Electromagnetics in the Healthcare Industry

Electromagnetic fields are present in many of the medical applications today. The technology is becoming more complex and it is therefore vitally important that the tools and procedures perform at their maximum capacity and always yield precise and accurate data. In this article we look at how FEKO has been involved in this industry and the future prospects for technology.

MRI Scanner Design Electromagnetic simulation software has become an indispensable tool in the development and analysis of magnetic resonance imaging (MRI) systems. Motivated by better image resolution, improved signal to noise ratio (SNR) and reduced scan times, the trend to design systems that operate at higher static (B0) field strengths comes with new challenges. The increase in static field strength implies a proportional increase in the radiofrequency (B1) field. As the wavelength becomes comparable to the electrical size of the patient and the MRI geometry, it becomes more difficult to achieve a homogeneous field distribution that is required for good image quality.

The increase in operating frequency also has implications for patient safety as the conductivity of the tissues is proportional to the frequency, making the tissue more susceptible to absorbing EM energy. The close proximity of transmit arrays to the body can also be responsible for higher absorption of the RF energy in the body. FEKO offers several methods that are ideal for the investigation of these aspects of MRI system design and safety, reducing the number of prototype iterations and achieving more robust system design. It is also an ideal virtual test bench for researchers to explore new concepts critical to the advancement of MRI technology.

Higher Frequencies The use of Magnetic Resonance Imaging (MRI) in medical diagnosis continues to grow. In order to achieve better image resolution, the trend in MRI is toward higher frequencies, e.g. 300 MHz for 7 Tesla magnetic fields. At such frequencies, the RF coil radius is of the same order of magnitude as the wavelength in the human tissue. Long gone are the days when an RF coil could be designed with quasi-static techniques or by going through many cycles of prototype adjustment. Full-wave electromagnetic simulators are the backbone of RF birdcage coil design today.

A 7T low-pass MRI coil with head phantom showing B1+ field distribution, simulated with hybrid MoM/FEM. The head phantom (provided by humanbodymodels.com, a Simpleware product) has an average tetrahedral size of 6.3 mm.

Given the fact that RF MRI coils always operate at one particular frequency, it is natural to employ a frequency-domain simulation method. Two such methods are the Method of Moments (MoM) and the Finite Element Method (FEM). While the MoM is efficient and accurate to solve the empty coil or coil with a generic phantom, the hybrid MoM/FEM offers an especially useful solution for accurate simulation of MRI systems with anatomical models. The MoM is well suited to the solution of the curved metallic geometries of the coil and the FEM for the modelling of the conductive tissue in human phantoms. The FDTD method also offers efficient solutions to coils with anatomical models due its straightforward approach to discretisation of the models and may be more suitable for those cases where the FEM memory requirements become large.

Safety Considerations with MRI The number of people with medical implants, such as pacemakers, replacement for various joints and monitoring devices is increasing steadily. At the same time, more and more people undergo MRI examinations for diagnostic purposes. During an MRI scan, high RF fields are applied, which may induce strong currents on the metallic implants. This can lead to increases in the local and averaged specific absorption rate (SAR) and temperature in the close proximity of the device. This could result in the patient experiencing localised tissue damage or burns. For this reason, rigorous compliance testing is required in order to certify an implant is MRI compatible. Antenna Design and Application (or rather “Deployment”)

Above: MICS band receive antenna simulations for wireless endoscope application

The electrically small antennas that are typically used for body mounted or implanted devices must achieve the gain requirements to sustain robust and reliable telemetry. Antennas are optimised for limited geometric volume, achieving the antenna gain to maintain link budgets for wireless telemetry. This is complicated by the fact that the device must work either in close proximity to, or be implanted inside the body. The body is an inherently challenging environment for antenna design due to the lossy properties of its tissues, resulting in communication signals being absorbed in the body rather than being radiated. Antenna performance must therefore be tested for a broad range of usage scenarios including diversity of biological systems with respect to different patient age, gender, height, BMI, etc. In some cases patient-specific treatment planning is required, e.g. in hyperthermia where electromagnetics is used to heat tumours.

Human Phantom Models While generic or homogeneous phantoms are well suited for efficient simulation, more realistic anatomical phantoms are required for system verification and safety analysis. A range of phantoms, meshed for use with either MoM or FEM can be downloaded from the FEKO website. The phantoms can also be easily remeshed as a voxel mesh for simulation with FDTD in FEKO. Additional high quality anatomical mesh models are also available through Simpleware.

Above: A 7 element surface array coil for spinal imaging simulated with the hybrid MoM/FEM method: the array is tuned and matched at 3T and the spacing is optimised to reduce coupling between elements. The anatomical phantom (provided by humanbodymodels.com, a Simpleware product) contains 13 different tissues and is meshed with 1.36 million elements with an average size of 8.3 mm.

The Future of Remote Patient Monitoring Venture capitalists are investing huge sums of money into Remote Patient Monitoring (RPM) due to tight budgets in healthcare and also overcrowding in hospitals. According to a study by Kalorama Information, the US market for advanced patient monitoring has grown from $3.9 billion in 2007 to $ 8.9 billion in 2011 and is forecast to reach $ 20.9 billion by 2016. Wearable sensors that communicate wirelessly with our healthcare practitioners will become standard practice. Ingested capsules that can be controlled remotely may soon deliver our medication. Micro-cameras and micro-robots will swim through our bloodstreams, turning previously complex surgical procedures into minimally invasive events. The unwiring of these and other technologies is being driven by the need for mobile-based solutions, the ubiquity of all-things-mobile.

In many cases, these technologies come with their own class of design challenges. The devices must conform to size and comfort constraints while sustaining wireless communications and wireless powering. Furthermore, stringent requirements must be met concerning patient safety (SAR and temperature), interference with other medical equipment (EMC and EMI) and wireless spectrum. By applying simulation-driven design methodologies, these scenarios can be tested at an early prototype stage, resulting in more robust end products. Similarly, compliance to safety and EMC standards can be estimated early, increasing the likelihood that the final product will comply.

HyperWorks and Healthcare The pressure is on to deliver quality health care in the most cost-effective way possible. It is becoming necessary, due to various factors to monitor patients outside of clinical settings. Apart from ensuring that the patient maintains a quality of life it can also significantly minimise costs for the patient. With a suite of products Hyperworks is set to become a leader in the design of high quality healthcare products.

Peter Futter
Peter Futter

About Peter Futter

Peter Futter is a senior application specialist for electromagnetic (EM) solutions at Altair Engineering, based in Stellenbosch, South Africa. He has diverse expertise in numerical simulation for a range of Electromagnetic applications, technical support and product strategy and business development in a competitive global market place. His technical specialties include antenna design, integration and placement, electromagnetic compatibility (EMC).