bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodosimetry of interstitial light delivery to solid tumors
Authors: Fenning M. C. , Brown D. Q. , Chapman J. D.
Published: March 2003

Both anaplastic and well‐differentiated Dunning prostate adenocarcinomas were illuminated in anesthetized Fischer X Copenhagen rats by single‐fiber and multiple‐fiber illuminators. Each illuminator consisted of a 2‐cm laterally diffusing optical fiber placed within a plastic brachytherapy needle which was implanted into a tumor. Light attenuation coefficients for various wavelengths were obtained from measures of the radial falloff of intensity with distance from single fibers. These coefficients served as input to a 2‐dimensional (2‐D) photodosimetry computer code which calculated relative light intensities in planes perpendicular to single‐fiber and various multiple‐fiber configurations. These calculations assumed uniform optical property of tissue throughout each tumor, uniform and equal illuminance from diffusing fibers, and precise needle implantation. Relative light intensities along specific tumor tracks were measured and compared with those predicted by the 2‐D photodosimetry code. Agreement within ±14% was observed for all configurations studied. Variations in relative intensity in tumor planes perpendicular to a standard seven‐fiber illuminator were determined as a function of the distance between the implanted needles. Light wavelengths of 700 nm and greater produced relatively uniform light fields (approximately ±20%) in the R3327‐AT tumor with needle spacings of at least 1 cm. The addition of two fibers at the periphery of this illuminator (a nine‐fiber illuminator) improved the uniformity of light delivery to the encompassed tumor volumes. The importance of precision photodosimetry for interstitial applications of photodynamic therapy is discussed.
Post therapy imaging in high dose I‐131 radioimmunotherapy patients
Authors: Eary Janet F. , Pollard Kenneth R. , Durack Lawrence D. , Bice Alden N. , Lewellen Thomas K. , Matthews Dana , Press Oliver W. , Nelp Wil B. , Appelbaum Frederick R. , Bernstein Irwin
Published: March 2003

The biodistribution of a trace‐labeled I‐131 antibody is used to predict the biodistribution of a high dose I‐131 antibody for therapy. Internal radiation dose estimates derived from the trace‐labeled antibody have been used to determine the I‐131 doses in a phase I escalating dose therapy trial for hematologic malignancy. To confirm the hypothesis that the distribution of a trace‐ and high‐dose labeled antibodies are similar, both trace (7–11 mCi, 259–407 MBq) and high‐dose (100–800 mCi, 3700–29600 MBq) I‐131 radiolabeled antibody infusion were imaged in 12 patients who were treated for leukemia or lymphoma. With specialized imaging techniques using lead attenuation sheets, clearance data from organs were obtained from serial gamma camera images. Biological clearance half times of I‐131 from both trace and therapy level doses were in agreement. An exception was a patient who developed human antimouse antibody before therapy, and subsequently had rapid clearance of the therapy dose. The method was feasible, yielded reproducible results, and provided critical data for relating therapy toxicity to radiation absorbed dose estimates.
The relation between wedge factors in air and water
Authors: Bar‐Deroma Raquel D. , Bjärngard Bengt E.
Published: March 2003

The wedge factor (WF) for an x‐ray beam is defined as the ratio of doses with and without the wedge at a point on the central axis, in water. It is a function of depth d, field size s, and collimator setting c. It is demonstrated that WF can be calculated as WF(d,s,c)=WFairAw(d)Hw(c)/ A0(d)H0(c), where WFair is a constant factor, namely, the ratio of the incident fluences measured “in air” with and without the wedge for reference conditions, e.g., at isocenter for a 10×10 cm2 collimator setting. Aw(d) and A0(d) are attenuation factors, and Hw(c) and H0(c) head‐scatter factors, with and without the wedge and normalized to reference conditions. This expression is based on the approximation that the scatter factor is the same with and without the wedge. The field size, which affects the scatter factor, is therefore not an important variable for the wedge factor using this approach. The above relation has been evaluated at 6 and 25 MV for various depths (d≤25 cm) and field sizes (s≤25 cm) for built‐in wedges (wedge angle approximately 60°). The agreement was better than ±1.5%. This formulation of the wedge factor is easily implemented in calculations of dose and monitor settings.
A doubly achromatic asymmetric three‐magnet 90° deflection system
Authors: Houwen Wang
Published: March 2003

A doubly achromatic asymmetric three‐magnet beam deflection system is described which is suitable for deflecting low energy electron linac beams with high energy spread through approximately 90°. It consists of three dipoles: two small antisymmetric magnets and one approximately 90° magnet. The doubly achromatic relations for the system are given. The configuration has the specific advantage over all other doubly achromatic systems of minimizing the magnet dimension in the direction opposite to the exit trajectory, and is particularly desirable in a high energy medical electron linac, for it reduces rotation radius of gantry of the machine.
Digitization requirements in mammography: Effects on computer‐aided detection of microcalcifications
Authors: Chan Heang‐Ping , Niklason Loren T. , Ikeda Debra M. , Lam Kwok Leung , Adler Dorit D.
Published: March 2003

We have developed a computerized method for detection of microcalcifications on digitized mammograms. The program has achieved an accuracy that can detect subtle microcalcifications which may potentially be missed by radiologists. In this study, we evaluated the dependence of the detection accuracy on the pixel size and pixel depth of the digitized mammograms. The mammograms were digitized with a laser film scanner at a pixel size of 0.035 mm ×0.035 mm and 12‐bit gray levels. Digitization with larger pixel sizes or fewer number of bits was simulated by averaging adjacent pixels or by eliminating the least significant bits, respectively. The SNR enhancement filter and the signal‐extraction criteria in the computer program were adjusted to maximize the accuracy of signal detection for each pixel size. The overall detection accuracy was compared using the free response receiver operating characteristic curves. The results indicate that the detection accuracy decreases significantly as the pixel size increases from 0.035 mm × 0.035 mm to 0.07 mm × 0.07 mm (P<0.007) and from 0.07 mm × 0.07 mm to 0.105 mm × 0.105 mm (P<0.002). The detection accuracy is essentially independent of pixel depth from 12 to 9 bits and decreases significantly (P<0.003) from 9 to 8 bits; a rapid decrease is observed as the pixel depth decreases further from 8 to 7 bits (P<0.03) or from 7 to 6 bits (P<0.02). Although the results are expected to depend on the noise and spatial resolution properties of the x‐ray screen‐film system and the film digitizer, as well as on the image processing methods, this study demonstrates that very high spatial resolution (a pixel size of 0.035 mm × 0.035 mm or smaller) will be required for digital mammographic systems in order to capture the very subtle microcalcifications detectable on screen‐film mammograms.
Noise analysis in real‐time portal imaging. I. Quantization noise
Authors: Rajapakshe R. , Shalev S.
Published: March 2003

The limit at which quantization noise becomes dominant in video‐based real‐time portal imaging has been studied. Quantization noise due to truncation in integer frame averaging is shown to be dominant over the input analog‐to‐digital converter (A/D) quantization noise, unless image addition is used in video‐based real‐time portal imaging systems. Portal images acquired with the Newvicon camera by averaging more than 64 frames are found to be dominated by the quantization noise due to truncation. It has shown that the signal‐to‐noise ratio (SNR) is limited to 886.8 when using an 8‐bit A/D with digital frame averaging, but higher values can be achieved with digital frame addition. It is also shown that digital frame addition together with 16‐bit processing can achieve higher contrast resolution than digital frame averaging and 8‐bit processing.
Verification of absorbed doses determined with thimble and parallel‐plate ionization chambers in clinical electron beams using ferrous sulphate dosimetry
Authors: Van der Plaetsen A. , Seuntjens J. , Thierens H. , Vynckier S.
Published: March 2003

Absorbed dose values determined with the commonly applied NACP and PTW/Markus parallel‐plate chambers and the cylindrical NE2571 Farmer chamber were compared to values obtained with ferrous sulphate dosimetry in a number of electron beams. For the ionometry with the parallel‐plate chambers the dose‐to‐air chamber factor ND (or Ngas) was derived from a 60Co beam calibration free in air with an additional buildup layer of 0.54 g cm−2 graphite as proposed by the protocol for electron dosimetry published by the Netherlands Commission on Radiation Dosimetry. For the product kattkm in this calibration geometry values of 0.980±0.003 [1 standard deviation (s.d.)] and 0.993±0.004 (1 s.d.) were obtained for the parallel‐plate NACP and PTW/Markus chambers, respectively. The behavior of the fluence perturbation correction factor pf versus the mean electron energy at depth was deduced for the flat PTW/Markus and cylindrical NE2571 chamber by comparison with the NACP chamber, for which pf was assumed unity. Our results show a small but significant energy dependence of pf for the PTW/Markus chamber. The absorbed dose values, determined ionometrically with the different chambers considered in the study using the experimentally determined kattkm and pf values, are systematically 0.5% higher than those obtained with ferrous sulphate dosimetry adopting 352×10−6 m−2 kg−1 Gy−1 for εmG. The performed comparative study confirms also that for the NACP chamber pf is unity independent of the electron energy down to a mean energy at depth of 2 MeV.
A Clarkson's sector integration routine for personal computers
Authors: Steidley K. D. , Gamper C. , Greener A. , Caggiano A.
Published: March 2003

A Clarkson's sector integration scheme has been implemented on a personal computer to calculate mean scatter‐air ratio and mean tissue‐air ratio for external photon beams of irregular shape. To find where a given radial line from the point of interest intersects the edge of the radiation field, polygon clipping algorithms have been adapted from the field of image processing. The program has calculation run times of ∼1 s on an 80387 with a math coprocessor for an integration of 36 sectors. The software was validated by comparison to program irreg, where for various squares and clinical shapes, the agreement was typically 0.7%. The source code is available from the authors.
Photoacoustic ultrasound
Authors: Kruger Robert A.
Published: March 2003

Differential absorption has been detected and localized in three‐dimensions by recording the photoacoustic pulses that were produced when short‐duration (∼1 μs) pulses of electromagnetic energy were absorbed regionally within a turbid medium. These absorption sites were localized with a spatial resolution of ∼6 mm within a 20×20×7.5‐cm3 volume of 0.3% Liposyn® solution, a highly scattering medium. A Xenon flashlamp, delivering a nominal 1 μs pulse of broadband light, was used to irradiate the Liposyn® solution. Photoacoustic echoes were recorded with a focused, ultrasonic transducer, tuned to a nominal frequency of 0.5 MHz. The spatial resolution that was demonstrated is consistent with the expected ultrasonic properties of the transducer. Improved spatial resolution can be expected with shorter‐duration radiation exposure and higher‐frequency transducer designs. The technique is generalizable to any electromagnetic energy range (including long‐wavelength microwaves) that penetrates the medium and produces differential, regional absorption of energy.
X‐ray tube potential, filtration, and detector considerations in dual‐energy chest radiography
Authors: Gauntt David M. , Barnes Gary T.
Published: March 2003

The effect of x‐ray tube potential and prepatient and interdetector filtration in single exposure dual energy chest imaging has been studied employing a carefully benchmarked model. The analysis utilized published methodology. Noise in simulated lung and mediastinum fields of the aluminum (bone) and Lucite (soft tissue) images were studied at fixed entrance skin exposure (ESE) for commonly employed sandwich detector and sandwich imaging plate configurations. Our results indicate noise in the lung increases slowly with tube potential above 120 kVp, while noise in the mediastinum decreases rapidly. Also, at high tube potential (≥120 kVp) adding moderate amounts of prepatient K‐edge filtration (≅100 mg/cm2) while optimizing imaging conditions for the lung tends to decrease noise in the lungs by ≅30% while increasing noise in the mediastinum by a similar amount. Without K‐edge prepatient filtration, noise in the lung is minimized with Cu interdetector filter weights near 400 mg/cm2. In the mediastinum noise is minimized with heavier interdetector filter and prepatient K‐edge filter weights. Prepatient K‐edge filter weights that minimize image noise in either field can increase the tube loading by factors ranging from 10 to 1010. Systems designed with sandwich detectors using commercially available phosphors and coating weights can produce contrast‐to‐noise ratios (CNRs) as high as 50% of the theoretical limit (defined as an optimized system with a totally absorbing rear detector).
Synthetic images by subspace transforms I. Principal components images and related filters
Authors: Sychra J. J. , Bandettini P. A. , Bhattacharya N. , Lin Q.
Published: March 2003

The principal component (PC) approach offers compressions of an image sequence into fewer images and noise suppressing filters. Multiple MR images of the same tomographic slice obtained with different acquisition parameters (i.e., with different TR,TE, and flip angles), time sequences of images in nuclear medicine, and cardiac ultrasound image sequences are examples of such input image sets. In this paper noise relationships of original and linearly transformed image sequences in general, and specifically of original, PC, and PC‐filtered images are discussed. As the spinoff, it introduces locally weighted PC transforms and filters, nonlinear PC's, and a single‐image based filter for suppression of noise. Examples illustrate increased perceptibility of anatomical/functional structures in PC images and PC‐filtered images, including extraction of physiological functional information by PC loading curves. Generally, the more correlated the original images are, the more effective is the PC approach.
Backscattering and transmission through a high Z interface as a measure of electron beam energy
Authors: Das Indra J. , Bushe Harry S.
Published: March 2003

The ratio of electron beam transmission (Tr) and backscattering (Bs) is used as a measure of beam energy. With a proper thickness of high Z medium placed at the surface of a low Z homogeneous medium, it is observed that the function log(Tr/Bs) is a linear function of electron energy. This method is applied to electrons produced with scattering foil and pulse‐swept (scanned) beams for the nominal energies ranging from 6 to 20 MeV. This technique provides a unique alternative method which is easy, simple, and accurate within ±3% for estimation of electron beam energy.
Optimization of beam parameters for dual‐energy digital subtraction angiography
Authors: Van Lysel Michael S.
Published: March 2003

Dual‐energy digital subtraction angiography (DSA) is immune to the misregistration artifacts which plague conventional temporal subtraction DSA. However, since the signal‐to‐noise ratio (SNR) of dual‐energy DSA is lower than temporal subtraction it is important to optimize the dual‐energy technique with respect to the SNR. This study investigates the optimization of x‐ray exposure parameters for the low‐ and high‐energy x‐ray beams, drawing upon our experience with dual‐energy DSA studies in 20 cardiac patients. It is shown that the SNR is optimized, with respect to patient exposure, when the image intensifier (II) exposure for the high‐energy beam is approximately 5 times higher than the low‐energy beam. The unequal distribution of exposure between the low‐ and high‐energy beams is achieved by selecting different video apertures for the two beams, and is therefore referred to as the “dual‐aperture” technique. The dual‐aperture technique achieves the same SNR as the single‐aperture technique (which uses equal II exposures for the two beams) with 45% less patient exposure. In addition, the dual‐aperture technique enhances the ability to maintain an optimum low‐energy kVp, which increases the dual‐energy signal amplitude. The optimum high‐beam filter thickness for use with the dual‐aperture approach is determined. The optimum filter thickness, which is dependent upon the desired tube loading, is found to range from 1.3 to 2.0 mm Cu for tube loads ranging from 188 to 688 J/beam pair.
The use of radiochromic film in treatment verification of dynamic stereotactic radiosurgery
Authors: Ramani R. , Lightstone A. W. , Mason D. L. D. , O'Brien P. F.
Published: March 2003

Standard silver‐based films are usually too sensitive to be used as direct indicators of dose in dynamic radiosurgery because of optical saturation. This paper describes the use of a new radiochromic film to measure 6‐MV radiosurgery doses and dose distributions in a head phantom. Dose calibration of the radiochromic film was performed in the range of 2.3–50.2 Gy using light of 632‐ and 530‐nm wavelengths. Radiosurgery dose distributions were measured using the radiochromic film in a head phantom undergoing the same treatment as a patient, and were compared with the planned distributions. For an example case (nominal 2.0‐cm‐diam cone), film measurement verified the calculated dose distribution in one plane. The simple measurement technique described led to experimental uncertainties of ±0.1 cm for the 90% and 50% isodose lines, ±0.3 cm for the 20% line, and ±0.5 cm for the 10% line. Isocenter dose was measured with an uncertainty of ±3%. Refinements to the technique should allow more precise measurements. It is concluded that the radiochromic film, with some limitations, is a convenient and useful tool for dynamic radiosurgery quality assurance.
Scatter compensation in digital chest radiography using the posterior beam stop technique
Authors: Lo Joseph Y. , Floyd Carey E. , Baker Jay A. , Ravin Carl E.
Published: March 2003

A new scatter compensation technique for computed radiography based on posterior beam stop (PBS) sampled scatter measurements and the bicubic spline interpolation technique was proposed. Using only a single exposure, both the clinical image and an array of scatter measurements, which were interpolated into a smooth scatter‐only image, were simultaneously acquired. The scatter was subtracted from the clinical image to generate the primary‐only image. To gauge the accuracy of scatter estimation, both quantitative and interpolation errors were evaluated. The PBS measurements were compared against the standard beam stop method at 16 locations in an anatomical phantom, resulting in quantitative errors of 2.7% relative to the scatter or 6.8% relative to the primary. Also measured were the interpolation error over 64 interpolation sample locations and 64 midpoint sample locations in the anatomical phantom. The combined interpolation error was 1.9% relative to the scatter or 8.0% relative to the primary. At the interpolation sample locations, the errors were identical between the phantom radiograph and digital portable chest radiographs from five patients. By summing the quantitative and interpolation errors in quadrature, the overall error of the PBS SISTER (scatter interpolation‐subtraction technique for radiography) method was 3.3% relative to the scatter or 10% relative to the primary, which was adequate for dual‐energy imaging purposes (less than 10% error relative to the scatter or 20% relative to the primary). The change of image contrast, noise, and signal‐to‐noise ratio (SNR) at six locations in the anatomical phantom were quantitatively analyzed. Contrast and noise were equally enhanced in all anatomical regions, resulting in approximately the same SNR before and after compensation. The average contrast over all six locations increased 2.8 times, average noise increased 4.9 times, and average SNR barely decreased to 99%. This technique therefore provided accurate scatter compensation by custom measurements of each patient, preserved the SNR, required only one exposure with no dose increase, and performed at low computational cost.
Attenuation correction in PET using single photon transmission measurement
Authors: deKemp Robert A. , Nahmias Claude
Published: March 2003

The use of single photon transmission measurement with a rotating rod source has been evaluated to measure the attenuation correction factors in positron emission tomography (PET). The singles projections are resampled into the coincidence geometry using the detector positions and the rod source location. A nonparalyzable dead time correction algorithm was developed for the block detectors used in the McMaster PET scanner. This enables accurate attenuation correction factors (ACFs) to be computed using a wide range of source strengths for transmission scanning. Transaxial resolution is approximately 6 mm, which is comparable to emission scanning performance. Axial resolution is about 25 mm, with only crude source collimation. ACFs are underestimated by as much as 10% due to increased cross‐plane scatter, compared to coincidence transmission accuracy. The response of the correction factors to object density is within 15%, when comparing singles transmission measurement to current coincidence transmission measurement. The major advantage of using singles transmission measurement is a dramatically increased count rate. A factor of 7 increase in count rate over coincidence scanning is possible with a 2‐mCi transmission rod source. Uniformity of 2% in the transmission images is possible with this source strength and a 2‐min acquisition. There are no randoms counted in singles transmission scans, which makes the measured count rate vary linearly with source activity. Singles detector dead time losses are approximately 6% in the detectors opposite a 2‐mCi rod source.
Reducing electron contamination for photon beam‐quality specification
Authors: Li X. Allen , Rogers D. W. O.
Published: March 2003

The percentage depth dose at 10 cm in a 10×10‐cm2 photon beam at an SSD of 100 cm, %dd(10), is a better beam‐quality specifier for radiotherapy beams than the commonly used values of or nominal accelerating potential. The presence of electron contamination affects the measurement of %dd(10) but can be removed by the use of a 0.1‐cm lead filter, which reduces surface dose from contaminant electrons from the accelerator by more than 95% for radiotherapy beams with energies from 60Co to 50 MV. The filter performs best when it is placed immediately below the head. An electron‐contamination correction factor is introduced to correct for electron contamination from the filter and air. It converts the %dd(10) which includes the electron contamination with the filter in place [hereafter %dd(10)m], into %dd(10) for just the photons in the filtered beam. The correction factor is a linear function of %dd(10)m for all filtered beams with %dd(10)m>70%. A small correction for the photon filtering effect converts the pure photon %dd(10) for the filtered beam into that for the unfiltered beam, which can be used to determine stopping‐power ratio. Calculations show that the values of water‐to‐air stopping power ratio in the unfiltered beam are related to the values of %dd(10)m in the filtered beam by a cubic function. The uncertainty of stopping‐power ratios in unfiltered beams for the same value of the %dd(10)m is within 0.2% for all beams.
Collision avoidance in computer optimized treatment planning
Authors: Humm J. L.
Published: March 2003

Of major concern in fully automated computerized treatment delivery is the possibility of gantry/couch or gantry/patient collisions. In this work, software has been developed to detect collisions between gantry and couch or patient for both transaxial and noncoplanar treatment fields during the treatment planning process. The code uses the gantry angles, turntable angles, and position of the couch surface relative to the isocenter supplied by the planner for the prescribed radiation fields. In addition, the maximum patient anterior–posterior and lateral separations are entered in order to model the patient outline by a conservative cylindrical ellipse. By accessing a database containing the precise mechanical dimensions of the therapy equipment, 3D analytical geometry is used to test for collisions between gantry/patient and gantry/couch for each treatment field. When collisions are detected, the software inspects the use of an extended distance treatment, by recalculating and testing for collisions, with the couch at a greater distance from the collimator along the direction of the central axis. If a collision is avoided at extended distance, the lateral, longitudinal, and vertical motions of the couch are recorded for entry into the treatment plan, or else a warning message is printed, together with the nearest permissible collision‐free gantry angle. Upon inspection, the planner can either elect to use the calculated closest permissible gantry angle or reject the plan. The software verifies that each proposed treatment field is safe, but also that the transition between fields is collision‐free. This requires that the sequence of the treatment fields be ordered, preferably into a sequence which minimizes the delivery time compatible with patient safety. A separate program, referred to as a segment manager, has been developed to optimize the sequence of the treatment planner's radiation fields. When collisions are possible on the transition between treatment fields, intermediate couch positions are inserted into the treatment delivery procedure to ensure a continual safe distance between both the couch and patient and the moving gantry. Treatment segments are organized according to gantry angle, beginning at 180°, and sequentially stepping clockwise around the patient. The use of extended distance treatment is proposed as a means of increasing the safety zone between the patient and the gantry. As the gantry rotates, one proposal to maximize patient safety would maintain the couch at extended distance. In this case, the couch prescribes a circle around the isocenter, which is maintained at 180° out of phase with the gantry motion.
A spatial‐frequency dependent quantum accounting diagram and detective quantum efficiency model of signal and noise propagation in cascaded imaging systems
Authors: Cunningham I. A. , Westmore M. S. , Fenster A.
Published: March 2003

The detective quantum efficiency (DQE) is a system parameter that can be used to accurately describe image noise transfer characteristics through many imaging systems. A simpler approach used by some investigators, particularly when evaluating new ideas and system designs, is to describe the system as a series of cascaded stages. Each stage may correspond to either an increase in the number of quanta (e.g., conversion from x‐ray to optical quanta in a radiographic screen), or a loss (a detection or coupling probability). The number of secondary quanta at each stage per incident primary quantum is given by the product of all preceding gains, and can be displayed graphically for convenient interpretation. The stage with the fewest quanta is called the “quantum sink,” limiting the pixel signal‐to‐noise ratio to less than the square root of the number of quanta per pixel. This conventional zero‐spatial‐frequency “quantum accounting diagram” (QAD), however, neglects the spatial spreading of secondary quanta and can seriously underestimate image noise. It is shown that this problem is avoided with the introduction of a spatial‐frequency dependent QAD, expressed as the product of the gains and squared modulation‐transfer functions (MTF) of each stage. A generalized expression is developed for the DQE of a cascaded imaging system that is dependent only on the gain, gain Poisson excess (related to the variance), and MTF, of each stage. A direct relationship is then shown to exist between the DQE and values in the QAD. The QAD of a hypothetical system consisting of a charge‐coupled device camera and a scintillating screen is evaluated as an illustrative example. The conventional zero‐frequency analysis suggests two quantum sinks occur with approximately equal importance: one in the number of x rays, and one in the number of optical quanta. The spatial‐frequency dependent analysis, however, shows the optical quantum sink becomes severe and dominates at nonzero frequencies. The necessary increase in gain or optical numerical aperture required to prevent the optical quantum sink for spatial frequencies of interest is determined from the QAD analysis. The visual impact of this nonzero spatial‐frequency quantum sink is shown in images generated using a Monte Carlo simulation of the cascading process.
Automated segmentation of regions of interest on hand radiographs
Authors: Cheng Shirley N. C. , Chan Heang‐Ping , Niklason Loren T. , Adler Ronald S.
Published: March 2003

Most radiologists do not use texture information contained in the trabecular patterns of hand radiographs to diagnose erosive changes and demineralization due to systemic inflammatory diseases that affect the skeletal system. However, high‐resolution digitization achievable by a laser digitizer now makes it possible to access texture information that may not be perceived visually. We are studying the feasibility of computer‐assisted early detection of these processes with particular attention to patients with hyperparathyroidism. In this paper the methods used to extract a region of interest (ROI) for texture analysis are discussed. The techniques include multiresolution sensing, automatic adaptive thresholding, detection of orientation angle, and projection taken perpendicular to the line of least second moment. The methods were tested on a database of 50 pairs of hand radiographs. We segmented the middle and the index fingers with an average success rate of 83% per hand. For the segmented finger strips, we located ROIs on both the middle and the proximal phalanges correctly over 84% of the times. Texture information was collected in the form of a concurrence matrix within the ROI. This study is a prelude to evaluating the correlation between classification based on texture analysis and diagnosis made by experienced radiologists.
Two‐dimensional registration of magnetic resonance brain images
Authors: Fitchard Edward F. , Narayana Ponnada A.
Published: March 2003

A fully automated two‐dimensional image registration technique based on cross correlation is presented. This technique is evaluated using magnetic resonance images of human brain. Results indicate that this algorithm is capable of accurately estimating both linear and angular offsets.
Objective performance characteristics of a new asymmetric screen‐film system
Authors: Van Metter Richard , Dickerson Robert
Published: March 2003

A study of the objective imaging characteristics of a new asymmetric screen‐film system is presented herein. The system is characterized by high x‐ray absorption asymmetric screens, and a low‐noise, high‐contrast asymmetric film having near‐zero crossover. Comparisons are made with the imaging characteristics of two widely used conventional screen‐film systems. Sensitometry, modulation transfer function, and noise power spectra were measured using standard methods. Granularity, noise equivalent quanta, and detective quantum efficiency were computed from these. The new screen‐film system has an average gradient at lung‐field densities between the two conventional systems studied, while the mediastinum contrast exceeds both conventional systems. The lung‐field modulation transfer function half bandwidth of the new asymmetric system exceeds that of both conventional systems by 60%. At mid exposures the detective quantum efficiency of the new asymmetric system is comparable to those of the conventional systems studied. However, the exposure range over which detective quantum efficiency remains high is substantially wider.
The accuracy of SPECT brain activation images: Propagation of registration errors
Authors: Sychra J. J. , Pavel D. G. , Chen Y. , Jani A.
Published: March 2003

Functional single photon emission computed tomography (SPECT) images of brain activation are based on a comparison of base line and activation images. The correctness of the functional images depends, among other factors, on the accurate spatial registration (alignment) of the base line and activation image data. The relationship between the registration errors and the errors of the resulting functional images is studied. It is shown that misregistration errors as small as a shift by pixel or rotation by 1° result in 5%–10% errors of the pixel values of functional SPECT images of regional blood flow (the ratio and the relative difference images).
Comparison of NIST and manufacturer calibrations of 90Sr+90Y ophthalmic applicators
Authors: Soares C. G.
Published: March 2003

Since the resumption of the NIST calibration service for 90Sr+90Y beta‐particle ophthalmic applicators, 65 sources have been calibrated using the revised technique [C. G. Soares, Med. Phys. 18, 787–793 (1991)]. For 59 of these sources, the manufacturer's calibration results were available for comparison to the NIST calibration results. The 59 sources represent eight different manufacturers, only one of which is still selling new sources. Manufacturer calibration dates range from the present back to 1954. The results of the comparisons are presented, broken down by both manufacturer and calibration date; there are interesting and significant trends in both, with average differences of 20% not uncommon. The obsolete unit, “roentgen‐equivalent‐beta” (reb), in which some of the manufacturer calibrations are expressed, is discussed, and a factor (0.00982 Gy reb−1) for its conversion to absorbed dose is suggested.
A transmission‐map‐based scatter correction technique for SPECT in inhomogeneous media
Authors: Welch Andy , Gullberg Grant T. , Christian Paul E. , Datz Frederick L. , Morgan Hugh T.
Published: March 2003

In this paper a method of modeling the distribution of scattered events in emission projection data is developed and applied. This method is based on the use of a transmission map to define the inhomogeneous scattering object. The key point is the use of the set of line integrals calculated as part of the attenuation correction technique, as the basis of a model of the distribution of scattered events. The probability of a photon being scattered through a given angle and being detected in the emission energy window is approximated using a Gaussian function. The parameters of this Gaussian are determined using Monte Carlo generated parallel‐beam scatter line spread functions from a nonuniformly attenuating phantom. The model is incorporated into a two‐dimensional projector–backprojector and used with the Expectation‐Maximization‐Maximum‐Likelihood algorithm for the reconstruction of fan‐beam phantom data. The correction is shown to perform well for a phantom that varies slowly in the axial direction. For the more clinically realistic situation of a torso phantom, the method produces improvements in terms of blood pool to myocardium contrast, but does not restore the contrast to the level exhibited in a reconstruction from “scatter free” data.
Depth dose flattening of electron beams using a wire mesh bolus
Authors: Alasti H. , Galbraith D. M.
Published: March 2003

A common problem with low‐energy electron beams (<15 MeV) is their low surface dose when the incident electrons are monodirectional. This makes it difficult to deliver a uniform dose to tumor with any precision, limiting the clinical usefulness of such beams. A practical method is presented for greatly increasing the tissue depth enclosed by the 95% isodose region, while delivering the entire dose in a single uninterrupted treatment. Beam modification is achieved by placing a wire mesh of high atomic number (Z) on the treatment surface throughout the treatment. Electron beams of energies 6, 9, and 13 MeV are modified to produce a near uniform dose from the surface to the original depth of maximum, approximately doubling the depth enclosed by the 95% isodose. These beams have a step‐function‐like depth dose and can be arranged to deliver a constant dose to tumor located at varying depths while simultaneously sparing deeper tissues which are also located at varying depths. A single mesh design was found to be suitable for all energies.
Calculation of dose distributions in the vicinity of high‐Z interfaces for photon beams
Authors: Sauer Otto A.
Published: March 2003

In the vicinity of interfaces between materials of different atomic number Z, extremes in absorbed dose occur for high‐energy photon irradiations. The spatial extension of the effects is within the range of 1 cm, which may not be ignorable from the radiobiological point of view. At the front side of a high‐Z slab a maximum is observed, whereas at the exit side a small buildup zone of the dose occurs, e.g., for a 5 MV beam, in front of a water/iron interface, the enhancement is about 30% of that to the homogeneous medium. The reduction at the back of the iron slab is about 16% for this energy, but vanishes with increasing energy. For high‐energy photons this effect is mainly caused by the strong atomic number dependence of the scattering power for secondary electrons. The amount and extent of the scattering effects have been measured for aluminum and for iron slabs embedded in water or PMMA. The experimental data are in good agreement with Monte Carlo calculated values. Therefore the data form a reliable base to test the performance of commonly used treatment planning algorithms. The convolution or superposition method is used to calculate dose distributions. To account for the Z dependence of the scattering and the stopping power of the secondary electrons, corrections are applied to the energy deposition kernels. The boundary crossing of energy deposition kernels can be treated only in an approximate manner. However, the algorithm developed improves the accuracy of the dose calculation in the vicinity of interfaces significantly.
A review of proton beam radiation therapy
Authors: Miller Daniel W.
Published: March 2003

Beams of protons have been used for radiation treatment for approximately 40 years. This paper provides a short summary of activities from institutions that have contributed to the field over this period. The paper reviews some of the basic interactions of protons with matter that are relevant to radiation treatment and summarizes current knowledge pertaining to the response of biological systems to proton beam radiation. The technical means of producing and delivering proton treatment beams are discussed along with examples of the implementation of these methods at treatment facilities around the world. Finally, the clinical dosimetry of proton beams is summarized and sample data are presented.
An investigation of the effect of finite system resolution and photon noise on the bias and precision of dynamic cardiac SPECT parameters
Authors: Welch Andy , Smith Anne M. , Gullberg Grant T.
Published: March 2003

The exchange of a radionuclide (and similarly of oxygen) between blood and the myocardium is a dynamic process. Dynamic SPECT offers the possibility of directly quantifying the kinetic parameters which describe this process. In this paper we investigate and quantify the effect of typical SPECT system resolution and photon counting statistics on the bias and precision of dynamic cardiac SPECT parameters. System resolution and photon noise are only two of the image degrading processes which occur in SPECT. Therefore, the bias and precision quoted should be viewed as a lower limit on those which could be expected with an experimental study. Dynamic SPECT projection data are simulated using a realistic human torso phantom. Data are simulated assuming both perfect system resolution and a system resolution typical of a clinical SPECT system. A triple detector SPECT system which acquires a full set of projection data in 10 s using continuous detector motion is modelled. Kinetic parameters are estimated using a number of myocardial regions of interest. The results show that the rate constant characterizing the washin of activity into the myocardium is more sensitive to region of interest position than is the washout rate constant. The bias and precision of the dynamic parameters are estimated from multiple realizations of projection data exhibiting various noise levels. The main effect of increased photon noise in the projection data is to decrease the precision of the estimated parameters. However, there is also some evidence that for high noise levels the bias of the parameters may also be affected.
Evaluation of water‐equivalent plastics as phantom material for electron‐beam dosimetry
Authors: Thomadsen Bruce , Constantinou Chris , Ho Anthony
Published: March 2003

This investigation evaluated samples of three phantom materials designed as substitutes for water for electron beam calibration and depth ionization measurements. Two of the materials are commercially available (photon–electron Solid Water and Plastic Water), while the third (Homat) is not. Applying the values for water for all factors used in the calibration protocol of the American Association of Physicists in Medicine [Task Group 21, Med. Phys. 10, 741–771 (1983)] results in a discrepancy in calculated peak dose rates. Eliminating this discrepancy requires the additional inclusion of a multiplicative correction factor of approximately 1.015 for beam energies below 10 MeV, 1.01 for beam energies between 10 and 12 MeV, and 1.005 for beam energies above 12 MeV. Measurements for R50 and extrapolated range may be made in these materials with no corrections. Some improvement can be made in the performance of the phantom material by optimizing the match to water specifically for electron beams without regard for photon beam response. As with all radiation oncology apparatus, calibration phantoms need acceptance testing before routine use.
The dose distribution produced by a 32P‐coated stent
Authors: Prestwich W. V. , Kennett T. J. , Kus F. W.
Published: March 2003

The dose distribution surrounding uniform cylinders of 32P has been calculated in order to obtain information relevant to the utilization of radioactive stents. The radiological impact on two major components of vascular tissue, smooth muscle, and endothelial cells is also considered.
Proton radiography as a tool for quality control in proton therapy
Authors: Schneider Uwe , Pedroni Eros
Published: March 2003

Proton radiography is investigated for its use as a quality control tool in proton therapy. Images were produced both with range and range uncertainty information of protons passing through phantoms (Alderson phantom and a sheep's head). With the range images the correct positioning of the patient with respect to the beam could be verified. The range uncertainty images were used to quantitatively detect range variations of protons passing through inhomogeneities in the patient. These measurements can be used to indicate critical situations during proton therapy or to determine the safety margin around the tumor volume. With the range information the precision of different calibrations of computer tomography Hounsfield values to relative proton stopping power, used for proton treatment planning, was determined. It is found that the precision in range can be improved by a detailed analysis of the calibration data obtained from tissue‐substitute measurements, by a factor of 2.5. The resulting range errors are in the order of the positioning precision (∼1 mm).
A fractal approach to the segmentation of microcalcifications in digital mammograms
Authors: Lefebvre Françoise , Benali Habib , Gilles René , Kahn Edmond , Di Paola Robert
Published: March 2003

This paper presents a computerized method for the automated segmentation of individual microcalcifications in a region of interest (ROI) known to contain a cluster in digital mammograms. Mammographic parenchyma can be accurately modeled with the fractal approach, but not areas with microcalcifications. The digitized image is divided into 16×16‐pixel overlapping windows and those accurately modeled by the fractal model are eliminated. The next steps include local thresholding of the ROIs using an iterative method, the elimination of some of the artifacts and identification of the clustered microcalcifications using a clustering algorithm. The evaluation was performed on 81 simulated clusters superimposed on normal mammographic backgrounds and on a representative database of 408 real mammograms. Microcalcification locations were identified by two radiologists independently. These locations were compared to those found by the computer algorithm. An average of 59% of the simulated microcalcifications and 69% of the microcalcifications common to both radiologists were detected. The algorithm described provides a fully automated method for the segmentation of individual microcalcifications in an area of the mammogram known to contain a cluster.
Minimization of target positioning error in accelerator‐based radiosurgery
Authors: Low Daniel A. , Li Zuofeng , Drzymala Robert E.
Published: March 2003

The stereotactic radiosurgery system used at the Mallinckrodt Institute of Radiology is patterned after that developed at the Joint Center for Radiation Therapy (Brigham & Women's Hospital, Boston, MA) and uses the Brown–Roberts–Wells computed tomography (CT) stereotactic system. The patient's head is attached to a stand that rotates with the treatment couch. The irradiation is conducted using a set of converging arcs of irradiation. Because of mechanical limitations, no accelerator or treatment couch is capable of placing the center of the radiation beam at precisely the same point for all gantry and couch angles and a compromise must be made when locating the nominal isocenter. The stand settings are checked by placing a radiopaque QA sphere at the desired target location. The QA sphere is imaged using a series of eight films exposed at a set of couch and gantry angles that encompass the treatment angles. The distances between the QA sphere image and the center of the radiation field indicate if the correct coordinates were set on the stand and if the radiation beam converges to a sufficiently small region (<0.1‐cm diameter) for treatment. A mathematical procedure has been developed to use the film‐measured position errors to determine a stand offset that will minimize the distance between the accelerator isocenter and the target. The technique is capable of reducing the average placement error, as measured by imaging the QA sphere, to 0.035 cm with a maximum deviation of 0.07 cm.