FIELD OF THE INVENTION

The present invention relates to a medical ultrasound excited thermography apparatus for thermal inspecting of a portion of a skin over a bone of a subject. The present invention further relates to a medical ultrasound excited thermography method for thermal analysing of a portion of a skin over a bone of a subject.

BACKGROUND OF THE INVENTION

In the field of medical imaging systems X-ray, CT, MRI or ultrasound imaging are utilized as diagnosis systems, in particular for diagnosing bone fractures. X-ray imaging and CT imaging have a reduced resolution so that hairline and stress fractures cannot be detected and, further, the imaging produce harmful ionizing radiation and usually require operation by skilled persons. X-ray, CT and MRI image systems are further expensive and not anywhere, in particular in poor rural areas available for medical diagnosis. The less expensive ultrasound image systems however are less suitable for diagnostics on bone fractures and anomalies and demand an even higher degree skilled person to operate.

Ultrasound excited thermography as e.g. disclosed by WO 2011/045695 Al heats a tissue volume selectively by absorption of the ultrasound and is visualized using magnetic resonance imaging (MRI), which is a complex and highly specialized diagnostic method.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide improved inspecting of a bone of a subject with low technical effort and improved image quality.

The object of the present invention is solved by the subject-matter of the independent claims; further embodiments are incorporated in the dependent claims.

According to one aspect of the present invention, a medical ultrasound excited thermography apparatus for thermal inspecting of a portion of a skin over a bone of a subject, comprising: an ultrasound transducer including a plurality of transducer elements adapted to emit ultrasound waves to the portion of the subject to be imaged,

an infrared camera adapted to provide infrared image data of the portion of the subject,

- a frequency filter adapted to filter the infrared image data on the basis of a modulation frequency of the ultrasound waves, and

a modulator connected to the ultrasound transducer.

The modulator is adapted to modulate the ultrasound waves emitted by the ultrasound transducer at a modulation frequency, and the frequency filter is adapted to filter the infrared image data at the modulation frequency. This is a possibility to provide a precise lock-in thermogram.

According to another aspect of the present invention, a method for thermal inspecting of a portion of a subject, in particular the skin over a bone of a subject is provided, comprising the steps of:

- transmitting ultrasound waves to the portion of the subject to be imaged,

receiving infrared waves of the portion of the subject

filtering the infrared image data on the basis of a modulation frequency, and providing infrared image data of the portion of the subject by means of an infrared camera.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method has similar and all identical preferred

embodiments as the claimed device and as defined in the dependent claims.

The present invention is based on the idea of introducing thermal heating into the portion of the subject to be inspected by means of ultrasound waves, which are provided by the ultrasound transducer or ultrasound emitter. The ultrasound waves are reflected at anatomical interfaces, defects and in particular at bone fractures so that a selected slight heating can be achieved in the portion of the subject, which can be imaged by means of the infrared camera. A thermal contrast is generated by a bone fracture or defect, since bone tissue mainly reflects ultrasound with lower amount of attenuation with respect to a fracture with a low reflection and high attenuation. Since ultrasound attenuation is proportional to temperature increase a thermal contrast is created. Since the infrared image data is filtered at the modulation frequency of the ultrasound waves, a lock-in thermography can be provided having a necessary low noise equivalent temperature difference (NETD) so that a low temperature difference in the portion of the subject can be imaged with high quality and high reliability. Consequently, defects in the portion of the subject can be inspected and detected with low technical effort and high image quality.

In an exemplary embodiment, the frequency filter comprises a fast Fourier transformation (FFT) filter. This is a possibility to improve the resolution of the inspection via the thermal imaging.

In a further exemplary embodiment, the FFT filter is adapted to perform a Fourier transformation at the modulation frequency. This is a possibility to reduce the noise equivalent temperature difference and to provide a corresponding high quality imaging.

In an exemplary embodiment, the frequency filter is synchronized with the modulator. In particular, the frequency filter and the modulator are connected to each other and preferably controlled by a control unit. This is a possibility to synchronize the frequency filter to the modulator with low technical effort.

In a further exemplary embodiment, the medical apparatus comprises a defect indicator adapted to indicate a defect in the portion of the subject based on the infrared image data. This is a possibility to indicate a defect to an operator with low technical effort.

In an exemplary embodiment, the medical apparatus further comprises a display device adapted to display the image data filtered by the frequency filter. This is a possibility to provide an infrared image to the user to detect a defect in the portion of the subject.

In an exemplary embodiment, the medical apparatus is formed as a medical imaging apparatus for thermal imaging of a portion of a subject.

In an exemplary embodiment, the frequency filter comprises an amplitude filter adapted to filter an amplitude of the infrared image data. This is a possibility to display a temperature profile of a portion to be imaged with low technical effort and to display the defects in the portion of the subject.

In a further exemplary embodiment, the frequency filter comprises a phase filter adapted to filter a phase of the image data. This is a possibility to display possible defects in the imaged portion of the subject precisely with low technical effort.

In a further exemplary embodiment, the frequency filter comprises a lock-in amplifier adapted to provide the phase or the amplitude of the image data based on modulated sine and cosine wave signals. This is a possibility to filter the image data with low technical effort.

In a further exemplary embodiment, the modulator is an amplitude modulator adapted to modulate an amplitude of the ultrasound waves. This is a possibility to heat the portion of the subject by means of ultrasound waves and wherein the corresponding infrared waves can be filtered at the modulation frequency with low technical effort. Further, the heated tissue can be detected more easily and in a deeper region of the portion of the subject.

In an exemplary embodiment, the modulation frequency of the ultrasound waves is 0,001 Hz to 1 Hz. This is a possibility to effectively modulate the ultrasound waves, since thermal conduction in the body acts as a low-pass filter for the fracture heating.

In an exemplary embodiment, the ultrasound transducer is adapted to emit the ultrasound waves at a frequency of 0.5 to 30 MHz. There is a possibility to slightly heat the tissue of the subject in the portion to be imaged easily with low technical effort. In a further preferred embodiment, the frequency of the ultrasound waves is 1 to 2 MHz and most preferred 1.7 MHz.

In an exemplary embodiment, the medical apparatus further comprises a rest unit which is adapted to receive a portion of the subject. There is a possibility to fix the portion of the subject to be imaged to apply the ultrasound waves precisely to the portion and to precisely image the heated portion by means of the infrared camera. There is a possibility to improve the precision of the thermal inspection based on the thermal images.

In an exemplary embodiment, the ultrasound transducer is connected by an acoustic coupling element to the portion of the subject. The acoustic coupling element is preferably formed as a gel or fluid filled bag or a fluid container in order to provide the acoustic coupling between the ultrasound transducer and the portion of the subject. There is a possibility to provide an acoustic coupling to the portion of the subject with low technical effort and to improve the transmission of the ultrasound waves to the portion of the subject.

In a further exemplary embodiment, the ultrasound transducer comprises an array of transducer elements, which are formed as piezo elements or capacitive

micromachined ultrasonic transducer (CMUT) transducer elements. There is a possibility to transmit ultrasound waves with low technical effort.

As mentioned above, the present invention provides a possibility to inspect a portion of the subject, in particular a bone of the subject precisely with low technical effort, since the portion of the subject like tissue or bone marrow can be slightly heated with low technical effort and the temperature difference of deeper portions of the subject conducted towards the skin can be detected in the infrared image data which is filtered at the modulation frequency, i.e. the lock-in frequency. Hence, possible cracks, fractures or abnormal structures in the portion of the subject can be inspected with low technical effort. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter. In the following drawings

Fig. 1 shows a schematic representation of a medical ultrasound excited thermography apparatus for thermal inspecting of the skin of a subject;

Fig. 2 shows a detailed schematic drawing of the medical ultrasound excited thermography apparatus for thermal inspecting of a portion of the subject; and

Fig. 3 shows a thermal image of the portion of the subject captured by the medical apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a schematic representation of a medical ultrasound excited thermography apparatus generally denoted by 10. The medical apparatus 10 is applied to inspect a volume of an anatomical site, in particular an anatomical site of a patient 12. The medical apparatus 10 comprises an ultrasound transmitter (i.e. ultrasound transducer) 14 having an ultrasound array including a multitude of transducer elements for transmitting ultrasound waves. The transducer elements are formed as piezo transducer elements or capacitive micro machined ultrasound transducer elements (CMUT), which operate at a frequency of 1 to 30 MHz and preferably at a frequency of 1.7 MHz so that the ultrasound transmitter 14 emits ultrasound waves having a frequency of 1 to 30 MHz and preferably 1.7 MHz.

The medical apparatus 10 further comprises a modulator 16, which is associated to the ultrasound transducer of the ultrasound transmitter 14 and to the infrared camera. The modulator 16 modulates the ultrasound waves emitted by the ultrasound transmitter 14 at a modulation frequency by modulating the amplitude of the ultrasound wave so that the ultrasound waves are correspondingly amplitude modulated at the modulation frequency. In another embodiment, the modulator switches the ultrasound transducer on and off in order to provide modulated ultrasound waves.

The medical apparatus 10 further comprises an infrared camera 18, which receives infrared radiation 20 from the skin over the anatomical site of the patient 12 and provide corresponding infrared image data of the skin over the anatomical site. The infrared camera 10 typically detects the infrared radiation 20 at a wavelength of 8 to 14 μιη and a noise equivalent temperature difference (NETD) of 0.08 K. The infrared camera 18 provides the infrared image data as images of a skin 22 over the anatomical site of the patient 12.

The medical apparatus 10 comprises a frequency filter 24 connected to the infrared camera 18 which receives the infrared image data and filters the infrared image data correspondingly at the modulation frequency of the ultrasound waves. The frequency filter 24 is synchronized with the modulator 16 so that a frequency filter 24 filters the infrared image data correspondingly at the modulation frequency. In the embodiment shown in Fig. 1, the modulator 16 and the frequency filter 24 are connected to each other in order to synchronize the frequency filter 24 and the modulator 16. In another embodiment, the frequency filter 24 and the modulator 16 are connected to a common control unit in order to synchronize the frequency filter 24 and the modulator 16 to the same modulation frequency.

The medical apparatus 10 is formed as an ultrasound excited lock- in thermography apparatus, wherein the anatomical site is excited by the ultrasound waves and the overlying skin is imaged by the infrared camera 18, wherein the image data is filtered by the frequency filter 24 at the modulation frequency of the ultrasound waves. By means of the frequency filtering at the modulation frequency, a NETD of less than 0.01 K can be achieved in a reasonable time. The modest (few K) maximum allowed body heating (by ultrasound) along with the temperature attenuation by thermal conduction from the fracture to the skin requires a low NETD.

The frequency filter may be formed as a FFT filter in order to provide the filtered thermal images.

In an alternative embodiment, the frequency filter may comprise a lock- in amplifier which multiplies the thermal images with a modulated square wave or modulated sine and cosine wave and provides an amplitude thermogram based on the multiplexed thermograms.

The medical apparatus 10 shown in Fig. 1 comprises a display 26 for displaying the thermal images to a user. In an alternative embodiment, the thermal images are processed and analyzed in an image processor of a fracture or defect indicator. If the fracture or defect indicator detects an image pattern, which corresponds to a fracture or a defect in general, the fracture or defect indicator indicates to the user that a fracture or defect has been detected. The fracture or defect indicator may be provided with or without the display 26 for displaying the thermal image.

The lock- in amplifier (not shown in fig. 1) multiplies the thermograms with a modulated sine and cosine wave as shown in equation 1 and 2: S = Intensity (t _i ,x,y) - sin( t _i ) ( 1 ) i

and

S-9o°c ^{= ~} ∑ Intensity(t _i ,x,y) - cos(t _; ) (2) i

wherein the amplitude of the thermogram is calculated by:

In an alternative embodiment, the phase of the thermogram can be calculated by: = tan^ (4)

The (amplitude or phase) thermogram of the skin can displayed and/or processed into the fracture indicator.

During the examination or inspection of the patient 12, the anatomical site, in this particular case shown in Fig. 1 a bone of an arm of the patient 12 is excited by means of the ultrasound waves emitted by the ultrasound probe 14. The ultrasound waves are reflected at tissue-bone interfaces such as bone fractures and absorbed in the bone marrow so that the temperature of the bone marrow is increased. The temperature rise at the used power and frequency (1 - 30 MHz) is less than allowed maximum of e.g. 1,5 °C above the physiological level of about 37 °C. The temperature rise at the skin of the subject 12 is much lower than the temperature rise in the bone. The infrared camera 18 receives the infrared radiation 20 emitted from the so heated anatomical site, i.e. from the skin 22 of the patient 12. Due to the modulation of the ultrasound waves at the modulation frequency (0,001 Hz - 1 Hz) and the corresponding filtering of the infrared image data provided by the infrared camera 18 at the modulation frequency, the temperature rise in the anatomical site and in particular in the bone at the anatomical site can be detected and imaged so that any defect in the anatomical site forming an interface for the ultrasound wave can be visualized and detected.

Hence, the diagnosis of a bone fracture and other defects in the anatomical site can be analyzed by means of the medical apparatus 10 precisely with low technical effort.

Fig. 2 shows a detailed schematic diagram of the medical apparatus 10 for inspecting a bone fracture of the patient 12 by thermal imaging of the overlying skin.

Identical elements are denoted by identical reference numerals, wherein here merely the differences are explained in detail. The ultrasound transmitter 14 comprises an ultrasound transducer array 28 including the transducer elements for emitting ultrasound waves 30 to the anatomical site 32 of the patient 12 to be imaged. The ultrasound waves 30 are modulated at the modulation frequency by means of the modulator 16, wherein the ultrasound waves 30 are partially reflected at tissue-bone interfaces 34 as schematically shown in Fig. 2. The bone marrow in bone 36 is selectively heated at the fracture 38 by the ultrasound wave 30. The heat diffuses from the fracture, which is essentially an (intermitting) planar heat source, into the body, also to the overlying skin. The overlying skin of anatomical site 32 emits infrared radiation 20 corresponding to the respective local skin temperature. Hence, the bone 36 is thermally imaged via the overlying skin of anatomical site 32.

Due to the filtering of the infrared image data 39 at the modulation frequency, i.e. the modulation frequency of lock- in thermography, small temperature differences of the overlying skin can be imaged so that a temperature rise of the anatomical site 32 of less than e.g. 1.5 °C above the physiological level can be used for the thermal imaging. Further, due to the lock- in thermography fractures and defects up to a depth of over 2.5 cm can be detected by the thermal imaging.

The ultrasound transmitter 14 is preferably connected to the skin 22 of the patient 12 by means of an acoustic coupling element 40 which may be formed of a gel (filled bag), a fluid (filled bag) or a fluid container for acoustic incoupling of the ultrasound waves 30 into the anatomical site 32. The medical apparatus 10 may further comprise a rest unit 41 which is adapted to receive and fix the anatomical site 32 of the patient 12 with respect to the infrared camera 18 so that a high resolution image of the infrared radiation 20 can be captured.

The frequency filter 24 may comprise an amplitude filter 42 and/or a phase filter 44. The amplitude filter 42 is adapted to filter an amplitude of the infrared image data and to provide corresponding image data to the display unit 26 for displaying the amplitude distribution of the infrared waves 20. The phase filter is adapted to filter a phase of the image data or of the infrared radiation 20 and to provide corresponding phase filter image data to the display unit 26 in order to display the distribution of the phase of the infrared radiation 20. The thermograms may be processed into some fracture indicator or measure.

The modulation frequency is preferably implemented as the inverse value of the measurement time. A modulation at the heart rate of the patient 12 is inadvisable to avoid artefacts in the thermograms by any blood conversion pulsation. In Fig. 3 a schematic thermal image of the skin over a bone 36 heated by ultrasound waves 30 is schematically shown.

The thermal image 46 shows the overlying skin of the bone 36 with a local amplitude hotspot due to a temperature fluctuation of the underlying bone marrow at the fracture 38 as imaged by the infrared camera 18. The hotspot comprises a temperature amplitude of approximately 30 m in this case. As shown in Fig. 3, allowable temperature amplitudes below e.g. 1.5 K can be easily detected and the resulting (fracture depth dependent) temperature amplitude of the overlying skin can be imaged and displayed to a greater depth of the fracture with lower skin temperature amplitude due to the lock-in thermography provided by the medical apparatus 10 according to the present invention.

The reflection coefficient which is the ratio of the intensity of the reflected wave to the incident wave is low for soft tissue interfaces such as the liver or the kidney so that approximately 1% of the ultrasound energy is reflected for ultrasound frequencies of 1 to 2 MHz. At the muscle-bone interface 34 approximately 60% of the ultrasound energy is reflected and for interfaces of soft tissue to air approximately 99% is reflected. This is the reason why the ultrasound waves 30 are usually not used as a primary imaging modality for bones, the digestive tract and lungs.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limiting the scope.