Adjustment of rotation and saturation effects (AROSE) for CEST imaging

Endogenous CEST signal usually has low specificity due to contaminations from the magnetization transfer contrast (MTC) and other labile protons with overlapping or close Larmor frequencies. We propose to improve CEST signal specificity with adjustment of rotation and saturation effects (AROSE).

The AROSE approach measures the difference between CEST signals acquired with the same average irradiation power but largely different duty cycles, for example, a continuous wave or a high duty cycle pulse train versus a low duty cycle pulse train with a flip angle φ. Simulation, phantom, and in vivo rodent studies were performed to evaluate the characteristics of the AROSE_φ signal.

Simulation and experimental results show that AROSE_2π is a low-pass filter that can suppress fast exchanging processes (e.g., >3000 s^-1 ), whereas AROSE_π is a band-pass filter suppressing both fast and slow exchange (e.g., <30 s^-1 ) rates. For other φ angles, the sensitivity and the exchange-rate filtering effect of AROSE_φ falls between AROSE_π and AROSE_2π . AROSE can also minimize MTC and improve the Larmor frequency selectivity of the CEST signal. The linewidth of the AROSE_1.5π spectrum is about 60% to 65% when compared to the CEST spectrum measured by continuous wave. Depending on the needs of an application, the sensitivity, exchange-rate filtering, and Larmor frequency selectivity can be adjusted by varying the flip angle, duty cycle, and average irradiation power.

Compared to conventional CEST signals, AROSE can minimize MTC and improve exchange rate filtering and Larmor frequency specificity.