|ECR 2015 / C-1007|
|Virtual Biopsy and Three Dimensional Ultrasound for Radio Frequency Ablation of Thyroid Nodules|
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Virtual Biopsy guided RFA treatment (Fig. 2) was performed in all the five scheduled patients for thermal ablation of benign thyroid nodules. One nodule was treated in each subject. The Virtual Biopsy was used with the fusion between the real-time 2D scan and the already acquired 3D US volumes of the same patient. In two patients the 3D US acquisition was performed using the 3D motorized probe BL433 while the 2D US guidance of the RFA electrode and the acquisitions of the 2D US scans for the real-time fusion imaging with the 3D US, was performed with the 2D linear array LA523.
In the remaining patients the LA533 and LA332 probes (for 2 subjects and 1 subject, respectively) were used for needle guidance using the Virtual Biopsy tool and also for 3D Pan acquisitions of thyroid volumes. The number of volumes acquired was dependent by the size of the nodule to be treated (2 volumes was the average). LA533 and LA332 probes have a dual-possibility hand grip design, pinch grip and palmar grip (appleprobe design), in order to provide a neutral wrist position. This resource represented an additional operator’s comfort option during long RFA sessions. Moreover, the presence of a single tracking sensor on the probe enabled the operator to handle the transducer in both pinch grip and palmar grip .
Due to the small array width of the LA332 probe (36 mm) which enabled high maneuverability on short necks, and due to its low frequency and high depth scanning capability, the probe was used with the TPView, enlarged field of view imaging technology, in order to obtain the scanning sector size of a convex, with the improved coupling capability of a linear probe.
The scanning velocity during Virtual Navigator 3D Pan acquisitions didn’t affect the reconstruction as the probe spatial position was recorded at more than 50Hz, in this way it was guaranteed a perfect re-alignment of the acquired US [9,10].
Virtual Navigator 3D Pan volumes were acquired scanning longitudinally the neck surface. Two US volumes (with 12 seconds scan time for each US 3D acquisition) were fused together with 3D Pan tool, in order to obtain a panoramic volume of almost half thyroid containing the target nodule. The obtained Pan volume was achieved with the Auto gluing algorithm. A surface shift after the US volumes gluing was noted: the reason of this shift can be found in the pressure applied on the probe during the neck volume acquisitions. Different tissue densities of the neck areas can lead to different compressions during scanning. The Auto gluing algorithm works recognizing and matching the inner structures of the scanned volumes (focused on the re-alignment of inner structures) and leaving a discontinuity reconstruction only at the surface level, considered the “less interesting” part of the reconstructed volume.
The target thyroid nodule and its surrounding tissues were examined also performing ElaXto. Elastosonography was performed for tissue stiffness evaluation of the nodule and the surrounding thyroid areas. In terms of elasticity, the nodule, with respect to the surrounding thyroid parenchyma, resulted harder, as shown in the ElaXto color coded map. The elastosonography evaluation of the nodule and the surrounding thyroid parenchyma stiffness was performed also during real-time simultaneous visualization of 2D US scan, fused with the glued US volume. Elastosonography helped the operator to clearly detect the target nodule, being stiffer than the thyroid surrounding parenchyma. Bi-dimensional US ElaXto examination was performed in different directions, scanning the nodule on several planes containing different nodule views, in order to include the whole area around the nodule (Fig. 3).
Virtual Navigator 3D Pan acquisitions (Fig. 4) were performed taking care of maintaining an overlapping region among the different US volume acquisitions and to limit the shadowing effect as much as possible, avoiding poor probe-tissue coupling with consequent reduction in image quality, in order to obtain high quality B-Mode imaging in all the examined volumes.
The complete duration of the US thyroid examination was increased by 4 minutes in average, due to the US volume acquisitions. The MCS was used and positioned on the patient’s sternal heads conjunction.
Custom color volumetric ball targets, visible on both 2D US and 3D Pan volume, were used in order to better identify the interesting areas. Color Doppler, Power Doppler, Pulsed Wave Doppler evaluations were performed also during real-time simultaneous visualization of 2D US scan fused with the glued US volumes, in order to make a hemodynamic assessment of the nodule and of the surrounding areas. Elastosonography was used in order to recognize thyroid stiffer regions.
The Virtual Biopsy tracked RFA treatments were performed with 6 cm electrodes (in 2 patients) and 10 cm electrodes (in 3 patients), depending on the size and depth of the nodule to be treated. Both RFA tools were electromagnetically tracked within the Virtual Navigator reference space using the CIVCO VTrax. A proper bending of the electrode was performed by the operator (in order to overcome obstacles to properly ablate the nodule) securely keeping the RFA electrode tip within the width of the Virtual Biopsy virtual needle graphical representation (“arrow-shaped” region).
At the moment of the insertion of the RFA electrode, due to tissue compression and to minor neck movements of the patient, a minor fine tuning was necessary in order to maintain properly fused together the 3D acquisition with the 2D real-time scanning.
CEUS was performed before the treatment to check nodule vascularization and exact dimensions. 3D US acquisitions were preformed both regarding CEUS and also regarding conventional B-Mode acquisitions in order to acquire the volume for volumetric measurements and in order to use the pre-treatment volume during the US guidance of the RFA tool. Especially during the gas out phase of the thermal ablation treatment, the real-time fusion imaging of the pre-treatment 3D acquisition was used for visual control of the area already treated with respect to the nodule area still to be treated.
The Virtual Biopsy needle tip virtual visualization was used in order to help the operator during the moving shot technique (Fig. 5) so that he/she is able to move frequently the RFA electrode tip securely also during the gas out phase of the treated tissues. The gas out creates a physical limit for the US propagation which did not allow the operator to clearly see directly the real electrode tip, therefore relying mostly only on its virtual representation offered by the Virtual Biopsy technology.
After the treatment, the nodule was re-acquired with the 3D probe in three 3 patients and with the 3D Pan technology on 2 patients, in order to compare the volume of the nodule before and after the RFA treatment. This procedure was performed in B-Mode modality and also with CEUS (Fig. 6).
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