Anisotropy in the speed of sound in ivory

Name: K.Sindi, N.L.Bubb. D.L.Gutteridge  and J.A.Evans

Affiliation: Leeds Dental Institute and Medical Physics, Leeds Institute of Genetics Health and Therapeutics

Title: Anisotropy in the speed of sound in ivory

Objective:

There is growing interest in the use of ultrasound for the imaging and characterisation of teeth. Two quantities, the speed of ultrasound (SOS) and its attenuation, are normally measured when carrying out such studies. However, anisotropy in the speed of sound as a result of structural directionality has been shown in other tissues. This study aims to test the hypothesis that this applies to dentine.

Methods:

Measurement of SOS require relatively large samples and hence our pilot study used ivory as being a dentine analogue allowing various thicknesses to be measured (4, 6, and 9 mm). The measurements were carried out using a reflection method with a 2.25 MHz transducer. To assess the role of the structure we measured the SOS in three orthogonal directions and also after demineralisation with lactic acid (6 %). The SOS was measured along the X axis before and after demineralisation. The effect of acid demineralisation was assessed on a different sample which was a disc of 4 mm thickness and 9 mm diameter. Following the measurements the samples were imaged using an electron microscope in order to identify the orientation of the tubules within the ivory.

Results:

The SOS varied according to the direction of the tubules in the sample. The SOS values shown are the average values for all 3 thicknesses in each direction.

The images from the SEM demonstrate that the tubules are aligned along the X direction. The values obtained pre- and post-demineralisation were 4444 and 4000 ±177 m s^-1 respectively.

Conclusion:

The results support the hypothesis that SOS measurements in teeth require a careful awareness of structure and orientation.