3D Reconstruction Of Human Enamel Using High Frequency Ultrasound

More and more attention has been paid to the health of human teeth.Many tooth diseases are related to the loss of tooth tissue and erosion,but can be controlled with proper treatment.Therefore,early detection and timely treatment is critical to prevent the deterioration of the disease.Ultrasound has been widely used in medical practice such as diagnosis,non-destructive testing and treatment.Due to its short wavelength and associated high resolution,high frequency ultrasound is more suitable for the applications concerning tooth.Although ultrasonography in dentistry can trace back to the early 1960's,it has not been developed as an efficient diagnostic technique.Many exploration and research has been carried out by scholars and organizations on 3D ultrasound imaging in dental application,but the outcomes are not satisfactory so far.Some of the imaging systems are very complex.Most of them need to dedicatedly prepare the tooth samples,which changed the original stucture of the samples and in turn reduced the practicality of the results.To overcome these shortcomings,this dissertation introduces a new scanning system which collects structural information of tooth surface and enamel-dentine junction.A 3D motorized scanning stage was used to control the moving trajectories of an A-mode transducer so as to acquire the echoes from the surface of a tooth.A frequency-based compensation method was applied to 1-dimentional signals.Then a new denosing algorithm of 2-dimentional ultrasound images based on Q-learning was introduced.3-dimentional images of tooth enamel could be achieved and no special preparation was required before the scanning.The measurements of enamel thickness were compared with those of CT scanning data.Four permanent molars provided by from Guanghua College of Stomatology,Sun Yat-sen University were scanned and then 3D reconstructed.The 3D reconstruction and measurements of enamel thickness show that the imaging system and the imaging method can well reproduce the enamel layer and can provide the dentists with full 3D view of tooth enamel.The main research contents of this paper can be divided into three aspects: design and implementation of a 3D reconstruction system;a frequency-based gain compensation algorithm applied to 1D echo signals;a Q-Learning-based fusion filtering algorithm to improve the image quality applied to 2D ultrasound images.Design and implementation of a 3D reconstruction system.The components of a 3D ultrasound reconstruction system were studied ? Several scanning methods and their advantages and disadvantages were compared and analyzed.A mechanical scanning system was developed based on the research which consisted of a single element focused transducer with a center frequency of 50 MHz,a general purpose ultrasonic pulser-receiver,a 3D translation device with its control box and control card,an acquisition card and a computer.Besides these hardware,the software which control the data acquisition,RF signal processing,2D image denosing and 3D visualization was developed.Introduce of a novel attenuation compensation algorithm.Linear autoregressive(AR)model was applied to the estimation of echo signal attenuation parameter based on existing gain compensation algorithms.The compensated signal can used to build images with better quality.Then a nonstationary AR algorithm was introduced which replaced the AR parameters with the combinations of small amount of base functions and provided even more improvement to image quality.Introduce of a new denosing algorithm of ultrasound images based on Q-learning.Three kinds of filters(median filter,wavelet transform filter and speckle reducing anisotropic diffusion filter)were fused to implement a two-dimensional image filtering algorithm.The results of these three filters were weighted and the agent chose next action(change of the weights)according to the reward(image quality evaluation)until the optimal solution was found.Subjective and objective evaluation shows that this algorithm has good effects and general adaptability.