Localized 4.7 T Proton Magnetic Resonance Spectroscopy in Neonatal Encephalopathy: Implementation, Safety and Preliminary Interpretation of Results

Abstract

Magnetic resonance (MR) techniques have revolutionized the assessment of brain development and injury in the newborn. MR studies at 3 T or above are becoming widespread; however, given the high cost and complexity of high-field MR systems, it is important to assess their benefits and limitations for clinical practice.

MR spectroscopy (MRS) studies at high field carry potential benefits such as improved signal-to-noise ratio (SNR) and chemical shift dispersion compared with 1.5 T. However, safety is a very important consideration as radiofrequency (RF) tissue heating and acoustic noise also increase with field strength. Furthermore, increased chemical-shift dependence of the position of the volume of interest (VOI), shorter transverse and longer longitudinal relaxation times constitute additional challenges.

We describe the first in-vivo, proton MRS studies of newborn human brain at 4.7 T. Nine infants with neonatal encephalopathy, all ventilated and with continuous physiological monitoring, were studied. We used L-FOCI localization, mostly with a VOI centred on the thalami.

The absence of 4.7 T results from healthy infants made it impossible to interpret pathological metabolite peak-area ratios. However, concentration ratios and concentrations should be field-strength independent and interpretable using control values acquired at lower field. Of eight infants with concentration-ratio measurements, seven had low [N-acetylaspartate (NAA)]/[choline (Cho)] and three increased [lactate]/[NAA]. Of the two infants with ‘concentration’ measurements both had low [NAA] and one reduced [creatine].

We used an adult coil and consequent RF power restrictions had repercussions for the minimum magnetization recovery time (TR). A smaller, neonatal coil should improve SNR and, with less RF power, relax TR constraints thereby enabling fuller exploitation of the potential of high-field MRS.