Simultaneous measurement of T₁ /B₁ and pharmacokinetic model parameters using active contrast encoding (ACE)-MRI

The aim of this study was to assess the feasibility of combining dynamic contrast enhanced-magnetic resonance imaging (DCE-MRI) with the measurement of the radiofrequency (RF) transmit field B₁ and pre-contrast longitudinal relaxation time T₁₀ . A novel approach has been proposed to simultaneously estimate B₁ and T₁₀ from a modified DCE-MRI scan that actively encodes the washout phase of the curve with different amounts of T₁ and B₁ weighting using multiple flip angles and repetition times, hence referred to as active contrast encoding (ACE)-MRI. ACE-MRI aims to simultaneously measure B₁ and T₁₀ , together with contrast kinetic parameters, such as the transfer constant K^trans , interstitial space volume fraction v_e and vascular space volume fraction v_p . The proposed method was tested using numerical simulations and in vivo studies with mouse models of breast cancer implanted in the flank and mammary fat pad, and glioma in the brain. In the numerical simulation study with a signal-to-noise ratio of 10, both B₁ and T₁₀ were estimated accurately with errors of 5.1 ± 3.5% and 12.3 ± 8.8% and coefficients of variation (CV) of 14.9 ± 8.6% and 15.0 ± 5.0%, respectively. Using the same ACE-MRI data, the kinetic parameters K^trans , v_e and v_p were also estimated with errors of 14.2 ± 8.3% (CV = 13.5 ± 4.6%), 14.7 ± 9.9% (CV = 13.3 ± 4.5%) and 14.0 ± 9.3% (CV = 14.0 ± 4.5%), respectively. For the in vivo tumor data from 11 mice, voxel-wise comparisons between ACE-MRI and DCE-MRI methods showed that the mean differences for the five parameters were as follows: ΔK^trans  = 0.006 (/min), Δv_e  = 0.016, Δv_p  = 0.000, ΔB₁  = -0.014 and ΔT₁  = -0.085 (s), which suggests a good agreement between the two methods. When compared with separately measured B₁ and T₁₀ , and DCE-MRI estimated kinetic parameters as a reference, the mean relative errors of ACE-MRI estimation were B₁  = -0.3%, T₁₀  = -8.5%, K^trans  = 11.4%, v_e  = 14.5% and v_p  = 4.5%. This proof-of-concept study demonstrates that the proposed ACE-MRI method can be used to estimate B₁ and T₁₀ , together with contrast kinetic model parameters.