Abstract
Introduction: Whilst conventional Magnetic Resonance Imaging (MRI) is universally used as the method of choice for examining the boundaries of the intervertebral disc clinically it gives little information about the internal structure of the disc. This is largely due to the fact that the normal resolution of such devices (typically 1mm in plane and 3mm out of plane) is just too large to resolve structures and pathologies of interest.
Aim: This work aims to describe the appearance of normal and pathological discs when imaged using a high resolution system. It then tests the hypothesis that a degeneration grading scale based upon such observations corresponds well with the graded appearance of the sectioned disc.
Method: 13 lumbar discs from 7 non-chondrodystrophoid dogs (age 2–10 yr, mean 5.7 yr) were employed in this study. They were imaged using a small bore 0.5T research imaging system using a T2* weighted pulse sequence (TR=500ms, TE=17ms), a 60mm field of view, 1 mm slice thickness, in plane resolution was 230 μm. A grading scale based on the standard visual scale was developed for grading these images.
Results: The outer and middle annulus had a strongly banded appearance with adjacent lamellae having high and low intensities (in spite of there similar chemical composition). The inner annulus (and frequently all the posterior annulus) had a uniform high intensity appearance as did the nucleus. Frequently, there has a well defined dark boundary between the annulus and nucleus. Increasing degeneration lead to disorder of the annulus structure and non-uniformity in the nucleus. Statistical comparison of the visual and MRI grading scales were extremely good (α=0.90–0.95) except for the posterior annulus (α=0.26).
Conclusion: Many features of the MRI appearance of discs at high resolution, such as the banded structure of the annulus, were not expected and must be due to some subtle physical processes. Care must therefore be taken with the interpretation of such images, in particular to assessment of hydration. Grading of high resolution images corresponded well to the ‘gold standard’ of visual appearance on sectioning. However, this scale is totally different to that used to grade discs using conventional clinical MRI.
Correspondence should be addressed to Carlos Widgerowitz, Honorary Secretary BORS, Division of Surgery and Oncology, Section of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Tort Centre, Dundee DD1 9SY, Scotland.
