5B-2 3D Imaging of Teeth Using High Frequency Ultrasound

It was shown in the late 1960's that the internal structures of teeth could be investigated using ultrasonic pulse echo techniques with 4 MHz contact probes. However, the low frequency limited the resolution of the system and therefore the thickness of dental structures which could be observed. More recent reports have increased frequencies into the region of 10 to 20 MHz. With such frequencies, the resolution in enamel is improved to 0.5 mm. However, the average thickness of enamel in human teeth is around 1.5 mm, implying that even the improved resolution is still inadequate for detailed images and diagnosis. As well as the considerations about the resolution of the system, it has been shown that the attenuation and losses due to acoustic boundaries in tooth structures are detrimental to image reconstruction, with potentially useful information lost or degraded. Therefore, it is essential to have maximum energy transfer into, and back out from, the tooth. The work presented here introduces a novel high frequency focused ultrasound transducer operating at 35 MHz. In order to avoid the natural complexities of the human tooth in the experiment, human incisors were prepared so that only one layer of enamel and dentine were present. The sample was then immersed in distilled water on a translation stage and an x-y raster ultrasound scan was performed. A number of signal processing algorithms were applied to the raw data including correction of distortion and position via correlation and high and low bandpass filtering. The image processing application IMAGEJ was then used to reconstruct a 3D representation and rotation of the processed dataset. The individual A-scans which in turn create the B-scans and 3D images are of a much higher resolution in both the temporal and spatial domain than previously published. The 35 MHz operating frequency gives a resolution of 0.19 mm in the enamel layer, which is at a useful level for the detection of dental caries and more specifically acid erosion. The high frequency also produces a spotsize of 110 mum which allows for accurate localisation in the individual A-scans. The results are believed to be the first known 3D high resolution ultrasound images of the enamel-dentine junction.