Tissue, such as a benign or malignant tumor, organ, or other body region may be treated invasively by surgically removing the tissue, with minimal invasion or non-invasively by using, for example, thermal ablation. Both approaches may effectively treat certain localized conditions, but involve delicate procedures to avoid destroying or damaging otherwise healthy tissue.
Thermal ablation, as may be accomplished using focused ultrasound, has particular appeal for treating diseased tissue surrounded by or neighboring healthy tissue or organs because the effects of ultrasound energy can be confined to a well-defined target region. Ultrasonic energy may be focused to a zone having a cross-section of only a few millimeters due to relatively short wavelengths (e.g., as small as 1.5 millimeters (mm) in cross-section at one Megahertz (1 MHz)). Moreover, because acoustic energy generally penetrates well through soft tissues, intervening anatomy often does not impose an obstacle to defining a desired focal zone. Thus, ultrasonic energy may be focused at a small target in order to ablate diseased tissue while minimizing damage to surrounding healthy tissue.
To focus ultrasonic energy toward a desired target, drive signals may be sent to an acoustic (preferably piezoelectric) transducer having a number of transducer elements such that constructive interference occurs at the focal zone. At the target, sufficient acoustic energy may be delivered to heat tissue until necrosis occurs, i.e., until the tissue is destroyed. Preferably, tissue along the path through which the acoustic energy passes (the “pass zone”) outside the focal zone is heated only minimally, if at all, thereby minimizing damage to tissue outside the focal zone.
Typically, ultrasonic energy is delivered according to a treatment plan, often based on a predefined model of the target and the patient's anatomy. However, because the human body is flexible and certain organs move (due to breathing, for example), treatment delivered as multiple sonications over time (even when delivered within seconds of each other) may require interim adjustments to targeting and/or to one or more treatment parameters to compensate for movement of the target. Indeed, absorption of ultrasound energy may itself change the shape and/or location of the target through swelling, for example, necessitating similar changes. This creates a significant challenge given the need to avoid damage to healthy tissue while still achieving complete ablation of the target.
One approach for tracking a moving anatomical target uses an imaging device to capture periodic images of the target and compare the target's location in the image to the treatment plan. In such cases, clearly visible landmarks of the target (or organ in which the target is located) may be used to identify the target within every imaging frame. However, the intra-frame motion of the target may lead to substantial targeting inaccuracy. This uncertainty is further compounded by any computational lags associated with capturing and analyzing the imaging data. Typically, the more accurate the image, the more computationally intensive the image processing becomes, resulting in a longer latency—meaning there is an inherent uncertainty about the actual location of the target at the point in time when the data is actually available to be viewed and/or acted upon.
Accordingly, there is a need for systems and methods for effectively focusing acoustic energy in a manner that does not adversely affect surrounding tissue and can be administered in a timely fashion while considering movement of both the target and the surrounding tissue.