Susceptibility weighted imaging of cartilage canals in porcine epiphyseal growth cartilage ex vivo and in vivo.

Susceptibility weighted imaging (SWI) is a recent technique, used primarily for neuroimaging that uses subtle differences in magnetic susceptibility values between different tissues to generate contrast (1-3). As opposed to standard, magnitude-only, proton density, T1 or T2 weighted imaging. SWI also incorporates the use of phase data in contrast generation (4). The phase data contain information on local susceptibility changes between tissues (high spatial frequency phase changes) as well as on static inhomogeneous background magnetic fields (low spatial frequency phase changes). The latter phase components may mask the tissue-related phase information (1,4,5), however, by applying an appropriate high pass filter (4) or using alternative methods for “phase unwrapping” (6,7), the unwanted low frequency component may be filtered out. Phase contrast and SWI are used mainly in imaging of brain vasculature (1,2), iron content (8), direct visualization of brain anatomy (3,5,9), hemorrhages and brain lesions (10,11), and quantification of oxygen saturation (12). The difference of ~0.2 ppm (13,14) in magnetic susceptibility between deoxygenated blood and surrounding tissue leads to the accumulation of a phase difference between the venous blood and surrounding tissues (1). The resulting contrast due to phase accumulation is more easily depicted at higher field strengths since the amplitude of susceptibility induced local field variations is linearly proportional to the main magnetic field (4). Alternatively, the higher signal to noise ratios (SNR) available at higher field strengths can be used to shorten data acquisition time or to achieve higher spatial resolution while maintaining phase contrast at a same level (4). Additionally, the source of the SWI contrast can be of subvoxel size and still contribute to detectable signal differences (4). The vasculature of the developing skeleton particularly that of epiphyseal growth cartilage has a critical role in supplying nutrients. The vasculature in epiphyseal growth cartilage is confined to “cartilage canals,” channels that contain both arterioles and venules. Larger canals have been reported to range from 0.2 to 0.6 mm (average 0.3 mm) in piglets of 1–4 weeks of age with the vessels in the canals ranging from 0.01 to 0.16 mm in diameter (15). In animal models, it has been shown that a disruption of the cartilage canal blood supply causes tissue necrosis, which in turn may hinder ossification leading to the development of osteochondrosis (16,17). While this pathogenesis has been reported in animal models using histological evaluations, similar investigations have not been reported in humans due, in large part, to lack of noninvasive investigational tools. Only a few publications have described imaging of cartilage canals using MRI. Babyn et al. reported visualization of porcine cartilage canals in 3 mm biopsy cores with T2-weighted high-resolution MR imaging (18). Jaramillo et al. (19) later demonstrated the use of Gadolinium enhanced imaging of the cartilage canals in piglets, followed shortly by Barnewolt et al. (20), demonstrating distinct age dependent patterns of vascular canal orientation and distribution in human infants. Other reports have also demonstrated the use of Gadolinium-enhanced visualization of cartilage canals (15,21). The science of visualizing cartilage canals, however, is still in its infancy. Imaging of cartilage canals using intravenous Gadolinium contrast agent is limited by the short useful scanning time window and the SNR and resolution achievable; specifically, visualization of the network of smaller vessels using Gadolinium appears very limited (15,20). Furthermore, especially when imaging children, tissue inherent contrast is preferred over the administration of exogeneous contrast agents. To date, the intricate system of cartilage canals has only been visualized using ex vivo animal systems. The only anatomical studies demonstrating high-resolution images of the cartilage canals have used gold standard histological methods or intravenous contrast injection followed by tissue clearing and photography (16,22). An imaging method for utilizing inherent tissue contrast for high-resolution imaging of cartilage canals without the administration of intravenous contrast agent is highly desirable and would allow in vivo assessment of cartilage canal blood supply in normal and diseased epiphyseal cartilage. The purpose of this study was to investigate the feasibility of using SWI contrast to visualize cartilage canals in a porcine model, both ex vivo and in vivo. The first hypothesis of the study was that SWI would enable imaging and three dimensional (3D) visualization of the cartilage canals in postmortem specimens of epiphyseal growth cartilage. To test the hypothesis, specimens of porcine distal femur and distal humerus were imaged ex vivo using SWI at 9.4 T. After high-field ex vivo investigation, the second hypothesis was that SWI is also feasible for imaging the cartilage canals in vivo. To test this hypothesis and explore the in vivo feasibility, a live piglet was imaged at 7.0 T. Since 7.0 T is not yet clinically approved, an additional ex vivo experiment was conducted at 3.0 T to estimate the feasibility of using this technique at a clinically available magnetic field strength.