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 µm 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.