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

Abstract

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 T₁ measurements that can be obtained as part of a standard clinical imaging protocol applied to tumor patients.

Purpose: To generate T₁ 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 T₁ 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 T₁ measurements from the MPRAGE and the SR acquisitions obtained in vitro at 7-T.

Results: The slope of the linear model was 0.966 (R² = 0.9934). MPRAGE-based T₁ values measured in the rat brain were 1723 msec in the thalamus. MPRAGE-based T₁ 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 μmol.

Data conclusion: The T₁ 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.

Keywords: Gd-based nanoparticle; MPRAGE; T1 map; theranostic agent; tumor.