Ileana Hancu

B1 Mapping by Bloch-Siegert Shift

A novel method for amplitude of radiofrequency field (B  1+ ) mapping based on the Bloch‐Siegert shift is presented. Unlike conventionally applied double‐angle or other signal magnitude–based methods, it encodes the B1 information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy, and robustness. The Bloch‐Siegert frequency shift is caused by irradiating with an off‐resonance radiofrequency pulse following conventional spin excitation. When applying the off‐resonance radiofrequency in the kilohertz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the magnitude of B  12 . Adding gradient image encoding following the off‐resonance pulse allows one to acquire spatially resolved B1 maps. The frequency shift from the Bloch‐Siegert effect gives a phase shift in the image that is proportional to B  12 . The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off‐resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch‐Siegert B1 mapping with 25 sec/slice is demonstrated to be quantitatively comparable to a 21‐min double‐angle map. As such, this method enables robust, high‐resolution B  1+ mapping in a clinically acceptable time frame. Magn Reson Med 63:1315–1322, 2010.

A Concentration-Independent Method to Measure Exchange Rates in PARACEST Agents

The efficiency of chemical exchange dependent saturation transfer (CEST) agents is largely determined by their water or proton exchange kinetics, yet methods to measure such exchange rates are variable and many are not applicable to in vivo measurements. In this work, the water exchange kinetics of two prototype paramagnetic agents (PARACEST) are compared by using data from classic NMR line‐width measurements, by fitting CEST spectra to the Bloch equations modified for chemical exchange, and by a method where CEST intensity is measured as a function of applied amplitude of radiofrequency field. A relationship is derived that provides the water exchange rate from the X‐intercept of a plot of steady‐state CEST intensity divided by reduction in signal caused by CEST irradiation versus 1/ω12, referred to here as an omega plot. Furthermore, it is shown that this relationship is independent of agent concentration. Exchange rates derived from omega plots using either high‐resolution CEST NMR data or CEST data obtained by imaging agree favorably with exchange rates measured by the more commonly used Bloch fitting and line‐width methods. Thus, this new method potentially allows access to a direct measure of exchange rates in vivo, where the agent concentration is typically unknown. Magn Reson Med 63:625–632, 2010.

CEST and PARACEST MR contrast agents

In this review we describe the status of development for a new class of magnetic resonance (MR) contrast agents, based on chemical exchange saturation transfer (CEST). The mathematics and physics relevant to the description of the CEST effect in MR are presented in an appendix published in the online version only. We discuss the issues arising when translating in vitro results obtained with CEST agents to using these MR agents in in vivo model studies and in humans. Examples are given on how these agents are imaged in vivo. We summarize the status of development of these CEST agents, and speculate about the next steps that may be taken towards the demonstration of CEST MR imaging in clinical applications.

Three-dimensional triple-quantum-filtered 23Na imaging of in vivo human brain

A scheme for the generation of three‐dimensional, triple‐quantum–filtered (TQ) sodium images from normal human brain is presented. In this approach, a three‐pulse, six‐step, coherence transfer filter was used in conjunction with a fast twisted projection imaging sequence to generate spatial maps of the TQ signal across the entire brain. It is demonstrated, theoretically as well as experimentally, that the use of the three‐pulse coherence filter leads to TQ sodium images in which the dependence of the image intensity on the spatial variation of the flip angle is less pronounced than it is in the “standard,” four‐pulse, TQ filter. Correction for the variation of the TQ signal intensity across the field of view because of radio‐frequency (RF) inhomogeneity is straightforward with this approach. This imaging scheme allows the generation of RF inhomogeneity–corrected, TQ, sodium images from human brain at moderate field strength (3.0 T) in times acceptable for routine clinical examinations (20 minutes). Magn Reson Med 42:1146–1154, 1999.

Loss of Cell Ion Homeostasis and Cell Viability in the Brain: What Sodium MRI Can Tell Us

This chapter demonstrates the use of sodium magnetic resonance imaging (MRI) as a noninvasive, in vivo means to assess metabolic changes that ensue from loss of cell ion homeostasis due to cell death in the brain. The chapter is organized in two sections. In the first section, the constraints imposed on the imaging methods by the nuclear magnetic resonance (NMR) properties of the sodium ion are discussed and strategies for avoiding their potential limitations are addressed. The second section illustrates the use of sodium MRI for monitoring focal brain ischemia in permanent and temporary primate models of endovascular middle cerebral artery occlusion.

A Multislice Gradient Echo Pulse Sequence for CEST Imaging

Chemical exchange–dependent saturation transfer and paramagnetic chemical exchange–dependent saturation transfer are agent‐mediated contrast mechanisms that depend on saturating spins at the resonant frequency of the exchangeable protons on the agent, thereby indirectly saturating the bulk water. In general, longer saturating pulses produce stronger chemical and paramagnetic exchange–dependent saturation transfer effects, with returns diminishing for pulses longer than T1. This could make imaging slow, so one approach to chemical exchange–dependent saturation transfer imaging has been to follow a long, frequency‐selective saturation period by a fast imaging method. A new approach is to insert a short frequency‐selective saturation pulse before each spatially selective observation pulse in a standard, two‐dimensional, gradient‐echo pulse sequence. Being much less than T1 apart, the saturation pulses have a cumulative effect. Interleaved, multislice imaging is straightforward. Observation pulses directed at one slice did not produce observable, unintended chemical exchange–dependent saturation transfer effects in another slice. Pulse repetition time and signal‐to noise ratio increase in the normal way as more slices are imaged simultaneously. Magn Reson Med, 2010.

Optimized glutamate detection at 3T.

To identify the pulse sequence and acquisition parameters that result in the most accurate and repeatable measurements of glutamate (Glu) concentration in the brain at 3T.

1H MR spectroscopy using TE averaged PRESS: a more sensitive technique to detect neurodegeneration associated with Alzheimer's disease.

A sensitive proton magnetic resonance spectroscopy (1H MRS) acquisition scheme that is capable of discriminating between normal controls and a group of patients with early Alzheimers disease (AD) is presented. The performance of this newly developed method, TE averaged PRESS (PRESS‐J), in detecting neurodegeneration associated with early AD is compared with that of short echo time (TE) PRESS. A stepwise discriminant function analysis is used to construct discriminant functions for both pulse sequences. These functions are each composed of a single predictor: the N‐acetyl aspartate (NAA)/creatine (Cr) ratio for PRESS‐J, and the NAA/myoInositol (mI) ratio for PRESS. We observed lower P‐values, higher areas under the receiver operating characteristic curves, and higher sensitivity at a given specificity for the PRESS‐J discriminating function in comparison with the PRESS discriminating function. The higher sensitivity of PRESS‐J is due to decreased variability when the singlets are fit in the spectra. This increased sensitivity enables new MR applications and, among other benefits, allows for smaller group sizes in drug trials, which can significantly reduce the cost of such trials. Magn Reson Med 53:777–782, 2005.

The case of the missing glutamine

A theoretical study was performed to determine the accuracy and repeatability of multiple one‐dimensional pulse sequences in the quantification of glutamine concentration at 3 T. Variable repeatability (12% to > 50%) and significant absolute error (−50% to +70%) were noted for the eight pulse sequences considered. Data acquired in vivo using three of the pulse sequences used for simulation matched the predicted repeatability well; among the pulse sequences considered, point‐resolved spectroscopy (TE = 80 ms) offered minimal error and acceptable repeatability (12%) for brain glutamine measurements. Following correction for the expected bias of each pulse sequence, consistent glutamine measurements, in the 1‐mM range, were reported with the three sequences. An explanation for the mismatch between in vivo 1H MRS and in vitro 13C/1H MRS at high field was attempted. Copyright

Accelerated spectroscopic imaging of hyperpolarized C-13 pyruvate using SENSE parallel imaging.

The ability to accelerate the spatial encoding process during a chemical shift imaging (CSI) scan of hyperpolarized compounds is demonstrated through parallel imaging. A hardware setup designed to simultaneously acquire 13C data from multiple receivers is presented here. A system consisting of four preamplifiers, four gain stages, a transmit coil, and a four receive channel rat coil was built for single channel excitation and simultaneous multi‐channel detection of 13C signals. The hardware setup was integrated with commercial scanner electronics, allowing the system to function similar to a conventional proton multi‐channel setup, except at a different frequency. The ability to perform parallel imaging is demonstrated in vivo. CSI data from the accelerated scans are reconstructed using a self‐calibrated multi‐spectral parallel imaging algorithm, by using lower resolution coil sensitivity maps obtained from the central region of k‐space. The advantages and disadvantages of parallel imaging in the context of imaging hyperpolarized compounds are discussed. Copyright