Ultrashort echo time magnetization transfer (UTE-MT) imaging and modeling: magic angle independent biomarkers of tissue properties

MRI biomarkers such as T-2, T-2* and T-1rho have been widely used, but are confounded by the magic angle effect. The purpose of this study is to investigate the use of the two-dimensional ultrashort echo time magnetization transfer (UTE-MT) sequence for potential magic angle independent MR biomarkers. Magnetization transfer was investigated in cadaveric Achilles tendon samples using the UTE-MT sequence at five MT powers and five frequency offsets ranging from 2 to 50kHz. The protocol was applied at five sample orientations ranging from 0 to 90 degrees relative to the B-0 field. The results were analyzed with a two-pool quantitative MT model. Multiple T-E data were also acquired and mono-exponential T-2* was calculated for each orientation. Macromolecular proton fractions and exchange rates derived from UTE-MT modeling did not appreciably change between the various orientations, whereas the T-2* relaxation time demonstrated up to a sixfold increase from 0 degrees to 55 degrees. The UTE-MT technique with two-pool modeling shows promise as a clinically compatible technique that is resistant to the magic angle effect. This method provides information on the macromolecular proton pool that cannot be directly obtained by other methods, including regular UTE techniques.