Summary
The Live Volumetric Imaging (LVI) catheter is capable of real-time 3D intracardiac echo (ICE) imaging, uniquely providing full volume sectors with deep penetration depth and high volume frame rate. The key enabling technology in this catheter is an integrated piezoelectric micromachined ultrasound transducer (pMUT), a novel matrix phased array transducer fabricated using semiconductor microelectromechanical systems (MEMS) manufacturing techniques. This technology innovation may enable better image guidance to improve accuracy, reduce risk, and reduce procedure time for transcatheter intracardiac therapies which are currently done with limited direct visualization of the endocardial tissue. Envisioned applications for LVI include intraprocedural image guidance of cardiac ablation therapies as well as transcatheter mitral and aortic valve repair.
1
Background
Available modalities for interventional echocardiography include 3D transthoracic (TTE) and transesophageal (TEE) echo as well 2D ICE. The use of 3D echo is increasing for transcatheter aortic and mitral valve surgeries, as this provides more accurate assessment of valve annulus for prosthesis sizing as well as more intuitive views for intraprocedural guidance. The well-documented limitations of 2D compared to 3D ultrasound include: mental reconstruction required of 3D structures from multiple 2D slices; geometric assumptions for quantitative measurements leading to inaccuracy and variability, especially for asymmetric or abnormal shapes; difficulty in visualizing structural dynamics (especially cardiac) in a single plane; patient anatomy and required probe orientation precluding acquisition of optimal view. While image quality of 3D TEE is generally adequate, procedural logistics are more complicated. Additional personnel including an echocardiographer and anesthesiologist must be present for the procedure, and patient risk and discomfort are increased as TEE requires general anesthesia and esophageal intubation.
Three-dimensional ICE would provide a catheter-based interventional imaging modality providing 3D views that can be reconfigured in real time. For example, volume views can be rotated to provide a different directional perspective, and any arbitrary plane or multiple planes within the volume can be displayed in real-time without movement or repositioning of the imaging catheter. Furthermore, ICE imaging allows targets to be viewed in the near field from the right atrium with better resolution compared to TEE and TTE. An illustration of 3D ICE is shown in Fig. 1 .
2
Technology overview
LVI catheters contain novel pMUT matrix array transducers that are manufactured using semiconductor MEMS microfabrication techniques. Matrix transducer arrays enable azimuth and elevation phasing to produce real-time 3D ultrasound images. Semiconductor processing enables miniaturization and more efficient manufacturing such that smaller element size and higher element density can be achieved more easily than with conventional machined ceramic transducers. Matrix pMUT arrays with 256 to 512 elements have been fabricated that fit within a catheter lumen of 11F or less. Conventional 2D ICE catheters contain linear transducer arrays with 64 elements that are machined from bulk piezoelectric ceramic materials. The pMUT arrays also contain a piezoelectric layer for good acoustic performance; however, rather than individually cutting and assembling transducer arrays from ceramic sheets, pMUT technology is fabricated using semiconductor photolithographic techniques. Hundreds of transducer arrays can be produced in one silicon wafer and thousands of arrays in one batch of wafers. Furthermore, taking advantage of semiconductor economies of scale can reduce component manufacturing cost of the transducer array. Photographs of a pMUT matrix array and cable assembly as well as a silicon wafer containing the arrays are shown in Fig. 2 .
The pMUT arrays are fabricated in silicon wafers and consist of piezoelectric unimorph membranes that form the active transducer elements. A lead zirconate titanate (PZT) thin film and metal electrodes are deposited on the silicon wafer and photolithographically patterned to form individual array elements. The silicon substrate is etched from the back side under each element to form a silicon membrane layer under the PZT and electrode layers. PZT is the same piezoelectric material used in conventional ceramic transducers. Because a thin film layer is used, pMUT elements possess higher element capacitance than ceramic transducers, enabling lower source impedance even for small matrix array elements to reduce parasitic losses due to cable loading.
Transducer operation for pMUT elements also differs from conventional ceramic transducers. The membrane elements in a pMUT array operate in a unique “flexure mode” enabling acoustic pressure output that is comparable to bulk ceramic transducers . The piezoelectric film excites the natural resonance of the membrane structure to produce flextensional motion across the element width and buckling of the membrane, with resonance frequency inversely proportional to element width. Frequency range of 4 to 20 MHz has been produced using element widths of 50 to 130 μm. Dimensions in this range can be achieved easily using semiconductor photolithographic processes, whereas the smaller dimensions required for matrix array elements are more challenging for mechanical dicing operations used to produce bulk ceramic transducer arrays. Images of a pMUT element cross section as well as membrane deflection are shown in Fig. 3 .
