Elsevier

NeuroImage

Volume 21, Issue 2, February 2004, Pages 757-767
NeuroImage

Optimisation of the 3D MDEFT sequence for anatomical brain imaging: technical implications at 1.5 and 3 T

https://doi.org/10.1016/j.neuroimage.2003.09.062Get rights and content

Abstract

An algorithm for the optimisation of 3D Modified Driven Equilibrium Fourier Transform (MDEFT) sequences for T1-weighted anatomical brain imaging is presented. Imaging parameters are optimised for a clinical whole body scanner and a clinical head scanner operating at 1.5 and 3 T, respectively. In vivo studies show that the resulting sequences allow for the whole brain acquisition of anatomical scans with an isotropic resolution of 1 mm and high contrast-to-noise ratio (CNR) in an acceptable scan time of 12 min. Typical problems related to the scanner-specific hardware configurations are discussed in detail, especially the occurrence of flow artefacts in images acquired with head transmit coils and the enhancement of scalp intensities in images acquired with phased array receive coils. It is shown both theoretically and experimentally that these problems can be avoided by using spin tagging and fat saturation.

Introduction

There is a wide range of MRI sequences for T1-weighted structural brain imaging. The purpose of these techniques is the acquisition of anatomical images with a high spatial resolution and good tissue contrast. In functional neuroimaging, anatomical scans of this kind are used as a reference for the functional scans, which usually display a low spatial resolution and reduced tissue contrast. Among the most common techniques for structural imaging are T1-weighted FLASH sequences Frahm et al., 1986, Haase, 1990 and MP-RAGE sequences Deichmann et al., 2000, Mugler and Brookeman, 1990.

The modified driven equilibrium Fourier transform (MDEFT) sequence is widely used for the acquisition of anatomical images with T1 weighting at high field strengths (Ugurbil et al., 1993) because of its advantageous contrast characteristics. The applicability at field strengths as low as 1 T has likewise been demonstrated (Norris and Redpath, 1997). The purpose of the present work is the optimisation of a 3D MDEFT sequence (Lee et al., 1995) for T1-weighted anatomical imaging with whole brain coverage and an isotropic resolution of 1 mm for two types of clinical scanners: a 1.5 T whole body scanner equipped with a whole body transmit coil and different types of receive coils, and a 3 T head scanner equipped with a single-element transmit–receive head coil. For each scanner, imaging parameters are optimised for maximum contrast-to-noise ratio (CNR) between white matter (WM) and grey matter (GM), for acceptable values of the signal-to-noise ratio (SNR) in WM and GM, and for CSF suppression. A total experimental duration of 12 min is chosen. The parameter optimisations are performed theoretically and tested in vivo on both scanners. In addition, there are detailed discussions on several hardware specific problems, for example, the occurrence of flow artefacts due to the use of transmit coils with spatially limited sensitivities and the enhancement of the scalp signal due to the use of phased-array receive systems. Solutions to these problems such as spin tagging and fat suppression are discussed and tested.

In general, this work is designed as a pragmatic guide to the implementation of 3D MDEFT sequences for high-resolution anatomical brain imaging on clinical scanners at different field strengths.

Section snippets

General sequence structure

For optimisation of the MDEFT sequence, expressions must be derived which enable calculation of the achievable SNR and CNR for a specific choice of parameters. The point spread function (PSF) must also be evaluated for each tissue type to determine whether contrast blurring might occur. In this section, the general structure of the 3D MDEFT sequence used in the present work is described.

The matrix size during image acquisition is given by NR (Read), N2D (2D Phase) and N3D (3D Phase, Partition).

Methods and materials

All experiments were performed either on a 1.5 T Sonata whole body scanner or on a 3 T Allegra head scanner (Siemens Medical Systems, Erlangen, Germany).

On the 1.5 T scanner, a whole-body coil was used for transmission. The coil displays a moderate B1 inhomogeneity, and the dependence of B1 on the distance x from the isocentre can be expressed approximately by the formulaB1∼1−P×10−5(x/mm)2where P assumes the values 1.9 in head/foot direction and 1.5 in anterior/posterior direction (on both

Experiment 1 (1.5 T, standard head coil)

The average SNR values were 27.3 ± 1.7 (WM) and 16.9 ± 1.2 (GM). The average CNR was 10.4 ± 0.6. A representative axial slice for a single volunteer is depicted in Fig. 4a, showing several WM and GM structures, such as corpus callosum, caudate nucleus, internal capsule, putamen and thalamus. The receiver characteristics of the head coil are clearly visible in WM areas: in the internal capsule, the signal intensity is relatively high, but there are signal and contrast losses in the anterior and

Discussion and conclusion

The results show that 3D MDEFT sequences, which were optimised with the help of the theory presented, yield anatomical images with high contrast and low noise levels at both field strengths considered. There is no evidence of contrast blurring. The use of an eight-element phased-array receiver system increases the CNR considerably, allowing for the detection of relatively small anatomical structures. The CNR for scans acquired on a 3 T scanner with a standard single-receiver head coil using the

Acknowledgements

This work was supported by the Wellcome Trust.

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