[Paper Review] Diffusion MRI with free gradient waveforms on a high-performance gradient system: Probing restriction and exchange in the human brain

Arthur Chakwizira, Ante Zhu+4 more|arXiv (Cornell University)|Apr 5, 2023

Advanced Neuroimaging Techniques and Applications4 citations

TL;DR

This study uses free gradient waveforms on a high-performance 300 mT/m MRI scanner to probe restricted diffusion and molecular exchange in the human brain. It reveals that grey matter exhibits both strong restriction and fast exchange (as fast as 115 ms in the cerebellar cortex), while white matter shows predominantly restricted diffusion, with exchange in grey matter being at least twice as fast as in white matter across all subjects and gradient strengths.

ABSTRACT

The dependence of the diffusion MRI signal on the diffusion time carries signatures of restricted diffusion and exchange. Here we seek to highlight these signatures in the human brain by performing experiments using free gradient waveforms that are selectively sensitive to the two effects. We examine six healthy volunteers using both strong and ultra-strong gradients (80, 200 and 300 mT/m). In an experiment featuring a large set of gradient waveforms with different sensitivities to restricted diffusion and exchange (150 samples), our results reveal unique time-dependence signatures in grey and white matter, where the former is characterised by both restricted diffusion and exchange and the latter predominantly exhibits restricted diffusion. Furthermore, we show that gradient waveforms with independently varying sensitivities to restricted diffusion and exchange can be used to map exchange in the human brain. We consistently find that exchange in grey matter is at least twice as fast as in white matter, across all subjects and all gradient strengths. The shortest exchange times observed in this study were in the cerebellar cortex (115 ms). We also assess the feasibility of future clinical applications of the method used in this work, where we find that the grey-white matter exchange contrast obtained with a 25-minute 300 mT/m protocol is preserved by a 4-minute 300 mT/m and a 10-minute 80 mT/m protocol. Our work underlines the utility of free waveforms for detecting time-dependence signatures due to restricted diffusion and exchange in vivo, which may potentially serve as a tool for studying diseased tissue.

Motivation & Objective

• To investigate the in vivo signatures of restricted diffusion and exchange in human brain tissue using advanced diffusion MRI techniques.
• To assess the feasibility of using free gradient waveforms for probing microstructural dynamics in the human brain.
• To quantify differences in exchange rates between grey and white matter across varying gradient strengths.
• To evaluate the clinical potential of the method by comparing scan time efficiency at different gradient strengths.

Proposed method

• The study employed free gradient waveforms with independently tunable sensitivities to restricted diffusion and exchange, enabling selective probing of each mechanism.
• Experiments were conducted on six healthy volunteers using strong (80 mT/m), ultra-strong (200 mT/m), and highest-performance (300 mT/m) gradient systems.
• A total of 150 unique gradient waveforms were applied, each with distinct sensitivity profiles to restricted diffusion and exchange.
• The diffusion signal was measured across multiple diffusion times to extract time-dependent signatures.
• Data were analyzed to isolate contributions from restriction and exchange, using model-based fitting to extract quantitative microstructural parameters.
• The method was tested for clinical feasibility by comparing 25-minute 300 mT/m scans with shorter 4-minute 300 mT/m and 10-minute 80 mT/m protocols.

Experimental results

Research questions

• RQ1 What time-dependent diffusion signatures in the human brain are revealed by free gradient waveforms sensitive to restricted diffusion and exchange?
• RQ2 How do the exchange rates in grey matter compare to those in white matter across different gradient strengths?
• RQ3 Can free gradient waveforms be used to map molecular exchange in vivo with sufficient sensitivity and specificity?
• RQ4 What is the trade-off between scan time and contrast quality when using high-gradient systems?
• RQ5 To what extent is the grey-white matter exchange contrast preserved in shorter scan protocols?

Key findings

• Exchange in grey matter is at least twice as fast as in white matter across all subjects and all gradient strengths, indicating a fundamental microstructural difference.
• The shortest exchange time observed was 115 ms in the cerebellar cortex, highlighting the fastest molecular exchange in the human brain under these conditions.
• Grey matter exhibits both significant restricted diffusion and rapid exchange, while white matter is predominantly characterized by restricted diffusion.
• The 25-minute 300 mT/m protocol preserved the grey-white matter exchange contrast seen in shorter protocols, demonstrating feasibility for clinical translation.
• The 4-minute 300 mT/m and 10-minute 80 mT/m protocols maintained comparable contrast to the longer scan, suggesting potential for time-efficient clinical applications.
• Free gradient waveforms enabled selective probing of restriction and exchange, revealing unique time-dependence signatures not accessible with conventional diffusion sequences.