Non-uniformity of electromagnetic fields created by excitation coils in MRI scanners is an important factor, which must be measured and characterized in order to deliver adequate images and comply with safety regulations. This issue gradually becomes more and more important with commissioning of ever more powerful magnets and, consequently, higher radio frequencies with higher absorption rate in human body. With technological advance in thermal imaging, traditional measurement techniques based on discrete thermal sensors may soon be superseded by non-invasive optical methods, offering far greater flexibility and precision. The article introduces new technique of measuring spatial distribution of superficial electromagnetic field created by radio-frequency (RF) coils. The experiments described in this paper were performed using a General Electric 1.5 Tesla MRI scanner. Thermal images were acquired using a Testo 875 thermal camera, with a spatial resolution of 3 mm at a distance of 1 m, temperature range of 0-280°C, and sensitivity of 80 mK at 30°C. Solid-state phantoms of the size 700x300'5 mm3 were made from polymethylmethacrylate and shock-resistant polystyrene-dielectric low thermal conductivity materials opaque in infrared. The new method of measuring non-uniformity of fields created by RF coils is based on long-term stability of thermal map imprinted into plastic during scanning, proportionality of RF intensity to temperature variations, absence of optical artifacts due to opacity of phantoms, and subsequent numerical analysis of the entire digital map of temperature over the 700x300 mm2 area of phantoms. The new method may prove feasible, convenient, accurate, inexpensive, and handy. The acquired thermal maps were converted into digital maps and analyzed numerically, computing principal statistical parameters such as peak-to-peak variation, real mean square, average, and most probable values.
Keywords: Magnetic resonance imaging, phantoms, coils, thermal imaging