Simultaneous QSM and metabolic imaging of the brain using SPICE

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Quantitative susceptibility mapping (QSM) has been widely used in recent years for in vivo mapping of tissue magnetic susceptibility 1, a biomarker useful for detecting intracranial hemorrhages 2, quantifying blood vessel oxygenation 4, and examining iron accumulation in the deep gray matter (GM) 7. MR spectroscopic imaging (MRSI) has also been recognized as a unique tool to obtain molecule‐specific information, allowing in vivo characterization of biochemical properties of tissues 8. QSM and MRSI, therefore, provide complementary information useful for many research and clinical applications, such as study of brain metabolism 9, diagnosis and characterization of neurological disorders 15, and assessment of therapeutic efficacy 19.
Currently, QSM and MRSI are carried out in separate scans, both of which often require long data acquisition times, limiting their practical use in clinical and research applications. Conventional QSM methods acquire a series of JOURNAL/mrim/04.02/01445475-201801000-00002/math_2MM1/v/2017-12-21T175206Z/r/image-png ‐weighted images using a high‐resolution multi‐echo gradient echo (GRE) sequence, which typically takes 5–10 min to cover the whole brain. MRSI scans usually take even longer (e.g., a conventional chemical shift imaging scan can take up to 30 min to cover a single 2D slice with an in‐plane resolution of approximately 4 × 4 mm2). Although a number of methods have been proposed to accelerate QSM and MRSI scans 21, no simultaneous QSM and MRSI acquisitions have, to the best of our knowledge, ever been attempted yet.
This work reports the feasibility of simultaneous QSM and high‐resolution metabolite mapping from a single 1H‐MRSI scan using a new MRSI technique known as spectroscopic imaging by exploiting spatiospectral correlation (SPICE) 23. This capability is made possible by exploiting the fact that tissue susceptibility information is naturally encoded in the water spectroscopic signals from an MRSI scan if no water suppression is applied (Fig. 1). Conventional MRSI acquisitions usually apply several RF pulses to suppress the water and lipid signals, almost completely eliminating the susceptibility information. This problem is nicely overcome with SPICE, which uses ultrashort echo time (TE)/short pulse repetition time (TR) acquisitions without water suppression 23. Although SPICE has been used mainly for metabolic imaging, we extend its data acquisition and processing scheme to make it more suitable for simultaneous QSM and metabolic imaging. More specifically, in data acquisition, we use dual‐density sparse sampling and ramp sampling to extend k‐space coverage to the level often required by QSM, while maintaining a reasonable SNR for the spatiospectral encodings used for metabolic imaging. In data processing, we take advantage of the larger number of echoes generated by the SPICE sequence (and any readily available anatomical priors) to produce high‐quality QSM; we use a union‐of‐subspaces model to reconstruct the metabolite spatiospectral distributions. These features together allow us to obtain tissue susceptibility maps of the brain at a voxel size of 1.8 × 1.8 × 2.4 mm3 (or slightly smaller if the effect of the anatomical constraints is also taken into account) and metabolite maps at a nominal voxel size of 2.4 × 2.4 × 2.4 mm3 from a single 7‐min scan. A more detailed description of our acquisition and processing schemes is given subsequently, which is followed by some representative experimental results to demonstrate the performance of the proposed method.

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