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Processing and visualizing three-dimensional ultrasound data

¹ Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ and ² Department of Radiology, University of Cambridge, Addenbrooke's Hospital, Cambridge CB2 2QQ, UK

View larger version (17K): [in a new window]   Figure 1. 3D ultrasound acquisition protocols. Dedicated freehand probes contain motors that sweep a 2D B-scan over the area of interest: rotational and fan shaped geometries are common. Freehand systems rely on the clinician to guide a conventional 2D probe over the area of interest. The relative positions of the B-scans can be tracked using an add-on spatial locator or, alternatively and less accurately, estimated from the B-scan images themselves.

View larger version (18K): [in a new window]   Figure 2. Sampling and interpolation. The diagram shows the intersection of two B-scans and a reslice plane with a voxel array. The circles represent individual pixels on the various planes. Consider the shaded voxel. If nearest neighbour interpolation were used to construct the voxel array, the intensity of this voxel would be set according to the nearby pixel on B-scan n. Again assuming nearest neighbour interpolation, the same intensity would be attached to the reslice pixel which lies entirely inside this voxel. With direct visualization, however, the intensity of the reslice pixel would be inherited from the nearby pixel on B-scan n+1. Thus, the intermediate voxel representation causes visualization artefacts, which are avoided by direct rendering.

View larger version (20K): [in a new window]   Figure 3. Direct reslice rendering. For nearest neighbour interpolation up to a maximum distance d, we consider a box of width 2d centred on each B-scan, and calculate the intersection of this box with the reslice plane. Each reslice pixel within the intersection polygon is shaded according to the intensity of the corresponding B-scan pixel. The process is repeated for all the B-scans in the data set, with a z-buffer ensuring that only those B-scans pixels closest to the reslice plane contribute to the final rendering.

View larger version (136K): [in a new window]   Figure 4. Direct 3D ultrasound visualization examples. This 3D ultrasound data set comprises 652 B-scans like the one at the bottom left of the figure. The planar and non-planar reslices are interpolated directly from these B-scans, using nearest neighbour interpolation. In this example, each "reslice" is in fact a narrow volume rendering of width 3 mm: this makes the resulting image far less dependent on the precise positioning of the plane or surface, and usefully highlights bony structures in this obstetric example. The B-scan display shows the intersection with the reslice volume, while the planar reslice display shows the intersection with both the B-scan plane and the non-planar reslice volume. All three displays are shown together in the multiplanar/non-planar reformat, in their correct relative positions. This display can be rotated interactively to give a better impression of the 3D geometry.

View larger version (32K): [in a new window]   Figure 5. Cardiac gating for Doppler 3D ultrasound visualization. For this examination of the popliteal artery, the cardiac pulse was estimated by simply counting the number of coloured pixels in each B-scan. After tagging each B-scan with its cardiac phase, meaningful visualizations can be achieved, at whatever phase, by selecting only those B-scans with matching phase. Movies can be constructed from sequences of renderings, like the ones above, at progressive phases.

View larger version (54K): [in a new window]   Figure 6. Artefacts in (a) 2D and (b) 3D ultrasound. The context afforded by a B-scan, in particular the known direction of insonification, allows many artefacts to be spotted by the expert eye. The disorientation of a 3D reslice makes it far harder to distinguish artefacts from true anatomical features.

View larger version (46K): [in a new window]   Figure 7. Interpolating a surface through multiaxial contours. A distance field is calculated on the plane of each contour, encoding the shortest distance from each point on the plane to the contour. The shape of each contour is represented by a set of maximal disks [28]. Correspondences between these disks are used to define interpolation directions between the planes [8]. The distance field is interpolated in these directions: the zero-crossings of the resulting 3D field define the surface. The regularized marching tetrahedra algorithm [29] provides a triangulation of the surface, which can be visualized using standard surface rendering techniques. The enclosed volume is readily calculated from the triangular mesh representation [9].

View larger version (60K): [in a new window]   Figure 8. Inter-B-scan registration for a high resolution scan of the forearm. Pairwise registration of neighbouring B-scans suppresses artefacts caused by varying probe contact pressure. To avoid drift in this accumulated registration process, a post-processing step warps the entire data set to ensure that the first and last B-scans are in their correct positions as indicated by the position sensor [20].

View larger version (49K): [in a new window]   Figure 9. Combined inter-B-scan and intersweep registration. The figure shows reslices through a data set of the human breast, part of an experimental radiotherapy planning protocol following lumpectomy. The uncorrected reslice exhibits probe pressure and respiratory artefacts, as well as a clear misregistration between the two sweeps. Both types of artefact are effectively removed by the combined registration process.

View larger version (94K): [in a new window]   Figure 10. Comparison of corrected 3D ultrasound data with CT data. A reslice through the breast data is shown superimposed on a corresponding reslice through CT data of the same breast, acquired at the same time. The figure reveals excellent correspondence between the corrected ultrasound data and the non-contact CT image. The area targeted for radiotherapy is outlined in white.