Robust and semantic needle detection in 3D ultrasound using orthogonal-plane convolutional neural networks.

PURPOSE: During needle interventions, successful automated detection of the needle immediately after insertion is necessary to allow the physician identify and correct any misalignment of the needle and the target at early stages, which reduces needle passes and improves health outcomes. METHODS: We present a novel approach to localize partially inserted needles in 3D ultrasound volume with high precision using convolutional neural networks. We propose two methods based on patch classification and semantic segmentation of the needle from orthogonal 2D cross-sections extracted from the volume. For patch classification, each voxel is classified from locally extracted raw data of three orthogonal planes centered on it. We propose a bootstrap resampling approach to enhance the training in our highly imbalanced data. For semantic segmentation, parts of a needle are detected in cross-sections perpendicular to the lateral and elevational axes. We propose to exploit the structural information in the data with a novel thick-slice processing approach for efficient modeling of the context. RESULTS: Our introduced methods successfully detect 17 and 22 G needles with a single trained network, showing a robust generalized approach. Extensive ex-vivo evaluations on datasets of chicken breast and porcine leg show 80 and 84% F1-scores, respectively. Furthermore, very short needles are detected with tip localization errors of less than 0.7 mm for lengths of only 5 and 10 mm at 0.2 and 0.36 mm voxel sizes, respectively. CONCLUSION: Our method is able to accurately detect even very short needles, ensuring that the needle and its tip are maximally visible in the visualized plane during the entire intervention, thereby eliminating the need for advanced bi-manual coordination of the needle and transducer.

Quantitative imaging performance of frequency-tunable capacitive micromachined ultrasonic transducer array designed for intracardiac application: Phantom study.

Commercially available intracardiac echo (ICE) catheters face a trade-off between viewing depth and resolution. Frequency-tunable ICE probes would offer versatility of choice between penetration or resolution imaging within a single device. In this phantom study, the imaging performance of a novel, frequency-tunable, 32-element, 1-D CMUT array integrated with front-end electronics is evaluated. Phased-array ultrasound imaging with a forward-looking CMUT probe prototype operated beyond collapse mode at voltages up to three times higher than the collapse voltage (-65 V) is demonstrated. Imaging performance as a function of bias voltage (-70 V to -160 V), transmit pulse frequency (5-25 MHz), and number of transmit pulse cycles (1-3) is quantified, based on which penetration, resolution, and generic imaging modes are identified. It is shown that by utilizing the concept of frequency tuning, images with different characteristics can be generated trading-off the resolution and penetration depth. The penetration mode provides imaging up to 71 mm in the tissue-mimicking phantom, axial resolution of 0.44 mm, and lateral resolution of 0.12 rad. In the resolution mode, axial resolution of 0.055 mm, lateral resolution of 0.035 rad, and penetration depth of 16 mm are measured. These results show what this CMUT array has the potential versatile characteristics needed for intracardiac imaging, despite its relatively small transducer aperture size of 2 mm × 2 mm imposed by the clinical application.

Near-Field Ultrasound Imaging During Radiofrequency Catheter Ablation: Tissue Thickness and Epicardial Wall Visualization and Assessment of Radiofrequency Ablation Lesion Formation and Depth.

BACKGROUND: Safe and successful radiofrequency catheter ablation depends on creation of transmural lesions without collateral injury to contiguous structures. Near-field ultrasound (NFUS) imaging through transducers in the tip of an ablation catheter may provide important information about catheter contact, wall thickness, and ablation lesion formation. METHODS AND RESULTS: NFUS imaging was performed using a specially designed open-irrigated radiofrequency ablation catheter incorporating 4 ultrasound transducers. Tissue/phantom thickness was measured in vitro with varying contact angles. In vivo testing was performed in 19 dogs with NFUS catheters positioned in 4 chambers. Wall thickness measurements were made at 222 sites (excluding the left ventricle) and compared with measurements from intracardiac echocardiography. Imaging was used to identify the epicardium with saline infusion into the pericardial space at 39 sites. In vitro, the measured exceeded actual tissue/phantom thickness by 13% to 20%. In vivo, NFUS reliably visualized electrode-tissue contact, but sensitivity of epicardial imaging was 92%. The chamber wall thickness measured by NFUS correlated well with intracardiac echocardiography (r=0.86; P<0.0001). Sensitivity of lesion identification by NFUS was 94% for atrial and 95% for ventricular ablations. NFUS was the best parameter to predict lesion depth in right and left ventricle (r=0.47; P<0.0001; multiple regression P=0.0025). Lesion transmurality was correctly identified in 87% of atrial lesions. CONCLUSIONS: NFUS catheter imaging reliably assesses electrode-tissue contact and wall thickness. Its use during radiofrequency catheter ablation may allow the operator to assess the depth of ablation required for transmural lesion formation to optimize power delivery.

Preclinical Testing of Frequency-Tunable Capacitive Micromachined Ultrasonic Transducer Probe Prototypes.

In intracardiac echocardiography (ICE) it may be beneficial to generate ultrasound images acquired at multiple frequencies, having the possibility of high penetration or high-resolution imaging in a single device. The objective of the presented work is to test two frequency-tunable probe prototypes in a preclinical setting: a rigid probe having a diameter of 11 mm and a new flexible and steerable 12-Fr ICE catheter. Both probes feature a forward-looking 32-element capacitive micromachined ultrasonic transducer array (aperture of 2 × 2 mm2) operated in collapse mode, which allows for frequency tuning in the 6-MHz-18-MHz range. The rigid probe prototype is tested ex vivo in a passive heart platform. Images of an aortic valve acquired in high-penetration (6 MHz), generic (12 MHz) and high-resolution (18 MHz) mode combine satisfying image quality and penetration depth between 2.5 cm and 10 cm. The ICE catheter prototype is tested in vivo using a porcine animal model. Images of an aortic valve are acquired in the 3 imaging modes with the ICE catheter placed in an ascending aorta at multiple depths. It was found that the combination of the forward-looking design and frequency-tuning capability allows visualizing intracardiac structures of various sizes at different distances relative to the catheter tip, providing both wide overviews and detailed close-ups.

Medical Instrument Detection in 3-Dimensional Ultrasound Data Volumes.

Ultrasound-guided medical interventions are broadly applied in diagnostics and therapy, e.g., regional anesthesia or ablation. A guided intervention using 2-D ultrasound is challenging due to the poor instrument visibility, limited field of view, and the multi-fold coordination of the medical instrument and ultrasound plane. Recent 3-D ultrasound transducers can improve the quality of the image-guided intervention if an automated detection of the needle is used. In this paper, we present a novel method for detecting medical instruments in 3-D ultrasound data that is solely based on image processing techniques and validated on various ex vivo and in vivo data sets. In the proposed procedure, the physician is placing the 3-D transducer at the desired position, and the image processing will automatically detect the best instrument view, so that the physician can entirely focus on the intervention. Our method is based on the classification of instrument voxels using volumetric structure directions and robust approximation of the primary tool axis. A novel normalization method is proposed for the shape and intensity consistency of instruments to improve the detection. Moreover, a novel 3-D Gabor wavelet transformation is introduced and optimally designed for revealing the instrument voxels in the volume, while remaining generic to several medical instruments and transducer types. Experiments on diverse data sets, including in vivo data from patients, show that for a given transducer and an instrument type, high detection accuracies are achieved with position errors smaller than the instrument diameter in the 0.5-1.5-mm range on average.

Frequency Tuning of Collapse-Mode Capacitive Micromachined Ultrasonic Transducer.

The information in an ultrasound image depends on the frequency that is used. In a clinical examination it may therefore be beneficial to generate ultrasound images acquired at multiple frequencies, which is difficult to achieve with conventional transducers. Capacitive micromachined ultrasonic transducers (CMUTs) offer a frequency response that is tunable by the bias voltage. In this study we investigate this frequency tunability for ultrasonic imaging. We characterized a CMUT array operated at bias voltages up to three times higher than the collapse-voltage. All elements of the array were connected to a single transmit and receive channel through a bias circuit. We quantified the transmit-receive and transmit sensitivity as a function of frequency for a range of bias voltages. Impulse response measurements show that the center frequency is modifiable between 8.7MHz and 15.3MHz with an applied bias voltage of -50V to -170V. The maximum transmit sensitivity is 52kPa/V at a center frequency of 9.0MHz with an applied bias voltage of -105V. The -3dB transmit range in center frequency accessible with the variable bias voltage is 6.7-15.5MHz. This study shows that a collapse-mode CMUT can operate efficiently at multiple center frequencies when the driving pulse and the bias voltage are optimized. We demonstrate the usefulness of frequency tuning by comparing images at different optimal combinations of driving frequency and bias voltage, acquired by linearly moving the transducer across a tissue mimicking phantom.

High-resolution resonant and nonresonant fiber-scanning confocal microscope.

We present a novel, hand-held microscope probe for acquiring confocal images of biological tissue. This probe generates images by scanning a fiber-lens combination with a miniature electromagnetic actuator, which allows it to be operated in resonant and nonresonant scanning modes. In the resonant scanning mode, a circular field of view with a diameter of 190 µm and an angular frequency of 127 Hz can be achieved. In the nonresonant scanning mode, a maximum field of view with a width of 69 µm can be achieved. The measured transverse and axial resolutions are 0.60 and 7.4 µm, respectively. Images of biological tissue acquired in the resonant mode are presented, which demonstrate its potential for real-time tissue differentiation. With an outer diameter of 3 mm, the microscope probe could be utilized to visualize cellular microstructures in vivo across a broad range of minimally-invasive procedures.