Technical Note
Ring Array Transducers for Real-Time 3-D Imaging of an Atrial Septal Occluder

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Abstract

We developed new miniature ring array transducers integrated into interventional device catheters such as used to deploy atrial septal occluders. Each ring array consisted of 55 elements operating near 5 MHz with interelement spacing of 0.20 mm. It was constructed on a flat piece of copper-clad polyimide and then wrapped around an 11 French O.D. catheter. We used a braided cabling technology from Tyco Electronics Corporation to connect the elements to the Volumetric Medical Imaging (VMI) real-time 3-D ultrasound scanner. Transducer performance yielded a –6 dB fractional bandwidth of 20% centered at 4.7 MHz without a matching layer vs. average bandwidth of 60% centered at 4.4 MHz with a matching layer. Real-time 3-D rendered images of an en face view of a Gore Helex septal occluder in a water tank showed a finer texture of the device surface from the ring array with the matching layer.

Introduction

Previously, we developed lead zirconate titanate (PZT)-based matrix array catheter transducers (Lee et al. 2004) and endoscopes (Pua et al., 2004, Light et al., 2005) for real-time three-dimensional (3-D) ultrasound imaging. More recently, we have fabricated forward viewing ring array catheters for 3-D image guidance of interventional devices such as the vena cava filter and aortic aneurysm stent graft (Light et al. 2008) and trans-apical heart valves (Light et al. 2011). Other laboratories are developing side viewing 3-D catheters (Lee et al. 2011) and forward-viewing capacitive micromachined ultrasonic transducers (CMUT) ring arrays for electrophysiologic applications and real-time 3-D intravascular ultrasound (IVUS) (Degertekin et al., 2006, Yeh et al., 2006; Khuri-Yakub and Oralkan 2011).

Atrial septal defects (ASD) comprise up to 7% of total congenital heart lesions and as much as 25% of congenital heart disease in adults (Kaplan 1993). ASDs may go undetected for decades. If the ASD is small and the patient is asymptomatic, no action is taken. Otherwise, treatment options include open-heart surgery to repair the hole or catheter deployment of an atrial septal occluder. This minimally invasive procedure is often preferred by patients. The procedure is typically done under fluoroscopic guidance combined with transesophageal echo (TEE) and Doppler ultrasound (Ko et al. 2009) as well as two-dimensional (2-D) intracardiac echo (ICE) catheters (Hijazi et al. 2009) and even 3-D TEE (Perk et al. 2009). However, since the procedure already calls for the insertion of a catheter, we believe a catheter-based transducer capable of generating real-time 3-D images integrated into the occluder deployment kit might reduce the need for the other imaging modalities with the associated x-ray exposure of fluoroscopy and complexity and discomfort of TEE ultrasound. Real-time 3-D ultrasound enables continuous monitoring of cardiac structures and interventional devices before, during and after deployment. However, the integration of 3-D ultrasound imaging catheters into the deployment kits of interventional devices presents major challenges because of the requirement for dozens of active transducer channels to improve 3-D image quality and the severe fabrication difficulties in electrical connection to the submillimeter 2-D array elements. It is also important not to disrupt the original design of the device deployment kit. Figure 1 shows a schematic of an integrated ring array transducer, connected to the scanner via the system flex circuit and septal occluder deployment catheter. The real-time 3-D ultrasound pyramidal scan originates from the distal tip of the device, with the occluder exiting through the lumen. In this article, we describe the fabrication of such ring arrays operating near 5 MHz and the acquisition with a commercial scanner (Volumetrics Medical Imaging, Inc., Durham, NC, USA) of real-time 3-D ultrasound images of the Gore Helex septal occluder (W.L. Gore & Associates, Inc. Newark, DE, USA).

Section snippets

Materials and Methods

We modified a commercial catheter deployment kit for our ring array transducers. This product uses an 8.5 Fr sheath (I.D.) resulting in an approximately 11 French (O.D.) catheter. This outer diameter determined the ring array diameter. We previously described ring array transducers using a small flexible circuit connected to our scanner system cable using Tyco Medical cabling (Tyco Electronics, Wilsonville, OR) (Light et al. 2009).

Figure 2 shows a schematic of the construction steps to build

Results

Figure 5 shows a typical pulse echo response (a) and power spectrum (b) of the control transducer without matching layer. For 12 elements mean center frequency is 4.7 MHz and the –6 dB bandwidth is 20%.

Figure 6 shows the pulse-echo response (a) and spectrum (b) of a single element from the ring array transducer with matching layer. For 12 elements mean center frequency is 4.28 MHz and the –6 dB bandwidth is 68%. The pulse echo sensitivity increased by 5% compared with the control array. Figure 7

Discussion and Conclusions

We developed new miniature ring array transducers integrated into interventional device catheters such as used to deploy atrial septal occluders. Such a catheter-based transducer capable of generating real-time 3-D images integrated with the occluder deployment kit might reduce the need for x-ray fluoroscopy and TEE ultrasound during the occluder deployment.

The ring arrays consisted of 55 elements operating near 5 MHz with interelement spacing of 0.20 mm. The development of matching layers and

Acknowledgments

This research was supported by NIH grant HL089507.

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