Medical researchers apply high-speed imaging to simulate blood flow
Vascular surgeon uses a PC-based digital video system to measure blood-flow parameters in simulations of diseased vessels.
By Lawrence J. Curran, Contributing Editor
Dr. Timothy Liem, a vascular surgeon and assistant professor of surgery at the University of Missouri School of Medicine (Columbia, MO), has applied high-speed imaging to gain a better understanding of blood flow in an effort to combat the arterial disease atherosclerosis. He uses a digital video system to record images at a rate of 500 to 1000 frame/s. Then he analyzes the images to investigate the simulations of blood flow through arteries afflicted by the disease.
Atherosclerosis--a form of arteriosclerosis--is characterized by the deposition of plaques containing cholesterol and lipids on the innermost layer of the walls of large- and medium-sized arteries, resulting in impaired blood circulation. Liem`s department studies hemodynamics-the forces involved in blood circulation through arteries and branching blood vessels-using a flow-visualization system to help measure pressure differences and shear stresses at certain locations in blood vessels.
The high-speed imaging system, developed by Redlake Imaging Corp. (Morgan Hill, CA), is linked to the department`s flow-simulation apparatus for careful control of fluid pressure and flow rate. The fluid is either methyl salicylate or a 4% concentration of glutaraldehyde-fixed red blood cells. The PC-based imaging system measures the velocity, viscosity, and density of the fluid, and the computer calculates the shear stress at selected locations from that data. "Low-shear stresses and high shear-stress gradients are thought to be associated with atherosclerosis," Liem says.
In the elaborate digital video system, flow rates are rapid and high-speed imaging is essential to capture what is happening in the flow simulation for later slow-motion analysis. For the research by Liem, the University of Missouri department starts with either human or animal (often pig) arteries, applies formalin and glutaraldehyde to fix them, and then uses ethanol to dry the vessels.
The artery is then bathed in oil of wintergreen (methyl salicylate) to make it transparent and is attached to the flow-simulation apparatus in a backlit, glass specimen dish placed under a MotionScope PCI 2000S camera head from Redlake (see Fig. 1). Lastly, the artery is attached to a rigid metal frame for stability, and both the artery and frame are bathed in methyl salicylate within the glass dish (see Fig. 2).
A key ingredient for capturing the flow dy namics is the introduction into the fluid of polystyrene microspheres ranging in diameter from 50 to 250 µm. Capturing the course of the micro- spheres through the visualization system is the activity that enables the PC-based video imaging system to measure velocity, density, and viscosity, from which the important shear stresses are calculated.
The essential elements in the flow-visualization system are the Redlake Imaging MotionScope PCI 2000S system; a Redlake MotionScope PCI camera control/memory board residing in a 400-MHz Dimension XPS-R PC from Dell Computer Corp. (Round Rock, TX); Redlake`s MotionScope PCI application software, which provides camera-control and basic motion-analysis functionality; and a SpeedWriter compact-disk recordable drive supplied by Smart and Friendly (Chatsworth, CA). A standard boom stand holds the camera, while the specimen dish is backlit by a 250-W ProLight from Lowel-Light Manufacturing Inc. (Brooklyn, NY; see Fig. 3).
Motion Engineering Co. Inc. (MEC; Indianapolis, IN) is the system integrator--the company Liem first approached with his need for high-speed digital imaging. He was aware of the earlier work in hemodynamics performed by an investigator in Japan using 16-mm film. However, Liem became convinced that such an approach was cumbersome because the lighting and timing were intricate, and sometimes several days were required to develop the film to view results (see "Shedding light on motion analysis on p. 41").
Opting for a digital video solution, he found both Redlake Imaging and Eastman Kodak listed on the Internet as vendors of high-speed cameras. Liem eventually connected with Redlake through MEC, the Redlake reseller/integrator for his location. After evaluating equipment from both firms, Liem chose the Redlake system because the company`s software integrates true images and velocities to calculate the important shear strengths.
The MotionScope PCI monochrome digital camera head incorporates a solid-state high-speed shutter. Don Thomas, Redlake vice president of marketing, says that with the integration of cabling, PCI card, memory, and software, the PCI product series comprises a dedicated high-speed imaging system ready to plug and play in a personal computer. "Images are first captured by the camera and held in memory on the MotionScope PCI board. The images can be archived on the hard drive or on removable media, such as a Zip or Jazz drive, for later slow-motion analysis," he adds.
Conventional professional video cameras record at 30 to 50 frames/s. Thomas says that the MotionScope systems can record up to 8000 frames/s using shutter speeds to 10 µs and resolutions to 480 ¥ 420 ¥ 8-bit pixels per frame. Currently, Liem is recording at 500 to 1000 frames/s.
System-integrator MEC added the boom stand for Liem`s application. The boom stand comes with a horizontal arm that is attached to a mounting plate that can be adjusted to help fine-tune the position of the camera.
Liem usually records for two seconds, or about 2000 images at a time, as the microspheres traverse through the artery (see Fig. 3). The images are stored on the PCI card in volatile memory for immediate analysis in slow motion. They can also be stored as AVI files for later analysis. The MotionScope PCI system allows the use of a PC in this visualization system, which costs less than $20,000 instead of the $50,000 equipment often associated with university research.
Liem`s department did not alter the system after receiving it but is considering adding some software that will enable the taking of still photos, annotated with coordinates and velocity profiles, from the displayed screen images.
FIGURE 1. To investigate atherosclerosis, Dr. Timothy Liem, an assistant professor of surgery at the University of Missouri School of Medicine (Columbia, MO), uses a Redlake Imaging MotionScope PCI 2000S high-speed video system housed in a 400-MHz Dell Dimension XPS-R Pentium II PC tower (1). The camera head and C-mount lens (2) are connected to the PC tower by a PCI cable (3). The PC includes a Smart and Friendly 4X-write, 12X-read, compact-disk drive for image-storage purposes (4). A Lowel-Light ProLight is used for back lighting (5), and a boom stand (6) secures the camera head above a specimen dish that holds the artery (7).
FIGURE 2. Transparent artery setup shows the fluid and polystyrene microspheres flowing at high speed during a blood-flow simulation.
FIGURE 3. The MotionScope PCI high-speed imaging system is installed in the laboratory of the University of Missouri School of Medicine. The camera head is mounted above the specimen dish that holds the artery used in the blood-flow simulations of the dynamics of atherosclerosis.
Shedding light on motion analysis
Before adopting high-speed digital video imaging, vascular-surgeon Dr. Timothy Liem considered the use of 16-mm film to record and analyze simulated blood flow in his hemodynamics research at the University of Missouri School of Medicine (Columbia, MO). He soon became convinced, however, that it would be too slow to capture the rapidly moving polystyrene microspheres he tracks as they course through arteries in the blood-flow simulations.
His initial inclination toward film has a solid foundation in history. Film-based motion analysis was probably initially implemented in 1879 when California millionaire and former governor Leland Stanford hired photographer Edward Muybridge to conduct the famous running-horse experiment. Stanford and others questioned if a running horse ever assumed the posture often depicted in paintings: front legs thrust forward and rear legs backward with all four hooves off the ground. Stanford offered Muybridge his steed Occidental for the experiment.
Working in Palo Alto, CA, Muybridge set up 24 cameras in a long shed with controlled lighting. As Occidental ran the length of the shed, Muybridge triggered the cameras by breaking a string attached to each shutter. The resulting rapidly-recorded image sequence allowed viewers to observe, measure, and understand object motion that was too fast for the eye to perceive. The photos revealed that at one instant, all of Occidental`s hooves were indeed off the ground, but that the hooves were suspended underneath, rather than being thrust out in front and behind. Through this rudimentary motion analysis, Stanford verified running-horse movements.
In the 1970s, motion analysis came into a new era when high-speed video systems were launched with the introduction of enhanced-speed reel-to-reel tape systems. The video industry grew as the advantages of high-speed motion analysis became apparent, despite the equipment`s high price.
In the early 1990s, digital high-speed video systems were introduced. These systems recorded high-speed images into solid-state memory and eliminated many of the problems associated with the tape-based systems. But these systems were large and often priced at $50,000 to $100,000. Redlake Imaging Corp. (Morgan Hill, CA) introduced its MotionScope series of digital high-speed cameras in 1994. Then, in 1998, the company released its MotionScope PCI series, which sells for about $12,000, depending on the configuration and options, for use with personal computers.