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Optothermal profile of an ablation catheter with integrated microcoil for
MR-thermometry during Nd:YAG laser interstitial thermal therapies of the liver—An
experimental and theoretical study
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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.
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
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
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
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
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
The results from this initial assessment indicate that microcoils can be safely
integrated with ablation catheters and ensure that the complete necrosis of
tumor can still be achieved.
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