Purpose: AGuIX is a recently developed, gadolinium-based, theranostic agent that, in conjunction with radiotherapy, shows promise in the treatment of tumors. We developed a new protocol for quantifying AGuIX nanoparticle uptake based on the magnetization prepared rapid (two) gradient echo (MP2RAGE) sequence and compare this protocol against the standard quantification technique, a variable flip angle (VFA) MR method.
Methods: Determination of nanoparticle concentration from MR images requires T1 mapping and calculation of AGuIX relaxivity. A phantom containing 12 vials of unique gels each with a known T1 value (Eurospin II TO5) was used to obtain and compare VFA and MP2RAGE generated T1 maps. A 3T scanner (MAGNETOM Vida, Siemens Healthcare) was used to image the Eurospin phantom using both VFA and MP2RAGE sequences. T1 maps were calculated from sequence outputs and regions containing materials of differing T1 values were averaged and compared against ground truth T1 values. T1 mapping of a second phantom containing vials of known concentrations of AGuIX measured the relaxivity constant. Example patient data (pre- and post- AGuIX administration) was used as proof of concept.
Results: The MP2RAGE generated T1 maps of the contrast phantom yielded significantly less error than VFA (380ms average error reduction). The relaxivity constant for AGuIX was calculated to be 5.96 mM-1s-1 and was used to calculate concentration for pre- and post- AGuIX T1 maps of a patient with brain metastases. The resulting average concentration in the tumor was 0.084 mg/L, similar to previously published results using a different MRI scanner and protocol (~0.1 mg/ml).
Conclusion: We have developed a new, accurate method to quantify AGuIX concentration using the MP2RAGE sequence. These methods provide the foundation from which to fully characterize the patterns of uptake and distribution of AGuIX in ongoing and future clinical trials.