Matthew Nichols

MRI of the human brain at 130 microtesla

Significance We describe MRI in a magnetic field of 130 μT with signals detected from prepolarized protons with a superconducting quantum interference device (SQUID). We report measurements of the longitudinal relaxation time T1 of brain tissue, blood, scalp fat, and cerebrospinal fluid. Using a combination of inversion recovery and multiple echoes, we form 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 T1-contrast obtainable at ultralow frequencies. We further present 2D brain images of T1 and the transverse relaxation time T2 showing that, as expected, they exhibit similar contrast. We present in vivo images of the human brain acquired with an ultralow field MRI (ULFMRI) system operating at a magnetic field B0 ∼ 130 μT. The system features prepolarization of the proton spins at Bp ∼ 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 T1 of brain tissue, blood, and scalp fat at B0 and Bp, and cerebrospinal fluid at B0. We use these T1 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 T1 contrast obtainable at ULF. We further present 2D images of T1 and the transverse relaxation time T2 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 T1ρ contrast imaging (T1 in the rotating frame) at fields of 1.5 T and above.

Observation of 2D Fermionic Mott Insulators of ^{40}K with Single-Site Resolution.

We report on the site-resolved observation of characteristic states of the two-dimensional repulsive Fermi-Hubbard model, using ultracold ^{40}K atoms in an optical lattice. By varying the tunneling, interaction strength, and external confinement, we realize metallic, Mott-insulating, and band-insulating states. We directly measure the local moment, which quantifies the degree of on-site magnetization, as a function of temperature and chemical potential. Entropies per particle as low as 0.99(6)k_{B} indicate that nearest-neighbor antiferromagnetic correlations should be detectable using spin-sensitive imaging.