Joachim Tinschert

Recent advances of ultrasound imaging in dentistry – a review of the literature

Ultrasonography as an imaging modality in dentistry has been extensively explored in recent years due to several advantages that diagnostic ultrasound provides. It is a non-invasive, inexpensive, painless method and unlike X-ray, it does not cause harmful ionizing radiation. Ultrasound has a promising future as a diagnostic imaging tool in all specialties in dentistry, for both hard and soft tissue detection. The aim of this review is to provide the scientific community and clinicians with an overview of the most recent advances of ultrasound imaging in dentistry. The use of ultrasound is described and discussed in the fields of dental scanning, caries detection, dental fractures, soft tissue and periapical lesions, maxillofacial fractures, periodontal bony defects, gingival and muscle thickness, temporomandibular disorders, and implant dentistry.

High-Frequency Ultrasound as an Option for Scanning of Prepared Teeth: An in Vitro Study

Because of its ability to non-invasively capture hard structures behind soft tissue, high-frequency ultrasound (HFUS)-assisted microscanning could be a patient-friendly and promising alternative for digitization of prepared teeth. However, intra-oral HFUS microscanners for taking digital impressions of prepared teeth are still not available in the clinical setting. Because working range, scanner size, scanning time, surface reconstruction accuracy and costs are major factors in such a system, our overall objective is to minimize hardware efforts and costs while maintaining the accuracy of the surface-reconstructed tooth model in the range 50 μm. In the work described here, we investigated the accuracy of tooth impression taking using a single-element HFUS microscanner with only three translational degrees of freedom under the restriction that only one occlusal scan is performed per tooth. As in favor of time and scanning efforts the data density is expected to be low, the surface reconstruction process is linked to a model-based surface reconstruction approach using a thin spline robust point matching algorithm to fill data gaps. A priori knowledge for the model is generated based on the original HFUS measurement data. Three artificial teeth and one human molar were prepared and scanned using an extra-oral HFUS laboratory microscanner that was built to test and evaluate different scanning setups. A scanner with three translational degrees of freedom was used to scan the teeth from an occlusal direction. After application of the proposed thin-spline robust point matching algorithm-based reconstruction approach, reconstruction accuracy was assessed by comparing the casts with a control group scanned with an extra-oral laser-scanning system. The mean difference between the reconstructed casts and the optical control group was in the range 14-53 μm. The standard deviation was between 21 and 52 μm. This let us assume that the suggested approach can help to decrease hardware efforts while maintaining the robustness of the 3-D surface reconstruction process for future HFUS-based intra-oral scanners.

Accuracy assessment of high frequency 3D ultrasound for digital impression-taking of prepared teeth

Silicone based impression-taking of prepared teeth followed by plaster casting is well-established but potentially less reliable, error-prone and inefficient, particularly in combination with emerging techniques like computer aided design and manufacturing (CAD/CAM) of dental prosthesis. Intra-oral optical scanners for digital impression-taking have been introduced but until now some drawbacks still exist. Because optical waves can hardly penetrate liquids or soft-tissues, sub-gingival preparations still need to be uncovered invasively prior to scanning. High frequency ultrasound (HFUS) based micro-scanning has been recently investigated as an alternative to optical intra-oral scanning. Ultrasound is less sensitive against oral fluids and in principal able to penetrate gingiva without invasively exposing of sub-gingival preparations. Nevertheless, spatial resolution as well as digitization accuracy of an ultrasound based micro-scanning system remains a critical parameter because the ultrasound wavelength in water-like media such as gingiva is typically smaller than that of optical waves. In this contribution, the in-vitro accuracy of ultrasound based micro-scanning for tooth geometry reconstruction is being investigated and compared to its extra-oral optical counterpart. In order to increase the spatial resolution of the system, 2nd harmonic frequencies from a mechanically driven focused single element transducer were separated and corresponding 3D surface models were calculated for both fundamentals and 2nd harmonics. Measurements on phantoms, model teeth and human teeth were carried out for evaluation of spatial resolution and surface detection accuracy. Comparison of optical and ultrasound digital impression taking indicate that, in terms of accuracy, ultrasound based tooth digitization can be an alternative for optical impression-taking.

High Frequency (75MHz) Ultrasound based Tooth Digitization using Sparse Spatial Compounding

Over the last decade, extra- and intraoral optical scanning for computer integrated manufacturing (CIM) of dental restorations became the focus of interest. Despite the fact that intraoral systems are getting more and more accurate, their invivo accuracy is influenced by the presence of oral fluids. Moreover, subgingival preparation margins need to be uncovered invasively prior to the scan and powder may be required to cope with different translucency and reflectivity of target materials. High frequency ultrasound (HFUS) based intraoral micro-scanning could be an alternative technology for optical impression taking. However, for accurate 3D teeth geometry reconstruction, homogenously distributed echoes of the occlusal and lateral tooth surfaces as well as margins are required. Whereas HFUS phased array technology for 3D image compounding is not yet available, mechatronic single element based transceiver concepts at most require 5 degrees of freedom (4 in case of synthetic aperture focusing). To overcome these drawbacks, a sparse spatial compounding (SSC) technique is being investigated which makes use of only a limited number of additional scanning directions under a fixed incidence angle allowing for simplification of the final micro-scanning kinematic. Measurements with extracted prepared human molar teeth have been carried by using an extraoral HFUS-SSC scanner. The results demonstrate that with SSC almost homogenously distributed spatial data coverage of tooth surface points can be achieved.

Sensitivity analysis of synthetic aperture focusing based on the virtual source point for high-frequency ultrasound imaging

In a previous work we investigated the use of the synthetic aperture focusing technique based on a virtual point source (VSAFT) for high frequency ultrasound-based tooth surface reconstruction in the context of CAD/CAM-based dental restorations. However, discrepancies between real and assumed values of the transducers and coupling medias parameters could affect the performance of the VSAFT. In the framework of this work an analysis has been conducted to investigate the influence of wrong assumptions concerning speed of sound and location of the virtual point source (VPS) with respect to spatial resolution and SNR after processing with VSAFT in case of using a focused (f#=2) high frequency (75MHz) ultrasound probe. Additionally, a comparison between common apodization windows (Hamming, triangle and cosine) and the coherence factor method (CF) has been carried out. The results show that already 3% deviation of the VPS position from their real value causes a significant deterioration of the achieved 6-dB lateral resolution after VSAFT processing. The best lateral resolution and SNR have been achieved with the CF method. Furthermore, the acoustic focus is more appropriate for VSAFT compared to the geometric pendant.