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Purpose: To correlate the accumulated thermal dose (ATD) with lesion size in magnetic resonance (MR)‐guided focused ultrasound (MRgFUS) thalamotomy to help guide future clinical treatments. Materials and Methods: Thirty‐six patients with medication‐refractory essential tremor were treated using a commercial MRgFUS brain system (ExAblate 4000, InSightec) in a 3T MR scanner (MR750, GE Healthcare). Intraoperative MR‐thermometry was performed to measure the induced temperature and thermal dose distributions (thermal coefficient = −0.00909 ppm/°C). The ATD was calculated over multiple sonications with appropriate corrections for spatial‐shifting artifacts. The ATD profile sizes obtained for dose values of 17, 40, 100, 200, and 240 cumulative equivalent minutes at 43°C (CEM) were correlated with the corresponding lesion sizes measured via axial T1‐ and T2‐weighted MR images acquired 1 day post‐treatment. Results: Of a total of 232 included sonications, 83 required corrections for off‐resonance‐induced spatial‐shifting artifacts (correction range = [1.1,2.2] mm). The mean lesion sizes measured on T2‐weighted MR images (6.2 ± 1.3 mm, mean ± SD) were 15% larger than those measured on corresponding T1‐weighted MR images (5.3 ± 1.2 mm, mean ± SD). The ATD values that provided the best correlations with the measured lesion sizes on T2‐ and T1‐weighted MR images were 100 and 200 CEM, respectively. Conclusion: The ATD was correlated with lesion size measured 1 day following MRgFUS thalamotomy for essential tremor. These data provide useful information for predicting brain lesion size and determining treatment endpoints in future clinical MRgFUS procedures. [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]

Purpose: MR thermometry based on the proton resonance frequency shift (PRFS) is the most commonly used method for the monitoring of thermal therapies. As the chemical shift of water protons is temperature dependent, the local temperature variation (relative to an initial baseline) may be calculated from time-dependent phase changes in gradient-echo (GRE) MR images. Dynamic phase shift in GRE images is also produced by time-dependent changes in the magnetic bulk susceptibility of tissue. Gas bubbles (known as “white cavitation”) are frequently visualized near the RF electrode in ultrasonography-guided radio frequency ablation (RFA). This study aimed to investigate RFA-induced cavitation’s effects by using simultaneous ultrasonography and MRI, to both visualize the cavitation and quantify the subsequent magnetic susceptibility-mediated errors in concurrent PRFS MR-thermometry (MRT) as well as to propose a first-order correction for the latter errors. Methods: RF heating in saline gels and in ex vivo tissues was performed with MR-compatible bipolar and monopolar electrodes inside a 1.5 T MR clinical scanner. Ultrasonography simultaneous to PRFS MRT was achieved using a MR-compatible phased-array ultrasonic transducer. PRFS MRT was performed interleaved in three orthogonal planes and compared to measurements from fluoroptic sensors, under low and, respectively, high RFA power levels. Control experiments were performed to isolate the main source of errors in standard PRFS thermometry. Results: Ultrasonography, MRI and digital camera pictures clearly demonstrated generation of bubbles every time when operating the radio frequency equipment at therapeutic powers (>=30 W). Simultaneous bimodal (ultrasonography and MRI) monitoring of high power RF heating demonstrated a correlation between the onset of the PRFS-thermometry errors and the appearance of bubbles around the applicator. In an ex vivo study using a bipolar RF electrode under low power level (5 W), the MR measured temperature curves accurately matched the reference fluoroptic data. In similar ex vivo studies when applying higher RFA power levels (30 W), the correlation plots of MR thermometry versus fluoroptic data showed large errors in PRFS-derived temperature (up to 45 °C absolute deviation, positive or negative) depending not only on fluoroptic tip position but also on the RF electrode orientation relative to the B0 axis. Regions with apparent decrease in the PRFS-derived temperature maps as much as 30 °C below the initial baseline were visualized during RFA high power application. Ex vivo data were corrected assuming a Gaussian dynamic source of susceptibility, centered in the anode/cathode gap of the RF bipolar electrode. After correction, the temperature maps recovered the revolution symmetry pattern predicted by theory and matched the fluoroptic data within 4.5 °C mean offset. Conclusions: RFA induces dynamic changes in magnetic bulk susceptibility in biological tissue, resulting in large and spatially dependent errors of phase-subtraction-only PRFS MRT and unexploitable thermal dose maps. These thermometry artifacts were strongly correlated with the appearance of transient cavitation. A first-order dynamic model of susceptibility provided a useful method for minimizing these artifacts in phantom and ex vivo experiments. [ABSTRACT FROM AUTHOR]