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Dec 2001

Volume 28, Issue 12, pp. 2391-2593

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POINT/COUNTERPOINT: The World Wide Web is a net disservice to medical physicists in developing countries

Kwan-Hoong Ng, Azam Niroomand-Rad, and William R. Hendee, Moderator

Med. Phys. 28, 2391 (2001); http://dx.doi.org/10.1118/1.1421374 (3 pages) | Cited 1 time

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© 2001 American Association of Physicists in Medicine.
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89.20.Hh World Wide Web, Internet
87.90.+y Other topics in biological and medical physics (restricted to new topics in section 87)

RADIATION IMAGING PHYSICS: Application of the mutual information criterion for feature selection in computer-aided diagnosis

Georgia D. Tourassi, Erik D. Frederick, Mia K. Markey, and Carey E. Floyd, Jr.

Med. Phys. 28, 2394 (2001); http://dx.doi.org/10.1118/1.1418724 (9 pages) | Cited 31 times

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The purpose of this study was to investigate an information theoretic approach to feature selection for computer-aided diagnosis (CAD). The approach is based on the mutual information (MI) concept. MI measures the general dependence of random variables without making any assumptions about the nature of their underlying relationships. Consequently, MI can potentially offer some advantages over feature selection techniques that focus only on the linear relationships of variables. This study was based on a database of statistical texture features extracted from perfusion lung scans. The ultimate goal was to select the optimal subset of features for the computer-aided diagnosis of acute pulmonary embolism (PE). Initially, the study addressed issues regarding the approximation of MI in a limited dataset as it is often the case in CAD applications. The MI selected features were compared to those features selected using stepwise linear discriminant analysis and genetic algorithms for the same PE database. Linear and nonlinear decision models were implemented to merge the selected features into a final diagnosis. Results showed that the MI is an effective feature selection criterion for nonlinear CAD models overcoming some of the well-known limitations and computational complexities of other popular feature selection techniques in the field.© 2001 American Association of Physicists in Medicine.
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87.57.N- Image analysis

RADIATION IMAGING PHYSICS: Computerized classification of suspicious regions in chest radiographs using subregion Hotelling observers

Alan H. Baydush, David M. Catarious, Jr., Joseph Y. Lo, Craig. K. Abbey, and Carey E. Floyd, Jr.

Med. Phys. 28, 2403 (2001); http://dx.doi.org/10.1118/1.1420402 (7 pages) | Cited 4 times

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We propose to investigate the use of subregion Hotelling observers (SRHOs) in conjunction with perceptrons for the computerized classification of suspicious regions in chest radiographs for being nodules requiring follow up. Previously, 239 regions of interest (ROIs), each containing a suspicious lesion with proven classification, were collected. We chose to investigate the use of SRHOs as part of a multilayer classifier to determine the presence of a nodule. Each SRHO incorporates information about signal, background, and noise correlation for classification. For this study, 225 separate Hotelling observers were set up in a grid across each ROI. Each separate observer discriminates an 8 by 8 pixel area. A round robin sampling scheme was used to generate the 225 features, where each feature is the output of the individual observers. These features were then rank ordered by the magnitude of the weights of a perceptron. Once rank ordered, subsets of increasing number of features were selected to be used in another perceptron. This perceptron was trained to minimize mean squared error and the output was a continuous variable representing the likelihood of the region being a nodule. Performance was evaluated by receiver operating characteristic (ROC) analysis and reported as the area under the curve (AZ). The classifier was optimized by adding additional features until the AZ declined. The optimized subset of observers then were combined using a third perceptron. A subset of 80 features was selected which gave an AZ of 0.972. Additionally, at 98.6% sensitivity, the classifier had a specificity of 71.3% and increased the positive predictive value from 60.7% to 84.1%. Preliminary results suggest that using SRHOs in combination with perceptrons can provide a successful classification scheme for pulmonary nodules. This approach could be incorporated into a larger computer aided detection system for decreasing false positives. © 2001 American Association of Physicists in Medicine.
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87.59.B- Radiography
87.57.N- Image analysis

RADIATION IMAGING PHYSICS: Mathematical morphology in computerized analysis of angiograms in age-related macular degeneration

Angela Barthes, John Conrath, Monique Rasigni, Mouloud Adel, and Jean-Pierre Petrakian

Med. Phys. 28, 2410 (2001); http://dx.doi.org/10.1118/1.1420735 (10 pages) | Cited 6 times

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This paper deals with image processing of numerical retinal angiograms in order to facilitate diagnosis and follow-up in age-related macular degeneration (AMD) which is currently the main cause of blindness in industrialized countries. A computerized scheme using principally mathematical morphology operators is proposed for detecting and counting drusen, which are precursor lesions of the ocular fundus. In order to check the feasibility of this approach, results relative to 58 retinal images are compared with those given by three retinal specialists independently. From manual counting measures it is found that interobserver correlation coefficients lie in the range 0.71–0.78. On the other hand, a correlation coefficient of 0.89 is obtained when the average of the three expert countings is compared with the drusen number given by the computerized method. This coefficient is improved from 0.89 to 0.93 by processing only frames captured immediately after the appearance of a dye consecutive to intraveinous sodium fluorescein injection. Compared to the manual analysis which is, among other things, tedious and time consuming, the computerized analysis is both quicker and more objective. Validation by the practitioner is however necessary, given possible detection errors. The proposed computerized scheme for detecting and counting drusen may be easily automated and so should prove useful in clinical studies which involve high volume analysis of retinal angiograms. © 2001 American Association of Physicists in Medicine.
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87.63.L- Visual imaging
87.57.N- Image analysis
42.66.Ct Anatomy and optics of eye
02.00.00 Mathematical methods in physics
87.10.-e General theory and mathematical aspects

RADIATION IMAGING PHYSICS: Comparison of light and x-ray sensitometric responses of double-emulsion films for different processing conditions

Christian Blendl and Egbert Buhr

Med. Phys. 28, 2420 (2001); http://dx.doi.org/10.1118/1.1418723 (7 pages) | Cited 2 times

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The effects of different film processing conditions on light and x-ray sensitometric responses were compared for a variety of double-emulsion x-ray films. The processing conditions were altered by changes of the developer temperature. Three different exposure variants were applied: x-ray sensitometry using two stepped neutral density attenuators between film and screens, simultaneous double-sided light sensitometry, and single-sided light sensitometry. 13 different types of double-emulsion x-ray films were investigated, among them three asymmetric films. In the special case of exposing the asymmetric films with the single-sided light sensitometer, a method was investigated where each side of the film is exposed at different locations and the sum effect is analyzed. From each sensitometric curve shape two parameters, the logarithmic speed (log S) and the average gradient (G), were evaluated. The results of this study can be summarized as follows: (1) Single-sided and double-sided light sensitometers revealed almost equal changes of log S when the processing conditions are altered. Thus, single-sided light sensitometers can serve as a substitute for double-sided light sensitometers provided that suited exposure methods are used and appropriate sensitometric parameters are evaluated. (2) Light sensitometry quantitatively indicated changes of the film processing that affect the x-ray speed. Hence, light sensitometry is a useful method to monitor changes in film processing. © 2001 American Association of Physicists in Medicine.
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87.59.B- Radiography

RADIATION IMAGING PHYSICS: Microwave-induced thermoacoustic tomography: Reconstruction by synthetic aperture

Dazi Feng, Yuan Xu, Geng Ku, and Lihong V. Wang

Med. Phys. 28, 2427 (2001); http://dx.doi.org/10.1118/1.1418015 (5 pages) | Cited 13 times

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We have applied the synthetic-aperture method to linear-scanning microwave-induced thermoacoustic tomography in biological tissues. A nonfocused ultrasonic transducer was used to receive thermoacoustic signals, to which the delay-and-sum algorithm was applied for image reconstruction. We greatly improved the lateral resolution of images and acquired a clear view of the circular boundaries of buried cylindrical objects, which could not be obtained in conventional linear-scanning microwave-induced thermoacoustic tomography based on focused transducers. Two microwave sources, which had frequencies of 9 and 3 GHz, respectively, were used in the experiments for comparison. The 3 GHz system had a much larger imaging depth but a lower signal–noise ratio than the 9 GHz system in near-surface imaging. © 2001 American Association of Physicists in Medicine.
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87.63.-d Non-ionizing radiation equipment and techniques
87.57.N- Image analysis
43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
87.19.Pp Biothermics and thermal processes in biology
43.80.Qf Medical diagnosis with acoustics

RADIATION IMAGING PHYSICS: Detection of tumorigenesis in rat bladders with optical coherence tomography

Yingtian Pan, John P. Lavelle, Sheldon I. Bastacky, Susan Meyers, Georgi Pirtskhalaishvili, Mark L. Zeidel, and Daniel L. Farkas

Med. Phys. 28, 2432 (2001); http://dx.doi.org/10.1118/1.1418726 (9 pages) | Cited 21 times

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Optical coherence tomography (OCT) is a novel technique that enables noninvasive cross-sectional imaging of biological tissues. Because of its high resolution (∼10 μm), superior dynamic range (140 dB in our case) and up to 2–3 mm penetration depth, OCT is potentially useful for noninvasive screening of superficial lesions. Bladder cancer arises within the transitional epithelium. Despite the ability to visualize the epithelium via cystoscopy, it is often difficult to detect early epithelial cancers and to determine their penetration to the underlying layers. To investigate the potential of OCT to enhance imaging of bladder cancers and other epithelial lesions, we applied OCT to normal and diseased bladder epithelium, and correlated the results with histological findings. OCT images of porcine bladder (a close homolog of human bladder) confirm the ability of this method to image human tissues. To determine whether OCT can track the course of bladder cancer, a standard rat model of bladder cancer in which Fisher rats are exposed to methyl-nitroso-urea (MNU), was followed both with OCT and histological studies. Our results show that the micro morphology of porcine bladder such as the urothelium, submucosa and muscles is identified by OCT and well correlated with the histological evaluations. OCT detected edema, inflammatory infiltrates, and submucosal blood congestion as well as the abnormal growth of urothelium (e.g., papillary hyperplasia and carcinomas). By contrast, surface imaging, which resembles cystoscopy, provided far less sensitivity and resolution than OCT. This is the first OCT study of any tumor documented in a systematic fashion, and the results suggest the potential of OCT for the noninvasive diagnosis of both bladder inflammatory lesions and early urothelial abnormalities, which conventional cystoscopy often misses, by imaging characterization of the increases in urothelial thickening and backscattering. However, because of the depth limitation, OCT may have limited applications in staging the invasion of higher-state urothelial cancers, especially for papillary carcinomas. © 2001 American Association of Physicists in Medicine.
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87.63.L- Visual imaging
87.19.X- Diseases

RADIATION THERAPY PHYSICS: Delivery of intensity-modulated radiation therapy with a conventional multileaf collimator: Comparison of dynamic and segmental methods

Chen-Shou Chui, Maria F. Chan, Ellen Yorke, Spiridon Spirou, and C. Clifton Ling

Med. Phys. 28, 2441 (2001); http://dx.doi.org/10.1118/1.1418018 (9 pages) | Cited 37 times

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Intensity-modulated radiation therapy (IMRT) can be delivered with a conventional multileaf collimator (MLC), either in dynamic mode (DMLC) or in segmental mode (SMLC, also known as “step-and-shoot”). The advantage of DMLC is its ability to deliver the desired intensity profile produced by inverse planning with a high degree of fidelity. The SMLC method, on the other hand, resembles treatment with multiple static fields, and can be more easily verified. However, the use of SMLC requires that the desired profile be approximated by discrete levels of intensity, which may lead to degradation in the delivered dose distribution. Clearly, the results of SMLC delivery depend on the number of levels and the spatial resolution of the intensity distribution. In this work, we compare the DMLC method and the SMLC method employing different numbers of levels and different spatial resolutions. Three disease sites were studied: prostate, nasopharynx, and breast, with three cases for each. In general, a 5- to 10-level SMLC plan produced results comparable to that from a DMLC plan. The target coverage is improved by increasing the number of levels while critical organs are better protected with finer spatial resolutions. The beam-on-time (MUs) requirement for SMLC was approximately 20% less than DMLC, but the delivery time (in minutes) was about twice as long. Thus, the choice depends on many factors including machine capability, quality assurance, target coverage, critical organ protection, beam-on-time, delivery time, and other clinical considerations. © 2001 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.56.Da Ancillary equipment

RADIATION THERAPY PHYSICS: Improved leaf sequencing reduces segments or monitor units needed to deliver IMRT using multileaf collimators

Mark Langer, Van Thai, and Lech Papiez

Med. Phys. 28, 2450 (2001); http://dx.doi.org/10.1118/1.1420392 (9 pages) | Cited 32 times

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Leaf sequencing algorithms may use an unnecessary number of monitor units or segments to generate intensity maps for delivery of intensity modulated radiotherapy (IMRT) using multiple static fields. An integer algorithm was devised to generate a sequence with the fewest possible segments when the minimum number of monitor units are used. Special hardware related restrictions on leaf motion can be incorporated. The algorithm was tested using a benchmark map from the literature and clinical examples. Results were compared to sequences given by the routine of Bortfeld that minimizes monitor units by treating each row independently, and the areal or reducing routines that use fewer segments at the price of more monitor units. The Bortfeld algorithm used on average 58% more segments than provided by the integer algorithm with bidirectional motion and 32% more segments than did an integer algorithm admitting only unidirectional sequences. The areal algorithm used 48% more monitor units and the reducing algorithm used 23% more monitor units than did the bidirectional integer algorithm, while the areal and reducing algorithms used 23% more segments than did the integer algorithm. Improved leaf sequencing algorithms can allow more efficient delivery of static field IMRT. The integer algorithm demonstrates the efficiencies possible with an improved routine and opens a new avenue for development. © 2001 American Association of Physicists in Medicine.
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87.56.Da Ancillary equipment

RADIATION THERAPY PHYSICS: Matching tomographic IMRT fields with static photon fields

A. Sethi, L. Leybovich, N. Dogan, and B. Emami

Med. Phys. 28, 2459 (2001); http://dx.doi.org/10.1118/1.1418238 (7 pages) | Cited 2 times

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The matching of abutting radiation fields presents a challenging problem in radiation therapy. Due to sharp penumbra of linear accelerator beams, small (1–2 mm) errors in field positioning can lead to large (>30%) hot or cold spots in the abutment region. With head and neck immobilization devices (thermoplastic mask/aquaplast) an average setup error of 3 mm has been reported. Therefore hot or cold spots approaching 50% of the prescription dose may occur along the matchline. Although abutting radiation fields have been investigated for static fields, there is no reported study regarding matching of tomographic IMRT and static fields. Compared to static fields, the matching of tomographic IMRT fields with static fields is more complicated. Since IMRT and static fields are planned on separate treatment planning computers, the dose in the abutment region is not specified. In addition, commonly used techniques for matching fields, such as feathering of junctions, are not practical. We have developed a method that substantially reduces dose inhomogeneity in the abutment region. In this method, a “buffer zone” around the matchline was created and was included as part of the target for both IMRT and static field plans. In both fields, a small dose gradient (⩽3%/mm) in the buffer zone was created. In the IMRT plan, the buffer zone was divided into three sections with dose varying from 83% to 25% of prescription dose. The static field dose profile was modified using either a specially designed physical (hard) or a dynamic (soft) wedge. When these modified fields were matched, the combined dose in the abutment region varied by ⩽10% in the presence of setup errors spanning 4 mm (±2 mm) when the hard wedge was used and 10 mm (±5 mm) with the soft wedge. © 2001 American Association of Physicists in Medicine.
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87.53.Bn Dosimetry/exposure assessment
87.56.-v Radiation therapy equipment
87.59.bd Computed radiography
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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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