to a mathematical expression. Absorbed fractions for point sources were obtained by integrating the kernels over spheres. Photon dose kernels and absorbed fractions were generated for the following radionuclides: I‐123, I‐124, I‐125, I‐131, In‐111, Cu‐64, Cu‐67, Ga‐67, Ga‐68, Re‐186, Re‐188, Sm‐153, Sn‐117m, Tc‐99m. The Monte Carlo simulation was verified by comparing the dose kernels to published monoenergetic photon kernels. Further validation was obtained by generating an I‐125 brachytherapy seed kernel and comparing it with published data. Since Monte Carlo simulation was initialized by sampling from the complete photon spectra of these radionuclides, interpolation between monoenergetic kernels and absorbed fractions was not required. The absorbed‐fraction due to uniform spherical distributions can be directly applied for use in internal dosimetry. In addition, the kernels can be used as input for three‐dimensional internal dosimetry calculations.
Analytic representation of the dose from a 32P‐coated stent
Authors: Prestwich W. V.
Published: March 2003

The dose along the radial direction located at the midplane of a radioactive stent, simulated by a uniform cylinder of 32P, is represented by an analytical function consisting of the sum of two modified exponentials. This procedure reproduces values obtained from numerical integration, for which no closed form exists, to within 5% for distances up to 6 mm from the wall and for stent diameters from 2–6 mm.
Use of a multileaf collimator as a dynamic missing‐tissue compensator
Authors: Geis Paul , Boyer Arthur L. , Wells Nathan H.
Published: March 2003

A dynamic multileaf collimator (D‐MLC) was used to investigate the feasibility of producing missing‐tissue compensators. The modulation of the x‐ray field in two dimensions produced by conventional physical compensators was mimicked by delivering a sequence of D‐MLC‐shaped subfields. A method is introduced to calculate monitor units (MU) for dynamically compensated fields that is analogous to and expands upon methods used for conventional compensating filter MU calculations. In this investigation, the tissue deficit at the surface of an anatomical phantom was measured using a Moiré camera. The tissue deficit data were used to generate a series of D‐MLC subfields that, delivered in sequence, provided the compensated treatment. Film was used to integrate the dose delivered to a specified depth of compensation. Isodose distributions were measured for uncompensated fields, fields compensated with a conventional lead or plastic filter, and fields compensated with the D‐MLC. A comparison of the dose distributions shows the compensation achieved with the dynamic compensating filter is comparable to that achieved using conventional physical compensating filters.
In phantom determination of collimator scatter factor
Authors: Lam Kwok L. , Ten Haken Randall K.
Published: March 2003

The collimator scatter factor Sc has generally been measured in air using an ionization chamber inside a buildup cap or mini‐phantom. Here, Sc was measured in phantom at 10 cm depth for 6 and 15 MV photons with square collimator settings of 2.5–40.0 cm. The results were consistent with in air measurements with a mini‐phantom to within 0.4%. In the measurements, a series of Cerrobend field shaping blocks were used to define the field size in the phantom while the collimator settings were varied from the field size in the phantom to twice that value. Corrections of up to 2% for scattered radiation from the added Cerrobend field shaping blocks were necessary. Since a buildup cap or mini‐phantom is not used, the smallest field size that can be measured is limited only by the size of the detector and the measurement is performed with full scatter resembling the treatment condition of a patient.
Characterizing output for dynamic wedges
Authors: Liu Chihray , Zhu Timothy C. , Palta Jatinder R.
Published: March 2003

The output factor for the dynamic wedge, unlike that for the physical wedge, is a complex function of the field dimension along the moving jaw and wedge angle. The large change in output (varying as much as 40% for 45° and 60° wedge angles) can be attributed clearly to the segmented treatment tables (STTs), which specify cumulative monitor unit weighting as a function of jaw position, y. We found that the output factor (in air or water) on the central axis for the dynamic wedge can be characterized by multiplying the output factor (in air or water) for an open field by a normalization factor, which is determined from the STTs, thus indicating that collimator scatter is similar for both the dynamic wedge field and the open field. The introduction of the normalization factor decreases the commissioning time for dynamic wedges significantly and is useful for quality assurance.
Dosimetric considerations for a multileaf collimator system
Authors: Palta Jatinder R. , Yeung Daniel K. , Frouhar Vincent
Published: March 2003

A commercial linear accelerator with a factory‐fitted multileaf collimator (MLC) was commissioned for clinical use. Measurements made of dosimetric parameters included central axis depth‐dose, field‐size factors, wedge factors, penumbra, and leaf leakage for the 6‐MV and 15‐MV photon beams available on this accelerator. The depth‐dose characteristics, output factors, and transmission factors were similar to those reported in the literature for a machine by the same manufacturer with a standard treatment head. Because of scalloping, the effective penumbra for the MLC was 3 to 4 mm wider than that for the conventional collimator jaws. The output for the fields shaped by the MLC was generally lower than that for similar fields shaped with Lipowitz's metal (Cerrobend). The magnitude of the difference was field‐size dependent and ranged from 0.5% to 4.5% for open shaped fields, increasing to 1% to 5% in the presence of wedges. Further analysis of this observation has shown it to be primarily due to differences in the scattered radiation from the collimator head.
Bremsstrahlung production at 50 MeV in different target materials and configurations
Authors: Sätherberg Anders , Andreo Pedro , Karlsson Mikael
Published: March 2003

A combination of Monte Carlo, convolution, and experimental techniques have been used to investigate bremsstrahlung production at 50 MeV in full‐range targets to produce narrow elementary photon beams for scanning. Calculations using the ITS 3.0 Monte Carlo system for various target designs, including particle transport through the treatment head of an MM50 racetrack microtron and a water phantom, have been compared to experimental dose profiles from narrow photon beams at 10‐cm depth in water. A reduction in the ITS 3.0 default substep size has been found necessary even for incomplete agreement, in consistency with the findings of Faddegon and Rogers [Nucl. Instrum. Meth. A 327, 556–565 (1993)] for a different experimental setup and energy using the previous version of ITS. Results show that the calculated shape of the tail of dose distributions from narrow photon beams agrees well with measurements, but CYLTRAN/ITS 3.0 fails to reproduce the central part of the distribution. The discrepancy at small angles, reported previously for EGS4 and ITS 2.1 simulations, possess a limitation to Monte Carlo simulations of narrow photon beams used in scanned systems of clinical accelerators. Radial dose profiles have been calculated by convolution of the energy fluence at the exit of the target with one polyenergetic Monte Carlo calculated dose kernel and also a database consisting of ten different dose kernels corresponding to different monoenergetic photon pencil beams for comparison. The agreement with the much slower fully detailed Monte Carlo calculations was better when using the database kernels than the polyenergetic kernel. Results for the mean energy, mean polar angle, and energy fluence at different depths within various targets have been obtained. These are discussed in the context of the design characteristics of bremsstrahlung targets with emphasis on their utilization for scanning photon beam techniques.
Use of routine quality assurance procedures to detect the loss of a linear accelerator primary scattering foil
Authors: Davis M. G. , Nyerick C. E. , Horton J. L. , Hogstrom K. R.
Published: March 2003

The effects of the mechanical loss of a stainless steel primary scattering foil on a 12‐MeV electron beam from a dedicated intraoperative electron accelerator are discussed. Routine quality assurance tests, including dose output constancy, energy constancy, and beam uniformity (flatness and symmetry), were used to determine the nature of the malfunction when it occurred. It is concluded that these quality assurance checks, if done with the frequencies recommended by the AAPM Task Group 40 Report [Med. Phys. 21, 581–619 (1994)] and repeated at the time of occurrence, are sufficient to detect loss of an electron scattering foil.
Robust three‐dimensional object definition in CT and MRI
Authors: Bland Peyton H. , Meyer Charles R.
Published: March 2003

This work describes the application of an object definition algorithm to the medical imaging environment for the task of automated detection of anatomical boundaries in three dimensions in the presence of low spatial frequency nonstationarities. We have chosen the Liou–Jain algorithm and have modified it for use with 3D medical image datasets and extended it by including a recruitment operator that corrects for the algorithm's inherent volume underestimation. The algorithm avoids problems in both traditional statistical segmentation and 2D techniques and elegantly bridges the gap between traditional gradient‐based edge finding and regression‐based segmentation techniques. Results are shown for MRI datasets from the human abdomen and brain and for a CT dataset of a liver tumor, as well as an MRI scan of a glioma in a rat brain. For comparison, the human abdomen dataset was processed by a multivariate, statistical classifier. The results demonstrate the statistical technique's susceptibility to low spatial frequency nonstationarities due to rf field inhomogeneity; the Liou–Jain algorithm is shown to be immune to this effect. Further, the results show spatial consistency as a result of inherent characteristics of the algorithm. Volumes identified by the algorithm are visualized and assessed qualitatively in three dimensions. Quantitative accuracy of the algorithm's volume estimates is assessed by the use of a phantom. This work demonstrates that this technique is effective in automatically detecting anatomical organ and lesion surfaces in 3D medical datasets that are corrupted by low spatial frequency nonstationarity and in obtaining volume estimates.
A new way of averaging with applications to MRI
Authors: Madore Bruno , Henkelman R. Mark
Published: March 2003

Averaging is often used to increase the quality of an image degraded by noise or artifacts. A method is developed in which several degrees of freedom are introduced in the averaging process, this freedom making possible the choice of different weighting factors for different portions of the Fourier space. If a weighting factor is associated with each line of a magnetic resonance acquisition, we show that we obtain some freedom to eliminate motion artifacts. The process minimizes a quantity called the gradient energy over a region of interest in the image plane. A processed image is obtained from a mosaic of such regions of interest scanned over the whole image plane. The method is shown to yield greater motion artifact suppression in magnetic resonance images than that achieved with regular averaging. The main strength of the method is probably its ability to diminish the intensity of unstructured artifacts which are usually poorly managed by other postprocessing methods of artifacts suppression.
Measured scattered x‐ray energy spectra for simulated irradiation geometries in diagnostic radiology
Authors: Marshall N. W. , Faulkner K. , Warren H.
Published: March 2003

In this study, scattered x‐ray distributions were produced by irradiating an anthropomorphic pelvis phantom under fluoroscopic conditions using incident beams generated at tube potentials between 61 kVp and 112 kVp. Both overcouch and undercouch x‐ray tube orientations were used when irradiating the phantom. The energy spectrum of the scattered x rays was measured with a germanium x‐ray detector. Two measurement geometries were employed: (i) the detector placed at 90° to the incident x‐ray beam to determine the energy spectra incident on the trunk region of staff and (ii) the detector placed at 45° to the x‐ray beam, measuring spectra incident on the head and neck region. The effect of irradiation area on the scattered spectra was also investigated. Spectral distributions, along with mean energy and half‐value layer (HVL) in mm Al, are presented for each spectrum. Energy spectra measured at 90° to the incident x‐ray beam were found to have HVLs approximately 10%–15% greater than the corresponding primary incident spectrum, for both overcouch and undercouch irradiations. The magnitude of the irradiation area had negligible effect on the mean energy and HVL of the spectra.
Characterization and correction of pulse pile‐up in simultaneous emission–transmission computed tomography
Authors: Wu Xiang , Brown J. Keenan , Kalki Kathrin , Hasegawa Bruce H.
Published: March 2003

We have developed an emission–transmission CT (ETCT) system capable of both single‐photon emission computed tomography (SPECT) imaging and x‐ray transmission CT imaging using a common photon counting detector. In principle, SPECT and x‐ray CT projection data can be acquired simultaneously with the ETCT system; however, doing so results in contamination of the SPECT projection data due to pulse pile‐up caused by the relatively high x‐ray fluence rate. In this study, we characterize the effects of pulse pile‐up for simultaneous ETCT imaging through computer simulation and experimental studies. We demonstrate that pulse pile‐up in the SPECT energy window can be well approximated by a simple quadratic relationship between the pile‐up rate and the x‐ray fluence rate for sufficiently small x‐ray fluence rates. Using this quadratic relationship, we developed a simple pile‐up correction scheme that subtracts the pile‐up counts from the emission data and also truncates the exterior regions of the emission projection data. Analysis of difference images and profiles indicate that this method permits us to reconstruct SPECT images with no apparent noise or resolution degradation in comparison to those obtained via sequential emission and transmission scans.
Scatter and attenuation correction for 111In based on energy spectrum fitting
Authors: Kaplan Mitchell S. , Miyaoka Robert S. , Kohlmyer Steven K. , Haynor David R. , Harrison Robert L. , Lewellen Thomas K.
Published: March 2003

A combined scatter and attenuation correction that does not require a transmission scan is proposed for 111In imaging. Estimates of the unscattered intensity at both 171 and 245 keV are obtained by fitting the observed energy spectrum at each pixel or region of interest using the measured scatter‐free spectrum and a simple model for scatter. The scatter model for the 171 keV peak accounts for scatter contributed by both the 171 and 245 keV emissions. After correcting for scatter, the attenuation is estimated from the observed ratio of photopeak intensities using the known difference in attenuation at the two emission energies and a model based on a point source in water. Accurate scatter correction is a prerequisite for the success of this method because scatter from the higher energy emission will otherwise contaminate the lower photopeak. This differential attenuation method (DAM) of estimating attenuation is demonstrated and calibrated using a series of point source measurements with a wedge‐shaped attenuator. The observed absolute and differential attenuation are in good agreement with the narrow‐beam linear attenuation coefficients for water. Estimates of precision suggest a depth resolution of 1.0–2.5 cm for realistic count densities over the clinically relevant depth range (0–25 cm). The accuracy of DAM in a more realistic attenuation environment is assessed using a hot sphere inside the anthropomorphic data spectrum torso phantom viewed from several angles (with differing attenuation). Finally, the potential of DAM for SPECT attenuation correction was investigated by computer simulation using the SIMSET Monte Carlo software. Preliminary results based on measured planar data and simulated SPECT data indicate that DAM can improve the quality and quantitative accuracy of 111In images. In one SPECT simulation study, the average error in tumor to soft‐tissue ratios was reduced from 32% for uncorrected data to 8% for data corrected with DAM. However, the technique is susceptible to significant noise amplification and can cause substantial streak artifacts in low‐count SPECT studies if sufficient smoothing of the depth estimates is not performed.
Blood flow cooling and ultrasonic lesion formation
Authors: Kolios Michael C. , Sherar Michael D. , Hunt John W.
Published: March 2003

This article examines lesion formation using focused ultrasound and demonstrates how blood flow may affect lesion dimensions using a theoretical model. The effects of blood flow on temperature distributions during ultrasonic lesioning are examined for both regional cooling by the microvasculature and localized cooling due to thermally significant vessels. Regional cooling was critically assessed using two models: the Pennes bioheat transfer equation and the scalar effective thermal conductivity equation. Localized cooling was modeled by adding an advective term in the heat diffusion equation in regions enclosed by thermally significant vessels. A finite difference approach was used to solve the basic equations of heat transfer in perfused tissues in cylindrical coordinates. The extent of the lesioned tissue was determined by the accumulated thermal dose at each location. The size of the lesion was then calculated from the boundaries of the thermal isodose curves generated by the simulations. The results were compared to published in vivo lesion data in rat liver. It was shown that even for short ultrasound exposure times (∼8 s), blood flow may play an important role in the thermal dose distribution.
A bound on the energy resolution required for quantitative SPECT
Authors: Heanue J. A. , Brown J. K. , Tang H. R. , Hasegawa B. H.
Published: March 2003

Scattered radiation is one of several physical perturbations that limit the accuracy of quantitative measurements in single‐photon emission computed tomography (SPECT). Improvement in detector energy resolution leads to a reduction of scatter counts and a corresponding improvement in the quantitative accuracy of the SPECT measurement. In this study, simulated SPECT projections of a simple myocardial perfusion phantom were used to investigate the effect of detector energy resolution on the data. The phantom consists of a spherical shell of radionuclide within a 15 cm radius water‐filled cylinder. Each projection contains on the order of 3×105 counts. The results demonstrate that a full‐width, half‐maximum energy resolution of 3–4 keV is sufficient to render the error due to scatter insignificant compared to the uncertainty due to photon statistics in this case. Further simulations verify that because smaller objects produce less scatter, they can be imaged accurately with degraded energy resolution. These results are useful when designing prototype systems that utilize solid‐state detectors and low‐noise electronics to achieve improved energy resolution.
Testing of dynamic multileaf collimation
Authors: Chui Chen‐Shou , Spirou Spiridon , LoSasso Thomas
Published: March 2003

It has been shown that intensity‐modulated fields have the potential to deliver optimum dose distributions, i.e., high dose uniformity in the target and lower doses in the surrounding critical organs. One way to deliver such fields is by using dynamic multileaf collimation (DMLC). This capability is already available in research mode on some treatment machines. While much effort has been devoted to developing algorithms for DMLC, the mechanical reliability of this new treatment delivery mode has not been fully studied. In this work, we report a series of tests designed to investigate the mechanical aspects of DMLC and their implications on dosimetry. Specifically, these tests were designed to examine (1) the stability of leaf speed, (2) the effect of lateral disequilibrium on dose profiles between adjacent leaves, (3) the significance of acceleration and deceleration of leaf motion, (4) the effect of positional accuracy and rounded‐end of the leaves, and (5) create a simple test pattern that may serve as a basis for routine quality assurance checks. Results of these tests are presented. The implications on dosimetry and consideration for the design of leaf motion are discussed.
A new radiotherapy surface dose detector: The MOSFET
Authors: Butson Martin J. , Rozenfeld Anatoly , Mathur Jagdish N. , Carolan Martin , Wong Tony P. Y. , Metcalfe Peter E.
Published: March 2003

Radiotherapy x‐ray and electron beam surface doses are accurately measurable by use of a MOSFET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200‐μm in diameter and consists of a 0.5‐μm Al electrode on top of a 1‐μm SiO2 and 300‐μm Si substrate. Results for % surface dose were within ±2% compared to the Attix chamber and within ±3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10×10‐cm and 40×40‐cm field size for 6‐MV x rays at 100‐cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate retrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitable in vivo dosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.)
Dual‐slice spiral versus single‐slice spiral scanning: Comparison of the physical performance of two computed tomography scanners
Authors: Liang Yun , Kruger Robert A.
Published: March 2003

In this paper we deal with two types of spiral scanners; one is a single‐slice spiral scanner, while the other employs dual‐slice technology into spiral scanning. Physical performance parameters, including image noise, contrast resolution, spatial resolution (transversal and longitudinal), and radiation exposure are measured. Computer simulations based on two interpolation methods (180° and 360° linear interpolation) are also used in evaluating the slice‐sensitivity profile (SSP) and noise. The results show that the noise behaves in the same way for both types of scanners. The noise change, relative to that of the standard scan with the same scanning parameters, depends solely on the interpolation algorithm. Table speed and scanner geometry (either single slice or dual slice) have no effect on the noise value. For the given table speed, as well as individual detector collimation (slice width) the dual‐slice scan results in better longitudinal resolution (SSP) compared to a single‐slice scan if the scan is obtained with nonoverlapping slices (pitch greater than 2). This is because the dual‐slice scan obtains twice the number of nonoverlapped projections for the same length, which reduces the degradation of the slice profile by using more densely arranged projections (in the longitudinal direction) for the interpolation. In the dual‐slice scanner the workable scan rate is extended up to pitch 4 compared to a pitch of 2 for the single‐slice scanner. Therefore, the dual‐slice spiral scanner is preferred in applications requiring an increased scan rate with comparative image quality.
Radiation dose and image quality of double‐loaded cassettes
Authors: Kelsey Charles A. , Mettler Fred A. , Sullivan Lisa M.
Published: March 2003

Some U.S. hospitals double‐load x‐ray cassettes for certain procedures. Loading two films in the same cassette for portable emergency room (ER), intensive care unit (ICU), or operating room radiographs provides both the referring clinicians and the radiologists with immediate images. Our study demonstrates a cost increase of 15%, an increase in air kerma for a chest x ray from 0.12 to 0.35 mGy (12–35 mrad), slight differences in optical density, image contrast, and spatial resolution under double‐loading conditions. Our study shows that double loading cassettes may improve patient care by economically expediting the communication of radiographic findings. The decision to double load portable ICU or ER cassettes must be based on a balance of factors.
Basic imaging properties of a new screen‐film system for chest radiography
Authors: Higashida Yoshiharu , Murakami Yasunori , Yoshida Akira , Kastuda Noboru , Moribe Nobuyuki , Bussaka Hiromasa , Hamada Tomohiro , Yoshida Yukito , Hidetoshi Miyake , Takahashi Mutsumasa
Published: March 2003

To evaluate the potential clinical usefulness of a new screen‐film system (advanced screen‐film system; AD system) for chest radiography, its fundamental imaging properties compared with a conventional screen‐film system (HR‐4/HR‐S) were investigated. The basic imaging properties were evaluated by measuring characteristic (H&D) curves, relative speeds, MTFs (modulation transfer functions), WS (Wiener spectra), and x‐ray attenuations of screens. The detail visibilities and pathological details of various diseases in chest radiographs of patients were evaluated subjectively. The film gradient of the AD system was slightly lower at low radiographic density, and higher at high density, as compared with a conventional screen‐film system. The screen speed of the AD system was 212% greater than that of the conventional system, and the film speed was 53% that of the conventional film. As the result, the total speed of the AD system was slightly higher compared with the conventional system. The spatial resolution of the AD system was comparable to or slightly lower than that of the conventional system. The noise level of the AD system was considerably lower than that of the conventional system at low (D=0.5) and middle (D=1.0) radiographic density levels. However, it was high at high radiographic density (D=1.8). The radiographic densities in the underpenetrated areas with the AD system were greater than those of the conventional system when the lung densities are matched comparable. Improvement in noise level with the AD system at low and middle density levels may be useful for detection of various diseases in chest radiographs.
Correction of motion artifacts in linogram and projection reconstruction MRI using geometry and consistency constraints
Authors: Gai Neville , Axel Leon
Published: March 2003

Motion results in various artifacts such as blurring and streaks in clinical imaging of subjects based on reconstruction from projections. We model subject motion‐induced artifacts due to scaling, translational and rotational motion. A correction algorithm based on the Ludwig–Helgason consistency conditions is derived here. These conditions are satisfied whenever the projection data are consistent. We apply the algorithm to simulated data collected on linogram (LR) and projection reconstruction (PR) geometries, and to real PR geometry data, in magnetic resonance imaging (MRI). The results show that motion‐induced in‐plane, interview artifacts can be reduced with application of the algorithm. The algorithm is general enough to be applied to certain other cases arising in tomographic imaging.
Transmission imaging of large attenuators using a slant hole collimator on a three‐headed SPECT system
Authors: King Michael A. , Luo Dershan , Dahlberg Seth T. , Villegas Bernard J. , Penney Bill C. , Morgan Hugh T.
Published: March 2003

By combining conjugate views, truncation‐free attenuation profiles of patients can be obtained by using slant hole collimators on three‐headed SPECT systems. The alterations in reconstruction algorithms necessary for use with slant hole collimators and potential image artifacts are discussed. Based on an evaluation of the size of objects that can be imaged without truncation and the size of the overlap region in the conjugate views, a 15° slant angle was determined to be optimal. Studies with a 30° slant hole collimator verified the ability of slant hole transmission imaging to provide accurate, truncation‐free attenuation maps of a 56 cm lateral width phantom. The center of rotation was determined to be dependent on the slant angle and radius of rotation of the slant collimator. These studies also demonstrated that the spatial resolution in the transaxial plane of the attenuation maps depends on radius of rotation of the slant hole collimator, but does not depend on the radius of rotation of an uncollimated transmission source. A multiline transmission source was investigated for use with estimating the attenuation map in Tc‐99m labeled sestamibi perfusion imaging.
Radiation therapy dosimetry using magnetic resonance imaging of polymer gels
Authors: Maryanski M. J. , Ibbott G. S. , Eastman P. , Schulz R. J. , Gore J. C.
Published: March 2003

Further progress in the development of polymer gel dosimetry using MRI is reported, together with examples of its application to verify treatment plans for stereotactic radiosurgery and high dose rate brachytherapy. The dose distribution image produced in the tissue‐equivalent gel by radiation‐induced polymerization, and encoded in the spatial distribution of the NMR transverse relaxation rates (R2) of the water protons in the gel, is permanent. Maps of R2 are constructed from magnetic resonance imaging data and serve as a template for dose maps, which can be used to verify complex dose distributions from external sources or brachytherapy applicators. The integrating, three‐dimensional, tissue‐equivalent characteristics of polymer gels make it possible to obtain dose distributions not readily measured by conventional methods. An improved gel formulation (BANG‐2) has a linear dose response that is independent of energy and dose rate for the situations studied to date. There is excellent agreement between the dose distributions predicted using treatment planning calculations and those measured using the gel method, and the clinical practical utility of MRI‐based polymer gel dosimetry is thereby demonstrated.
Calibration of the delayed‐gamma neutron activation facility
Authors: Ma R. , Zhao X. , Rarback H. M. , Yasumura S. , Dilmanian F. A. , Moore R. I. , Lo Monte A. F. , Vodopia K. A. , Liu H. B. , Economos C. D. , Nelson M. E. , Aloia J. F. , Vaswani A. N. , Weber D. A. , Pierson R. N. , Joel D. D.
Published: March 2003

The delayed‐gamma neutron activation facility at Brookhaven National Laboratory was originally calibrated using an anthropomorphic hollow phantom filled with solutions containing predetermined amounts of Ca. However, 99% of the total Ca in the human body is not homogeneously distributed but contained within the skeleton. Recently, an artificial skeleton was designed, constructed, and placed in a bottle phantom to better represent the Ca distribution in the human body. Neutron activation measurements of an anthropomorphic and a bottle (with no skeleton) phantom demonstrate that the difference in size and shape between the two phantoms changes the total body calcium results by less than 1%. To test the artificial skeleton, two small polyethylene jerry‐can phantoms were made, one with a femur from a cadaver and one with an artificial bone in exactly the same geometry. The femur was ashed following the neutron activation measurements for chemical analysis of Ca. Results indicate that the artificial bone closely simulates the real bone in neutron activation analysis and provides accurate calibration for Ca measurements. Therefore, the calibration of the delayed‐gamma neutron activation system is now based on the new bottle phantom containing an artificial skeleton. This change has improved the accuracy of measurement for total body calcium. Also, the simple geometry of this phantom and the artificial skeleton allows us to simulate the neutron activation process using a Monte Carlo code, which enables us to calibrate the system for human subjects larger and smaller than the phantoms used as standards.
The application of transit dosimetry to precision radiotherapy
Authors: Hansen V. N. , Evans P. M. , Swindell W.
Published: March 2003

A method of using electronic portal imaging (EPI) for transit dosimetry is described. In this method, a portal image of the treatment field is first aligned with a digitally reconstructed radiograph (DRR) to geometrically relate the computed tomography (CT) scan, used to generate the DRR, with the EPI. Then the EPI is corrected for scatter within the patient to yield a map of primary fluence striking the detector. This is backprojected through the planning CT data set to yield a distribution of primary fluence within the patient. This distribution is then convolved with dose deposition kernels to yield a map of dose delivery within the patient. Such a distribution may be compared with the dose distribution resulting from the original treatment plan in order to evaluate the adequacy of the treatment. This method has been evaluated using a humanoid phantom. We find the transit dosimetry relative dose distribution when compared with film and thermoluminescent dosimeter (TLD) measurements and compared with our planning system to agree within 2% in the pelvic region of a humanoid phantom.
Angular‐dependent coherent scatter measured with a diagnostic x‐ray image intensifier‐based imaging system
Authors: Westmore M. S. , Fenster A. , Cunningham I. A.
Published: March 2003

Low‐angle scatter of x rays at diagnostic energies is primarily coherent. This coherence gives rise to interference effects resulting in x‐ray diffraction patterns that are characteristic of the scattering material. A method is described of imaging these low‐angle (0°–10°) x‐ray diffraction properties of tissue specimens using a diagnostic x‐ray beam and image intensifier‐based system. The coherent‐scatter cross sections of several materials measured this way are presented. It is shown theoretically that the measurements made with this system can be expressed as the mono‐energetic cross section “blurred” by the x‐ray spectrum using a linear superposition integral. Experimental results using aluminum powder confirm this. Using a 70 kVp x‐ray beam filtered with gadolinium to reduce the spectral width, materials such as water, Lucite, and hydroxyapatite all have significantly different diffraction patterns. The cross sections determined from this analysis form the basis of a unique method of characterizing and identifying tissue samples according to their atomic structure rather than x‐ray attenuation properties.
PbF2 compared to Al2O3 and AlF3 to produce an epithermal neutron beam for radiotherapy
Authors: Liu Hungyuan B.
Published: March 2003

An evaluation of moderating materials is presented for boron neutron captured therapy. PbF2 was considered and its performance was compared to that of Al2O3 and AlF3. PbF2 is found quite effective as a moderator for medical epithermal neutron beams. (AIP)