Viewing 2D and 3D representations side-by-side
Navigating in real time
Arbitrary viewing angles and slices
The Volume Browser runs in two parts, the client and the server. The interactions between the two are unique, a hybrid between a central server and a stand-alone system. This hybrid relationship provides high quality 3D image data (larger than the client could handle on its own) with to up to 40 different connections at once.
Figure 1: Diagram of hybrid client-server relationship
The client is a small application that installs on your computer. The PSC has developed clients for SGI IRIX, Sun Solaris, Linux, Windows 95, Windows 98, Windows ME, Windows 2000 and Mac OS X. The client communicates with server over the internet to display the 3D images stored on the server, and allowing you to change your view interactively.
The Volume Server stores the large volumetric datasets in its memory and provides the client only with the image data it needs. The image data is transmitted first at coarse resolution, then fills in with higher resolution as motion and view adjustment slow.
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PVB has two main windows � the left window displays the 2D slice information while the right window presents the 2D slice in the proper scale and orientation within the 3D context. The 3D display includes anatomical structure surfaces, including the overlying skin. The intersection of surfaces and the slicing plane can be viewed in both the 2D and 3D windows. Surfaces, represented as meshes, are retrieved for display via HTTP.
Figure 2: An example from the Visible Human Project. The PSC Volume Browser window displays slice in detail, while VB Context window displays the slice within the body's shape.
The Volume Browser allows you to interactively view an image by rotating and slicing the image, and zooming in and out. Unlike other software capable of providing volume images, the Volume Browser allows you to change views in an instant- no waiting for images to reload or new angles to be calculated. Real time navigation is made possible by the client-server relationship and the coarse-to-fine development of image resolution.
The Volume Server operates exclusively from a pre-constructed hierarchical memory-resident data structure based on 8*8*8 volume cubes, with the finest cube formed by an 8*8*8 set of voxels, the next level representing 8*8*8 lowpass-filtered half-resolution voxels, and so forth to coarser and coarser levels. The data on the volume server is maintained at four levels of detail, corresponding to 2x zoom scales from 1 to 1/16th resolution. The client provides continuous zooming by sampling and interpolating from these base resolutions.
Figure 3: The server provides a coarse resolution image immediately, and then refines the image as real-time view adjustment slows.
The Volume Browser allows you to view the volume from any angle, and slice through the image to view an angled surface across the interior of the image. There are three standardized viewing angles: sagittal (from the side), coronal (from the front), and transverse (from above). The Volume Browser allows you to move from these standard views to best explore the image.
The Volume Browser provides tools for user-controlled segmentation and labeling, which are used by expert anatomists to trace and capture anatomical structures. For example, points and Catmull-Rom splines can be added to the volume and viewed in conjunction with slice data. Arbitrary slice navigation allows you to create segments in the natural plane of an object rather than the plane defined by the original imaging technique. Multiple splines may be placed in the volume and saved to disk.
Figure 5: An example from the Visible Human Project. The PSC Volume Browser window shows an sagittal view showing a spline in green created around the liver. The VB Context window shows the viewable slice in relation to the body and the segemented liver and kidneys.
The PSC Volume Server (PVS) is responsible for efficiently and dynamically delivering volume data for interactive, uncorrelated navigation by up to 40 simultaneous users. The ability to hanlde 40 users at once is a result of compression and the client-server relationship.
The Volume Browser can be remotely and interactively driven using a collaborative mode that uses standard socket connections to a collaborative server. In collaborative mode one browser acts as the master to send a series of commands to the other members of the group, which then perform these commands, allowing demonstrations to a distributed group of users — for example, instructors interacting with students distributed across a laboratory or at separate sites.
Collaborative viewing of the National Library of Medicine's Visible Human data has been used in anatomy classrooms. In this setting, groups of anatomy students simultaneously referred to different parts of the Visible Human data to help with their studies of actual cadaver anatomy. This project was done in collaboration with Dr. Brian Athey and his team at the University of Michigan. The actual Visible Human data was created by Vic Spitzer and his team at the University of Colorado's Center for Human Simulation.