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Medical Ultrasonic Imaging: Using Carbon Graphite Beyond Mechanical Applications

Carbon graphite’s mechanical properties are unparalleled by any other commercially available material. Its ability to self-lubricate, combined with its incredible oxidation resistance and mechanical strength has undoubtedly made it one of the most versatile engineered materials on the market.

In fact, carbon graphite is so versatile that it is not restricted to solely mechanical applications like shaft bearings or mechanical seals. The use of carbon graphite as an acoustic matching layer in transducers for medical ultrasonic probes is a prime example of how this material is useful in unique, non-mechanical applications.


Medical ultrasonic probes are used to construct images of tissues and organs within the human body in a safe and controlled manner. Unlike alternative imaging techniques such as X-rays or CT scans, ultrasound imaging poses no risk of harmful ionizing radiation exposure and can produce images in real-time. Ultrasonic imaging is commonly used to visualize the heart, assess bone strength, monitor fetal growth, view lung tissue… the list goes on.

As the technology backing ultrasonic imaging progresses, more and more ultrasound applications are being made available to medical professionals. This in turn fuels even further technical advancement, resulting in the rapid technological growth we currently see in the field.

So where do the benefits of carbon graphite factor in? No rubbing occurs in ultrasonic transducers, so carbon graphite’s self-lubricating ability isn’t necessarily beneficial. The probes do not see extreme temperatures and they are not exposed to caustic chemicals, so carbon graphite’s oxidation and chemical resistance don’t come into play.

Instead, the density and thickness of the carbon graphite is what matters, since these two properties directly influence the material’s acoustic impedance. The acoustic impedance is the resistance to movement that an ultrasonic wave experiences as it passes through a certain material. In order to understand why acoustic impedance is an important property, we must first take a look at the basic operating premise of medical ultrasonic probes.

How Medical Ultrasonic Probes Work

  1. The probe is pressed against the patient’s skin, pointing towards the area of interest.
  2. An ultrasonic wave is transmitted from the piezoelectric transducer (which converts electrical charge into ultrasound). The backing material ensures the ultrasound is directed towards the area of interest.
  3. The ultrasound travels through the carbon graphite matching layer. The matching layer has an acoustic impedance that is about halfway between the impedance of the piezoelectric transducer and the impedance of human skin. This helps to minimize the amount of reflection at the interface between the probe and the patient, thus maximizing the transmission of ultrasound into the patient.
  4. At the area of interest, some ultrasonic energy is reflected back to the probe, and some is refracted/attenuated. 
  5. The reflected wave travels back through the carbon graphite matching layer, which also serves to reduce reflection of ultrasound coming back into the probe and thus, maximize the transmission of ultrasound back into the probe.
  6. The transducer collects the reflected ultrasound and, with the aid of a computer, generates an image from the reflected ultrasonic waves.



The carbon graphite matching layer optimizes the transmission of ultrasonic energy to and from the probe, resulting in a clearer image. This optimization is nearly entirely dependent on the acoustic impedance of the material. Medical ultrasound probe manufacturers can alter and optimize this impedance not only by changing the thickness of the matching layer, but also by experimenting with different grades of carbon graphite. By impregnating with different materials and experimenting with different base carbons, manufacturers are able to change critical material properties that affect impedance, namely density.

Since the final density of the material is dependent on both the base carbon type and the impregnation type, impregnated carbon graphite can be manufactured in a very wide range of impedances. This gives ultrasonic probe manufacturers a lot of flexibility in their design. As a result of the wide range of impedances available, a big effort has been made in recent years to characterize and optimize the performance of different carbon graphite grades in ultrasonic applications.

Development of medical ultrasonic probes has progressed in leaps and bounds due in part to the optimization of matching layers in the probe’s design. Experimentation with new grades of carbon graphite has yielded impressive results which ultimately have a direct impact on many lives. The advancement of this technology means that doctors will be able to more effectively detect, diagnose, and treat any number of ailments and save lives in the process.

Metcar creates custom carbon graphite parts and materials for a wide variety of applications, including ultrasonic probes. Get in touch with the specs for your application and get a custom carbon part to optimize your application today.

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