Interventional real-time ultrasound imaging with an integrated electromagnetic field generator

Miniaturized electromagnetic tracking enables efficient ultrasound-navigated needle insertions

Ultrasound (US) has gained popularity as a guidance modality for percutaneous needle insertions because it is widely available and non-ionizing. However, coordinating scanning and needle insertion still requires significant experience. Current assistance solutions utilize optical or electromagnetic tracking (EMT) technology directly integrated into the US device or probe. This results in specialized devices or introduces additional hardware, limiting the ergonomics of both the scanning and insertion process. We developed the first ultrasound (US) navigation solution designed to be used as a non-permanent accessory for existing US devices while maintaining the ergonomics during the scanning process. A miniaturized EMT source is reversibly attached to the US probe, temporarily creating a combined modality that provides real-time anatomical imaging and instrument tracking at the same time. Studies performed with 11 clinical operators show that the proposed navigation solution can guide needle insertions with a targeting accuracy of about 5 mm, which is comparable to existing approaches and unaffected by repeated attachment and detachment of the miniaturized tracking solution. The assistance proved particularly helpful for non-expert users and needle insertions performed outside of the US plane. The small size and reversible attachability of the proposed navigation solution promises streamlined integration into the clinical workflow and widespread access to US navigated punctures.

Multimodal registration of ultrasound and MR images using weighted self-similarity structure vector

PROPOSE

Multimodal registration of 2D Ultrasound (US) and 3D Magnetic Resonance (MR) for fusion navigation can improve the intraoperative detection accuracy of lesion. However, multimodal registration remains a challenge because of the poor US image quality. In the study, a weighted self-similarity structure vector (WSSV) is proposed to registrate multimodal images.

METHOD

The self-similarity structure vector utilizes the normalized distance of symmetrically located patches in the neighborhood to describe the local structure information. The texture weights are extracted using the local standard deviation to reduce the speckle interference in the US images. The multimodal similarity metric is constructed by combining a self-similarity structure vector with a texture weight map.

RESULTS

Experiments were performed on US and MR images of the liver from 88 groups of data including 8 patients and 80 simulated samples. The average target registration error was reduced from 14.91 ± 3.86 mm to 4.95 ± 2.23 mm using the WSSV-based method.

CONCLUSIONS

The experimental results show that the WSSV-based registration method could robustly align the US and MR images of the liver. With further acceleration, the registration framework can be potentially applied in time-sensitive clinical settings, such as US-MR image registration in image-guided surgery.

Recent Advances in Tracking Devices for Biomedical Ultrasound Imaging Applications

With the rapid advancement of tracking technologies, the applications of tracking systems in ultrasound imaging have expanded across a wide range of fields. In this review article, we discuss the basic tracking principles, system components, performance analyses, as well as the main sources of error for popular tracking technologies that are utilized in ultrasound imaging. In light of the growing demand for object tracking, this article explores both the potential and challenges associated with different tracking technologies applied to various ultrasound imaging applications, including freehand 3D ultrasound imaging, ultrasound image fusion, ultrasound-guided intervention and treatment. Recent development in tracking technology has led to increased accuracy and intuitiveness of ultrasound imaging and navigation with less reliance on operator skills, thereby benefiting the medical diagnosis and treatment. Although commercially available tracking systems are capable of achieving sub-millimeter resolution for positional tracking and sub-degree resolution for orientational tracking, such systems are subject to a number of disadvantages, including high costs and time-consuming calibration procedures. While some emerging tracking technologies are still in the research stage, their potentials have been demonstrated in terms of the compactness, light weight, and easy integration with existing standard or portable ultrasound machines.

Reattachable fiducial skin marker for automatic multimodality registration

Purpose

Fusing image information has become increasingly important for optimal diagnosis and treatment of the patient. Despite intensive research towards markerless registration approaches, fiducial marker-based methods remain the default choice for a wide range of applications in clinical practice. However, as especially non-invasive markers cannot be positioned reproducibly in the same pose on the patient, pre-interventional imaging has to be performed immediately before the intervention for fiducial marker-based registrations.

Methods

We propose a new non-invasive, reattachable fiducial skin marker concept for multi-modal registration approaches including the use of electromagnetic or optical tracking technologies. We furthermore describe a robust, automatic fiducial marker localization algorithm for computed tomography (CT) and magnetic resonance imaging (MRI) images. Localization of the new fiducial marker has been assessed for different marker configurations using both CT and MRI. Furthermore, we applied the marker in an abdominal phantom study. For this, we attached the marker at three poses to the phantom, registered ten segmented targets of the phantom’s CT image to live ultrasound images and determined the target registration error (TRE) for each target and each marker pose.

Results

Reattachment of the marker was possible with a mean precision of 0.02 mm ± 0.01 mm. Our algorithm successfully localized the marker automatically in all ($$n=201$$ n = 201 ) evaluated CT/MRI images. Depending on the marker pose, the mean ($$n=10$$ n = 10 ) TRE of the abdominal phantom study ranged from 1.51 ± 0.75 mm to 4.65 ± 1.22 mm.

Conclusions

The non-invasive, reattachable skin marker concept allows reproducible positioning of the marker and automatic localization in different imaging modalities. The low TREs indicate the potential applicability of the marker concept for clinical interventions, such as the puncture of abdominal lesions, where current image-based registration approaches still lack robustness and existing marker-based methods are often impractical.

Technical Note: Assessment of electromagnetic tracking systems in a surgical environment using ultrasonography and ureteroscopy instruments for percutaneous renal access

PURPOSE

Electromagnetic tracking systems (EMTSs) have been proposed to assist the percutaneous renal access (PRA) during minimally invasive interventions to the renal system. However, the influence of other surgical instruments widely used during PRA (like ureteroscopy and ultrasound equipment) in the EMTS performance is not completely known. This work performs this assessment for two EMTSs [Aurora^® Planar Field Generator (PFG); Aurora^® Tabletop Field Generator (TTFG)].

METHODS

An assessment platform, composed by a scaffold with specific supports to attach the surgical instruments and a plate phantom with multiple levels to precisely translate or rotate the surgical instruments, was developed. The median accuracy and precision in terms of position and orientation were estimated for the PFG and TTFG in a surgical environment using this platform. Then, the influence of different surgical instruments (alone or together), namely analogic flexible ureterorenoscope (AUR), digital flexible ureterorenoscope (DUR), two-dimensional (2D) ultrasound (US) probe, and four-dimensional (4D) mechanical US probe, was assessed for both EMTSs by coupling the instruments to 5-DOF and 6-DOF sensors.

RESULTS

Overall, the median positional and orientation accuracies in the surgical environment were 0.85 mm and 0.42° for PFG, and 0.72 mm and 0.39° for TTFG, while precisions were 0.10 mm and 0.03° for PFG, and 0.20 mm and 0.12° for TTFG, respectively. No significant differences were found for accuracy between EMTSs. However, PFG showed a tendency for higher precision than TTFG. AUR, DUR, and 2D US probe did not influence the accuracy and precision of both EMTSs. In opposition, the 4D probe distorted the signal near the attached sensor, making readings unreliable.

CONCLUSIONS

Ureteroscopy- and ultrasonography-assisted PRA based on EMTS guidance are feasible with the tested AUR or DUR together with the 2D probe. More studies must be performed to evaluate the probes and ureterorenoscopes' influence before their use in PRA based on EMTS guidance.

Polhemus EM tracked Micro Sensor for CT-guided interventions

PURPOSE

Electromagnetic (EM) tracking is a key technology in image-guided therapy. A new EM Micro Sensor was presented by Polhemus Inc.; it is the first to enable localization of medical instruments through their trackers. Different field generators (FGs) are available by Polhemus, one being almost as small as a sugar cube. As accuracy and robustness of tracking are known challenges to using EM trackers in clinical environments, the goal of this study was a standardized assessment of the Micro Sensor in both a laboratory (lab) and a computed tomography (CT) environment.

METHODS

The Micro Sensor was assessed by means of Hummel et al.'s standardized protocol; it was assessed in conjunction with a Polhemus Liberty tracker and three FGs - with edge lengths of 1 (TX1), 2 (TX2), and 4 (TX4) inches. Precision as well as positional and rotational accuracy were determined in a lab and a CT suite. Distortions by four different metallic cylinders and tracking of two typical medical instruments - a hypodermic needle and a flexible endoscope - were also tested.

RESULTS

A jitter of 0.02 mm or less was found for all FGs in the different environments, except for the TX2 FG for which no valid data could be obtained in the CT. Errors of 5 cm distance measurements were 0.6 mm or less for all FGs in the lab. While the distance errors of the TX1 FG were only slightly increased up to 1.6 mm in the CT, those of the TX4 FG were found to be up to around 10% of the measured distance (5.4 mm on average). The mean orientation error was found to be 0.9^° /0.5^° /0.1^° for the TX4/TX2/TX1 FG in the lab. In the CT environment, rotation errors were in the same range: less than 1.2^° /0.1^° for the TX4/TX1 FG. Deviation under the presence of metallic cylinders stayed below 1 mm in most cases. Precision and orientational accuracy do not seem to be affected by instrument tracking and stayed in the same range as for the other measurements whereas distance errors were slightly increased up to 1.7 mm.

CONCLUSION

This study shows that accurate tracking of medical instruments is possible with the new Micro Sensor; it demonstrated a jitter of 0.01 mm or less, position errors below 2 mm, and rotation errors of less than 0.3^° . As with other EM trackers, errors increase when large tracking volumes with ranges of up to 50 cm are required in clinical environments. For smaller tracking volumes with ranges of up to 15 cm, a high accuracy and robustness was found. This is interesting especially for the TX1 FG which can easily be placed in close vicinity to the region of interest.

Ultrasound-Guided Biopsies of Bone Lesions Without Cortical Disruption Using Fusion Imaging and Needle Tracking: Proof of Concept

OBJECTIVE

To assess the technical feasibility and safety of combined fusion imaging and needle tracking under ultrasound guidance to target bone lesions without cortical disruption.

MATERIALS AND METHODS

Between January 2016 and March 2016, seven patients underwent US-guided biopsy of bone lesions without cortical disruption. Targeted bone lesions were measuring more than 1.5 cm with a thin cortex, a trans-osseous pathway not exceeding 2 cm and without any adjacent vulnerable structures. First three procedures were performed in the CT suite to aid the needle tracking where necessary (group 1), the remaining four procedures were performed in the US suite (group 2). In group 1, deviation from the real position of the bone trocar (estimated on CT) was compared to the virtual position (estimated on the fusion CT-US images). In both group, procedure data and histopathological results were collected, and compared to the suspected diagnosis and follow-up.

RESULTS

Mean procedure duration was 44 min. Total number of synchronisation points for combined fusion imaging were 3.3 on average. In group 1, mean deviation between the virtual and real CT coordinates was 5.3 mm on average. All biopsies yielded adequate quality analysable bone sample. Histopathological analysis revealed malignancy in three cases, non-specific inflammation in two cases, and normal bone in two cases. The four benign results were confirmed as true negative results. There were no immediate or post-procedural complications.

CONCLUSION

The use of combined fusion imaging and needle tracking ultrasound guidance to target bone lesions without cortical disruption seems technically feasible, provided the patient and lesion selection is appropriate.

First clinical use of the EchoTrack guidance approach for radiofrequency ablation of thyroid gland nodules

PURPOSE

Percutaneous radiofrequency ablation (RFA) of thyroid nodules is an alternative to surgical resection that offers the benefits of minimal scars for the patient, lower complication rates, and shorter treatment times. Ultrasound (US) is the preferred modality for guiding these procedures. The needle is usually kept within the US scanning plane to ensure needle visibility. However, this restricts flexibility in both transducer and needle movement and renders the procedure difficult, especially for inexperienced users. Existing navigation solutions often involve electromagnetic (EM) tracking, which requires placement of an external field generator (FG) in close proximity of the intervention site in order to avoid distortion of the EM field. This complicates the clinical workflow as placing the FG while ensuring that it neither restricts the physician's workspace nor affects tracking accuracy is awkward and time-consuming.

METHODS

The EchoTrack concept overcomes these issues by combining the US probe and the EM FG in one modality, simultaneously providing both real-time US and tracking data without requiring the placement of an external FG for tracking. We propose a system and workflow to use EchoTrack for RFA of thyroid nodules.

RESULTS

According to our results, the overall error of the EchoTrack system resulting from errors related to tracking and calibration is below 2 mm. Navigated thyroid RFA with the proposed concept is clinically feasible. Motion of internal critical structures relative to external markers can be up to several millimeters in extreme cases.

CONCLUSIONS

The EchoTrack concept with its simple setup, flexibility, improved needle visualization, and additional guidance information has high potential to be clinically used for thyroid RFA.

Ultrasound-navigated radiofrequency ablation of thyroid nodules with integrated electromagnetic tracking: comparison with conventional ultrasound guidance in gelatin models

PURPOSE

A thyroid-like gelatin model was used to determine potential superiority of a new navigation system for ultrasound (US)-guided electrode insertion called EchoTrack, featuring a US probe with an integrated electromagnetic field generator, in comparison with conventional US when performing radiofrequency ablation.

METHODS

In order to compare 20 navigated ablations with 20 ablations under conventional US guidance, a thyroid-like gelatin model was used. In each group, 10 in-plane and 10 out-of-plane punctures were performed. Metal seeds measuring 8.5 [Formula: see text] 1.8 mm served as ablation targets. The number of redirections until final electrode placement, targeting accuracy and electrode placement time were measured.

RESULTS

The number of redirections could be significantly ([Formula: see text]) reduced from 2.7 ± 1.3 in the conventional group to 0.2 ± 0.5 in the EchoTrack group. Accuracy increased from 3.9 ± 4.7 to 2.0 ± 1.9 mm. The total placement time increased from 39 ± 20.5 to 79.2 ± 26 s.

CONCLUSIONS

EchoTrack is able to reduce the redirections needed to place the electrode in comparison with conventional US and provides high placement accuracy. Our new navigation system has high potential to reduce the risk of harming critical structures and to improve guidance during ablation of difficult nodules, as treatment planning as well as the safety of out-of-plane punctures are improved.