The present invention relates to apparatus for monitoring the ablation of tissue during surgery, to determine whether or not all the target tissue, or even that target tissue has been ablated.

It has been known for some time that strongly focused beams of ultrasound at a frequency of about 1 MHz can be used to ablate, that is destroy, selected volumes of living human or animal tissue at some depth in the body, apparently without damage to the overlying tissues. This characteristic is now being investigated for surgical applications.

Ablated regions of tissue, or ablative lesions, can sometimes be visualised by means of ultrasonic B-scan (pulse-echo) imaging procedures. Using this method, a surgeon looks at the produced image and makes a subjective judgment on whether or not he has made a sufficient lesion. This is not, however, a reliable method of making such a determination since limitations of the imager and/or the human eye mean that it is not always possible to detect from these images whether or not changes have occurred, and further these images do not necessarily represent the changes that have occurred in the tissue. Frequently, it is not possible to make the right judgment this way and a patient may have to be recalled later for further surgery.

It was unclear what specific changes in tissue properties, related to the ablation process, give rise to detectable changes in ultrasonic echo properties of the tissue. The present invention is a result of an investigation into this feature and a search for other

potentially useful and available ultrasonic signals that would complement the simple echo amplitude signals used in conventional imaging.

It is an aim of the present invention to provide apparatus which is able to overcome disadvantages of the prior art and to provide non-invasive and near instantaneous information on both the occurrence and location of tissue alteration resulting, preferably, from focused ultrasonic surgery.

Further it is an aim that the information should be presented in a more readily comprehensible form than that provided solely by studying an image of an area of tissue by eye, and instead, or as well, it should be of a quantative nature.

According to one aspect of the present invention there is provided apparatus for monitoring surgical ablations comprising means for monitoring backscattering and/or attenuation coefficients of a candidate ablation region and a posterior region adjacent thereto.

Preferably, the coefficient monitoring means further comprises means for monitoring backscattering and/or attenuation coefficient of an adjacent region anterior to said candidate region.

Preferably this apparatus is combined with ablating means, possibly in the form of a lesioning transducer, for near immediate feedback.

According to another aspect of the present invention, there is provided a method of monitoring an ablation of a candidate region of tissue, said method comprising the step of:

monitoring backscattering and/or attenuation coefficients of the candidate region and a posterior region adjacent thereto.

The method may also comprise the step of monitoring backscattering and/or attenuation coefficients of an adjacent region anterior said candidate region, and/or the step of comparing monitored coefficients for the same region or regions from before and after a period of ablation.

The present invention makes use of the fact that the absorption co-efficient of tissue changes when it is ablated and a lesion forms.

The present invention will be further described, by way of non-limitative example, with reference to the accompanying drawings, in which:-

Figure 1 shows the configuration geometry of a lesioning transducer and imaging transducer relative to tissue volumes, together with a schematic representation of the control circuitry, according to one embodiment of the present invention; and

Figures 2a and 2b show a series of transmitted and received signal sequences in the transducers of Figure 1.

Figure 1 illustrates a configuration of two associated ultrasonic transducer assemblies, a strongly focused lesioning transducer (2) and a weakly focused imaging transducer (4) , in an array (1) , and is generally similar to arrangements used in some forms of commercial extracorporeal ultrasonic lithotriptors. The lesioning transducer (2) is controlled by a treatment processor (7) .

This in turn is controlled by the display/controls (9) which the surgeon uses to control the process. The imaging transducer (4) is controlled by an imaging processor (6) which again is controlled by the surgeon using the display/controls (9) . A memory (8) is used to store images and pulse echoes received by the imaging transducer (4) . The focus of the array (1) is shifted by a motor (3) . This is controlled by the position controller (5) which reacts in response to signals from the imaging processor (6) and the display/controls (9) . The lesioning transducer (2) is generally arranged to be co-axial and confocal with the imaging transducer (4) and brings energy to a focus within the lesioning region (14) of a tissue (10) . Additionally the imaging transducer (4) emits a pencil-like beam of pulses that irradiate the lesioning region (14) as well as an adjacent anterior region (12) and an adjacent posterior region (16) in the same order as those regions are encountered by the pulses, that is first the anterior region (12) , then the lesioning region (14) and finally the posterior region (16) . In this arrangement the same transducer receives pulse echoes from those regions in the same time sequence order.

Ultrasonic radiation of different frequencies may be used according to the depth of surgery. Just below the skin a frequency of around 7 MHz may be used. On the other hand, with deep surgery, frequencies of around 1 to 3 MHz may be more appropriate.

The effect of the strongly focused ultrasonic radiation is to heat up the tissue. This may result in the "cooking" of tissue in the lesioning volume (14) . The reflective characteristics of such affected tissue change and this effect can be detected quickly. With deep surgery, and especially around frequencies of 1.5 to 2 MHz, ultrasonic radiation can also result in the

stimulation of a phenomenon known as cavitation in a target volume. Cavitation is caused by the breakdown of the tissue or liquid and the lowering of local pressure. It occurs when the ultrasound radiation boils liquids within the fluids and vapour expands to form microscopic bubbles within the tissue. These can vibrate at characteristic frequencies and result in characteristic acoustic emission in all directions. This can also be received by either or both of the two transducers (2,4) .

Figures 2a and 2b show an example of a time sequence of signals transmitted and received by the different transducers. Figure 2a shows two lines of signals. The first indicates the signals transmitted by the lesioning transducer (2) and the second indicates those it receives. Similarly Figure 2b shows the signals transmitted and received by the imaging transducer (4) .

By the operation of the controls (9) , or by an automatic operation in the imaging processor (6) an imaging pulse (20) is transmitted by the imaging transducer (4) before any lesioning radiation is transmitted by the lesioning transducer (2) . This first imaging pulse (20) results in three pulse-echoes (22,24,26) received by the imaging transducer (4) from the three tissue regions (12,14,16) shown in Figure 1. These first received pulse- echoes (22,24,26) are stored in the memory (8) to be used to provide a datum to decide the effect of subsequent lesioning. Subsequently lesioning irradiation (28) is transmitted by the lesioning transducer (2) as controlled by the surgeon, or possibly automatically; and focused on a particular volume (14) of the tissue (10) . This causes changes in the physiology of the tissue in that volume (14) and may eventually stimulate cavitation. If cavitation occurs then it results in an acoustic emission which is received by both

the transducers (2,4) and which can be stored in the memory (8). Either the signals (30,32) as are, or selected frequency components of these signals can be indicative of the occurrence of cavitation. The emissions can be displayed on the display (9) and if these signals are noticed they can be used as an indication that lesioning has been effected. Alternatively the processors can determine whether the signals indicate lesioning.

At some point after the lesioning radiation has been transmitted, and possibly after cavitation emissions have been detected, a further imaging pulse (34) is transmitted from the imaging transducer (4) , again either as determined by the surgeon or automatically, which results in a new set of pulse echoes (36, 38, 40) in which the amplitudes of at least some of the echo pulses will have changed. These echoes too can be stored in the memory.

Alternatively, continuous power may be applied, for example for 1 to 2 seconds, and during that time at, for instance, 1/10 second intervals, there may be 10 μs of imaging.

The image plane which provides the echoes is a plane in the direction the ultrasound is sent. To build up a three-dimensional image a sweeping motion (either by rotation and/or translation) can be used.

Comparisons can then be made in the imaging processor (6) between the amplitudes of received signals to determine whether the changes indicate sufficient ablation in the lesioned volume (14) . All these signals and this whole sequence may be repeated if the comparison shows that further ablation is required.

Ablation of tissue results in changes in its attenuation coefficients. The attenuation of ultrasound includes a combination of power absorption and backscattering, with absorption accounting for up to 90% of it.

The amplitudes of backscattered signals (38,24) from the target lesioning volume (14) , before and after lesioning, or the amplitudes of the ratios between the signals (38/36, 24/22) from the target lesioning volume (14) and the anterior volume (12) before and after lesioning, are compared. These indicate changes resulting from the lesioning process which show themselves in the alteration of the tissue echogenicity, or back scattering co-efficient. Correspondingly, comparisons of the amplitudes of signals (40,26) from the posterior volume (16) or of the amplitudes of the ratios between the signals (40/36, 26/22) from the posterior volume (16) and the anterior volume (12) , before and after lesioning, indicate changes which show in an alteration in the attenuation coefficient of the target lesioning volume (14).

In this procedure the anterior tissue volume (12) , closest to the imaging transducer, is, by definition unaffected by the lesioning process and thus the amplitude of the signal echoed back from it will not be affected by that process. Therefore the signals (22,36) from the anterior region (12) are, in principle, identical to each other and may be used as a baseline. Thuε, in principle and within the limits of measurement accuracy, the position of the anterior limit of observable lesioning damage is given as the maximum distance from, in this case, the imaging transducer (2) for which the before and after echo signals (22,36) remain equal.

All or any of these signals may be displayed on a monitor in the display (9) , as they are shown in Figures 2a and 2b, or may be displayed for the surgeon as relative amplitudes or in any other desired form. It is therefore possible for the surgeon to see the results of the lesioning process almost immediately after it has occurred and before moving on to the next area of tissue which needs ablating. The surgeon may control all the aspects of the imaging and lesioning. And thus the imaging may be interleaved with the lesioning vintil the results are as required.

Normally the surgeon, having decided that an area is sufficiently ablated will control the array (1) to focus on a new target volume using the controls. However, if the deciding process is automatic, the position controller can be instructed by the imaging processor (6) instead.

The signals exemplified in Figures 2a and 2b may be replaced with others or may be repeated in various permutations.

There may be more than one imaging transducer and/or different transducers may transmit and receive the imaging and echo pulses for imaging. Further, a separate transducer may be introduced to provide specific reception of a characteristic component of the cavitation-related acoustic emission and may be tuned to the frequency of that emission, that is, for example, 0.5 times the driving frequency. Also, there is no need for the imaging and lesioning transducers to be coaxial. The imaging transducer may be arranged to scan either a plane or a volume containing the axis of the lesioning transducer (2) , in which case the sets of data or some combination of them, derived from the above procedures may be displayed

as two-dimensional or three-dimensional maps of tissue alteration features, for example as overlays on conventional pulse-echo image maps of tissue anatomy.