AbdEl-Monem El-Sharkawy

Absolute Temperature Monitoring Using RF Radiometry in the MRI Scanner

Temperature detection using microwave radiometry has proven value for noninvasively measuring the absolute temperature of tissues inside the body. However, current clinical radiometers operate in the gigahertz range, which limits their depth of penetration. We have designed and built a noninvasive radiometer which operates at radio frequencies (64 MHz) with ~100-kHz bandwidth, using an external RF loop coil as a thermal detector. The core of the radiometer is an accurate impedance measurement and automatic matching circuit of 0.05 Omega accuracy to compensate for any load variations. The radiometer permits temperature measurements with accuracy of plusmn0.1degK, over a tested physiological range of 28degC-40 degC in saline phantoms whose electric properties match those of tissue. Because 1.5 T magnetic resonance imaging (MRI) scanners also operate at 64 MHz, we demonstrate the feasibility of integrating our radiometer with an MRI scanner to monitor RF power deposition and temperature dosimetry, obtaining coarse, spatially resolved, absolute thermal maps in the physiological range. We conclude that RF radiometry offers promise as a direct, noninvasive method of monitoring tissue heating during MRI studies and thereby providing an independent means of verifying patient-safe operation. Other potential applications include titration of hyper- and hypo-therapies

SSFP-based MR thermometry

Of the various techniques employed to quantify temperature changes by MR, proton resonance frequency (PRF) shift‐based phase‐difference imaging (PDI) is the most accurate and widely used. However, PDI is associated with various artifacts. Motivated by these limitations, we developed a new method to monitor temperature changes by MRI using the balanced steady‐state free precession (balanced‐SSFP) pulse sequence. Magnitude images obtained with the SSFP pulse sequence were used to find the PRF shift, which is proportional to temperature change. Spatiotemporal temperature maps were successfully reconstructed with this technique in gel phantom experiments and a rabbit model. The results show that the balanced‐SSFP‐based method is a promising new technique for monitoring temperature. Magn Reson Med 52:704–708, 2004.

Animal magnetic resonance imaging and acoustic noise.

MRI scanner noise can cause discomfort and potential damage to human hearing. MRI is used extensively in animal research and, increasingly, as a diagnostic tool in veterinary medicine, generally with no hearing protection. Effects of noise could be detrimental for research animals that will be used after scanning or for companion animals that will be returned to owners. Though safe exposure standards have not been determined for most animals, it is important to consider that animals exposed to scanner noise may suffer hearing damage. To study potential animal MRI adverse hearing effects, we measured sound levels produced by several animal scanning protocols in a commercial 3T scanner. Using hearing threshold data for research or companion animals, we estimate weighted sound pressure levels and compare these to levels at which damage occurs in humans. SPLs were above 90 dB on many scans and often exceeded 100 dB. Animals may be exposed to these levels intermittently for 1 h or more. Exposure to these sound...