Design, Development, and Evaluation of Focused Ultrasound Arrays for Transesophageal Cardiac Ablations
Open Access
- Author:
- Lee, Hotaik
- Graduate Program:
- Acoustics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 15, 2006
- Committee Members:
- Nadine Barrie Smith, Committee Chair/Co-Chair
Victor Ward Sparrow, Committee Member
Thomas B Gabrielson, Committee Member
Keefe B Manning, Committee Member
Anthony A Atchley, Committee Member - Keywords:
- focused ultrasound
transesophageal ablation
atrial fibrillation
HIFU - Abstract:
- The ultimate purpose of this dissertation is the evaluation of the feasibility of transesophageal cardiac surgery in arrhythmia treatment, using therapeutic ultrasound energy without the requirement for surgical incisions or blood contact. Atrial fibrillation (AF) is the most common cardiac arrhythmia, affecting over 2.2 million Americans. One effective treatment is cardiac ablation, which shows a high rate of success in treating paroxysmal AF. As a prevailing modality for this treatment, catheter ablation using radiofrequency has been effective, but there is measurable morbidity and significant costs and time associated with this invasive procedure for permanent or persistent AF. To address these issues, a transesophageal ultrasound applicator for noninvasive cardiac ablations has been designed, developed and evaluated in this dissertation. Focused ultrasound for thermal ablation has gained interest for decades due to its noninvasive characteristics. Since the esophagus is close to the posterior of the left atrium, its position makes it attractive for the incision-less surgery of the selected area of the heart using ultrasound. The overall goal of this study is to bring an applicator as closely as possible to the heart in order to effectively deliver ultrasound energy, and create electrically isolating lesions in myocardial tissue, replicating the currently used Maze procedure. The Maze procedure is a surgical operation that treats AF by creating a grid of incisions resulting in non-conductive scar tissue in the atria. The initial design of an ultrasound applicator capable of creating atrial lesions from the esophagus, involved evaluating sound pressure fields within layers of the esophagus and myocardium. Based on the multiple factors of the simulation results of transducer arrays, current transesophageal medical devices, and the throat anatomy, a focused ultrasound transducer that can be inserted into the esophagus has been designed and tested. In this study, a two-dimensional sparse phased array with flat tapered elements was found to be adequate as a transesophageal ultrasound applicator. The spatially sparse array uses 64 active elements operating at a frequency of 1.6 MHz sampled from 195 (15 by 13) rectangular elements. With this applicator, the size and position of the ablation targets can be controlled by changing the electrical power and phase to the individual elements for ultrasound beam focusing and steering. The magnetic resonance-compatible probe head housing is 19 mm in diameter and incorporates an acoustic window. For the verification of the suggested design, a prototype array with an acoustic impedance matching layer was constructed, and tested using exposimetry and ex vivo experiments. Experimental results indicated that the array could focus and steer the beam with an angle within ±10° inside the tissue. Also, the array can deliver sufficient power to the focal point to produce ablation while not damaging nearby tissue outside the target area. The results demonstrated a potential application of the ultrasound applicator to transesophageal cardiac surgery in atrial fibrillation treatment.