Presentations

Extraction of quantitative parameters of brain tissue microstructure is possible with DTI, inhomogeneous MT (ihMT), and MWF MRI. We examine the ability of the parameters from these techniques to distinguish ex-vivo specimens of the cerebellum from donors with MS relative to non-MS tissue. Significant differences were found for DTI and MT/ihMT metrics in WM, but only for MT/ihMT in GM. Differences between quantitative MT parameters from fits to ihMT data were not significant.

Application of neuromelanin MRI (NM-MRI) in the brainstem has been utilized in studies of neurological disorders such as Parkinson's disease. Acquisition of two types of MT preparation for inhomogeneous MT, MT applied at a single offset and then dual off-resonance frequencies, presents an advancement to NM-MRI by providing an acquisition with a myelin sensitive signal for complementary information. We demonstrate this in ex-vivo brainstem samples at 9.4 T, which allows comparison with other MRI microstructural techniques, as well as in-vivo at 3 T, where MT images provide a contrast comparable to NM-MRI by fast-spin-echo.

Inhomogeneous MT (ihMT) MRI was applied in a genetic mouse model of glioblastoma to assess its potential to provide useful complementary information regarding the true extent of tumor infiltration. Data were acquired to produce maps of MT and ihMT ratios, which were analyzed based on regions of interest relative to brain tissue contralateral to the tumor and progression with time. The tumor was characterized by significantly lower MT and ihMT. Reduction in peripheral white matter ihMT was taken to indicate demyelinating processes associated with glioblastoma. Thus, ihMT might be used to inform on disease progression in tumor adjacent brain tissue.

Inhomogeneous magnetization transfer (ihMT) is predominantly associated with myelin imaging and shows promise in detection and monitoring myelin related disorders, including multiple sclerosis (MS). Extraction of quantitative parameters from ihMT data (qihMT) would ideally provide information independent of the ihMT sequence. We used Cramer Rao lower bound analysis to develop an optimized acquisition protocol for qihMT and simulations to test the fidelity of the protocol. Preliminary application to central nervous system samples showed differences in free pool longitudinal relaxation rate R_1a, bound pool transverse relaxation time T_2b, and dipolar relaxation time T_1d between specimens from donors with and without MS.

Quantitative magnetization transfer (MT) was carried out using inhomogeneous MT (ihMT) data, following optimization of an acquisition protocol. The optimized protocol was applied in ex-vivo brain tissue from a donor with multiple sclerosis (MS), as well as tissue containing an MS plaque. Parameter maps output from quantitative MT showed white and grey matter contrast, as well as a lower restricted or bound pool fraction in the area of the MS plaque from histology. Results from quantitative MT using ihMT were compared with those using selective inversion recovery, as well as quantitative outputs from diffusion tensor MRI.

Inhomogeneous magnetization transfer (ihMT) effects have been readily observed in myelinated structures. The advent of low duty-cycle ihMT to increase the signal allows application of ihMT in other tissues. In this work, we explore the feasibility of applying ihMT in non-myelinated tissues such as the heart, liver, and kidneys of mice. This is achieved using a radial, ultra-short echo-time acquisition for greater motion robustness. The results demonstrate a measurable ihMT signal outside the central nervous system. Thus the microstructure of such tissues might be assessed based on the dipolar order contribution to ihMT.

Standard MT and ihMT ratio (ihMTR) measures can be sensitive to B₁ and T₁, making them less specific to tissue microstructure. Using the inverse signal, i.e. one divided by the signal, and a high flip-angle reference image in calculation of an ihMTR metric has been proposed as a metric with improved insensitivity to T₁ and B₁ in steady-state gradient-echo sequences. We present a modified method for use in prepared sequences such as magnetization prepared rapid gradient echo (MPRAGE). The sensitivity of ihMT MPRAGE metrics to T₁ and B₁ was tested using simulations and acquisitions in brains of healthy volunteers.

The inhomogeneous magnetization transfer (ihMT) technique has shown myelin sensitivity, and is understood to be dependent on power and the dipolar relaxation time parameter, T_1D, which is longer in myelinated tissues. Implementation of ihMT can be adapted to provide a smaller, but non-negligible signal from other, relatively short T_1D tissues. Simulations showed a measurable ihMT signal, achieved from fixed low duty cycle MT preparations with high B₁ pulses, decayed with pulse width at a rate dependent on T_1D. Thus short, high B₁ pulses were implemented to acquire ihMT data from ex-vivo samples of rat heart, kidney, and tail tendon, demonstrating the feasibility of short T_1D imaging.

Perfusion imaging is a promising application for hyperpolarized tracers, as they provide high signal with no endogenous background. Hyperpolarized 13C labeled tert-butanol is a freely diffusible perfusion agent with long T1 and T2 relaxation times in vivo. Prior work has shown that tert-butanol can be polarized to 5-10% using dynamic nuclear polarization through addition of glycerol as a glassing agent. Here we investigate a formulation based on a water/sucrose/tert-butanol mixture that yields a 1.6-fold improvement in polarization, and illustrate its use in 3D cerebral perfusion imaging in rats.