Medical Device Link.

Originally Published MEM Fall 2003

MANUFACTURING

New x-ray technology offers medical electronics manufacturers a high-resolution, high-sensitivity imaging option for inspection of electronics and more.

Gil Zweig

Real-time x-ray inspection has gained worldwide use as a rapid and effective tool for quality assurance in highly technical industries such as electronic circuit manufacturing. However, it has not yet been widely used in medical device manufacturing, for two primary reasons: the material characteristics of medical devices and the limitations of traditional real-time x-ray detectors. Many critical medical devices are fabricated from rubbers, plastics, and ceramics. These materials have low x-ray absorption and consequently do not lend themselves to most real-time x-ray inspection methods. Such materials must be x-ray imaged with low settings of the x-ray tube anode voltage to obtain x-ray absorption sufficient to yield a satisfactory image.

Most real-time x-ray imaging detectors do not respond to these low voltages, in either resolution or gray-scale accuracy, particularly when the devices are placed close to the detector. Film-based x-ray imaging systems do have the accuracy required, but these have been precluded from use because of the time required to obtain an x-ray film image.

Traditional X-Ray Detectors

Figure 1. Typical x-ray detectors: a cesium iodide image intensifier, a linear-array detector, and a flat-panel detector. (click to enlarge)

Real-time or fluoroscopic x-ray inspection systems are found in a number of applications, including food processing, mail security, and electronic component quality assurance. The detectors fall into one of three categories: cesium iodide image intensifiers, linear-array detectors, or flat-panel detectors (see Figure 1). Because the intrinsic resolution of these detectors is relatively low, the detectors require geometric magnification (positioning the device close to the x-ray source) to see fine details.

Additionally, these detectors do not image well at the lower voltages necessary to inspect materials with low radiographic density, such as those frequently used in medical device fabrication. This is most likely the reason that many device manufacturers have not readily adopted such inspection as part of a quality assurance program.

Real-Time System Requirements

Many critical devices are composed of materials with low radiographic density, such as polymers. Even within the category of polymers, there is significant variation in radiographic density among different materials of the same thickness.

Figure 2. A radiographic resolution target (left) and an x-ray image of resolution target (right). (click to enlarge)

X-Ray Image Resolution. X-ray image resolution, defined in terms of line pairs per millimeter (lp/mm), describes the x-ray image of a resolution target, an example of which is seen in Figures 2a and 2b. The resolution of the image is then expressed in terms of how many line pairs can be observed clearly in one millimeter of space on the x-ray. The intrinsic resolution of an x-ray detector is the resolution achieved when the target is placed in contact with the detector itself. The three x-ray detectors mentioned previously each demonstrate an intrinsic resolution of approximately 3 lp/mm.

MXRA X-Ray Imaging Technology

Figure 3. MXRA x-ray camera.

MXRA x-ray imaging technology describes a proprietary x-ray detector design that provides a significantly higher resolution (10 to 20 lp/mm) than traditional systems are capable of producing. The higher resolution results from unique design of the phosphor scintilator and optics used in the MXRA (see Figure 3). At the same time, this design responds to the low x-ray voltages, in the range of 15 to 30 kV, which are required to image low-density materials. A particularly valuable feature of this technology is the ability to position medical devices in close proximity to the camera surface, either manually or on a conveyor belt. This feature supports compact and cost-effective system design.

MXRA technology has been shown to be an effective quality assurance tool in a number of medical device applications, including packaged surgical staples, molded catheters, ceramic components, and electronic circuits.

Figure 4. Surgical staple cartridge (left). MXRA x-ray image showing missing titanium staples (right).

Packaged Surgical Staples. When used in the production of packaged surgical staples, for example, real-time x-ray inspection can reveal whether all the titanium staples in a surgical stapler cartridge are in place after the cartridges have been packaged (see Figure 4). The low atomic weight of titanium makes its detection more difficult than other metals and, for this reason, MXRA technology has been particularly effective.

Molded Catheters. Uniform blood-flow passages in catheters are critical. Any deformation in the passageway provides a stagnation region where blood can clot. Real-time x-ray inspection of the molded portions of the catheter can determine whether unacceptable deformation has taken place during the molding process.

Ceramic Components. Ceramic components have found widespread use in medical device manufacturing. Many ceramic compounds exhibit low x-ray absorption properties, making x-ray inspection difficult. MXRA x-ray technology is particularly well suited to ceramics due to its high sensitivity at low x-ray voltages. As a result, defects such as cracks can be readily detected.

Figure 5. X-ray image of a seal around electronic connector pins.

Electrical Connector Pins. Connector pins can be difficult to inspect using traditional x-ray technology. Real-time x-ray inspection identifies problems such as voids in the seal material. Such voids could allow gas to seep past the connector pins (see Figure 5).

System Design

Figure 6. Image magnification between 3 and 20 times.

An additional benefit of this technology is its capacity for internal optical magnification (see Figure 6). With this feature, the system becomes, in effect, an x-ray microscope with variable magnification. Because of the detector's high resolution, a medical device does not have to be moved toward the x-ray source to achieve geometric magnification. Instead, magnification can be achieved electro-optically.

Figure 7. A manual MXRA x-ray inspection system (left) and a conveyorized MXRA x-ray inspection installation (right). (click to enlarge)

Typical Configurations. With high-resolution, high-sensitivity x-ray detection capability and the ability to achieve magnification without having to move the medical device toward the x-ray source, a greater range of system design options becomes available. For example, desktop-sized systems can be used for manual positioning and inspection of devices. In addition, a compact system design is easy to customize for conveyorized inspection (see Figure 7).

Conclusion

With the introduction of new MXRA x-ray camera technology and its high-resolution, high-sensitivity x-ray imaging capabilities, medical electronics manufacturers now have a rapid and effective way to ensure the quality of sealed devices and packages. The technology has been found to be effective in detecting defects in catheters, surgical staples, ceramics, precision moldings, and electronic circuits. The camera design lends itself to compact, cost-effective, and flexible inspection system designs.

Gil Zweig is president of Glenbrook Technologies Inc. (Randolph, NJ). He can be reached at 973-361-8866.

Copyright ©2003 Medical Electronics Manufacturing