Traditional film/screen mammograms are obtained using Molybdenum or Rhodium target x-ray tubes. The energy spectrum from these sources matches the limited latitude of film/screen systems. For digital imaging systems, the latitude is linear over a wide range of exposures and arbitrary H&D curves can be obtained with image processing. This allows the recorded contrast to noise ratio (CNR) to be optimized by considering a wide range of radiographic techniques. For this work, we modeled the radiographic process for a digital (amorphous selenium) mammography system. The optimal CNR relative to dose was determined for several target/filter combinations, for a wide range of kVp values, and for varying breast thickness. The target/filter combinations included: Mo/Mo, Mo/Rh, Rh/Rh, W/Al, W/Mo, W/Ag, and W/Sn. As breast thickness increased, the use of a tungsten target with a tin filter resulted in a 34% improvement in CNR for the same dose to the breast when compared to the use of a Molybdenum target with a Molybdenum filter. Notably, the W/Sn target/filter combination resulted in a significantly lower mA-s for the same breast dose (2/3 to 1/5 lower for a breast thickness from 4 to 8cm). In mammography applications, use of a Tungsten tube rather than the traditional Molybdenum tube should lead to significant reductions in exposure time and tube heat while maintaining similar image quality and dose.
We present a non-linear (polynomial) transformation to minimize scattering of data points around normal tissue clusters in a normalized MRI feature space, in which normal tissues are clustered around pre-specified target positions. This transformation is motivated by non-linear relationship between MRI pixel intensities and intrinsic tissue parameters (e.g., T1, T2, PD). To determine scattering amount, we use ratio of summation of within-class distances fro clusters to summation of their between-class distances. We find the transformation by minimizing the scattering amount. Next, we generate a 3D visualization of the MRI feature space and define regions of interest (ROI's) on clusters seen for normal and abnormal tissues. We use these ROI's to estimate signature vectors (cluster centers). Finally, we use the signature vectors for segmenting and characterizing tissues. We used simulation, phantom, and brain MRI to evaluate the polynomial transformation and compare it to the linear transformation. In all studies, we were able to identify clusters for normal and abnormal tissues and segment the images. Compared to the linear method, the non-linear approach yields enhanced clustering properties and better separation of normal and abnormal tissues. ON the other hand, the linear transformation is more appropriate than the non-linear method for capturing partial volume information.
We present a method for exploring the relationship between the image segmentation results obtained by an optimal feature space method and the MRI protocols used. The steps of the work accomplished are as follows. (1) Three patients with brain tumors were imaged by a 1.5T General Electric Signa MRI System, using multiple protocols (T1- and T2-weighted spin- echo and FLAIR). T1-weighted images were acquired before and after Gadolinium (Gd) injection. (2) Image volumes were co- registered, and images of a slice through the center of the tumor were selected for processing. (3) Nine sets of images were defined by selecting certain MR images (e.g., 4T2's + 1T1, 4T2's + FLAIR, 2T2's + 1T1). (4) Using the images in each set, the optimal feature space was generated and images were segmented into normal tissues and different tumor zones. (5) Segmentation results obtained using different MRI sets were compared. We found that the locations of the clusters for the tumor zones and their corresponding regions in the image domain changed to some extent as a function of the MR images (MRI protocols) used. However, the segmentation results for the total lesion and normal tissues remained almost unchanged.
KEYWORDS: Functional magnetic resonance imaging, Linear filtering, Signal to noise ratio, Composites, Image segmentation, Image filtering, Brain, Image analysis, Model-based design, Magnetic resonance imaging
This paper presents development and application of an optimal linear filter for delineation of activated areas of the brain from functional MRI (fMRI) time series data. The steps of the work accomplished are as follows. (1) Delineation of activated areas is formulated as an optimal linear filtering problem. In this formulation, a linear filter (image combination method) is looked for, which maximizes the signal-to-noise ratio (SNR) of the activated areas subject to the constraint of removing inactivated areas from the image. (2) An analytical solution for the problem is found. (3) Image pixel vectors and expected time series pattern (signature) for inactivated pixels are used to calculate the weighting vectors numerically. (4) The segmented image by the proposed method is compared to those generated by the conventional methods (correlation, t- statistic, and z-statistic). Visual qualities of the images as well as their SNR's are compared. The optimal linear filter outperforms the conventional methods of fMRI analysis based on improved SNR and contrast-to-noise ratio (CNR) of the images generated by the proposed method compared to those generated by the other methods. In addition, this method does not require a priori knowledge of the fMRI response to the paradigm for its application. The method is linear and most of the work is done analytically, thereby numerical implementation and execution of the method are faster than the conventional methods.
This paper presents development and application of an automated scheme to minimize user dependency of the eigenimage filter. The steps of the new method are as follows: (1) User defines sample regions of interest on central location of the volume and generates the corresponding eigenimages. (2) Original images are segmented using a self organizing data analysis technique. (3) Regions for the tissue types are automatically found from the segmentation results. (4) Signature vectors are estimated from these regions and are compared with those obtained from the user initialization. If they are similar, these signature vectors are used to run eigenimage filtering. If they are not similar, the clustering parameters are adjusted and the procedure is repeated until similar signatures are found. (5) Next slice is loaded and signature vectors from the previous slice are used to get initial eigenimages. Cluster analysis is used to generate regions. The procedure described in the previous step is repeated until similar signatures are found. Then, final eigenimages are obtained. (6) Previous step is repeated until the last slice of the volume is analyzed. (7) Volume of each tissue type is estimated from the resulting eigenimages. Details and significance of each step are explained. Experimental results using simulation, phantom, and brain images are presented.
This paper presents development and performance evaluation of an MRI feature space method. The method is useful for: identification of tissue types; segmentation of tissues; and quantitative measurements on tissues, to obtain information that can be used in decision making (diagnosis, treatment planning, and evaluation of treatment). The steps of the work accomplished are as follows: (1) Four T2-weighted and two T1-weighted images (before and after injection of Gadolinium) were acquired for ten tumor patients. (2) Images were analyed by two image analysts according to the following algorithm. The intracranial brain tissues were segmented from the scalp and background. The additive noise was suppressed using a multi-dimensional non-linear edge- preserving filter which preserves partial volume information on average. Image nonuniformities were corrected using a modified lowpass filtering approach. The resulting images were used to generate and visualize an optimal feature space. Cluster centers were identified on the feature space. Then images were segmented into normal tissues and different zones of the tumor. (3) Biopsy samples were extracted from each patient and were subsequently analyzed by the pathology laboratory. (4) Image analysis results were compared to each other and to the biopsy results. Pre- and post-surgery feature spaces were also compared. The proposed algorithm made it possible to visualize the MRI feature space and to segment the image. In all cases, the operators were able to find clusters for normal and abnormal tissues. Also, clusters for different zones of the tumor were found. Based on the clusters marked for each zone, the method successfully segmented the image into normal tissues (white matter, gray matter, and CSF) and different zones of the lesion (tumor, cyst, edema, radiation necrosis, necrotic core, and infiltrated tumor). The results agreed with those obtained from the biopsy samples. Comparison of pre- to post-surgery and radiation feature spaces confirmed that the tumor was not present in the second study but radiation necrosis was generated as a result of radiation.
Accurate and reproducible volume calculations are essential for diagnosis and treatment evaluation for many medical situations. Current techniques employ planimetric methods that are very time consuming to obtain reliable results. The reproducibility and accuracy of these methods depend on the user and the complexity of the volume being measured. We have reported on an algorithm for volume calculation that uses the Eigenimage filter to segment a desired feature from surrounding, interfering features. The pixel intensities of the resulting image have information pertaining to partial volume averaging effects in each voxel preserved thus providing an accurate volume calculation. Also, the amount of time required is significantly reduced, as compared to planimetric methods, and the reproducibility is less user dependent and is independent of the volume shape. In simulations and phantom studies the error in accuracy and reproducibility of this method were less than 2%. The purpose of this study was to determine the reproducibility of the method for volume calculations of the human brain. Ten volunteers were imaged and the volume of white matter, gray matter, and CSF were estimated. The time required to calculate the volume for all three tissues was approximately one minute per slice. The inter- and intra-observer reproducibility errors were less than 5% on average for all volumes calculated. These results were determined to be dependent on the proper selection of the ROIs used to define the tissue signature vectors and the non-uniformity of the MRI system.
Fast, accurate, and reproducible volume estimation is vital to the diagnosis, treatment, and evaluation of many medical situations. We present the development and application of a semi-automatic method for estimating volumes of normal and abnormal brain tissues from computed tomography images. This method does not require manual drawing of the tissue boundaries. It is therefore expected to be faster and more reproducible than conventional methods. The steps of the new method are as follows. (1) The intracranial brain volume is segmented from the skull and background using thresholding and morphological operations. (2) The additive noise is suppressed (the image is restored) using a non-linear edge-preserving filter which preserves partial volume information on average. (3) The histogram of the resulting low-noise image is generated and the dominant peak is removed from it using a Gaussian model. (4) Minima and maxima of the resulting histogram are identified and using a minimum error criterion, the brain is segmented into the normal tissues (white matter and gray matter), cerebrospinal fluid, and lesions, if present. (5) Previous steps are repeated for each slice through the brain and the volume of each tissue type is estimated from the results. Details and significance of each step are explained. Experimental results using a simulation, a phantom, and selected clinical cases are presented.
We present development and application of a feature extraction method for magnetic resonance imaging (MRI), without explicit calculation of tissue parameters. We generate a three-dimensional (3-D) feature space representation of the data, in which normal tissues are clustered around pre-specified target positions and abnormalities are clustered somewhere else. This is accomplished by a linear minimum mean square error transformation of categorical data to target positions. From the 3-D histogram (cluster plot) of the transformed data, we identify clusters and define regions of interest (ROIs) for normal and abnormal tissues. There ROIs are used to estimate signature (feature) vectors for each tissue type which in turn are used to segment the MRI scene. The proposed feature space is compared to those generated by tissue-parameter-weighted images, principal component images, and angle images, demonstrating its superiority for feature extraction. The method and its performance are illustrated using a computer simulation and MRI images of an egg phantom and a human brain.
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