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Power Optimization of In-vivo Chemical Exchange Saturation Transfer (CEST) contrast for pH weighted Imaging

complicated due to the presence of conventional MT effects. We model the CEST process using an empirical formula considering both the saturation transfer and spillover effects at low to intermediate power. The model is shown to be able to correctly describe the effects of irradiation on endogenous CEST process and allowed us to achieve a significant improvement of the contrast when using pH-weighted imaging in an acute ischemia animal model. MATERIALS AND METHODS Adult male Wister rats (n = 3) received MRI one hour after permanent middle cerebral artery occlusion (MCAO) at 4.7 T. Multi-parametric imaging including cerebral blood flow (CBF), APT, apparent diffusion constant (ADC), and T2 maps were acquired to delineate the ischemic area. Single slice images were acquired using one shot EPI (slice thickness 2 mm, FOV 32x32 mm2, matrix 64x64). In the CEST experiments, exchangeable amide protons at the offset of +3.5ppm from the water resonance were selectively saturated (CW pulse, 4s) and the irradiation power varied from 0.25, 0.5, 0.75, 1, 1.5, 2, to 3 µ T. A reference MT image at the offset -3.5ppm and a control image (S0) without the RF saturation pulses were also acquired. The pH weighted image were processed using the equation: MTRasym(3.5ppm) = 100%×(Ssat(-3.5ppm) - Ssat(3.5ppm))/S0. RESULTS The APT contrast between the contralateral brain and CBF deficit area was analyzed as a function of irradiation power (Fig. 1). This contrast increased initially with the RF power to 2.5%, then decreased with further power increase, which we attribute to concomitant direct water saturation (spillover) and conventional MT effects. Ensemble exchange rate and pool ratio were fitted to be 13.6 s-1 and 746, respectively, in agreement with reported values2. It shows that the maximal APT contrast (2.5%) during acute ischemia can be acquired with an irradiation power of 0.75 µ T at 4.7T. It is a significant improvement over previously reported value (1.9%) acquired at suboptimal conditions (1.5 µ T). Fig. 2 shows the CBF and pH-weighted maps using the optimal irradiation pulse on a representative animal. The CBF map showed significant flow reduction across the ischemic right cerebral hemisphere. The pH-weighted image reflects pH reduction across the CBF deficit area (hypoperfused area with CBF two standard deviations or more reduced from the mean of the contralateral area). Area with most severe CBF reduction (red arrows) appeared as hypointense on the pH-weighted image. pH-weighted images acquired at suboptimal conditions showed much reduced contrast both within CBF deficits areas and between deficits and contralateral area. DISCUSSION