Patent Description:
In Extracorporeal Shock Wave Lithotripsy (ESWL), concrements, such as, for example, urinary stones or gallstones are disintegrated by focussed shock waves. <CIT> discloses a lithotripter with a device for detecting the quality of a coupling interface between a shock wave therapy head and a patient's body. Today, ESWL is a standard therapy in urolithiasis mainly competing with ureterorenoscopy and percutaneous nephrolithotomy as alternative procedures. Therapeutic devices utilising other acoustic energy sources, such as, for example, focussed or high-intensity ultrasound may be used to treat other medical disorders or for cosmetic purposes.

<FIG> shows a simplified schematic illustration of a typical lithotripter treatment head <NUM> comprising, inter alia, a shock wave source <NUM> including a shock wave focussing element (e.g. acoustic lens) <NUM>, an imaging unit (for example an ultrasound imaging system, not shown) and a coupling device <NUM> between the shock wave source <NUM> and the patient's body <NUM>.

During treatment between two- to three thousand shock waves are usually delivered at a frequency of <NUM> to <NUM>, thus, the total treatment duration is about <NUM> to <NUM>. The focussing element <NUM> (i.e. the acoustic lens) focusses the shock waves at a focal point <NUM>, wherein the focal distance of the focal point <NUM> is fixed by the acoustic lens' geometry, typically between <NUM> and <NUM> from the acoustic lens <NUM>. Prior to activating the shock wave source <NUM>, the target <NUM> (e.g. urinary stone) within the patient's body <NUM> is aligned with the focal point <NUM> of the acoustic lens <NUM>. Once activated, the shock waves are transmitted from the shock wave source <NUM> via a water-filled bellows or cushion (i.e. coupling device) <NUM> into the patient's body <NUM> to concentrate its energy onto target <NUM>. The bellows or cushion <NUM> is pressed into contact with the patient's body <NUM> by simply increasing the pressure within the bellows or cushion <NUM>. A coupling medium <NUM>, such as a coupling gel, is applied to the contact surface of the bellows or cushion <NUM> "bridging" the interface between the bellows or cushion <NUM> and the skin surface of the patient's body <NUM> and therefore improving acoustic coupling for the shock waves.

Various studies have shown that even a few air bubbles <NUM> (or other disturbances in the coupling interface <NUM>) within the coupling gel <NUM> can considerably reduce the efficacy of the shock waves. Due to its significant lower impedance, air fully blocks the shock waves and causes disturbances of the field, resulting in a significant loss of energy at the focal point. In practice, these air bubbles <NUM> may already be in the coupling gel <NUM> when provided by the manufacturer. More often, air bubbles may enter the coupling gel <NUM> when the coupling gel <NUM> is squeezed out of its container during application. Further, when arranging the bellows or cushion <NUM> into contact with the patient's body <NUM>, air bubbles <NUM> may be trapped in the coupling interface <NUM> either at the initial setup at the beginning of the treatment, or in case the contact between the bellows or cushion <NUM> and the patient's body <NUM> is temporarily interrupted during the treatment. In addition to air bubbles <NUM> being trapped in the coupling gel <NUM>, the coupling interface <NUM> may be disturbed if the contact pressure between the bellows or cushion <NUM> is too low and/or if there is only partial contact between the patient's body <NUM> and the bellows or cushion <NUM>. Other potentially severe disturbances may arise, if the bellows or cushion <NUM> do not fit snugly against the patient's body <NUM> causing wrinkles or folds in the bellows or cushion contact surface. Further disturbances of the coupling interface <NUM> may be caused by any clothing or bed linen that gets trapped between the bellows or cushion <NUM> and the patient's body <NUM>.

Once the therapy head <NUM> (i.e. including bellows or cushion <NUM>) is arranged into contact with the patient's body <NUM>, it is generally not possible to visually inspect the surface of the coupling interface <NUM>. Thus, some commercially available lithotripters may comprise a surveillance camera <NUM> (i.e. a video camera) that is typically integrated within the shock wave source <NUM> allowing visualisation (i.e. imaging) of a region of interest (ROI) of the coupling interface <NUM> that is displayed on a monitor <NUM> (e.g. Dornier OptiCouple®). A light source <NUM> (e.g. spot light) may be provided to illuminate the coupling interface <NUM>. The light source <NUM> may be any suitable light source (e.g. white LED) and may be positioned separate from the camera <NUM> so as to provide a desired illumination characteristic. Through the surveillance camera <NUM>, an operator can monitor the coupling interface <NUM> and detect disturbances, such as, for example, trapped air bubbles <NUM>, an incomplete coupling or folds/wrinkles of the bellows or cushion <NUM> contact surface etc. and the operator may remove any of these disturbances by simply wiping over the contact surface of the bellows or cushion <NUM>.

Alternatively, an inline ultrasound scanner, i.e. a diagnostic ultrasound transducer integrated into the shock wave source <NUM>, may be used to monitor the coupling interface <NUM>. However, inline ultrasound scanner only allow detection in the actual scanning plane, so that a <NUM>° rotation of the ultrasound transducer is required for a complete scan of the entire area of the coupling interface <NUM>, wherein a video camera <NUM> allows immediate and real-time inspection of the entire area of the coupling interface <NUM>.

In use, the coupling quality of the coupling interface <NUM> should be checked especially after the initial treatment setup is complete, i.e. the patient is suitably positioned relative to the therapy head <NUM> and the bellows or cushion <NUM> is brought into contact with the patient's body <NUM>. Often, the coupling quality of the coupling interface <NUM> remains unaffected during the actual treatment, however, the coupling quality of the coupling interface <NUM> may be disturbed by inadvertent patient movement, or by repositioning of the patient in order to adjust the shock wave target position, or by changing the coupling settings for whatever reason.

Consequently, it is crucial that the operator regularly inspects the coupling interface <NUM> throughout the entire duration of the treatment. Though, since a typical treatment can last between <NUM> and <NUM>, regular inspection of the coupling interface <NUM> can be a rather fatiguing task, especially as disturbance may only occur infrequently.

Currently available monitoring systems are based on image processing to evaluate the size of the coupling area blocked by air inclusions in the coupling gel by differentiating between areas that represent air from areas that represent no air, such that, if the size of the air inclusions is above a certain threshold, the operator may be alarmed. However, this approach comes with considerable practical problems. For example, the differentiation between pixels that represent air and pixels that represent the gel is non-trivial and depends very much on the present image contrast, illumination of the coupling interface, the camera settings, the colour of the applied coupling gel and even the patient's skin colour. Also, hairs, pigmental moles or tattoos are likely to affect the evaluation. Also, the bellows or cushion of the coupling device <NUM> are used to adapt for different penetration depths, i.e. changing the distance between camera <NUM> and the coupling interface <NUM>, so that the images size of the air inclusion appears different with varying distance between the camera <NUM> and the coupling interface <NUM>, making it difficult to determine an absolute value of the air inclusion. Further, the amount of the entrapped air is only of limited use when describing the impact on the transmission, i.e. air bubbles near the shock wave centre axis is known to affect the transmission more than air bubbles that are further away from the central axis, because the transmittal shock wave energy flux density is at its maximum at the centre axis. Also, numerous small air bubbles may affect the transmission more than a single air bubble of the same size, because the scattering surface of a bubble is larger than the bubble itself.

Therefore, it is an object of the present invention to provide an improved method and device for monitoring a coupling quality of the coupling interface <NUM> between a lithotripter and a patient. In particular, it is an object of the present invention to provide automatic image-based surveillance of a coupling quality of the coupling interface <NUM> with improved reliability and ease of use for the operator.

According to a first aspect, there is provided a method for continuously monitoring a coupling quality of a coupling interface between an acoustic energy source of a therapeutic device and a body surface area of a patient, comprising the steps of:.

Preferably, the method may further comprise the step of:
(f) repeating steps (c) to (e) for at least one other image of said plurality of images that is temporally spaced apart from said predetermined first image and said at least one second image.

This provides the advantage of an automated objective and reliable method for continuously and unambiguously monitoring changes in the quality of a coupling interface between the therapy head <NUM> and the patient. In particular, the present invention provides the advantage of detecting any changes in a predetermined image parameter that represents a coupling quality throughout the entire treatment duration, irrespective of any variations in the system settings or treatment setup for individual patients. That is, the detected change is somewhat normalised for the conditions existing at the time of treatment, therefore, making the automatic method more accurate and reliable over existing monitoring techniques.

Advantageously, said at least one first image characteristic may be any one of a tonal image distribution, a frequency spectrum and an image feature characteristic. In one embodiment, said tonal image distribution may be a histogram of a probability distribution function of image brightness of any one of said plurality of images. Preferably, said frequency spectrum may be a 2D Fourier spectrum of any one of said plurality of images. In another embodiment, said image feature characteristic may be a length of one or more edge features detected by an edge detection algorithm within any one of said plurality of images. Advantageously, said length may be a total length of said one or more edge features. Even more advantageously, said edge detection algorithm may utilise a Sobel operator.

Advantageously, said quantitative parameter may be based on a cross-correlation between said at least one first image characteristic of said predetermined first image and said at least one first image characteristic of said at least one second image.

In one embodiment, said quantitative parameter may be a difference in length between the length of said one or more edge features detected in said first image and the length of said one or more edge features detected in any one of said at least one second image.

Advantageously, said at least one second image and said at least one other image of said plurality of images may be a sequence of images subsequent to said predetermined first image and spaced apart at a predetermined time interval.

Advantageously, said signal may be a visual and/or audible signal.

Advantageously, said predetermined area may be adaptable during use. This provides the advantage that the ROI can change (e.g. size) or "move" its position so as to optimise the predetermined area (in case of patient movement etc.). The adaptable ROI may be provided by an algorithm within the image processor, wherein relevant parameters for the optimisation of the ROI may be set by the operator.

According to another aspect of the present invention, there is provided a device for continuously monitoring a coupling quality of a coupling interface between an acoustic energy source of a therapeutic device and a body surface area of a patient, comprising:.

Advantageously, said imaging system may comprise any one of an optical camera and a sonograph.

According to yet another aspect of the present invention, there is provided a computer readable storage medium having embodied thereon a computer program, when executed by a computer processor that is configured to perform the method according to the first aspect of the present invention.

Example embodiments of the description will now be described, by way of example only, with reference to the accompanying drawings, in which:.

The described example embodiment relates to an Extracorporeal Shock Wave Lithotripter (ESWL) and, in particular, to monitoring / surveillance of a coupling interface between the therapy head of the Lithotripter and the patient's body in order to detect changes of the coupling quality to then notify the operator in case the change exceeds a predetermined threshold. It is understood by the person skilled in the art that the present invention is not limited to shock wave lithotripters as described in the specific example but is equally applicable to other therapeutic devices using any other suitable acoustic energy source (e.g. shock waves, ultrasound).

Certain terminology is used in the following description for convenience only and is not limiting. In particular, it should be appreciated that the terms 'determine', 'calculate' and 'compute' and variations thereof may be used interchangeably and include any type of methodology, process, mathematical operation or technique. The terms 'generating' and 'adapting' may also be used interchangeably describing any type of computer image processing. In addition, the term 'pixel' is understood to mean a digital picture element, or the smallest unit of a display memory that can be controlled.

Further, unless otherwise specified (e.g. by providing a temporal order), the use of ordinal adjectives, such as, "first", "second", "third" etc. merely indicate that different instances of like objects are being referred to and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking or in any other manner.

Referring now to <FIG>, a portion of a commercial Lithotripter system is illustrated schematically (i.e. the therapy head). The arrangement is suited for detecting the quality of the coupling interface <NUM> and provide the method of the present invention. In particular, <FIG> shows a cross section of the therapy head <NUM> when operably coupled to the patient's body <NUM>. The therapy head <NUM> comprises a shock wave source <NUM> including an acoustic lens <NUM> (usually immersed in a water bath <NUM>) and a coupling device, such as, for example a cushion or bellows <NUM> (typically filled with water <NUM>). A coupling gel <NUM> is evenly applied to a contact surface of the bellows <NUM> forming a coupling interface <NUM> with the patient's body <NUM>. The rim <NUM> indicates the boundary of the contact surface with the bellows <NUM>. A video camera <NUM> is positioned so as to acquire images (e.g. digital images) of a region of interest of the coupling interface <NUM>. Typically, the video camera <NUM> is integrated into the shock wave source <NUM> so as to allow continuous monitoring of the coupling interface <NUM>. A monitor <NUM> is provided to display real-time images (snap-shot or video) for visual inspection. A light source <NUM> (e.g. white LED) is provided at the rim of the lens <NUM>. In this particular example, the light source <NUM> may be radially spaced apart from the camera <NUM> (e.g. <NUM>°) so as to provide a relative inhomogeneous illumination in order to display occurring air bubbles either relatively bright or dark. Such inhomogeneous illumination can be advantageous when identifying or detecting air bubbles within the method of the present invention.

During treatment setup, the therapy head <NUM> is positioned so that the target area <NUM> is within the focal point <NUM> of the acoustic lens <NUM>. The bellows <NUM> (with applied coupling gel <NUM>) is then inflated to move into contact with the patient's body <NUM> forming a coupling interface <NUM>, preferably without trapped air bubbles <NUM> or other disturbances. The operator visually inspects the image of the coupling interface <NUM> for any detrimental disturbances (e.g. air bubbles <NUM>, hairs <NUM>, folds or wrinkles etc.) and may wipe over the contact surface of the bellows <NUM> so as to remove such trapped air bubbles <NUM> and/or folds.

A region of interest (ROI) may be determined by the operator (e.g. user defined) or by an image processing algorithm (depending on image parameters set by the operator). The determined ROI may either be static during operation (e.g. a set window size) but may also be adapted during operation (e.g. an image processing function may adjust the ROI size and/or position during operation in accordance with pre-set parameters of the interface <NUM>).

Once the coupling quality is acceptable, shock wave treatment commences, typically for a duration of <NUM> to <NUM> during which inadvertent patient movement may cause a change in the coupling quality of the coupling interface <NUM>, for example, air gets trapped again in the coupling gel <NUM>, body hair <NUM> may be arranged so as to provide a disturbance, or folds or wrinkles in the bellows <NUM> contact surface may be effected by the movement.

As can be understood by a person skilled in the art, it would be difficult to either determine from a single image, whether or not, the coupling quality if sufficient for the whole duration of the treatment. Also, there may only be partial coupling caused by insufficient coupling pressure (i.e. the pressure within the bellows <NUM>) or disturbances attributable to a particular treatment situation (e.g. when treating a small child). Pigmental moles or hairs may be difficult to distinguish from trapped air bubbles <NUM> at different lighting. Depending on the present lighting, the brightness of trapped air bubbles <NUM> may vary between individual air bubbles (i.e. some air bubbles appear brighter and others appear darker relative to the background) and compared to the coupling gel <NUM>.

As illustrated in <FIG>, the present invention provides a method for automatically and unambiguously monitor the coupling interface <NUM> and detect any changes to the quality of the coupling interface <NUM>, as well as, visually and/or audibly indicate when the detected change exceeds a predetermined threshold, irrespective of the nature of the disturbances, the current setup or treatment situation.

During operation (i.e. when the initial coupling quality is acceptable and shock wave therapy has commenced), at least one first reference image is obtained (i.e. stored in a suitable storage medium) from the coupling interface <NUM> and subjected to image processing <NUM> to extract at least one image characteristic <NUM>, such as, for example, one or more characteristic parameter(s) and/or one or more characteristic function(s). The reference image <NUM> may be selected at a predetermined time (preferably the first image) and/or having a predetermined minimum coupling quality based on the chosen image characteristic. The image characteristic utilised may be any one of a distribution function (histogram) of brightness, a total edge length detected in the image, or a spatial Fourier spectrum.

During the duration of the treatment, subsequent images are continuously obtained from the coupling interface <NUM> at a predetermined time interval and each one is subjected to image processing <NUM> so as to extract the at least one image characteristic (e.g. total edge length, brightness histogram or Fourier spectrum etc). The extracted image characteristic <NUM> is then compared to the image characteristic of the predetermined reference image <NUM> using a comparator. Further, the comparator <NUM> comprises specific values for a maximum deviation from any one of the selectable image characteristics, any one of which may be utilised to determine a significant change from the reference image. Thus, in case the image characteristic of any one of the subsequent images exceeds the maximum deviation from the image characteristic of the reference image, a signal is triggered to notify the operator <NUM>. The signal may be any one of an audible or visual alarm.

The time interval between subsequent images may simply be the frequency of the video camera <NUM>, or a specific time interval may be set by the operator, for example, the time interval of obtained subsequent images may be in line with the frequency of the shock waves, or any other interval suitable to continuously monitor the coupling interface <NUM> and detect significant changes of a coupling quality.

Further, it is understood by the person skilled in the art, that the operator may select any one of the available image characteristics prior to the start of the treatment, to be used for detecting changes in the coupling quality of the coupling interface <NUM>. Also, the maximum deviation for each of the available image characteristics may be individually set by the operator and/or may be pre-set in the comparator during manufacture.

<FIG> illustrates the method of the present invention when using (e.g. pre-selecting) a distribution function (histogram) of brightness. <FIG> represents the reference image and <FIG> represents one of the subsequently obtained images. The reference numerals used are in line with the method illustrated in <FIG>, i.e. image displayed on the monitor <NUM>, extracted image characteristic of reference image <NUM> and a subsequent image <NUM>, and action of comparator <NUM>. The image shows an elliptical coupling interface illuminated by a spot light on its left side. A small air bubble is shown on the left side of the reference image, which may be acceptable to the operator, as it is not at the centre axis of the shock waves. The reference image also includes hairs <NUM>. As discussed earlier, an image processing unit of the system calculates a histogram of the brightness. The x-axis of the histogram represents image brightness (<NUM>=black, <NUM>=white), and the y-axis of the histogram represents the number of pixels. The black background is suppressed by setting the number of black pixels to zero. Though, a relevant region of interest may be defined by the operator. As is understood by the person skilled in the art, the method of the present invention does not require the reference image to be without any disturbances at all. The subsequent image obtained shows additional disturbances, e.g. trapped air bubbles <NUM>. The extracted histogram differs from the histogram for the reference image. In particular, the rather smooth brightness distribution of the reference image shows additional maxima and minima in the histogram of the subsequently obtained image. The differences are identified by the comparator, for example, by using known cross correlation techniques.

<FIG> illustrates the method when using total edge length as the image characteristic. In particular, the image processing unit applies an edge detection filter to enhance those image pixels which represent edges within the image, e.g. air bubbles or hairs show edges. The edge detection is independent of brightness changes of the background (i.e. coupling gel). Again, the method of the present invention does not require the reference image to be without any disturbances (i.e. edges at all). In this particular example, a horizonal and vertical Sobel kernel was convoluted with the image data. <FIG> represents the reference image and <FIG> represents one of the subsequently obtained images. The additional edges found in the subsequent image present additional disturbances (e.g. air bubbles <NUM>) which are identified by the comparator, e.g. by counting the number of pixels representing edge (i.e. pixels with a brightness above a set threshold). If the total number of pixels or total length of edges exceeds a predetermined maximum deviation from the edges of the reference image, a visual and/or audible signal is trigged to notify the operator.

<FIG> illustrates the method when using a spatial Fourier spectrum as the image characteristic. Here, the image processor applies, for example, a <NUM>-dimensional Fourier transform (FFT) to the image data. The Fourier transform contains all image information, wherein image variations are translated into spatial frequencies. In this particular example, the low frequencies are at the image centre of the FFT representation. <FIG> represents the reference image and <FIG> represents one of the subsequent images. The additional disturbances shown in the subsequent image induce changes in the frequency distribution of the FFT. The changes in the FFT can be identified by the comparator, in particular, by using a range of Fourier coefficients that are most sensitive to additional disturbances.

Claim 1:
A computer-implemented method for continuously monitoring a coupling quality of a coupling interface (<NUM>) between an acoustic energy source (<NUM>) of a therapeutic device (<NUM>) and a body surface area (<NUM>) of a patient, comprising the steps of:
(a) obtaining a plurality of images of at least one predetermined first area of the coupling interface (<NUM>);
(b) extracting at least one first image characteristic of a predetermined first image of said plurality of images;
(c) extracting said at least one first image characteristic of at least one second image of said plurality of images, said at least one second image being temporally spaced apart from said predetermined first image;
(d) determining a quantitative parameter corresponding to a difference between said at least one first image characteristic of said predetermined first image and said at least one first image characteristic of said at least one second image, and
(e) actuating a signal if said quantitative parameter exceeds a predetermined reference threshold, wherein said predetermined reference threshold is a maximum deviation from said at least one first image characteristic of said predetermined first image.