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

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.

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.

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 ( $p\ge 0.223$ ).

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.