HIFU PROBE

FIELD OF THE INVENTION

The instant invention relates to a method for maximising the efficiency of high intensity focused ultrasound (HIFU) treatments, in particular HIFU treatments employing mobile probes.

BACKGROUND OF THE INVENTION WO 2009/081339 describes a method and a system for tracking and guiding HIFU beams. This system involves the use of acoustic radiation force impulse (ARFI) imaging to detect the focal position of a HIFU beam relative to a target area. The system of WO 2009/081339 is able to relocate the mobile probe and to adjust its focal length in order to finely position the focal point of the beam in the target region for the treatment.

However, this solution has a number of drawbacks. Several probe positions are often available, which all give a fine positioning of the focal point inside the target region. A choice then has to be made between these probe positions .

Furthermore, it is often dangerous to bring HIFU transducer elements too close to some patient tissues of high absorption as these tissues not only reduce the energy delivered to the target region but also heat significantly when penetrated by ultrasounds thus increasing the temperature of the insonificated region.

The scattering of ultrasounds by boundaries in the insonificated region also causes energy loss by refraction and reflexion and increases power requirements; thereby also increasing the temperature of the insonificated region .

As a high temperature of the insonificated region might lead to patient burning hazard, it should be carefully avoided.

The instant invention has notably for object to improve the situation.

SUMMARY OF THE INVENTION

To this aim, according to the invention, such an apparatus for determining optimal positions of a HIFU probe, said HIFU probe being mobile and being adapted for treating a target region, comprises:

- a mobile probe;

- a central unit comprising a memory and a processor,

wherein said memory stores a database of probe positions and at least one image of a region separating the probe from the target region;

wherein said processor,

- segments said at least one image in order to identify a set of tissues and boundaries,

- evaluates at least one score for each probe position of the database, based on the set of tissues and boundaries, said score being indicative of an ultrasound energy loss along the path of ultrasounds between said probe position and the target region, and

- identify at least one optimal probe position according to said scores.

This way, at least one optimal probe position can be determined that minimizes the ultrasound energy loss along the path of ultrasounds to the target region 24. The efficiency of a HIFU treatment can thus be maximized and the patient hazard minimized. The insonification of certain areas of a patient can also be avoided

The apparatus can further comprise a displacement device to move the mobile probe to the optimal probe position .

According to another aspect of the invention, there is provided a method for determining optimal positions of a HIFU probe, said HIFU probe being mobile and being adapted for treating a target region, said method comprising at least the steps of :

segmenting at least one image of a region separating the probe from the target region in order to identify a set of tissues and boundaries;

evaluating at least one score for each probe position of a database of probe positions, based on the set of tissues and boundaries, said score being indicative of an ultrasound energy loss along the path of ultrasounds between said probe position and the target region 24; and identify at least one optimal probe position according to said scores.

In some embodiments, one might also use one or more of the following features:

the method further comprises a step of moving the probe to the optimal probe position;

- the step of evaluating comprises an operation of comparing, for each probe position of the database of probe positions, a location of a focal point associated with said probe position with a location of the target region;

- the step of evaluating comprises computing a set of criteria;

- the step of evaluating comprises an operation of determining, for each probe position of the database of probe positions, based on said set of criteria and a set of criteria thresholds, a set of validated transducer elements from a set of transducer elements of the mobile probe;

- the step of evaluating comprises an operation of determining, based on said set of criteria and a set of criteria thresholds, a set of validated probe positions from the database of probe positions;

- the step of evaluating comprises an operation of computing at least one score, based on at least one criterion taken in the list comprising at least said set of criteria and a number of validated transducer elements of the mobile probe;

- at least one criterion is based on the relative distances or angles of elements taken in the list comprising the set of tissues and boundaries and transducer elements of the mobile probe;

- at least one criterion is based on an absorption of tissues from the set of tissues and boundaries;

- at least one criterion is based on a scattering of boundaries from the set of tissues and boundaries;

the set of criteria comprises at least one element from the list comprising a distance of the transducer element to tissues of the insonificated region, an angle of the transducer element with boundaries of the insonificated region, absorption of tissues of the insonificated region, a distance between tissues of the insonificated region and the target region, a distance between boundaries of the insonificated region and a thickness of tissues of the insonificated region.

According to yet another aspect of the invention there is provided a non-transitory computer-readable storage device comprising instructions which, when they are executed on a processor of an apparatus, perform the above described method.

According to yet another aspect of the invention there is provided a computer program comprising instructions which, when they are executed on a processor of an apparatus, perform the above described method.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will readily appear from the following description of its embodiments, provided as non-limitative examples, and of the accompanying drawings.

On the drawings :

- Figure 1 illustrates an apparatus for maximising efficiency of HIFU treatment of a target region 24 according to an embodiment of the invention,

- Figure 2 illustrates a method for maximising efficiency of HIFU treatment of a target region 24 according to an embodiment of the invention,

- Figure 3 details an evaluating step of a method according to an embodiment of the invention,

- Figure 4 details a memory of an apparatus according to an embodiment of the invention.

- Figure 5 is a flowchart of a method according to an embodiment of the invention.

- Figure 6 is an illustration of images together with a set of tissues and boundaries displayed to an operator during a segmenting step according to an embodiment of the invention.

- Figure 7a is an illustration of several images acquired by Computed tomography and showed in inversed gray scale, white area being related to a low signal and black areas to a high signal, the images being used in a segmenting step according to an embodiment of the invention .

- Figure 7b is an illustration of several images acquired by Magnetic Resonance Imaging and showed in inversed gray scale, white area being related to a low signal and black areas to a high signal, the images being used in a segmenting step according to an embodiment of the invention .

Figure 7c is an illustration of a merge of several images acquired by Computed tomography and Magnetic Resonance Imaging and showed in inversed gray scale, white area being related to a low signal and black areas to a high signal, the merge being used in a segmenting step according to an embodiment of the invention.

On the different figures, the same reference signs designate like or similar elements. DETAILED DESCRIPTION

Figure 1 shows a schematic view of a high intensity focused ultrasound (HIFU) system 1 or apparatus being used on a patient 2.

As a non-limitative example, the HIFU system illustrated on figure 1 is performing HIFU treatment of a brain region of the patient 2. However, it should be noted that other parts of a human body can be treated in a very similar way. The HIFU system 1 can also be employed for the treatment of muscles, organs or any other tissues, living or artificial .

In the example of figure 1, the patient 2 have a head 20 with a brain 21 surrounded by a skull 22; itself surrounded by a skin 23. Brain 21 comprises a desired treatment region also called a target region 24 or a treated region 24.

An apparatus 1 can comprise a mobile probe 10 and a central unit 11.

The mobile probe 10 may include for instance:

- a probe armature 101 that may have a general concave shape in order to surround the patient's head in the example of figure 1 ;

- an array of HIFU transducer elements 102, for instance a two dimensional array following the shape of the armature 101;

- a balloon of degassed liquid 103 surrounded by a membrane 104, the balloon 103 being located between the array of HIFU transducers 102 and the patient 2, the membrane being in contact with the array of transducers on one side of the balloon and with the patient's skin 23 on another .

HIFU system 1 may also include a motorised displacement device 12 adapted to translate the mobile probe 10 along one or more directions of space, advantageously along the three directions X, Y, Z of space. The motorised displacement device 12 may be further adapted to rotate the mobile probe 10 around one or more directions of space, advantageously around the three directions X, Y, Z of space.

In some embodiments of the invention, the displacement device 12 is thus able to move the mobile probe 10 along six degrees of freedom.

In other embodiments, the mobile probe 10 may be manually moved by an operator.

The central unit 11 of the HIFU system 1 may include for instance a processor 110, a memory 111 and a graphics processing unit 112, also known as a GPU, the processor 111 being connected to the memory and the mobile probe 10 and controlling the HIFU transducers.

The processor 110 and/or the graphics processor unit 112 of the central unit 11 might also be connected to a micro-computer 4 for controlling the central unit 11, monitoring the HIFU treatment and displaying information to an operator, for instance a medical practitioner or a health care provider.

Reference will now be made in detail to embodiments of the invention which are described below, by referring to the accompanying figures. In this regards, embodiments of the invention may be embodied in several different forms and should not be construed as being limited to embodiments set forth in the following.

Some embodiments of the invention, hereafter described with reference to flowchart or block diagrams, will be understood as being implementable by computer readable instructions. These computer readable instructions can be provided to a processor of a general purpose computer, a special purpose processor or computer, or other programmable data processing circuit or circuits such that the instructions executed by the processor or circuit create modules that implement the operations and steps specified in the flowcharts blocks.

This way, embodiments of the present invention can be implemented through a non-transitory computer-readable storage device comprising any medium or media permitting the storing and transmission of the computer readable instructions or code.

The computer readable code can be recorded and transferred on a non-transitory computer-readable storage device in known ways and the device can include recording media, such as magnetic storage media and optical recording media. The storage device can also include transmission media such as media carrying or controlling carrier waves as well as elements of the Internet or of a computer network. Thus, the device may be a defined and measurable structure carrying or controlling a signal or information, such as a device carrying a bitstream, for example, according to embodiments of the present invention. The non- transitory computer-readable storage device may also be a distributed network, so that the computer program is stored, transferred and executed in a distributed fashion. By way of example only, the processor can include a circuit or a computer processor, and processing circuits may be distributed or be included in a single circuit.

Referring now to figure 2, an implementation of a method for maximising efficiency of HIFU treatment according to the present invention is illustrated.

This method includes at least three main steps:

a) a step of segmenting (310) images of an insonificated region 105 to identify a set of tissues and boundaries;

b) a step of evaluating (320), for each probe position of a database of probe positions, a score related to an energy loss through the path of ultrasound;

c) a step of identifying (330) an optimal probe position which minimizes said energy loss. These steps might be preceded by other steps such as, for instance, a preliminary acquisition of images of the patient's tissues or a selection of an area to treat, said selection being performed by a physician or a health care provider.

These steps might also be followed by other steps such as, for instance, a displacement of the mobile probe, a computation of HIFU sequences including for instance aberrations correction and a HIFU treatment performed with said mobile probe.

The three main steps will now be described in further details.

1. Step of segmenting (310) : step a

During the step of segmenting, at least one image of an insonificated region 105 is segmented to identify a set of tissues and boundaries.

An insonificated region 105 is a region at least partially penetrated by ultrasounds during a HIFU treatment .

An insonificated region 105 might include a target region 24 but might also be a distant region, for instance a region on the path of ultrasounds.

Said image or images of insonificated region 105s might have been preliminary acquired by performing a tomography of the patient using an imaging device.

As a matter of example, figure 7a is an illustration showing several slices of tomography acquired using a Computed Tomography (CT) imaging device

The image of figure 7a and 7b are showed in inversed gray scale compared to the classical use of gray scale: white areas are related to a low signal and black areas to a high signal.

Figure 7b is an illustration showing several slices of tomography acquired using a Magnetic Resonance Imaging (MRI) device, showed in inversed gray scale, white area being related to a low signal and black areas to a high signal .

With further reference to figure 7c, in a variant, said image or images of insonificated region 105s might be computed from a combination of several tomographies of the patient performed using several imaging devices.

The weight of each tomography in said combination might be adjustable by an operator or by the processor 110 or the graphics processor unit 112.

Advantageously, said tomographies might be registered, the image being thus a multimodality tomography fusion .

Figure 7c is an illustration showing an image computed from a combination of slices of tomography acquired using a Computed Tomography (CT) imaging device, and of slices of tomography acquired using a Magnetic Resonance Imaging (MRI) device.

Images shown on figure 7c are thus combination of the slices of tomography shown on figure 7a and 7b.

The image of figure 7c is shown in inversed gray scale, white area being related to a low signal and black areas to a high signal.

The weight of each tomography in said combination can be adjusted by an operator.

The images of figure 7c are thus multimodality tomography fusion images.

Said imaging devices can for example perform Computed tomography (CT) , Single-photon emission computed tomography (SPECT), Magnetic resonance imaging (MRI), Positron emission tomography (PET) , Ultrasound tomography or employ any other technology capable of imaging a volume or a section of the inside of a body.

The step of segmenting (310) may then comprise the delimitation of various tissues and/or boundaries appearing on said image. Depending on the technology used to acquire said images, the imaging device and the parameters of the tomography, said image or images might contain more or less information (for instance an image might contain one or more measurements of different natures for each location in the imaged region) which may be of a variety of natures (measurements of X-ray attenuation for CT, measurements of various relaxation times for MRI, ...) .

In a first embodiment of the invention, the processor 110 and/or the graphics processor unit 112 of the central unit 11 or of the micro-computer 4 might be able to identify tissues and/or boundaries in the insonificated region 105 based on the information provided in the images.

Several known methods of image segmentation may be employed to this aim, including thresholding, clustering, compression-based methods, histogram-based methods, edge detection, region-growing methods, split-and-merge methods, methods based on partial differential equations, graph partitioning methods, watershed transformation, model-based segmentation, multi-scale segmentation, neural networks segmentation or the like.

In other embodiments of the invention, said image or images can be presented to an operator, such as a physician or a health care provider, for instance on a screen of a micro-computer 4. Said images may be displayed as grey scale or multi-dimensional (coloured) images and said operator may be able to identify manually or semi- automatically tissues and/or boundaries in the imaged regions .

Segmentation might be performed automatically as described in the above first embodiment and subsequently validated, modified and/or completed by an operator.

Images of the insonificated region 105 may comprise slices of tomography taken along a single or a plurality of planes. Using several slices allows for segmenting the imaged region along various directions or planes of space.

Processor 110 and/or graphics processor unit 112 may be able to further compute a multi-dimensional segmentation of the imaged regions in tissues and/or boundaries by linking together or extruding one or several segmentations performed along one or two dimensions by processor 110, graphics processor unit 112 or said operator .

The set of tissues and/or boundaries obtained in the step of segmenting can comprise tissues and/or boundaries of the target region 24.

It can also comprise tissues and/or boundaries of regions to be avoided 25.

Said regions to be avoided 25 are regions that should not be penetrated by ultrasounds during a HIFU treatment .

As a matter of example, figure 6 shows several images together with a set of tissues and boundaries obtained during a segmenting step (310) .

The image of figure 6 is shown in inversed gray scale, white area being related to a low signal and black areas to a high signal.

Tissues and/or boundaries of a region to be avoided 25 are identified together with tissues and/or boundaries of a target region 24.

After said delimitation or segmentation of tissues and/or boundaries, various ultrasound parameters can be associated to each of said tissues and/or boundaries.

These ultrasound parameters can, for example, comprise absorption, attenuation, scattering, reflexion or refraction of said tissues and/or boundaries.

These ultrasound parameters can for instance be taken at specific ultrasound frequencies.

Said specific ultrasound frequencies can be in a range from 100 kHz to 10 MHz.

Advantageously, said specific ultrasound frequencies can belong to a range from 200 kHz to 2 MHz.

2. Step of evaluating (320) : step b

Based on the set of tissues and/or boundaries, scores indicative of ultrasound energy losses along a path of ultrasounds between the probe and the target region 24 can then be computed during a step of evaluating (320) .

The memory 111 of the central unit 11 of the micro- computer 4 of the HIFU system 1 can store a database 50 of probe positions 51.

Each stored probe positions 51 of the database may comprise a localisation of the mobile probe along one or more directions of space, advantageously along three directions X, Y, Z of space.

Each stored probe positions 51 may further comprise a rotational state of the mobile probe 10 around one or more directions of space, advantageously around three directions X, Y, Z of space.

Each stored probe position 51 may further comprise a focalisation state of the mobile probe 10.

In an embodiment of the invention, the database can comprise several stored probe positions 51.

In another embodiment of the invention, the database can comprise at least ten stored probe positions 51.

In yet another embodiment of the invention, the database can comprise at least fifty stored probe positions 51.

It is thus to be noted that, in the following, the term "position" of the probe or the transducer elements of the probe is to be understood as encompassing the localisation, the rotational state and the focalisation state of said probe or transducer elements.

2.1. Operation of comparing (321): operation b.l The step of evaluating (320) can comprise a first operation (321) of comparison.

In the comparing operation (321) a location of an ultrasound focal point associated with each probe position 51 is compared with a location of the target region 24.

Ultrasound focal point locations might be part of stored probe positions 51 or might be computed in real time during the operation of comparison (321) .

In a variant, ultrasound focal point locations might be computed using the tissues and/or boundaries of insonificated region 105s segmented during step a (310), in order to improve the accuracy of the focal point location estimation .

A criterion of proximity of said ultrasound focal point location with the target region 24 may then be computed, being indicative of the distance between said focal point and the target region 24.

In a first embodiment of the invention, said proximity criterion can be compared to a proximity threshold and, in the event the distance between said ultrasound focal point location and the target region 24 is higher than said threshold, the associated probe position will not be taken into account.

In a variant, said proximity criterion might be employed in subsequent operations performed during the evaluating step (320) and detailed further below.

The step of comparing (321) can further comprise the comparison of the location of an ultrasound focal point associated with each probe position 51 with locations of one or several regions to be avoided 25.

Advantageously, locations of said one or several regions to be avoided 25 might be further compared with the location of the insonificated region 105.

A second criterion of proximity of the insonificated region with said one or several regions to be avoided 25 can be computed, being indicative of the distance between the insonificated region and said one or several regions to be avoided 25.

In a variant, said second criterion can be indicative of the distance between the ultrasound focal point location and said one or several regions to be avoided 25.

In an embodiment of the invention, said second proximity criterion can be compared to a proximity threshold and, in the event the criterion is higher or lower than said threshold, the associated probe position will not be taken into account.

In a variant, said second proximity criterion might be employed in subsequent operations performed during the evaluating step (320) and detailed further below.

2.2. Operation of determining (322): operation b.2

In a second operation (322), a set of validated probe positions and/or a set of validated transducer elements of the mobile probe for each probe position, may be determined, based on a set of criteria thresholds.

Several criteria can be defined for each probe position and/or each transducer element of the mobile probe. Said criteria may be defined based on the set of tissues and boundaries and the position of the transducers elements of the mobile probe in each probe position 51.

Some criteria can for instance be related to the absorption or scattering of tissues and/or boundaries.

Some criteria may also be related to the relative distances or angles between elements taken in the list comprising the set of tissues and boundaries, position of transducer elements of the mobile probe and the mobile probe .

The set of criteria may further comprise for instance a distance of a transducer element to tissues of the insonificated region 105, an angle of a transducer element with boundaries of the insonificated region 105, an absorption of tissues of the insonificated region 105, a distance between tissues of the insonificated region 105 and the target region 24, a distance between boundaries of the insonificated region 105 and a thickness of tissues of the insonificated region 105, and the relative distance or angle between tissues, boundaries and the mobile probe.

A very simple example of such a criterion may be a criterion named abs and defined by the formula:

abs = t*a

wherein t is the thickness of a tissue layer having an absorption coefficient (X . This criterion increases with the absorption of said tissue layer and can be used to quantify the ultrasounds energy lost when ultrasounds go through said tissue layer.

One or several criteria thresholds may also be associated to some criteria. Such a threshold can define a maximal and/or a minimal value for an associated criterion.

For instance, a threshold associated to a distance between a transducer element and a tissue can be used to specify a minimal value of said distance.

For each probe position and/or each transducer element of each probe position, one criterion or several criteria are then determined.

Some criterion can then be compared to one or several criteria thresholds associated to said criterion, if such thresholds exist.

These comparisons may give a set of validation results associated with each probe position or each transducer element of each probe position.

Such a validation result may be a boolean result of a comparison between said criterion and a threshold associated with said criterion.

A validation result can, for instance, be negative if the criterion doesn't not satisfy the threshold; being for instance lower than a minimal threshold or higher than a maximal threshold. Said result can be positive if the criterion satisfies the threshold.

In some embodiments of the invention, if at least one validation result in a set of validation results is negative, the probe position and/or the transducer element, associated with said set of validation results doesn't satisfy the complete set of criteria and may not be used during the HIFU treatment.

In some embodiments, by repeating this operation for each mobile probe position 51 of the database 52 and/or for each transducer element of each mobile probe position 51 of the database 52, a set of validated probe positions can be determined.

In variants, a set of invalidated transducer elements and/or a set of invalidated probe positions might be determined in the same manner.

2.3. Operation of computing (323): operation b.3

In a third operation (323), at least one score can be computed for some mobile probe positions of the database .

Such scores may be indicative of the ultrasound energy loss along the path of ultrasound between the probe and the target region 24.

In some embodiments said scores can increase with the energy loss along the path of ultrasound while in other embodiments said scores can decrease with the energy loss along the path of ultrasound.

The main sources of ultrasound energy loss are:

- material absorption, and

- coupling between material (ultrasound reflexion and refraction)

As a matter of example, a system for maximizing HIFU treatment efficiency can thus advantageously:

- minimize the thickness of high absorption tissues such as bones (for instance skull) on the path of ultrasound (absorption criterion) , and

- minimize the incidence angle of the ultrasound with boundaries (for instance scalp/skull interface) , said angle being related to the angle between said interface and transducer elements (angle criterion) .

The proximity criterion, the number of validated or invalidated transducer elements and the set of criteria can thus be employed to build a formula for the calculation of a score indicative of the ultrasound energy loss along the path of ultrasound between the probe and the target region 24 when the probe is in a mobile probe position of the database .

In some embodiment, scores are evaluated for each probe position of the set of validated probe positions.

In another embodiment, scores are evaluated for each probe position of the database of probe positions.

A very simple example of such a formula, defining a score that increases with the energy loss, can be for instance :

Wherein

Angle is an average angle of the transducers elements of the mobile probe with regard to the scalp/skull interface, and

NumberOfProbeElementsOn is a number of validated transducer elements determined in the determining operation (322) .

A variety of other ways of defining a score according to the invention might be employed through various combinations of elements taken in a list of criteria comprising said proximity criterion, number of validated or invalidated transducer elements and set of criteria .

Said score should be indicative of the energy loss along the path of ultrasound between the probe and the target region 24 and thus the elements of said criteria list can advantageously be combined in an energetically relevant way. Criteria increasing with the energy loss and criteria decreasing with the energy loss can thus be differentiated and combined according to their relation with energy loss.

In the case the score is defined as increasing with the energy loss, a criterion increasing with the energy loss can thus be included in the formula defining said score in a way such as an increase of said criterion will lead to an increase of said score. A criterion decreasing with the energy loss would be included in a way such as an increase of said criterion will lead to a decrease of said score .

In the case the score is defined as decreasing with the energy loss, the situation would be the opposite.

As a matter of example, a criterion thickness , evaluating the thickness of the skull at a given location of said skull, may increase with the energy loss as the skull is a high absorption tissue.

A score defined by the above formula (1) being a score that increases with the energy loss, and the criterion thickness also increasing with the energy loss, said criterion may thus be included in a way such as an increase of criterion thickness will lead to an increase of said score. This criterion could thus be included in the above formula (1) as:

Angle * thickness

score = r ( 2 )

max NumberO^robeElementsOn,!)

Exact formulas defining scores and relations between criteria (addition, subtraction, multiplication, division, square roots, logarithm, powers and any other relevant mathematical operation) can be adapted to each embodiment in order to give relevant scores given the relative importance of each criterion in the energy loss along the path of ultrasound.

It is thus to be noted that a variety of formulas might be employed for defining scores without departing from the invention.

In some embodiments, a formula defining said score may be a very simple formula such as formula (2) .

In variants, the formula can be more elaborated.

In some embodiments, a set of positions of the database might be determined by selecting a set of mobile probe positions fulfilling a proximity criterion or a threshold on the number of validated or invalidated transducer elements.

A score might then be evaluated for each positions of this set.

Operations in the step of evaluating (320) might be performed in any order and in particular, might be performed in a different order than the above described order .

Step of identifying (330) : step c

In a third step (330), at least one optimal probe position can be identified using said scores.

If a score, for instance, increases with an energy loss, a probe position with a lowest score may be identified as an optimal probe position.

In a variant, several positions, having a score that is within a tolerance interval of the lowest score may be identified as optimal positions.

If a score decreases, for instance, with an energy loss, a probe position with a highest score may be identified as an optimal probe position. Again, in a variant, several positions, having a score that is within a tolerance interval of the highest score may be identified as optimal positions.

An identified optimal position may then be presented to a physician on a screen of the micro-computer 4.

In the case several positions are identified as optimal positions, said identified positions may be ordered by their associated scores and shown to an operator.

A choice can be made between said several positions either by a physician, an algorithm, for instance a random algorithm, or by using one or several additional criteria.

A subsequent fourth step (340), step d, can comprise a displacement of the mobile probe to an identified optimal position.

Said displacement may be performed manually by an operator or physician, automatically by the displacement device 12 of the HIFU system 1, or semi-automatically .

In the case the displacement is performed manually by an operator, a screen of the micro-computer 4 can show a correct position to the operator, indications on how to reach this position or instructions to guide said operator.