BISTRO: An outer‐volume suppression method that tolerates RF field inhomogeneity

Abstract

A technique is described for performing frequency-selective signal suppression with a high degree of tolerance to RF field inhomogeneity. The method is called B 1 -insensitive train to obliterate signal (BISTRO). BISTRO consists of multiple amplitude-and frequency-modulated (FM) pulses interleaved with spoiler gradients. BISTRO was developed for the purpose of accomplishing bandselective signal removal, as in water suppression and outer-volume suppression (OVS), in applications requiring the use of an inhomogeneous RF transmitter, such as a surface coil. In the present work, Bloch simulations were used to illustrate the principles and theoretical performance of BISTRO. Its performance for OVS was evaluated experimentally using MRI and spectroscopic imaging of phantoms and in vivo animal and human brain. By using FM pulses featuring offset-independent adiabaticity, BISTRO permitted high-quality, broadband suppression with one (or two) discrete borders demarcating the edge ( Key words: adiabatic pulse; pulse sequence; volume localization; outer-volume suppression; frequency modulation A technique to accomplish band-selective signal suppression is a common need in many types of NMR experiments. Examples of such application include chemicalshift selective suppression of water or fat resonances and outer-volume suppression (OVS) in localized spectroscopy and imaging. In the most common approach, signal suppression is achieved by exploiting one or more frequency-selective pulses that convert the initial magnetization M 0 into transverse magnetization M xy that is subsequently dephased by pulsed B 0 gradients (1-11). In OVS techniques, M xy is dephased after being excited in slabs at selected locations and orientations. RF pulses used to excite the unwanted magnetization are commonly the amplitude-modulated (AM) variety. Such OVS approaches generally provide excellent signal suppression, provided the RF field is relatively homogeneous so that each position in the sample experiences a flip angle of 90°. When the flip angle deviates from 90°, some longitudinal magnetization M z remains, giving incomplete signal suppression. The projection presaturation (PP) method of and related techniques (7,8) solve this problem to a certain degree by employing a series of identical, small flip-angle pulses. For example, by repeating the small flip 32 times, the PP technique can offer high-quality suppression (M z / M 0 Ͻ 0.01) on resonance despite an approximately fivefold variation of RF amplitude. Although this degree of compensation for RF inhomogeneity is more than sufficient for experiments performed with volume coils, it is inadequate for RF transmission with a surface coil, which typically subjects the sample to a Ͼ10-fold variation in RF amplitude. Furthermore, because the frequency response profile produced by AM pulses is highly dependent on flip angle, RF transmission with a surface coil can lead to out-of-slice signal excitation, and hence to an undesirable reduction of M 0 in the tissue region(s) to be detected.To overco...

The contribution of GABA to glutamate/glutamine cycling and energy metabolism in the rat cortexin vivo

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Proton T2 relaxation study of water, N‐acetylaspartate, and creatine in human brain using Hahn and Carr‐Purcell spin echoes at 4T and 7T

“…In the PRESS sequence, a five-lobe /2 sinc pulse and an optimized pulse (22) were used for excitation and refocusing, respectively, with T p ϭ 2 ms. To improve localization, PRESS was preceded by outer-volume suppression based on a B 1 -insensitive train to obliterate signal (BIS-TRO) in three dimensions (23).…”

Evidence for a “metabolically inactive” inorganic phosphate pool in adenosine triphosphate synthase reaction using localized 31P saturation transfer magnetic resonance spectroscopy in the rat brain at 11.7 T

“…To be less sensitive to B1 inhomogeneity, selective saturation of g-ATP consisted of a BISTRO pulse train of hyperbolic secant pulses (50-ms each duration, 100-Hz bandwidth) with 5-ms gradient spoiling between pulses. 15 Each spectrum was acquired in 34 min, keeping total examination time under 4 h. To control for direct saturation effects on the spectra, two other rats were scanned using the same protocol with the selective saturation placed symmetrical to g-ATP resonance relative to Pi, at all t sat . No detectable signal loss was observed on the dominant Pi peak, demonstrating the absence of significant RF bleed-over effect in our experimental conditions.…”