Magnetization Transfer Based Tissue Contrast

In Magnetization Transfer (MT) experiments, the spectral intensities from a series of spectra are recorded as a function of the resonance offset of an RF irradiation field. The (low level) irradiation saturates the spins that resonate at that particular frequency. Through dipolar couplings and spin exchange, the saturation is spread over a larger frequency range, leading to a distinct shape for the MT curve. The MT thus reflects the underlying frequency spread of the resonance line, which is not observed in a direct detection of the resonance.

The T₂ of macromolecular protons in tissue lie in the range of 10’s to 100’s of microseconds, which prevents their direct detection in vivo. However, the MT experiment is sensitive over this order of magnitude range and is available as a tool for characterizing tissue provided that the pathophysiology directly alters the motional dynamics of magnetically coupled  macromolecular protons. We have investigated quantitative interpretation of MT data and generated maps of the MT parameters of ex vivo plaque at 2.0 T  that correlates with histology. The T₂^s maps indicate that the fibrous plaque regions have the lowest T₂^s (≤10 µs) compared to necrotic core, and that necrotic core has the lowest rate of magnetization transfer.

Three of fifteen MT images acquired with MT saturation pulse resonant offset frequencies of 48, 23 and 5 kHz are displayed in the figure below (panels A-C), respectively. Note the difference in image contrast depending on the setting of the saturation pulse offset frequency. When the pulse is near the resonant frequency (panel C) much of the plaque tissue is dark due to direct saturation of the water proton resonance. Above 50 kHz the MT contrasted images are essentially proton density weighted images. Panel D shows the cross-relaxation spectra of individual pixels in three locations as indicated in Panel A.  MT parameter maps of T₂^s and RT^ls in panels  E and F, respectively. These two parameter maps contain specific information:  T₂^s reflects the overall “rigidity” of the tissue, and RT^ls has value ranges specific to chemical groups. These parameter maps have contrast distinct from T₁, T₂ and PD and contain structural correlations in each map for this advanced plaque.

(A-C) MT images of ex vivo carotid plaque obtained at solid saturation pulse frequency offsets of 48, 23, and 5 kHz, respectively. (D) Single pixel cross-relaxation spectra in plaque regions defined in A. (E-F) Resulting MT parameter maps fitting per pixel to binary spin bath model and Gaussian lineshape function.