TitreRF heating reduction associated to an MR endoluminal coil at 3T
Publication TypeConference Paper
AuthorsVerret, J-M, Pilleul, F, Rabrait, C, Beuf, O
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Purpose / Introduction MR endoluminal coils increase the Signal-to-Noise Ratio (SNR), allowing better distinction of the layers of the bowel: a crucial criterion for diagnosis of bowel diseases [1]. But they present a risk of burn for the patient. Radio Frequency (RF) electric field may cause important heating due to local concentrations of the E-field along the conductive cable and coil [2]. In this paper, the efficiency of RF traps toward heating suppression was assessed. The particular case of a 225 cm long receiver cable was with a cable path configuration tested to be in the worst case scenario. Subjects and Methods Experiments were performed on a GE DVMR750 3T system (General Electric Medical Systems). Endoluminal coil is a double loop coil prototype (60 mm length, 6 mm width) developed at CREATIS and was placed in 1.5 % agar gel phantom, an adequate medium for heat propagation. Four optic fiber probes were taped on the endoluminal coil: 3 at the distal extremity and 1 at the proximal base and connected to a temperature measurement device (Opsens Tempsens: 0.1 °C resolution, 0.357 Hz sampling rate). A FIESTA was performed with Acquisition Time: 1min36, 35°flip angle ; FOV :48*48 cm2, matrix 128*128, 250 KHz Bandwidth, TE/TR : 0.988/2.7ms. The receiver cable was an RG 58 coaxial cable 225cm long incorporating between0 and 5 RF traps. Inside the MRI bore, the receiver cable path passed through the maxima of the electric field, located with an electric dipole associated to a LED (Agilent HLMP 4015) Results Maximum temperature elevations measured at different locations in function of the number of traps are summarized in Table1.

Maximum Temperature rise in °C ( 5 measurements)
# RF trap Distal 1 (mean, std) Distal 2 (mean, std)  Distal 3 (mean,std) Proximal 1 (mean,std)
0 14.66,1 8.13, 0.48  7.57, 083 1.55, 00.17
1 8.5, 0.25 6.35, 0.18  5.02, 0.2 0.92, 0.2
2 1.56, 0.45 0.89, 0.2 0.96, 0.25 ~0 
3 0.94, 0.09 0.55, 0.08 0.59, 0.07 ~0 
4 0.52, 0.03 0.38, 0.7 0.42, 0.08 ~0 
5 0.30, 0.03 ~0  ~0  ~0 

Discussion/Conclusion RF heating is a local phenomenon which occurs mostly at distal extremity. Without RF traps, observed maximum temperature increase was 14.66 ± 1°C which would cause necrosis. In the worst case scenario, 5 RF traps efficiently suppressed heating for a cable length of 225 cm References [1] Pilleul, F, 2005, Lyon 1 Thesis [2] C. Armenean et al., 2004, Magn Reson Med 1200-1206

RF heating reduction associated to an MR endoluminal coil at 3T