Detection and correction of frequency instabilities for volumetric ¹H echo‐planar spectroscopic imaging

Abstract

Spectral quality in 1 H magnetic resonance spectroscopic imaging (MRSI) critically depends on the stability of the main magnetic field. For echo-planar MRSI implemented at 3 T, temperature variation in the passive steel shims of the magnet system can lead to a significant drift in the resonance frequency. A method is presented that incorporates interleaved measurement of the instantaneous resonance frequency of a reference water signal into a volumetric MRSI sequence and allows correction for the drift during postprocessing. Key words: in vivo MR spectroscopic imaging; brain; instrumental instabilities; frequency drift; EPSI Even small instrumental instabilities can limit the data quality for MR spectroscopy and spectroscopic imaging (MRSI). For example, the small field drift of the main magnetic field, B 0 , of superconducting magnets (Ͻ0.1 ppm/hr) can result in a measurable frequency shift over the course of a MRSI acquisition that results in suboptimal water suppression, broadening of spectral lines, and loss of phase coherence (1). An additional source of instability that can occur on higher-field MRI systems, i.e., with B 0 Ն 3 T, is heating of the gradient coil former and thermally coupled passive steel shims, due to either high shim currents or strong magnetic field gradients, which can cause a change in the resonance frequency that is several-fold higher than the magnet field stability (2). An elegant solution was proposed for single-voxel spectroscopy using an interleaved, nonlocalized water reference signal to obtain a frequency estimate and applying a negative feedback in real time to the Z0 shim coil current (2). However, implementation of this interleaved nonlocalized B 0 measurement is undesirable for spectroscopic imaging of the brain, where it is also desirable to obtain a localized and phase-encoded water reference signal, which can be used for measurement and correction of local B 0 variations of the metabolite data (3). The acquisition of both localized and nonlocalized B 0 measurements and execution of the real-time B 0 correction are time consuming and would lengthen the minimum TR of the pulse sequence (2) and thus the total acquisition time.In this paper, we show that a spatially varying drift of the resonance frequency occurs during the prolonged use of strong, oscillating readout gradients for a volumetric echo-planar spectroscopic imaging (3D EPSI) pulse sequence on a 3-T magnet and present a method for detection and correction of this effect, as well as frequency shifts induced due to subject motion. METHODSA volumetric proton EPSI acquisition sequence has been implemented on a 3-T Trio system (Siemens Medical Systems, Germany) as illustrated in Fig. 1. The sequence is similar to that described earlier (4), with the incorporation of an interleaved water reference acquisition. Data were acquired in normal human brain with TR ϭ 1710 msec, TE ϭ 70 msec, and TI ϭ 198 msec for global inversionrecovery used for lipid suppression. Additional timings shown in Fig. 1 include TR wr ϭ 591 msec, t...

Spatiotemporal magnetic field monitoring for MR

“…However, the accuracy and compactness of the phase expansion [2] depend on the judicious choice of this basis. It should be designed such that the phase evolution can be accurately captured with few basis functions and hence determined with few probes.…”

“…They are therefore neglected, as commonly done in MRI. 2 Strictly speaking, the magnitude operation leading to Eq. [1] couples the dynamic and static field components, causing small terms that change fast in both space and time.…”

“…where 2 When exposed to this field, the object's transverse magnetization with the gyromagnetic ratio γ accrues the phase…”

“…In order to combine such a map consistently with the dynamic phase model, as done in Eq. [2], the frequency mapping must be performed in the same reference state as the probe calibration.…”

Proton MR spectroscopy of the brain at 3 T: an update

“…These drifts can be corrected by the method recently proposed by Ebel and Maudsley [53], which allows for obtaining both a nonlocalized B 0 measurement and a fully phase-encoded water reference signal, and then for correcting the frequency drift during postprocessing using the measured values. The correction largely improves spectral quality, removing lineshape distortions and recovering metabolite signal loss.…”