Ablation is a medical procedure where part of the heart, a tumor or, in general, any dysfunctional tissue is ablated so as to treat a medical disorder. In particular, such a procedure may be performed by means of catheters suitable for generating electromagnetic energy (in the range of optics, radio frequencies or microwaves) or ultrasonic energy.
In the minimally-invasive treatment of cardiac arrhythmias, for example, the radio frequency (RF) ablation catheter is the most commonly used therapy tool; in fact, it is referred to as the so-called “gold standard” in trials of novel ablation catheter designs. The last generation RF ablation catheters, so-called irrigation catheters, employ active cooling technologies, which consist in infusing an irrigation fluid through the catheter so as to cool the ablation electrode; this reduces the incidence of coagulum formation, produces a more controlled ablation and allows for a higher power delivery, the latter resulting in larger and deeper lesions.
In particular, there are two types of active cooling technologies: (i) closed-loop; and (ii) open-loop. In closed-loop irrigation, the irrigation fluid circulates within the catheter without being released into the blood. On the contrary, in open-loop irrigation, the irrigation fluid flows through the catheter and exits through tiny holes, called irrigation holes, usually placed around the tip of the catheter. Between the two technologies, the open-loop irrigated RF ablation is the one that reduces more the incidence of coagulum formation, as the irrigation fluid is directly in contact with the ablation electrode, the blood and the tissue surface. Yet, open-loop irrigated ablation procedures (electromagnetic—as well as ultrasound-based) still have significant drawbacks.
One major drawback relates to actively controlling the ablation settings during treatment. Currently, the therapist relies on his own expertise to determine the optimal parameters for ablation, e.g., power, temperature and duration. Note that these settings vary largely, due to sizable intra-patient differences of thickness of the local heart wall, perfusion, blood pressure and velocity, heart rhythm, etc. Although a highly-skilled therapist is able to succeed with this approach, it is not always the case, and there are serious consequences for the patient when an error occurs.
The two major therapy-related problems result from either the under-heating or the over-heating of the site. In the case of under-heating, the tissue is not sufficiently coagulated to form the arrhythmia-blocking lesion desired by the therapist. This can lead to persistent or recurring symptoms in the patient and the requirement for subsequent treatment(s), longer periods of hospitalization and greater risks of stroke and embolism. The other extreme, over-heating, either causes rupturing of the tissue at the treatment site, releasing potentially life-threatening particles into the blood stream, or causes damages to neighboring organs and tissues. In the case that other organs are affected, fistulas can develop and these are often life-threatening (e.g., a fistula in the esophagus has roughly a 75% mortality rate).
One of the options for monitoring the ablation process is the use of ultrasound technology, which can give information of the ablation effect on the tissue below the surface. WO 2009/032421 A2 discloses a catheter for ultrasound-guided ablation, where ablation electrodes are displaced on an outer catheter and ultrasound transducers are displaced in an inner catheter. Due to the relative displacement between electrodes and transducers, a number of ablation electrodes may interfere with the ultrasound propagation, thus degrading the overall image quality. In one embodiment, such an issue is avoided by reducing the thickness of the ablation electrodes. However, still a degraded signal-to-noise ratio and dynamical range is expected, due to the reflection and attenuation caused by the acoustical window. Specifically, these reverberations show up overlapping the relevant cardiac structures in the ultrasound data, thus requiring substantial post-processing.
Hence, there is the need for a solution that overcomes the aforementioned disadvantages and provides more adequate control of the ablation process; this would prevent injury and death from under-heating and over-heating in ablation procedures.