Maria I. Altbach

Compression of electrocardiogram signals using JPEG2000

JPEG2000 is the latest international standard for compression of still images. Although the JPEG2000 codec is designed to compress images, we illustrate that it can also be used to compress other signals. As an example, we illustrate how the JPEG2000 codec can be used to compress electrocardiogram (ECG) data. Experiments using the MIT-BIH arrhythmia database illustrate that the proposed approach outperforms many existing ECG compression schemes. The proposed scheme allows the use of existing hardware and software JPEG2000 codecs for ECG compression, and can be especially useful in eliminating the need for specialized hardware development. The desirable characteristics of the JPEG2000 codec, such as precise rate control and progressive quality, are retained in the presented scheme. The goal of this paper is to demonstrate the ECG application as an example. This example can be extended to other signals that exist within the consumer electronics realm.

Plasmalemmal pH-gradients in drug-sensitive and drug-resistant MCF-7 human breast carcinoma xenografts measured by 31P magnetic resonance spectroscopy

31p Magnetic resonance spectroscopy (MRS) was employed to investigate tumor pH in xenografts of drug-sensitive and drug-resistant MCF-7 human breast carcinoma cells. Measured extracellular pH values were found to be lower than the intracellular pH in all three tumor types investigated. The magnitude of this acid-outside plasmalemmal pH gradient increased with increasing tumor size in tumors of two drug-resistant variants of MCF-7 cells, but not in tumors of the parent (drug-sensitive) cells. The partitioning of weak-base or weak-acid drug molecules across the plasma membrane of a tumor cell is dependent upon the acid-dissociation constant (pKa) of the drug as well as the plasmalemmal pH gradient. A large acid-outside pH gradient, such as those seen in MCF-7 xenografts, can exert a protective effect on the cell from weak-base drugs such as anthracyclines and Vinca alkaloids, which have pKa values of 7.5 to 9.5. The possibility of enhancing the therapeutic efficacy of weak-base drugs by dietary or metabolic manipulation of the extracellular pH, in order to reduce or reverse the plasmalemmal pH gradient, deserves investigation.

T2 mapping from highly undersampled data by reconstruction of principal component coefficient maps using compressed sensing

Recently, there has been an increased interest in quantitative MR parameters to improve diagnosis and treatment. Parameter mapping requires multiple images acquired with different timings usually resulting in long acquisition times. While acquisition time can be reduced by acquiring undersampled data, obtaining accurate estimates of parameters from undersampled data is a challenging problem, in particular for structures with high spatial frequency content. In this work, principal component analysis is combined with a model‐based algorithm to reconstruct maps of selected principal component coefficients from highly undersampled radial MRI data. This novel approach linearizes the cost function of the optimization problem yielding a more accurate and reliable estimation of MR parameter maps. The proposed algorithm—reconstruction of principal component coefficient maps using compressed sensing—is demonstrated in phantoms and in vivo and compared with two other algorithms previously developed for undersampled data. Magn Reson Med, 2012.

High‐resolution diffusion imaging with DIFRAD‐FSE (Diffusion‐Weighted Radial Acquisition of Data With Fast Spin‐Echo) MRI

A novel MRI method, DIFRAD‐FSE (diffusion‐weighted radial acquisition of data with fast spin‐echo), is demonstrated that enables rapid, high‐resolution multi‐shot diffusion‐weighted MRI without significant artifacts due to motion. Following a diffusion‐weighting spin‐echo preparation period, multiple radial lines of Fourier data are acquired using spin‐echo refocusing. Images can be reconstructed from the radial data set using a magnitude‐only filtered back‐projection reconstruction algorithm that removes phase errors due to motion. Results from human brain imaging demonstrate the ability of DIFRAD‐FSE to acquire multiple radial lines of Fourier data each TR period without significant artifacts due to relaxation and to produce high‐resolution diffusion‐weighted MR images without significant artifacts from motion. Magn Reson Med 42:11–18, 1999.

Radial fast spin-echo method for T2-weighted imaging and T2 mapping of the liver

To evaluate a multishot radial fast‐spin echo (RAD‐FSE) method developed to improve the quality of abdominal T2‐weighted imaging as well as the characterization of focal liver lesions.

Processing of radial fast spin-echo data for obtaining T2 estimates from a single k-space data set.

Radially acquired fast spin‐echo data can be processed to obtain T2‐weighted images and a T2 map from a single k‐space data set. The general approach is to use data at a specific TE (or narrow TE range) in the center of k‐space and data at other TE values in the outer part of k‐space. With this method high‐resolution T2‐weighted images and T2 maps are obtained in a time efficient manner. The mixing of TE data, however, introduces errors in the T2‐weighted images and T2 maps that affect the accuracy of the T2 estimates. In this work, various k‐space data processing methods for reconstructing T2‐weighted images and T2 maps from a single radial fast spin‐echo k‐space data set are analyzed in terms of the accuracy of T2 estimates. The analysis is focused on the effect of image artifacts, object dependency, and noise on the T2 estimates. Results are presented in computer‐generated phantoms and in vivo. Magn Reson Med, 2005.

Magnetic Resonance Imaging During Untreated Ventricular Fibrillation Reveals Prompt Right Ventricular Overdistention Without Left Ventricular Volume Loss

Background—Most out-of-hospital ventricular fibrillation (VF) is prolonged (>5 minutes), and defibrillation from prolonged VF typically results in asystole or pulseless electrical activity. Recent visual epicardial observations in an open-chest, open-pericardium model of swine VF indicate that blood flows from the high-pressure arterial system to the lower-pressure venous system during untreated VF, thereby overdistending the right ventricle and apparently decreasing left ventricular size. Therefore, inadequate left ventricular stroke volume after defibrillation from prolonged VF has been postulated as a major contributor to the development of pulseless rhythms. Methods and Results—Ventricular dimensions were determined by MRI for 30 minutes of untreated VF in a closed-chest, closed-pericardium model in 6 swine. Within 1 minute of untreated VF, mean right ventricular volume increased by 29% but did not increase thereafter. During the first 5 minutes of untreated VF, mean left ventricular volume increased by 34%. Between 20 and 30 minutes of VF, stone heart occurred as manifested by dramatic thickening of the myocardium and concomitant substantial decreases in left ventricular volume. Conclusions—In this closed-chest swine model of VF, substantial right ventricular volume changes occurred early and did not result in smaller left ventricular volumes. The changes in ventricular volumes before the late development of stone heart do not explain why defibrillation from brief duration VF (<5 minutes) typically results in a pulsatile rhythm with return of spontaneous circulation, whereas defibrillation from prolonged VF (5 to 15 minutes) does not.

Fast decomposition of water and lipid using a GRASE technique with the IDEAL algorithm.

Three‐point Dixon techniques achieve good lipid‐water separation by estimating the phase due to field inhomogeneities. Recently it was demonstrated that the combination of an iterative algorithm (iterative decomposition of water and fat with echo asymmetry and least‐squares estimation (IDEAL)) with a fast spin‐echo (FSE) three‐point Dixon method yielded robust lipid‐water decomposition. As an alternative to FSE, the gradient‐ and spin‐echo (GRASE) technique has been developed for efficient data collection. In this work we present a method for lipid‐water separation by combining IDEAL with the GRASE technique. An approach to correct for errors in the lipid‐water decomposition caused by phase distortions due to the switching of the readout gradient polarities inherent to GRASE is presented. The IDEAL‐GRASE technique is demonstrated in phantoms and in vivo for various applications, including pelvic, musculoskeletal, and (breath‐hold) cardiac imaging. Magn Reson Med 57:1047–1057, 2007.

View-ordering in radial fast spin-echo imaging

Radial MRI sequences are frequently used to obtain images with reduced sensitivity to motion. To decrease imaging time, multiple spin‐echo acquisitions can be incorporated into radial sequences. In this case, different radial lines of Fourier data have different TE times and the resulting images can contain streaking artifacts due to T2 decay. The streaking is not only dependent on the T2 of the object and the timing of the data acquisition, but also on the order in which radial lines are collected (view order). The view ordering can easily be controlled to minimize artifacts due to T2 decay as well as motion. Four view‐ordering techniques are presented and evaluated for the radial FSE sequence. Magn Reson Med 51:768–774, 2004.

T2 relaxometry with indirect echo compensation from highly undersampled data

To develop an algorithm for fast and accurate T2 estimation from highly undersampled multi‐echo spin‐echo data.