RESUMEN
Microscale, continuous-profile, diffractive lenses have been fabricated and characterized. Lenses designed to operate at λ0 = 405 nm were created by focused ion beam milling of a glass substrate. The micro-structured profile was analysed by confocal microscopy and optical performance was quantified by measurements of the transmitted laser beam profile. Lenses of size 125 µm × 125 µm, containing up to 18 annuli and focusing at 400 µm, 450 µm and 500 µm have been made. Measured focused beams were in excellent agreement with the predicted performance. A maximum diffraction efficiency of 84 % and side-lobe suppression down to the 10-4 level can be achieved. The suitability of the lenses for interfacing with trappedion systems is outlined.
RESUMEN
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.