The reference section contains a list of many visualization publications documenting and describing historic developments, fundamental algorithms and systems, advanced visualization paradigms, and a variety of useful biological and clinical applications. Biomedical image visualization involves the transformation and presentation of and interaction with multidimensional medical and/or biological image data sets. In discussing this subject, it is instructive at the outset to provide some definitions, especially to differentiate between the commonly used terms "imaging" and "visualization."
3D imaging primarily refers to acquiring digital samples of objects distributed throughout 3-space, i.e., in x y, z dimensions, usually but not necessarily with isotropic spacing (equal separation in all three directions). The term is often generalized to include processing, displaying, and analyzing such 3D data sets (however, 3D visualization is a better term for the gestalt). A 3D image or 3D imaging process can sometimes (not always) be synthesized by appropriate successive 2D steps, but ideally the image is acquired simultaneously in three dimensions and the imaging process (function) is applied congruently in three dimensions.
Multimodal imaging generally refers to the use of different imaging systems (e.g., CT, MRI, PET) to acquire images of the same object (e.g., a patient brain), providing complementary and more complete information about the object than can be obtained from any single image type (unimodal); the term may also be used to describe a spatiotemporal "fusion" (integration, combining) of images of the same object obtained from different imaging systems, determined by spatially and/or temporally registering the different images with sophisticated mathematical algorithms so that their individual samples all align in space and/or time.
Real time visualization, in computer display applications, implies a frame refresh/update rate sufficiently high to avoid perception of "jerkiness" or stutter (conversely, a "smooth" display) and is generally accepted to be 15-30 frames per second. This means that the display system must compute and display each complete new view in approximately 75 msec or less. In data collection, generally video rates are considered real time, i.e., 30 frames/sec.
Interactive visualization refers to sufficiently high response time and repetition rate of the system that senses a user action of some type (e.g., mouse movement, key press, wand motion) and computes a corresponding result (e.g., updating the view on the screen) so that the user will perceive (near) instantaneous response to his/her actions. This generally requires a response/repetition rate of 10-20/sec. However, interactivity is dependent on the application or procedure, i.e., higher response rates are needed for highly dynamic situations (for example, catheter positioning) and lower rates for more static activity (e.g., tumor approach).
3D visualization generally refers to transformation and display of 3D objects so as to effectively represent the 3D nature of the objects. Such displays range from shaded graphics in 2D display devices (sometimes referred to as 2 1/2-D), to stereoscopic-type displays requiring the aid of special glasses, to autostereoscopic and/or holographic 3D displays requiring no physical aids, to "immersive" displays that project the viewer "into" the scene, such as in virtual reality environments. But the term visualization as used in computer imaging also explicitly includes the capability to manipulate and analyze the displayed information. Additionally, this term implies inclusion of cognitive and interpretive elements.
Interactive visualization (near real time) and advanced display technologies (3D and beyond) open new realms into the practice of medicine by permitting the images obtained from modern medical imaging systems to be directly displayed and manipulated with intuitive immediacy and with sufficient detail and speed so as to evoke sensorial experience similar to that of real experience. Such interactive 3D environments allow physicians to "enter the visualizations" to take up any viewpoint, to see dynamic functional processes as well as detailed anatomy, to make accurate on-line measurements, and to manipulate and control interventional processes. The value of such visualization technology in medicine will derive more from the enhancement of real experience than from the simulation of reality [12,55].
Visualizable objects in medicine extend across a vast range of scale, from individual molecules and cells, through the varieties of tissue and interstitial interfaces, to complete organs, organ systems, and body parts, and include functional attributes of these systems, such as biophysical, biomechanical, and physiological properties [8,28,30,37,47,54,72,75]. Medical applications include accurate anatomy and function mapping, enhanced diagnosis, and accurate treatment planning and rehearsal. However, the greatest potential for revolutionary innovation in the practice of medicine lies in direct, fully immersive, real-time multisensory fusion of real and virtual information data streams into an online, real-time visualization during an actual clinical procedure. Such capabilities are not yet available to the general practitioner. However, current advanced computer image processing research has recently facilitated major progress toward fully interactive 3D visualization and realistic simulation. The continuing goals for development and acceptance of important visualization display technology are (1) improvement in speed, quality and dimensionality of the display and (2) improved access to the data represented in the display through interactive, intuitive manipulation and measurement of the data represented by the display. Included in these objectives is determination of the quantitative information about the properties of anatomic tissues and their functions that relate to and are affected by disease. With these advances in hand, there are several important clinical applications possible to be delivered soon that will have a significant impact on medicine and study of biology.
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