ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

2. INTRODUCTION
Intervertebral disc nutrition is based on diffusive processes of small molecules such as
oxygen and glucose. Among the anatomical structures which are involved in nutrition of
the disc cells, the vertebral endplates play a critical role and have been therefore
comprehensively investigated in previous studies (1,2). The mostly studied parameters
are endplate thickness, density and porosity of the bony portion of the endplate, density
and size of the vascular buds and marrow channels, and permeability.
Some studies were targeted to the investigation of the in vivo diffusion behavior of
small solutes through the endplates and inside the intervertebral disc (3). Serial postcontrast
magnetic resonance imaging (MRI) was widely employed to this aim, by using
nonionizing contrast agents with molecular weights lower than 1 kD (Gadodiamide or
Gadoteridol) which may well replicate the diffusion behavior of essential nutrients. In
these studies, both healthy and degenerated discs and endplates were investigated, thus
potentially allowing for the determination of a correlation between nutrient diffusion
and intervertebral disc degeneration.
However, the interpretation of contrast-enhanced MRI data remains of concern.
Assuming that the T1 signal enhancement is proportional to the concentration of the
contrast agent, the observation of the its time behavior would allow for the investigation
of the in vivo diffusive properties of the disc by using an inverse approach, in which the
tissue diffusion coefficients are optimized to best fit the measured concentration
profiles. To this purpose, an inverse finite element method based on gadoliniumenhanced
MRI was developed.
3. MATERIALS AND METHODS
90 lumbar intervertebral discs from 32 patients suffering from low back pain were
scanned with contrast-enhanced MRI. 22 discs were not included in the study due to
evident endplate defects such as Schmorl's nodes. Therefore, 68 discs were investigated.
2 discs were evaluated as grade 1 of the Pfirrmann scale (4), 24 disc as grade 2, 26 as
grade 3 and 15 as grade 4. For every disc, the T1 signal enhancement 5 minutes, 10
minutes, 2, 4 and 6 hour after injection of the contrast medium was determined.
Assuming that the relative T1 signal enhancement is proportional to the concentration of
the contrast agent, a one-dimensional finite element model of the diffusion of
gadolinium through the endplates and disc was set up for all investigated discs (Fig. 1).
Figure 1. One-dimensional finite element model of a lumbar intervertebral disc