APPLICATION NOTE

Stent wall thickness
measurement
Stents must be extremely well manufactured and free of critical defects.
Inspection of such devices, leading to their acceptance or rejection, is a
critical step in the overall manufacturing process. One of the most tricky
inspection tasks is the wall thickness measurement. Such thickness is a
critical dimension that has to fit into a tight tolerance for an acceptable stentthat will be implanted into a human body.
There are two methods that have been used so far to measure the wall
thickness of the stents. The most widely used method consists of imaging
the sidewall with a low magnification microscope. Ideally, the sidewalls
must be placed perpendicularly to the optical axis so that the entire wall
thickness can be displayed within the field of view. However, this technique
has two drawbacks. The main one is that it is difficult to determine precisely
where are located the limits of the sidewalls of a polished stent, because theycorrespond to the rounded edges of the struts. The second disadvantage isthat, because of the cylindrical shape of the stents, there are many positions
in which the upper struts hide the sidewalls that should be measured.

Figure 1. Standard optical method used so far to measure the wall thickness of a stent.

APPLICATION NOTE STENT WALL THICKNESS MEASUREMENT

1The second method consists of inserting the stent in a mandrel that fits toits inner surface. Then a sensor is used to measure the wall thickness as theheight difference between the surface of the mandrel and the outer surface ofa contiguous strut. Such sensor may be either a contact tip or a single-point
optical probe. This second method has also two drawbacks. The main one is
the gap that may arise between the mandrel and the inner surface of the stent
due to a bad fitting of the mandrel size or a local bending of the strut whose
wall thickness is being measured. This effect always leads to an overestimationof the wall thickness. The second disadvantage of this method is that it is very
slow since the measurements are point-to-point and, as a result, only very few
positions can be measured within a reasonable inspection time slot.

BackIllumination

Figure 2. Diagram of the new optical technique for the acquisition of the unrolled

sidewall images, showing the measurement position and the illumination setup

Sensofar Medical has developed a new optical technique for the measurementof wall thickness of stents based on the acquisition and analysis of unrolled
images of the sidewalls. These images are obtained by focusing the opticalhead at a laterally displaced position in which it is possible to image a certain
amount of the cross section of the sidewalls, as it is shown in Fig. 2. The
illumination system is a proprietary setup that combines back and side lightsources. We have also developed a new geometrical model that takes into
account the effect of the rounded edges of the polished struts on the wall
thickness measurement.
This new method makes it possible to obtain wall thickness results alongcontinuous regions of the sidewalls acquired in the unrolled images. In addition,
it is not necessary to use a mandrel for acquiring the unrolled images, but even
if a mandrel were used, the wall thickness results would be insensitive to theexistence of gaps between the mandrel and the inner surface of the stent.
The new method has been tested with different models of stents currently
available in the market.
APPLICATION NOTE STENT WALL THICKNESS MEASUREMENT2

■ Measuring wall thickness and edge radius using Sensofar Medical Q sixsystem

The unrolled image of the sidewalls is acquired by focusing the optical headof the Q six on a fixed measurement point in which the radius of the stentforms an angle α with the vertical axis z and at the midpoint of the sidewall,as shown in Figure 2.
The unrolled image of the sidewalls is obtained in the same way as the unrolled
images of the outer and inner surfaces. The roller stage of the Q six rotates thestent at a calibrated speed, while the camera is acquiring single-pixel lines at
a fixed frame rate. The resulting unrolled images are built by setting all theselines top-bottom together.
These unrolled images show green pixels when there is no strut blocking
the back illumination. The position A in the unrolled image is acquired when
the outer surface of a strut reaches the position of the optical axis position.
Continuing the rotation of the stent, position B is acquired when the edgebetween the outer surface and the sidewall crosses the optical axis. From thatpoint, the acquisition of the sidewall begins. Stent rotation and line acquisition
continue until the bottom part of the sidewall is reached. This position, where
the edge between the sidewall and the inner surface of the stent reaches theoptical axis, is defined as the C position. In addition, when the unrolled imageof the sidewall of an electro-polished stent is acquired, a bright stripe appears
in the sidewall region. We refer to this bright stripe as “Halo” and it is produced
by a direct reflection of the side illumination on the rounded edge of the stent.

CA
HALO

HALO

BA

EDGE
RADIUSBCWALL
THICKNESS

Figure 3. Unrolled sidewall image obtained with the Q six. The analysis of thedistribution of the grey levels in these images (red line in the left image) allows todetermine the limits of the strut (points A and C), the transition between the outersurface and the sidewall (point B) and the position of the Halo, from which the radiusof the edge can be obtained.

APPLICATION NOTE STENT WALL THICKNESS MEASUREMENT3The unrolled images of the sidewall obtained from the lines acquired between
positions B and C contain the information about the thickness of the strut.
Therefore, the upper position B and the lower position C should appear wellfocused in that unrolled image. Figure 4 shows the depth of focus required in
the optical system to obtain a well-focused unrolled sidewall image.

Figure 4: Acquisition of the sidewall (left image) and the required depth of focus (rightimage) of the optical system to obtain well focused B and C transitions

The unrolled images of the sidewalls are processed to detect the transitions
that define the limits of the strut (points A & C), the transition from outer
to lateral surface (point B), and the position of the Halo. The wall thickness
and the edge radius are accurately determined from the relative positions ofthese points using a proprietary geometrical correction model that takes into
account the effect of the rounded edges of the polished struts.
The results of wall thickness end edge radius are obtained and displayed ina few seconds all along the sidewalls or in predefined regions of interest, as
it can be seen in Fig. 5. Automatic inspection routines are used to set themeasurement positions. Summary tables with numerical results are always
available for quick navigation through the inspection results, reducing theAcceptance/Rejection decision time and increasing inspection throughput.

APPLICATION NOTE STENT WALL THICKNESS MEASUREMENT

4Figure 5. Wall thickness results obtained with the Q six in predefined regions of interest

After proper calibration, the wall thickness results obtained with this new
optical technique will be accurate, assuming that the following conditions aremet:■ The sidewall is radial■ The shape of the edges is circular■ The radii of the outer and inner surface edges are very similar
Any deviation from the above conditions should be checked by measuring the“True Thickness” of the struts using the 3D topography measurement capabilityof the Q six and setting a “Radius Factor” value that matches the wall thickness
result to the true value within a maximum deviation of 1 micron.

■ Conclusions
The new optical technique developed by Sensofar Medical has been validatedwith different real stent models. The wall thickness results obtained have
proven to be repeatable and accurate enough to meet the GR&R standard
requirements of stent manufacturers. The new technique is extremely fast andreliable and clearly outperforms existing methods that have been used so far
to measure the wall thickness of stents.

APPLICATION NOTE STENT WALL THICKNESS MEASUREMENT

5SENSOFAR is a leading-edge technology
company that has the highest quality standards
within the field of surface metrology
Sensofar Medical provides state-of-the-art technology for theinspection of implantable medical devices and components as wellas leading-edge solutions for R&D worldwide, with each systemdesigned to incorporate the highest quality standards within the field.
Sensofar Group’s Headquarters is located in Barcelona, also known asa technology and innovation hub in Europe. The Group is represented
in over 30 countries through a global network of partners and has its
own offices in Asia, Germany and the United States.
HEADQUARTERS

SENSOFAR MEDICAL | BARCELONA (Spain) | T. +34 93 700 14 92 | info@sensofar.com
SENSOFAR ASIA | SHANGHAI (China) | T. +86 021 51602735 | info.asia@sensofar.com
SENSOFAR USA | NEWINGTON (USA) | T. +1 617 678 4185 | info.usa@sensofar.com

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Copyright © 2020 SENSOFAR MEDICAL. All rights reserved. The information in this publication is based on SENSOFAR’s internal research and knowledge at the time of printing and is subject to change without notice. Appearance of products may vary.

AN00-05A-EN-Stent Side Walls Q six

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