T1ρ, T2ρ: Applications
Weitian Chen1
1The Chinese University of Hong Kong, China

Synopsis

Keywords: Contrast mechanisms: Rotating frame relaxometry

MRI can be used for non-invasive imaging and characterization of metabolites and macromolecules in human tissues based on chemical exchange (CE) and magnetization transfer (MT). It is popular to use off-resonance saturation radiofrequency (RF) pulses to study CE and MT based contrasts. Alternatively, relaxation times in the presence of a spin-lock field can also be used to measure CE and MT signal. This leads to many promising applications of spin-lock MRI, including, but not limited to, diagnosis of brain cancer, Alzheimer’s’ disease, demyelination, MSK diseases, cardiac diseases, and fibrosis.

MRI can be used for non-invasive imaging and characterization of metabolites and macromolecules in human tissues based on chemical exchange (CE) and magnetization transfer (MT). It is popular to use off-resonance saturation radiofrequency (RF) pulses to study CE and MT based contrasts. Principally, off-resonance saturation RF pulse is applied to saturate the protons bound to macromolecules or labile protons associated with metabolites, which exchange with free water protons due to MT and CE effects. This results in water signal attenuation, which allows indirect measurement of metabolites and macromolecules.

Alternatively, spin-lock-based techniques can also be used to measure CE and MT signal. Spin-lock is often achieved by applying a radiofrequency (RF) pulse to create an effective spin-lock field along the magnetization, which results in MR signal relaxation with a time constant T1rho. Spin-lock MRI can also be performed so that the magnetization is perpendicular to the spin-lock field, which results in MR signal relaxation with a time constant T2rho.

Multiple methods have been developed to measure chemical exchange effect based on spin-lock.

  • T1rho dispersion. For on-resonance spin-lock, T1rho value depends on the applied amplitude of the spin-lock RF pulse, known as T1rho dispersion effect. This effect has been used to probe chemical exchange effect.
  • Composite R1rho-R2. Similar to T1rho dispersion, the composite R1rho and R2 acquisition can be used remove dipole-dipole terms and measure the chemical exchange effect.
  • Chemical exchange spin-lock (CESL). In CESL, the spin-lock is applied at a range of resonance frequency offsets. This allows analysis like MTR asymmetry used in CEST to measure chemical exchange effect.
  • One advantage of spin-lock over saturation RF pulse is that spin-lock can be used to quantify relaxation rate. R1rho (=1/T1rho) measured at multiple resonance frequency offsets has been shown to reduce contamination from water pool when measuring chemical exchange effect.
  • Dynamic glucose enhanced imaging. Glucose has labile protons which can be detected based on chemical exchange effect. Dynamic T1rho mapping can be used for dynamical glucose enhanced imaging after glucose uptake.
Recently, methods have also been reported to measure the magnetization transfer effect based on a spin-lock approach. During the off-resonance spin-lock, the magnetization transfer affects the R1rho relaxation rate. This effect can be controlled by the sequence parameters such as spin-lock RF amplitude and the resonance frequency offset. By measuring multiple R1rho values with the same contribution from the water pool but different contributions from the MT pool, the water pool signal can be removed. The resulting signal is only associated with MT parameters and can be used to quantify MT parameters.

The measurement of metabolites and macromolecules based on spin-lock techniques have various promising applications. In this lecture, we will review applications of spin-lock MRI in brain cancer, demyelination, Alzheimer’s diseases, multiple sclerosis, cartilage characterization, cardiac imaging, and fibrosis imaging.

Acknowledgements

The author would like to acknowledge the ISMRM organization committee and the funding support from the Chinese University of Hong Kong, the Research Grants Council of the Hong Kong SAR, and the Innovation and Technology Commission of the Hong Kong SAR.

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Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)