Characterization of Susceptibility Artifacts in MR-thermometry PRFS-based during Laser Interstitial Thermal Therapy

Magnetic Resonance Thermometry (MRT) is demonstrating huge abilities to guide laser interstitial thermal therapy (LITT) in several organs, such as the brain. Among the methods to perform MRT, Proton Resonance Frequency (PRF) shift holds significant benefits, like tissue independence. Despite its potential, PRF shift-based MRT holds significant challenges affecting the accuracy of reconstructed temperature maps. In particular, susceptibility artifacts due to gas-bubble formation are an important source of error in temperature maps in MRT-guided LITT. This work presents the characterization of the susceptibility artifacts in MRT-guided LITT and the measurement of its size. LITT was performed in gelatin-based phantoms, at 5 W, 2 W, 1 W, and 0.5 W under MRI guidance with a 1.5 T clinical MRI scanner. Temperature images were obtained with a 3D EPI (Echo-planar imaging) prototype sequence. Areas of temperature errors were defined as zones of negative temperature variation $\boldsymbol{< -2^{\circ}\mathrm{C}}$. Moreover, we have analyzed the artifact shape in sagittal, axial and coronal planes. The analysis demonstrates a double-lobe shape for the susceptibility artifact mainly distributed in the sagittal plane. Also, the higher laser power caused a bigger artifact area. Temperature errors of $\boldsymbol{\sim 80^{\circ}\mathrm{C}}$ proved the necessity to avoid susceptibility artifact generation during MRT-guided LITT. The analysis of the influence of the laser power on the artifact has suggested that using low laser power (0.5 W) helps avoid this measurement error.