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