Sebastian Kozerke

Modified Look-Locker inversion recovery (MOLLI) for high-resolution T1 mapping of the heart.

A novel pulse sequence scheme is presented that allows the measurement and mapping of myocardial T1 in vivo on a 1.5 Tesla MR system within a single breath‐hold. Two major modifications of conventional Look‐Locker (LL) imaging are introduced: 1) selective data acquisition, and 2) merging of data from multiple LL experiments into one data set. Each modified LL inversion recovery (MOLLI) study consisted of three successive LL inversion recovery (IR) experiments with different inversion times. We acquired images in late diastole using a single‐shot steady‐state free‐precession (SSFP) technique, combined with sensitivity encoding to achieve a data acquisition window of <200 ms duration. We calculated T1 using signal intensities from regions of interest and pixel by pixel. T1 accuracy at different heart rates derived from simulated ECG signals was tested in phantoms. T1 estimates showed small systematic error for T1 values from 191 to 1196 ms. In vivo T1 mapping was performed in two healthy volunteers and in one patient with acute myocardial infarction before and after administration of Gd‐DTPA. T1 values for myocardium and noncardiac structures were in good agreement with values available from the literature. The region of infarction was clearly visualized. MOLLI provides high‐resolution T1 maps of human myocardium in native and post‐contrast situations within a single breath‐hold. Magn Reson Med 52:141–146, 2004.

Compressed sensing in dynamic MRI.

Recent theoretical advances in the field of compressive sampling—also referred to as compressed sensing (CS)—hold considerable promise for practical applications in MRI, but the fundamental condition of sparsity required in the CS framework is usually not fulfilled in MR images. However, in dynamic imaging, data sparsity can readily be introduced by applying the Fourier transformation along the temporal dimension assuming that only parts of the field‐of‐view (FOV) change at a high temporal rate while other parts remain stationary or change slowly. The second condition for CS, random sampling, can easily be realized by randomly skipping phase‐encoding lines in each dynamic frame. In this work, the feasibility of the CS framework for accelerated dynamic MRI is assessed. Simulated datasets are used to compare the reconstruction results for different reduction factors, noise, and sparsity levels. In vivo cardiac cine data and Fourier‐encoded velocity data of the carotid artery are used to test the reconstruction performance relative to k‐t broad‐use linear acquisition speed‐up technique (k‐t BLAST) reconstructions. Given sufficient data sparsity and base signal‐to‐noise ratio (SNR), CS is demonstrated to result in improved temporal fidelity compared to k‐t BLAST reconstructions for the example data sets used in this work. Magn Reson Med 59:365–373, 2008.

Cardiac SSFP imaging at 3 Tesla

Balanced steady‐state free precession (SSFP) techniques provide excellent contrast between myocardium and blood at a high signal‐to‐noise ratio (SNR). Hence, SSFP imaging has become the method of choice for assessing cardiac function at 1.5T. The expected improvement in SNR at higher field strength prompted us to implement SSFP at 3.0T. In this work, an optimized sequence protocol for cardiac SSFP imaging at 3.0T is derived, taking into account several partly adverse effects at higher field, such as increased field inhomogeneities, longer T1, and power deposition limitations. SSFP contrast is established by optimizing the maximum amplitude of the radiofrequency (RF) field strength for shortest TR, as well as by localized linear or second‐order shimming and local optimization of the resonance frequency. Given the increased SNR, sensitivity encoding (SENSE) can be employed to shorten breath‐hold times. Short‐axis, long‐axis, and four‐chamber cine views obtained in healthy adult subjects are presented, and three different types of artifacts are discussed along with potential methods for reducing them. Magn Reson Med 51:799–806, 2004.

4D flow MRI.

Traditionally, magnetic resonance imaging (MRI) of flow using phase contrast (PC) methods is accomplished using methods that resolve single‐directional flow in two spatial dimensions (2D) of an individual slice. More recently, three‐dimensional (3D) spatial encoding combined with three‐directional velocity‐encoded phase contrast MRI (here termed 4D flow MRI) has drawn increased attention. 4D flow MRI offers the ability to measure and to visualize the temporal evolution of complex blood flow patterns within an acquired 3D volume. Various methodological improvements permit the acquisition of 4D flow MRI data encompassing individual vascular structures and entire vascular territories such as the heart, the adjacent aorta, the carotid arteries, abdominal, or peripheral vessels within reasonable scan times. To subsequently analyze the flow data by quantitative means and visualization of complex, three‐directional blood flow patterns, various tools have been proposed. This review intends to introduce currently used 4D flow MRI methods, including Cartesian and radial data acquisition, approaches for accelerated data acquisition, cardiac gating, and respiration control. Based on these developments, an overview is provided over the potential this new imaging technique has in different parts of the body from the head to the peripheral arteries. J. Magn. Reson. Imaging 2012;36:1015–1036.

k-t PCA: temporally constrained k-t BLAST reconstruction using principal component analysis.

The k‐t broad‐use linear acquisition speed‐up technique (BLAST) has become widespread for reducing image acquisition time in dynamic MRI. In its basic form k‐t BLAST speeds up the data acquisition by undersampling k‐space over time (referred to as k‐t space). The resulting aliasing is resolved in the Fourier reciprocal x‐f space (x = spatial position, f = temporal frequency) using an adaptive filter derived from a low‐resolution estimate of the signal covariance. However, this filtering process tends to increase the reconstruction error or lower the achievable acceleration factor. This is problematic in applications exhibiting a broad range of temporal frequencies such as free‐breathing myocardial perfusion imaging. We show that temporal basis functions calculated by subjecting the training data to principal component analysis (PCA) can be used to constrain the reconstruction such that the temporal resolution is improved. The presented method is called k‐t PCA. Magn Reson Med, 2009.

High-Resolution Magnetic Resonance Myocardial Perfusion Imaging at 3.0-Tesla to Detect Hemodynamically Significant Coronary Stenoses as Determined by Fractional Flow Reserve

OBJECTIVES The objective of this study was to compare visual and quantitative analysis of high spatial resolution cardiac magnetic resonance (CMR) perfusion at 3.0-T against invasively determined fractional flow reserve (FFR). BACKGROUND High spatial resolution CMR myocardial perfusion imaging for the detection of coronary artery disease (CAD) has recently been proposed but requires further clinical validation. METHODS Forty-two patients (33 men, age 57.4 ± 9.6 years) with known or suspected CAD underwent rest and adenosine-stress k-space and time sensitivity encoding accelerated perfusion CMR at 3.0-T achieving in-plane spatial resolution of 1.2 × 1.2 mm(2). The FFR was measured in all vessels with >50% severity stenosis. Fractional flow reserve <0.75 was considered hemodynamically significant. Two blinded observers visually interpreted the CMR data. Separately, myocardial perfusion reserve (MPR) was estimated using Fermi-constrained deconvolution. RESULTS Of 126 coronary vessels, 52 underwent pressure wire assessment. Of these, 27 lesions had an FFR <0.75. Sensitivity and specificity of visual CMR analysis to detect stenoses at a threshold of FFR <0.75 were 0.82 and 0.94 (p < 0.0001), respectively, with an area under the receiver-operator characteristic curve of 0.92 (p < 0.0001). From quantitative analysis, the optimum MPR to detect such lesions was 1.58, with a sensitivity of 0.80, specificity of 0.89 (p < 0.0001), and area under the curve of 0.89 (p < 0.0001). CONCLUSIONS High-resolution CMR MPR at 3.0-T can be used to detect flow-limiting CAD as defined by FFR, using both visual and quantitative analyses.

Array compression for MRI with large coil arrays

Arrays with large numbers of independent coil elements are becoming increasingly available as they provide increased signal‐to‐noise ratios (SNRs) and improved parallel imaging performance. Processing of data from a large set of independent receive channels is, however, associated with an increased memory and computational load in reconstruction. This work addresses this problem by introducing coil array compression. The method allows one to reduce the number of datasets from independent channels by combining all or partial sets in the time domain prior to image reconstruction. It is demonstrated that array compression can be very effective depending on the size of the region of interest (ROI). Based on 2D in vivo data obtained with a 32‐element phased‐array coil in the heart, it is shown that the number of channels can be compressed to as few as four with only 0.3% SNR loss in an ROI encompassing the heart. With twofold parallel imaging, only a 2% loss in SNR occurred using the same compression factor. Magn Reson Med 57:1131–1139, 2007.

Dynamic contrast-enhanced myocardial perfusion MRI accelerated with k-t sense.

In the k‐t sensitivity encoding (k‐t SENSE) method spatiotemporal data correlations are exploited to accelerate data acquisition in dynamic MRI studies. The present study demonstrates the feasibility of applying k‐t SENSE to contrast‐enhanced myocardial perfusion MRI and using the speed‐up to increase spatial resolution. At a net acceleration factor of 3.9 (k‐t factor of 5 with 11 training profiles) accurate representations of dynamic signal intensity (SI) changes were achieved in computer simulations. In vivo, 5× k‐t SENSE was compared with 2× SENSE (identical acquisition parameters except for in‐plane spatial resolution = 1.48 × 1.48 mm2 vs. 2.64 × 2.64 mm2, respectively). In 10 volunteers no differences in myocardial SI profiles were found (relative peak enhancement = 151% vs. 149.7%, maximal upslope = 12.9%/s vs. 13.3%/s for 2× SENSE and 5× k‐t SENSE, respectively, all P > 0.05). Overall image quality was similar, but endocardial dark rim artifacts were reduced with k‐t SENSE. Signal‐to‐noise ratio (SNR) in the myocardium was greater with 5× k‐t SENSE by a factor of 1.36 ± 0.23 at peak contrast enhancement with the relative yield decreasing with increasing dynamics in the object in accordance to theory. Higher nominal acceleration factors of up to 10‐fold were shown to be feasible in computer simulations and in vivo. Magn Reson Med 58:777–785, 2007.

Heart motion adapted cine phase-contrast flow measurements through the aortic valve

A method for magnetic resonance cine velocity mapping through heart valves with adaptation of both slice offset and angulation according to the motion of the valvular plane of the heart is presented. By means of a subtractive labeling technique, basal myocardial markers are obtained and automatically extracted for quantification of heart motion at the valvular level. The captured excursion of the basal plane is used to calculate the slice offset and angulation of each required time frame for cine velocity mapping. Through‐plane velocity offsets are corrected by subtracting velocities introduced by basal plane motion from the measured velocities. For evaluation of the method, flow measurements downstream from the aortic valve were performed both with and without slice adaptation in 11 healthy volunteers and in four patients with aortic regurgitation. Maximum through‐plane motion at the aortic root level as calculated from the labeled markers averaged 8.9 mm in the volunteers and 6.5 mm in the patients. The left coronary root was visible in 2–4 (mean: 2.2) time frames during early diastole when imaging with a spatially fixed slice. Time frames obtained with slice adaptation did not contain the coronary roots. Motion correction increased the apparent regurgitant volume by 5.7 ± 0.4 ml for patients with clinical aortic regurgitation, for an increase of approximately 50%. The proposed method provides flow measurements with correction for through‐plane motion perpendicular to the aortic root between the valvular annulus and the coronary ostia throughout the cardiac cycle. Magn Reson Med 42:970–978, 1999.

Optimizing spatiotemporal sampling for k-t BLAST and k-t SENSE: Application to high-resolution real-time cardiac steady-state free precession

In k‐t BLAST and k‐t SENSE, data acquisition is accelerated by sparsely sampling k‐space over time. This undersampling in k‐t space causes the object signals to be convolved with a point spread function in x‐f space (x = spatial position, f = temporal frequency). The resulting aliasing is resolved by exploiting spatiotemporal correlations within the data. In general, reconstruction accuracy can be improved by controlling the k‐t sampling pattern to minimize signal overlap in x‐f space. In this work, we describe an approach to obtain generally favorable patterns for typical image series without specific knowledge of the image series itself. These optimized sampling patterns were applied to free‐breathing, untriggered (i.e., real‐time) cardiac imaging with steady‐state free precession (SSFP). Eddy‐current artifacts, which are otherwise increased drastically in SSFP by the undersampling, were minimized using alternating k‐space sweeps. With the synergistic combination of the k‐t approach with optimized sampling and SSFP with alternating k‐space sweeps, it was possible to achieve a high signal‐to‐noise ratio, high contrast, and high spatiotemporal resolutions, while achieving substantial immunity against eddy currents. Cardiac images are shown, demonstrating excellent image quality and an in‐plane resolution of ∼2.0 mm at >25 frames/s, using one or more receiver coils. Magn Reson Med 53:1372–1382, 2005.