FIELD OF THE INVENTION

The present invention relates, generally, to systems and methods involving multiple ultrasound transducers. In particular, various embodiments are directed to maintaining the coherence between multiple ultrasound transducers.

BACKGROUND

Focused ultrasound (i.e., acoustic waves having a frequency greater than about 20 kilohertz) can be used to image or therapeutically treat internal body tissues within a patient. For example, ultrasonic waves may be used to ablate tumors, eliminating the need for the patient to undergo invasive surgery. During tumor ablation, a piezoceramic transducer is placed externally to the patient, but in close proximity to the tissue to be ablated (i.e., the target). The transducer converts an electronic drive signal into mechanical vibrations, resulting in the emission of acoustic waves. The transducer may be shaped so that the waves converge in a focal zone. Alternatively or additionally, the transducer may be formed of a plurality of individually driven transducer elements whose phases can each be controlled independently from one another. Such a “phased-array” transducer facilitates steering the focal zone to different locations by adjusting the relative phases between the transducers. Magnetic resonance imaging (MRI) may be used to visualize the patient and target, and thereby to guide the ultrasound beam.

The effectiveness of ultrasound therapy depends on the properties of the focus (such as the accuracy of the focus location and/or the sharpness and shape of the focal zone). After a focus having the desired properties is created, it is important to maintain it during treatment. This, in turn, depends on persistent coherence among the ultrasound waves that generate the focus. For purposes hereof, two waves are considered to be coherent if they have substantially the same frequency and the difference between their phases is substantially constant.

Conventionally, approaches for ensuring coherence among ultrasound waves emitted from various elements of a single transducer array have been proposed. For example, the geometric locations of the transducer elements with respect to each other and with respect to the transducer body may be measured using, for example, acoustic transmission in the laboratory as part of the manufacturing process. During treatment, the location of the transducer body is determined (e.g., using MRI trackers); as such, the location of each transducer element can be derived from the location of the transducer body. The coherence between the ultrasound waves can be achieved by maintaining the relative locations and/or orientations of the transducer elements with respect to each other.

In some situations, the ultrasound therapy may implement more than one transducer array to provide high ultrasound power and/or sufficient intensity from two distinct acoustic windows. The above-described conventional approach, however, is applicable for maintaining coherence between the ultrasound waves emitted by transducer elements within a transducer array only; it cannot ensure coherence between waves emitted from different transducer arrays.

Accordingly, there is a need for an approach that ensures coherence between ultrasound waves that are emitted from elements of different transducers arrays during treatment.

SUMMARY

Various embodiments of the present invention provide systems and methods for ensuring coherence between ultrasound waves emitted from elements of different transducer arrays by maintaining a constant relative spatial arrangement (e.g., positions and/or orientations) of transducer elements in each transducer array. This may be achieved by limiting the possible geometric locations of the transducer arrays. In various embodiments, two or more transducer arrays are affixed to a connecting structure having predefined constraints. For example, the connecting structure may include an external frame having a limited number of options for positioning the transducer arrays. Locational trackers, such as MRI trackers, may be placed on the transducer arrays to facilitate determining their relative locations with respect to one another and to an anatomic region of interest in a patient's body. This information and the predefined constraints of the connecting structure may together accurately identify the location of the anatomic region of interest and the locations of the transducer elements in the transducer arrays in three-dimensional space. Based on the identified locations of the anatomic region of interest and the transducer elements, an ultrasound transmission configuration (e.g., amplitude, frequency and/or phase) associated with each transducer element may then be computed to ensure that the ultrasound waves emitted by the transducer elements in two or more transducer arrays are coherent at the anatomic region of interest.

Accordingly, in one aspect, the invention pertains to a method for maintaining coherence of waves emitted from multiple ultrasound transducer arrays connected via a connecting frame, each transducer array having multiple transducer elements. In various embodiments, the method includes (a) determining relative locations of the transducer arrays with respect to one another and the connecting frame; (b) determining a location of the connecting frame relative to an anatomic region of interest; (c) determining a spatial arrangement of the transducer elements in each transducer array with respect to the anatomic region of interest based on steps (a) and (b); and (d) based on the determination in step (c), adjusting a transmission configuration (e.g., an amplitude, a frequency and/or a phase) of the transducer elements in the plurality of transducer arrays to achieve a desired focusing property with respect to the anatomic region of interest while maintaining coherence therebetween. In one implementation, the method further includes determining spatial parameters (e.g., locations and/or orientations) of the transducer elements in each transducer array. In addition, step (a) and/or step (b) may be determined using an MR image. In some embodiments, the anatomic region of interest includes a target region for treatment.

Step (a) may include attaching a location tracker to each of the ultrasound transducer arrays and determining relative locations of the locational trackers with respect to one another and the connecting frame. In one embodiment, the method further includes attaching a secondary location tracker to the connecting frame. Step (a) may include determining an orientation angle between two of the transducer arrays from multiple allowed orientation angles. In addition, step (b) may determine a three-dimensional location of the connecting frame relative to the anatomic region of interest, and step (c) may determine a three-dimensional spatial arrangement of the transducer elements in each transducer array with respect to the anatomic region of interest.

In another aspect, the invention relates to a system for maintaining coherence of ultrasound waves during a procedure. In various embodiments, the system includes (a) multiple transducer arrays; (b) multiple retention arms each for receiving one of the transducer arrays; (c) a connecting frame for receiving and mechanically retaining the arms in fixed, known angular relation to each other; and (d) a processor configured to (i) determine relative locations of the transducer arrays with respect to one another and the connecting frame; (ii) determine a location of the connecting frame relative to an anatomic region of interest; (iii) determine a spatial arrangement of the transducer elements in each transducer array with respect to the anatomic region of interest based on the relative locations of the transducer arrays and the location of the connecting frame relative to the anatomic region of interest; and (iv) based on the determined spatial arrangement of the transducer elements in each transducer array with respect to the anatomic region of interest, adjust a transmission configuration (e.g., an amplitude, a frequency and/or a phase) of the transducer elements in the transducer arrays to achieve a desired focusing property with respect to the anatomic region of interest while maintaining coherence therebetween. In one implementation, the processor is further configured to determine spatial parameters (e.g., locations and/or orientations) of the transducer elements in each transducer array.

The system may include multiple locational trackers (e.g., MRI trackers), each in fixed relation to one of the transducer arrays, and a tracking system (e.g., an MR apparatus) for tracking the locational trackers. The processor may be further configured to determine relative locations of the locational trackers with respect to one another and the connecting frame. In addition, the processor may be configured to determine an orientation angle between two of the transducer arrays from multiple allowed orientation angles. In some embodiments, the system further includes an MR apparatus. The relative locations of the transducer arrays with respect to one another and to the connecting frame and/or the location of the connecting frame relative to the anatomic region of interest may then be determined using an MR image. In one implementation, the anatomic region of interest includes a target region for treatment.

In addition, the transducer arrays may be movable along lengths of the retention arms; the system may further include means for mechanically retaining the transducer arrays at selected positions therealong. For example, the transducer arrays may be movable along the lengths of the retention arms only in discrete increments. Alternatively, the transducer arrays may be continuously movable along the lengths of the retention arms. In one embodiment, the connecting frame is configured to provide a series of discrete, angularly displaced mounting locations for the retention arms. In another embodiment, the connecting frame is configured to provide continuous, angularly displaced mounting locations for the retention arms.

As used herein, the term “substantially” means ±10%, and in some embodiments, ±5%. In addition, the term “element” means either an individual transducer in an array or an independently drivable portion of a single transducer. Reference throughout this specification to “one example,” “an example,” “one embodiment,” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present technology. Thus, the occurrences of the phrases “in one example,” “in an example,” “one embodiment,” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, routines, steps, or characteristics may be combined in any suitable manner in one or more examples of the technology. The headings provided herein are for convenience only and are not intended to limit or interpret the scope or meaning of the claimed technology.

