bluuebunny/crossref_metadata_2025 huggingface dataset:
Extrapolation chamber measurements of 90Sr+90Y beta‐particle ophthalmic applicator dose rates
Authors: Deasy J. O. , Soares C. G.
Published: March 2003

Aspects of extrapolation chamber dose‐rate measurements of 90Sr+90Y beta‐particle ophthalmic applicators are examined in this report, including the proper choice of collector electrode size, the gap width over which the measurement should be done, the effect of the entrance window materials, and the stopping‐power ratio. Experiments, a simple analytic model for the effect of chamber geometry and nonzero gap width, and more detailed Monte Carlo simulations were used. The variation of the planar flux density as a function of angle for a thick 90Sr+90Y source was measured and used as input for the model. From Monte Carlo simulation, the dose rate for tissue irradiation falls off by 8% between the surface and a depth of 7 mg/cm2. The derivative of chamber ionization as a function of gap width, needed for the dose‐rate calibration, increases rapidly as the gap width decreases, typically by a factor of about 2 between gap widths of 1.5 and 0.15 mm. About half of this change is due to ionizing electrons leaving the collection volume at the larger gap widths as shown by the analytic model; the rest of the change is due to ionizing electrons which backscatter from the collector electrode and its backing as shown by Monte Carlo simulations. The backscattering effect increases the derived surface dose by a factor of 1.46. A satisfactory dose‐rate extrapolation is obtained from gap widths of 0.1–0.25 mm, where the total ionization current is observed to be nearly linear in gap width.
Intraoperative beta probe: A device for detecting tissue labeled with positron or electron emitting isotopes during surgery
Authors: Daghighian Farhad , Mazziotta John C. , Hoffman Edward J. , Shenderov Peter , Eshaghian Behzad , Siegel Stefan , Phelps Michael E.
Published: March 2003

An intraoperative beta probe was designed, built, and tested for detection of radio‐labeled malignant tissues that has the advantage of being selectively sensitive to beta while insensitive to gamma radiation. Since beta radiation (electrons or positrons) has a short range in tissue, this probe is ideal for detecting tracers in tumors at the surface of the surgical field. This probe contains a plastic scintillation detector sensitive to beta rays and to a lesser degree some background gamma rays. A second detector counts spurious gamma rays and allows for their subtraction from the activity measured by the first detector. Sensitivity of the dual probe for I‐131 and F‐18 was measured to be 108 counts/s/kBq (4000 counts/s/μCi). The dual‐detector probe faithfully measured the 10:1 “tumor” to background ratio of radioactivity concentrations in a simulated environment of a tumor in the presence of intense background 511 keV photons. In another phantom experiment, simulating abdominal tumor deposits with various realistic I‐131 radioactive concentrations, the probe was able to accurately identify tumors of approximately 50 mg with a tumor/normal radioactivity concentration of 3/1 in 10 s.
High‐sensitivity GafChromic film dosimetry for 125I seed
Authors: Chiu‐Tsao Sou‐Tung , de la Zerda Alberto , Lin Jun , Kim Jae Ho
Published: March 2003

The dose response of high‐sensitivity GafChromic film to photons from 125I seeds for doses up to 200 Gy was established. The optical densities were measured using two types of densitometers: (a) a Macbeth spot densitometer with broadband light spectrum, and (b) an LKB He‐Ne laser scanning microdensitometer with red light of wavelength 632.8 nm. The net optical density was found to be a power function of dose with exponents of 0.858 and 0.997, for the Macbeth and LKB densitometers, respectively. Film sensitivity with the LKB densitometer was about double of that with the Macbeth densitometer. The dose measurements were performed using the high‐sensitivity GafChromic films for 125I model 6702 seed in solid water phantom. Each film was positioned parallel to the seed's long axis and centered at the seed's transverse axis. Films were exposed at various distances, ranging from contact to 3 cm from the seed center. The radiation dose delivered to the film center varied from 7 to 50 Gy, depending on the distance. The optical density at the film center was measured using both types of densitometers. Dose conversion was achieved with the established dose response curves for the respective densitometers. The dose values, along the seed's transverse axis obtained using both densitometers, were compared with each other, and also compared with published thermoluminescent dosimeter (TLD) data and Monte Carlo results. General agreement was found. It was concluded that the high‐sensitivity GafChromic film measurement is a feasible method for 125I seed dosimetry in solid water phantom. Furthermore, in order to investigate the energy dependence of the dose response of the high‐sensitivity GafChromic film, the dose responses of such film for photon energies, 660 and 1250 keV, emitted by 137Cs and 60Co, respectively, for doses up to 200 Gy were studied. The dose response curves, obtained with the LKB laser densitometer, were essentially linear for both radionuclides. However, those curves obtained with the Macbeth densitometer were fitted to power functions of dose, with an exponent of 0.839 and 0.849 for 137Cs and 60Co, respectively. The relative sensitivities for 125I and 137Cs compared to that for 60Co were found to be 0.56 and 0.93, respectively.
The effects of curved tissue layers on the power deposition patterns of therapeutic ultrasound beams
Authors: Fan Xiaobing , Hynynen Kullervo
Published: March 2003

A numerical model for calculating the ultrasonic power deposition in layered media was developed and experimentally tested. The new model takes into account the ultrasound wave reflection and refraction occurring at tissue interfaces. In order to study the effects of soft tissue interfaces on the resulting ultrasonic power deposition patterns, the tested model was applied to simulate four different treatments situations in ultrasound therapy (such as hyperthermia). It was found that in most cases the effect of soft tissue interfaces could be ignored during ultrasound treatments. However, strongly curved tissue interfaces such as those encountered during brain treatments could have significant effects on the location of the focus and the magnitude of local power deposition if sharply focused ultrasound beams are used. The maximum lateral displacement of the focus in a brain treatment was 3 mm. This is too large to be ignored during therapy and thus the effect of tissue interfaces should be taken into account when certain ultrasound treatments are planned and executed.
Evaluation of a device for indirect assessment of automatic exposure control systems
Authors: Rossi Raymond P.
Published: March 2003

A test device which can indirectly measure the performance of an automatic exposure control system and which offers the potential to reduce the time and film requirements for such tests has been evaluated. Evaluation results demonstrate that the device can successfully predict image optical density to within ±0.10 optical density units over a wide range of imaging technical factors and image optical densities.
A cone beam SPECT reconstruction algorithm with a displaced center of rotation
Authors: Li Jianying , Jaszczak Ronald J. , Wang Huili , Gullberg Grant T. , Greer Kim L. , Coleman R. Edward
Published: March 2003

A filtered backprojection (FBP) algorithm is derived based on Feldkamp's FBP algorithm for a cone beam geometry that has a displaced center of rotation. In cone beam single photon emission computed tomography (CB‐SPECT) the center of rotation displacement can degrade the reconstructed images. The center of rotation displacement of interest is mechanical shift, which is the displacement of the midplane of the cone beam collimator off the rotation center. Mechanical shift is characterized by two orthogonal components: the shift of the midplane of the cone beam collimator along the direction of the axis of rotation, and the distance between the midline of the cone beam collimator and the axis of rotation. This new algorithm corrects mechanical shift directly by incorporating mechanical shift into the algorithm. This new algorithm is evaluated using both Monte Carlo simulated data and experimentally acquired data. The results demonstrate that this algorithm is able to correct for blurring and the “doughnut” type artifacts caused by system mechanical shift and improve the image resolution.
The head‐scatter factor for small field sizes
Authors: Zhu Timothy C. , Bjärngard Bengt E.
Published: March 2003

The head‐scatter factors H were examined for four different linear accelerators and were found to be similar at field sizes larger than 3×3 cm2. Sharply reduced values for small collimator openings were observed for all the accelerators. It is concluded that the head‐scatter (or collimator‐scatter) factor has two major components. Scatter of photons in various structures in the beam path, especially the flattening filter, causes a slow (about 10%) increase with increased collimator opening. Insertion of a built‐in wedge may double this number. When the collimators are closed, they ultimately block photons from the periphery of the source. This may cause a considerable reduction of the primary photon fluence and typically affects fields smaller than 3×3 cm2. The effect can be used to estimate the source size, with results that correlate with the design of the bending magnet.
A multileaf delineator system for a radiotherapy simulator
Authors: Karlsson Mikael
Published: March 2003

Multileaf collimators are offered as an accessory to many accelerators for conformal radiotherapy and the dose planning systems are capable of calculating multileaf collimated beams. There is further a frequent desire to verify the radiation fields on a radiotherapy simulator prior to the actual treatment. For this purpose, an accessory to the simulator was developed (the multileaf delineator). With this unit the front end of the individual leaves are indicated both on the skin of the patient and in the x‐ray image. All anatomical structures both outside and inside the field edge are, however, still fully visible.
Dosimetry for asymmetric x‐ray fields
Authors: Kwa William , Kornelsen Richard O. , Harrison Richard W. , El‐Khatib Ellen
Published: March 2003

Conventional linear accelerators have four field‐defining jaws or collimators. Usually, one set of the two opposing jaws moves concurrently to define the field width and the other set defines the field length. The resultant square or rectangular field will have the field centerline coincide with the collimator axis. However, some modern linacs have independent collimators or jaws that can be set asymmetrically. In this case, one of the two opposing jaws can be closed down independently of the other one to define an asymmetric field of smaller dimension. The field center now does not coincide with the collimator axis. Asymmetric collimators have found many clinical applications, but have complicated the dosimetry for physicists. Data acquisition and treatment planning implementations are tedious and complicated. An algorithm has been developed to correct for the reduced dose in the smaller asymmetric field. The approach used is similar in principle to the Day's equivalent field calculation. The difference in dose between an asymmetric and a symmetric radiation field is accounted for by a correction factor that is a function of the asymmetric and symmetric field sizes, off axis distance, and depth of measurement. The correction method presented here applies only to the closing down of one independent jaw. Beam profiles for asymmetric fields are measured for both the 6 and 10 MV photon beams. For a 6 MV photon beam, the calculated doses at the asymmetric field center at depths of 2, 10, and 25 cm are within 0.2% of the measured doses for a 20×20 cm2 field blocked to a 5×20, 10×20, and 15×20 cm2 asymmetric field. The agreement between the calculated and measured doses for the 10 MV photon beam is, on average, within 0.5%. The calculated beam profiles for asymmetric fields are also found to be in good agreement with measurements for both the 6 and 10 MV photon beams. The calculation method has been implemented into an in‐house developed treatment planning system. Examples of isodose distributions for the 6 MV asymmetric fields show good agreement with the measured data.
On the fractal nature of trabecular structure
Authors: Chung Hsiao‐Wen , Chu Chia‐Chi , Underweiser Marian , Wehrli Felix W.
Published: March 2003

Fractal analysis has recently been suggested [Med. Phys. 20, 1611–1619 (1993)] as a means to characterize the structure of cancellous bone by measuring the fractal dimension using a box counting algorithm. This work re‐examines the possible fractal nature of such structures on nuclear magnetic resonance (NMR) images of cancellous bone by estimating the trabecular boundary length as a function of box size under various experimental conditions. On high‐resolution images (pixel sizes on the order of 50 μm) and signal‐to‐noise ratios of 30, the trabecular boundary turns out to be a smooth surface relative to the achievable resolution and is thus nonfractal. The fractal dimension of the trabecular structure is undefined and can vary significantly as a function of image signal‐to‐noise ratio. The present work further indicates the “apparent” fractal dimension obtained by box counting to be a reflection of marrow pore size. In conclusion, the results indicate that, at the currently achievable resolution, the box counting algorithm is not suitable for fractal analysis on images of cancellous bone and that the fractal appearance of the trabecular network reported previously is artifactual.
A variable‐resolution rotate‐only computed tomography scanner
Authors: Hangartner Thomas N.
Published: March 2003

The Rotoscan is a computed tomography scanner that combines the advantages of variable geometric resolution and adjustable size of measurement diameter of translate‐rotate scanners with the improved speed of rotate‐only scanners. Because of the small number of only 26 detectors used for this scanner, a special data collection scheme of multiple rotations with interleaved detector positions was employed. In order to avoid angular data interpolation after reordering of the projections from the fan‐ to a parallel‐beam geometry, the detectors were incrementally moved at a right angle to the centerline of the fan rather than rotated about the source. The measurement time of 40 s for one cross‐section is comparable to that of second‐generation systems. However, for longer measurement diameters, the measurement time for second‐generation systems increases, whereas that of the Rotoscan remains constant.
Mixed field dosimetry of epithermal neutron beams for boron neutron capture therapy at the MITR‐II research reactor
Authors: Rogus Ronald D. , Harling Otto K. , Yanch Jacquelyn C.
Published: March 2003

During the past several years, there has been growing interest in Boron Neutron Capture Therapy (BNCT) using epithermal neutron beams. The dosimetry of these beams is challenging. The incident beam is comprised mostly of epithermal neutrons, but there is some contamination from photons and fast neutrons. Within the patient, the neutron spectrum changes rapidly as the incident epithermal neutrons scatter and thermalize, and a photon field is generated from neutron capture in hydrogen. In this paper, a method to determine the doses from thermal and fast neutrons, photons, and the B‐10(n,α)Li‐7 reaction is presented. The photon and fast neutron doses are measured with ionization chambers, in realistic phantoms, using the dual chamber technique. The thermal neutron flux is measured with gold foils using the cadmium difference technique; the thermal neutron and B‐10 doses are determined by the kerma factor method. Representative results are presented for a unilateral irradiation of the head. Sources of error in the method as applied to BNCT dosimetry, and the uncertainties in the calculated doses are discussed.
Thick beryllium target as an epithermal neutron source for neutron capture therapy
Authors: Wang C.‐K. Chris , Moore Brian R.
Published: March 2003

Accelerator‐based intense epithermal neutron sources for Neutron Capture Therapy (NCT) have been considered as an alternative to nuclear reactors. Lithium (Li) has generally received the widest attention for this application, since the threshold energy is low and neutron yield is high. Because of the poor thermal and chemical properties of Li and the need for heat removal in the target, the design of Li targets has been quite difficult. Beryllium (Be) has been thought of as an alternative target because of its good thermal and chemical properties and reasonable neutron yield. However, in order to have a neutron yield comparable to that of a thick Li target bombarded with 2.5 MeV protons, the proton energy required for a thick Be target must be approaching 4 MeV. Consequently, the neutrons emitted are more energetic. In addition, a significant amount of high‐energy gamma rays, which is undesirable, will occur when Be is bombarded with low‐energy protons. Regardless of the more energetic neutrons and additional gamma rays, in this paper it is shown that it is possible to develop a high‐quality and high‐intensity epithermal neutron beam based on a thick Be target for NCT treatment. For a fixed proton current, the optimal Be‐target‐based beam (with 4‐MeV protons) can produce a neutron beam, with both quality and intensity slightly better than those produced by the optimal Li‐target‐based beam (with 2.5‐MeV protons). The single‐session NCT treatment time for the optimal Be‐target‐based beam is estimated to be 88 min for a proton current of 50 mA. The major advantage of a Be target over a Li target is the simplicity in design, construction, operation, and maintenance.
A technique of automating compensator design for lung inhomogeneity correction using an electron portal imaging device
Authors: Yin Fang‐Fang , Schell M. C. , Rubin Philip
Published: March 2003

A technique of automating compensator design for lung inhomogeneity correction using an electron portal imaging device (EPID) has been investigated. This technique utilizes exit‐radiation information as detected by an EPID to determine the thickness of the compensator desired. In this particular study, the compensator thickness is determined to provide a uniform gray‐level distribution (related to uniform exit‐dose distribution) in the region of the portal image to be compensated. Initially, a compensation characteristic curve, which relates the compensator thickness to the pixel value of the electronic portal image, is measured for both the Lead and Lipowitz compensator materials and a 6‐MV photon beam. Then, a chest‐treatment field is simulated using an anthropomorphic phantom. Based on the analysis of the profile (gray‐level distribution) across the lung and mediastinum regions in the electronic portal image, the average of pixel values within the mediastinum region is selected as the matching level and the regions to be compensated are determined. With the aid of the predetermined compensation characteristic curve and proper distance scaling, the compensator thickness at each pixel location is automatically calculated at the block tray level to correct lung inhomogeneity. In a simple test using a single anterioposterior (AP) chest field, the compensated profile in the electronic portal image presents a uniform gray‐level distribution (related to uniform exit dose) compared to the uncompensated profile. In the case of two opposed (AP/PA) photon beams, half of the calculated compensator thickness is used for each beam and a uniform midline dose is achieved based on measured data using TLDs (the ratio of standard deviation to mean dose is reduced from 7.5% to 2.5%). This technique is potentially useful to automate the compensator design for certain treatment fields, not only for tissue inhomogeneity, but also for the missing tissue.
Evaluation of an asymmetric screen‐film system for chest radiography
Authors: Morishita Junji , MacMahon Heber , Doi Kunio , Carlin Michael , Sukenobu Yoshiharu
Published: March 2003

To evaluate the potential utility of an asymmetric screen‐film system for chest radiography, its image quality and detail visibility compared with a conventional screen‐film system are investigated. The basic imaging properties were evaluated by measuring Hurter and Driffield curves, resolution properties, and noise Wiener spectra. The visibility of simulated anatomical and pathological details in radiographs of a chest phantom and normal anatomy in chest radiographs of patients were evaluated subjectively. The dynamic range of each system is comparable, though the asymmetric screen‐film system can provide an advantage over the conventional system due to a relative dose reduction of approximately 35% and higher resolution properties at high optical densities. The noise level of the asymmetric screen‐film system is slightly greater at low optical densities and much greater at high optical densities. However, the visibility of lung details with the asymmetric screen‐film system is slightly superior to the conventional screen‐film system despite the increase in noise. Mediastinal and retrodiaphragmatic details are similar, though marginally superior with the asymmetric screen‐film system. It is concluded that the asymmetric screen‐film system provided slightly superior image quality to the conventional screen‐film system for chest radiography, provided the average lung density is maintained at a higher level than is customary with conventional systems.
On the determination of the effective transmission factor for stainless steel ovoid shielding segments and estimation of their shielding efficacy for the clinical situation
Authors: Verellen Dirk , De Neve Wilfried , Van den Heuvel Frank , Storme Guy , Coen Veronique , Coghe Marc
Published: March 2003

Commercially available ovoid tubes for gynecological applications used in conjunction with the microSelectron‐HDR (Nucletron International B.V., Waardegelder 1, 3905 TH Veenendaal, The Netherlands) for 192Ir sources, allow for shielding. Publications concerning the transmission properties of these 4.5‐mm thick stainless steel (AISI number 303/304) shielding segments are scarce and not compatible for implementation in treatment planning. Therefore the effect of shielding on dose distribution is unknown. The effective transmission factor has been measured and implemented in the planning computations. Screening efficacy was evaluated on 20 actual treatment plans, analyzing dose reduction to critical tissue and comparing dose distribution in planes relevant for this particular application. Due to high transmission (effective transmission factor=0.85), stainless steel screening segments only provide low, local dose reductions of maximum 15%. A new approach with regard to optimization and source configuration is needed to reduce dose to vulnerable tissue, exploiting the screening segments to a maximum extent. Better shielding, especially at the midline (plane bisecting the ovoids) could be expected by using shielding segments with other geometrical characteristics.
Perceptual comparison of pulsed and continuous fluoroscopy
Authors: Aufrichtig Richard , Xue Ping , Thomas Cecil W. , Gilmore Grover C. , Wilson David L.
Published: March 2003

Pulsed fluoroscopy (hereafter called pulsed) at reduced acquisition rates, typically 15 acq/s (pulsed‐15), is proposed to reduce x‐ray dose in interventional procedures. However, since the human visual system (HVS) acts as a temporal low‐pass filter that interacts with such acquisitions, the proper dose for pulsed must be obtained in perception experiments. We determine the dose for low‐frame‐rate pulsed that gives visualization equivalent to that of conventional 30 acq/s fluoroscopy, hereafter called continuous. Computer‐generated phantoms are used. They consist of stationary, low‐contrast disks on a flat background containing Poisson noise that mimics quantum noise in fluoroscopy. Image sequences are displayed on the video tachistoscope, a device with considerable display flexibility. Three experimental paradigms are used. (1) In a paired‐comparison study, pulsed and continuous are displayed side‐by‐side on the same monitor, and the visibility of a contrast detail phantom is compared. (2) Using this same display, subjects record the minimally detectable disk contrast (the min‐contrast measurement). (3) In a four‐alternative forced‐choice experiment, a disk is placed in one of four positions, and the subject determines the position of the disk. The methods are complementary—the forced‐choice experiment properly eliminates the subjectivity of the observer threshold while the paired‐comparison study is much more time efficient. With regard to pulsed and continuous comparisons, remarkable similarity is found between the supra‐threshold experiments (1 and 2) and the detectability experiment (3); i.e., the average absolute differences in the equivalent‐perception dose as determined by the three measures is approximately 3%. No difference is found between interlaced and noninterlaced display. A relatively small dependence of dose savings on disk size is found with larger disks giving increased dose savings. Average dose savings of 22%, 38%, and 49% are found for pulsed‐15, pulsed‐10, and pulsed‐7.5, respectively.
Measurement of image intensifier system modulation transfer function by video signal analysis
Authors: McRobbie D. W. , Nieto‐Camero J. J.
Published: March 2003

A technique for the measurement of the modulation transfer function (MTF) of image intensifier systems by means of video signal analysis is presented. The MTF can be measured in both horizontal and vertical directions. The method is applicable to routine quality assurance testing in the field. The limitations on accuracy of the technique are investigated by computer simulation. Experimental results reported for several image intensifier systems correlate well with subjective assessments of limiting resolution and other parameters.
Effect of case selection on the performance of computer‐aided detection schemes
Authors: Nishikawa Robert M. , Giger Maryellen L. , Doi Kunio , Metz Charles E. , Yin Fang‐Fang , Vyborny Carl J. , Schmidt Robert A.
Published: March 2003

The choice of clinical cases used to train and test a computer‐aided diagnosis (CAD) scheme can affect the test results (i.e., error rate). In this study, we deliberately modified the components of our testing database to study the effects of this modification on measured performance. Using a computerized scheme for the automated detection of breast masses from mammograms, it was found that the sensitivity of the scheme ranged between 26% and 100% (at a false positive rate of 1.0 per image) depending on the cases used to test the scheme. Even a 20% change in the cases comprising the database can reduce the measured sensitivity by 15%–25%. Because of the strong dependence of measured performance on the testing database, it is difficult to estimate reliably the accuracy of a CAD scheme. Furthermore, it is questionable to compare different CAD schemes when different cases are used for testing. Sharing databases, creating a common database, or using a quantitative measure to characterize databases are possible solutions to this problem. However, none of these solutions exists or is practiced at present. Therefore, as a short‐term solution, it is recommended that the method used for selecting cases, and histograms or mean and standard deviations of relevant image features be reported whenever performance data are presented.
A filtered backprojection dose calculation method for inverse treatment planning
Authors: Holmes Timothy , Mackie T. Rock
Published: March 2003

An efficient method of calculating dose distributions in homogeneous media for megavoltage photons is described. The method is similar to filtered backprojection image reconstruction and is based on the analogy between external beam radiotherapy and SPECT image reconstruction. The filtered backprojection dose calculation significantly reduces the computation time for a large number of x‐ray beams compared to a conventional convolution dosimetry method. A factor of 20 reduction in computation time is demonstrated for a 2D implementation of the model. The method has proved useful for speeding up an inverse treatment planning algorithm for conformal radiotherapy, and has the potential to be implemented in the reconstruction hardware of a radiotherapy CT simulator. Results of computer simulations based on the model are presented.
A regional convolution kernel algorithm for scatter correction in dual‐energy images: Comparison to single‐kernel algorithms
Authors: Kruger David G. , Zink Frank , Peppler Walter W. , Ergun David L. , Mistretta Charles A.
Published: March 2003

Single kernel scatter correction algorithms are based on the model that the scatter field can be predicted by convolution of the primary intensity (Iprim) with a spatially invariant scatter point‐spread function (PSF). Practical limitations (Iprim unknown) suggest the substitution of the total detected intensity (Idet) for Iprim as the source image in the convolution. In regions of high scatter fraction (SF), Idet is a poor approximation of Iprim, thereby causing an overestimation of scatter originating in the region. This contributes to errors in estimating detected scatter in the mediastinum and neighboring regions. A technique using a regionally variable point‐spread function that significantly reduces RMS error in estimation of the primary image as compared to the single PSF method is investigated. The regionally variable convolution method employs a larger PSF in the mediastinum and a smaller PSF in the lungs to reduce the error in estimating the scatter throughout the image. The method to allow for patient differences has also been expanded and various implementations of these methods have been compared. Results show that the dual‐kernel algorithm is always more effective than an equivalent single‐kernel algorithm. The dual‐kernel algorithm using a predicted scatter fraction curve gives an overall RMS error in the primary of as low as 20.8% which is equivalent to 8.7% RMS error in the scatter. The dual‐kernel method using a predicted scatter fraction curve approaches the accuracy of the single‐kernel method using patient specific scatter measurements. Because using individual scatter measurements is a less desirable method for clinical use, we feel that the dual‐kernel algorithm which uses two region specific convolution kernels and a variable scatter fraction curve is the preferable method.
A generalized pencil beam algorithm for optimization of radiation therapy
Authors: Gustafsson Anders , Lind Bengt K. , Brahme Anders
Published: March 2003

An iterative pencil beam algorithm for optimization of multidimensional radiation therapy dose plans has been developed. The algorithm allows the use of both physical and radiobiological treatment objective functions and allows arbitrary sampling such as straight Cartesian grids with linear or nonlinear sampling functions or random sampling. The algorithm can account for and optimally combine almost all the degrees of freedom at an advanced radiotherapy clinic, such as different beam modalities and spectra, beam directions, beam fluence distributions, and time–dose fractionations. The algorithm allows for external charged and neutral beams as well as intracavitary and interstitial sources to be optimally combined. A quantity termed the generalized fluence vector is introduced, combining fluences and energy fluences from external beams as well as the radiation source densities of intracavity and interstitial sources or external source distributions. The positivity constraint on the generalized fluence can therefore be applied directly during the optimization procedure. The convergence properties and the required iteration time of the algorithm are discussed. Several examples with combinations of photon and electron beams of different energies and directions of incidence are presented. The optimization has been made with the treatment objective to maximize the probability of achieving tumor control without causing severe complications in healthy normal tissues.
Longitudinal resolution in volumetric x‐ray computerized tomography—Analytical comparison between conventional and helical computerized tomography
Authors: Wang G. , Vannier M. W.
Published: March 2003

The primary advantage of helical computerized tomography (CT) is the capability of scanning a complete anatomical volume in a single breath hold. Due to the table motion and subsequent interpolation process, the slice sensitivity profile (SSP) in helical CT is worse than the response function of the detector array. In this paper, image longitudinal resolution in volumetric x‐ray CT is analytically characterized, and a comparison made between conventional and helical CT. First, the SSPs are derived for both conventional and helical CT with the half‐scan interpolation method under the condition that the table increment and detector collimation are the same. Then, the corresponding transfer functions are obtained for bandwidth determination, which directly describe the spatial resolution. Both one‐tenth‐cutoff and mean‐square‐root measures are used to quantify the bandwidth. Although it appears that broadening the SSP in helical CT could adversely affect longitudinal resolution, it is proved that for a given x‐ray dose, helical CT allows substantially better longitudinal resolution than conventional CT due to its inherent retrospective reconstruction capability. To make full use of the potential of helical CT scan data, it is recommended that about five slices be reconstructed per table increment. Helical CT is superior in applications requiring a high longitudinal resolution.
Computerized detection of masses in digital mammograms: Automated alignment of breast images and its effect on bilateral‐subtraction technique
Authors: Yin Fang‐Fang , Giger Maryellen L. , Doi Kunio , Vyborny Carl J. , Schmidt Robert A.
Published: March 2003

An automated technique for the alignment of right and left breast images has been developed for use in the computerized analysis of bilateral breast images. In this technique, the breast region is first identified in each digital mammogram by use of histogram analysis and morphological filtering operations. The anterior portions of the tracked breast border and computer‐identified nipple positions are selected as landmarks for use in image registration. The paired right and left breast images, either from mediolateral oblique or craniocaudal views, are then registered relative to each other by use of a least‐squares matching method. This automated alignment technique has been applied to our computerized detection scheme that employs a nonlinear bilateral‐subtraction method for the initial identification of possible masses. The effectiveness of using bilateral subtraction in identifying asymmetries between corresponding right and left breast images is examined by comparing detection performances obtained with various computer‐simulated misalignments of 40 pairs of clinical mammograms. Based on free‐response receiver operating characteristic and regression analyses, the detection performance obtained with the automated alignment technique was found to be higher than that obtained with simulated misalignments. Detection performance decreased gradually as the amount of simulated misalignment increased. These results indicate that automatic alignment of breast images is possible and that mass‐detection performance appears to improve with the inclusion of asymmetric anatomic information but is not sensitive to slight misalignment.
The x‐ray fovea, a device for reducing x‐ray dose in fluoroscopy
Authors: Labbe Michael S. , Chiu Ming‐Yee , Rzeszotarski Mark S. , Bani‐Hashemi Ali R. , Wilson David L.
Published: March 2003

The x‐ray fovea (U.S. patents pending) is a device for reducing x‐ray dose to patients and operators during x‐ray fluoroscopy. It consists of a semitransparent collimator with an open, circular, central hole. The fovea collimator is placed at the exit of the x‐ray tube, and the attenuation of the peripheral x‐ray beam reduces x‐ray exposure to patients and operators. The shadow caused by the x‐ray fovea can be compensated using real‐time image processing hardware. Accurate compensation is demonstrated for both linearly and logarithmically acquired images using a model that accounts for beam hardening in the fovea collimator. The central fovea region has improved image quality due to reduced scatter and veiling glare from the periphery. From beam‐stop measurements, a 40% reduction in scatter plus veiling glare is measured using the fovea. A contrast improvement ratio of 1.5 is measured throughout the central region. In the compensated periphery, noise is increased by a factor of 1.66 because fewer photons are detected, but a small amount of temporal filtering compensates this degradation. The Roentgen area product (RAP) exposure to patients is reduced by ≊70%, while scattered exposure to operators is reduced by ≊60%.
Computing the modulation transfer function of a magnetic resonance imager
Authors: Steckner Michael C. , Drost Dick J. , Prato Frank S.
Published: March 2003

A new method for computing the modulation transfer function (MTF) of magnetic resonance (MR) imagers is presented. Previous attempts to compute the MTF of MR imagers used nonlinear magnitude reconstructed images, resulting in erroneous MTFs. By using complex domain images, the new method produces predisplay MTFs which describe the spatial frequency transfer characteristics of the entire image formation process, except the magnitude operator, eliminating the artifacts previously found in MR imager MTFs. The use of complex domain images results in two‐sided MTFs which differentiate the positive and negative frequencies associated with positive and negative phase encoding or positive and negative time relative to the echo formation. Experimental results are presented which confirm the theoretically predicted form of MR imager MTFs and the need for two‐sided MTFs.
Measurements of phantom scatter factors for small field sizes in high energy x rays
Authors: Haider Tawfiq K. , El‐Khatib Ellen E.
Published: March 2003

An alternate method to measure the phantom scatter factor in small fields is provided for high energy photon beams. The measurement technique is based on the density scaling theorem described by O'Connor [Phys. Med. Biol. 1, 352–369 (1957)]. The phantom scatter factor (Sp) is measured in balsa and cedar wood to give effective field sizes in 3×3 to 0.5×0.5 cm2 water. The extrapolated zero area phantom scatter factor from the average data of balsa and cedar is 0.45. This indicates that the variation of output is largely due to a sharp decrease in the phantom scatter factor for small field sizes.
On three‐dimensional dose calculation of photon beam with wedge filters
Authors: Yao Jonathan Y. , Ranganathan Giri
Published: March 2003

A wedge filter is one of the most commonly used beam modifying devices. The introduction of a wedge filter alters both the distribution of the primary photon fluence and the first and second scatter patterns. The effect of the wedge filter is normally expressed in terms of an exponential attenuation function that is used to modify the primary photon fluence. This paper describes a new three‐dimensional (3‐D) wedged field dose calculation algorithm. This algorithm automatically takes into account (1) the scatter distribution pattern correction due to the effect of wedge filters and (2) the fixed primary photon fluence correction, by utilizing a tissue maximum ratio table calculated from the measured percent depth dose of the wedge, a measured in‐plane scan and a measured cross‐plane scan of the largest rectangular wedge field size. As a result, the calculation accuracy achieved by this algorithm is better than the calculation accuracy achieved by other algorithms. The results show better than 1% accuracy of the wedge central axis percent depth dose calculation and better than 3% accuracy of the off‐axis dose calculation when compared with measurements.
Dosimetric problems at low monitor unit settings for scanned and scattering foil electron beams
Authors: Das Indra J. , Harrington James C. , Akber Syed F. , Tomer Alan F. , Murray James C. , Cheng Chee‐Wai
Published: March 2003

Electron beam dosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for different types of linear accelerators. It is observed that for the scattering foil units, the dose/MU is a smooth function of MU for all beam energies. Discrepancies in dose/MU are highest at the lowest MU. Significant variation (5%–245%) in dose linearity is observed among various linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units tested. On the contrary, for the scanned beam, there is no predictable pattern as dose/MU is random in nature and varies with time and beam energy. The maximum dosimetric error is observed for the highest energy beam where the beam width is most narrow. Using film, the beam uniformity was noticed to be very poor at low MU and high energy for scanned beams. The beam uniformity and dose linearity are random at low MU due to the random nature of the scan cycle. Under the adverse conditions, the deviation in dosimetric parameters was observed up to 200 MU.
An analytical expression for electron beam central axis depth doses
Authors: Wierzbicki Wieslaw , Podgorsak Ervin B.
Published: March 2003

An analytical expression based on four fitting parameters is proposed for a mathematical description of electron beam central axis depth dose distributions. The expression approximates well the measured electron beam data in the field size range from 4×4 cm2 to 25×25 cm2 and in the energy range from 6 to 20 MeV in all four regions of the electron depth dose curve: build‐up, dose maximum, dose fall‐off, and bremsstrahlung contamination.
Analysis of the spatial‐frequency‐dependent DQE of optically coupled digital mammography detectors
Authors: Maidment Andrew D. A. , Yaffe Martin J.
Published: March 2003

The effect of optical coupling efficiency on the spatial‐frequency‐dependent propagation of signal and noise is considered for x‐ray image detectors for digital mammography in which a phosphor screen is optically coupled to a charge‐coupled device (CCD) image array. For experimental purposes, optical coupling between a Gd2O2S:Tb phosphor screen and a CCD image array was provided by relay lenses. Neutral density filters were inserted between the lenses to vary the optical coupling efficiency without altering the inherent spatial resolution. The total coupling efficiency, defined as the number of electrons (e−) recorded in the CCD per x‐ray interaction in the phosphor, was calculated in each case. The modulation transfer function, and the contributions to the total noise power spectrum (NPS) of x‐ray quantum noise, secondary quantum noise, and inherent detector noise were measured as a function of coupling efficiency. These data were used to calculate the spatial‐frequency‐dependent detective quantum efficiency [DQE(f)]. The NPS due to x‐ray quantum noise had a significant spatial‐frequency dependence for coupling efficiencies of more than 9 e− per x‐ray interaction, but little spatial‐frequency dependence for coupling efficiencies of less than 2 e− per x‐ray interaction. These results indicate that to preserve high spatial‐frequency values of DQE(f), and to ensure that images are x‐ray quantum‐noise limited at high spatial frequencies, a coupling efficiency on the order of 10 e− per x‐ray interaction is required. This is contrary to the common belief, based on zero spatial‐frequency analysis, that a coupling efficiency on the order of 1 e− per x‐ray interaction is sufficient to acquire x‐ray quantum‐noise‐limited images.
A laboratory CT scanner for dynamic imaging
Authors: Drangova M. , Fenster A.
Published: March 2003

A high‐resolution laboratory CT scanner has been developed for imaging objects undergoing periodic motion. The scanner comprises an x‐ray image intensifier, optically coupled to a linear photodiode array. Gated time‐evolved projections of a single slice of the moving object are acquired, reformatted, and reconstructed. The resulting series of CT images shows the object at different phases of its motion cycle. The scanner has an adjustable field of view (FOV) and the resolution can be as high as 3.2 mm−1 (for the 40‐mm FOV). The spatial resolution depends on the inherent resolution of the scanner and on the object's velocity. For objects moving at 1 cm s−1, the spatial resolution is reduced by 9% in the direction of motion. The signal intensity in the reconstructed image is linear for materials with attenuation coefficients as high as 1.5 cm−1 (for a 90‐kVp x‐ray beam), with an average accuracy of ±0.02 cm−1. The average accuracy of circumference measurements made from the CT images is ±0.3 mm. Lastly, an application of this dynamic CT scanner to imaging excised human arterial specimens under simulated physiological pressure conditions is presented as an example.
An objective method for evaluating electronic portal imaging devices
Authors: Dong L. , Boyer A. L.
Published: March 2003

A statistical technique is described for analyzing the image of a contrast‐detail phantom acquired by a radiotherapy electronic portal imaging device. By using Student's t test, the image analysis algorithm computes which holes in the contrast‐detail phantom can be objectively resolved from the background signal. The specific pattern of the resolved holes and the total number of the resolved holes characterize the performance of the electronic portal imaging device. Image processing techniques are used to align the holes automatically. Because the use of this algorithm is completely automated, the procedure removes operator subjectivity from the evaluation.
A new computer method to quickly and accurately compute steady‐state temperatures from ferromagnetic seed heating
Authors: Indik Robert A. , Indik Julia H.
Published: March 2003

A new, very fast, yet accurate program, mgseed, has been developed that computes steady‐state temperatures from the three‐dimensional bioheat transfer equation due to heating by a ferromagnetic seed. Seeds have a self‐regulating power absorption characteristic such that their temperatures remain within a few degrees of their Curie transition point. The code is also very flexible, being able to model a seed of any orientation embedded in a tissue domain that can be inhomogeneous with respect to blood perfusion or thermal conductivity. mgseed uses multigrid (or multilevel) programming techniques as well as a finite volume discretization that exploits knowledge of the approximate shape of the temperature solution very near to a seed. These techniques allow the code to sample the seed very coarsely, requiring only one or two nodes to cross the seed. With these coarse samplings mgseed calculated very accurate temperatures in under 3 min of CPU time on a Sun Sparcstation 2. The accuracy of mgseed is demonstrated at different levels of perfusion by comparing its solution in a perpendicular plane that bisects the seed with the known analytical solution. The speed of mgseed is compared to other methods of solution and it is found that mgseed performs 14 times faster than successive over relaxation and conjugate gradient methods, and 2.5 times faster than a preconditioned (modified block incomplete Cholesky) conjugate gradient method. It is concluded that the techniques for discretization and solution incorporated into mgseed can greatly improve the flexibility and speed of hyperthermia treatment planning, which could ultimately lead to an increased level of control over treatment outcome.