Bernd Stoeckel

Brain tissue sodium concentration in multiple sclerosis: a sodium imaging study at 3 tesla

Neuro-axonal degeneration occurs progressively from the onset of multiple sclerosis and is thought to be a significant cause of increasing clinical disability. Several histopathological studies of multiple sclerosis and experimental autoimmune encephalomyelitis have shown that the accumulation of sodium in axons can promote reverse action of the sodium/calcium exchanger that, in turn, leads to a lethal overload in intra-axonal calcium. We hypothesized that sodium magnetic resonance imaging would provide an indicator of cellular and metabolic integrity and ion homeostasis in patients with multiple sclerosis. Using a three-dimensional radial gradient-echo sequence with short echo time, we performed sodium magnetic resonance imaging at 3 T in 17 patients with relapsing-remitting multiple sclerosis and in 13 normal subjects. The absolute total tissue sodium concentration was measured in lesions and in several areas of normal-appearing white and grey matter in patients, and corresponding areas of white and grey matter in controls. A mixed model analysis of covariance was performed to compare regional tissue sodium concentration levels in patients and controls. Spearman correlations were used to determine the association of regional tissue sodium concentration levels in T(2)- and T(1)-weighted lesions with measures of normalized whole brain and grey and white matter volumes, and with expanded disability status scale scores. In patients, tissue sodium concentration levels were found to be elevated in acute and chronic lesions compared to areas of normal-appearing white matter (P < 0.0001). The tissue sodium concentration levels in areas of normal-appearing white matter were significantly higher than those in corresponding white matter regions in healthy controls (P < 0.0001). The tissue sodium concentration value averaged over lesions and over regions of normal-appearing white and grey matter was positively associated with T(2)-weighted (P < or = 0.001 for all) and T(1)-weighted (P < or = 0.006 for all) lesion volumes. In patients, only the tissue sodium concentration value averaged over regions of normal-appearing grey matter was negatively associated with the normalized grey matter volume (P = 0.0009). Finally, the expanded disability status scale score showed a mild, positive association with the mean tissue sodium concentration value in chronic lesions (P = 0.002), in regions of normal-appearing white matter (P = 0.004) and normal-appearing grey matter (P = 0.002). This study shows the feasibility of using in vivo sodium magnetic resonance imaging at 3 T in patients with multiple sclerosis. Our findings suggest that the abnormal values of the tissue sodium concentration in patients with relapsing-remitting multiple sclerosis might reflect changes in cellular composition of the lesions and/or changes in cellular and metabolic integrity. Sodium magnetic resonance imaging has the potential to provide insight into the pathophysiological mechanisms of tissue injury when correlation with histopathology becomes available.

Advantages of parallel imaging in conjunction with hyperpolarized helium—A new approach to MRI of the lung

Hyperpolarized helium (3He) gas MRI has the potential to assess pulmonary function. The non‐equilibrium state of hyperpolarized 3He results in the continual depletion of the signal level over the course of excitations. Under non‐equilibrium conditions the relationship between the signal‐to‐noise ratio (SNR) and the number of excitations significantly deviates from that established in the equilibrium state. In many circumstances the SNR increases or remains the same when the number of data acquisitions decreases. This provides a unique opportunity for performing parallel MRI in such a way that both the temporal and spatial resolution will increase without the conventional decrease in the SNR. In this study an analytical relationship between the SNR and the number of excitations for any flip angle was developed. Second, the point‐spread function (PSF) was utilized to quantitatively demonstrate the unconventional SNR behavior for parallel imaging in hyperpolarized gas MRI. Third, a 24‐channel (24ch) receive and two‐channel (2ch) transmit phased‐array system was developed to experimentally prove the theoretical predictions with 3He MRI. The in vivo experimental results prove that significant temporal resolution can be gained without the usual SNR loss in an equilibrium system, and that the entire lung can be scanned within one breath‐hold (∼13 s) by applying parallel imaging to 3D data acquisition. Magn Reson Med, 2006.

Whole body traveling wave magnetic resonance imaging at high field strength: Homogeneity, efficiency, and energy deposition as compared with traditional excitation mechanisms

In 7 T traveling wave imaging, waveguide modes supported by the scanner radiofrequency shield are used to excite an MR signal in samples or tissue which may be several meters away from the antenna used to drive radiofrequency power into the system. To explore the potential merits of traveling wave excitation for whole‐body imaging at 7 T, we compare numerical simulations of traveling wave and TEM systems, and juxtapose full‐wave electrodynamic simulations using a human body model with in vivo human traveling wave imaging at multiple stations covering the entire body. The simulated and in vivo traveling wave results correspond well, with strong signal at the periphery of the body and weak signal deep in the torso. These numerical results also illustrate the complicated wave behavior that emerges when a body is present. The TEM resonator simulation allowed comparison of traveling wave excitation with standard quadrature excitation, showing that while the traveling wave B  +1 per unit drive voltage is much less than that of the TEM system, the square of the average B  +1 compared to peak specific absorption rate (SAR) values can be comparable in certain imaging planes. Both systems produce highly inhomogeneous excitation of MR signal in the torso, suggesting that B1 shimming or other parallel transmission methods are necessary for 7 T whole body imaging. Magn Reson Med 67:1183–1193, 2011.

Sodium long‐component T 2* mapping in human brain at 7 Tesla

Sodium (23Na) MRI may provide unique information about the cellular and metabolic integrity of the brain. The quantification of tissue sodium concentration from 23Na images with nonzero echo time (TE) requires knowledge of tissue‐specific parameters that influence the single‐quantum sodium signal such as transverse (T2) relaxation times. We report the sodium (23Na) long component of the effective transverse relaxation time T  2* values obtained at 7 T in several brain regions from six healthy volunteers. A two‐point protocol based on a gradient‐echo sequence optimized for the least error per given imaging time was used (TE1 = 12 ms; TE2 = 37 ms; averaged N1 = 5; N2 = 15 times; pulse repetition time = 130 ms). The results reveal that long T  2* component of tissue sodium (mean ± standard deviation) varied between cerebrospinal fluid (54 ± 4 ms) and gray (28 ± 2 ms) and white (29 ± 2 ms) matter structures. The results also show that the long T  2* component increases as a function of the main static field B0, indicating that correlation time of sodium ion motion is smaller than the time‐scale defined by the Larmor frequency. These results are a prerequisite for the quantification of tissue sodium concentration from 23Na MRI scans with nonzero echo time, will contribute to the design of future measurements (such as triple‐quantum imaging), and themselves may be of clinical utility. Magn Reson Med, 2009.

Aspects of Clinical Imaging at 7 T

The intrinsic improvements in signal-to-noise ratio, spectral dispersion, and susceptibility contrast with increasing static magnetic field strength, B 0, has spurred the development of MR technology from its very first application to clinical imaging. With maturing magnet, RF, and gradient technology, the clinical community has seen the static magnetic field of clinical systems increase from 0.2 to 1.5 to 3.0 T. Today, the “high field” label for human MR research describes initial experiences with 7, 8, and 9.4T systems. While currently primarily research instruments, this technology is bound to cross the boundary into the clinical diagnostic arena as key technical issues are solved and the methodology proves itself for addressing clinical issues. In this chapter we discuss the particular advantages and disadvantages of ultra high field systems for clinical imaging as well as some of the immediate technological challenges that must be solved to derive the full benefit of the extraordinary sensitivity of these systems, which has been glimpsed from their research use.

Real-time cardiac MRI with radial acquisition and k-space variant reduced-FOV reconstruction

This work aims to demonstrate that radial acquisition with k-space variant reduced-FOV reconstruction can enable real-time cardiac MRI with an affordable computation cost. Due to non-uniform sampling, radial imaging requires k-space variant reconstruction for optimal performance. By converting radial parallel imaging reconstruction into the estimation of correlation functions with a previously-developed correlation imaging framework, Cartesian k-space may be reconstructed point-wisely based on parallel imaging relationship between every Cartesian datum and its neighboring radial samples. Furthermore, reduced-FOV correlation functions may be used to calculate a subset of Cartesian k-space data for image reconstruction within a small region of interest, making it possible to run real-time cardiac MRI with an affordable computation cost. In a stress cardiac test where the subject is imaged during biking with a heart rate of >100 bpm, this k-space variant reduced-FOV reconstruction is demonstrated in reference to several radial imaging techniques including gridding, GROG and SPIRiT. It is found that the k-space variant reconstruction outperforms gridding, GROG and SPIRiT in real-time imaging. The computation cost of reduced-FOV reconstruction is ~2 times higher than that of GROG. The presented work provides a practical solution to real-time cardiac MRI with radial acquisition and k-space variant reduced-FOV reconstruction in clinical settings.

Single breath-hold whole heart coronary MRA with isotropic spatial resolution using highly-accelerated parallel imaging with a 32-element coil array

Introduction Whole heart coronary MRA (CMRA) is typically performed with navigator gating because of the extensive data acquisition needed to achieve an isotropic spatial resolution on the order of 1-2 mm3 with full anatomic coverage (10-16 cm). Previous studies have shown that whole heart CMRA can be performed with either a single [1] or double [2,3] breath-hold (BH) approach using highlyaccelerated parallel imaging. The single breath-hold approach [1] acquires the coil sensitivity data immediately before and after the coronary MRA data within the same cardiac cycle, whereas the double BH approach acquires coil sensitivity data in a separate BH. The single BH approach lengthens the time between the T2 and fat suppression pulses to the image acquisition, and the double BH approach may suffer from misregistration. We propose to acquire the coil sensitivity and coronary MRA data in two separate cardiac phases (early systole and mid diastole, respectively) both within a single BH, in order to circumvent the aforementioned problems.