Abstract
Near infrared light between the wavelengths of 700 and 950 nanometers has a relatively low absorption in tissue, and light of these wavelengths is able to penetrate several centimetres into tissue. Absorption of light is primarily due to hemoglobin. The absorption spectra for oxy-hemoglobin and deoxy-hemoglobin are different, and therefore comparison of light absorption at different wavelengths allows an assessment of the relative concentrations of these two chromophores. Light penetrates bone as well as soft-tissue, and near infrared spectroscopy (NIRS) is potentially a relatively simple, low-cost technique for assessing perfusion in bone. However, although absorption of light is low, scattering is high, and the spatial resolution of the measurement is poor. Application of the technique to the study of bone perfusion requires consideration of the potential confounding absorption arising from adjacent tissues that may have higher perfusion.
A clinical problem of interest in our institute is that of vascular changes occurring in bone of patients with spinal cord injury (SCI), and the relationship of these changes to bone density changes. We have, therefore, concentrated on developing NIRS for measurement of the proximal tibia, which is a common site for fractures in these patients. In order to develop a probe for the measurement of bone, experiments were performed with phantoms containing infrared absorbing dyes. Numerical simulations were also performed using the Monte Carlo technique. One of the most important design considerations is the distance between the optode delivering light to the skin, and the collecting optode which detects light. It was found that a separation of 20 mm between the light source and detector was an optimum compromise for minimizing contributions from overlying skin and surrounding muscle, while still being able to detect light efficiently enough to measure dynamic changes in chromophore concentration.
We have now started to apply this technique clinically. Relative changes of oxy- and deoxy-hemoglobin concentration have been measured in response to a range of interventions. Comparison has been made of the effect of different interventions designed to modify perfusion of bone (neuro-muscular stimulation of the calf, intermittent pneumatic compression, low amplitude high frequency vibration, and venous tourniquet). We are studying vascular reactivity in chronic SCI patients and controls and we have also started to investigate the effect of daily neuro-muscular stimulation in acute SCI patients. Preliminary results of these clinical studies will be presented.
