MRI of the human brain at 130 microtesla

Abstract

We present in vivo images of the human brain acquired with an ultralow field MRI (ULFMRI) system operating at a magnetic field B 0 ∼ 130 μT. The system features prepolarization of the proton spins at B p ∼ 80 mT and detection of the NMR signals with a superconducting, second-derivative gradiometer inductively coupled to a superconducting quantum interference device (SQUID). We report measurements of the longitudinal relaxation time T 1 of brain tissue, blood, and scalp fat at B 0 and B p , and cerebrospinal fluid at B 0 . We use these T 1 values to construct inversion recovery sequences that we combine with Carr-Purcell-Meiboom-Gill echo trains to obtain images in which one species can be nulled and another species emphasized. In particular, we show an image in which only blood is visible. Such techniques greatly enhance the already high intrinsic T 1 contrast obtainable at ULF. We further present 2D images of T 1 and the transverse relaxation time T 2 of the brain and show that, as expected at ULF, they exhibit similar contrast. Applications of brain ULFMRI include integration with systems for magnetoencephalography. More generally, these techniques may be applicable, for example, to the imaging of tumors without the need for a contrast agent and to modalities recently demonstrated with T 1ρ contrast imaging (T 1 in the rotating frame) at fields of 1.5 T and above. . 3D magnetic field gradients specify a unique magnetic field and thus an NMR frequency or phase in each voxel of the subject, so that with appropriate signal decoding one can acquire a 3D image (4).Clinical MRI systems with B 0 = 1:5 T achieve a spatial resolution of typically 1 mm; 3-T systems are becoming increasingly widespread in clinical practice (5), offering a higher signal-tonoise ratio (SNR) and thus higher spatial resolution. Nonetheless, there is ongoing interest in less expensive MRI systems operating at lower fields. Commercially available 0.2-to 0.5-T systems based on permanent magnets offer both lower cost and wider patient aperture than their higher field counterparts, at the expense of spatial resolution. At the still lower field of 0.03 T maintained by a room temperature solenoid, Connolly and coworkers (6, 7) obtained clinically useful SNR and spatial resolution by prepolarizing the protons in a field B p of 0.3 T. Prepolarization (8) enhances the magnetization of the proton ensemble over that produced by the lower precession field; after the polarizing field is removed, the higher magnetization produces a correspondingly larger signal during its precession in B 0 . Using the same method, Stepisnik et al. (9) obtained MR images in the Earth's magnetic field (∼ 50 μT).In recent years there has been increasing interest (10-36) in NMR and MRI at fields ranging from a few nanotesla to the order of 100 μT. The enormous reduction in the detected signal amplitude compared with the high field value is overcome partly by using prepolarization and partly by detecting the signal with an untuned superconducting input circuit inductively coupl...

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Very Low Field MRI: A fast system compatible with Magnetoencephalography

“…Due to their limited field strength these systems would allow to image patient categories not compatible with clinical scanners (pregnant women, babies), offer an increased T 1 intrinsic contrast among different tissues [3], and are less sensitive to metal presence [4]. Additionally, the low field strength allows integration of MRI with other imaging modalities whose hardware is not compatible with high magnetic fields, such as Magnetoencephalography (MEG) [5,6,7]. Up to now ULF-MRI devices have demonstrated to be able to acquire images with resolution in the mm range [6,7,8,9,10] and reasonable SNR, to meet the criterion for adequate anatomical imaging [11], but with too long scan times.…”