T1 Mapping From MPRAGE Acquisitions: Application to the Measurement of the Concentration of Nanoparticles in Tumors for Theranostic Use

Background The measurement of the concentration of theranostic agents in vivo is essential for the assessment of their therapeutic efficacy and their safety regarding healthy tissue. To this end, there is a need for quantitative T-1 measurements that can be obtained as part of a standard clinical imaging protocol applied to tumor patients. Purpose To generate T-1 maps from MR images obtained with the magnetization-prepared rapid gradient echo (MPRAGE) sequence. To evaluate the feasibility of the proposed approach on phantoms, animal and patients with brain metastases. Study Type Pilot. Phantom/Animal model/Population Solutions containing contrast agents (chelated Gd3+ and iron nanoparticles), male rat of Wistar strain, three patients with brain metastases. Field Strength/Sequence A 3-T and 7-T, saturation recovery (SR), and MPRAGE sequences. Assessment The MPRAGE T-1 measurement was compared to the reference SR method on phantoms and rat brain at 7-T. The robustness of the in vivo method was evaluated by studying the impact of misestimates of tissue proton density. Concentrations of Gd-based theranostic agents were measured at 3-T in gray matter and metastases in patients recruited in NanoRad clinical trial. Statistical Tests A linear model was used to characterize the relation between T-1 measurements from the MPRAGE and the SR acquisitions obtained in vitro at 7-T. Results The slope of the linear model was 0.966 (R-2 = 0.9934). MPRAGE-based T-1 values measured in the rat brain were 1723 msec in the thalamus. MPRAGE-based T-1 values measured in patients in white matter and gray matter amounted to 747 msec and 1690 msec. Mean concentration values of Gd3+ in metastases were 61.47 mu mol. Data Conclusion The T-1 values obtained in vitro and in vivo support the validity of the proposed approach. The concentrations of Gd-based theranostic agents may be assessed in patients with metastases within a standard clinical imaging protocol using the MPRAGE sequence. Evidence Level 2. Technical Efficacy Stage 1.

Automated summary

This study aims to generate T1 maps from MR images using the MPRAGE sequence and evaluate its feasibility in phantoms, animal models, and patients with brain metastases. The results demonstrate that the MPRAGE sequence can be used as a method to measure the concentration of Gd-based theranostic agents within a standard clinical imaging protocol.