Stefan Heger

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.

User-interactive registration of bone with A-mode ultrasound

In this study, we investigated a registration technique based on a mechanically tracked A-mode ultrasound pointer for transcutaneous A-mode ultrasound pointer for transcutaneous, noninvasive palpation of bone surface. The principle has been exemplified for distal femur registration in total hip replacement surgery and has been evaluated in laboratory trials on a bone/soft-tissue model. Three different registration modes were demonstrated. Regarding clinical requirements, we constricted the areas of palpation to points and surfaces accessible through a minimal surgical portal in order to avoid additional trauma to soft tissue. Registration based solely on proximal surface points seems to be insufficient for clinical application. To avoid the invasive direct palpation of the condyles and epicondyles, we used the A-mode ultrasound pointer for noninvasive transcutaneous registration (Mode III). The resulting entire mean rms error was 0.59 mm (0.28/spl deg/), which is significantly better and more reliable than a registration exclusively in the proximal part of the femur. During the ultrasound registration process, the computer-based assistance tool gave visual feedback to the user to guide the alignment of the transducers beam axis to be perpendicular to the local bone surface. After five iterations, the final angle error of this approximation was in the range of /spl plusmn/2 degree and generally influenced by the contribution of the discretization angle error /spl Phi//sub DIS/. Based on the consideration of the thickness of the soft-tissue and the geometry of the ultrasound probe, it is suggested that this is a sufficient accuracy in order to ensure a reliable ultrasound-based digitization of the bone surface. The overall mean registration time using A-mode ultrasound, including palpation, and matching, seems to be tolerable. The algorithm and interface for user-interactive A-mode ultrasound-based registration seem to provide efficient support for robust and accurate registration.

Robot- and computer-assisted craniotomy (CRANIO): from active systems to synergistic man-machine interaction.

Abstract Computer and robot assistance in craniotomy/craniectomy procedures is intended to increase precision and efficiency of the removal of calvarial tumours, enabling the preoperative design and manufacturing of the corresponding implant. In the framework of the CRANIO project, an active robotic system was developed to automate the milling processes based on a predefined resection planning. This approach allows for a very efficient milling process, but lacks feedback of the intra-operative process to the surgeon. To better integrate the surgeon into the process, a new teleoperated synergistic architecture was designed. This enables the surgeon to realize changes during the procedure and use their human cognitive capabilities. The preoperative planning information is used as guidance for the user interacting with the system through a master—slave architecture. In this article, the CRANIO system is presented together with this new synergistic approach. Experiments have been performed to evaluate the accuracy of the system in active and synergistic modes for the bone milling procedure. The laboratory studies showed the general feasibility of the new concept for the selected medical procedure and determined the accuracy of the system. Although the integration of the surgeon partially reduces the efficiency of the milling process compared with a purely active (automatic) milling, it provides more feedback and flexibility to the user during the intra-operative procedure.

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.

Trackerless ultrasound-integrated bone cement detection using a modular minirobot in revision total hip replacement:

Abstract Medical robots are superior to freehand manipulation if an accurate, precise, and time-efficient implementation of a preplanned intervention is required. In the first part of this contribution a new modular minirobot for automatic ultrasound-based bone cement detection followed by subsequent cement milling in revision total hip replacement is presented. A minirobot integrated ultrasound module eliminates the need for external position tracking (e.g. by an optical system) as well as patient registration since the scanned contours can be directly provided within the robots coordinate system. Further, the modular minirobot concept allows kinematics, workspace, and mechanical parameters to be easily adapted to the requirements of related or even new surgical applications. In the experimental part, the impact of ultrasound module integration on the implementation of optimized scanning strategies is investigated and evaluated in a laboratory set-up. As wave mode conversion and refraction artefacts due to angular sound incidence influence the detection accuracy, the transducer alignment can be optimized with respect to the number of degrees of freedom (DOFs) provided by the minirobot. A model-based scanning approach using two degrees of freedom (2DOFs), three degrees of freedom (3DOFs), and four degrees of freedom (4DOFs) respectively is presented. For automated scanning path calculation, a 2DOF distal—proximal prescan has been performed to estimate the principal components of the cement cavitys geometry using either a model-based or a statistical approach. In a cadaver study, the model-based approach consistently outperformed the statistical approach. The 3DOFs and 4DOFs scanning strategies yielded a significantly higher scanning accuracy if compared with the 2DOFs approach whereas the 3DOFs approach represents a trade-off between system complexity and detection accuracy.

Surface-based determination of the pelvic coordinate system

In total hip replacement (THR) one technical factor influencing the risk of dislocation is cup orientation. Computer-assisted surgery systems allow for cup navigation in anatomy-based reference frames. The pelvic coordinate system most used for cup navigation in THR is based on the mid-sagittal plane (MSP) and the anterior pelvic plane (APP). From a geometrical point of view, the MSP can be considered as a mirror plane, whereas the APP can be considered as a tangent plane comprising the anterior superior iliac spines (ASIS) and the pubic tubercles. In most systems relying on the pelvic coordinate system, the most anterior points of the ASIS and the pubic tubercles are selected manually. As manual selection of landmark points is a tedious, time-consuming and error-prone task, a surface-based approach for combined MSP and APP computation is presented in this paper: Homologous points defining the MSP and the landmark points defining the APP are selected automatically from surface patches. It is investigated how MSP computation can benefit from APP computation and vice versa, and clinical perspectives of combined MSP and APP computation are discussed. Experimental results on computed tomography data show that the surface-based approach can improve accuracy.

A-mode ultrasound-based intra-femoral bone cement detection and 3D reconstruction in RTHR

Due to the difficulty of determining the 3D boundary of the cement-bone interface in Revision Total Hip Replacement (RTHR), the removal of the distal intra-femoral bone cement can be a time-consuming and risky operation. Within the framework of computer- and robot-assisted cement removal, the principles and first results of an automatic detection and 3D surface reconstruction of the cement-bone boundary using A-mode ultrasound are described. Sound propagation time and attenuation of cement were determined considering different techniques for the preparation of bone cement, such as the use of a vacuum system (Optivac®, Biomet). A laboratory setup using a rotating, standard 5-MHz transducer was developed. The prototype enables scanning of bisected cement-prepared femur samples in a 90° rotation range along their rotation axis. For system evaluation ex vivo, the distal femur of a human cadaver was prepared with bone cement and drilled (Ø 10 mm) to simulate the prosthesis cavity in a first approximation. The sample was cut in half and CT scanned (0.24 mm resolution; 0.5 mm distance; 0.5 mm thickness), and 3D voxel models of the manually segmented bone cement were reconstructed, providing the ground truth. Afterwards, 90° segments of each ex-vivo sample were scanned by the A-mode ultrasound system. To obtain better ultrasound penetration, we used coded signal excitation and pulse compression filtering. A-mode ultrasound signal detection, filtering and segmentation were accomplished fully automatically. Subsequently, 3D voxel models of each sample were calculated. Accuracy evaluation of the measured ultrasound data was performed by ICP matching of each ultrasound dataset (∼8000 points) to the corresponding CT dataset and calculation of the residual median distance error between the corresponding datasets. Prior to each ICP matching, an initial pre-registration was calculated using prominent landmarks in the corresponding datasets. This method yielded a median distance error in the region of 0.25 mm for the cement-bone interface in both femur halves.

A voice-coil actuated ultrasound micro-scanner for intraoral high resolution impression taking

Silicone based impression-taking of prepared teeth followed by plaster casting is well-established but potentially less reliable, error-prone and inefficient for newly emerging techniques such as computer aided design and manufacturing (CAD/CAM) of dental prosthetics. Intra-oral optical scanners have been introduced to increase efficiency of CAM but no breakthrough occurred so far. Oral liquids such as saliva, blood and sulcular fluid are still one of the main problems since the preparation area must be completely dry. Moreover, sub-gingival preparations need to be uncovered invasively prior to scanning and a reflecting powder coating is required in some cases. High frequency ultrasound (HFUS) has been recently introduced as an alternative to optical scanning. Ultrasound is less sensitive against oral fluids and in principal able to penetrate gingiva in a patient-friendly and cost-effective way. Although HFUS systems have been introduced for ophthalmology, dermatology or small animal imaging, none of them suits the challenging requirements and high accuracy demands for intra-oral micro-scanning of prepared teeth. For this reason, we conceived a new ultrasonic micro-scanning device based on a voice-coil actuated spherically focused HFUS transducer for intra-oral use. The system, which is designed for both highly dynamic accurate positioning and micrometer-resolution, is supplied with a sensor providing position feedback for motion control as well as the ultrasound trigger engine. In this contribution, we describe the set-up and evaluate the lateral displacement of the micro-scanners end-effector with respect to the oscillation rate using laser triangulation. The results are in good agreement to the requirements of an intra-oral ultrasound based micro-scanner.

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.