Synopsis

Multi-contrast quantitative mapping has become an important research direction for characterizing brain tissue in physiological and pathological conditions. A rapid quantitative mapping acquisition is fundamental for its application in clinical routine. In this study, we adapted the STrategically Acquired Gradient Echo (STAGE) protocol for a clinical 1.5T MRI scanner. Using STAGE, we reconstructed longitudinal and effective transverse relaxation time maps. We obtained values similar to those in the literature in different brain structures with high repeatability. These results suggest that STAGE can be used at 1.5T and provide a variety of quantitative maps with an acquisition time of 7 minutes.

Introduction

Quantitative mapping of longitudinal (T1), transverse and effective transverse (T2*) magnetic resonance (MR) relaxation times, proton spin density (PSD), and quantitative susceptibility mapping (QSM) are useful for characterizing brain tissue¹ in physiological and pathological conditions^1-8.

Usually, sequences for mapping T1, T2*, PSD and QSM are acquired separately^8-10, with long scanning time making translation into clinical practice difficult. The STrategically Acquired Gradient Echo (STAGE) method^11,12 provides T1- and PSD-weighted (T1W, PSDW) images; susceptibility weighted imaging (SWI); T1, PSD, and R2*(1/T2*) maps and QSM in 5 minutes at 3T and 7 minutes at 1.5T¹³. The STAGE sequences and quantitative maps generation were recently described^11,12 using 3 T scanners. This study describes a STAGE protocol for a 1.5 T scanner in order to 1) investigate if obtained T1 and T2* are similar to values reported by literature; 2) assess repeatability.

Methods

Subjects: 22 healthy subjects (HS, 36.4±19.3 years, 16 females). One HS (35 years, female), scanned on 10 different days.

MRI protocol: The scans were performed on a 1.5 T Siemens Magnetom Avanto scanner at IRCCS Fondazione Don Carlo Gnocchi ONLUS (Milan, Italy), using a 12-channel head coil. The STAGE protocol consisted of two double-echo gradient echo (GRE) SWI scans acquired with full flow compensation on each echo, with a total scan time of 7 minutes. Parameters, published at 3T^11,12, were adapted for 1.5 T (Table 1) as follows. We simulated the gray matter (GM)/white matter(WM) contrast, the T1 precision, and the GM SNR as a function of Flip Angle (FA), in order to identify two FAs that allowed to reach a good trade-off among the parameters.

Quantitative maps: Data were processed with an in-house Matlab (MathWorks) program^11,12. The T1, PSD, radiofrequency (RF) transmit, and RF receiver field maps were obtained from the two first echoes and the transmit and receiver field inhomogeneities were corrected¹¹. The SWI was generated from the second echo with the lower FA. An R2* (1/T2*) map was obtained averaging the R2* maps obtained from the two echoes of each scan.

Relaxation times extraction from tissues of interest: Non-brain tissue was removed from the T1W, using the Brain Extraction Toolbox¹⁴. Deep GM structures (Caudate Nucleus –CN; Putamen –Put; Thalamus –Thal; Globus Pallidus –GP) were segmented on T1W using FIRST toolbox¹⁵. The masks were eroded to exclude border regions.The CSF, WM, GM partial volume maps were obtained using FAST toolbox¹⁶ and thresholded to 0.95 to exclude the voxels affected by partial volume effect. The Substantia Nigra (SN) and the Red Nucleus (RN) were manually segmented on SWI images on the axial slice where the structures were prominent and the borders were well defined. The median T1 and R2* were computed within each structure mask. The coefficient of variation (COV) was computed for each measure, for the 22 subjects and for the same subject acquired 10 times separately.

Results

Discussion

We adapted the STAGE sequences for a 1.5 T scanner. In order to provide all the maps displayed in Figure 1, different sequences are usually added in a research setting. Decreasing the acquisition times is a strong requirement for translation into clinical setting. The strategic combination of multi-TE and multi-FA makes it possible to obtain all the maps by acquiring only two gradient echo SWI images in 7 minutes.

The T1 values in the segmented structures obtained using the STAGE protocol have a good consistency among subjects (Figure 3): the intersubject COV is between 2% and 8%, similarly to the physiologic variability seen in^17-19,21. The physiologic variability of R2* was higher, similarly to other work²². T1 and R2* repeatability was good: the COVs ranged from about 0 to less than 6%, similarly to the ones recently reported for a phantom²³. The semiautomatic segmentation increased reproducibility with respect to manual drawing.

Our T1 and R2* results are similar to those reported in previous studies using a 1.5 T scanner^17-22 (Figure 2), that used sequences specifically designed for obtaining T1 mapping.

Acknowledgements

References