Search

Purpose: To establish an interleaved multislice variant of the averaged magnetization inversion-recovery acquisitions (AMIRA) approach for 2D spinal cord imaging with increased acquisition efficiency compared with the conventional 2D single-slice approach(es), and to determine essential prerequisites for a working interleaved multislice AMIRA approach in practice., Methods: The general AMIRA concept is based on an inversion recovery-prepared, segmented, and time-limited cine balanced SSFP sequence, generating images of different contrast. For AMIRA imaging of multiple, independent slices in a 2D interleaved fashion, a slice loop within the acquisition loops was programmed. The former non-selective inversions were replaced with slice-selective inversions with user-definable slice thickness., Results: The thickness of the slice-selective inversion in 2D interleaved multislice AMIRA should be doubled compared with the manufacturer's standard setting to avoid an increased sensitivity to flow and pulsation effects particularly in the CSF. However, this solution also limits its practical applicability, as slices located at directly adjacent vertebrae cannot be imaged together. Successful interleaved two-slice AMIRA imaging for a "reference" in vivo protocol with 0.50 × 0.50 mm 2 in-plane resolution and 8-mm slice thickness is demonstrated, therefore halving its acquisition time per slice from 3 min down to 1.5 min., Conclusion: The investigated 2D interleaved two-slice AMIRA variant facilitates spinal cord imaging that maintains similar contrast and the same resolution as the conventional 2D single-slice AMIRA approach, but does so with a halved acquisition time., (© 2024 The Author(s). Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

Purpose: To present a new complex-valued B 1 + mapping method for electrical properties tomography using Carr-Purcell spin echoes., Methods: A Carr-Purcell (CP) echo train generates pronounced flip-angle dependent oscillations that can be used to estimate the magnitude of B 1 + . To this end, a dictionary is used that takes into account the slice profile as well as T 2 relaxation along the echo train. For validation, the retrieved B 1 , 0.34 S/m). Moreover, the phase of the first echo reflects the transceive phase. Overall, the CP echo train yields an estimate of the complex-valued B + map is compared with the actual flip angle imaging (AFI) method in a phantom (79 ε 0 , 0.34 S/m). Moreover, the phase of the first echo reflects the transceive phase. Overall, the CP echo train yields an estimate of the complex-valued B 1 + , allowing electrical properties tomography with both permittivity and conductivity. The presented method is evaluated in phantom scans as well as for in vivo brain at 3 T., Results: In the phantom, the obtained magnitude B 1 + and conductivity (0.35 ± 0.04) S/m values are in accordance with expectations. In the brain, the obtained electrical properties are also close to expectations. In addition to the retrieved complex B 0 and conductivity (0.35 ± 0.04) S/m values are in accordance with expectations. In the brain, the obtained electrical properties are also close to expectations. In addition to the retrieved complex B 1 + information, the decay of the CP echo trains also yields an estimate for T 2 ., Conclusion: The CP sequence can be used to simultaneously provide both B 1 + magnitude and phase estimations, and therefore allows for full reconstruction of the electrical properties., (© 2021 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

Purpose: To establish a novel approach for fast high-resolution spinal cord (SC) imaging using averaged magnetization inversion recovery acquisitions (AMIRA)., Methods: The AMIRA concept is based on an inversion recovery (IR) prepared, segmented, and time-limited cine balanced steady state free precession sequence. Typically, for the fastest SC imaging without any signal averaging, eight consecutive images in time with an in-plane resolution of 0.67 × 0.67 mm 2 and 6 mm to 8 mm slice thickness are acquired in 51 s. AMIRA does not require parallel acquisition techniques., Results: AMIRA measures eight images of remarkable tissue contrast variation between spinal cord gray (GM) and white matter (WM) and cerebrospinal fluid (CSF). Following the AMIRA concept, averaging the first IR contrast images not only improves the signal-to-noise ratio but also offers a surprising enhancement of the contrast-to-noise ratio between GM and WM, whereas averaging the last images considerably improves the contrast-to-noise ratio between WM and CSF. These observations are supported by quantitative data., Conclusion: The AMIRA concept provides 2D spinal cord imaging with multiple tissue contrasts and enhanced contrast-to-noise ratios with a typical 0.67 × 0.67 mm 2 in-plane resolution and a slice thickness between 4 mm and 8 mm acquired in only 1 to 2 min per slice. Magn Reson Med 79:1870-1881, 2018. © 2017 International Society for Magnetic Resonance in Medicine., (© 2017 International Society for Magnetic Resonance in Medicine.)

This article presents the basic principles of the superstimulated echo mechanism and shows preliminary results of its application to T1-weighted imaging with positive T1-contrast. A superstimulated echo scheme uses a preparation of square-wave modulated, periodically inverted z-magnetization, which after signal evolution during the mixing time TM is fully converted into transverse magnetization. This avoids the 50% signal loss of a conventional stimulated echo. Furthermore, its implementation as a preparation module for standard turbo spin echo (TSE) imaging allows producing images with positive T1-contrast., (Copyright © 2011 Wiley Periodicals, Inc.)

This article introduces an effective b-factor b(TSE) for turbo spin echo (TSE) sequences to quantify their inherent diffusion sensitivity. b(TSE) is investigated for a broad variety of two-dimensional- and three-dimensional-TSE sequences using constant and varying flip angles (transitions between pseudo steady states, SPACE, VISTA, Cube, etc.). The inherent TSE diffusion sensitivity becomes important for high-resolution protocols, which can lead to subtle contrast modifications or even fluid suppressions in a clinical setting or animal imaging regime. The b(TSE) values obtained considerably depend on the relaxation times and diffusion coefficient and, thus, on the tissue under observation. The fractional b(TSE) contributions per TSE imaging encoding axis are highly anisotropic. Further noteworthy effects such as decreasing b-factors along a TSE train are pointed out and explained. The results are also discussed in combination with recent findings regarding contrast properties and possible diffusion sensitivity of TSE sequences. Identical but well more pronounced b(TSE) effects are observed in the animal imaging regime due to smaller field of view and higher resolutions., (Copyright © 2012 Wiley Periodicals, Inc.)

Despite the existence of numerous motion correction methods, head motion during MRI continues to be a major source of artifacts and can greatly reduce image quality. This applies particularly to diffusion weighted imaging, where strong gradients are applied during long encoding periods. These are necessary to encode microscopic movements. However, they also make the technique highly sensitive to bulk motion. In this work, we present a prospective motion correction method where all applied gradients are adjusted continuously to compensate for changes of the object position and ensure the desired phase evolution in the image coordinate frame. Additionally, in phantom experiments this new technique is used to reproduce motion artifacts with high accuracy by changing the position of the imaging frame relative to the measured object. In vivo measurements demonstrate the validity of the new correction method., (Copyright © 2011 Wiley Periodicals, Inc.)

In this study, a method for whole-body diffusion-weighted imaging (wbDWI) during continuous table motion has been developed and implemented on a clinical scanner based on a short-Tau inversion recovery echo-planar DWI sequence. Unlike currently available multistation wbDWI, which has disadvantages such as long scanning times, poor image quality, and troublesome data realignment, continuously moving table wbDWI can overcome these technical problems while extending the longitudinal field of view in MRI systems. In continuously moving table wbDWI, images are acquired consecutively at the isocenter of the magnet, having less geometric distortions and various possibilities of spatial and temporal coverage of an extended field of view. The acquired images, together with an apparent diffusion coefficient analysis, show that continuously moving table wbDWI can be used by appropriately adapting the table velocity, scan range, radiofrequency coils, slice resolutions, and spatio-temporal acquisition schemes according to various clinical demands., (Copyright © 2011 Wiley-Liss, Inc.)

This work presents a new imaging sequence in which multiple slices are simultaneously excited and refocused in a spin-echo train. The multiple spin-echo trains are interleaved in such a manner that (i) the Carr-Purcell-Meiboom-Gill conditions are fulfilled at all times, and (ii) the signals from slices can be separated, preventing aliasing. This paper also demonstrates how the sequence may be used in a novel fat-water Dixon method that enables fast volume coverage. The technique is demonstrated in phantoms and in vivo., ((c) 2010 Wiley-Liss, Inc.)

Magnetization transfer effects represent a major source of contrast in multislice turbo spin echo sequences (TSE)/fast spin echo sequences. Generally, low refocusing flip angles have become common in such MRI sequences, especially to mitigate specific absorption rate problems. Since the strength of magnetization transfer effects is related to the radiofrequency power and therefore specific absorption rate applied, magnetization transfer induced signal attenuations are investigated for a variety of TSE sequences with low constant and variable flip angles. Noticeable differences between the sequences have been observed. In particular, fewer signal attenuations are observed for TSE with low flip angles such as hyperecho-TSE and smooth transitions between pseudo steady states-TSE, leading to contrast that is less dependent on the number of slices. It is shown that the strength of the magnetization transfer-induced signal attenuations can be understood and described by a physical framework, which is based on the mean square flip angle of a given TSE sequence., (Copyright (c) 2009 Wiley-Liss, Inc.)

This study investigates the contrast behavior of 2D inversion recovery (IR) prepared turbo spin echo (TSE) sequences that use RF pulse schemes with variable low flip angles (hyperTSE) to reduce RF power deposition. A framework of equations and calculations for adapting the sequence parameters is presented by which equivalent image contrast is achieved compared to conventional IR-TSE imaging. Although the inversion time (TI) and repetition time (TR) do not need to be changed, the echo time (TE) has to be prolonged such that the effective TE (TE(eff)) is preserved. Measurements in healthy volunteers confirmed this finding for IR-TSE sequences using different TIs: fluid attenuated inversion recovery (FLAIR), gray matter (GM)-white matter (WM)-IR, and short tau IR (STIR). The results demonstrate that hyperTSE sequences enable high-quality IR-prepared imaging with a considerably reduced specific absorption rate (SAR).

To overcome specific absorption rate (SAR) limitations of spin-echo-based MR imaging techniques, especially at (ultra) high fields, rapid acquisition relaxation enhancement/TSE (turbo spin echo)/fast spin echo sequences in combination with constant or variable low flip angles such as hyperechoes and TRAPS (hyperTSE) have been introduced. Due to the multiple spin echo and stimulated echo pathways involved in the signal formation, the contrast behavior of such sequences depends on both T2 and T1 relaxation times. In this work, constant and various variable flip angle sequences were analyzed in a volunteer study. It is demonstrated that a single effective echo time parameter TE(eff) can be calculated that accurately describes the overall T2 weighted image contrast. TE(eff) can be determined by means of the extended phase graph concept and is practically independent of field strength. Using the described formalism, the contrast of any TSE sequence can be predicted. HyperTSE sequences are demonstrated to show a robust and well-defined T2 contrast allowing clinical routine MRI to be performed with SAR reductions of typically at least 70%., ((c) 2006 Wiley-Liss, Inc.)

The article presents an algorithm for calculation of flip angles in multiecho experiments to generate echoes with predefined amplitudes based on the extended phase graph algorithm. The algorithm can be used to optimize the echo envelope and thus the point spread function (PSF) in hyperecho and TRAPS (transition into the pseudosteady state) experiments while minimizing the total RF power. Implementations at 3 T using echo trains with Gaussian and Lorentzian PSF demonstrate a reduction in RF power by a factor of 3-5 while maintaining high image quality., (Copyright 2003 Wiley-Liss, Inc.)

A variation of the rapid acquisition with relaxation enhancement (RARE) sequence (also called turbo spin-echo (TSE) or fast spin-echo (FSE)) is presented. This technique uses variable flip angles along the echo train such that magnetization is initially prepared into the static pseudo steady state (PSS) for a low refocusing flip angle (alpha < 180 degrees ). It is shown that after such a preparation, magnetization will always stay very close to the static PSS even after significant variation of the subsequent refocusing flip angles. This allows the design of TSE sequences in which high refocusing flip angles yielding 100% of the attainable signal are applied only for the important echoes encoding for the center of k-space. It is demonstrated that a reduction of the RF power (RFP) by a factor of 2.5-6 can be achieved without any loss in signal intensity. The contribution of stimulated-echo pathways leads to a reduction of the effective TE by a factor f(t), which for typical implementations is on the order of 0.5-0.8. This allows the use of longer echo readout times, and thus longer echo trains, for acquiring images with a given T(2) contrast., (Copyright 2003 Wiley-Liss, Inc.)