We present preliminary investigations that examine the feasibility of incorporating volumetric images generated using
digital tomosynthesis into brachytherapy treatment planning. The Integrated Brachytherapy Unit (IBU) at our facility
consists of an L-arm, C-arm isocentric motion system with an x-ray tube and fluoroscopic imager attached. Clinically,
this unit is used to generate oblique, anterior-posterior, and lateral images for simple treatment planning and dose
prescriptions. Oncologists would strongly prefer to have volumetric data to better determine three dimensional dose
distributions (dose-volume histograms) to the target area and organs at risk. Moving the patient back and forth to CT
causes undo stress on the patient, allows extensive motion of organs and treatment applicators, and adds additional time
to patient treatment. We propose to use the IBU imaging system with digital tomosynthesis to generate volumetric
patient data, which can be used for improving treatment planning and overall reducing treatment time. Initial image data
sets will be acquired over a limited arc of a human-like phantom composed of real bones and tissue equivalent material.
A brachytherapy applicator will be incorporated into one of the phantoms for visualization purposes. Digital
tomosynthesis will be used to generate a volumetric image of this phantom setup. This volumetric image set will be
visually inspected to determine the feasibility of future incorporation of these types of images into brachytherapy
treatment planning. We conclude that initial images using the tomosynthesis reconstruction technique show much
promise and bode well for future work.
KEYWORDS: Performance modeling, Solid modeling, Visual process modeling, Tumor growth modeling, Databases, Image filtering, Digital mammography, Mammography, Breast cancer, Human vision and color perception
Human vision models have been shown to capture the response of the visual system; their incorporation into the classification stage of a Computer Aided Detection system could improve performance. This study seeks to improve the performance of an automated breast mass detection system by using the Watson filter model versus a Laguerre Gauss Channelized Hotelling Observer (LG-CHO). The LG-CHO and the Watson filter model were trained and tested on a 512x512 ROI database acquired from the Digital Database of Screening Mammography consisting of 800 total ROIs; 200 of which were malignant, 200 were benign and 400 were normal. Half of the ROIs were used to train the weights for ten LG-CHO templates that were later used during the testing stage. For the Watson filter model, the training cases were used to optimize the frequency filter parameter empirically to yield the best ROC Az performance. This set of filter parameters was then tested on the remaining cases. The training Az for the LG-CHO and the Watson filter was 0.896 +/- 0.016 and 0.924 +/- 0.014 respectively. The testing Az for the LG-CHO and Watson filter was 0.849 +/- 0.019 and 0.888 +/- 0.017. With a p-value of 0.029, the difference in testing performance was statistically significant, thus implying that the Watson filter model holds promise for better detection of masses.
We present preliminary investigations that examine the feasibility of incorporating digital tomosynthesis into radiation oncology practice with the use of kilovoltage on-board imagers (OBI). Modern radiation oncology linear accelerators now include hardware options for the addition of OBI for on-line patient setup verification. These systems include an x-ray tube and detector mounted directly on the accelerator gantry that rotate with the same isocenter. Applications include cone beam computed tomography (CBCT), fluoroscopy, and radiographs to examine daily patient positioning to determine if the patient is in the same location as the treatment plan. While CBCT provides the greatest anatomical detail, this approach is limited by long acquisition and reconstruction times and higher patient dose. We propose to examine the use of tomosynthesis reconstructed volumetric data from limited angle projection images for short imaging time and reduced patient dose. Initial data uses 61 projection images acquired over an isocentric arc of twenty degrees with the detector approximately fifty-four centimeters from isocenter. A modified filtered back projection technique, which included a mathematical correction for isocentric motion, was used to reconstruct volume images. These images will be visually and mathematically compared to volumetric computed tomography images to determine efficacy of this system for daily patient positioning verification. Initial images using the tomosynthesis reconstruction technique show much promise and bode well for effective daily patient positioning verification with reduced patient dose and imaging time. Additionally, the fast image acquisition may allow for a single breath hold imaging sequence, which will have no breath motion.
The purpose of this paper is to present a new segmentation routine developed for mammographic masses. We previously developed a computer-aided detection (CAD) system for mammographic masses that employed a simple but imprecise segmentation procedure. To improve the systems performance, an iterative, linear segmentation routine was developed. The routine begins by employing a linear discriminant function to determine the optimal threshold between estimates of an objects interior and exterior pixels. After applying the threshold and identifying the objects outline, two constraints are applied to minimize the influence of extraneous background structures. Each iteration further refines the outline until the stopping criterion is reached. The segmentation algorithm was tested on a database of 181 mammographic images that contained forty-nine malignant and fifty benign masses. A set of suspicious regions of interest (ROIs) was found using the previous CAD system. Twenty features were measured from the regions before and after applying the new segmentation routine. The difference in the features discriminatory ability was examined via receiver operating characteristic (ROC) analysis. A significant performance difference was observed in many features, particularly those describing the object border. Free-response ROC (FROC) curves were utilized to examine how the overall CAD system performance changed with the inclusion of the segmentation routine. The FROC performance appeared to be improved, especially for malignant masses. When detecting 90% of the malignant masses, the previous system achieved 4.4 false positives per image (FPpI) compared to the post-segmentation systems 3.7 FPpI. At 85%, the respective FPpI are 4.1 and 2.1.
Bi-plane correlation imaging (BCI) is a new imaging approach that utilizes angular information from a bi-plane digital acquisition in conjunction with computer assisted detection (CAD) to reduce the degrading influence of anatomical noise in the detection of subtle lesions in planar images. An anthropomorphic chest phantom, supplemented with added nodule phantoms (5-13 mm at the image plane), was imaged from different posterior projections within a ±12° range by moving the x-ray tube vertically and horizontally with respect to the detector. Each image was analyzed using a basic front-end single-view CAD algorithm. The correlation of the suspect lesions from the PA view with those from each of the oblique views was examined using a priori knowledge of the acquisition geometry. The correlated suspect lesions were registered as positive. Using an optimum --3° vertical geometry and processing parameters, BCI resulted in 62.5% sensitivity, 1.5 FP/image, and 0.885 PPV. The corresponding values from the observer experiment were 56% sensitivity, 10.8 FP/image, and 0.45 PPV, respectively. Compared to single-view CAD results, the BCI reduced sensitivity by 20%. However, the corresponding reduction in FPs was notably higher (94%) leading to 140% improvement in the PPV. Changes in processing parameters could result in higher PPV and lower FP/image at the expense of lower sensitivity. Similar findings were indicated for small (5-9 mm) and large (9-13 mm) nodules, but the relative improvement was significantly higher for smaller nodules. (The research was supported by a grant from the NIH, R21CA91806.)
KEYWORDS: Databases, Mammography, CAD systems, Computer aided diagnosis and therapy, Cancer, Data modeling, Medical imaging, Solid modeling, Statistical analysis, Breast cancer
We propose to investigate the use of a Laguerre-Gauss Channelized Hotelling Observer (LG-CHO) for the basis of a computer aided detection scheme for masses in mammography. A database of 1320 regions of interest was selected from the DDSM database collected by the University of South Florida. The breakdown of the cases was: 656 normals, 307 benigns, and 357 cancers. For the detection task, cancer and benign cases were considered positive and normal was considered negative. A 25 channel LG-CHO was designed to best classify regions as containing a mass or not. Application of this LG-CHO to the database gave a ROC area under the curve of 0.936 and a partial area of 0.648. Additionally, at 98% sensitivity the classifier had a specificity of 44.8% and a positive predictive value of 64.2%. Preliminary results suggest that using a LG-CHO can provide a strong backbone for a CAD scheme to help radiologists with detection. These initial results should be able to be incorporated into a larger CAD system for higher performance either as a false positive reduction scheme or as an initial filter used for mass detection.
In this paper, we present preliminary results from a highly sensitive and specific CAD system for mammographic masses. For false positive reduction, the system incorporated features derived from shape, fractal, and channelized Hotelling observer (CHO) measurements. The database for this study consisted of 80 craniocaudal mammograms randomly extracted from USF's digital database for screening mammography. The database contained 49 mass findings (24 malignant, 25 benign). To detect initial mass candidates, a difference of Gaussians (DOG) filter was applied through normalized cross correlation. Suspicious regions were localized in the filtered images via multi-level thresholding. Features extracted from the regions included shape, fractal dimension, and the output from a Laguerre-Gauss (LG) CHO. Influential features were identified via feature selection techniques. The regions were classified with a linear classifier using leave-one-out training/testing. The DOG filter achieved a sensitivity of 88% (23/24 malignant, 20/25 benign). Using the selected features, the false positives per image dropped from ~20 to ~5 with no loss in sensitivity. This preliminary investigation of combining multi-level thresholded DOG-filtered images with shape, fractal, and LG-CHO features shows great promise as a mass detector. Future work will include the addition of more texture and mass-boundary descriptive features as well as further exploration of the LG-CHO.
We describe a probit regression approach for maximum-likelihood (ML) estimation of a linear observer template from human-observer data in two-alternative forced-choice experiments. Like a previous approach to ML estimation in this problem [Abbey & Eckstein, Proc. SPIE, Vol. 4324, 2001], our approach does not make any assumptions about the distribution of the images. The previous approach utilized a regularizing prior distribution to control the degrees of freedom in the problem. In this work, we constrain the observer template to be represented by a limited number of linear features. Standard methods of probit regression are described for estimating the feature weights, and hence the observer templates. We have used this probit regression method to estimate human-observer templates for the detection of a small (5mm diameter) round simulated mass embedded in digitized mammograms. Our estimated templates for detecting the mass contain a band of heavily weighted spatial frequencies from 0.08 to 0.3 cycles/mm. We show comparisons between the human-observer template data, and the templates of a number of linear model observers that have been investigated as perceptual models of the human.
This paper describes the development of a computer-aided diagnosis (CAD) tool for solitary pulmonary nodules. This CAD tool is built upon physically meaningful features that were selected because of their relevance to shape and texture. These features included a modified version of the Hotelling statistic (HS), a channelized HS, three measures of fractal properties, two measures of spicularity, and three manually measured shape features. These features were measured from a difficult database consisting of 237 regions of interest (ROIs) extracted from digitized chest radiographs. The center of each 256x256 pixel ROI contained a suspicious lesion which was sent to follow-up by a radiologist and whose nature was later clinically determined. Linear discriminant analysis (LDA) was used to search the feature space via sequential forward search using percentage correct as the performance metric. An optimized feature subset, selected for the highest accuracy, was then fed into a three layer artificial neural network (ANN). The ANN's performance was assessed by receiver operating characteristic (ROC) analysis. A leave-one-out testing/training methodology was employed for the ROC analysis. The performance of this system is competitive with that of three radiologists on the same database.
We propose to investigate a novel use of the Hotelling observer for the task of discrimination of solitary pulmonary nodules from a database of regions that were all deemed suspicious. A database of 239 regions of interest (ROIs) was collected from digitized chest radiographs. Each of these 256x256 pixel ROIs contained a suspicious lesion in the center for which we have a truth file. For our study, 25 separate Hotelling observers were set up in a 5x5 grid across the center of the ROIs. Each separate observer was designed to 'observe' a 15x15 pixel area of the image. Leave-one-out training was used to generate 25 output observer features. These 25 features were then narrowed down using a sequential forward searching linear discriminant analysis. The forward search was continued until the accuracy declined at 13 features and the subset was used as the input layer to an artificial neural network (ANN). This network was trained to minimize mean squared error and the output was the area under the ROC curve. The trained ANN gave an ROC area of .86. In comparison, three radiologists performed at ROC area indexes of .72, .79, and .83.
Recent developments in digital detectors have led to investigating the importance of grids in mammography. We propose to examine the use Bayesian Image Estimation (BIE) as a software means of removing scatter post acquisition and to compare this technique to a grid. BIE is an iterative, non- linear statistical estimation technique that reduces scatter content while improving CNR. Images of the ACR breast phantom were acquired both with and without a grid on a calibrated digital mammography system. A BIE algorithm was developed and was used to process the images acquired without the grid. Scatter fractions (SF) were compared for the image acquired with the grid, the image acquired without the grid, and the image acquired without the grid and processed by BIE. Images acquired without the anti-scatter grid had an initial SF of 0.46. Application of the Bayesian image estimation technique reduced this to 0.03. In comparison, the use of the grid reduced the SF to 0.19. The use of Bayesian image estimation in digital mammography is beneficial in reducing scatter fractions. This technique is very useful as it can reduce scatter content effectively without introducing any adverse effects such as aliasing caused by gridlines.
Previously, we have demonstrated the ability of Bayesian image estimation (BIE) to reduce scatter and improve image contrast to noise ratio (CNR) in chest radiography without degradation of resolution. Here, we compare the effectiveness of BIE to a standard 12:1 grid. Images of a geometric phantom with two inches of added polystyrene were obtained both with and without a 12:1, 150 lp/mm grid. Images were acquired with standard protocols: 120 kVp, 72 inch source to image distance, and PA positioning. Images were acquired on a calibrated photostimulable phosphor system. An image exposure was used corresponding to the same patient dose as when acquiring film/screen chest images using a phototimer. The image acquired without the grid was processed by BIE for 6 iterations. Contrast, noise, and CNR were calculated and compared for the image acquired with the grid and the BIE processed image in different regions. BIE processing improved image CNR by 200 to 350% over that provided by the anti-scatter grid for the different regions. BIE provides higher CNR than that of a 12:1 grid. Because of this increase in CNR, Bayesian processed images will show an increase in detectability of low contrast objects, such as subtle lung nodules.
Previously, we have shown the effectiveness of using Bayesian image estimation (BIE) to reduce scatter and increase the contrast to noise ratio (CNR) in digital chest radiography without degradation of resolution. Here, we investigate the incorporation of a spatially varying scatter model. Previously, BIE used a simple model for scatter, where scatter was modeled as a spatially invariant radial exponential with a single full-width at half-maximum and magnitude. This invariance resulted in some overcompensation and some undercompensation. A new spatially varying scatter model, where each pixel can have a different scatter kernel magnitude, was incorporated into BIE and used to reduce scatter in quantitative chest radiographs. Scatter fractions were reduced to less than 3% in the lung and mediastinum at 8 iterations. The original BIE technique only reduced scatter fractions to less than 2% in the lung and 38% in the mediastinum. CNR was improved by approximately 60% in the lung region and 200% in the mediastinum. No degradation of resolution was measured. Visual inspection showed improvement of image quality. Incorporation of a spatially varying scatter model into BIE reduces scatter to levels which far exceed those provided by an anti-scatter grid and can increase CNR without loss of resolution.
Previously, we have shown the effectiveness of using Bayesian image estimation (BIE) to reduce scatter and increase the contrast to noise ratio (CNR) in digital chest radiography. Here, we investigate the use of BIE to reduce scatter and increase CNR on digital mammographic data. Calibrated photostimulable phosphor digital images were obtained of the American College of Radiologists (ACR) mammographic phantom at several different exposures. An iterative Bayesian estimation algorithm was used to process this data. Residual scatter fractions (RSF) and CNR were computed. Resolution was visually inspected. These results were compared to those of a mammogram acquired at standard clinical imaging parameters using an anti-scatter grid for scatter reduction. On average, at all exposure levels. BIE reduced scatter fractions from 57% to 6%, while a grid only reduced RSF to 31%. At similar exposure levels, BIE processing improved CNR to 21.6, while a grid produced images with a CNR of 15.8. At an exposure level of 37% less than the standard exposure, BIE improved CNR to 18.9. A visual assessment of resolution using the objects in the phantom showed no reduction of resolution. In some images, phantom masses appeared more readily apparent. BIE processing of mammographic data can reduce scatter and increase image CNR. This type of image processing may potentially allow for decreased radiation dose to the patient with no loss of image quality. BIE as a method for scatter compensation in mammography is very promising. This preliminary work shows improvement in CNR to values greater than that of a standard grid.
Statistical estimation techniques for medical images require well-founded models for the data formation and acquisition process. In our work, we attempt to quantify the appropriate empirical noise model for two alternative digital chest radiography modalities. In this study, we acquired image sets from the same clinical x-ray unit using photostimulable phosphor plates (PPP) and film/screen. The images were acquired in the range of 1-9 mR. The PPP data, at 1760 by 2140 pixels, were transferred directly to our workstations for analysis, and the films were digitized at 2048 by 2480 pixels (0.2 mm resolution for both modalities). 1024 by 1024 regions of interest (ROIs) without edge effects or artifacts were then extracted. Using Fourier transform methods, the noise power spectra were formed for all the images with smoothing applied near the axes to exclude frequency components due to beam profile, detector inhomogeneity and other structural noise. The noise power spectra were then integrated to find the variance at the exposure in question. This corresponds to an estimate of the non-structural variance. The data for all images obtained with one method were finally combined to form the variance function (variance as a function of the mean) for the modality. The data show that for the range of exposure considered, the PPP and digitized film modalities exhibit significant differences in noise structure. Subtle differences in the noise power spectra were seen. Of particular interest to our application, however, the variance function in the two cases was seen to differ in its functional form. The data from the photostimulable phosphor plates correspond well to a linear model (variance proportional to the mean) throughout most of the range investigated, whereas the digitized film variance function corresponds better to a higher-degree polynomial fit over the range considered. This study indicates that separate models may be warranted for different sources of digital chest radiographs and also suggests the type of assumption that, based on empirical findings, may be useful in such a framework.
Previously, we have shown that Maximum Likelihood Expectation Maximization (MLEM) can be used to effectively estimate a scatter reduced image in digital chest radiography; however, the MLEM technique is known to increase image noise. A MLEM-median (ML- median) technique has been implemented that follows each MLEM iteration with a 3x3 median filter for noise reduction. Subjective image quality of the scatter reduced ML-median processed image was improved over the original measured image with enhanced visualization of the retrocardiac region and the mediastinum. In both the mediastinum and the lung region, contrast was significantly improved, while percent noise (noise) was only slightly increased over that of the measured image. The contrast-to-percent noise ratio (CNF) in these regions was increased 130 percent, on average. ML-median processing was compared to Bayesian Image Estimation that incorporated a Gibb's prior. CNR for the ML-median technique was increased 16.5 percent and 49.7 percent in the lung and mediastinum regions, respectively, over that of the Bayesian technique. The effect of ML-median processing on resolution was also examined.
We developed a hybrid artificial neural network for scatter compensation in digital portable chest radiographs. The network inputs an image region of interest (ROI), and outputs the scatter estimate at the ROI's center. We segmented each image into four regions by relative detected exposure, then trained a separate Adaline (adaptive linear element) or adaptive filter for each region. We produced a spatially varying hybrid Madaline (mulitple Adaline) by combining outputs from weight matrices of different sizes trained for different durations. The network was trained with 20 patient or 1280 examples, then evaluated with another 5 patients or 320 examples. Scatter estimation errors were not very different, ranging from the Adaline's 6.9 percent to the hybrid Madaline's 5.5 percent. Primary errors (more relevant to quantitative radiography techniques like dual energy imaging) were 43 percent for the Adaline, reduced to 27 percent for the Madaline, and further reduced to 19 percent for the hybrid Madaline. The trained weight matrices, which act like convolution filters, resembled the shape and magnitude of scatter point spread functions. All networks outperformed conventional convolution-subraction techniques using analytical kernels. With its spatially varying neural network model, the hybrid Madaline provided the most accurate and robust estimation of scatter and primary exposures.
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