Effect of particle size on liver MRI R2* relaxometry: Monte Carlo simulation and phantom studies.

Purpose: To investigate the effect of particle size on liver R2*$$ {\mathrm{R}}_2^{\ast } $$ 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 R2*$$ {\mathrm{R}}_2^{\ast } $$, 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. R2*$$ {\mathrm{R}}_2^{\ast } $$ measurements in the phantom were made at both 1.5 T and 3.0 T. Finally, differences in R2*$$ {\mathrm{R}}_2^{\ast } $$ predictions or measurements were evaluated across varying particle sizes. Results: In the simulation study, strong linear and positively correlated relationships were observed between R2*$$ {\mathrm{R}}_2^{\ast } $$ predictions and particle concentrations across varying particle sizes and magnetic field strengths (r≥0.988$$ r\ge 0.988 $$). The relationships were affected by iron sphere size (p<0.001$$ p<0.001 $$), where smaller iron sphere size yielded higher predicted R2*$$ {\mathrm{R}}_2^{\ast } $$, whereas fat droplet size had no effect on R2*$$ {\mathrm{R}}_2^{\ast } $$ predictions (p≥0.617$$ p\ge 0.617 $$) for constant total fat concentration. Similarly, the phantom study showed that R2*$$ {\mathrm{R}}_2^{\ast } $$ measurements were relatively sensitive to iron sphere size (p≤0.004$$ p\le 0.004 $$) unlike fat droplet size (p≥0.223$$ p\ge 0.223 $$). Conclusion: Liver R2*$$ {\mathrm{R}}_2^{\ast } $$ 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 R2*$$ {\mathrm{R}}_2^{\ast } $$ relaxometry in vivo, and enable improved quantitative MRI phantom design. [ABSTRACT FROM AUTHOR]