20 WEEK 3D ULTRASOUND FACE REGISTRATION

However, we do consider a miniature inertial orientation sensor to guide the matching algorithms, increasing their speed and reliability. This system does not require an inconvenient external position sensor attached to the probe as much of the information required to calculate the probe trajectory can be inferred by matching the 3D blocks of recorded data. The aim of this work is then to incorporate the benefits from the existing strategies, as previously described, to produce a clinical system that offers unique advantages in a clinical setting. The disadvantage to this method is that it requires the pre-processing of volumes to remove artefacts. This, combined with the fact that position sensors may not be well suited to routine clinical use, has led to the development of a modified version of 3D SIFT (scale-invariant feature transform) to globally register ultrasound volumes, which, rather than using an intensity-based approach, uses common features (blobs and corners) to match two volumes without using any position tracker. However, image-based similarity measures can be very overlap-dependent. In the literature, the majority of the mosaicing systems rely on an external position sensor as an input to the image-based techniques.

20 WEEK 3D ULTRASOUND FACE REGISTRATION

It is possible to register volumes using the image data alone however, in order to improve the speed and robustness of the registration algorithm, a simpler method using a reduced search space is often considered. These all result in poor registration performance. Furthermore, there are acoustic artefacts relating to variations in the resolution, attenuation and the material properties in the propagation path of the sound waves.

The main difficulties associated with ultrasound data are that there is an irregular sampling of the acquisition space and the anatomy's appearance varies with the direction of insonification. However, registering ultrasound volumes is a difficult task. In the majority of matching algorithms, the notion of a similarity measure is considered between two volumes, where the target volume is kept stationary while the position of the source volume is changed iteratively until an extreme value of the similarity measure designates a registration point.

A possible solution to stitching is to consider an automatic image-based registration technique that uses a matching algorithm to align overlapping data. The mosaicing will also provide the ultrasound practitioner with a compounded volume of higher quality and the ability to visualise the anatomical structures from a variety of angles. This will potentially have many clinical advantages in visualising structures that are too large for a single volume.

With the ability to obtain a 3D data set, the next evolutionary step in 3D ultrasound is to create an extended field of view by stitching several volumes together. The third group involves sweeping the 1D transducer array through a known trajectory using a mechanical device. The second group of techniques, called freehand 3D ultrasound, involves letting the clinician sweep the 1D transducer array manually across the subject, coupled with some sort of a tracking device to measure its trajectory. Because dense arrays of 2D transducers are difficult to build, alternative approaches have been developed using 1D transducer arrays to produce 2D B-scan images at known locations in 3D space. This can record 3D data directly by using different combinations of elements to scan the volume of interest. The most obvious one is to have a 2D array of transducer elements in contact with the patient.

Providing 3D extended-field-of-view ultrasound may aid the interpretation of ultrasound images by demonstrating the anatomical relations of pathology pictorially to clinicians. It is well recognised that ultrasound image acquisition and interpretation is more operator-dependent than other imaging modalities such as CT or MRI. The combination of 3D ultrasound and extended field of view may facilitate a wider use of 3D ultrasound by allowing ultrasound practitioners to image larger structures. In our experience at a tertiary referral centre, 3D ultrasound is not routinely used despite it being readily available. Other research groups have explored 3D ultrasound in areas such as cardiac assessment and evaluation of the foetal brain using a single 3D volume. The main clinical applications are at present in obstetrics, in areas such as assessment of cleft lip. Three-dimensional (3D) ultrasound has become increasingly popular in recent years, providing an efficient framework for safe, low-cost volumetric imaging.

20 week 3d ultrasound face