Medical Imaging

In medical technology and industrial measurement technology, sonography is an established field of analysis. The use of ultrasonic transducers in the form of ultrasonic arrays is of decisive importance for these imaging techniques. The majority of ultrasound arrays manufactured in medical technology today use piezoelectric ceramic lead zirconate titanate (PZT), exploiting the reverse piezoelectric effect, to generate sound. However, high-frequency, high-resolution or contactless air-coupled arrays based on PZT are expensive and complex to manufacture. In addition, the toxic materials used are only sustainable to a limited extent (RoHS conformity).

Capacitive micromechanically manufactured ultrasonic transducers (CMUT) open up new possibilities here. Micromechanical manufacturing processes enable the economic production of corresponding ultrasound arrays for the first time. Furthermore, the high miniaturisation enables the use of CMUTs in invasive applications (e.g. intravascular ultrasound, IVUS). The results of the developments to date indicate a good property profile for CMUTs for the production of high-frequency arrays. The CMUT devices offer:

• very high acoustic bandwidth
• extremely low mechanical coupling between the elements
• good adaptability to water and air
• integration together with electronic components (ASIC)
• no toxic materials (RoHS conformity)

High bandwidth and low coupling between channels are fundamental condition for imaging that can match the standards of conventional ultrasound imaging. Fraunhofer IPMS' highly integrated MEMS technology makes it possible for the first time to connect the signals of an array on site with readout electronics to achieve a simple and compact contacting of the elements. By using the monolithic connection technology between sensor and electronics, highly planar surfaces can be realised as a contact to the medium.

With a confocal fluorescence laser scanning microscope, the sample is irradiated point by point and the fluorescence radiation excited in the sample is measured. In addition to capturing horizontal sectional images, this technology enables the production of 3D models of structured surfaces and fluorescent samples. The main areas of application are in biological and medical research as well as industrial quality assurance. However, due to the areas of application in research, these are usually complex stationary and correspondingly cost-intensive instruments.

Fraunhofer IPMS has therefore developed a robust and portable MEMS-based fluorescence laser scanning microscope using standard optics. This is made possible by integrating a 2D microscanning mirror developed in-house.

Improved imaging in point-of-care diagnostics requires very compact systems with high optical magnification, which are realised by combining multi-aperture optics with novel MEMS drive principles. Systems with optical resolution in the sub-μm range are possible, which are fabricated by integration at wafer level. This allows robust diagnostics while reducing the time required for a single analysis. The basic optical imaging principle requires relative movement of the multi-aperture optics to the object for magnified image acquisition.

A first fully functional demonstrator still uses piezo components for this purpose. In order to further miniaturise the system, these will be replaced by special MEMS actuators, so-called inchworm drives, for which a patent application has now been filed and which use the NED drive principle developed at Fraunhofer IPMS. The integration of these MEMS actuators into the demonstrator will take place in the near future.