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Dec 2008

Volume 35, Issue 12, pp. 5203-5960

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POINT/COUNTERPOINT: There is currently enough evidence and technology available to warrant taking immediate steps to reduce exposure of consumers to cell-phone-related electromagnetic radiation

Vini G. Khurana, M.D. Ph.D., John E. Moulder, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 35, 5203 (2008); http://dx.doi.org/10.1118/1.3013548 (4 pages) | Cited 1 time

Online Publication Date: 6 November 2008

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Abstract Unavailable
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87.50.S- Radiofrequency/microwave fields effects
87.19.xj Cancer
87.19.L- Neuroscience
87.50.C- Static and low-frequency electric and magnetic fields effects

RADIATION IMAGING PHYSICS: Direct-conversion flat-panel imager with avalanche gain: Feasibility investigation for HARP-AMFPI

M. M. Wronski and J. A. Rowlands

Med. Phys. 35, 5207 (2008); http://dx.doi.org/10.1118/1.3002314 (12 pages) | Cited 7 times

Online Publication Date: 6 November 2008

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The authors are investigating the concept of a direct-conversion flat-panel imager with avalanche gain for low-dose x-ray imaging. It consists of an amorphous selenium (a-Se) photoconductor partitioned into a thick drift region for x-ray-to-charge conversion and a relatively thin region called high-gain avalanche rushing photoconductor (HARP) in which the charge undergoes avalanche multiplication. An active matrix of thin film transistors is used to read out the electronic image. The authors call the proposed imager HARP active matrix flat panel imager (HARP-AMFPI). The key advantages of HARP-AMFPI are its high spatial resolution, owing to the direct-conversion a-Se layer, and its programmable avalanche gain, which can be enabled during low dose fluoroscopy to overcome electronic noise and disabled during high dose radiography to prevent saturation of the detector elements. This article investigates key design considerations for HARP-AMFPI. The effects of electronic noise on the imaging performance of HARP-AMFPI were modeled theoretically and system parameters were optimized for radiography and fluoroscopy. The following imager properties were determined as a function of avalanche gain: (1) the spatial frequency dependent detective quantum efficiency; (2) fill factor; (3) dynamic range and linearity; and (4) gain nonuniformities resulting from electric field strength nonuniformities. The authors results showed that avalanche gains of 5 and 20 enable x-ray quantum noise limited performance throughout the entire exposure range in radiography and fluoroscopy, respectively. It was shown that HARP-AMFPI can provide the required gain while maintaining a 100% effective fill factor and a piecewise dynamic range over five orders of magnitude (10−7–10−2 R/frame). The authors have also shown that imaging performance is not significantly affected by the following: electric field strength nonuniformities, avalanche noise for x-ray energies above 1 keV and direct interaction of x rays in the gain region. Thus, HARP-AMFPI is a promising flat-panel imager structure that enables high-resolution fully quantum noise limited x-ray imaging over a wide exposure range.
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87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
87.59.C- Fluoroscopy
87.59.B- Radiography
85.60.Dw Photodiodes; phototransistors; photoresistors
85.30.Tv Field effect devices

RADIATION IMAGING PHYSICS: Three-dimensional linear system analysis for breast tomosynthesis

Bo Zhao and Wei Zhao

Med. Phys. 35, 5219 (2008); http://dx.doi.org/10.1118/1.2996014 (14 pages) | Cited 25 times

Online Publication Date: 6 November 2008

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The optimization of digital breast tomosynthesis (DBT) geometry and reconstruction is crucial for the clinical translation of this exciting new imaging technique. In the present work, the authors developed a three-dimensional (3D) cascaded linear system model for DBT to investigate the effects of detector performance, imaging geometry, and image reconstruction algorithm on the reconstructed image quality. The characteristics of a prototype DBT system equipped with an amorphous selenium flat-panel detector and filtered backprojection reconstruction were used as an example in the implementation of the linear system model. The propagation of signal and noise in the frequency domain was divided into six cascaded stages incorporating the detector performance, imaging geometry, and reconstruction filters. The reconstructed tomosynthesis imaging quality was characterized by spatial frequency dependent presampling modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) in 3D. The results showed that both MTF and NPS were affected by the angular range of the tomosynthesis scan and the reconstruction filters. For image planes parallel to the detector (in-plane), MTF at low frequencies was improved with increase in angular range. The shape of the NPS was affected by the reconstruction filters. Noise aliasing in 3D could be introduced by insufficient voxel sampling, especially in the z (slice-thickness) direction where the sampling distance (slice thickness) could be more than ten times that for in-plane images. Aliasing increases the noise at high frequencies, which causes degradation in DQE. Application of a reconstruction filter that limits the frequency components beyond the Nyquist frequency in the z direction, referred to as the slice thickness filter, eliminates noise aliasing and improves 3D DQE. The focal spot blur, which arises from continuous tube travel during tomosynthesis acquisition, could degrade DQE significantly because it introduces correlation in signal only, not NPS.
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87.63.lm Image enhancement
42.30.Wb Image reconstruction; tomography

VISION 20/20: Planning and delivery of intensity-modulated radiation therapy

Cedric X. Yu, Christopher J. Amies, and Michelle Svatos

Med. Phys. 35, 5233 (2008); http://dx.doi.org/10.1118/1.3002305 (9 pages) | Cited 15 times

Online Publication Date: 6 November 2008

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Intensity modulated radiation therapy (IMRT) is an advanced form of external beam radiation therapy. IMRT offers an additional dimension of freedom as compared with field shaping in three-dimensional conformal radiation therapy because the radiation intensities within a radiation field can be varied according to the preferences of locations within a given beam direction from which the radiation is directed to the tumor. This added freedom allows the treatment planning system to better shape the radiation doses to conform to the target volume while sparing surrounding normal structures. The resulting dosimetric advantage has shown to translate into clinical advantages of improving local and regional tumor control. It also offers a valuable mechanism for dose escalation to tumors while simultaneously reducing radiation toxicities to the surrounding normal tissue and sensitive structures. In less than a decade, IMRT has become common practice in radiation oncology. Looking forward, the authors wonder if IMRT has matured to such a point that the room for further improvement has diminished and so it is pertinent to ask what the future will hold for IMRT. This article attempts to look from the perspective of the current state of the technology to predict the immediate trends and the future directions. This article will (1) review the clinical experience of IMRT; (2) review what we learned in IMRT planning; (3) review different treatment delivery techniques; and finally, (4) predict the areas of advancements in the years to come.
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87.53.Kn Conformal radiation treatment
87.53.Bn Dosimetry/exposure assessment
87.55.dk Dose-volume analysis

RADIATION IMAGING PHYSICS: Image artifacts in digital breast tomosynthesis: Investigation of the effects of system geometry and reconstruction parameters using a linear system approach

Yue-Houng Hu, Bo Zhao, and Wei Zhao

Med. Phys. 35, 5242 (2008); http://dx.doi.org/10.1118/1.2996110 (11 pages) | Cited 26 times

Online Publication Date: 6 November 2008

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Digital breast tomosynthesis (DBT) is a three-dimensional (3D) x-ray imaging modality that reconstructs image slices parallel to the detector plane. Image acquisition is performed using a limited angular range (less than 50 degrees) and a limited number of projection views (less than 50 views). Due to incomplete data sampling, image artifacts are unavoidable in DBT. In this preliminary study, the image artifacts in DBT were investigated systematically using a linear system approximation. A cascaded linear system model of DBT was developed to calculate the 3D presampling modulation transfer function (MTF) with different image acquisition geometries and reconstruction filters using a filtered backprojection (FBP) algorithm. A thin, slanted tungsten (W) wire was used to measure the presampling MTF of the DBT system in the cross-sectional plane defined by the thickness (z-) and tube travel (x-) directions. The measurement was in excellent agreement with the calculation using the model. A small steel bead was used to calculate the artifact spread function (ASF) of the DBT system. The ASF was correlated with the convolution of the two-dimensional (2D) point spread function (PSF) of the system and the object function of the bead. The results showed that the cascaded linear system model can be used to predict the magnitude of image artifacts of small, high-contrast objects with different image acquisition geometry and reconstruction filters.
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87.59.E- Mammography
87.64.Aa Computer simulation

MAGNETIC RESONANCE PHYSICS: Development of a quantitative method for analysis of breast density based on three-dimensional breast MRI

Ke Nie, Jeon-Hor Chen, Siwa Chan, Man-Kwun I. Chau, Hon J. Yu, Shadfar Bahri, Tiffany Tseng, Orhan Nalcioglu, and Min-Ying Su

Med. Phys. 35, 5253 (2008); http://dx.doi.org/10.1118/1.3002306 (10 pages) | Cited 29 times

Online Publication Date: 6 November 2008

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Breast density has been established as an independent risk factor associated with the development of breast cancer. It is known that an increase of mammographic density is associated with an increased cancer risk. Since a mammogram is a projection image, different body position, level of compression, and the x-ray intensity may lead to a large variability in the density measurement. Breast MRI provides strong soft tissue contrast between fibroglandular and fatty tissues, and three-dimensional coverage of the entire breast, thus making it suitable for density analysis. To develop the MRI-based method, the first task is to achieve consistency in segmentation of the breast region from the body. The method included an initial segmentation based on body landmarks of each individual woman, followed by fuzzy C-mean (FCM) classification to exclude air and lung tissue, B-spline curve fitting to exclude chest wall muscle, and dynamic searching to exclude skin. Then, within the segmented breast, the adaptive FCM was used for simultaneous bias field correction and fibroglandular tissue segmentation. The intraoperator and interoperator reproducibility was evaluated using 11 selected cases covering a broad spectrum of breast densities with different parenchymal patterns. The average standard deviation for breast volume and percent density measurements was in the range of 3%–4% among three trials of one operator or among three different operators. The body position dependence was also investigated by performing scans of two healthy volunteers, each at five different positions, and found the variation in the range of 3%–4%. These initial results suggest that the technique based on three-dimensional MRI can achieve reasonable consistency to be applied in longitudinal follow-up studies to detect small changes. It may also provide a reliable method for evaluating the change of breast density for risk management of women, or for evaluating the benefits/risks when considering hormonal replacement therapy or chemoprevention.
Show PACS
87.61.Tg Clinical applications
87.19.xj Cancer
87.57.nm Segmentation
07.05.Pj Image processing
02.60.Ed Interpolation; curve fitting

RADIATION THERAPY PHYSICS: On the use of HDR 60Co source with the MammoSite® Radiation Therapy System

D. Baltas, G. Lymperopoulou, and N. Zamboglou

Med. Phys. 35, 5263 (2008); http://dx.doi.org/10.1118/1.3002312 (6 pages) | Cited 2 times

Online Publication Date: 6 November 2008

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This work summarizes Monte Carlo results in order to evaluate the potential of using HDR 60Co sources in accelerated partial breast irradiation (APBI) with the MammoSite® applicator. Simulations have been performed using the MCNP5 Monte Carlo Code, in simple geometries comprised of two concentric spheres; the internal consisting of selected concentrations, C, of a radiographic contrast solution in water (Omnipaque 300™) to simulate the MammoSite balloon and the external consisting of water to simulate surrounding tissue. The magnitude of the perturbation of delivered dose due to the radiographic contrast medium used in the MammoSite® applicator is calculated. At the very close vicinity of the balloon surface, a dose build-up region is observed, which leads to a dose overestimation by the treatment planning system (TPS) which depends on Omnipaque™ 300 solution concentration (and is in order of 2.3%, 3.0%, and 4.5%, respectively, at 1 mm away from the balloon - water interface, for C=10%, 15%, and 20%). However, dose overestimation by the TPS is minimal for points lying at the prescription distance (d=1 cm) or beyond, for all simulated concentrations and radii of MammoSite® balloon. An analytical estimation of the integral dose outside the CTV in the simple geometries simulated shows that dose to the breast for MammoSite® applications is expected to be comparable using HDR 60Co and 192Ir sources, and higher than that for 169Yb. The higher enegies of 60Co sources result to approximately twice radiation protection requirements as compared to 169Ir sources. However, they allow for more accurate dosimetry calculation with currently used treatment planning algorithms for 60Co sources, compared to 169Ir.
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87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment
87.55.dk Dose-volume analysis
87.19.xj Cancer
87.55.K- Monte Carlo methods

RADIATION THERAPY PHYSICS: A simplified method of four-dimensional dose accumulation using the mean patient density representation

Carri K. Glide-Hurst, Geoffrey D. Hugo, Jian Liang, and Di Yan

Med. Phys. 35, 5269 (2008); http://dx.doi.org/10.1118/1.3002304 (9 pages) | Cited 13 times

Online Publication Date: 6 November 2008

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The purpose of this work was to demonstrate, both in phantom and patient, the feasibility of using an average 4DCT image set (AVG-CT) for 4D cumulative dose estimation. A series of 4DCT numerical phantoms and corresponding AVG-CTs were generated. For full 4D dose summation, static dose was calculated on each phase and cumulative dose was determined by combining each phase’s static dose distribution with known tumor displacement. The AVG-CT cumulative dose was calculated similarly, although the same AVG-CT static dose distribution was used for all phases (i.e., tumor displacements). Four lung cancer cases were also evaluated for stereotactic body radiotherapy and conformal treatments; however, deformable image registration of the 4DCTs was used to generate the displacement vector fields (DVFs) describing patient-specific motion. Dose discrepancy between full 4D summation and AVG-CT approach was calculated and compared. For all phantoms, AVG-CT approximation yielded slightly higher cumulative doses compared to full 4D summation, with dose discrepancy increasing with increased tumor excursion. In vivo, using the AVG-CT coupled with deformable registration yielded clinically insignificant differences for all GTV parameters including the minimum, mean, maximum, dose to 99% of target, and dose to 1% of target. Furthermore, analysis of the spinal cord, esophagus, and heart revealed negligible differences in major dosimetric indices and dose coverage between the two dose calculation techniques. Simplifying 4D dose accumulation via the AVG-CT, while fully accounting for tumor deformation due to respiratory motion, has been validated, thereby, introducing the potential to streamline the use of 4D dose calculations in clinical practice, particularly for adaptive planning purposes.
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87.57.Q- Computed tomography
87.55.dk Dose-volume analysis
87.19.Wx Pneumodyamics, respiration
87.57.nj Registration
87.53.Ly Stereotactic radiosurgery
87.53.Kn Conformal radiation treatment

RADIATION IMAGING PHYSICS: Mean glandular dose estimation using MCNPX for a digital breast tomosynthesis system with tungsten/aluminum and tungsten/aluminum+silver x-ray anode-filter combinations

Andy K. W. Ma, Dimitra G. Darambara, Alexander Stewart, Spencer Gunn, and Edward Bullard

Med. Phys. 35, 5278 (2008); http://dx.doi.org/10.1118/1.3002310 (12 pages) | Cited 9 times

Online Publication Date: 6 November 2008

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Breast cancer screening with x-ray mammography, using one or two projection images of the breast, is an indispensible tool in the early detection of breast cancer in women. Digital breast tomosynthesis (DBT) is a 3D imaging technique that promises higher sensitivity and specificity in breast cancer screening at a similar radiation dose to conventional two-view screening mammography. In DBT a 3D volume is reconstructed with anisotropic voxels from a limited number of x-ray projection images acquired over a limited angle. Although the benefit of early cancer detection through screening mammography outweighs the potential risks associated with radiation, the radiation dosage to women in terms of mean glandular dose (MGD) is carefully monitored. This work studies the MGD arising from a prototype DBT system under various parameters. Two anode/filter combinations (W∕Al and W∕Al+Ag) were investigated; the tube potential ranges from 20 to 50 kVp; and the breast size varied between 4 and 10 cm chest wall-to-nipple distance and between 3 and 7 cm compressed breast thickness. The dosimetric effect of breast positioning with respect to the imaging detector was also reviewed. It was found that the position of the breast can affect the MGD by as much as 5% to 13% depending on the breast size.
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87.53.Bn Dosimetry/exposure assessment
87.19.xj Cancer
87.59.E- Mammography
87.57.rh Mammography
87.57.Q- Computed tomography

RADIATION IMAGING PHYSICS: Texture classification-based segmentation of lung affected by interstitial pneumonia in high-resolution CT

Panayiotis Korfiatis, Christina Kalogeropoulou, Anna Karahaliou, Alexandra Kazantzi, Spyros Skiadopoulos, and Lena Costaridou

Med. Phys. 35, 5290 (2008); http://dx.doi.org/10.1118/1.3003066 (13 pages) | Cited 7 times

Online Publication Date: 6 November 2008

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Accurate and automated lung field (LF) segmentation in high-resolution computed tomography (HRCT) is highly challenged by the presence of pathologies affecting lung borders, also affecting the performance of computer-aided diagnosis (CAD) schemes. In this work, a two-dimensional LF segmentation algorithm adapted to interstitial pneumonia (IP) patterns is presented. The algorithm employs k-means clustering followed by a filling operation to obtain an initial LF order estimate. The final LF border is obtained by an iterative support vector machine neighborhood labeling of border pixels based on gray level and wavelet coefficient statistics features. A second feature set based on gray level averaging and gradient features was also investigated to evaluate its effect on segmentation performance of the proposed method. The proposed method is evaluated on a dataset of 22 HRCT cases spanning a range of IP patterns such as ground glass, reticular, and honeycombing. The accuracy of the method is assessed using area overlap and shape differentiation metrics (dmean, drms, and dmax), by comparing automatically derived lung borders to manually traced ones, and further compared to a gray level thresholding-based (GLT-based) method. Accuracy of the methods evaluated is also compared to interobserver variability. The proposed method incorporating gray level and wavelet coefficient statistics demonstrated the highest segmentation accuracy, averaged over left and right LFs (overlap=0.954, dmean=1.080 mm, drms=1.407 mm, and dmax=4.944 mm), which is statistically significant (two-tailed student’s t test for paired data, p<0.0083) with respect to all metrics considered as compared to the proposed method incorporating gray level averaging and gradient features (overlap=0.918, dmean=2.354 mm, drms=3.711 mm, and dmax=14.412 mm) and the GLT-based method (overlap=0.897, dmean=3.618 mm, drms=5.007 mm, and dmax=16.893 mm). The performance of the three segmentation methods, although decreased as IP pattern severity level (mild, moderate, and severe) was increased, did not demonstrate statistically significant difference (two-tailed student’s t test for unpaired data, p>0.0167 for all metrics considered). Finally, the accuracy of the proposed method, based on gray level and wavelet coefficient statistics ranges within interobserver variability. The proposed segmentation method could be used as an initial stage of a CAD scheme for IP patterns.
Show PACS
87.57.Q- Computed tomography
87.57.R- Computer-aided diagnosis
87.57.nm Segmentation
87.57.cf Spatial resolution
87.19.X- Diseases
87.19.Wx Pneumodyamics, respiration
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
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Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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