Microwave tissue ablation technique and equipment
The experiments were conducted with a high-power MWA system with a maximum generator power of 150 W. The system was equipped with a pump for perfusion-cooling of the applicator shaft. The MW generator (ECO-100E2, Nanjing ECO Medical Instrument Co., China) worked at a frequency of 2.45 GHz. All experiments were conducted with a 14-G MW applicator (ECO-100AI13C, Nanjing ECO Medical Instrument Co., China) with a shaft length of 15 cm. The applicator is composed of a shaft consisting of titanium alloy and a ceramic tip with a length of 18 mm. A 4-m long coaxial cable connects the antenna with the generator enabling the generator to be positioned safely outside the MR scanner room during ablation.
MRI protocol and artefact evaluation
Artefact evaluation was conducted in a 1.5-T short bore scanner (Magnetom ESPREE, Siemens Healthineers, Erlangen, Germany) with a horizontal main magnetic field (B0) and a four-channel body-array surface coil. The MW applicator was placed in an MRI phantom consisting of a Plexiglas box filled with a 0.2% gadolinium solution (Gadovist, Bayer Healthcare, Berlin, Germany). The phantom was positioned at the magnet isocentre and enabled a deflection of the applicator relative to B0 between 0° and 90°. The measurements were performed with three different sequences:
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1)
A three-dimensional T1-weighted volume interpolated breath-hold examination (T1W-VIBE) with chemically selective fat-saturation pulse, performed with flip angle of 10°, repetition time (TR) of 6.2 ms, echo time (TE) of 1.61 ms, bandwidth of 457 Hz/pixel, slice thickness of 1 mm, field of view (FOV) 192 × 192 mm, acquisition matrix 192 × 192, and reconstruction matrix 192 × 192;
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2)
A two-dimensional T1-weighted, fast low-angle shot (T1W-FLASH) gradient-echo sequence with periodic chemically selective fat-saturation pulses and flip angle of 70°, TR of 122 ms, TE of 4.36 ms, bandwidth of 139 Hz/pixel, slice thickness 4 mm, FOV 192 × 192 mm, acquisition matrix 192 × 192, and reconstruction matrix 192 × 192;
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3)
A two-dimensional T2-weighted turbo spin-echo (T2W-TSE) sequence with TR of 3750 ms, TE of 129 ms, flip angle 145°, echo train length 29, bandwidth 260 Hz/pixel, slice thickness 4 mm, FOV 192 × 192 mm, acquisition matrix 192 × 192, reconstruction matrix 384 × 384.
The following factors were systematically varied: sequence type (T1W-VIBE, T1W-FLASH, T2W-TSE), applicator orientation to B0 (0°, 45°, 90°), slice orientation with respect to the applicator (axial, coronal, sagittal), and encoding direction (phase encoding direction or frequency encoding direction being perpendicular to the long axis of the applicator) resulting in a total of 36 artefact measurements. Artefact analysis of the acquired images was performed with the open-source software ImageJ (http://rsb.info.nih.gov/ij). Artefacts were defined according to the American Society for Testing Materials as deviation of ± 30% from the median signal intensity around the applicator [20]. Imaging analyses were conducted in consent of two readers (AG and RH).
The artefact diameters were measured at the applicator tip and the applicator shaft. The tip location error (TLE) was assessed on images with coronal or sagittal orientation in relation to the applicator. The TLE describes the deviation from the measured distance between the distal end of the tip and the Plexiglas model and the actual set 10 mm distance. A positive TLE correlates with an overestimation of the applicator position in the long axis direction [21].
Ablation protocols and ablation zone evaluation
All ablations were performed ex vivo at room temperature using three fresh bovine livers (Bos Taurus) obtained from a local abattoir. Before positioning of the MW applicator, large hepatic veins were explored with a metal probe to avoid close positioning. The ablation durations were 5, 10, and 15 min (maximum recommended duration). Ablations were conducted with a power of 80 W and 120 W (maximum recommended power for liver ablation with a 14-G antenna). Each combination was repeated four times resulting in a total of 24 ablations. Results were only made available to the manufacturer after completion of the experiments.
After ablation, the antenna was replaced by a metal bar serving as guidance to dissect the liver along the ablation zone. For further measurements, the ablation zone was photographed (Canon, EOS 350D, Tokyo, Japan). The ablation zone diameter along the antenna insertion axis was defined as long-axis diameter (LAD). The largest diameter of the ablation zone perpendicular to the LA was defined as short-axis diameter (SAD) (Fig. 1). Dimensions were determined using callipers by measuring the perimeters of the white coagulation zone. The volume of the ablation zone was calculated using the ellipsoid formula for diameters (Volume = π/6*LA*(SAD)2). The shape of the ablation zone was determined by calculating the sphericity index (SI) = SAD/LAD.
Statistical analysis
Acquired data were analysed with the statistical software JMP 13 (SAS Institute, Cary, NC, USA). To compare the TLE, shaft artefact diameter and tip artefact diameter in terms of sequence type, angulation to B0, slice orientation and encoding direction analysis of variance (ANOVA) was performed. The assumptions of variance homogeneity and normal distribution were checked. In the case of heterogeneity of variances, the Welch ANOVA test was used. If a significant overall effect was found, post hoc between-group comparisons were performed following the closed testing procedure and by using the Student t test [22, 23]. In case of comparing only two parameters, the Student t test was used instead of ANOVA.
To compare the effect of a different ablation power on the SAD, volume, and SI with respect to the ablation duration (5, 10, and 15 min) unpaired t tests was used. Results were displayed as mean ± standard deviation (SD). A p value < 0.05 was considered statistically significant for all tests.
