Search

Laser ablation (LA) is a minimally invasive technique for the treatment of tumors as an alternative to surgical resection. The light absorbed by tissue is converted into heat, and causes irreversible cell damage when temperatures higher than 60 °C are reached. The knowledge in real time of temperature may be particularly beneficial for adjusting laser settings applied during treatment and to be notified in real time about its end-point. As a consequence, several techniques for temperature monitoring within the tissue have been investigated along the last decades. In the field of LA, particularly attractive are non-invasive methods. Among these techniques, thermometry based on the analysis of Magnetic Resonance Imaging (MR-thermometry) has gaining large acceptance in this field. MR-thermometry allows estimating the temperature variation thanks to the thermal dependence of several MRI parameters, among others the most promising are T 1 relaxation time, and proton resonance frequency shift. The aim of this study is to assess the sensitivity of MRI thermometry using three T 1 -weighted sequences (i.e., Inversion Recovery Turbo-FLASH, IRTF, Saturation Recovery Turbo-FLASH, SRTF, and FLASH) using an 1.5-T MR scanner on healthy swine pancreases undergoing LA. The reference temperature was measured by MRI-compatible fiber optic sensors (fiber Bragg grating sensors). The sensitivity of the proposed techniques was estimated and compared. The thermal sensitivity of the three sequences was −1.47 ± 0.08 °C −1 , −0.95 ± 0.05 °C −1 , and −0.56 ± 0.04 °C −1 for IRTF, SRTF and FLASH, respectively. Results show that the proposed technique may be adequate for temperature monitoring during LA. [ABSTRACT FROM AUTHOR]

Thin-film micro-coils are integrated with commercially available ablation catheters, for MR-thermometry during laser interstitial thermal therapies (LITTs). The coils are formed on a flexible polyimide substrate and consist of a two-turn electroplated copper inductor and integrated parallel plate capacitors for tuning and matching. Their performance was assessed during Nd:YAG laser ablations in a static phantom study carried out in a 3 T clinical scanner. Further moving phantom studies were performed to calculate errors due to motion. The temperature accuracy is improved by 1.5–10 times in a radius matching the dimensions of the lesions typically treated. Resolution of 1 mm can be maintained during motion by using short acquisition time sequences while the SNR remains sufficient for accurate MR-thermometry. The temperature error on a moving un-heated phantom under respiratory gating does not exceed 1 °C. This demonstration suggests a possible improvement of the overall control of MR-guided LITTs by local temperature monitoring. [ABSTRACT FROM AUTHOR]

Purpose: Flexible microcoils integrated with ablation catheters can improve the temperature accuracy during local MR-thermometry in Nd:YAG laser interstitial thermal therapies. Here, the authors are concerned with obtaining a preliminary confirmation of the clinical utility of the modified catheter. They investigate whether the thin-film substrate and copper tracks of the printed coil inductor affect the symmetry of the thermal profile, and hence of the lesion produced. Methods: Transmission spectroscopy in the near infrared was performed to test for the attenuation at 1064 nm through the 25 jum thick Kapton substrate of the microcoil. The radial transmission profile of an infrared high-power, light emitting diode with >80% normalized power at 1064 nm was measured through a cross section of the modified applicator to assess the impact of the copper inductor on the optical profile. The measurements were performed in air, as well as with the applicator surrounded by two types of scattering media; crystals of NaCl and a layer of liver-mimicking gel phantom. A numerical model based on Huygens-Fresnel principle and finite element simulations, using a commercially available package (COMSOL Multiphysics), were employed to compare with the optical measurements. The impact of the modified optical profile on the thermal symmetry was assessed by examining the high resolution microcoil derived thermal maps from a Nd:YAG laser ablation performed on a liver-mimicking gel phantom. Results: Less than 30% attenuation through the Kapton film was verified. Shadowing behind the copper tracks was observed in air and the measured radial irradiation correlated well with the diffraction pattern calculated numerically using the Huygens-Fresnel principle. Both optical experiments and simulations, demonstrate that shadowing is mitigated by the scattering properties of a turbid medium. The microcoil derived thermal maps at the end of a Nd:YAG laser ablation performed on a gel phantom in a 3 T scanner confirm that the modified irradiation pattern does not disrupt the thermal symmetry, even though, unlike tissue, the gel is minimally scattering. Conclusions: The results from this initial assessment indicate that microcoils can be safely integrated with ablation catheters and ensure that the complete necrosis of the liver tumor can still be achieved. [ABSTRACT FROM AUTHOR]