Optimizing circular ring arrays for forward-looking IVUS imaging

Deep learning assisted sparse array ultrasound imaging

This study aims to restore grating lobe artifacts and improve the image resolution of sparse array ultrasonography via a deep learning predictive model. A deep learning assisted sparse array was developed using only 64 or 16 channels out of the 128 channels in which the pitch is two or eight times the original array. The deep learning assisted sparse array imaging system was demonstrated on ex vivo porcine teeth. 64- and 16-channel sparse array images were used as the input and corresponding 128-channel dense array images were used as the ground truth. The structural similarity index measure, mean squared error, and peak signal-to-noise ratio of predicted images improved significantly (p < 0.0001). The resolution of predicted images presented close values to ground truth images (0.18 mm and 0.15 mm versus 0.15 mm). The gingival thickness measurement showed a high level of agreement between the predicted sparse array images and the ground truth images, as indicated with a bias of -0.01 mm and 0.02 mm for the 64- and 16-channel predicted images, respectively, and a Pearson's r = 0.99 (p < 0.0001) for both. The gingival thickness bias measured by deep learning assisted sparse array imaging and clinical probing needle was found to be <0.05 mm. Additionally, the deep learning model showed capability of generalization. To conclude, the deep learning assisted sparse array can reconstruct high-resolution ultrasound image using only 16 channels of 128 channels. The deep learning model performed generalization capability for the 64-channel array, while the 16-channel array generalization would require further optimization.

Concentric-ring arrays for forward-viewing ultrasound imaging

Purpose

Current ultrasound (US)-image-guided needle insertions often require an expertized technique for clinicians because the performance of tasks in a three-dimensional space using two-dimensional images requires operators to cognitively maintain the spatial relationships between the US probe, the needle, and the lesion. This work presents forward-viewing US imaging with a ring array configuration to enable needle interventions without requiring the registration between tools and targets.

Approach

The center-open ring array configuration allows the needle to be inserted from the center of the visualized US image, providing simple and intuitive guidance. To establish the feasibility of the ring array configuration, the design parameters causing the image quality, including the radius of the center hole and the number of ring layers and transducer elements, were investigated.

Results

Experimental results showed successful visualization, even with a hole in the transducer elements, and the target visibility was improved by increasing the number of ring layers and the number of transducer elements in each ring layer. Reducing the hole radius improved the region's image quality at a shallow depth.

Conclusions

Forward-viewing US imaging with a ring array configuration has the potential to be a viable alternative to conventional US image-guided needle insertion methods.

Design, Implementation, and Medical Applications of 2-D Ultrasound Sparse Arrays

An ultrasound sparse array consists of a sparse distribution of elements over a 2-D aperture. Such an array is typically characterized by a limited number of elements, which in most cases is compatible with the channel number of the available scanners. Sparse arrays represent an attractive alternative to full 2-D arrays that may require the control of thousands of elements through expensive application-specific integrated circuits (ASICs). However, their massive use is hindered by two main drawbacks: the possible beam profile deterioration, which may worsen the image contrast, and the limited signal-to-noise ratio (SNR), which may result too low for some applications. This article reviews the work done for three decades on 2-D ultrasound sparse arrays for medical applications. First, random, optimized, and deterministic design methods are reviewed together with their main influencing factors. Then, experimental 2-D sparse array implementations based on piezoelectric and capacitive micromachined ultrasonic transducer (CMUT) technologies are presented. Sample applications to 3-D (Doppler) imaging, super-resolution imaging, photo-acoustic imaging, and therapy are reported. The final sections discuss the main shortcomings associated with the use of sparse arrays, the related countermeasures, and the next steps envisaged in the development of innovative arrays.

Dependence of Temperature Rise on the Position of Catheters and Implants Power Sources Due to the Heat Transfer into the Blood Flow

This work provides a numerical analysis of heat transfer from medical devices such as catheters and implants to the blood flow by considering the relative position of such power sources to the vessel wall. We have used COMSOL Multiphysics® software to simulate the heat transfer in the blood flow, using the finite element method and Carreau-–Yasuda fluid model (a non-Newtonian model for blood flow). The location of the power source is changed (from the center to near the wall) in the blood vessel with small steps, while the blood flow takes different velocities. The numerical simulations show that when the catheter/implant approaches the vessel wall, the temperature increases linearly for ~90% of the radial displacement from the centerline position to the vessel wall, while for the last 10% of the radial displacement, the temperature increases exponentially. As a result, the temperature is increased significantly, when changing the position of the catheter/implant from the centerline to the area adjacent to the vessel wall.

Sparse Convolutional Beamforming for 3-D Ultrafast Ultrasound Imaging

Real-time 3-D ultrasound (US) provides a complete visualization of inner body organs and blood vasculature, crucial for diagnosis and treatment of diverse diseases. However, 3-D systems require massive hardware due to the huge number of transducer elements and consequent data size. This increases cost significantly and limit both frame rate and image quality, thus preventing the 3-D US from being common practice in clinics worldwide. A recent study presented a technique called sparse convolutional beamforming algorithm (SCOBA), which obtains improved image quality while allowing notable element reduction in the context of 2-D focused imaging. In this article, we build upon previous work and introduce a nonlinear beamformer for 3-D imaging, called COBA-3D, consisting of 2-D spatial convolution of the in-phase and quadrature received signals. The proposed technique considers diverging-wave transmission and achieves improved image resolution and contrast compared with standard delay-and-sum beamforming while enabling a high frame rate. Incorporating 2-D sparse arrays into our method creates SCOBA-3D: a sparse beamformer that offers significant element reduction and, thus, allows performing 3-D imaging with the resources typically available for 2-D setups. To create 2-D thinned arrays, we present a scalable and systematic way to design 2-D fractal sparse arrays. The proposed framework paves the way for affordable ultrafast US devices that perform high-quality 3-D imaging, as demonstrated using phantom and ex-vivo data.

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

OBJECTIVE

The current standard of care for peripheral chronic total occlusions involves the manual routing of a guidewire under fluoroscopy. Despite significant improvements in recent decades, navigation remains clinically challenging with high rates of procedural failure and iatrogenic injury. To address this challenge, we present a proof-of-concept robotic guidewire system with forward-viewing ultrasound imaging to allow visualization and maneuverability through complex vasculature.

METHODS

A 0.035" guidewire-specific ultrasound transducer with matching layer and acoustic backing was designed, fabricated, and characterized. The effect of guidewire motion on signal decorrelation was assessed with simulations and experimentally, driving the development of a synthetic aperture beamforming approach to form images as the transducer is steered on the robotic guidewire. System performance was evaluated by imaging wire targets in water. Finally, proof-of-concept was demonstrated by imaging an ex vivo artery.

RESULTS

The designed custom transducer was fabricated with a center frequency of 15.7 MHz, 45.4% fractional bandwidth, and 31 dB SNR. In imaging 20 μm wire targets at a depth of 6 mm, the lateral -6 dB target width was 0.25 ± 0.03 mm. The 3D artery reconstruction allowed visualization of vessel wall structure and lumen.

CONCLUSION

Initial proof-of-concept for an ultrasound transducer-tipped steerable guidewire including 3D image formation without an additional sensor to determine guidewire position was demonstrated for a sub-mm system with an integrated ultrasound transducer and a robotically-steered guidewire.

SIGNIFICANCE

The developed forward-viewing, robotically-steered guidewire may enable navigation through occluded vascular regions that cannot be crossed with current methods.

Wideband 2-D sparse array optimization combined with multiline reception for real-time 3-D medical ultrasound

Three-dimensional (3-D) ultrasound medical imaging provides advantages over a traditional 2-D visualization method. However, the use of a 2-D array to acquire 3-D images may result in a transducer composed of thousands of elements and a large amount of data in the front-end, making it impractical to implement high volume rate imaging and individually control all elements with the scanner. This paper proposes an original approach, valid for wideband operations centered on the design center frequency, to maintain a limited number of active elements and firing events, while preserving high resolution and volume rate. A 7 MHz 2-D array is composed of two circular concentric subparts. In the inner footprint the elements are distributed following a regular grid, while in the outer subpart a sparse non-grid solution is adopted. The inner circular dense array is composed of 256 elements with a pitch of 0.5λ. The overall footprint, delimited by the outer subpart, is equivalent to a 256-element array with a pitch of 1.5λ. All the elements of the inner subpart are activated in transmission. Following an optimization procedure, both subparts, including a subset of the elements placed in the inner footprint (i.e., sparse on-the-grid array) and the elements spread over the outer subpart (i.e., sparse off-the-grid array) are used to receive. A total number of 256 elements, defined by the sum of elements distributed in the inner and outer subparts, is fixed in reception. The proposed approach implies a multiline reception strategy, where for each transmission 3 × 3 firing events occur in reception. The sparse receive array is optimized by using a simulated annealing optimization. An original cost function is designed specifically to achieve successful results in wideband conditions. The receive array is optimized in order to obtain consistent results for different signal bandwidths of the excitation pulse. For all the desired bandwidths, the optimized array will provide the recovery of the lower lateral resolution of the transmission phase and, at the same time, a significant reduction of the undesired side lobe raised in the 3-D two-way beam pattern. The 3-D two-way beam pattern analysis reveals that the proposed solution is able to guarantee a lateral resolution of 1.35 mm at a focus depth of 25 mm for the three fractional signal bandwidths of interest (i.e., 30%, 50% and 70%) considered in the optimization process. The undesired side lobes are successfully suppressed especially when, as a consequence of the multiline strategy, non-coincident steering angles are used in transmission and reception. Moreover, thanks to the firing scheme adopted, a high-volume rate of 63 volumes per second may be achieved at the focus depth. The volume rate decreases to 32 volumes per second at twice the focal depth. Phantom image simulations show that the proposed method maintains a satisfactory and almost uniform image quality in terms of resolution and contrast for all the signal bandwidths of interest.

Gap-filling method for suppressing grating lobes in ultrasound imaging: Experimental study with deep-learning approach

Sparse arrays reduce the number of active channels that effectively increases the inter-element spacing. Large inter-element spacing results in grating lobe artifacts degrading the ultrasound image quality and reducing the contrast-to-noise ratio. A deep learning-based custom algorithm is proposed to estimate inactive channel data in periodic sparse arrays. The algorithm uses data from multiple active channels to estimate inactive channels. The estimated inactive channel data effectively reduces the inter-element spacing for beamforming, thus suppressing the grating lobes. Estimated inactive element channel data was combined with active element channel data resulting in a pseudo fully sampled array. The channel data was beamformed using a simple delay-and-sum method and compared with the sparse array and fully sampled array. The performance of the algorithm was validated using a wire target in a water tank, multi-purpose tissue-mimicking phantom, and in-vivo carotid data. Grating lobes suppression up to 15.25 dB was observed with an increase in contrast-to-noise (CNR) for the pseudo fully sampled array. Hypoechoic regions showed more improvement in CNR than hyperechoic regions. Root-mean-square error for unwrapped phase between fully sampled array and the pseudo fully sampled array was low, making the estimated data suitable for Doppler and elastography applications. Speckle pattern was also preserved; thus, the estimated data can also be used for quantitative ultrasound applications. The algorithm can improve the quality of sparse array images and has applications in small scale ultrasound devices and 2D arrays.

Contrast and Volume Rate Enhancement of 3-D Ultrasound Imaging Using Aperiodic Plane Wave Angles: A Simulation Study

Three-dimensional plane wave imaging (PWI) with a 2-D array has been studied for ultrafast volumetric imaging in medical ultrasound. Compared to 2-D PWI, 3-D PWI requires the transmission of an increased number of plane waves (PWs) to scan a volume of interest and achieve transmit dynamic focusing in both the lateral and elevational directions. To reduce the number of PW angles for a given 2-D angular range by mitigating the grating lobe level, we propose two aperiodic patterns of PW angles: concentric rings with a uniform radial interval and the well-known sunflower pattern. Both patterns are validated to provide uniform angle distributions without regular periodicity, and thereby reduce the grating lobe level compared to a periodic angle distribution with the same number of PW angles. Simulation studies show that the aperiodic patterns enhance the contrast of B-mode images by approximately 3-6 dB over all depths. This enhancement implies that the aperiodic angle sets can increase the volume rate by approximately 2-6 times compared to the periodic angle set at the same contrast and spatial resolution.

A 40-MHz Ultrasound Transducer with an Angled Aperture for Guiding Percutaneous Revascularization of Chronic Total Occlusion: A Feasibility Study

Complete blockage of a coronary artery, called chronic total occlusion (CTO), frequently occurs due to atherosclerosis. To reopen the obstructed blood vessels with a stent, guidewire crossing is performed with the help of angiography that can provide the location of CTO lesions and the image of guidewire tip. Since angiography is incapable of imaging inside a CTO lesion, the surgeons are blind during guidewire crossing. For this reason, the success rate of guidewire crossing relies upon the proficiency of the surgeon, which is considerably reduced from 69.0% to 32.5% if extensive calcification, not penetrated by a guidewire, exists in CTO lesions. In this paper, a recently developed 40-MHz forward-looking intravascular ultrasound (FL–IVUS) transducer to visualize calcification within CTO lesions is reported. This transducer consists of a single element angled aperture and a guidewire passage. The aperture is spherically deformed to have a focal length of 3 mm in order to improve spatial resolution of FL–IVUS images. The angle between the beam direction and the axis of rotation is designed to be 30° to effectively visualize calcification within a CTO lesion as well as the blood vessel wall. The experimental results demonstrated that the developed FL–IVUS transducer facilitates visualization of calcification within CTO lesions and makes it possible to help the surgeon make decisions about whether to push the guidewire in order to cross the lesion or to change the surgical procedure.

Experimental 3-D Ultrasound Imaging with 2-D Sparse Arrays using Focused and Diverging Waves

Three dimensional ultrasound (3-D US) imaging methods based on 2-D array probes are increasingly investigated. However, the experimental test of new 3-D US approaches is contrasted by the need of controlling very large numbers of probe elements. Although this problem may be overcome by the use of 2-D sparse arrays, just a few experimental results have so far corroborated the validity of this approach. In this paper, we experimentally compare the performance of a fully wired 1024-element (32 × 32) array, assumed as reference, to that of a 256-element random and of an “optimized” 2-D sparse array, in both focused and compounded diverging wave (DW) transmission modes. The experimental results in 3-D focused mode show that the resolution and contrast produced by the optimized sparse array are close to those of the full array while using 25% of elements. Furthermore, the experimental results in 3-D DW mode and 3-D focused mode are also compared for the first time and they show that both the contrast and the resolution performance are higher when using the 3-D DW at volume rates up to 90/second which represent a 36x speed up factor compared to the focused mode.

Wideband 2-D Array Design Optimization With Fabrication Constraints for 3-D US Imaging

Ultrasound (US) 2-D arrays are of increasing interest due to their electronic steering capability to investigate 3-D regions without requiring any probe movement. These arrays are typically populated by thousands of elements that, ideally, should be individually driven by the companion scanner. Since this is not convenient, the so-called microbeamforming methods, yielding a prebeamforming stage performed in the probe handle by suitable custom integrated circuits, have so far been implemented in a few commercial high-end scanners. A possible approach to implement relatively cheap and efficient 3-D US imaging systems is using 2-D sparse arrays in which a limited number of elements can be coupled to an equal number of independent transmit/receive channels. In order to obtain US beams with adequate characteristics all over the investigated volume, the layout of such arrays must be carefully designed. This paper provides guidelines to design, by using simulated annealing optimization, 2-D sparse arrays capable of fitting specific applications or fabrication/implementation constraints. In particular, an original energy function based on multidepth 3-D analysis of the beam pattern is also exploited. A tutorial example is given, addressed to find the N _e elements that should be activated in a 2-D fully populated array to yield efficient acoustic radiating performance over the entire volume. The proposed method is applied to a 32 ×32 array centered at 3 MHz to select the 128, 192, and 256 elements that provide the best acoustic performance. It is shown that the 256-element optimized array yields sidelobe levels even lower (by 5.7 dB) than that of the reference 716-element circular and (by 10.3 dB) than that of the reference 1024-element array.

Microbubble-mediated intravascular ultrasound imaging and drug delivery

Intravascular ultrasound (IVUS) provides radiation-free, real-time imaging and assessment of atherosclerotic disease in terms of anatomical, functional, and molecular composition. The primary clinical applications of IVUS imaging include assessment of luminal plaque volume and real-time image guidance for stent placement. When paired with microbubble contrast agents, IVUS technology may be extended to provide nonlinear imaging, molecular imaging, and therapeutic delivery modes. In this review, we discuss the development of emerging imaging and therapeutic applications that are enabled by the combination of IVUS imaging technology and microbubble contrast agents.

PMN-PT single-crystal high-frequency kerfless phased array

This paper reports the design, fabrication, and characterization of a miniature high-frequency kerfless phased array prepared from a PMN-PT single crystal for forward-looking intravascular or endoscopic imaging applications. After lapping down to around 40 μm, the PMN-PT material was utilized to fabricate 32-element kerfless phased arrays using micromachining techniques. The aperture size of the active area was only 1.0 × 1.0 mm. The measured results showed that the array had a center frequency of 40 MHz, a bandwidth of 34% at -6 dB with a polymer matching layer, and an insertion loss of 20 dB at the center frequency. Phantom images were acquired and compared with simulated images. The results suggest that the feasibility of developing a phased array mounted at the tip of a forward-looking intravascular catheter or endoscope. The fabricated array exhibits much higher sensitivity than PZT ceramic-based arrays and demonstrates that PMN-PT is well suited for this application.

Single-chip CMUT-on-CMOS front-end system for real-time volumetric IVUS and ICE imaging

Intravascular ultrasound (IVUS) and intracardiac echography (ICE) catheters with real-time volumetric ultrasound imaging capability can provide unique benefits to many interventional procedures used in the diagnosis and treatment of coronary and structural heart diseases. Integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with front-end electronics in single-chip configuration allows for implementation of such catheter probes with reduced interconnect complexity, miniaturization, and high mechanical flexibility. We implemented a single-chip forward-looking (FL) ultrasound imaging system by fabricating a 1.4-mm-diameter dual-ring CMUT array using CMUT-on-CMOS technology on a front-end IC implemented in 0.35-μm CMOS process. The dual-ring array has 56 transmit elements and 48 receive elements on two separate concentric annular rings. The IC incorporates a 25-V pulser for each transmitter and a low-noise capacitive transimpedance amplifier (TIA) for each receiver, along with digital control and smart power management. The final shape of the silicon chip is a 1.5-mm-diameter donut with a 430-μm center hole for a guide wire. The overall front-end system requires only 13 external connections and provides 4 parallel RF outputs while consuming an average power of 20 mW. We measured RF A-scans from the integrated single- chip array which show full functionality at 20.1 MHz with 43% fractional bandwidth. We also tested and demonstrated the image quality of the system on a wire phantom and an ex vivo chicken heart sample. The measured axial and lateral point resolutions are 92 μm and 251 μm, respectively. We successfully acquired volumetric imaging data from the ex vivo chicken heart at 60 frames per second without any signal averaging. These demonstrative results indicate that single-chip CMUT-on-CMOS systems have the potential to produce realtime volumetric images with image quality and speed suitable for catheter-based clinical applications.