Quantitation of the susceptibility difference between trabecular bone and bone marrow: Experimental studies

TLDR

The susceptibility difference between trabecular bone and soft tissue creates magnetic field inhomogeneities that shorten transverse relaxation times. The study aims to quantify how different relaxation mechanisms affect signal intensity in gradient‑echo images of bone marrow in the presence of trabecular bone. The authors measured T1, T2, and apparent T relaxation times in dried human vertebrae of varying bone density immersed in saline or an oil‑water emulsion, correlating the results with bone density. They found that T1 and T2 are largely independent of bone density, while the apparent transverse relaxation rate 1/T increases with density at 0.2 s⁻¹ mg⁻¹ cc⁻¹ due to dephasing, with similar density‑related effects observed for fat protons and shorter T values in the epiphysis versus diaphysis in vivo. © 1991 Academic Press, Inc.

Abstract

Abstract In this study we quantify the effects of different relaxation mechanisms on the signal intensity in gradient‐echo images of tissue such as bone marrow in the presence of trabecular bone. The susceptibility difference between trabecular bone and soft tissue produces distortions in the magnetic lines of force which induce strong inhomogeneities in the static magnetic field. Diffusion of tissue protons in such magnetic field gradients produce a shortening of the transverse relaxation time T 2 , while the dephasing of the transverse magnetization due to susceptibility differences produces a shortening of the apparent relaxation time T as demonstrated in gradient‐echo images. We have used specimens of dried human vertebrae with different bone densities immersed in either saline to simulate tissue water or an emulsion of oil and water to simulate bone marrow to quantify these relaxation mechanisms in vitro . We have measured the MR relaxation times T 1 , T 2 , and T of protons within the trabecular spaces and correlated their variations with trabecular bone density. We have found that in vitro , at 1.5 T, the relaxation times T 1 and T 2 do not show significant variations with bone density and there are no significant contributions to the transverse relaxation rate due to the diffusion of tissue water in the magnetic field gradients. However, the relaxation rate, 1/ T , of saline in the presence of trabecular bone increases at a rate of 0.2 s −1 mg/cc due to the dephasing of the transverse magnetization in the magnetic field inhomogeneities. Similar bone density‐related T variations were observed for fat protons within the trabeculae where the chemical‐shift‐induced modulations of signal intensity in an oil‐water emulsion have been separated from the susceptibility‐induced relaxation effects. In addition, we have verified these effects in vivo and quantified in vivo variations in fat and water relaxation rates of bone marrow in the epiphysis and diaphysis in the appendicular skeleton of normal volunteers and found that both fat and water T are shorter in the epiphysis compared to the diaphysis, which correlates well with previous observations. © 1991 Academic Press, Inc.