Heart of the matter

Balloon angioplasty provides cardiologists with a means to treat blockages of the arteries caused by coronary heart disease.

Jan 1st, 2008
Th Cholton

Machine vision inspects the surfaces of medical stents

Conard Holton

Balloon angioplasty provides cardiologists with a means to treat blockages of the arteries caused by coronary heart disease. To perform the procedure, cardiologists insert a long, thin tube with a balloon-tipped catheter into the artery. After the catheter reaches the blocked artery, the balloon is inflated, pushing any plaque against the artery wall. Because the artery wall may become weakened by the procedure, a small metal tube or stent is used to stop any reblocking of the artery.

These stents are composed of expandable wire forms or perforated tubes that are often coated with a drug. While these so-called drug-eluting stents are similar to conventional stents, their chemical coating requires a consistent and even distribution of the drug on the stent.

“Because of the critical nature of these devices,” says Markus Tarin, president and CEO of MoviMED (Irvine, CA), “the U.S. Food and Drug Administration (FDA; Rockville, MD) mandates 100% inspection. This procedure, currently being performed using stereomicroscopes is, of course, subjective and subject to operator fatigue.” Manual inspection is made more difficult because each stent is fragile, flexible, and highly reflective. Because of the high magnification required, the operator is only presented with a limited depth of field so that only approximately one third of the image is in focus at any particular time. To properly inspect each stent, 100% surface inspection of both the inside and outside diameter must be made. “Normally,” says Tarin, “this process requires between 140 and 160 different images to be analyzed.”

Tasked with automating this inspection process, Tarin and his colleagues considered using either linescan or area-array cameras. “While the use of area-array cameras would have resulted in easier stent-part fixturing,” says Tarin, “more than 170 partially focused 2-D images would need to be captured-a process that could have taken longer than 10 seconds.” Using a linescan camera, however, results in a single-image acquisition time of less than 1 s at the expense of a more-complex part fixturing.

Mandrel movement

In the system designed by MoviMED, each individual stent is mounted on a custom glass mandrel. This is used to keep the stent in focus as it is imaged by the linescan camera and also acts as a backlight for the imaging system. “Since the field of view (FOV) needed to be a maximum of 35 mm and the smallest feature to be resolved was 5 µm,” says Tarin, “the minimum camera resolution required was 35 mm/5 µm or 7000 pixels. For this reason, MoviMED chose an 8k × 1 Camera Link linescan camera from Dalsa (Waterloo, ON, Canada) to capture images of the stent as it was rotated on the mandrel. With an FOV of 35 mm, this camera provided a minimum feature measurement of 35 mm/8192 or 4.3 µm/pixel.”

Because stents have highly reflective, specular surfaces, composed of electro-polished stainless steel, and each stent is rarely straight to within ±50 µm, illumination and choice of the correct lens played a critical role in the system design. The specular nature of the stent surface means that it must be illuminated by a highly diffused and multidirectional light. To do this, a custom-developed illumination source was used. This enabled the system to differentiate specular, diffuse, or absorptive features on the surface of the stent.

Similarly, because the size of the image sensor of the Camera Link camera was 57.34 mm, a large-format lens was required. With a field of view of 35 mm, the primary magnification required for this lens was 57.34/35 or 1.68. Because each stent can move within ±50 µm within the FOV of the camera, there is an associated magnification change when conventional lenses are used. For this reason, Tarin and his colleagues chose a telecentric lens from Edmund Optics (Barrington, NJ) to substantially reduce such magnification effects.

As images are captured, they are buffered by the frame grabber and transferred to the host CPU where they are reformatted as single images using NI LabVIEW software. Complete images of a single stent are then displayed to the operator on a high-resolution LCD. With such a system, operators are relieved of using tiring, microscope-based methods, and aging baby-boomers are relieved of worry about less-than-perfect medical devices implanted in their arteries.

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CONARD HOLTON is editor in chief of Vision Systems Design; e-mail: cholton@pennwell.com.

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