Effect of particle size on liver MRI R 2 * relaxometry: Monte Carlo simulation and phantom studies

Abstract

Purpose: To investigate the effect of particle size on liver $R_2^{*}$ by Monte Carlo simulation and phantom studies at both 1.5 T and 3.0 T.

Methods: Two kinds of particles (i.e., iron sphere and fat droplet) with varying sizes were considered separately in simulation and phantom studies. MRI signals were synthesized and analyzed for predicting $R_2^{*}$ , based on simulations by incorporating virtual liver model, particle distribution, magnetic field generation, and proton movement into phase accrual. In the phantom study, iron-water and fat-water phantoms were constructed, and each phantom contained 15 separate vials with combinations of five particle concentrations and three particle sizes. $R_2^{*}$ measurements in the phantom were made at both 1.5 T and 3.0 T. Finally, differences in $R_2^{*}$ predictions or measurements were evaluated across varying particle sizes.

Results: In the simulation study, strong linear and positively correlated relationships were observed between $R_2^{*}$ predictions and particle concentrations across varying particle sizes and magnetic field strengths ( $r\ge 0.988$ ). The relationships were affected by iron sphere size ( ), where smaller iron sphere size yielded higher predicted $R_2^{*}$ , whereas fat droplet size had no effect on $R_2^{*}$ predictions ( $p\ge 0.617$ ) for constant total fat concentration. Similarly, the phantom study showed that $R_2^{*}$ measurements were relatively sensitive to iron sphere size ( $p\le 0.004$ ) unlike fat droplet size ( $p\ge 0.223$ ).

Conclusion: Liver $R_2^{*}$ is affected by iron sphere size, but is relatively unaffected by fat droplet size. These findings may lead to an improved understanding of the underlying mechanisms of $R_2^{*}$ relaxometry in vivo, and enable improved quantitative MRI phantom design.

Keywords: Monte Carlo simulations; fat droplet; iron; liver R2*; particle size; phantom.