Direct B₀ field monitoring and real‐time B₀ field updating in the human breast at 7 tesla

Abstract

Large dynamic fluctuations of the static magnetic field (B 0 ) are observed in the human body during MR scanning, compromising image quality and detection sensitivity in several MR imaging and spectroscopy sequences. Partially, these dynamic B 0 fluctuations are due to physiological motion such as breathing, but other sources of temporal B 0 field fluctuations are also present in the MR system (e.g., eddy currents). Especially at ultrahigh field (!7 T), the increased susceptibility effects lead to large B 0 field variations over time. Direct measurement and correction of these temporal field variations of up to 70 Hz will lead to a significant reduction of artifacts and improved measurement stability/reproducibility. For direct measurement of the temporally changing B 0 field, a simple field probe was developed, that was placed in proximity to the tissue of interest. In this work, it is shown how such a field probe system can be used to monitor temporal B 0 field variations in the human body during MRI and magnetic resonance spectroscopy. Furthermore, it is shown how the acquired temporal B 0 field information can drive a dynamic shim module to directly correct the B 0 magnetic field in real time. Magn Reson Med 67:586-591, 2012. V C 2011 Wiley Periodicals, Inc.Key words: 7 T; field correction; frequency correction; realtime shimming; breast Variation of the static magnetic field (B 0 ), and the resulting phase and frequency variations during MRI and magnetic resonance spectroscopy (MRS) sequences, can lead to a range of artifacts including ghosting, signal loss, phase corruption, and line broadening. Several sources of dynamic field variations during routine MRI scanning can be identified and are either attributed to scanner instability (e.g., eddy currents and drift), motion of the body, and the consequently changing susceptibility distribution in the magnet. Expansion of the chest during breathing not only causes severe artifacts in tissue close to the chest, such as the breast (1), but also affects imaging in body parts at further distance. In fact, breathinginduced phase changes are currently a limiting factor in high-resolution imaging of the brain (2). However, breathing is not the only source of these spatio-temporal field variation. Also cardiac pulsation (3) and motion of the extremities (4) can be a limiting factor in accurate MR image acquisition. As susceptibility effects scale with field strength, extreme field variations are expected at ultrahigh field (!7 T) resulting in uncontrolled frequency alterations. B 0 field variations of several hertz in the brain (2,5,6) and up to 40 Hz in the breast have been reported (7,8) at ultrahigh field. If not corrected, these B 0 fluctuations can lead to problems with localization and non-Lorentzian line broadening in MRS (9) and can cause severe signal loss or ghosting artifacts, for example in susceptibility-weighted imaging (2,4), diffusion tensor imaging (10), temperature measurements (3), phase-based perfusion imaging (11), spectroscopic imaging or spectra...

1H-MR Spectroscopy in the Human Spinal Cord

“…19,30 Nevertheless, whether the use of respiratory triggering/gating is feasible is yet to be investigated because it might lead to severe scanning-time prolongation. More advanced solutions such as real-time B0 shim updating, as recently demonstrated for breast MR spectroscopy, 38 are also promising for spinal cord MR spectroscopy.…”

Padé optimization of noise-corrupted magnetic resonance spectroscopic time signals from fibroadenoma of the breast

“…Contrary to these expectations, however, the diagnostic accuracy of MRS in distinguishing benign breast lesions from breast cancer was not found to be superior in studies using 3 T scanners [53,54] compared to those using 1.5 T scanners [51]. There have been some published data using even higher field (4 or 7 T) MR scanners [52,[55][56][57][58][59]. Whereas tCho levels were significantly elevated in breast cancer in an investigation [56] of 500 in vivo breast spectra using a 4 T scanner, tCho was also reported as being present in many benign breast lesions and in normal breast.…”

Explicit extraction of absorption peak positions, widths and heights using higher order derivatives of total shape spectra by nonparametric processing of time signals as complex damped multi-exponentials

“…These attempts were motivated by the insufficient accuracy of Fourier-based in vivo MRS and MRSI with clinical (1.5 or 3T) MR scanners for identifying cancers and distinguishing these from benign lesions [43][44][45]. However, at higher field strengths, measured linewidths are generally wider due to increased susceptibility effects [46].…”

“…However, at higher field strengths, measured linewidths are generally wider due to increased susceptibility effects [46]. Overall, the use of higher field scanners has frequently failed to provide the anticipated improved cancer detection [26,[43][44][45]. Moreover, the enormous costs [47] would preclude such an approach for widespread applications, especially in screening/surveillance.…”