A Robotically Steerable Guidewire With Forward-Viewing Ultrasound: Development of Technology for Minimally-Invasive Imaging

A 1.11 mm2 IVUS SoC With -Range Plane Wave Transmit Beamforming at 40 MHz

Intravascular ultrasound (IVUS) imaging catheters are significant tools for cardiovascular interventions, and their use can be expanded by realizing IVUS imaging guidewires and microcatheters. The miniaturization of these devices creates challenges in SNR due to the need for higher frequencies to provide adequate resolution. An integrated IVUS system with transmit beamforming can mitigate these limitations. This work presents the first practical highly integrated system-on-a-chip (SoC) with plane wave transmit beamforming at 40 MHz for IVUS on guidewire or microcatheters. The front-end circuitry has a 20-channel ultrasound transmitter (Tx) and receiver (Rx) array interfaced with a capacitive micromachined ultrasound transducer (CMUT) array. During each firing, all 20 Tx are excited with the same analog delay with respect to each other, which can be continuously adjusted between 0 and 10 ns in two directions, generating a steerable plane wave in a range of +/-50 for a phased array at 40 MHz. The unit delays are generated via a voltage-controlled delay line (VCDL), which only needs two external controls, one tuning the unit delay and the other determining the steering direction. The SoC is fabricated using a 180-nm high-voltage (HV) CMOS process and features a slender active area of 0.3 mm 3.7 mm. The proposed SoC consumes 31.3 mW during the receiving mode. The beamformer's functionality and the SoC's overall performance were validated through acoustic characterization and imaging experiments.

2D array imaging system for mechanically-steered, forward-viewing ultrasound guidewire

Approximately 4 million people with peripheral artery disease (PAD) present with critical limb ischemia each year, requiring urgent revascularization to avoid loss of limb. Minimally-invasive (i.e. endovascular) revascularization is preferable due to increased recovery time and increased risk of complications associated with open surgery. However, 40% of people with PAD also have chronic total occlusions (CTOs), resulting in > 20% of revascularization procedures failing when CTOs are present. A steerable robotic guidewire with integrated forward-viewing imaging capabilities would allow the guidewire to navigate through tortuous vasculature and facilitate crossing CTOs in revascularization procedures that currently fail due to inability to route the guidewire. The robotic steering capabilities of the guidewire can be leveraged for 3D synthetic aperture imaging with a simplified, low element count, forward-viewing 2D array on the tip of the mechanically-steered guidewire. Images can then be formed using a hybrid beamforming approach, with focal delays calculated for each element on the tip of the guidewire and for each physical location to which the robotically-steered guidewire is steered. Unlike synthetic aperture imaging with a steerable guidewire having only a single element transducer, an array with even a small number of elements can allow estimation of blood flow and physiological motion in vivo. A miniature, low element count 2D array transducer with 9 total elements (3 × 3) having total dimensions of 1.5 mm × 1.5 mm was designed to operate at 17 MHz. A proof-of-concept 2D array transducer was fabricated and characterized acoustically. The developed array was then used to image a wire target, a peripheral stent, and an ex vivo porcine iliac artery. Images were formed using the described synthetic aperture beamforming strategy. Acoustic characterization showed a mean resonance frequency of 17.6 MHz and a -6 dB bandwidth of 35%. Lateral and axial resolution were 0.271 mm and 0.122 mm, respectively, and an increase in SNR of 4.8 dB was observed for the 2D array relative to the single element case. The first 2D array imaging system utilizing both mechanical and electronic steering for guidewire-based imaging was developed and demonstrated. A 2D array imaging system operating on the tip of the mechanically-steered guidewire provides improved frame rate and increases field of view relative to a single element transducer. Finally, 2D array and single element imaging were compared for introduced motion errors, with the 2D array providing a 46.1% increase in SNR, and 58.5% and 17.3% improvement in lateral and axial resolution, respectively, relative to single element guidewire imaging.

High-Frequency, 2-mm-Diameter Forward-Viewing 2-D Array for 3-D Intracoronary Blood Flow Imaging

Coronary artery disease (CAD) is one of the leading causes of death globally. Currently, diagnosis and intervention in CAD are typically performed via minimally invasive cardiac catheterization procedures. Using current diagnostic technology, such as angiography and fractional flow reserve (FFR), interventional cardiologists must decide which patients require intervention and which can be deferred; 10% of patients with stable CAD are incorrectly deferred using current diagnostic best practices. By developing a forward-viewing intravascular ultrasound (FV-IVUS) 2-D array capable of simultaneously evaluating morphology, hemodynamics, and plaque composition, physicians would be better able to stratify risk of major adverse cardiac events in patients with intermediate stenosis. For this application, a forward-viewing, 16-MHz 2-D array transducer was designed and fabricated. A 2-mm-diameter aperture consisting of 140 elements, with element dimensions of 98×98×70 μ m ( w×h×t ) and a nominal interelement spacing of 120 μ m, was designed for this application based on simulations. The acoustic stack for this array was developed with a designed center frequency of 16 MHz. A novel via-less interconnect was developed to enable electrical connections to fan-out from a 140-element 2-D array with 120- μ m interelement spacing. The fabricated array transducer had 96/140 functioning elements operating at a center frequency of 16 MHz with a -6-dB fractional bandwidth of 62% ± 7 %. Single-element SNR was 23 ± 3 dB, and the measured electrical crosstalk was - 33 ± 3 dB. In imaging experiments, the measured lateral resolution was 0.231 mm and the measured axial resolution was 0.244 mm at a depth of 5 mm. Finally, the transducer was used to perform 3-D B-mode imaging of a 3-mm-diameter spring and 3-D B-mode and power Doppler imaging of a tissue-mimicking phantom.

Endovascular Detection of Catheter-Thrombus Contact by Vacuum Excitation

OBJECTIVE

The objective of this work is to introduce and demonstrate the effectiveness of a novel sensing modality for contact detection between an off-the-shelf aspiration catheter and a thrombus.

METHODS

A custom robotic actuator with a pressure sensor was used to generate an oscillatory vacuum excitation and sense the pressure inside the extracorporeal portion of the catheter. Vacuum pressure profiles and robotic motion data were used to train a support vector machine (SVM) classification model to detect contact between the aspiration catheter tip and a mock thrombus. Validation consisted of benchtop accuracy verification, as well as user study comparison to the current standard of angiographic presentation.

RESULTS

Benchtop accuracy of the sensing modality was shown to be 99.67%. The user study demonstrated statistically significant improvement in identifying catheter-thrombus contact compared to the current standard. The odds ratio of successful detection of clot contact was 2.86 (p = 0.03) when using the proposed sensory method compared to without it.

CONCLUSION

The results of this work indicate that the proposed sensing modality can offer intraoperative feedback to interventionalists that can improve their ability to detect contact between the distal tip of a catheter and a thrombus.

SIGNIFICANCE

By offering a relatively low-cost technology that affords off-the-shelf aspiration catheters as clot-detecting sensors, interventionalists can improve the first-pass effect of the mechanical thrombectomy procedure while reducing procedural times and mental burden.

The CathEye: A Forward-Looking Ultrasound Catheter for Image-Guided Cardiovascular Procedures

Catheter based procedures are typically guided by X-Ray, which suffers from low soft tissue contrast and only provides 2D projection images of a 3D volume. Intravascular ultrasound (IVUS) can serve as a complementary imaging technique. Forward viewing catheters are useful for visualizing obstructions along the path of the catheter. The CathEye system mechanically steers a single-element transducer to generate a forward-looking surface reconstruction from an irregularly spaced 2-D scan pattern. The steerable catheter leverages an expandable frame with cables to manipulate the distal end independently of vessel tortuosity. The tip position is estimated by measuring the cable displacements and used to create surface reconstructions of the imaging workspace with the single-element transducer. CathEye's imaging capabilities were tested with an agar phantom and an ex vivo chronic total occlusion (CTO) sample while the catheter was confined to various tortuous paths. The CathEye maintained similar scan patterns regardless of path tortuosity and was able to recreate major features of the imaging targets, such as holes and extrusions. The feasibility of forward-looking IVUS with the CathEye is demonstrated in this study. The CathEye mechanism can be applied to other imaging modalities with field-of-view (FOV) limitations and represents the basis for an interventional device fully integrated with image guidance.

Supervised segmentation for guiding peripheral revascularization with forward-viewing, robotically steered ultrasound guidewire

BACKGROUND

Approximately 500 000 patients present with critical limb ischemia (CLI) each year in the U.S., requiring revascularization to avoid amputation. While peripheral arteries can be revascularized via minimally invasive procedures, 25% of cases with chronic total occlusions are unsuccessful due to inability to route the guidewire beyond the proximal occlusion. Improvements to guidewire navigation would lead to limb salvage in a greater number of patients.

PURPOSE

Integrating ultrasound imaging into the guidewire could enable direct visualization of routes for guidewire advancement. In order to navigate a robotically-steerable guidewire with integrated imaging beyond a chronic occlusion proximal to the symptomatic lesion for revascularization, acquired ultrasound images must be segmented to visualize the path for guidewire advancement.

METHODS

The first approach for automated segmentation of viable paths through occlusions in peripheral arteries is demonstrated in simulations and experimentally-acquired data with a forward-viewing, robotically-steered guidewire imaging system. B-mode ultrasound images formed via synthetic aperture focusing (SAF) were segmented using a supervised approach (U-net architecture). A total of 2500 simulated images were used to train the classifier to distinguish the vessel wall and occlusion from viable paths for guidewire advancement. First, the size of the synthetic aperture resulting in the highest classification performance was determined in simulations (90 test images) and compared with traditional classifiers (global thresholding, local adaptive thresholding, and hierarchical classification). Next, classification performance as a function of the diameter of the remaining lumen (0.5 to 1.5 mm) in the partially-occluded artery was tested using both simulated (60 test images at each of 7 diameters) and experimental data sets. Experimental test data sets were acquired in four 3D-printed phantoms from human anatomy and six ex vivo porcine arteries. Accuracy of classifying the path through the artery was evaluated using microcomputed tomography of phantoms and ex vivo arteries as a ground truth for comparison.

RESULTS

An aperture size of 3.8 mm resulted in the best-performing classification based on sensitivity and Jaccard index, with a significant increase in Jaccard index (p < 0.05) as aperture diameter increased. In comparing the performance of the supervised classifier and traditional classification strategies with simulated test data, sensitivity and F1 score for U-net were 0.95 ± 0.02 and 0.96 ± 0.01, respectively, compared to 0.83 ± 0.03 and 0.41 ± 0.13 for the best-performing conventional approach, hierarchical classification. In simulated test images, sensitivity (p < 0.05) and Jaccard index both increased with increasing artery diameter (p < 0.05). Classification of images acquired in artery phantoms with remaining lumen diameters ≥ 0.75 mm resulted in accuracies > 90%, while mean accuracy decreased to 82% when artery diameter decreased to 0.5 mm. For testing in ex vivo arteries, average binary accuracy, F1 score, Jaccard index, and sensitivity each exceeded 0.9.

CONCLUSIONS

Segmentation of ultrasound images of partially-occluded peripheral arteries acquired with a forward-viewing, robotically-steered guidewire system was demonstrated for the first-time using representation learning. This could represent a fast, accurate approach for guiding peripheral revascularization.

Fluoroscopic Image-Based 3-D Environment Reconstruction and Automated Path Planning for a Robotically Steerable Guidewire

Cardiovascular diseases are the leading cause of death globally and surgical treatments for these often begin with the manual placement of a long compliant wire, called a guidewire, through different vasculature. To improve procedure outcomes and reduce radiation exposure, we propose steps towards a fully automated approach for steerable guidewire navigation within vessels. In this paper, we utilize fluoroscopic images to fully reconstruct 3-D printed phantom vasculature models by using a shape-from-silhouette algorithm. The reconstruction is subsequently de-noised using a deep learning-based encoder-decoder network and morphological filtering. This volume is used to model the environment for guidewire traversal. Following this, we present a novel method to plan an optimal path for guidewire traversal in three-dimensional vascular models through the use of slice planes and a modified hybrid A-star algorithm. Finally, the developed reconstruction and planning approaches are applied to an ex vivo porcine aorta, and navigation is demonstrated through the use of a tendon-actuated COaxially Aligned STeerable guidewire (COAST).

Dual-Resonance (16/32 MHz) Piezoelectric Transducer With a Single Electrical Connection for Forward-Viewing Robotic Guidewire

Peripheral artery disease (PAD) affects more than 200 million people globally. Minimally invasive endovascular procedures can provide relief and salvage limbs while reducing injury rates and recovery times. Unfortunately, when a calcified chronic total occlusion is encountered, ~25% of endovascular procedures fail due to the inability to advance a guidewire using the view provided by fluoroscopy. To enable a sub-millimeter, robotically steerable guidewire to cross these occlusions, a novel single-element, dual-band transducer is developed that provides simultaneous multifrequency, forward-viewing imaging with high penetration depth and high spatial resolution while requiring only a single electrical connection. The design, fabrication, and acoustic characterization of this device are described, and proof-of-concept imaging is demonstrated in an ex vivo porcine artery after integration with a robotically steered guidewire. Measured center frequencies of the developed transducer were 16 and 32 MHz, with -6 dB fractional bandwidths of 73% and 23%, respectively. When imaging a 0.2-mm wire target at a depth of 5 mm, measured -6 dB target widths were 0.498 ± 0.02 and 0.268 ± 0.01 mm for images formed at 16 and 32 MHz, respectively. Measured SNR values were 33.3 and 21.3 dB, respectively. The 3-D images of the ex vivo artery demonstrate high penetration for visualizing vessel morphology at 16 MHz and ability to resolve small features close to the transducer at 32 MHz. Using images acquired simultaneously at both frequencies as part of an integrated forward-viewing, guidewire-based imaging system, an interventionalist could visualize the best path for advancing the guidewire to improve outcomes for patients with PAD.