Axel Haase

Generalized autocalibrating partially parallel acquisitions (GRAPPA)

In this study, a novel partially parallel acquisition (PPA) method is presented which can be used to accelerate image acquisition using an RF coil array for spatial encoding. This technique, GeneRalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) is an extension of both the PILS and VD‐AUTO‐SMASH reconstruction techniques. As in those previous methods, a detailed, highly accurate RF field map is not needed prior to reconstruction in GRAPPA. This information is obtained from several k‐space lines which are acquired in addition to the normal image acquisition. As in PILS, the GRAPPA reconstruction algorithm provides unaliased images from each component coil prior to image combination. This results in even higher SNR and better image quality since the steps of image reconstruction and image combination are performed in separate steps. After introducing the GRAPPA technique, primary focus is given to issues related to the practical implementation of GRAPPA, including the reconstruction algorithm as well as analysis of SNR in the resulting images. Finally, in vivo GRAPPA images are shown which demonstrate the utility of the technique. Magn Reson Med 47:1202–1210, 2002.

Partially Parallel Imaging With Localized Sensitivities (PILS)

In this study a novel partially parallel acquisition method is presented, which can be used to accelerate image acquisition using an RF coil array for spatial encoding. In this technique, Parallel Imaging with Localized Sensitivities (PILS), it is assumed that the individual coils in the array have localized sensitivity patterns, in that their sensitivity is restricted to a finite region of space. Within the PILS model, a detailed, highly accurate RF field map is not needed prior to reconstruction. In PILS, each coil in the array is fully characterized by only two parameters: the center of coils sensitive region in the FOV and the width of the sensitive region around this center. In this study, it is demonstrated that the incorporation of these coil parameters into a localized Fourier transform allows reconstruction of full FOV images in each of the component coils from data sets acquired with a reduced number of phase encoding steps compared to conventional imaging techniques. After the introduction of the PILS technique, primary focus is given to issues related to the practical implementation of PILS, including coil parameter determination and the SNR and artifact power in the resulting images. Finally, in vivo PILS images are shown which demonstrate the utility of the technique. Magn Reson Med 44:602–609, 2000.

VD-AUTO-SMASH imaging.

Recently a self‐calibrating SMASH technique, AUTO‐SMASH, was described. This technique is based on PPA with RF coil arrays using auto‐calibration signals. In AUTO‐SMASH, important coil sensitivity information required for successful SMASH reconstruction is obtained during the actual scan using the correlation between undersampled SMASH signal data and additionally sampled calibration signals with appropriate offsets in k‐space. However, AUTO‐SMASH is susceptible to noise in the acquired data and to imperfect spatial harmonic generation in the underlying coil array. In this work, a new modified type of internal sensitivity calibration, VD‐AUTO‐SMASH, is proposed. This method uses a VD k‐space sampling approach and shows the ability to improve the image quality without significantly increasing the total scan time. This new k‐space adapted calibration approach is based on a k‐space–dependent density function. In this scheme, fully sampled low‐spatial frequency data are acquired up to a given cutoff‐spatial frequency. Above this frequency, only sparse SMASH‐type sampling is performed. On top of the VD approach, advanced fitting routines, which allow an improved extraction of coil‐weighting factors in the presence of noise, are proposed. It is shown in simulations and in vivo cardiac images that the VD approach significantly increases the potential and flexibility of rapid imaging with AUTO‐SMASH. Magn Reson Med 45:1066–1074, 2001.

Rapid three-dimensional MR imaging using the FLASH technique.

Fast low-angle shot (FLASH) imaging is a new technique for rapid magnetic resonance (MR) imaging that reduces acquisition times to seconds while retaining spatial resolution. This article deals with a three-dimensional (3D) variant of the FLASH method that allows the recording of a 3D-data set of 128 X 128 X 128 pixels within an acquisition time of only 4 min. The method is demonstrated using a 2.35 T 40 cm bore MR system. Experiments are carried out on rabbit head and human extremities. Depending on the field of view, the isotropic resolution is 1 mm or even less leading to cross-sectional images with a 1 mm slice thickness. In principle, FLASH imaging techniques are applicable to any MR system without the need of major hardware modifications. However, high-speed computers, large storage capacity, and rapid image display routines greatly facilitate an advantageous use of the 3D-FLASH variant.

Inversion recovery TrueFISP: Quantification of T1, T2, and spin density

A novel procedure is proposed to extract T1, T2, and relative spin density from the signal time course sampled with a series of TrueFISP images after spin inversion. Generally, the recovery of the magnetization during continuous TrueFISP imaging can be described in good approximation by a three parameter monoexponential function S(t) = Sstst(1‐INV exp(‐t/T  *1 ). This apparent relaxation time T  *1 ≤ T1 depends on the flip angle as well as on both T1 and T2. Here, it is shown that the ratio T1/T2 can be directly extracted from the inversion factor INV, which describes the relation of the signal value extrapolated to t = 0 and the steady‐state signal. Analytical expressions are given for the derivation of T1, T2, and relative spin density directly from the fit parameters. Phantom results show excellent agreement with single point reference measurements. In human volunteers T1, T2, and spin density maps in agreement with literature values were obtained. Magn Reson Med 51:661–667, 2004.

Stimulated echo imaging

Abstract A new form of NMR imaging is described using stimulated echoes. The technique, dubbed STEAM ( sti mulated e cho acquisition m ode) imaging, turns out to become a versatile tool for multipurpose NMR imaging. Stimulated echoes can be excited by a sequence of at least three rf pulses, which in the basic experiment have flip angles of 90° or less. Thus no selective or nonselective 180° pulses are needed, which eliminates a variety of problems associated with such pulses in conventional spin-echo NMR imaging. Further advantages of STEAM imaging are concerned with the functional flexibility of an imaging sequence comprising three pulses and three intervals and the possibility of “storing” information prepared during the first interval into the form of longitudinal magnetization during the second interval. In general, the applied rf power is considerably reduced as compared to spin-echo-based imaging sequences. Here the general principles of the technique are outlined and first applications to multislice imaging of directly neighboring slices are demonstrated. Subsequent papers will be concerned with modifications of the basic STEAM sequence which, for example, allow multiple chemical-shift-selective (CHESS) imaging, complete imaging of the spin-lattice relaxation behavior, diffusion imaging, and single-shot real-time imaging.

Inversion recovery snapshot FLASH MR imaging.

Snapshot fast low angle shot (FLASH) magnetic resonance (MR) imaging techniques have been developed to enable real time imaging of MR parameters. The method is based on a 64 x 128 FLASH tomogram acquired within less than 200 ms. This work describes snapshot FLASH MR using a single 180 degrees pulse prior to the acquisition of a series of FLASH images. The experiment creates continuous dynamic inversion recovery (IR) T1 contrast in successive images. The total acquisition time of 16 images displaying the IR behavior is less than 4 s. Representative snapshot FLASH IR MR images of the abdomen of healthy rats and of an implanted hepatic tumor are illustrated.

Dobutamine-Stress Magnetic Resonance Microimaging in Mice Acute Changes of Cardiac Geometry and Function in Normal and Failing Murine Hearts

Abstract— The aim of this study was to assess the capability of MRI to characterize systolic and diastolic function in normal and chronically failing mouse hearts in vivo at rest and during inotropic stimulation. Applying an ECG-gated FLASH-cine sequence, MRI at 7 T was performed at rest and after administration of 1.5 &mgr;g/g IP dobutamine. There was a significant increase of heart rate, cardiac output, and ejection fraction and significant decrease of end-diastolic and end-systolic left ventricular (LV) volumes (P <0.01 each) in normal mice during inotropic stimulation. In mice with heart failure due to chronic myocardial infarction (MI), MRI at rest revealed gross LV dilatation. There was a significant decrease of LV ejection fraction in infarcted mice (29%) versus sham mice (58%). Mice with MI showed a significantly reduced maximum LV ejection rate (P <0.001) and LV filling rate (P <0.01) and no increase of LV dynamics during dobutamine action, indicating loss of contractile and relaxation reserve. In 4-month-old transgenic mice with cardiospecific overexpression of the &bgr;1-adrenergic receptor, which at this early stage do not show abnormalities of resting cardiac function, LV filling rate failed to increase after dobutamine stress (transgenic, 0.19±0.03 &mgr;L/ms; wild type, 0.36±0.01 &mgr;L/ms;P <0.01). Thus, MRI unmasked diastolic dysfunction during dobutamine stress. Dobutamine-stress MRI allows noninvasive assessment of systolic and diastolic components of heart failure. This study shows that MRI can demonstrate loss of inotropic and lusitropic response in mice with MI and can unmask diastolic dysfunction as an early sign of cardiac dysfunction in a transgenic mouse model of heart failure.

T1 maps by K-space reduced snapshot-FLASH MRI

The T1 maps evaluated from k-space reduced Snapshot fast low angle shot (FLASH) images provide high contrast parameter images for tissue characterization in vivo of any body region. An algorithm for computing T1 values that allows a fast and reliable evaluation of T1 maps and yields reproducible values of tissue parameters in MR imaging is presented. The algorithm combined with the Snapshot FLASH inversion recovery imaging sequence permits a precise determination of T1 values, even for T1 times as low as 50 ms. Comparison with a spectroscopical inversion recovery method on identical phantoms demonstrates the accuracy of this technique. With its total acquisition time of approximately 2 s, IR Snapshot FLASH is fast enough to be used in monitoring fast T1 dynamics.

High-resolution MRI with cardiac and respiratory gating allows for accurate in vivo atherosclerotic plaque visualization in the murine aortic arch

Genetically engineered mouse models provide enormous potential for investigation of the underlying mechanisms of atherosclerotic disease, but noninvasive imaging methods for analysis of atherosclerosis in mice are currently limited. This study aimed to demonstrate the feasibility of MRI to noninvasively visualize atherosclerotic plaques in the thoracic aorta in mice deficient in apolipoprotein‐E, who develop atherosclerotic lesions similar to those observed in humans. To freeze motion, MR data acquisition was both ECG‐ and respiratory‐gated. T1‐weighted MR images were acquired with TR/TE ∼1000/10 ms. Spatial image resolution was 49 × 98 × 300 μm3. MRI revealed a detailed view of the lumen and the vessel wall of the entire thoracic aorta. Comparison of MRI with corresponding cross‐sectional histopathology showed excellent agreement of aortic vessel wall area (r = 0.97). Hence, noninvasive MRI should allow new insights into the mechanisms involved in progression and regression of atherosclerotic disease. Magn Reson Med 50:69–74, 2003.