Synopsis

Keywords: Contrast mechanisms: Rotating frame relaxometry, Cardiovascular: Myocardium

T_1ρ mapping is rapidly emerging as a promising tool for non-invasive tissue characterization in cardiac MRI and beyond. T_1ρ relaxation is sensitive to molecular interactions in the low frequencies range, from tenths to thousands of Hz, making it a great candidate to probe the interactions betweeen water and macromolecules without the use of external contrast agents. In cardiac MR, T_1ρ mapping has shown diagnostic value for both ischemic and non-ischemic cardiomyopathies. In this educational we will explore the origin of T_1ρ relaxation, discuss how to measure myocardial T_1ρ times and give a brief overview of current applications and challenges.

Background

Recently, rotating-frame relaxation methods, like T_1ρ, have gained prominence for their potential to provide insight into the macro-molecular environment of biological tissues without having to inject external contrast agents [1]. Rotating-frame relaxation refers to the relaxation processes measured during continuous RF irradiation, as opposed to conventional relaxation probed during free decay. Thus, rotating-frame relaxation reflects different physical interactions compared with conventional relaxation times. Specifically, it has shown higher sensitivity than native T₁ or T₂ to interactions between water and macro-molecules.Compared with contrast-based techniques, rotating-frame relaxation mapping entails less risk, better cost-effectiveness, and the potential for repeated assessments over time. These advantages have led to agrowing number of technical and clinical studies [1,2].Specific signal preparations, during which the RF field is kept at a moderately high value for long durations (usually 10-50 times longer than for RF excitation pulses) are used to induce rotating-frame relaxation. These RF pulses are called spin-locking pulses because they are not used to excite the magnetization but to lock it along their axis. Spin-locking effectively suppresses certain mechanisms that would conventionally induce relaxation under free-decay conditions. Thus, during spin-lock preparations, longitudinal and transverse relaxation are relative to the spin-locking RF pulse, instead of the main magnetic field, and are called T_1ρ and T_2ρ, instead of T₁ and T_2.

Acknowledgements

No acknowledgement found.