Adaptations in trabecular bone microarchitecture in Olympic athletes determined by 7T MRI.

The National Institutes of Health (NIH) consensus panel on osteoporosis defined it as a “skeletal disorder characterized by compromised bone strength predisposing a person to an increased risk of fracture” (1). Though bone mineral density (BMD), as assessed by dual energy X-ray absorptiometry (DEXA), is the parameter currently used to diagnose osteoporosis and assess fracture risk, many studies have concluded that trabecular bone microarchitecture is as important as BMD in determining bone strength and fracture risk (2–5). In recent years, high-resolution micro-MRI (~100–200 µm) has become a promising noninvasive technique to evaluate and produce images of trabecular bone structure (6,7). Furthermore, the development of novel postprocessing techniques, such as fuzzy distance transform (FDT) (8,9) and digital topological analysis (DTA) (10–12), has provided researchers with the means to quantitatively assess trabecular bone morphology and topology. Until now, most of these studies have been performed at 1.5T, which can be limited by long scan times and low signal-to-noise ratio (SNR). In recent years, there has been increasing interest in imaging at higher field strengths for musculoskeletal applications (13–15). Scanning at higher field strengths has the potential to provide improved SNR and spatial resolution and to decrease the amount of time required for scanning (16). Disadvantages of scanning at higher field strengths also exist, such as stronger magnetic susceptibility and larger chemical shift artifacts. For example, the chemical shift difference between water and fat resonance at 7T is ~1040 Hz compared to ~440 Hz at 3T and ~220 Hz at 1.5T (16). At a bandwidth of 130 Hz (used in this study), this corresponds to a chemical shift of eight pixels at 7T, vs. 3.4 pixels at 3T and 1.7 pixels at 1.5T. Furthermore, changes in relaxation time (increase in T1 and decrease in T2 as the static field strength increases) may lead to decreased contrast resolution. Thus, pulse sequence parameters have to be reoptimized if they are to be utilized at higher field strength. The goals of our current study were the following: 1) to utilize a whole-body 7T MRI system to produce high-resolution in vivo images of trabecular bone at the knee; and 2) to determine the feasibility of using 7T MRI to detect differences in trabecular bone microarchitecture among subjects by analyzing the images with FDT and DTA. For our subjects, we chose to compare a small group of elite athletes, members of the 2004 U.S. Olympic Fencing Team, and a group of age- and weight-matched controls who maintained normal levels of physical activity. The members of the Olympic fencing team are unique in that they have subjected their lower extremities to intense mechanical loading over several years. Wolff’s Law states that bone is deposited in accordance with the mechanical forces placed upon it (17). We therefore hypothesized that trabecular bone quality in these elite athletes would demonstrate improvements in structural parameters that allow it to withstand elevated mechanical loads, and we hypothesized that these features would be detectable by 7T MRI.