MRI of the human brain at 130 microtesla

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.