Shalmali Dharmadhikari MS, Presenter: Nothing to Disclose

Navin Bansal PhD, Abstract Co-Author: Nothing to Disclose

Radio-frequency (RF)-induced heating is a safety concern in MRI, especially in the presence of metallic implants, which may cause serious tissue heating. Specific absorption rate (SAR) is the rate of energy deposited per unit mass of tissue and is used to assess RF safety. MR thermometry using TmDOTP5- and water is proposed as a novel method for safety evaluation of SAR.

MR experiments were performed on a 9.4-T system. A cylindrical phantom containing 50 mM TmDOTP5- in 4% agarose gel was used. Calibration constants (C0, C1) were obtained by acquiring water and TmDOTP5- spectra to calculate the difference in frequency shift (ΔF) and by measuring temperature with fiber optic probe. 3D 1H water and TmDOTP5- images were collected using a gradient echo sequence. 3D absolute temperature images were calculated from phase shifts (ΔΦ) and frequency shifts in TmDOTP5- and water images using equations: ΔF= [(ΔΦ/2πζ)+(fref - f)]/f0 and T = (ΔF-C0)/C1. 3D SAR and cooling constant images were computed on a voxel-by-voxel basis by curve-fitting data to the differential model dT/dt = -k(T-Ta)+(S/c) where dT/dt is the rate of change of temperature (°Cs-1), k is the cooling constant (s-1), T and Ta are phantom and ambient temperatures (°C) and c is the specific heat capacity of the phantom material (Jkg-1°C-1) . SAR measurements with the same sequence were also repeated with a bare-ended insulated wire immersed in the phantom. SAR for a T1ρ imaging sequence with a spin lock (SL) pulse of varying length was also evaluated.

Due to the strong chemical shift dependence of TmDOTP5- on temperature compared to water, absolute temperature images with high temperature and spatial resolution were obtained. The SAR images showed higher SAR near the walls of the phantom. The T1ρ sequence showed higher mean SAR with increasing SL pulse duration. The SAR images for phantom with conductor showed higher SAR near the bare end.

MR thermometry with TmDOTP5- enables high resolution temperature measurement which is essential for accurate SAR measurement. The use of TmDOTP5- for SAR measurement in phantoms for evaluating implant safety is invaluable. Its application in human studies awaits further investigation.

Due to its high temperature and spatial resolution, MR thermometry with TmDOTP5- can be a valuable tool for determining the SAR of implants, pulse sequences, coils, etc. for assuring patient safety.

Dharmadhikari, S, Bansal, N, Evaluation of SAR Using MR Thermometry with Thulium1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(Methylene Phosphonate) (TmDOTP5-) for RF Safety in MRI.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11002408.html