A few years ago, I wrote a message on Biomch-L on the topic of low back pain (LBP) and the potential use of imaging techniques to predict disc damage before it actually occurs. Although a number of colleagues showed some interest but none was strong enough to lead to a substantial collaboration or even discussion. That did not act as a total deterrent for me and now I would like to re-open the discussion on this topic to see what other colleagues think. This time I can also report the results of some crude experiments which we carried out in Southampton, which might enthuse more colleagues.

First I would begin by reiterating an assumption that: prior to tissue failure, say in the annulus of an inter-vertebral disc, there must be some change in the normal biomechanical characteristics of that disc.

The loading response of body tissue can be used as an indicator of their functional performance and therefore state of health. This is understood from biomechanical studies that have demonstrated altered mechanical properties of excised diseased tissue through in-vitro experiments. It is also plausible to expect in-vivo biomechanical properties to be directly related to structural integrity and functional performance and that under normal conditions of use, structural damage and/or dysfunction only follow deteriorations in the biomechanical properties. The challenge is therefore to detect this deterioration and since it can be hypothesised that the lifecycle of any deterioration in the biomechanical properties of tissue (and that can be any tissue such as muscle tissue, cardiac or vascular, etc.) is a gradual process that takes place over a number of days, weeks or even months, it is then reasonable to aim for discovering and developing a sensitive but totally non-disruptive (e.g. non-invasive) technique for detecting the deterioration of normal biomechanical characteristics of tissue.

A technique to predict tissue damage can have a profound effect on preventive treatments be it through physical interventions or medicinal and can open the door for many new areas of research that aim to maintain health as opposed to relieve symptoms.

In view of the above, when an intervertebral disc fails it does so because its biomechanical properties deteriorate such that the loading response of the annulus falls short of the demands placed upon it.

If it were possible to measure the in vivo loading response of the disc with a non-invasive method then it would be possible to work out a measure of its functional performance. In the example of an inter-vertebral disc a measure of the biomechanical performance could be obtained by measuring the internal pressures or stress contours, and/or deformations or strains that arise as a result of an applied load, e.g. a deformation of the spinal motion segment. Developing a technique to make these measurements non-disruptively and non-invasively is inevitably reliant on existing technologies as the starting point and MRI is a natural first choice.

Our attempt in Southampton was initially inspired by the wish to assess the sensitivity and demonstrate the potential of MRI to measure very small loading responses. At its simplest, this meant measuring loading responses that result from loads which do not cause any structural damage and fall within normal loading patterns and limits. However, to make the load measurable the human body would have to be invaded or the experiment conducted in-vitro. An alternative was to consider a model that could reasonably demonstrate the potential of MRI in measuring loading responses.

Following trials of several members of the citrus family and a kiwi a small onion or shallot seemed to confirm that use of diffusion weighted imaging sequences can capture the biomechanical response that is elicited by small external pressure. An inflatable bag, similar to what is used in blood pressure measurements was wrapped around the onion and placed in the coil inside the scanner. One end of a plastic tube was connected to the inlet port of the bag (a barbed plastic nipple) and the other end was extended through a small window to the adjacent room and connected to a pneumatic pump and a pressure gauge. The circuit included a hand operated valve to create an open circuit when pressure had to be reduced. It was hypothesised that as pressure builds up the outermost layer of the onion is deformed inwardly to the next layer and the thin film of fluid that is between these layers is pressurised. Further increases of the pressure deforms the second layer and the thin film of fluid that exists under it and the third layer. Further increases of the pressure may have a combined consequence in terms of deforming deeper layers and their associated thin fluid films and deforming the already deformed layers further by squashing them in addition to bending.

Our image sequences show the area under pressure as lighting up and upon removal of the pressure (i.e. deflating the bag) returning to normal. Although the pressure was very low to confirm the onion’s structure was intact, it was dissected after the experiment.

I have a series of photographs that I can send to anyone interested.

Some of Our brilliant colleagues and eminent Professors in the Department of Applied Mathematics in Southampton and Nottingham worked on developing a preliminary model which would allow you to take the MR image parameters and work out the strains within the structure. We have not validated this but that is an obvious next step and then developing the model further, if any colleagues find this of interest do not hesitate to contact me or to just repeat the work and take it further.

thanks

Hamid




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Dr. H Rassoulian BSc, MSc, PhD, CSci, MIPEM, CEng, FIMechE, CS
Visiting Clinical Scientist
Clinical Neurosciences
School of Medicine, University of Southampton
Southampton General Hospital
Southampton SO16 6YD
Email: HamidR@soton.ac.uk