Publication date: 12 February 2008
Although it now represents a massive opportunity, the medical market can present large barriers to entry for electronics companies. Beyond the regulatory requirements – which are, understandably, not insignificant – designers are faced with unique challenges that test their skill and ingenuity. The combination of harsh environments (both physical and electrical) and a need for extreme accuracy means meeting those challenges requires a level of design expertise that only comes with experience.
ML Electronics has been active in the medical market for 12 years and for the last four years the design of medical devices has represented around 50% of its business. As a design and manufacturing company it is able to provide the level of service necessary to not only comply with the stringent regulatory demand but also the support needed, in a market where product lifecycles are measured in years instead of months.
It is not unusual for some products to remain in the medical market for more than 10years and with continual development they can benefit from the latest in digital and analogue technology. It is also largely due to these technologies that new and exciting methodologies continue to emerge in the medical market, which someday may lead to faster and more accurate diagnosis of life threatening disease. One such methodology is electrical impedance tomography, an imaging technique now finding applications within many vertical sectors, including medical.
Essentially, it works by measuring the electrical impedance in a subject, across a spectrum of frequencies. The impedance measured is, in turn, used to create an image, by comparing the relative differences in conductivity and permitivity across specific regions of the subject.
The technique is effective for a variety of subjects and is already used in the industrial sector (see Figure 1). It is also being developed for geological purposes, an example being the investigation of riverbed erosion.
When applied in a medical application, real time scans of body and bone tissue can be captured in a non intrusive way for diagnostic purposes. Through this form of tomography and spectroscopy it is possible to discern, from the variations in conductivity and permitivity between multiple points on the body, the composition of body matter.
Once rendered it is hoped that a trained eye, or perhaps even an electronic observer, could then determine whether the body tissue is healthy or suffering from a variety of conditions, including cancer.
Although largely still in the experimental stage, electrical impedance tomography (EIT) is an application area that continues to develop and one where ML Electronics has some experience. EIT is an example of an application that is directly enabled by developments in electronics. Through the use of digital signal processing (DSP) and advanced analogue/digital converters it is possible to create systems with between five and 500 sensors.
The number of sensors used in any given application depends largely on that application; a geological application – where accuracy is more important than sample rate – could realistically deploy hundreds of sensors. In a medical application there are physical and practical limits to the number of sensors used. In addition, the key requirement is more likely to be real time response, so fewer sensors may be used but over a much smaller area.
As each sensor acts as a transmitter and a receiver, acquiring data from the sensor array involves excitation across a spectrum of frequencies using as many transmit/receive permutations as can be catered for.
For a system with over 500 sensors, then, it is clear why a full scan with every possible combination of sensors and across multiple frequencies can take a significant amount of time. In addition, neighbouring sensors will receive a stronger signal than those situated further away, so it is also essential to know where each sensor is located relative to all others.
Only a few years ago, this kind of application would have required multiple DSPs running under full load to achieve even a few frames per second. Today it can be accomplished using one of the latest powerful and highly integrated devices available, such as the Texas Instruments DaVinci DSP. Of course, the digital signal processing element is only part of the system. A critical factor in this kind of application is the design of the analogue front end.
Interfacing to the sensors alone requires a high degree of analogue design expertise. In a geological application the sensors may be measuring signals in the nV range, which require extremely accurate and stable amplification before being passed to the ADC stage. In a medical application the signals received may be slightly larger (although still sub-mV) but the environment holds other inherent challenges. The main enemy in this kind of environment is noise, which is difficult to avoid given the scale of the signals and the conditions under which they are acquired. In this respect, an operating room or medical environment can actually present more of a challenge than a riverbank. With electrodes attached directly to the skin using long wires acting as antenna, it isn’t difficult to see why keeping noise out of the system is a major challenge.
It now becomes clear why generating the excitation signals, at a range of frequencies, then receiving the resultant signal and processing the date requires significant analogue and digital design expertise.
While the DSP is able to help compensate for unwanted noise in the system, the overall accuracy hinges predominantly on the quality of the analogue design and the components used. Developments in the analogue sector means it is now much more affordable to access high quality analogue components. The industry as a whole has benefited greatly from the investment in developing analogue solutions for the much larger audio market, with accuracy becoming more important for ‘audiophiles’, improvements made to analogue devices targeting the audio market are directly benefiting other sectors, including medical. The performance of operational amplifiers has also improved dramatically over recent years and noticeably in low power devices. This is particularly important in the EIT application, where there can be literally hundreds of op-amps used.
However, as stated earlier, using the best components doesn’t guarantee the best results and if the analogue design doesn’t provide enough accuracy the digital element will simply be processing noise instead of signal.
Achieving a high signal to noise ratio (SNR) is aided, again, by op-amp development; with 10s of volts of common mode noise present in the system, a high common mode rejection ratio (CMRR) is essential. Only through good analogue design can the benefits of good SNR and CMRR be maintained throughout the system, without that it is simply a case of ‘garbage in, garbage out’. No amount of complex DSP can compensate for bad data.
As developments at the silicon level continue to benefit the system, the future promises to deliver even greater potential for this kind of application. A key factor in the results that can be obtained through any kind of medical imaging application is resolution and we look forward to the introduction of even better ADCs and faster, more accurate op amps.
With those developments it will be possible to deploy more sensors in an EIT system, which in turn will require more powerful DSPs to process the data – however this development seems inevitable given the rapid evolution of DSP technology.
Beyond that it isn’t a massive leap to deploy electronics on the sensors themselves, enabling even greater resolution through distributed processing. Once there, the next logical step may be to integrate the sensors in to implantable devices, which will help overcome a large number of signal acquisition challenges and improve SNR even further.
In the meantime, the benefits of EIT as a non intrusive procedure that offers greater accessibility than, for instance, MRI scans, indicate it will have an important role to play in future diagnostic and preventative medicine.
I pride myself on being able to work from first principles through to a working design. My main expertise lies in analogue and power design and have been responsible for the design of inverters and power supplies for high and low power Naval systems operating at 50 – 400Hz with ratings of ten’s of kW’s
As one of MLE’s first employees, having previously been Technical Director for G & R Electronics, I have been involved with most of the products developed. These include being the lead engineer in a complex power supply based system involving 3 – 1Kw power converters with exceptionally low EMC profiles delivering precise currents for plasma generation. This product is used by surgeons to coagulate bleeding tissue. I have been heavily involved in designing parallel capable power supplies and inverters from 250W – 10Kw. I have over 30 year’s EMC experience in designing products for Military applications.
Additionally, I am responsible for the team within MLE that identifies and researches new technology ensuring that our clients have the latest technology where appropriate. I have a keen personal interest in alterative power technologies.
