|ECR 2015 / C-1007|
|Virtual Biopsy and Three Dimensional Ultrasound for Radio Frequency Ablation of Thyroid Nodules|
|This poster is published under an open license. Please read the disclaimer for further details.|
Methods and materials
A. Subject Predisposition
Five patients (4 female, 1 male) with benign thyroid nodules (mean age = 57, range = 42-70) underwent US examination and RFA ablation of the thyroid nodule, after signing a written informed consent. The RFA was performed using a RF Generator-M2004 (equipped with pump RFP 100 and jar for system cooling) with 6 cm and 10 cm radio frequency electrodes RFT-0710N (RF Medical Co., Ltd., Seoul, Korea).
The subject was lying on the examination bed, with the neck slightly hyper-extended and positioned on a head support, in order to keep the neck as stable as possible (Fig. 1).
All the subjects were anesthetized by a US-guided percutaneous approach at the level of the targeted area, using Naropina 7.5 mg (AstraZeneca S.p.A., Basiglio, Milano, Italy).
B. Image Acquistion and RFA Guidance
For all the examinations and RFA guidance, an Esaote MyLabTwice US system (Esaote S.p.A., Genova Italy), equipped with Virtual Navigation option , allowing real-time image fusion of 3D US with 2D US scans, was employed. Moreover, Esaote LA523, LA533, LA332 Linear array probes and BL433 Volumetric Linear array probe (LA523 - Operating Bandwidth: 4-13 MHz; LA332 - Operating Bandwidth: 3-11 MHz; LA533 - Operating Bandwidth: 3-13 MHz; BL433 - Operating Bandwidth: 4-13 MHz) with different reusable tracking brackets with sensor mounted (Esaote Virtual Navigator electromagnetic sensor receiver dedicated support for LA523 and BL433; CIVCO 639-042 for LA533; CIVCO 639-031for LA332 - CIVCO Medical Solutions, Kalona, Iowa, USA) were used. Virtual Navigator tracking of the RFA electrode and real-time fusion imaging between 3D and 2D US data on the US system was possible by an electromagnetic tracking system, consisting of a transmitter on a fixed position, a small receiver mounted on the US probe through a dedicated support and the MCS, applied on the patient’s skin close as much as possible to the examined area (in this case, the patient’s sternal heads conjunction). A proper disc support for the sensor and a blockage with plaster strips were used in order to maintain the MCS as steady as possible. The transmitter, whose position is considered to be the origin of the reference space system, corrected by the data coming from the MCS, was kept steady by a proper support, while the position and the orientation of the US probe in the created 3D space was provided by the receiver unit. The same electromagnetic tracking system, provided for the US probe, was used also for the RFA electrodes tracking. The receiver support used was a CIVCO VirtuTrax (VTrax) Instrument Navigator (CIVCO Medical Solutions, Kalona, Iowa, USA). The magnetic field produced by Virtual Navigator EM tracking system is stronger at the transmitter site and it fades with distance from the transmitter: the magnetic field is lower than the Earth’s magnetic field at a distance of 78 cm from the transmitter, therefore the MCS movement freedom was possible within 78 cm. The EM transmitter was properly positioned to keep all sensors (i.e., US probe, MCS and VTrax) in the most homogeneous region, sited around 50 cm from the source. A non-metallic table was used to reduce as much as possible the interferences with the created electromagnetic field. The MC precision test was already performed and described in a previously published study .
The treatment area was a sterile zone which required the use of sterile covers for the US probes and also for the VTrax tool (CIV-Flex Sterile Covers, CIVCO Medical Solutions, Kalona, Iowa, USA).
C. Three-Dimensional US, Fusion Imaging and RFA Procedure
Before starting the Virtual Biopsy and the three-dimensional US (motorized probe or 3D Pan) procedures, a check of the accuracy of the electromagnetic field was performed: the same point coordinates were measured twice in two different spatial orientations by a dedicated registration pen, with the electromagnetic sensor mounted in. Precision lower than 0.2 cm was considered acceptable.
Virtual Biopsy, enabled by the Virtual Navigator technology, gave to the operator the possibility to plan the RFA electrode path even before its insertion. The RFA electrode insertion was guided in plane and out of plane with proper graphical indications in both situations. The Virtual Biopsy was used considering the single plane 2D US scan alone or considering also the fusion between the 3D Pan acquisition and the 2D Us scan, in order to enlarge the field of view and to have three-dimensional view of the examined and treated area.
A particular visualization tool of the Virtual Biopsy, the Intelligent Positioning system, allowed to activate a sort of viewfinder at the level of the tip of the RFA electrode, in order to help the operator to reach the desired target.
The three-dimensional US acquisition was performed both using a 3D motorized linear probe, electromagnetically tracked in order to enable the fusion imaging between the acquired 3D volume and the 2D US real-time acquisitions, and also using the 3D Pan technology which enabled the use of the same 2D probe for the acquisition of free-hand electromagnetically tracked US volumes.
The 3D Pan tool, based on the electromagnetic field positioning capabilities of Virtual Navigator technology and already employed in other clinical applications [9,10], enabled the gluing of different 3D US neck volumes and the navigation within. The operator had the possibility to use the usual 2D transducers (LA533 and LA332) for volume acquisitions and then to shift to the 2D real-time fusion imaging with the pre-acquired 3D US volume, without any re-synchronization procedure between 3D and 2D views.
A thick layer of sterile US gel (Aquasonic 100, Parker Laboratories Inc, Fairfield, New Jersey, USA) was used to ensure a complete coupling between the transducer and the examined subject’s skin, to avoid black cones and dark areas on the US image and to prevent excessive pressure on the examined area, in order not to change the neck tissue shape and position.
Custom color ball targets were placed on the acquired 2D scans directly or on the 3D US volume, in order to identify the thyroid areas that have to be scanned and treated more precisely, applying different tools for increased diagnostic confidence. Patients underwent US guided RFA treatments for benign thyroid nodules and also Elastosonography around the US-visible nodule, Color Doppler or Power Doppler plus low mechanical index CEUS in order to study the vascularization of the thyroid nodule and its volume. SonoView US contrast media was administered before and after the treatment (Bracco S.p.A., Milano, Italy).
3D Pan reconstruction and gluing algorithm of different US volumes could work using two different processes: “Preview” made a 3D global reconstruction, based only on the geometric and position information given by the probe position and orientation within the Virtual Navigator electromagnetic field, while “Auto”, in addition to the information coming from the tracking system, performed a data analysis focused on tissue structure recognition, in order to find the best matching among the volumes. This could be particularly useful to compensate small movements, due to breathing and/or little tissue compression caused by the US probe during scanning. Major tissue deformation leads to a failure of the automatic gluing process.
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