The acoustic lens design and in vivo use of a multifunctional catheter combining intracardiac ultrasound imaging and electrophysiology sensing

Abstract

A multifunctional 9F intracardiac imaging and electrophysiology mapping catheter was developed and tested to help guide diagnostic and therapeutic intracardiac electrophysiology (EP) procedures. The catheter tip includes a 7.25-MHz, 64-element, side-looking phased array for high resolution sector scanning. Multiple electrophysiology mapping sensors were mounted as ring electrodes near the array for electrocardiographic synchronization of ultrasound images. The catheter array elevation beam performance in particular was investigated. An acoustic lens for the distal tip array designed with a round cross section can produce an acceptable elevation beam shape; however, the velocity of sound in the lens material should be approximately 155 m/s slower than in tissue for the best beam shape and wide bandwidth performance. To help establish the catheter's unique ability for integration with electrophysiology interventional procedures, it was used in vivo in a porcine animal model, and demonstrated both useful intracardiac echocardiographic visualization and simultaneous 3-D positional information using integrated electroanatomical mapping techniques. The catheter also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures.

Fig. 1

The functional dimensions of the 9F HockeyStick catheter.

Fig. 2

The general cable connection scheme for the combination catheter. The left panel shows the nondisposable trunk cable between the imaging system and the patient table, and the two separate connection paths for EP mapping and for imaging. The right panel shows the distal end of an early mechanical prototype steered in its minimum bend radius position.

Fig. 3

The tip region of the HockeyStick catheter. The flex folds back upon itself at the distal tip so that there are actually two flex layers under the array, with an adhesive layer between them. The photo shows a HockeyStick catheter tip region with a tapered lens design.

Fig. 4

The array, lens, and elevation field geometry. A single intra-lens acoustic path is shown that helps to produce a new Huygen’s point on the lens surface. This new calculated point describes a complex vector pressure normal to the lens surface. The Z-component of the complex pressure is then used to produce the pressure estimate in the Y-Z field.

Fig. 5

The elevation-depth plane at x = 0 showing the serial computation apertures with transducer array and lens surface sample points. The plot in the left panel shows the sampling used for the points with a tapered lens surface, and the right panel shows the sample points in the full round lens case.

Fig. 6

The simulated two-way elevation beams of the tapered lens at 0.7-mm thickness (top row) and the full round lens at 1.22-mm thickness (bottom row) with lens materials of six different speeds (columns) at 7.25 MHz and 5% fractional bandwidth transmission signals. All beam plots are normalized at every depth, and plot dimensions are 10 mm in elevation, and 1.5 to 37.5 mm in depth (relative to the array top surface under the lens) with the white at −6 dB and gray at −12 dB beam contours. Both the tapered and the full round lens speeds are in meters per second and relative to a tissue medium speed. The relative speeds for tapered lens are: (a) −500, (b) −300, (c) −100, (d) 0, (e) +300, and (f) +500; and for the round lens: (g) −300, (h) −200, (i) −100, (j) 0, (k) +100, and (l) +200.

Fig. 7

The computational summary result of lens shape, speed, and operational frequency on elevation beam quality. A comparison is shown of normalized (based on the case at the central point) two-way elevation beam energy sums on the center (z) axis for center frequencies of 5.25, 7.25, and 9.25 MHz and 37°C tissue environment conditions with both a full round lens with radius 1.5 mm and center lens thickness of 1.22 mm (solid lines), and a tapered lens of thickness 0.7 mm (0.52 mm less thick than the round lens, dashed lines) with a variation in lens speed relative to the medium speed. The lens attenuation is fixed at 10 dB/cm/MHz and the medium attenuation is assumed to be 0.52 dB/cm/MHz. The simulations were done at 5% FBW and for depths from z = 1.5 to 37 mm.

Fig. 8

The simulated response of the elevation beam relative pressure at the focal peak as a function of the lens thickness. Three lens material candidates were examined in lens thicknesses from 0.2 mm to a full round lens at 1.22 mm.

Fig. 9

The HockeyStick catheter array with an RP 6400 full round lens configuration. 1-D KLM model result is shown at upper left; pulse-echo test results from the second device built, HS-2, are shown at lower left. The right panel shows a standard target phantom result for phased-array imaging with an early prototype to a 60-mm depth.

Fig. 10

Simulation and laboratory comparisons of a full round lens beam performance at 37°C (a)–(e) and 21°C (f)–(j) water bath conditions. The Schlieren images (a) and (f) show elevation focusing with a narrow band signal at 7.25 MHz at warm and cool temperatures with image dimensions of 10-mm total elevation width and a depth of 37 mm. The simulation plot sets [(b), (c), (e), and (g), (h), (j)] show the expected one-way elevation beam results for the same plot dimensions as the Schlieren, but with the normalized-at-every-depth plot intensity ranges of 18 dB, 18 dB, and 60 dB, respectively. The elevation beam profiles (d) and (i) show a normalized comparison of predicted profiles (solid) and hydrophone data (points) collected at a depth of 20 mm with a narrow-band 4-MHz test signal. The calculated elevation peak pressures normalized to a no-lens condition in a cool bath are −5.9 dBw and −9.5 dBw for the warm and cool baths, respectively.

Fig. 11

A HockeyStick catheter prototype is shown at top left with a total of four EP sensor electrodes, one of which is located at the distal tip. A fluoroscopic view of the same catheter is shown at bottom left. The sketch at the right portrays the HockeyStick in the right atrium guiding an RF ablation catheter.

Fig. 12

A HockeyStick catheter image of the atrio-ventricular sulcus region in the right atrium of a pig during an active RF ablation procedure. The ablation catheter itself, the lesion site, and bubbles forming during prolonged ablation are clearly visible.

Fig. 13

The anterior view fluoroscopic image at the top shows the NavX mapping catheter looped on itself in the left ventricle (LV) of a pig, while the HockeyStick is shown in the RA. The two views at the bottom show the 3-D volume of a pig LV mapped by a NavX electroanatomical mapping catheter; the HockeyStick catheter (labeled “HS”) is in the RA while the slightly smaller NavX catheter is seen in the left ventricle. Both catheters were independently steered and could be tracked in a continuous 3-D mode very easily.