Maarten Frank Hoes MSc, Presenter: Nothing to Disclose

Joyce Gerda Riek Bomers MSc, Abstract Co-Author: Nothing to Disclose

Christiaan Gerardus Overduin BSC, Abstract Co-Author: Nothing to Disclose

Eva Rothgang, Abstract Co-Author: Support, Siemens AG

Jurgen J. Futterer MD, PhD, Abstract Co-Author: Nothing to Disclose

Frank De Lange PhD, Abstract Co-Author: Nothing to Disclose

MR-guidance offers superior soft tissue contrast and monitoring capabilities over US-guidance during cryotherapy. MR-thermometry during focal cryotherapy may improve ablation control and aid in preventing harm to nearby critical structures. PRFS thermometry seems the best applicable method due to its high sensitivity, linear calibration and tissue-type independence1. This study focuses on the feasibility and accuracy of PRFS thermometry in proximity of the ice ball.

MR images were acquired on a 1.5T MR scanner (MAGNETOM Avanto, Siemens, Erlangen, Germany). A 17 gauge cryo needle (IceRod, Galil Medical, Yokneam, Israel) and four fiber-optic temperature sensors (T1, Neoptix, Québec, Canada) were inserted parallel to each other in a tissue equivalent agar phantom. The sensors were positioned at 0.5 cm intervals on one side of the cryo needle. Several freeze-thaw cycles were performed. Phase changes were measured in the coronal plane using a gradient echo sequence (TE/TR/FA=5/51/25° and resolution 2.3x2.3x5mm). Temperature maps were calculated using software built on the IFE framework2.

In the MR images, the ice ball was observed to elliptically expand, eventually covering all four sensors. In contrast to the temperature readings, PRFS thermometry only demonstrated a temperature decrease –even to sub zero temperatures- outside the ice ball in the direction along B0. No temperature gradient was observed perpendicular to the cryo needle. This directional dependency was not influenced by changing the phase encoding direction.

The differences observed between the temperature readings and PRFS thermometry are attributable to the susceptibility contrast between frozen and unfrozen tissue. This disturbs the local magnetic field, thus causing temperature underestimation in line with and overestimation perpendicular to the B0 field3. The clinical usability of PRSF thermometry during focal cryoablations depends on the successful implementation of an on-line correction for the susceptibility artifact. 1 Rieke et al., 2008, JMRI 2 Rothgang et al., 2010, Proc SPIE MI 3 Kickhefel et al., 2011, Magn Reson Mater Phy  

The addition of PRFS thermometry during MR-guided cryotherapy may improve ablation control and aid in preventing harm to nearby critical structures.

Hoes, M, Bomers, J, Overduin, C, Rothgang, E, Futterer, J, De Lange, F, Towards Proton Resonance Frequency Shift (PRFS) Thermometry during Focal MR-guided Cryotherapy.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12043883.html