Intrascanner and interscanner variability of magnetization transfer‐sensitized balanced steady‐state free precession imaging

Recently, a new and fast three‐dimensional imaging technique for magnetization transfer ratio (MTR) imaging has been proposed based on a balanced steady‐state free precession protocol with modified radiofrequency pulses. In this study, optimal balanced steady‐state free precession MTR protocol parameters were derived for maximum stability and reproducibility. Variability between scans was assessed within white and gray matter for nine healthy volunteers using two different 1.5 T clinical systems at six different sites. Intrascanner and interscanner MTR measurements were well reproducible (coefficient of variation: c_v < 0.012 and c_v < 0.015, respectively) and results indicate a high stability across sites (c_v < 0.017) for optimal flip angle settings. This study demonstrates that balanced steady‐state free precession MTR not only benefits from short acquisition time and high signal‐to‐noise ratio but also offers excellent reproducibility and low variability, and it is thus proposed for clinical MTR scans at individual sites as well as for multicenter studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

High‐resolution 3D radial bSSFP with IDEAL

Radial trajectories facilitate high‐resolution balanced steady state free precession (bSSFP) because the efficient gradients provide more time to extend the trajectory in k‐space. A number of radial bSSFP methods that support fat–water separation have been developed; however, most of these methods require an environment with limited B₀ inhomogeneity. In this work, high‐resolution bSSFP with fat–water separation is achieved in more challenging B₀ environments by combining a 3D radial trajectory with the IDEAL chemical species separation method. A method to maintain very high resolution within the timing constraints of bSSFP and IDEAL is described using a dual‐pass pulse sequence. The sampling of a unique set of radial lines at each echo time is investigated as a means to circumvent the longer scan time that IDEAL incurs as a multiecho acquisition. The manifestation of undersampling artifacts in this trajectory and their effect on chemical species separation are investigated in comparison to the case in which each echo samples the same set of radial lines. This new bSSFP method achieves 0.63 mm isotropic resolution in a 5‐min scan and is demonstrated in difficult in vivo imaging environments, including the breast and a knee with ACL reconstruction hardware at 1.5 T. Magn Reson Med 71:95–104, 2014. © 2013 Wiley Periodicals, Inc.     

Balanced alternating steady-state elastography.

A conventional balanced steady-state free precession (b-SSFP) sequence scheme was modified such that the dynamic equilibrium becomes very sensitive to small cyclic displacements, generating two distinct and alternating steady states. This novel technique is proposed for the visualization of propagating transverse acoustic shear waves, as used in MR elastography (MRE) to determine the mechanical properties of materials or in vivo soft tissue. Experiments with tissue-like agarose gel phantoms and simulations demonstrate that the novel sequence offers an increase in phase sensitivity by about one order in magnitude compared to standard motion-encoding methods. In addition, the new method benefits from the very short acquisition times achieved by b-SSFP protocols.     

Alternate ascending/descending directional navigation approach for imaging magnetization transfer asymmetry

A new method for imaging magnetization transfer (MT) asymmetry with no separate saturation pulse is proposed in this article. MT effects were generated from sequential two‐dimensional balanced steady‐state free precession imaging, where interslice MT asymmetry was separated from interslice blood flow and magnetic field inhomogeneity with alternate ascending/descending directional navigation (ALADDIN). Alternate ascending/descending directional navigation provided high‐resolution multislice MT asymmetry images within a reasonable imaging time of ～3 min. MT asymmetry signals measured with alternate ascending/descending directional navigation were 1–2% of baseline signals (N = 6), in agreement with those from the conventional methods. About 70% of MT asymmetry signals were determined by the first prior slice. The frequency offset ranges in this study were >8 ppm from the water resonance frequency, implying that the MT effects were mostly associated with solid‐like macromolecules. Potential methods to make alternate ascending/descending directional navigation feasible for imaging amide proton transfer (～3.5 ppm offset from the water resonance frequency) were discussed. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Cardiovascular magnetization transfer ratio imaging compared with histology: A postmortem study

Cardiovascular magnetization transfer ratio (MTR) imaging by steady state free precession is a promising imaging method to assess microstructural changes within the myocardium. Hence, MTR imaging was correlated to histological analysis. Three postmortem cases were selected based on a suspicion of myocardial infarction. MTR and T₂‐weighted (T_2w) imaging was performed, followed by autopsy and histological analysis. All tissue abnormalities, identified by autopsy or histology, were retrospectively selected on visually matched MTR and T_2w images, and corresponding MTR values compared with normal appearing tissue. Regions of elevated MTR (up to approximately 20%, as compared to normal tissue), appearing hypo‐intense in T_2w‐images, revealed the presence of fibrous tissue in microscopic histological analysis. Macroscopic observation (autopsy) described scar tissue only in one case. Regions of reduced MTR (up to approximately 20%) corresponded either to (i) the presence of edema, appearing hyperintense in T_2w‐images and confirmed by autopsy, or to (ii) inflammatory granulocyte infiltration at a microscopic level, appearing as hypo‐intense T_2w‐signal, but not observed by autopsy. Findings from cardiovascular MTR imaging corresponded to histology results. In contrast to T_2w‐imaging, MTR imaging discriminated between normal myocardium, scar tissue and regions of acute myocardial infarction in all three cases. J. Magn. Reson. Imaging 2014;40:915–919 . © 2013 Wiley Periodicals, Inc.     

Frequency stabilization using infinite impulse response filtering for SSFP fMRI at 3T.

The steady-state free precession (SSFP) method has been shown to exhibit strong potential for distortion-free functional magnetic resonance imaging (fMRI). One major challenge of SSFP fMRI is that the frequency band corresponding to the highest functional sensitivity is extremely narrow, leading to substantial loss of functional contrast in the presence of magnetic field drifts. In this study we propose a frequency stabilization scheme whereby an RF pulse with small flip angle is applied before each image scan, and the initial phase of the free induction decay (FID) signals is extracted to reflect temporal field drifts. A simple infinite impulse response (IIR) filter is further employed to obtain a low-pass-filtered estimate of the central reference frequency for the upcoming scan. Experimental results suggest that the proposed scheme can stabilize the frequency settings in accordance with field drifts, with oscillation amplitudes of <0.5 Hz. Phantom studies showed that both slow drifts and fast fluctuations were prominently reduced, resulting in less than 5% signal variations. Visual fMRI at submillimeter in-plane resolution further demonstrated 15% activation signals that were nicely registered in the microvessels within the sulci. It is concluded that the IIR-filtered frequency stabilization is an effective technique for achieving reliable SSFP fMR images at high field strengths.     

Gradient echo imaging

Magnetic resonance imaging (MRI) based on gradient echoes is used in a wide variety of imaging techniques and clinical applications. Gradient echo sequences form the basis for an essential group of imaging methods that find widespread use in clinical practice, particularly when fast imaging is important, as for example in cardiac MRI or contrast‐enhanced MR angiography. However, the term “gradient echo sequence” is somewhat unspecific, as even images acquired with the most common sequences employing the gradient echo for data acquisition can significantly differ in signal, contrast, artifact behavior, and sensitivity to, eg, flow. This is due to the different use of sequence timing and basic sequence building blocks such as spoiler gradients or specific radiofrequency (RF) pulse phase patterns. In this article the basic principles of gradient echo formation compared to spin echo imaging are reviewed and the properties of gradient echo imaging in its simplest form (TR ? T₂) are described. Further, the most common three variants of fast gradient echo sequences (TR < T₂), namely, unbalanced gradient echo, RF spoiled gradient echo, and balanced steady state free precession; are discussed. For each gradient echo sequence type, examples of applications exploiting the specific properties of the individual technique are presented. J. Magn. Reson. Imaging 2012;35:1274–1289. © 2012 Wiley Periodicals, Inc.     

T2 measurement of the human myocardium using a T2-prepared transient-state TrueFISP sequence.

A fast and motion-insensitive technique suitable for myocardial BOLD contrast imaging is presented. The method, termed T2-TrueFISP, combines T2 magnetization preparation with steady-state free precession (SSFP) imaging for T2 relaxation mapping of the myocardium in healthy volunteers. The T2 contrast-to-noise ratio (CNR) was optimized with the use of transient-state TrueFISP readout and half-Fourier readout with linear phase encoding. Single-slice myocardial T2-weighted image was obtained within one heartbeat, and a single slice T2 map of the myocardium was obtained in under 5-7 s. A respiratory navigator-gating method was incorporated for serial measurements and signal averaging, with the subjects breathing freely. The mean myocardial T2 relaxation time measured in 12 healthy volunteers was 54 +/- 5.7 ms. Regional variations of T2 values across the myocardium were 7%. Temporal variations across serial T2 measurements in a transmural region covering approximately 0.5 cc of the left ventricular (LV) wall were 3.6% without signal averaging (number of excitations (NEX) = 1) and 1.7% with signal averaging (NEX = 10). According to our preliminary results, the T2-TrueFISP method is expected to provide a robust and sensitive tool for clinical application of myocardial BOLD contrast imaging.