Computer Graphics

Intimately related to the issues of image processing are the techniques by which medical and biological images are dis­played with enough realism to achieve the intended results but with enough efficiency to be used in actual clinical situa­tions. Algorithms and programs for accurately portraying anatomy and, to some extent, function have improved stead­ily, sometimes exceeding the ability of the hardware to meet the demands. Fortunately, the well-known advances in per­formance and cost of advanced graphics hardware, including general-purpose computers as well as special-purpose graph­ics processors, have provided the platforms necessary for im­plementation of state-of-the-art graphics techniques.

The display of two-dimensional images is, in principle, straightforward on a computer output screen with multiple colors or gray levels per pixel. The display programs provide an interface between the user, the image, and the graphics hardware and software of the computer so that one pixel of the image is translated to one pixel of the video screen. Com­plications arise when there is a mismatch between the image and the screen, so that image pixels must be removed or dis­play pixels must be interpolated. A further complication for the developer of either two — or three-dimensional graphics software is the plethora of data file formats that exist (43). Fortunately, many public domain or proprietary software packages provide excellent format conversion tools, but some experimentation is frequently required to use them properly.

The development of methods for efficient and realistic ren­dering of three-dimensional images continues to be an area of ongoing research. Early work reduced anatomic structures to wire frame models (44), and that technique is still sometimes used for previewing and rapid manipulation on hardware that is not sufficiently powerful for handling full images in real time or near real time. Several methods require the identifi­cation of surfaces through image segmentation, as described above. The surfaces can be triangulated and displayed as es­sentially two-dimensional structures in three dimensions (45). After initial processing, this is a rather efficient display method, but much of the three-dimensional information is lost. Alternately, the image can be reduced to a series of volu­metric structures that can be rendered by hardware special­ized for their reproduction (46). One of the most realistic, but computationally expensive, three-dimensional rendering methods is ray tracing, in which an imaginary ray of light is sent through the structures and is attenuated by the opacity of the anatomic structures that it encounters along the way (47). Different effects can be emphasized by modifying the dy­namic range of the pixels in the image—that is, by changing the relationship between the opacity of the image and the pixel value to be displayed on the screen.

Figure 6. A composite of eight magnetic resonance and isochronal surface images from the second activation wavefront after an unsuccessful defibrillation shock. The electrical data were acquired from about 60 plunge needles with endocardial and epicardial electrodes inserted through the left and right ventricles of the heart of an experimental animal. Successive iso — chrones (left to right, top to bottom) are shown at 6 ms intervals. Visualization techniques that allow the superposition of function and anatomy are very helpful in understanding the relation­ships between variables and how they affect physiological mechanisms, and they can potentially lead to improved diagnosis and therapy. Reprinted from Ref. 49, with permission. Copyright CRC Press, Boca Raton, FL.

Medical computer graphics are at their most useful when it is possible to superimpose images from more than one mod­ality into a single display or to superimpose functional infor­mation acquired from biochemical, electrical, thermal, or other devices onto anatomical renderings. As an example of the former, images from positron emission tomography (PET) scans, which reflect metabolic activity, can be displayed on anatomy acquired by magnetic resonance imaging. The com­bination provides a powerful correlation between structure and function, but the technical challenges of registering im­ages from two different devices or taken at different times are significant (48). An example of the combination of functional and anatomic data is the superposition of electrical activity, either intrinsic or externally applied, of the heart onto realis­tic cardiac anatomy. This kind of technique can provide new insights into the mechanisms and therapy of cardiac arrhyth­mias (49). Figure 6 is a sequence of still frames from a video showing the progression of a wavefront of electrical activation across a three-dimensional cardiac left ventricle after an un­successful defibrillation shock.

Computer graphics and image processing, along with ad­vanced imaging technologies, are making a significant impact in medical knowledge and practice and have the potential for many more applications. A combination of traditional CAD/ CAM visualization and advanced imaging can be used for ef­fective assessment of quality of fit of orthopedic prostheses (50). Capabilities and functionality have increased dramati­cally with the advent of advanced graphics hardware and commercial software packages aimed at scientists and clini­cians who are not graphics experts. Full realization of the benefits of these systems will require further advances in these areas, along with adaptation to the needs of clinicians and the constraints of the changing health care climate (51).

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