In vivo T1 mapping of neonatal brain tissue at 64 mT

Abstract

Purpose: Ultralow-field (ULF) point-of-care MRI systems allow image acquisition without interrupting medical provision, with neonatal clinical care being an important potential application. The ability to measure neonatal brain tissue T₁ is a key enabling technology for subsequent structural image contrast optimization, as well as being a potential biomarker for brain development. Here we describe an optimized strategy for neonatal T₁ mapping at ULF.

Methods: Examinations were performed on a 64-mT portable MRI system. A phantom validation experiment was performed, and a total of 33 in vivo exams were acquired from 28 neonates with postmenstrual age ranging from 31⁺⁴ to 49⁺⁰ weeks. Multiple inversion-recovery turbo spin-echo sequences were acquired with differing inversion and repetition times. An analysis pipeline incorporating inter-sequence motion correction generated proton density and T₁ maps. Regions of interest were placed in the cerebral deep gray matter, frontal white matter, and cerebellum. Weighted linear regression was used to predict T₁ as a function of postmenstrual age.

Results: Reduction of T₁ with postmenstrual age is observed in all measured brain tissue; the change in T₁ per week and 95% confidence intervals is given by dT₁ = -21 ms/week [-25, -16] (cerebellum), dT₁ = -14 ms/week [-18, -10] (deep gray matter), and dT₁ = -35 ms/week [-45, -25] (white matter).

Conclusion: Neonatal T₁ values at ULF are shorter than those previously described at standard clinical field strengths, but longer than those of adults at ULF. T₁ reduces with postmenstrual age and is therefore a candidate biomarker for perinatal brain development.

Keywords: gray matter; neonatal; relaxometry; ultralow-field MRI; white matter.

FIGURE 1

Pulse‐sequence diagram of inversion‐recovery 3D turbo spin echo. The sequence consists of an adiabatic inversion pulse, a turbo spin echo readout train, and a center‐frequency navigator

FIGURE 2

Results of the contrast phantom validation experiment.

FIGURE 3

Results of the homogeneity phantom validation experiment

FIGURE 4

Three slices from a single neonate (gestation 33⁺², postmenstrual age 34⁺⁰). A, Source images for each T₁‐weighted acquisition. B,C, M₀ and T₁ maps. D–F, Raw datapoints (blue dots) and predicted signals (red lines/cross) based on measured T₁ and M₀. The blue datapoint/red cross away from the curve corresponds to the TR = 3000 ms datapoint, and therefore belongs to a separate inversion‐recovery curve.

FIGURE 5

The effect of motion correction on T₁ maps of a neonate born at 38⁺³ gestation and imaged at 41⁺⁰ postmenstrual age. A,B, Coronal and axial T₁ maps for three iterations of the reconstruction pipeline. C,D, Coronal and axial relative RMS error maps between the input images and predicted output images at each iteration

FIGURE 6

T₁ maps of 8 subjects over the full age range of neonates scanned in this study

FIGURE 7

T₁ versus postmenstrual age. The color of each datapoint indicates the acquisition protocol from Table 1