Using a small, solid-state laser and digital particle image velocimetry (DPIV), our group at the University of California, Berkeley is investigating atherosclerosis, a leading cause of death in the United States (see Figure 1). By tracking fluid moving through a simulated artery, we hope to gain a better understanding of the complex blood flow patterns associated with atherosclerosis and how they influence the risk of stroke and other prevalent ailments.

Particle image velocimetry

Traditional instruments such as the hotwire and laser-Doppler anemometer are one-point measurement techniques, and thus do not reveal instantaneous spatial structures of blood flows. Particle image velocimetry (PIV) produces two-dimensional flow-field measurements rather than point measurements, giving this technique significant advantages over other methods for examining and quantifying blood flows.

In particle image velocimetry, a pulsed laser illuminates particles introduced into a fluid flow, causing them to either reflect light or fluoresce. A time-gated camera generates a sequence of images of the glowing particles, allowing researchers to track the motion and better understand movement within the fluid. Digital particle image velocimetry (DPIV) replaces film-based cameras and shutter systems with electronic imaging. The speed and efficiency of the digital approach allows scientists to effectively analyze a vast number of images, a prerequisite to studying complex blood flows.

Dual-head PIV Laser

Designed specifically for DPIV applications, the MiniLase PIV from New Wave Research (Fremont, CA) is a Q-switched, frequency-doubled, dual-head laser that produces 5-6 ns pulses (see Figure 2). Incorporating a potassium titanyl phosphate (KTP) doubling crystal, the laser has good conversion efficiency -- from 90 mJ of input at 1064 nm, the system produces 50 mJ of output at 532 nm. The output power is stable to ±4% for 95% of the shots.

The laser heads, each operating at 15 Hz, can be fired independently of each other with a user-controlled delay between pulses. This dual-head design is one of the most important benefits of the laser. In PIV, two successive images are captured, and the particles in one image correlated with the particles in the next. The elapsed time between two pulses from a single laser firing at 15 Hz is 67 ms. At typical flow velocities, the particles would move too far to track from one image to the next — in essence, PIV with such a laser would be a single-point method. The dual-head design is necessary to permit users to compare subsequent images with sufficiently short time delays to track particle motion.

The turnkey system can be operated internally via controls located on its remote control panel or through external TTL control. At roughly 14 in. x 6 in. x 3 in., and 10 lb., the laser head is compact enough to be easily transported around the lab, allowing researchers to perform DPIV analyses without having to move or disrupt the experiment site. The laser's portability also allows it to be mounted on a tripod near the experiment, reducing the risks in the lab that come from having to propagate a laser beam over a long distance.

Fluid flow in an artery

To model a partially obstructed human carotid artery, we built polycarbonate test section incorporating two symmetric, gaussian-shaped inserts that simulate an atherosclerotic lesion, creating an 80% blockage in the arterial flow. We inserted this test section into an adjustable flow-generation apparatus with a constant-head tank that produces steady flow through the test section. A linear driving system induces a pressure pulse that represents a human heartbeat (see Figure 3).

FIGURE 3: In the Berkeley PIV experiment, the laser illuminates a test section designed to mimic a partially occluded artery. A constant-head tank maintains desired flow rate and a linear drive imparts a pressure pulse that simulates the human heartbeat.

To study fluid flow, we first check the flow by injecting red dye into the test section above the occlusion. Next, we seed the flow field with 10-mm-diameter, silver-coated glass spheres, and illuminate them with the PIV laser. Mirrors direct the laser beam to the test section and subsequently through a cylindrical lens to create a sheet of laser light with a divergence of less than 4 mrad.

A 480-x-1134-pixel charge-coupled device (CCD) camera captures the glowing, scattered-light particles as they move, and records the images on a laser video disc at 30 frames per second (see Figure 4). The images are then digitized and stored on a computer, then processed to yield velocity measurements.

We perform DPIV measurements downstream from the constriction to determine regions of separation, and magnitude and direction of wall-shear stresses. Software processes the data, dividing the images up into small regions and analyzing them to produce a velocity vector in each region. Once the entire velocity field is constructed, we can determine vorticity and wall-shear-stress measurements to better understand the physics of arterial flow .

Digital particle image velocimetry experiments such as the ones underway at the University of California, Berkeley are crucial to understanding blood flows. The resultant flow visualization data are beneficial for identifying vortex generation, jet structures, regions of flow recirculation and the effects of pulsatility. Small, solid-state lasers play an important role in bearing this information and promise to help scientists gain an enhanced comprehension of atherosclerosis.