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HOPE (Half-intensity with macrOmolecule-suPprEssion): Ultra-short TE MRS without macromolecules on 3 T, 4 T and 9.4 T1McLean Hospital; Harvard Medical School, Belmont, MA, United States, 2National Institute on Drug Abuse, Baltimore, MD, United States
Synopsis
The strong and overlapping macromolecule (MM) signals remains as one of the major technical challenges for metabolites quantification using ultra-short TE MRS. HOPE (Half-intensity with macrOmolecule-suPprEssion), a simple but effective MM suppression MRS method based on SPECIAL, was proposed and tested on human 3T, 4T and animal 9.4T. HOPE has no additional pulse or cycling compared to SPECIAL but with the same short TE. With the similar SNR level as STEAM, HOPE achieves additional benefit with substantial suppression of MM signals and more accurate quantifications of MM overlapped metabolites such as lactate and GABA.
Introduction
It has been demonstrated that the ultra-short spin echo time (TE) proton MRS methods greatly enhanced the reliabilities detecting smaller and frequency overlapping compounds in the brains of both rodents (1,2) and human (3,4) because J modulation of scalar coupled metabolites are greatly reduced. However, the strong and overlapping macromolecule (MM) signals remains as the major technical challenge for metabolites quantification using short TE MRS (5). MM resonances have been reported to vary with age and cerebral location (6,7), as well as composition of gray/white matter (7,8). Large inter-subject variance of MM has also been reported (9). Mathematical (10) and experimental (11,12) estimates of MM are the two most widely used methods to account for the MM contributions, but there are still difficulties to address individual MM characteristics. In this study, HOPE (Half-intensity with macrOmolecule-suPprEssion), an ultra-short TE MRS sequence based on SPECIAL (13,14) to achieve effective MM suppression, was proposed (15) and implemented on 3 T, 4 T and 9.4 T to demonstrate its capability to detect MM overlapped compounds such as lactate (Lac) and GABA.Methods
The mechanism of HOPE is demonstrated in Fig. 1: a delay is inserted after the inversion localization pulse of SPECIAL. With a well-adjusted delay time, the MM resonances are mostly subtracted out in the difference spectrum, with metabolite peaks remains with approximately half intensity because MMs’ T1 are much shorter than metabolites. No additional pulse or cycling is needed compared to SPECIAL.
The feasibility of HOPE was tested on the Siemens 3 T Trio human scanner with a 32-channel birdcage coil, the Agilent 4 T human scanner with a quadrature surface coil, and Bruker 9.4 T animal scanner with a volume coil as transmitter and surface coil as receiver. The TE on 3 T, 4 T and 9.4 T were 8 ms,10 ms and 6.5 ms respectively. 5 and 4 healthy subjects went through the scan on 3 T and 4 T scanners respectively, with the MRS voxel on the anterior cingulate cortex (ACC) region (30 x 20 x 30 mm3). 6 rats were scanned on the 9.4 T animal scanner with MRS voxel on the prefrontal cortex (3 x 5 x 4 mm3). On all scanners 256 averages were acquired with TR = 3 s for the water suppressed spectrum and 16 averages for water spectrum. Average-by-average frequency and phase corrections were performed in the matlab based MRS toolkit FID-A (16). The eddy current corrections and fitting quantification was performed in LCModel (10) with the simulated macromolecule peaks accounting for the baseline.
Results and discussions
A series of inversion-recovery times (IRs) from 20 ms (regular SPECIAL) to 1 s were tested and the representative spectra on 3 T and 9.4 T were shown in Fig. 2 together with their baselines. MM signals gradually reduced and the baseline became smoother as IR increased, while the MM overlapping metabolites such as Lac and GABA became more resolved. A longer IR achieves better MM suppression however with lower SNR. With balancing MM suppression and SNR, IRs of 900 ms, 1000 ms and 800 ms were used for the scans on 3 T, 4 T human scanners and 9.4 T animal scanner respectively (Fig. 3). Lac doublet at 1.3 ppm was much better resolved with MM suppression on all field strengths. With suppression of MM near 2.0 ppm, the GABA resonances at 1.9 ppm were less overlapped and increasingly resolved as the field strength increases. The LCModel quantification results in Fig. 4 follow the same trend: the GABA concentrations measured with 4.0 T and 9.4 T were similar while the CRLB reduced as the static magnetic field increased. On the other hand, Lac measurements on different platforms were very consistent. With a similar measurement of PCC on 3 T of the same subjects, it was interesting to observe a significantly lower Lac level compared to ACC (Fig. 5, N=5, paired t-test) and the result is pending with further verification. NAA+NAAG was used as reference for all results above.Conclusion
HOPE, a simple but effective MM suppression MRS sequence with ultra-short TE was proposed and tested on a variety of magnetic field strengths. With the same SNR level as STEAM, HOPE achieves additional benefit with substantial suppression of MM signals and more accurate quantifications of MM overlapped metabolites. With better spectral resolution and higher SNR on human 7 T, it would be very promising to resolve all proton measurable metabolites without MM contaminations in one single scan.Acknowledgements
No acknowledgement found.References
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