A Simple Multi-Parametric qMRI Phantom

Abstract

Quantitative MRI phantoms allow researchers to test the accuracy and precision of their methods on simplified chemical systems. While several well-defined phantoms exist for reproducible T1, T2, diffusion, and magnetization transfer behavior, options for more complex systems are limited. A system phantom capable of simultaneously producing tissue-relevant T1, multi-exponential T2, magnetization transfer (MT), and diffusion characteristics is of great interest, particularly for model-based qMRI methods. Here, a formulation of cross-linked polyvinylpyrrolidone (PVP) and bovine serum albumin (BSA) is evaluated as an easily synthesized, inexpensive qMRI phantom. Phantoms of varying PVP and BSA concentrations were scanned using 1) inversion-prepared CPMG on a 400 MHz NMR spectrometer for multi-exponential T2 and T1, 2) selective inversion recovery at 400 MHz for MT characterization, and 3) pulsed gradient spin echo on a 7T imaging system for apparent water diffusion. Phenomenological models relating each qMRI parameter to PVP and BSA concentration were generated via multiple linear regression fitting. PVP introduces a chemically-shifted signal component with T2 < 25 ms, while BSA provides a macromolecular proton pool that exchanges magnetization transfer with water. The PVP:BSA phantoms characterized here can enable researchers to control 1) free water T2 from 45 - 90 ms, 2) a short-lived signal pool making up 3 - 12% of the total signal with T2 = 8 - 25 ms, 3) water T1 from 1.2 - 1.9 s, 4) apparent diffusion coefficients from 1.0 - 1.8 μm2/ms, and 5) macromolecular pool signal proportion from 6-12%. Future work should incorporate field strength effects into qMRI parameter model equations to extend the phantoms’ applicability to clinical field strengths.