The Ultrasound Visualization Pipeline - Scientific Visualization: Uncertainty, Multifield, Biomedical, and Scalable Visualization - page 274

Fig. 24.4 Using MagiCut to Clip the volume, generating a clear view to the desired structure [ 1 ]
directly on the 3D rendered scene. Each voxel is then projected onto the screen and
removed if it lies within the marked area. The third clipping tool is based on the
distance from a single mouse-click on the view-plane. A hemispherical wave front
is propagated from the seed-point and stops when the voxels reach a user-specified
threshold. Figure 24.4 shows an example of clipping implemented in the GE Voluson
machines [ 1 ].
24.5 Registration
Merging ultrasound with other modalities can be very beneficial. While ultrasound
provides high resolution images at a high frame-rate, other modalities, such as MRI
or CT can provide information complimentary to the ultrasound images. Data reg-
istration is the process of transforming different modalities into the same reference
frame to achieve as much comprehensive information about the underlying struc-
ture as possible. While CT and MRI are typically pre-operative imaging techniques,
ultrasound can easily be performed live during surgery. For instance, the radiation
from CT is dangerous and the large electro magnets in an MRI scanner require that
everything in the room is non-magnetic. Recently Curiel et al. built a non-magnetic
ultrasound scanner for proper simultaneous intra-operative imaging [ 11 ]. Though
there was some electric interference between the two modalities, the technique is
promising, although availability will most likely be very low.
Nikas et al. published an evaluation of the application of co-registered 2D ul-
trasound and MRI for intra-operative navigation [ 47 ]. Ultrasound based navigation
shows promising results due to live acquisition at high frame rates and easy portabil-
ity. For prostate brachytherapy a combination of ultrasound and co-registered CT can
be used, as shown by Fuller et al. [ 17 ]. Existing commercial products apply optical
tracking for intra-operative navigation during neurosurgery [ 71 ]. Figure 24.5 shows
how ultrasound and CT can be blended together into a single reference frame [ 7 ].
Registration can be divided into two different types: Rigid and non-rigid. Rigid
registration can be used to quickly obtain a registration between two modalities and
is suitable for rigid anatomies such as the skull. A common approach to register two
images is to search for the transformation which minimizes a difference function,