Water–fat separation in spiral magnetic resonance fingerprinting for high temporal resolution tissue relaxation time quantification in muscle

Kirsten Koolstra,Hermien E Kan,Peter Börnert,Thom T J Veeger,Andrew G Webb,Peter Koken

doi:10.1002/mrm.28143

Kirsten Koolstra, Hermien E Kan + Show 4 more

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https://doi.org/10.1002/mrm.28143

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Abstract

PurposeTo minimize the known biases introduced by fat in rapid T1 and T2 quantification in muscle using a single‐run magnetic resonance fingerprinting (MRF) water–fat separation sequence.MethodsThe single‐run MRF acquisition uses an alternating in‐phase/out‐of‐phase TE pattern to achieve water–fat separation based on a 2‐point DIXON method. Conjugate phase reconstruction and fat deblurring were applied to correct for B 0 inhomogeneities and chemical shift blurring. Water and fat signals were matched to the on‐resonance MRF dictionary. The method was first tested in a multicompartment phantom. To test whether the approach is capable of measuring small in vivo dynamic changes in relaxation times, experiments were run in 9 healthy volunteers; parameter values were compared with and without water–fat separation during muscle recovery after plantar flexion exercise.ResultsPhantom results show the robustness of the water–fat resolving MRF approach to undersampling. Parameter maps in volunteers show a significant (P < .01) increase in T1 (105 ± 94 ms) and decrease in T2 (14 ± 6 ms) when using water–fat‐separated MRF, suggesting improved parameter quantification by reducing the well‐known biases introduced by fat. Exercise results showed smooth T1 and T2 recovery curves.ConclusionWater–fat separation using conjugate phase reconstruction is possible within a single‐run MRF scan. This technique can be used to rapidly map relaxation times in studies requiring dynamic scanning, in which the presence of fat is problematic.

Highlights

Fast and accurate tissue relaxation time measurements in the presence of significant amounts of fat are relevant to muscle studies, but are challenging due to the known biases in the values obtained
We introduce a simple water–fat separation approach for Magnetic resonance fingerprinting (MRF), in which the dictionary size and matching algorithm remain unchanged compared with traditional MRF
Measurements in healthy volunteers showed that, using this technique, measured muscle water T1 values are increased and water T2 values are decreased compared to MRF without water–fat separation

Summary

| INTRODUCTION

Fast and accurate tissue relaxation time measurements in the presence of significant amounts of fat are relevant to muscle studies, but are challenging due to the known biases in the values obtained. We use an alternating in-phase/out-of-phase TE pattern to encode the chemical fat shift in the MRF acquisition, such that it can be combined with the well-established 2-point DIXON technique to separate water from fat signals.[23] This approach allows water–fat separation based on a single-run MRF scan that can help increase temporal resolution and reduce the risk of data corruption by motion or systemrelated inconsistencies. The blurring due to the main field inhomogeneity can be corrected for by applying CPR,[25] after which the signal equation (Equation 1) turns into In this process, the simulated readout trajectory can be used to generate a time map, describing at which time each k-space position was acquired. This is justified by the small difference in TEs used (1.15 ms), which would introduce a negligible change in signal amplitude due to T∗2 relaxation

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