MRI is becoming central to clinical decision-making in diseases involving the bone marrow, including spondyloarthritis and multiple myeloma. Both diseases cause alterations in marrow composition and structure, but there is currently no reliable method for quantifying these changes. Here, we describe a series of fat-water-bone phantoms enabling validation of proton density fat-fraction (PDFF) and R2* as measures of marrow composition and trabecular bone mineral density (BMD) respectively. Our data suggest that PDFF measurements can be achieved with excellent accuracy. Furthermore, there was a strong relationship between R2* and BMD, suggesting that R2* could be used to monitor BMD in vivo.