WO2013084093A1

WO2013084093A1 - Device for ultrasound imaging

Info

Publication number: WO2013084093A1
Application number: PCT/IB2012/056355
Authority: WO
Inventors: Ramon Quido ERKAMP; Emil George Radulescu; Sheng-Wen Huang; Caifeng Shan
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-12-07
Filing date: 2012-11-12
Publication date: 2013-06-13

Abstract

The present invention relates to a device for ultrasound imaging comprising an ultrasound transducer (102, 202, 302, 402, 502) for acquiring ultrasound images, a first holder (204, 312, 410, 510) for mechanically holding the ultrasound transducer and allowing movement of the ultrasound transducer through movement of the first holder, and a second holder (212, 214, 308, 404, 506, 512) for mechanically holding the first holder and/or the ultrasound transducer and for guiding and/or restricting movement of the ultrasound transducer through movement of the first holder. This provides a precise, reliable, fast and cost effective measurement of tissue layer boundaries of a body and/or estimation of the total fat mass and/or fat-free mass of a body.

Description

Device for ultrasound imaging

FIELD OF THE INVENTION

The present invention relates to a device for ultrasound imaging, in particular for determining actual tissue layer boundaries of a body and/or for estimating total values for fat and/or fat-free mass of a body.

BACKGROUND OF THE INVENTION

In the field of personal fitness appliances and personal health care it is desirable to get insight into a body's proportional composition of different tissue types. For this purpose it is necessary to distinguish several main tissues from each other. The most important tissues to detect from a health perspective are: fat mass and fat-free mass, lean body mass and muscle mass and a further discrimination of adipose tissue in subcutaneous and intra-abdominal adipose tissue. Low levels of physical activity and bad dietary habits can lead to poor physical fitness and in the long run result in lifestyle related diseases such as type 2 diabetes, hypertension, dyslipidemia, polycystic ovary syndrome, reproductive abnormalities, sexual dysfunction, heart disease, and metabolic syndrome. Medical professionals have to increasingly deal with these diseases. Having a method for quickly and reliably assessing a patient's level of physical fitness can help the professional to assess to what extent the physical fitness may be impacting the patient's health. Moreover, medically prescribed exercise intervention with fitness level and disease monitoring could be used to improve the patient's health and also document the effectiveness of the treatment.

A patient's body composition (fat versus muscle mass) is a good indicator of overall physical fitness. Commonly used solutions to detect tissue layers in body tissues use either modalities that are too complex to be used in a home setting like MRI scan, underwater weighting and skin fold measurements that require proper training to be meaningful or modalities that are too inconsistent to provide meaningful data such as bioelectrical impedance, which is very sensitive to the varying amount of water in the body. Furthermore these techniques are only capable of determining total mass of the selected tissue and do not provide insight into "on the spot" thicknesses of certain tissues. Other techniques involve either measurement with multi-beam and multi-focus ultrasound devices, but this involves heavy processing and costly hardware or makes prior assumptions about where the tissue layer should be. Due to the huge variation in body composition across the population such techniques cannot be applied widely.

A precise method for measuring body fat percentage is dual energy X-ray absorptiometry. The whole body is scanned twice, using energy beams with differing fat absorption rates. Digital subtraction and summation of these image volumes give fat volume versus total body volume. Due to the X-ray dose involved this method cannot be used for frequent monitoring.

A fairly accurate way of measuring a person's body fat percentage is by measuring the weight and volume of the person to find average tissue density. Together with assumptions on bone mass and knowing the densities of muscle and fat, a body fat measurement can be calculated. This method has good accuracy and is exceptionally consistent over multiple measurements. Unfortunately the procedure involves submerging the subject in a water tank, making the whole thing bulky, time consuming, and expensive.

A commonly used method for quickly estimating body fat percentage is based on skinfold tests. At several anatomical sites (between 3 and 7) the skin is pinched and slightly pulled, and a calliper is used to measure the thickness of these skinfolds and determine the subcutaneous fat layer thickness. These measurements are then put into a statistically derived formula that estimates the fat percentage. Although this is a relatively fast and also very cost effective method, it is not very repeatable. Some variables that influence the thickness measurement are how much skin is pinched, how hard it is pulled, how it is held by the hand, and the amount of pressure that is applied with the calliper. For these reasons there is a large variability in measurement values that also differs strongly depending on what person is performing the measurement.

Measuring body fat using ultrasound devices is disclosed for example in US

5,941,825. This method measures body fat by transmitting into a body ultrasound pulses, measuring at least one reflective distance, selecting the at least one reflective distance, which has the shortest distance to indicate the distance between the inner and outer border of subcutaneous fat tissue, wherein the selecting of the at least one reflective distance corrects for an ultrasound transmission parallax. It is asserted that this allows for a more convenient and precise measurement of layer thicknesses in an object.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a device for ultrasound imaging that particularly allows a precise, reliable, fast and cost effective measurement of tissue layer boundaries of a body and/or estimation of the total fat mass and/or fat-free mass of a body. Preferably, said device shall be configured to be easily and conveniently operated in a home setting.

According to the present invention a device is presented for ultrasound imaging comprising:

an ultrasound transducer for acquiring ultrasound images,

a first holder for mechanically holding the ultrasound transducer and allowing movement of the ultrasound transducer through movement of the first holder, and

a second holder for mechanically holding the first holder and/or the ultrasound transducer and for guiding and/or restricting movement of the ultrasound transducer through movement of the first holder.

The present invention is based on the idea to provide a device that is as cost effective and fast as the skinfold method but is also highly reliable and consistent in its measurements. This is achieved by mechanically sweeping an ultrasound imaging beam, particularly a single ultrasound imaging beam and slowly building up an ultrasound volume under the skin. Signal processing techniques are then preferably used, as proposed according to preferred embodiments, to determine the thickness of the subcutaneous fat layer at the scanned position. Preferably, the device is used directly at the same anatomical locations at which currently skinfold measurements are taken.

According to the present invention it is possible to detect tissue layers with a single element ultrasound transducer. Tissue layers of the body can, for instance, be detected by finding the peaks in the A- line signal. But the ID signal based detection is sensitive to data noises and is less reliable and consistent compared to 2D ultrasound image or 3D ultrasound volume based detection.

In a preferred embodiment the proposed device comprises a single element focused ultrasound transducer that is manually pushed and moved around against the skin. Alternatively, means for automatically moving the ultrasound transducer may be provided, i.e. the transducer motion is automated. In particular, a movement unit coupled to the first holder is provided in an embodiment for automatically moving the ultrasound transducer to predetermined positions, in particular along a predetermined trajectory.

Further, in an embodiment an attached cost effective movement sensor for sensing the movement and/or position of the ultrasound transducer with respect to a currently imaged object, the second holder or a housing of the device is provided. The movement sensor preferably measures the motion of the transducer relative to the skin. This device allows the slow build-up of either a 2D ultrasound image or a 3D volumetric view under the skin surface that is used to measure the thickness of the subcutaneous fat layer. Transmit pulses are preferably only sent when the movement sensor detects that the transducer is in motion to ensure that as low energy as possible is used to obtain the clinically relevant data.

Preferably, said movement sensor comprises an optical sensor, in particular attached to the ultrasound transducer or the first holder, for optically detecting movements of the ultrasound transducer. This provides a simple and cost effective, but precise way of obtaining movement or position information.

There are various embodiments for implementing said first and second holders. In a simple embodiment said first holder comprises a stick holding said ultrasound transducer at one end, wherein said second holder comprises one or more guidance rails for guiding the stick. A handle or joint may be provided at one end of the stick by which a user can manually move the stick and thus move the transducer, e.g. to shift it in a transversal direction and/or to change its angular inclination with respect to the skin.

In a first embodiment for implementing said second holder it comprises two orthogonally arranged guidance rails, wherein movement of the stick along one or both guidance rail(s) involves movement of at least one guidance rail in an angular direction. Thus, by the guidance rails, the movement of the stick and, thus, of the transducer is guided in some way although the user is rather free in determining the direction of movement.

Preferably, in this embodiment a rotational resistance measurement unit is arranged at one end of each guidance rail, wherein movement of the stick along one or both guidance rail(s) involves movement of at least one guidance rail in an angular direction involved by a movement of the stick along one or both guidance rail(s) leads to a change of resistance at the rotational resistance measurement unit arranged at one end of said guidance rail. In this way, the inclination and thus the movement and/or position of the transducer can be easily determined through the measured resistance(s).

In another quite simple embodiment said second holder comprises a rod arranged substantially orthogonal to the stick and being rotatable about its longitudinal axis. Again, the positioning is preferably measured by use of a rotational resistance measurement unit that is arranged at one end of the rod, wherein a movement of the stick involves a rotation of the rod leading to a change of resistance at the rotational resistance measurement unit. In still another embodiment said second holder comprises a cup and a flexible holding element inside the cup holding said stick and allowing a pivoting movement of the stick. Said flexible holding element may, for instance, be a membrane made from rubber or plastic, while other embodiments and materials may be usable that provide the desired function.

Preferably, in said embodiment the device further comprises one or more accelerometers arranged on the cup, the ultrasound transducer and/or the first holder for sensing movement of the ultrasound transducer with respect to the cup. Such accelerometers are very precise and cost efficient and measure the movement in a contactless manner.

As mentioned, the movement of the transducer may be effected manually or automatically. In both embodiments said second holder may further comprise a guidance unit for guiding the ultrasound transducer during movement. For instance, said guidance unit may comprise a guide plate having a spiral shaped channel into which a holding element attached to the ultrasound transducer extends causing movement along said spiral shaped channel when the ultrasound transducer is moved.

There are various options and further embodiments for using the device according to the present invention for determining actual tissue layer boundaries of a body and/or for estimating a person's fat- and/or fat-free mass. In a preferred embodiment the device is adapted for determining actual tissue layer boundaries of a body and said ultrasound transducer (also called probe) is adapted for acquiring two or more ultrasound images at adjacent positions of a surface of the body. The device further comprises

a converter for converting said ultrasound images separately to depth signals, wherein a depth signal is obtained by summing intensities of one of said ultrasound images along a line of substantially constant depth in the body,

- a detector for detecting a set of candidate tissue layer boundaries for an ultrasound image by thresholding the depth signal obtained for said ultrasound image,

a selection means for selecting from a set of candidate tissue layer boundaries a nearest candidate tissue layer boundary that is nearest to the surface of the body, and

a processing means for determining an actual tissue layer boundary from the nearest candidate tissue layer boundaries obtained for various ultrasound images.

According to another embodiment the device is adapted for estimating total fat- and/or fat-free mass of a body and comprises a device for determining actual tissue layer boundaries of a body and a body fat estimator for estimating the total fat- and/or fat-free mass of a body based on several actual tissue layer boundaries determined at different places of the body.

Different to the currently known devices of this art, such a device for determining actual tissue layer boundaries of a body and/or for estimating a person's fat- and/or fat-free mass acquires two or more ultrasound images at adjacent positions of the surface of the body and uses these images to determine a tissue layer boundary that appears spatially coherent on the acquired images.

The number of images acquired per position depends on how fast the user moves the transducer. For example, if moving slowly, multiple images might be acquired at one position. This can be detected by the movement detection means (e.g., used in computer mice) included in the device. Typically, the area is large enough to cover the body (part) that needs to be measured.

The user moves the device along a surface of the person and thus obtains ultrasound images from a larger area compared to acquiring only one ultrasound signal or image from one fixed position. This allows for a more reliable detection of tissue layer boundaries. It has been found that, if the user measures only at one fixed position, there could be a small local anomaly in the fat layer at that position and the device might falsely interpret this as a tissue boundary, thus yielding a false estimate of the fat layer. On the other hand, with such a device, the device is moved along an area on the surface of the body and several images are acquired. The local anomaly could be identified as an outlier and an accurate estimate be obtained. Because the several images are typically acquired at different time points, the images can also be referred to as frames of a video. Accordingly, it is also possible to use video processing methods for a more accurate identification of the tissue boundaries.

In a preferred embodiment the selection means is adapted to select the nearest candidate tissue layer boundary only from among those candidate tissue layer boundaries that have a tissue boundary width exceeding a minimum tissue boundary width. According to this embodiment, it is assumed that the actual tissue layer boundary which is to be determined has at least a certain minimum tissue boundary width. The tissue boundary width of candidate tissue layer boundaries could be determined for example by counting the number of pixels for which the depth signal is higher than the threshold.

By using this condition it is ensured that noise or small anomalies in the images are not falsely detected as tissue layer boundary. The minimum tissue boundary width can be a preset constant or it could be dependent on parameters such as e.g. the patient's age or weight. The minimum tissue boundary width could also be chosen depending on the resolution of the acquired ultrasound images.

In a preferred embodiment, said nearest candidate tissue layer boundaries are depth values and said means for determining an actual tissue layer boundary is based on averaging said nearest candidate tissue layer boundaries obtained for various ultrasound images.

In another preferred embodiment said processing means for determining an actual tissue layer boundary determines the actual tissue layer boundary based on the relative frequency of different nearest candidate tissue layer boundaries obtained for various ultrasound images, particularly by using the nearest candidate tissue layer boundary that occurs most frequently. Because ultrasound images are acquired at different adjacent positions, in general the depth values determined for these positions will be different. Using the average of these different depth values is the simplest way of determining one estimate of the actual tissue layer boundary. This approach is appropriate if the different depth values indeed correspond to the same tissue layer boundary. If, however, for some images false depth values are determined, for example because some of the images were corrupted by noise, it is appropriate to determine the actual tissue layer boundary based on the relative frequency of different depth values. For example, if for 20 ultrasound images a depth value of around 3 cm is determined, but for only three images a depth value of 10 cm is

determined, it is more sensible to reject the 10 cm depth values and determine the actual tissue layer boundary as 3 cm.

In a preferred embodiment the detector detects a set of candidate tissue layer boundaries for an ultrasound image by thresholding a weighted sum of said depth signal and a derivative of said depth signal. The weighting can also be such that the thresholding is performed only on the derivative signal.

For example in the case of high background image intensity the derivative of the depth signal may be more informative than the depth signal itself.

In a preferred embodiment, the ultrasound transducer is adapted for acquiring two or more ultrasound images at subsequent time points, wherein the device further comprises a visual tracking means for tracking tissue layer boundaries over images acquired at subsequent time points, wherein said visual tracking means is adapted to estimate a refined actual tissue layer boundary.

By making use of the temporal coherence (or continuity) between frames, tissue layer boundaries at each frame can be more accurately and reliably detected. For instance, looking at each individual frame, maybe there are too many uncertainties and it is ambiguous to decide where the tissue layer boundaries are. By tracking tissue layers across multiple frames, it becomes less uncertain or ambiguous to determine the tissue layers. In one embodiment, visual tracking algorithms can be used to track the deformation of the tissue layers in ultrasound videos. Multiple observations at frame 1...t-1 can be used to

estimate/track the tissue layer at frame t. For example, with particle filtering, the tissue layer detection can be formulated as

where x_t is the state of the tissue layer at frame t, and z_{1 :t} are the observations at frames 1 till t. This is described in more detail in Michael Isard and Andrew Blake, "CONDENSATION - Conditional Density Propagation for Visual Tracking", International Journal of Computer Vision, 29, 1, 5—28, (1998). A quantitative measurement, for example, the percentage or amount of fat or muscle mass, can be calculated from the ultrasound video.

According to a further aspect a device is presented that estimates a total fat- and/or fat-free mass of a body. A total body fat value can be estimated based on the several actual tissue layer boundaries that were determined at different places of the body as previously described.

In a preferred embodiment the total body fat value is estimated using a formula that involves a weighted sum of predetermined constants, an age of the person, a sum of actual tissue layer boundaries, a square of the sum of actual tissue layer boundaries, and/or a logarithm of the sum of actual tissue layer boundaries. Depending on the number of sites measured the following formulas for estimating body density (BD) can, for instance, be applied:

i) Method of Jackson & Pollock: "Generalized equations for predicting body density of men", British Journal of Nutrition (1978), 40: 497-504 Cambridge University Press:

For men:

7 site => BD = 1.11200000 - 0.00043499* (XI) + 0.00000055*(X1)² - 0.00028826 * (age)

BD = 1.21394 - 0.03101 *(log XI) - 0.00029 * (age)

3 site => BD = 1.1093800 - 0.0008267 * (X2) + 0.0000016 * (X2)² - 0.0002574 * (age)

BD 1.18860 - 0.03049 * (log X2) - 0.00027 * (age)

BD 1.1125025 - 0.0013125 * (X3) + 0.0000055 * (X3)² - 0.0002440 * (age) with: XI = Sum of chest, axilla, triceps, subscapula, abdomen, suprailium, thigh (in mm)

X2 = Sum of chest, abdomen, thigh (in mm)

X3 = Sum of chest, triceps and subscapula (in mm)

Age in years.

For women:

7 site => BD = 1.0970 - 0.00046971 *(X1) + 0.00000056* (XI)² - 0.00012828 * (age)

BD = 1.23173 - 0.03841 *(log XI) - 0.00015 * (age)

4 site => BD = 1.0960950 - 0.0006952 * (X2) + 0.0000011 * (X2)² - 0.00012828 * (age)

BD = 1.21993 - 0.03936 * (log X2) - 0.00011 * (age)

3 site => BBDD == 11..00999944992211 -- 00..00000099992299 ** ((XX33)) ++ 00..00000000002233 ** ( (X3)² - 0.0001392 * (age)

BD = 1.21389 - 0.04057 * (log X3) - 0.00016 * (age)

BD = 1.089733 - 0.0009245 * (X4) + 0.0000025 * (X4)² - 0.0000979 * (age) with:

XI = Sum of chest, axilla, triceps, subscapula, abdomen, suprailium, thigh (in mm)

X2 = Sum of triceps, abdomen, suprailium, thigh (in mm)

X3 = Sum of triceps, thigh, suprailium (in mm)

X4 = Sum of triceps, suprailium, abdomen (in mm)

Age in years.

ii) Method of A.W. Sloan:

BD = 1.1070 - 0.003845 * (thigh) - 0.001493* (iliac crest).

iii) The method of Siri et al. can be used for translating body density into body fat:

% Body Fat = (495 / Body Density) - 450.

The fat-free mass (FFM) can be calculated as FFM = Weight - FM.

In a further embodiment the device comprises a user interface for providing a user with instructions to place the transducer at certain locations on the body. This embodiment makes the device easier to operate and makes sure that the measurements that were determined at different places of the body are used correctly in above-mentioned formulas.

In a further embodiment, the device further comprises a means for detecting movement of the transducer, in particular movement of the transducer that is tangential to the surface of said body, for determining the relative positions of the acquired ultrasound images. Knowing the relative positions of the acquired ultrasound images enables the device to know the size of the area where the ultrasound images were acquired. This information could be used in a refined version of above-mentioned formulas. Alternatively, the device could detect false placement or false movement of the transducer and notify the user.

In a further embodiment, the device further comprises a means for comparing properties of said detected movement with properties of an expected movement. For example the device could notify the user if the transducer is being moved too fast.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 shows how the transducer is positioned on the surface of a body, which has two tissue layers,

Fig. 2 is a schematic block diagram of a device for estimating an actual tissue layer boundary according to an embodiment of the present invention,

Fig. 3 is a flowchart of the method,

Fig. 4 shows a schematic view of two ultrasound images, the corresponding depth signals, the candidate tissue layer boundaries, nearest tissue layer boundaries, and the actual tissue layer boundary,

Fig. 5A to Fig 5G illustrate the processing steps for obtaining a nearest candidate tissue layer boundary from an ultrasound image,

Fig. 6 is a schematic block diagram of a device for estimating a total body fat value according to another embodiment of the present invention,

Fig. 7 shows exemplary ultrasound images and A- line ultrasound signals,

Fig. 8 is a schematic block diagram of an ultrasound imaging according to the present invention,

Fig. 9 shows a first embodiment of a device for ultrasound imaging according to the present invention,

Fig. 10 shows a second embodiment of a device for ultrasound imaging according to the present invention,

Fig. 11 shows a third embodiment of a device for ultrasound imaging according to the present invention, and

Fig. 12 shows a fourth embodiment of a device for ultrasound imaging according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 shows an example of a probe 10 that is placed on the surface 12 of the person's body 14. The body has a first and a second tissue layer 16, 18, which are separated by a tissue layer boundary 20. The first tissue layer 16 is fat, the second tissue layer 18 is some other tissue, for example muscle. The ultrasound probe 10 has a transducer 22, which comprises a number of elements 24 for transmitting ultrasound 26 and receiving reflected ultrasound 28. Ultrasound can mainly get reflected either from tissue layer boundaries 20 or from local tissue inhomogeneities 30. Usually, only a small percentage of the transmitted ultrasound 26 is reflected, so that ultrasound gets reflected also from tissue layer boundaries 20 or tissue inhomogeneities 30 that are located deeper inside the body. The arrow 32 indicates the direction of increasing depth. The elements 24 of the transducer 22 are connected to a reconstruction unit 34, which computes a two-dimensional image.

Fig. 1 shows that the reconstruction unit 34 is located on the probe 10;

however, in general it can be located outside the probe 10. Although not explicitly shown, it is understood that the reconstruction unit 34 may also comprise a noise removal means, for example a noise removal means that is adapted to perform filtering or Otsu thresholding.

The user can move the probe 10 along a direction 38 that is tangential to the surface 12 of the body 14 and orthogonal to the plane of Fig. 1. For example, the user can slowly move the probe 10 along the user's belly in order to get a full measurement of the fat layer of the belly. The probe 10 comprises a tangential movement detection means 40, which can detect such tangential movement. The detection means 40 can be designed similar to the detection means that are used in computer mice, for example using an LED or laser with a corresponding photo detector. To determine the orientation of the ultrasound probe the probe further comprises an orientation sensor 42. While the user moves the ultrasound probe along the surface 12 of the body 14, the probe continuously acquires images 36. The images 36 thus correspond to adjacent positions on the surface 12 of the body 14. The images are typically 2D, but could also be 3D image volumes. The plurality of images 36 is sometimes also referred to as frames of an ultrasound video.

Fig. 2 shows a schematic block diagram of a device 8 according to an embodiment of the present invention; Fig. 3 shows a flowchart of the corresponding method.

In a first step S10, the probe 10 is positioned on the surface 12 of the body 14.

At step S12, images 36 are acquired with the probe 10. At step S14, the converter 44 converts some of these images to depth signals 46 by summing the intensities of the image 36 along a line that corresponds to essentially constant depths in the body. At step SI 6, the detector 48 uses thresholding of the depth signal 46 to detect candidate tissue layer boundaries 50. At step 20, the selection means 52 selects from a set of such candidate tissue layer boundaries 50 a nearest candidate tissue layer boundary 54 that is nearest to the surface 12 of the body 14. At step S20, the processing means 56 determines an actual tissue layer boundary 58 from said nearest candidate tissue layer boundaries, which were selected for various images 36. At step S22, the actual tissue layer boundary 58 is displayed on a display 60. In addition to the display 60, the device 8 may also comprise a user interface, e.g. for changing settings of the tissue layer measurement.

Fig. 4 shows a schematic view of how an actual tissue layer boundary 58 is determined from 2D ultrasound images 36. The images 36 are summed along lines that correspond to equal depths in the body 14. This conversion step 44 yields two depth signals 46. The depth signals 46 are shown in the figure as plots, where the horizontal axis corresponds to increasing depths within the body 14. The vertical axis corresponds to a higher value of the summed intensities. The threshold 62 is indicated with a dashed line. If the value of the depth signal 46 is higher than the threshold 62, a candidate tissue layer boundary 50 is detected at this position. The value of the threshold 62 can either be a fixed preset value or it can be dependent on the overall average intensity in the images 36. For example the threshold 62 could be designed as ten times the average intensity of one line corresponding to constant depth within the body.

For both of the images 36 two candidate tissue layer boundaries 50a, 50b are identified. The first candidate tissue layer boundary 50a is nearer to the surface of the body, however, it has a smaller width than the second candidate tissue layer boundary 50b. Because it is smaller than the required minimum width 64 it is rejected and the nearest candidate tissue layer boundary 54 is only chosen from among the remaining candidate tissue layer boundaries 50, in this case the second candidate tissue layer boundary 50b.

The processing means 56 determines the actual tissue layer boundary 58 by choosing the nearest candidate tissue layer boundary value 54 that occurs most frequently. If several depth values 54 occur with the same frequency, the average of those values is chosen as actual tissue layer boundary value 58.

Fig. 5A shows an acquired ultrasound image 36. The direction of increasing depth 32 is from top to bottom of the image, i.e., the top of the image corresponds to the surface 12 of the body 14. The image has rectangular dimensions, but in principle also other image dimensions would be possible. The image shows a fat layer 16, which is separated by a tissue layer boundary 20 from a second tissue layer 18. Fig. 5B shows the same ultrasound image 36 after a noise removal process which is performed using Otsu thresholding. Also shown in Figs. 5A and 5B is an example of a line 66 that corresponds to constant depth in the body 14.

Fig. 5C shows the depth signal 46 that is obtained by summing the noise- removed image 36 across horizontal lines 66. The direction of increasing depth 32 is now plotted horizontally from left to right.

Fig. 5D shows a derivative of the depth signal of Fig. 5C. The derivative in this case is computed as the absolute value of the mathematical derivative, i.e., it contains only positive values.

Fig. 5E shows the candidate tissue layer boundaries that are detected by thresholding a sum of the depth signal and the derivative depth signal. Subsequently, an outlier removal process takes place to remove candidates that spread only over a few lines (data points on the depth signal), for example by applying median filtering. At the interface between the probe 10 and the surface 12 of the body 14 ultrasound reflection 28 can occur. Although this is not visible in Fig. 5 A, it is clear that in principle this can lead to high intensities in the upper part (corresponding to an area near the surface of the body) of an image 36. It is understood that precautions are taken that these are not falsely identified as nearest candidate tissue layer boundary 54. For example the first two lines of the images 36 could be excluded from the nearest candidate tissue layer boundary detection. This is an engineering trick to avoid false detections due to the ultrasound reflection between the probe 10 and the surface 12 of the body 14. Generally this can be done by examining the first several lines of the images 36 to see if there is ultrasound refection between the probe 10 and the surface 12 of the body 14.

Fig. 5F shows the resulting candidate tissue layer boundaries 68 that have a tissue boundary width exceeding the minimum tissue boundary width 64.

Fig. 5G shows the nearest candidate tissue layer boundary 54 that was selected by the selection means.

The detection of nearest candidate tissue layer boundaries is performed in a similar way for ultrasound images 36 acquired from adjacent positions. This way, for every acquired ultrasound image 36 a nearest candidate tissue layer boundary can be determined. Alternatively, the above-mentioned conversion, detection, and selection can be applied only to a subset of the acquired images, for example only for images that were acquired from positions on the surface with at least a certain minimum distance between them. Fig. 6 shows an example of an embodiment of a device 70 for estimating a fat- and/or fat-free mass of a body. The body fat estimator 72 uses actual tissue layer boundary values 58 that are determined by the device 8 for determining actual tissue layer boundaries. The determined actual tissue layer boundaries 58 can be shown on the user interface 74. The user interface 74 also provides further information about the measurement process and gives the user instructions on how to use the device 70, for example where to place the probe and how to move it. The user interface can comprise a (touch) screen, LEDs, dedicated buttons, and/or a loudspeaker. The user can also provide the device 70 with information through the user interface 74. For example, the user could enter additional data like e.g. the age and gender of the patient amongst others. Further, the user can indicate whether he wants to perform a measurement e.g. at 3, 5 or 7 sites. Based on this selection, the body fat estimator 72 would use the appropriate formula. Finally, the user interface 74 shows the estimated fat- and/or fat-free mass or the estimated body density.

In the above methods and devices have been explained for ultrasound imaging, in particular for determining actual tissue layer boundaries of a body and/or for estimating a person's fat- and/or fat-free mass. In the following further embodiments of devices for ultrasound imaging according to the present invention shall be explained, by which the same or similar methods can be performed.

Figs. 7A and 7B show two examples of tissue layer detection with the A-line ultrasound signal. The curves 81, 91 correspond to the A-line signal in the ultrasound images 80, 90 along the vertical lines 82, 92. The peaks 83, 84 and 93, 94 detected in the ID signals 81, 91 indicate the tissue layers 85, 86 and 95, 96. Thus, it is possible to detect tissue layers with a single element ultrasound transducer, in particular by finding the peaks in the A-line signal. But the ID signal based detection is sensitive to data noises, and is less reliable and consistent compared to 2D ultrasound image based detection.

In the following several possible embodiments of the proposed device will be described that combine an ultrasound transducer with an (optional) movement sensor to measure its position relative to the skin or a housing or, at least, to measure motion of the ultrasound transducer. The transducer is preferably a mechanically focused single element transducer with either a curved surface for focusing or a flat element with a specially designed acoustic lens, or a combination of a curved surface and an acoustic lens. In an alternate embodiment, an annular array with a plurality of rings could be used, yielding a dynamic focusing along the acoustic axis. The (optional) movement sensor particularly provides information to verify if a large enough area or volume is covered. Preferably, transmit pulses are only sent when the movement sensor detects that the transducer is in motion.

A large enough volume shall be covered to increase the accuracy and robustness of the algorithm that detects the position of the layers. There are two aspects to this. First, the layers may not be uniformly thick, and at any given single spot the layers could be thicker or thinner than they are on average over a small local area. Considering a small area instead of a single line should improve the accuracy of measuring the average layer thickness. A2D image acquisition should even be sufficient to eliminate local outliers.

Second, in 3D B-mode volumes the interfaces between layers show up as surfaces of increased brightness where the average intensity of the speckle texture is brighter than the speckle texture above and below it. In a 2D image or 3D volume this layer boundary is easily visually identifiable, and algorithms can detect this by analyzing the average brightness over an area as a function of depth. If only a single line is available the local speckle brightness at the layer could by chance be low while speckle brightness at other depths by chance is high, which could compromise the robustness of layer detection.

A rough approximation for area size is that the area that needs to be scanned needs to be significantly larger (at least two times, preferably at least four times) than the speckle spot size. This is in turn related to the size of the beam at depth of interest. For example, at some defocused spots the beam could be 5mm x 5mm, so that an area of at least 1cm x 1cm, preferably more like 2cm x 2cm should be scanned.

Generally, exact positions of the transducer are not directly needed. But if it is desired to give each similar sized subvolume in the collected data equal weight to the average it is preferred to obtain the position of the transducer when data are acquired, also to make sure that some areas are not scanned multiple times or for a longer time than other areas so that more data is collected there then elsewhere. Preferably, this can be avoided by reasonably accurate tracking or some automated predictable motion pattern. Further, covering too small an area will compromise the robustness.

For a single element transducer, there is no need for beamforming circuitry, yielding a simple, cost-effective device. A block diagram of a complete ultrasound system 100 is shown in Fig. 8. It comprises an ultrasound transducer 102, a single channel pulser / receiver 104, an A/D converter 106, an IQ demodulator 108, a memory 110, a position encoder 112, an image reconstruction unit 114, an ultrasound image processing unit 116 and a display 118. According to an embodiment using this ultrasound system 100 individual lines of ultrasound imaging data are collected by the ultrasound transducer 102 and stored in the memory 110, preferably together with a timestamp of when each line was acquired.

Simultaneously the position of the transducer 102 as obtained by the movement sensor (called position encoder here) 112 as a function of time is recorded and stored in the memory 110. These two types of information in the memory then allow an image/volume to be reconstructed in the image reconstruction unit 114.

The ultrasound signal is a high frequency signal that is amplitude modulated. However, the signal of interest is not the high frequency itself, but rather the amplitude modulation of it (also called the envelope signal). The IQ demodulator 108, after reception in the receiver 104 and A/D conversion in the A/D converter 106, removes the high frequency component and extracts the envelope signal. This envelope signal has a much lower bandwidth, and can thus be sub-sampled to save memory without loosing any information. The collected envelope signal is then stored in the memory 110.

In the following embodiments several implementations allow the transducer to translate across the skin, giving a larger area of imaging data for the shallow depths but resulting in a larger device. Other implementations rely on pivoting the transducer, allowing a much smaller device at the cost of reduced data for shallow depth.

In a first embodiment an optical sensor is used to precisely track the transducer motion across a reflecting textured surface, such as the human skin. Such optical sensors are very low cost as they are mass produced. Hence, one or more existing optical sensor, as e.g. used in a conventional optical computer mouse, may be coupled to an existing single element transducer. To use this device, a little bit of ultrasound scanning gel would be applied to the area of interest, and the device, including first and second holding means as explained below, would be swept over the skin to cover a small area. The device surface that contacts the skin is flat to aid in maintaining an orientation perpendicular to the skin surface.

If maintaining a perpendicular orientation is a concern, a set of three ultrasound transducers could be used. One transducer faces straight down, and the others have a small tilt in lateral and elevational direction, respectively. The straight transducer should always be giving the smallest layer thickness.

Fig. 9 shows a two-dimensional variable resistor tracking ultrasound device 200 according to the present invention. In this embodiment the transducer 202 is only tilting and not translating, resulting in a volumetric sector image. The tissue (skin) location being measured is indicated by 14. The single element focussed ultrasound transducer 202 is held by a first holder 204 including a stick 206, a holding element 208 (including e.g. a ball joint) coupled to the transducer 202 and a handle 210. The first holder 204 can be moved by hand to change the tilt of the transducer 202. The stick 206 is sticking through two orthogonally placed and nested arches 212, 214 that have slots 216, 218 in them, said arches representing a second holder in the form of a kind of guidance rails.

At one end of the arches 212, 214 rotational variable resistors 220, 222 are attached. Moving the first holder 204 along the slot 218 will rotate the variable resistor 220 on the arch 212. Similarly, movement along the slot 216 rotates the other resistor 222. Thus, from the resistance values of the two variable resistors 220, 222 the angle of the transducer 202 can be directly determined, and also movement of the transducer can be recognized.

The device 200 (like all other embodiments of the device according to the present invention) may be mounted inside a small enclosed box (housing; not shown) that has a hole covered with a thin membrane where the transducer is located. Between the transducer and the membrane a small chamber is created that is filled with fluid or gel to provide the acoustic coupling.

To use this device, a little bit of ultrasound scanning gel would be applied to the membrane and the box is pressed onto the area of interest. Then, the first holder is moved around by hand to cover a sufficiently large area to perform the measurements.

Fig. 10 shows a one-dimensional variable resistor tracking ultrasound device 300 according to the present invention. In this embodiment the transducer 302 is only tilting in one dimension, resulting in a 2D sector image. Compared to the embodiment shown in Fig. 9, this design has reduced mechanical complexity and therefore reduced costs.

The plate 304 forms the underside of the device 300 that is pressed against the skin. Inside the plate 304 there is a cylindrical cavity 306 that holds the transducer element 302. The transducer 302 pivots on a rod 308 that is connected to a rotational variable resistor 310. The transducer 302 can be tilted by hand using the first holder 312 represented by a stick. Like in the embodiment shown in Fig. 9 the transducer 302 sits inside a small chamber that is filled with a fluid or gel and has a membrane towards the skin side. Scanning gel is applied to the membrane, the device is pressed onto the area of interest, and the first holder is moved back and forth by hand.

Fig. 11 shows an accelerometer based tracking ultrasound device 400 according to the present invention. In this embodiment the transducer 402 is held inside a cup 404 and accelerometer elements 406, 408 are placed on the transducer 402 and on the housing of the cup 404. These accelerometer elements 406, 408 can be very accurate and cost effective.

The interior of this cup 404 preferably has a soft rubber that holds the transducer 402 in place, but allows it to be pivoted at different angles by hand using the first holder 410 represented by a stick in this example. Alternatively, a ball joint (208 in Fig. 9) similar to that in the embodiment shown in Fig. 9 could be used to hold the transducer 402 in place. On the underside of the cup there is a thin membrane (not shown), and between the membrane and the transducer there exists a small cavity that is filled with ultrasound coupling gel. To use this device, a little bit of ultrasound scanning gel would be applied to the membrane and the box is pressed onto the area of interest. Then the handle is moved around by hand to cover a sufficiently large area to perform the measurements.

Fig. 12 shows still another embodiment of an automated moving ultrasound device 500 according to the present invention. Fig. 12A shows a bottom view, Fig. 12B shows a front view. If the transducer 502 is moved along a predefined path with a known velocity, there is no need for a movement sensor to detect the position and/or movement of the transducer.

The bottom of the device 500 would be pressed against the skin. The focussed transducer 502 has a beam (or extension / protrusion) 504 on the back that is sticking through a plate 506 with a spiral shaped slot 508. The slotted bracket 510 (representing the first holder) is attached to movement means 512, here an AC gearhead motor, and sweeps the transducer 502 along the spiral path.

In the front view shown in Fig. 12B the AC motor 512 can be seen. An AC motor is preferably used because it has a precisely defined rotational speed. Further, the protrusion 504 of the transducer 502 can be seen sticking through the slot 508 with a guide plate 514 attached to it. There is another such guide plate on the bottom side (not shown) of the spiral groove 508. These keep the transducer surface parallel to the groove plate 506.

The device 500 is preferably embedded in a liquid or gel and has a thin membrane at the bottom that is pressed against the skin. To use this device 500, a little bit of ultrasound scanning gel would be applied to the membrane and the box is pressed onto the area of interest. Then a button on the device is pressed to initiate the motor rotation. The transducer moves completely along the spiral path while A-lines are collected, and then the motor direction reverses and the same path is travelled back to the starting position.

To ensure roughly the same starting position in the spiral for each subsequent scan, a limit switch at the start of the spiral, a low power motor or slip coupling can be used and let it briefly run into the start of the spiral. Further, it can be relied upon operating the AC motor for equal amounts of time in forward and reverse direction.

It is to be noted that, of course, other trajectories can be used apart from the spiral trajectory as shown in Fig. 12, e.g. a meander-like trajectory. Further, such a groove plate can also be used without an automatic movement means so that the transducer is moved by hand. Still further, also in the other embodiments automatic movement means may be provided.

The invention can be used by the physician to get an immediate impression of the overall fitness level of the patient. It can also be used to monitor changes in the overall fitness level, for example as part of a medical exercise intervention. As this device will be easy to operate it could also be used in settings that are more in the area of preventative care and fitness coaching as opposed to medical treatment. In schools it could be used to alert students early if they are in danger of becoming obese, possibly coupled with education on the health consequences of obesity.

In health and fitness clubs it could be offered as an additional service to members who want to measure the effectiveness of their workout, and become a real motivation mechanism towards a healthier lifestyle. In professional sports it could be used by athletes and their trainers to optimize performance. The desired body composition depends on the particular sport that the athlete competes in (endurance marathon running, 100 meter sprint, long/short distance swimming, body building, bicycling, skiing, boxing, etc.).

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.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

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

Claims

CLAIMS:

1. A device for ultrasound imaging comprising:

an ultrasound transducer (102, 202, 302, 402, 502) for acquiring ultrasound images,

a first holder (204, 312, 410, 510) for mechanically holding the ultrasound transducer and allowing movement of the ultrasound transducer through movement of the first holder, and

a second holder (212, 214, 308, 404, 506, 512) for mechanically holding the first holder and/or the ultrasound transducer and for guiding and/or restricting movement of the ultrasound transducer through movement of the first holder.

2. The device as claimed in Claim 1, further comprising a movement sensor (112, 220, 222, 310, 406, 408) for sensing the movement and/or position of the ultrasound transducer with respect to a currently imaged object, the second holder or a housing of the device.

3. The device as claimed in Claim 2, wherein said movement sensor comprises an optical sensor, in particular attached to the ultrasound transducer or the first holder, for optically detecting movements of the ultrasound transducer.

4. The device as claimed in Claim 1, wherein said first holder comprises a stick

(206, 312, 410) holding said ultrasound transducer (202, 302, 402) at one end, wherein said second holder () comprises one or more guidance rails (212, 214) for guiding the stick.

5. The device as claimed in Claim 4, wherein said second holder comprises two orthogonally arranged guidance rails (212, 214) and wherein movement of the stick (206) along one or both guidance rail(s) involves movement of at least one guidance rail in an angular direction.

6. The device as claimed in Claim 5, wherein a rotational resistance

measurement unit (220, 222) is arranged at one end of each guidance rail and wherein movement of the stick along one or both guidance rail(s) involves movement of at least one guidance rail in an angular direction involved by a movement of the stick along one or both guidance rail(s) leads to a change of resistance at the rotational resistance measurement unit (220, 222) arranged at one end of said guidance rail.

7. The device as claimed in Claim 4, wherein said second holder comprises a rod (308) arranged substantially orthogonal to the stick (312) and being rotatable about its longitudinal axis.

8. The device as claimed in Claim 7, wherein a rotational resistance

measurement unit (310) is arranged at one end of the rod (308) and wherein a movement of the stick (312) involves a rotation of the rod (308) leading to a change of resistance at the rotational resistance measurement unit (310).

9. The device as claimed in Claim 4, wherein said second holder comprises a cup (404) and a flexible holding element inside the cup holding said stick (410) and allowing a pivoting movement of the stick.

10. The device as claimed in Claim 9, further comprising one or more accelerometers (406, 408) arranged on the cup (404), the ultrasound transducer (402) and/or the first holder (410) for sensing movement of the ultrasound transducer with respect to the cup.

11. The device as claimed in Claim 1 , further comprising a movement unit (512) coupled to the first holder (510) for automatically moving the ultrasound transducer (502) to predetermined positions, in particular along a predetermined trajectory.

12. The device as claimed in Claim 1, wherein said second holder comprises a guidance unit (506) for guiding the ultrasound transducer (502) during movement.

13. The device as claimed in Claim 12, wherein said guidance unit comprises a guide plate (506) having a spiral shaped channel (508) into which a holding element (504) attached to the ultrasound transducer (502) extends causing movement along said spiral shaped channel (508) when the ultrasound transducer (502) is moved.

14. The device as claimed in Claim 1, wherein said ultrasound transducer (102, 202, 302, 402, 502) is a single element focused ultrasound transducer.

15. The device as claimed in Claim 1, wherein said device is adapted for determining actual tissue layer boundaries of a body (14) and wherein said ultrasound transducer (10, 102, 202, 302, 402, 502) is adapted for acquiring two or more ultrasound images (36) at adjacent positions of a surface (12) of the body (14),

said device further comprising:

a converter (44) for converting said ultrasound images (36) separately to depth signals (46), wherein a depth signal (46) is obtained by summing intensities of one of said ultrasound images (36) along a line (66) of substantially constant depth in the body (14), - a detector (48) for detecting a set of candidate tissue layer boundaries (50) for an ultrasound image (36) by thresholding the depth signal (46) obtained for said ultrasound image (36),

a selection means (52) for selecting from a set of candidate tissue layer boundaries (50) a nearest candidate tissue layer boundary (54) that is nearest to the surface (12) ofthe body (14), and

a processing means (56) for determining an actual tissue layer boundary (58) from the nearest candidate tissue layer boundaries (54) obtained for various ultrasound images (36).