Effects of high intensity focused ultrasound on the intervertebral disc

Abstract: The purpose of this study was to study and try to optimise high intensity focused ultrasound treatment of the intervertebral disc.

We demonstrated that it was possible to heat intervertebral discs with HIFU delivered by 50 or 5 mm diameter ultrasound transducers.

Finite element studies indicated that it would be possible to heat the disc to a temperature of 65ºC or above through an extradiscal minimally invasive procedure with a 5 mm diameter ultrasound transducer.

Computational studies were in general validated by experiments. In vivo studies showed no sign of discomfort or abnormality to the treated animals. Histological changes due to collagen shrinkage in the discs were found, and no damage to the nerve or muscle could be found.

The attenuation coefficient in bovine annulus fibrosus was found to be in the range of 1.1 to 1.3 dB cm-1 MHz-1 and the absorption coefficient 0.4 to 0.7 dB cm-1 MHz-1, which is approximately half of the attenuation coefficient. This indicates that a significant proportion of the ultrasound is scattered, due to the anisotropic nature of the annulus fibrosus.

Using magnetic resonance thermometry in a bovine in vitro model, the possibility to measure the temperature distribution in the disc during HIFU treatment was demonstrated. This method can be used to evaluate future ultrasound transducers and treatment protocols.