Beam Filtration Research Articles

SU‐FF‐I‐141: Measuring Breast Density with Dual Energy Mammography: A Phantom Study

Purpose: To investigate the feasibility of measuring breast density in phantoms with dual energy mammography. Method and Materials: Glandular and adipose equivalent phantoms of uniform thickness were used for dual energy basis decomposition calibration. The dual energy mammography system used a tungsten anode x‐ray tube with a 50 μm rhodium beam filter for low energy images and a 300 μm copper beam filter for high energy images. Three different phantom studies were used to evaluate the technique. The first study consisted of phantoms with thicknesses of 25–85 mm in 5 mm steps with a mean density of 28%. The second study consisted of step phantoms designed to more closely mimic the shape of a female breast with maximal thicknesses of 30–70 mm at a fixed density of 50%. The third study consisted of 40 mm thick phantoms of different sizes (62.5, 125, 250 cm2) at a fixed density of 50% to assess the effect of breast size on density measurement. Results: The RMS errors in breast density measurements were 0.4% for the uniform phantoms, 3.3% for the step phantoms, and 4.7% for the phantoms of different sizes. Conclusion: The results of phantom studies indicate that dual energy mammography can be used to measure breast density with an RMS error of less than 5%. Dual energy mammography can potentially be implemented with minimal changes to the standard mammography exam.

Fourier X-ray Scattering Radiography Yields Bone Structural Information

To characterize certain aspects of the microscopic structures of cortical and trabecular bone by using Fourier x-ray scattering imaging. Protocols approved by the National Institutes of Health Animal Care and Use Committee were used to examine ex vivo the hind limb of a rat and the toe of a pig. The Fourier x-ray scattering imaging technique involves the use of a grid mask to modulate the cone beam and Fourier spectral filters to isolate the harmonic images. The technique yields attenuation, scattering, and phase-contrast (PC) images from a single exposure. In the rat tibia cortical bone, the scattering signals from two orthogonal grid orientations were compared by using Wilcoxon signed rank tests. In the pig toe, the heterogeneity of scattering and PC signals was compared between trabecular and compact bone regions of uniform attenuation by using F tests. In cortical bone, the scattering signal was significantly higher (P < 10(-15)) when the grid was parallel to the periosteal surface. Trabecular bone, as compared with cortical bone, appeared highly heterogeneous on the scattering (P < 10(-34)) and PC (P < 10(-27)) images. The ordered alignment of the mineralized collagen fibrils in compact bone was reflected in the anisotropic scattering signal in this bone. In trabecular bone, the porosity of the mineralized matrix accounted for the granular pattern seen on the scattering and PC images.

Characteristics comparison between a cyclotron-based neutron source and KUR-HWNIF for boron neutron capture therapy

At Kyoto University Research Reactor Institute (KURRI), 275 clinical trials of boron neutron capture therapy (BNCT) have been performed as of March 2006, and the effectiveness of BNCT has been revealed. In order to further develop BNCT, it is desirable to supply accelerator-based epithermal-neutron sources that can be installed near the hospital. We proposed the method of filtering and moderating fast neutrons, which are emitted from the reaction between a beryllium target and 30-MeV protons accelerated by a cyclotron accelerator, using an optimum moderator system composed of iron, lead, aluminum and calcium fluoride. At present, an epithermal-neutron source is under construction from June 2008. This system consists of a cyclotron accelerator, beam transport system, neutron-yielding target, filter, moderator and irradiation bed.In this article, an overview of this system and the properties of the treatment neutron beam optimized by the MCNPX Monte Carlo neutron transport code are presented. The distribution of biological effect weighted dose in a head phantom compared with that of Kyoto University Research Reactor (KUR) is shown. It is confirmed that for the accelerator, the biological effect weighted dose for a deeply situated tumor in the phantom is 18% larger than that for KUR, when the limit dose of the normal brain is 10 Gy-eq. The therapeutic time of the cyclotron-based neutron sources are nearly one-quarter of that of KUR. The cyclotron-based epithermal-neutron source is a promising alternative to reactor-based neutron sources for treatments by BNCT.

Optimization of the extraction facilities of an electron accelerator – bremsstrahlung source for the pulsed photonuclear method

The objective of the present work is to select the material and thickness of an electron-beam converter and filter and a bremsstrahlung collimator for designing the extraction facilities of an accelerator in an experimental setup for studying the photonuclear method. Numerical modeling of the conversion of electrons into bremsstrahlung in the electron-beam converter and filter and passage of the bremsstrahlung in the collimator is performed. The following main criteria are optimized: the neutron yield from the fissile material under bremsstrahlung, the radiation loads on the content of the irradiated container, and the radiation background. The data obtained are used to design and build the target block at the exit of the electron accelerator, including the beam converter and filter, and the bremsstrahlung collimator for an experimental model of a fissile-material detection facility based on the U-28 accelerator.

Decrease in Patient Radiation Exposure by a Tantalum Filter during Electrophysiological Procedures

Minimization of X-ray exposure remains a primary issue in cardiac electrophysiology. The effectiveness of X-ray beam filtration during cardiac electrophysiological procedures was therefore studied, using a 0.05-mm-thick tantalum (Ta) foil, as a filter on the primary X-ray beam, to reduce the patient dose without degradation of image quality. Preliminary tests were made on a phantom developed with polymethylmethacrylate, catheters, and guide wires. The filter was then used in patients during cardiac procedures. Identical patient images were obtained with and without the Ta filter insertion and the ratio between image quality and patient dose was evaluated. Changes in patient dose and signal-to-noise ratio, as image quality index were measured on the phantom and in patients. When the Ta filter was used, the mean effective individual patient dose decreased by 3 to 40% (mean reduction = 27%), with no perceivable difference in image quality estimated by electrophysiologists. This Ta filter may be useful to limit the radiation exposure of patients and operators during cardiac procedures.

Dose assessment from an online kilovoltage imaging system in radiation therapy

We have investigated the dosimetric properties of a commercial kilovoltage conebeam computerised tomography (kV-CBCT) system. The kV-CBCT doses weremeasured in 16 and 32 cm diameter standard cylindrical Perspex computerisedtomography (CT) and Rando anthropomorphic phantoms using 125 kVp and1.0–2.0 mA s per projection. We also measured skin doses using thermoluminescencedosimeters placed on the skin surfaces of prostate cancer patients undergoing kV–kVimage matching for daily set-up. The skin doses from kV–kV image matching ofprostate cancer patients on the anterior and lateral skin surfaces ranged from0.03 ± 0.01 to0.64 ± 0.01 cGy depending on the beam filtration and technique factorsemployed. The mean doses on the Rando phantom ranged from3.0 ± 0.1 to5.1 ± 0.3 cGy for full-fanscans and from 3.8 ± 0.1 to 6.6 ± 0.2 cGy for half-fan scans using 125 kVp and 2 mA s per projection. The isocentre cone beam doseindex (CBDI) in the 16 and 32 cm Perspex phantoms is 4.65 and 1.81 cGy, respectively (using a0.6 cm3 Capintec PR06C Farmer chamber) for full-fan scans, and the corresponding normalised CBDIs are 0.72 and0.28 cGy/100 mA s,respectively. The mean weighted CBDIs are 4.93 and 2.14 cGy, and the normalised weighted CBDIs are 0.76and 0.33 cGy/100 mA s for the 16 and 32 cm phantoms, respectively (full-fanscans). The normalised weighted CBDI for the half-fan scan is0.41 cGy/100 mA s for the 32 cm diameter phantom. All measurements of the CBDI using the0.6 cm3 Farmer chamber are within 2–5% of measurements taken with the 100 mm CT chamber.The CBDI technique and definitions can be used to benchmark CBCT systems and toprovide estimates of imaging doses to patients undergoing on-board imager (OBI)/CBCTimage guided radiation therapy.

Range-dependent geoacoustic inversion of vertical line array data using matched beam processing

This paper describes the results of range-dependent geoacoustic inversion using vertical line array data obtained from the 4th Matched Acoustic Properties and Localization Experiment conducted in the East Sea of Korea. The narrowband multitone continuous-wave signal from the towed source was analyzed to estimate the range-dependent geoacoustic properties along the radial track. The primary approach is based on the sectorwise inversion scheme. The inversion region up to 7.5 km from the vertical line array was divided into several segments, and the subinversions for each segment were performed sequentially. To reduce the dominance of low-angle arrivals, which bears little information for the bottom segment in question, matched beam processing with beam filtering was used for the cost function. The performance of proposed algorithm was tested using simulated data for an environment representative of the experimental site. The inversion results for the experimental data were consistent with the geophysical database and were validated from matched-field source localization using frequencies different from those used in the inversion.

Monitoring Requirements and Methods of Measurement of Total Filtration of X-Ray Beam in X-Ray Diagnostic Apparatuses

Monitoring Requirements and Methods of Measurement of Total Filtration of X-Ray Beam in X-Ray Diagnostic Apparatuses

A Systolic Array 2-D IIR Broadband RF Beamformer

A systolic architecture is proposed for the real-time implementation of broadband 2-D IIR beam filters having applications in ultra-wideband (UWB) radio frequency (RF) antenna arrays. Real-time throughputs of one-frame-per-clock-cycle are achieved. A finite-difference time-domain computational electromagnetic model of a typical indoor propagation environment is used to illustrate that the method significantly reduces the bit error rate of the simulated communication system in the presence of multi-user interference, thereby demonstrating the potential application of the architecture in RF communications.

Quantification of breast density with dual energy mammography: A simulation study

Breast density, the percentage of glandular breast tissue, has been identified as an important yet underutilized risk factor in the development of breast cancer. A quantitative method to measure breast density with dual energy imaging was investigated using a computer simulation model. Two configurations to measure breast density were evaluated: the usage of monoenergetic beams and an ideal detector, and the usage of polyenergetic beams with spectra from a tungsten anode x-ray tube with a detector modeled after a digital mammography system. The simulation model calculated the mean glandular dose necessary to quantify the variability of breast density to within 1/3%. The breast was modeled as a semicircle 10 cm in radius with equal homogenous thicknesses of adipose and glandular tissues. Breast thicknesses were considered in the range of 2-10 cm and energies in the range of 10-150 keV for the two monoenergetic beams, and 20-150 kVp for spectra with a tungsten anode x-ray tube. For a 4.2 cm breast thickness, the required mean glandular doses were 0.183 microGy for two monoenergetic beams at 19 and 71 keV, and 9.85 microGy for two polyenergetic spectra from a tungsten anode at 32 and 96 kVp with beam filtrations of 50 microm Rh and 300 microm Cu for the low and high energy beams, respectively. The results suggest that for either configuration, breast density can be precisely measured with dual energy imaging requiring only a small amount of additional dose to the breast. The possibility of using a standard screening mammogram as the low energy image is also discussed.

Singular value description of a digital radiographic detector: Theory and measurements

The H operator represents the deterministic performance of any imaging system. For a linear, digital imaging system, this system operator can be written in terms of a matrix, H, that describes the deterministic response of the system to a set of point objects. A singular value decomposition of this matrix results in a set of orthogonal functions (singular vectors) that form the system basis. A linear combination of these vectors completely describes the transfer of objects through the linear system, where the respective singular values associated with each singular vector describe the magnitude with which that contribution to the object is transferred through the system. This paper is focused on the measurement, analysis, and interpretation of the H matrix for digital x-ray detectors. A key ingredient in the measurement of the H matrix is the detector response to a single x ray (or infinitestimal x-ray beam). The authors have developed a method to estimate the 2D detector shift-variant, asymmetric ray response function (RRF) from multiple measured line response functions (LRFs) using a modified edge technique. The RRF measurements cover a range of x-ray incident angles from 0 degree (equivalent location at the detector center) to 30 degrees (equivalent location at the detector edge) for a standard radiographic or cone-beam CT geometric setup. To demonstrate the method, three beam qualities were tested using the inherent, Lu/Er, and Yb beam filtration. The authors show that measures using the LRF, derived from an edge measurement, underestimate the system's performance when compared with the H matrix derived using the RRF. Furthermore, the authors show that edge measurements must be performed at multiple directions in order to capture rotational asymmetries of the RRF. The authors interpret the results of the H matrix SVD and provide correlations with the familiar MTF methodology. Discussion is made about the benefits of the H matrix technique with regards to signal detection theory, and the characterization of shift-variant imaging systems.

A Speed-Optimized Systolic Array Processor Architecture for Spatio-Temporal 2-D IIR Broadband Beam Filters

For high-speed plane-wave filtering applications, real-time 2-D spatio-temporal linear-array broadband beam filters are required, operating at temporal frame rates in excess of hundreds of megahertz. The corresponding application specific VLSI circuits must have low critical-path latencies. A novel high-speed systolic array architecture for a first-order 2-D broadband frequency-planar spatio-temporal beam filter is proposed for this purpose and employs a field-programmable gate array (FPGA) circuit where the critical path latency is minimized by timing optimization of inter- and intra-parallel processor pipelines, together with 3-D look-ahead techniques. The method facilitates single-chip VLSI circuit implementations operating at real-time frame rates of several hundred megahertz.

Investigation of optimum X-ray beam tube voltage and filtration for chest radiography with a computed radiography system

The purpose of this study was to determine the optimum tube voltage and amount of added copper (Cu) filtration for processed chest radiographs obtained with an Agfa 75.0 Computed Radiography (CR) system. The contrast-to-noise ratio (CNR) was measured in the lung, heart/spine and diaphragm compartments of a validated chest phantom using various tube voltages and amounts of Cu filtration. The CNR was derived as a function of air kerma at the CR plate and with the effective dose. As rib contrast can interfere with detection of nodules in chest radiography, a tissue-to-rib ratio (TRR) was derived to investigate which tube voltages suppress the contrast of rib. Although processing algorithms affect the signal and noise in a way that is hard to predict, we found that, for a given set of processing parameters, the CNR was related to the plate air kerma and effective dose in a logarithmic manner (all R(2) >or=0.97). For imaging of the lung region, a low voltage (60 kVp) produced the highest CNR, whereas a high voltage (125 kVp) produced the highest TRR. In the heart/spine region, 80-125 kVp produced the highest CNR, while in the diaphragm region 60-90 kVp produced the highest CNR. For chest radiography with this CR system, the optimal tube voltage depends upon the region of interest. Of the filters tested, a 0.1 mm Cu thickness was found to provide a statistically significant increase in the CNR in the diaphragm region with tube potentials of 60 kVp and 80 kVp, without affecting the CNR in the other anatomical compartments.

Comparison of uniform amplitude beam and intercepted Gaussian beam superresolution theories

The general superresolution theories for uniform amplitude beams and intercepted Gaussian beams are investigated. For these two types of incident beam, both two-zone amplitude and pure-phase filters are adopted to provide specific numerical descriptions of their differences in superresolution performances. Simulated results of comparisons between their performances indicate that, with the same spot size ratio, the intercepted Gaussian beam achieves a higher central image brightness ratio and significantly lower side-lobe effect irrespective of the filter used.

MO‐A‐352‐01: Fluoroscopic Imaging Equipment

Modern fluoroscopy imaging equipment is designed to simultaneously monitor x‐ray beam attenuation, detector exposure rate, contrast‐to‐noise ratio, x‐ray generator parameters, and x‐ray tube loading. Knowledge of system design is essential to understanding equipment operation; and understanding equipment operation can facilitate optimization of performance. Under pre‐programmed control, fluoroscopy systems automatically make adjustments to the parameters controlling x‐ray production, beam filtration, detector signal output, digital image processing, and image presentation. The result is that reduction of patient dose does not necessarily require a loss of image quality.Optimizing performance begins with the determination of which system parameters are dynamically controlled, and the identification of the trigger points for changes in these parameters. Secondly, an understanding of how system parameters affect each other, affect image quality, and affect patient dose can lead to better choices in how the system should be set up for specific applications. This presentation will systematically describe the functions of the major system components including automatic exposure rate control, variable pulse‐rate fluoroscopy, generator operating curves, spectral beam filtration, and other factors that affect the ability of the imaging system to deliver optimum diagnostic images at moderate patient dose.Educational Objectives:1. Provide an understanding of modern fluoroscopic imaging system component design and functionality.2. Describe specific ways in which equipment settings can enhance or detract from optimum performance.

SU-GG-T-189: Comprehensive Evaluation of the Bow Tie Filter in the Elekta XVI CBCT System

Purpose: To evaluate the dosimetric and image quality performances of a newly‐released commercial bow‐tie filter (BTF) for use with the Elekta XVI® CBCT system. Method and Materials: Multiple phantoms, including the Catphan® and two uniform acrylic phantoms, were used for all measurements. Two commonly used clinical scanning protocols, head‐and‐neck and pelvis, were used to measure dose (central and peripheral) and image quality under the conditions of with and without the BTF. The total mAs parameters were varied to evaluate the influence on image quality in the presence of BTF during data collection. Results: Half value layers for 100 kV and 120 kV increased from 6.1 to 6.8 mm‐Al and 6.7 to 7.7 mm‐Al, respectively, when measured in the absence and presence of BTF. Dose by head‐and‐neck protocol and pelvis protocol, without BTF, were 0.13 and 2.35 cGy, whereas, with BTF, 0.09 and 1.51 cGy. The dose decreased by 31 % for head‐and‐neck and 36 % for pelvis in the presence of BTF. The presence of BTF did not improve the quality of image with default protocols. In fact, the uniformity values decreased more than 20 % from the original values. However, by increasing the total mAs improved the uniformity with BTF. BTF did not improve the spatial resolution even with increasing of mAs setting. Conclusion: A comprehensive evaluation of the newly‐released BTF for XVI CBCT system has been carried out. It was observed that the presence of BTF increases x‐ray beam filtration by 0.7 to 1 mm‐Al and the resultant dose decreased by 30 to 40 %. BTF does not show any improvement of image quality with current default clinical protocols. Further optimization of scanning and reconstruction parameters is required to appreciate the benefits of BTF.

TH‐C‐332‐07: Dual Energy Contrast‐Enhanced Breast Computed Tomography: A Phantom Study

Purpose: To investigate lesion enhancement with iodinated contrast material using dual energy breast computed tomography (CT). Method and Materials: The dual energy cone‐beam breast CT system consisted of a flat panel detector and an x‐ray tube installed on an optical bench. The x‐ray tube voltage and x‐ray beam filtration were switched at 30 Hz between 50 kVp (2.0‐mm Al filter) and 120 kVp (2.0‐mm Al + 0.8‐mm Ag filtration). A cylindrical breast phantom was constructed from polymethylmethacrylate (PMMA) and filled with oil and shredded PMMA. Five known concentrations of iodine (0.5, 1, 2, 4, 8 mgI/ml) were embedded in the phantom. The images were reconstructed with a Feldkamp filtered‐back‐projection algorithm. Dual energy subtraction was used to eliminate the contrast of oil and PMMA to enhance iodine signals. Results: The low energy scan could not distinguish PMMA and iodine concentrations of 0.5 and 1 mgI/ml. The high energy image could not distinguish PMMA and iodine concentration of 8 mgI/ml. After subtracting the PMMA, all five concentrations of iodine could be clearly visualized on the dual energy CT image. The dual energy iodine signal (CTDE) and the iodine concentration (CI) were related by (R = 0.998 and SEE=15.26) Conclusion: Dual energy contrast‐enhanced breast CT can potentially improve lesion.

MO-E-AUD C-07: Modeling Small Animal Micro Irradiator Orthovoltage Sources

Purpose: To model and compute spectral output, intensity, dose fall off, dose rate, and penumbra that can be achieved using industrial orthovoltage x‐rays sources used in small animal micro irradiators. Method and Materials: Using data from commercial sources as input parameters we investigated the performance of orthovoltage sources that can be used in small animal micro irradiators. We developed a pencil beam propagation code which was combined with a mouse digital phantom (MOBY‐John Hopkins Univ.) to simulate a complete small animal delivery system (source + animalmodel). We computed the source spectral output, beam filters, beam penumbra, soft tissue to bone dose ratio, dose fall off, and total dose delivered to the animalmodel. Sources covering the range of 150 to 450kVp with submillimeter focal spot were simulated. Results: We determined that a source of nominal maximum potential output of 320kVp and focal spot of 0.4×0.4 mm2 outperformed other available sources. We designed an optimum Thoraeus‐like filter to obtain a bremsstrahlung spectrum energy greater than 2 mm of Cu to increase skin spare and reduce bone dose. An average beam penumbra of 0.25mm and a dose rate of 40Gy/min were possible using this filtered beam. Higher energy sources would increase cost and shielding thickness. Lower energies sources showed limited intensities when they were aggressively filtered. Conclusion: We developed a numerical model to evaluate the radiation dose delivered by orthovoltage sources typically used in small animal irradiators. We concluded that when radiation quality, skin dose, bone to tissue dose ratio and animal throughput were considered, commercial orthovoltage sources of nominal energy of 320 keV were the best fit for conformal small animal micro irradiators. To obtain high conformality a submillimeter focal spot of 0.5×0.5 mm2 or less must be used. This work supported in part by NIH grant R21CA108677

SU‐GG‐I‐65: The Effect of Measurement Location On In‐Air CTDI Values

Purpose: To experimentally investigate the variation of CTDIair with radial distance from the scanner isocenter. Method and Materials: Radiation dose measurements were obtained on a Siemens Sensation 16 CT scanner operated with both the head and body beam filtration filters. Measurements were made of CTDIair (mGy/mAs) as a function of the radial distance r from the CT scanner isocenter. The radial profile CTDIair(r) was fit to a third order polynomial which permitted the average CTDI over any radius to be obtained analytically. Results: At r = 8 cm, values of CTDIair relative to the isocenter were 0.824 and 0.838 for the head and body filters, respectively; at r = 16 cm, the values of CTDIair relative to the isocenter were 0.521 and 0.546 for the head and body filters, respectively. Values of CTDIair averaged over an 8 cm radial distance were ∼0.91 × CTDIair(0), and averaged over a 16 cm radial distance were ∼ 0.74 × CTDIair(0). Conclusion: Quantification of variations of CTDIair(r) are of interest because: (a) this parameter quantifies the performance and effectiveness of beam shaping filters; (b) CTDIair data can be used to obtain Tissue Air Ratios that can estimate organ doses in CT.

MO‐E‐332‐05: The Effect of Copper Beam Filtration On the Transmission of Scattered X‐Rays Through a Typical Lead Barrier

Purpose: Standard use of copper beam filtration in modern cardiac catheterization and angiography systems can substantially change the x‐ray beam quality and ultimately have an impact on scattered x‐ray transmission through shielded barriers. A study was performed to investigate these effects. Method and Materials: Scatter was measured using broad‐beam geometry at 50 and 100 cm from the center of an 8″‐thick Lucite phantom with an 1800 cc ionization chamber. A Siemens Axiom Artis system was employed using 60, 81, 102 and 125 kVp with filtrations of 0, 0.1, 0.2, 0.3, 0.6 and 0.9mm of copper. The ionization chamber was wrapped in 1/16″‐thick lead to determine the transmission of scattered x‐rays through a typical shielded barrier. Results: Transmission at 125 kVp varied from 1.7×10−3 to 3.0×10−3 for beams with 0 to 0.9 mm of Cu, respectively. Similarly, transmission varied at 102 kVp from 1.2×10−3 to 2.2×10−3 and at 81 kVp from 7.1×10−4 to 9×10−4. Transmission at 60 kVp was about 7×10−4 for beams with 0 to 0.3 mm of Cu. Transmissions without Cu filtration at 125 and 102 kVp were measured to be about half the theoretical value reported by Simpkin and Dixon, which could result from variations in lead thickness. At 60 and 81 kVp transmissions without Cu filtration are more than an order of magnitude higher than that reported at 70 kVp by Simpkin and Dixon. Conclusion: Additional copper filtration can increase the barrier transmission up to a factor of two over unfiltered beams. However, it is unlikely that this amount of increase in transmission will significantly modify a shielding calculation. Further investigation is needed to determine the changes in typical workloads and scattered air kerma at different angles for these systems to understand the combined effect of these changes on barrier shielding calculations.