Radio-frequency induced heating of intra-cranial EEG electrodes: The more the colder?

Many neurological disorders are analyzed and treated with implantable electrodes. Many patients with such electrodes have to undergo MRI examinations - often unrelated to their implant - at the risk of radio-frequency induced heating. The number of electrode contact sites of these implants keeps increasing due to improvements in manufacturing and computational algorithms. Electrode grids with multiple receive channels couple to the RF fields present in MRI, but, due to their proximity, a combination of leads has a coupling response which is not a superposition of the individual leads' response. To investigate the problem of RF-induced heating of coupled multi-lead implants, temperature mapping was performed on a set of intra-cranial electroencephalogram (icEEG) electrode grid prototypes with increasing number of contact sites (1-16). Additionally, electric field measurements were used to investigate the radio-frequency heating characteristics of the implants in different media combinations, simulating the device being partially immersed inside the patient. MR measurements show RF-induced heating up to 19.6 K for the single electrode, reducing monotonically with larger number of contact sites to a minimum of 0.9 K for the largest grid. The SAR calculated from temperature measurements agrees well with electric field mapping: The same trend is visible for different insertion lengths, however, the energy dissipated by the whole implant varies with the grid size and insertion length. Thus, in the tested circumstances, a larger electrode number either reduced or had a similar risk of RF induced heating, indicating, that the size of electrode grids is a design parameter, which can be used to change an implants RF response and in turn to reduce the risk of RF induced heating and improve the safety of patient with neuro-implants undergoing MRI examinations.

Magnetic resonance imaging for pathobiological assessment and interventional treatment of the coronary arteries.

X-ray-based fluoroscopy is the standard tool for diagnostics and intervention in coronary artery disease. In recent years, computed tomography has emerged as a non-invasive alternative to coronary angiography offering detection of coronary calcification and imaging of the vessel lumen by the use of iodinated contrast agents. Even though currently available invasive or non-invasive techniques can show the degree of vessel stenosis, they are unable to provide information about biofunctional plaque properties, e.g. plaque inflammation. Furthermore, the use of radiation and the necessity of iodinated contrast agents remain unfavourable prerequisites. Magnetic resonance imaging (MRI) is a radiation-free alternative to X-ray which offers anatomical and functional imaging contrasts fostering the idea of non-invasive biofunctional assessment of the coronary vessel wall. In combination with molecular contrast agents that target-specific epitopes of the vessel wall, MRI might reveal unique plaque properties rendering it, for example, 'vulnerable and prone to rupture'. Early detection of these lesions may allow for early or prophylactic treatment even before an adverse coronary event occurs. Besides diagnostic imaging, advances in real-time image acquisition and motion compensation now provide grounds for MRI-guided coronary interventions. In this article, we summarize our research on MRI-based molecular imaging in cardiovascular disease and feature our advances towards real-time MRI-based coronary interventions in a porcine model.

La fluoroscopia con rayos X es la herramienta estándar para el diagnóstico y la intervención de coronariopatías. En los últimos años, la tomografía computarizada se ha convertido en una alternativa atraumática a la coronariografía, ya que se puede detectar la calcificación coronaria y ver a través de imágenes las luces de los vasos sanguíneos mediante el uso de medios de contraste yodados. Si bien las técnicas traumáticas o atraumáticas disponibles actualmente pueden mostrar el grado de la estenosis vascular, no pueden proporcionar información sobre las propiedades biofuncionales de la placa de ateroma, por ejemplo, inflamación de la placa de ateroma. Por otra parte, el uso de radiación y la necesidad de agentes de contraste yodados siguen siendo requisitos desfavorables. La resonancia magnética (RM) es una alternativa sin radiación a los rayos X que proporciona contrates de imagen con información anatómica y funcional, lo cual refuerza la idea del diagnóstico biofuncional atraumático de las paredes de los vasos coronarios. En combinación con medios de contraste molecular que actúan sobre epítopos específicos de las paredes de los vasos, la RM puede poner de manifiesto propiedades particulares de la placa de ateroma mediante su representación, por ejemplo, «vulnerabilidad y predisposición a rotura¼. La detección precoz de este tipo de lesiones puede facilitar un tratamiento a tiempo o preventivo antes de que tenga lugar una complicación coronaria grave.Además del diagnóstico por imagen, los avances en la adquisición de imágenes en tiempo real y la compensación del movimiento sirven de base para las intervenciones coronarias guiadas por RM. En este artículo, ofrecemos un resumen de nuestra investigación sobre imagen molecular con resonancia magnética en enfermedades cardiovasculares y presentamos nuestros avances hacia las intervenciones coronarias con RM en tiempo real en un modelo porcino.

It's the little things: On the complexity of planar electrode heating in MRI.

Neurological disorders are increasingly analysed and treated with implantable electrodes, and patients with such electrodes are studied with MRI despite the risk of radio-frequency (RF) induced heating during the MRI exam. Recent clinical research suggests that electrodes with smaller diameters of the electrical interface between implant and tissue are beneficial; however, the influence of this electrode contact diameter on RF-induced heating has not been investigated. In this work, electrode contact diameters between 0.3 and 4 mm of implantable electrodes appropriate for stimulation and electrocorticography were evaluated in a 1.5 T MRI system. In situ temperature measurements adapted from the ASTM standard test method were performed and complemented by simulations of the specific absorption rate (SAR) to assess local SAR values, temperature increase and the distribution of dissipated power. Measurements showed temperature changes between 0.8 K and 53 K for different electrode contact diameters, which is well above the legal limit of 1 K. Systematic errors in the temperature measurements are to be expected, as the temperature sensors may disturb the heating pattern near small electrodes. Compared to large electrodes, simulations suggest that small electrodes are subject to less dissipated power, but more localized power density. Thus, smaller electrodes might be classified as safe in current certification procedures but may be more likely to burn adjacent tissue. To assess these local heating phenomena, smaller temperature sensors or new non-invasive temperature sensing methods are needed.