SP-IDEAL water–fat separation in femur of dog. SP-IDEAL was performed on 6 SPGR echoes acquired with flip angle of 5°, TR of 40 ms, TE1 of 5.95 ms, and ?TE of 0.38 ms. D) Composite image (sum of water and fat images). Image source: J Nucl Med 2011;52(9):1482-1489.

Measuring bone marrow cellularity is typically done using a biopsy of the iliac crest, but a recent study published in the Journal of Nuclear Medicine indicates that MRI images may also adequately, and noninvasively, measure in vivo bone marrow cellularity.

Jose C. Pichardo, from the department of nuclear and radiological engineering at the University of Florida in Gainesville, and colleagues acquired spoiled gradient-echo in vivo images of the femur, humerus, upper spine and lower spine from two dogs using a clinical 3T MRI scanner.

They used single-peak iterative decomposition of water and fat with echo asymmetry and least square (SP-IDEAL) to derive fat fractions, and stimulated-echo acquisition mode spectra was acquired to perform multipeak IDEAL with precalibration (MP-IDEAL), according to the study.

After euthanizing the dogs imaged with the MRI, necropsy was performed on the cadavers, and bone cubes were extracted from the head, neck and shaft of the long bones. Cortical bone and excess soft tissue were removed from each bone cube and microscope slides were prepared to determine marrow cellularity in various regions of interest.

Researchers compared the SP-IDEAL images to necropsy photos and found excellent matching of fat distributions between the two.

“The excellent agreement between SP-IDEAL and histology is not surprising, because the spectroscopy results suggest that the abundance of methylene was relatively uniform in all bones in this study,” wrote Pichardo et al. “However, considering that methylene accounts for only approximately 75 percent of lipid content, the closeness of the agreement is quite surprising.

“Given that histology is the current gold standard for in vivo marrow cellarity determination, the close agreement between both methods indicates that SP-IDEAL can be used in lieu of histology,” the researchers noted. “SP-IDEAL is noninvasive and can be performed only at the iliac crest. In addition, because SP-IDEAL provides fat and water images of the complete bone, marrow cellularity changes can be assessed at multiple locations.”

While SP-IDEAL does not require the acquisition of MR spectroscopy (MRS) data, MP-IDEAL may provide a more accurate measurement of chemical fat fraction due to its account of multiple lipid resonances.

“Our MRS data indicate that although the relative abundance of methylene lipids is fairly uniform throughout the canine skeleton, the relative abundances of the remainder lipid species are highly variable,” the authors wrote. “Adipocyte segmentation of histology images of bone marrow provides a measure of a volumetric fat fraction, because it is invariant to the relative abundances of lipid species. SP-IDEAL accounts for only methylene lipids, which in the canine skeleton correspond to approximately 75 percent of lipid content. As such, SP-IDEAL will always underestimate the chemical fat fraction.

“The excellent agreement between the methods indicates that SP-IDEAL can be used to derive the adipocyte volume fraction data necessary for the estimation of patient-specific active bone marrow mass using published predictive equations,” the authors concluded.