Focused ultrasound makes it possible to treat deep tissues without direct access to these tissues. A focused ultrasound beam originating from a power transducer is concentrated towards a focus which is positioned on the target. This results in a double thermal and cavitation phenomenon. The tissue effect depends on the application of the ultrasound energy. Under certain conditions (moderate acoustic intensity), a thermal effect is obtained, under others (strong acoustic intensity), the cavitation effect predominates. The choice of treatment parameters (acoustic intensity and frequency, duration of firing, duration of pauses between the firings, spacing between the firings etc.) is made in order to avoid burning in the intermediate tissues, i.e., situated between the ultrasound source and the target. The designation “acoustic axis” of the transducer is given to a line joining the center of the transducer, or its center of symmetry (if it exists) and the focus. In the case of a plane transducer and electronic focusing, the acoustic axis is the axis perpendicular to the transducer plane, and passing through the focus; the acoustic axis can also generally be defined as passing through the focus and directed following the mean direction of ultrasound propagation.
The effect of each ultrasound pulse is generally limited to a small spatial zone, in which the intensity of the ultrasound field is strongest, and which is situated around the focus. The focal zone will typically have the shape of a cylinder 1.5 mm in diameter in a plane perpendicular to the acoustic propagation direction and 10 mm in length in the acoustic propagation direction.
This technique is particularly useful when the treatment must be precise, for example when the zone to be treated is close to sensitive organs to be preserved. This is the case for example with treatment of the prostate, in which the external sphincter must not be touched for fear of causing incontinence in the patient.
It has therefore been proposed to combine in one therapy appliance a treatment transducer and an imaging transducer; in fact ultrasound marking is useful because it is simple, inexpensive and emits no ionizing radiation. The imaging transducer is used, as its name indicates, to obtain an image of the zone to be treated. The treatment transducer, or power transducer, is used for the emission of the ultrasound intended for the treatment. From the quantitative point of view, the average power range for the imaging transducer is typically of the order of 0.1 to 1 W, while the average power range for the treatment transducer is typically of the order of 5 to 100 W. Moreover, the ultrasound pulses emitted for the imaging have a typical duration of 0.1 μs to 1 μs, whilst the therapy pulses last from 0.1 s to 20 s. In order to make it possible to visualize a volume containing the target, a displacement of the imaging transducer scanning plane can be provided.
Ultrasound therapy appliances combined with ultrasound scanning have been described. In EP-A-0 148 653, EP-A-0 162 735 and U.S. Pat. No. 5,431,621, an imaging transducer is accommodated in the center of a cap serving as treatment transducer; this cap has axial symmetry. The scanning plane of the imaging transducer contains the acoustic axis of the treatment transducer. The ultrasound imaging transducer can turn on its axis, but this is not the case with the treatment transducer. It is proposed in these documents to use the appliance to destroy renal calculi by shock waves or to treat tumors by hyperthermia.
WO-A-92 15253 describes a bevelled endorectal probe. The probe is mounted in rotation on a support and in translation along its longitudinal axis. The treatment transducer is fixed with respect to the probe body. The probe has an imaging transducer, which is fixed or mobile with respect to the treatment transducer. In all cases, the imaging transducer's scanning plane contains the focus of the treatment transducer.
EP-A-0-714 266 describes an endorectal probe suitable for treatment of the prostate. The probe comprises retractable therapy and imaging transducers. In the “imaging” position, the second transducer scans a plane containing the acoustic axis of treatment. The scanning plane is variable, as it can pivot about this axis. The treatment transducer does not turn about its acoustic axis, but about an axis which is parallel to the axis of the endorectal probe.
WO-A-89 07909 discloses, in FIG. 2, an extracorporeal treatment appliance comprising an imaging transducer and a treatment transducer. Each of the transducers is mounted at the end of a tube; the two tubes are mounted on a disk and extend perpendicularly to the plane of the disk. The disk is mounted in rotation in the appliance. The tube carrying the treatment transducer is approximately in the center of the disk; this tube is mobile in translation along its axis. At the end of the tube, the treatment transducer is mounted in rotation on an axis perpendicular to the axis of the tube. The treatment transducer thus has three degrees of freedom, in order to be oriented in all directions. The imaging transducer is mounted in analogous fashion; in all cases, the scanning plane of the imaging transducer contains the focus of the treatment transducer. The axis of rotation of the disk—which is the longitudinal axis of the tube carrying the treatment transducer—generally corresponds neither to the acoustic axis of the treatment transducer, nor to that of the imaging transducer; in fact, for a given treatment depth, the scanning movement of the target through the focal point is carried out by rotation of the treatment transducer about the axis perpendicular to the tube.
WO-A-95 02994 discloses, in FIG. 5, a probe suitable for visualizing and treating tissues situated in the probe's longitudinal axis, such as liver tumors or fibromas. This probe has an imaging transducer and a therapy transducer mounted back to back, the whole assembly being mounted in rotation at the end of the probe, about an axis perpendicular to the axis of the probe. The rotation of the probe body makes it possible to modify the scanning plane of the probe. The rotation of the transducers ensures scanning or treatment in the plane concerned. As in the preceding document, the axis of rotation of the probe body—which is the longitudinal axis of the probe—does not generally correspond to the acoustic axis of the treatment transducer. EP-A-0 273 180 discloses a probe of the same type.
These different appliances of the state of the art are only slightly or not at all suitable for the treatment of organs from outside the body, and for example for focused ultrasound treatment of the thyroid. A need therefore exists for an appliance which can treat organs such as the thyroid, by focused ultrasound, simply, with precision, and effectively.