WO2012007122A1

WO2012007122A1 - Medical x-ray system and process for determining a reference direction in such a system

Info

Publication number: WO2012007122A1
Application number: PCT/EP2011/003354
Authority: WO
Inventors: Franz Copf
Original assignee: Spontech Spine Intelligence Group AG
Current assignee: Spontech Spine Intelligence Group AG
Priority date: 2010-07-12
Filing date: 2011-07-06
Publication date: 2012-01-19
Anticipated expiration: 2013-01-12

Abstract

A medical X-ray system for examining body tissue comprises an X-ray source (18) configured to generate X-radiation (22), a detector (28) configured to detect X-radiation (22) that has passed through the body tissue, and a measuring device (32) configured to determine a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived. The measuring device comprises display means (40, 42; 56) that indicate the measured reference direction and that are arranged in the beam path between the X-ray source (18) and the detector (28) in such a manner that, upon transillumination of the body tissue with X-radiation (22), the display means (42; 56) are, together with the body tissue, projected onto the detector (28) and that the reference direction can be read off visually on an X-ray image formed on the detector (28).

Description

MEDICAL X-RAY SYSTEM AND METHOD FOR

DETERMINING A REFERENCE DIRECTION IN SUCH A SYSTEM

BACKGROUND7 OF THE INVENTION

1. Field of the invention

The invention relates to a medical X-ray system for examining body tissue. The invention further relates to a method for determining a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived, in a medical X-ray system.

2. Description of the prior art

In patients with back trouble so-called functional images of the vertebral column are frequently taken. Such a functional image usually consists of a succession of at least three radiographs of the vertebral column, in which the vertebral column is in a neutral state, in a state of maximal inclination and in a state of maximal reclination. Sometimes radio- graphs in which the vertebral column is in a state of maximal lateral flexion are used in addition. From such functional images the treating physician can draw conclusions as to the causes of the trouble and can propose suitable therapeutic measures . It is known (cf. for instance DE 20 2008 016 422 Ul) that the position of the organs in the human body depends on the orientation of said body. Since, on the one hand, the vertebrae in human beings bear a considerable part of the weight of the body and, on the other hand, the intervertebral discs are comparatively soft, the position of the vertebrae also changes significantly if the person changes the orientation of his/her body axis. For a highly accurate computer-assisted functional analysis of the vertebral column the influence of gravity on the position of the vertebrae should therefore not be ignored. Functional images of the vertebral column are frequently taken, however, without the orientation of the vertebral column at the time of the functional images being exactly ascertainable in retrospect. A substantial reason for this consists in the fact that the X-ray detectors of the X-ray appa- ratuses that are used for the functional images are not aligned exactly in relation to the gravitational direction or another reference direction. This is caused, for example, by uneven installation surfaces and also by deformations of supports and of other parts of the X-ray apparatus under its own weight.

In some X-ray apparatuses the diaphragms that are used for establishing the image section are rotatable about an axis of rotation that runs parallel to the normal to the image. In this way, not only the size but also the angular orientation of the image section can be established in such a way that exclusively the desired vertebrae are transilluminated by X- radiation. Image-processing software then automatically rotates the X-ray image in such a way that it can be observed with vertical and horizontal margins on a display screen or similar. On such an X-ray image it can no longer be discerned how the rectangular diaphragm had been orientated in three- dimensional space.

In connection with a digital intra-oral image sensor

DE 10 2006 020 931 Al proposes to provide said image sensor with an orientation sensor which registers the orientation of the intra-oral sensor at the time of irradiation. The measured data generated by the orientation sensor are passed on to image-processing software which rotates the image taken intra-orally in such a way that it can be represented in the correct orientation on a display unit. The orientation of the intra-oral cavity during the X-ray exposure, however, cannot be read off from the correctly represented image. From EP 1 175 592 Bl an X-ray system is known wherein the position of parts of the X-ray apparatus is likewise registered metrologically. In one embodiment several markings are appended to a C-arm apparatus which are registered optically by an external sensor. The sensor is, in turn, provided with an inclinometer which determines the position of the sensor in a three-dimensional coordinate system that is fixed with respect to gravitational force. The measured orientation of the C-arm apparatus in three-dimensional space is used for the purpose of correcting aberrations in the X-ray images that are to be ascribed to such changes of position and to deformations of structural parts of the C-arm apparatus which are a consequence of differing orientations in three-dimensional space .

However, these measures, known in the prior art, for deter- mining the orientation of parts of the X-ray system

metrologically and for using the results of measurement for the purpose of reorienting or rectifying the X-ray images by image-processing measures require a relatively high effort in terms of instrumentation and also in terms of software. In addition, the reoriented or rectified X-ray images do not give the treating physician any immediately discernible guides as to how the transilluminated tissue was oriented during the X-ray exposure.

SUMMARY OF THE INVENTION An object of the present invention is to provide a medical X- ray system for examining body tissue that rapidly enables a treating physician to place the tissue imaged on the X-ray images in relation to a reference direction, in particular the gravitational direction.

This object is achieved by a medical X-ray system which comprises an X-ray source configured to generate X-radiation, a detector configured to detect X-radiation that has passed through the body tissue, and a measuring device configured to determine a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived. The measuring device comprises dis- play means that indicate the measured reference direction and that are arranged in the beam path between the X-ray source and the detector in such a manner that, upon transillumination of the body tissue with X-radiation, the display means are, together with the body tissue, projected onto the detec- tor and that the reference direction can be read off visually on an X-ray image formed on the detector.

The invention is based on the observation that it is much simpler, and usually also more accurate, to project the display of the reference direction directly onto the X-ray image than to register the orientation of several components of the X-ray system in an elaborate manner metrologically and to derive the orientation of the body tissue therefrom. Since the treating physician can discern the orientation of the body tissue immediately on the X-ray image, no additional error- burdened steps are reguired such as are otherwise necessary in the course of a subseguent reconstruction of the exposure conditions .

The term "orientation" here denotes the angular alignment in three-dimensional space. Generally, however, only one of the three angles that are needed for full specification of the orientation in three-dimensional space is registered by the measuring device. On account of the high precision with which the reference direction is represented on the X-ray image, the X-ray system according to the invention can also be employed within the scope of computer-assisted functional analyses. For this pur- pose, the image of the display means on the X-ray image is registered and evaluated by image-processing software in order to determine the position of the reference direction and hence the orientation of the body tissue and to convert it into numerical values. If the body tissue is, for example, formed by parts of the skeleton, the numerical values acquired in this way can be used directly for a computer- assisted functional analysis of these parts.

A further advantage of the invention is that, if the measuring device is suitably designed, an X-ray system according to the invention can also be constructed by refitting retrospectively of an already existing X-ray apparatus. For this purpose no intervention has to be undertaken into the existing X-ray apparatus or into the software that is used for its operation .

As detector of the X-ray system, digital X-ray detectors or digitised storage films are suitable, but also classical X- ray films.

The reference direction may be, in particular, the gravitational direction. But many measuring devices for measuring the gravitational direction measure, strictly speaking, not the gravitational direction itself (i.e. the vertical) but rather a direction perpendicular thereto (i.e. the horizontal) . The gravitational direction can then, of course, be derived easily by forming the right angle from the horizontal. The derivation of the gravitational direction may, however, also be more elaborate, for instance if the measuring device does not utilise gravitational force itself for the measurement but, for example, measures the direction of the earth's magnetic field lines and derives the gravitational direction therefrom.

For ensuring that the reference direction on the X-ray image is visually readable, the display means may include a compo- nent having a transmissivity for X-radiation which is so small that the component is discernible on the X-ray image. In that case the component should be relocated spatially in an external fixed reference system if the orientation of the measuring device changes. This relocation is then also dis- cernible on the X-ray image and can be used for the purpose of making changes in the reference direction visually discernible .

In the simplest case the component is an elongated element, in particular a cord, a chain or a bar, hanging down from a transverse structure, and having a free end at which a plumb bob is attached. Such an elongated element generates a dash on the X-ray image, which indicates the gravitational direc^¬ tion immediately.

Such an elongated element is also an example for embodiments in which parts of the measuring direction used for the deter^¬ mination of the reference direction simultaneously constitute display means. Another example for this is a bubble tube which is filled with mercury. By reason of its high atomic number, mercury has a small transmissivity for X-radiation, as a result of which the gas bubble remaining in the bubble level stands out as a patch on the X-ray image. If a marker made of tungsten wire or of another roentgenopaque material is assigned to the bubble tube, an image of a type of spirit level arises on the X-ray image. A better readability is obtained if the measuring device includes a system of communicating tubes, the component being a liquid that is located in the system of communicating tubes. An (imaginary) connecting line between the levels in the two tubes then indicates the horizontal on the X-ray image. Here too the liquid may be mercury.

However, more elaborate apparatuses may be envisaged as measuring device, for example mechanical apparatuses that operate in accordance with the gyroscopic principle or inertial principle. Such apparatuses are frequently provided with an ana^¬ logue or digital output, at which the measurement results are made available in electronic form. Display means of the measuring device which are independent of the actual measuring apparatus then serve to indicate the results of measurement. The display means may for this purpose include, for example, mechanically adjustable diaphragm elements which are roentgenopaque and which indicate the reference direction on the X-ray image. In one embodiment an adjustable diaphragm is associated with the X-ray source, which diaphragm is configured to variably determine an image section of the X-ray image. The diaphragm is rotatable about an axis of rotation which runs at least substantially perpendicular to a plane in which a diaphragm aperture of the diaphragm extends. Diaphragms of such a type are, as already mentioned above, sometimes used in order to adapt the image section as optimally as possible to the tis^¬ sue to be transilluminated .

The measuring device may then have been integrated into the diaphragm in such a manner that it is, jointly with the diaphragm, rotatable about the axis of rotation, wherein the display means are located in the region of the diaphragm aperture .

The measuring device may, however, also be a sealed unit that is capable of being arranged in the beam path between the X- ray source and the detector but outside a housing of the X- ray source and of the detector. A measuring device designed in such a manner has the advantage that an already existing X-ray apparatus can also be retrofitted therewith subsequently, in order thereby to obtain an X-ray system according to the invention. In the simplest case the measuring device comprises a stand from which a plumb bob hangs down which is connected to the stand via an roentgenopaque elongated ele^¬ ment, as already elucidated above. The stand is then positioned in such a way that the elongated element is located in the beam path of the X-radiation and later appears as a vertical dash on the X-ray image. Another possibility for equipping an existing X-ray system retrospectively with a measuring device according to the invention consists in appending the measuring device to a carrier element that is capable of being detachably attached to a patient. The carrier element may be formed, for example, by a sticking plaster which is attached to the skin of a patient. If the X-ray exposure is carried out with a clothed patient, the carrier element may be provided with fastening means such as a Velcro fastener or similar, in order to attach the carrier element to an article of clothing of the pa- tient. Irrespective of the type of the fastening to the patient, such a carrier element can be attached to the patient in such a way that at least the display means of the measuring device are subjected to the X-radiation and are, jointly with the tissue to be examined, projected onto the X-ray im- age.

Subject of the invention is also an adhesive element for attaching to a patient, said adhesive element comprising a carrier element configured to be detachably attached to the skin of the patient or to an article of clothing worn by the pa- tient. The adhesive element further comprises a measuring device which is connected to the carrier element and which is configured to determine a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived. The measuring device includes display means that are configured to indicate the measured reference direction and that are designed in such a manner that, when the display means are arranged in a beam path between an X-ray source and a detector, upon transillu- mination of the body tissue the display means are, together with the body tissue, projected onto the detector and that the reference direction can be read off visually on the X-ray image formed on the detector.

Here too, the display means may include a component which has a transmissivity for X-radiation that is so small that the component is discernible on the X-ray image, and which is relocated spatially in an external fixed reference system if the orientation of the measuring device changes.

Furthermore, the measuring device may include a system of communicating tubes, the component being a liquid that is located in the system of communicating tubes.

The invention further provides a method for determining a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived, in a medical X-ray system for examining body tissue, the method comprising the following steps: a) providing an X-ray source configured to generate X- radiation; b) providing a detector configured to detect X-radiation that has passed through the body tissue; c) providing a measuring device configured to determine the reference direction, wherein the measuring device comprises display means that are configured to indicate the measured reference direction; d) arranging the display means in a beam path between the X-ray source and the detector; e) transilluminating the body tissue and (preferentially simultaneously) the display means with X-radiation; f) generating an X-ray image of the body tissue and of the display means, wherein the reference direction can be read off visually on the X-ray image.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be more readily understood with reference to the following detailed description of embodiments taken in conjunction with the accompanying drawings in which:

Figure 1 a schematic side view of an X-ray system according to the invention according to a first embodiment;

Figure 2 elements of an adjustable diaphragm which is part of the X-ray system shown in Figure 1;

Figure 3 a doubly angled tube which is filled with an roent^¬ genopaque liquid and integrated into the diaphragm;

Figure 4 the tube shown in Figure 3, but in a different orientation;

Figure 5 a top view of a diaphragm into which a measuring device according to the invention has been integrated;

Figure 6a schematically, an X-ray image that was taken with the aid of the X-ray system shown in Figure 1;

Figure 6b an X-ray image as in Figure 6a but that was taken with the diaphragm rotated;

Figure 7 a schematic side view of an X-ray system according to the invention according to a second embodiment, wherein the measuring device comprises an ancillary frame attached from outside to a housing of the X- ray apparatus;

Figure 8 an enlarged front view of the ancillary frame of the X-ray system shown in Figure 7;

Figure 9 schematically an X-ray image that was taken with the aid of the X-ray apparatus shown in Figure 7 ;

Figure 10 a top view of a sticking plaster to which a measuring device according to the invention has been appended;

Figure 11 schematically, an X-ray image on which parts of the measuring device shown in Figure 10 have been imaged.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

Figure 1 shows, in a schematic side view, an X-ray system according to the invention, which is denoted in its entirety by 10. The X-ray system 10 includes a stand 12, to which an arm 14 extending in the horizontal direction is attached in vertically displaceable manner. At its end the arm 14 bears a housing 16 in which an X-ray tube 17 is arranged. In the X- ray tube 17 a target, which is bombarded by an electron beam, forms an X-ray source 18 for the generation of X-radiation.

Furthermore, in the housing 16 a diaphragm 20 is arranged with which the dimensions of the image section can be established. The diaphragm 20 includes for this purpose several adjustable diaphragm elements which delimit the beam path of the X-radiation 22 which is indicated by dashed lines. The diaphragm 20 is capable of rotating, in a manner not represented in any detail, about a horizontal axis of rotation 24. The rectangular image section defined by the diaphragm 20 can in this way be rotated and consequently better adapted to the orientation of the tissue structures of a patient 30 that are of interest. The diaphragm 20 may also be integrated into the X-ray tube 17 and capable of rotating about the axis of rota^¬ tion 24 only together with the X-ray tube 17.

At its opposite end the arm 14 bears a holder 26 for an X-ray detector 28. The latter registers the X-radiation 22 that passes through the patient 30, and generates an X-ray image therefrom. In the case of the X-ray detector 28 it may be a question of a digital X-ray detector which immediately generates a digital X-ray image which, for example, can be ob- served on a display screen. Alternatively, the design of the X-ray detector 28 as a digital storage film or as a classical X-ray film enters into consideration.

The X-ray system 10 known to this extent differs from the state of the art in that in the housing 16 there is arranged on the diaphragm 20 a measuring device indicated at 32, the structure of which will be elucidated in more detail in the following with reference to Figures 2 to 5.

Figure 2 shows, in a top view, several diaphragm elements from which the diaphragm 20 is assembled. In the representa- tion shown in Figure 2 the diaphragm elements are arranged side by side; in the installed state they partly overlap and can, with the aid of a mechanism which is not represented, be adjusted relative to one another. In detail, the diaphragm 20 includes an upper transverse element 33a and a lower trans- verse element 33b as well as a left-hand lateral element 34a and a right-hand lateral element 34b. Additionally a left- hand and a right-hand masking element 36a and 36b, respectively, are provided, the position of which is not variable. The masking elements 36a, 36b are each provided with an ob- long window 38a and 38b, respectively, the positions of which correspond with cut-outs 39a, 39b in the lower transverse element 33b. The measuring device 32, which is shown in Figure 3 in a top view, consists of an approximately U-shaped pipe 40 which is made of glass or of another material that is transmitting for X-radiation. The pipe 40 is partly filled with mercury, as a result of which two menisci 44a, 44b are formed. The remaining space in the pipe 40 is filled with air or with another gas .

Since the two shanks of the pipe 40 constitute two communicating tubes, the two menisci 44a, 44b are always situated at the same height and consequently define a horizontal in the gravitational field of the Earth. If the tube 44 is tilted by an angle, as shown in Figure 4, the mercury 42 in the pipe 40 is relocated in such a way that the two menisci 44a, 44b continue to define a horizontal. The two ends of the pipe 40 may also have been connected to one another by an (optionally thinner) pipe, in order that air pockets do not arise which could impair the accuracy of measurement. The mercury 42 may also have been enclosed in a torus made of a material that is transmitting for X- radiation.

Figure 5 shows the diaphragm 20 with integrated measuring device 32 in the installed state. The diaphragm elements represented individually in Figure 2 are now arranged in such a way that a central diaphragm aperture 45, sealed all around, arises. The transverse elements 33a, 33b can be displaced in a first direction indicated by a double-headed arrow PI, and the lateral elements 34a, 34b in a second direction indicated by a double-headed arrow P2, so that the size and shape of the diaphragm aperture 45, and hence of the image section, can be changed.

The measuring device 32 is located on the rear of the diaphragm 20, facing away from the observer, and is for the most part concealed by the two masking elements 36a, 36b and the lower transverse element 33b. Only the two shanks of the pipe 40, with the mercury 42 contained therein, are uncovered by the windows 38a, 38b which have been provided in the masking elements 36a, 36b. Figure 6a. shows schematically a digital X-ray image, which was taken with the X-ray system described above, of three adjacent vertebrae Wl, 2, W3 of the patient 30. The image section indicated by a dashed line 46 has the same aspect ratio as the diaphragm aperture 45 which was established by the diaphragm 20.

On both sides of the image section 46 bright streaks 48a, 48b are discernible on the X-ray image. In this connection it is assumed that places at which no X-ray light impinges on the detector 28 are dark, whereas places exposed by X-ray light appear bright on the X-ray image. The bright streaks 48a, 48b arise where X-ray light 22 has passed through the windows 38a, 38b of the masking elements 36a and 36b, respectively, and in the process was not absorbed by the mercury 42 in the pipe 40 of the measuring device 32. The two lower concavely curved edges of the streaks 48a, 48b lie on a line that runs exactly horizontally. The region surrounding the image section 45 remains - apart from the streaks 48a, 48b - likewise unexposed and consequently dark, this being indicated in Figure 6a merely by cross-hatching. Figure 6b shows the X-ray image for the case where, in the course of taking the X-ray image, the diaphragm 20 was rotated by an angle about the axis of rotation 24. Image- processing software of the X-ray apparatus 10 has taken the rotating of the diaphragm 20 into account and rotated the im- age arising on the detector 28 in such a way that the edges of the image run horizontally and vertically. The vertebrae Wl, W2, W3 therefore appear in a different spatial orientation in the image section 46. But, by virtue of the reloca- tion of the mercury 42 in the pipe 40 as a consequence of the rotation about the axis of rotation 24, the streaks 48a, 48b on the X-ray image also have a different length. Here too, the position of the horizontal again results by a connection of the concavely curved lower ends of the streaks 48a, 48b. The treating physician can consequently establish directly on the X-ray image how the three vertebrae Wl, W2 , W3 were aligned with respect to the gravitational direction during the X-ray exposure. Figure 7 shows, in a representation based on Figure 1, a different exemplary embodiment wherein the measuring device 32 has not been integrated into the diaphragm 20 but is attached as a sealed unit to the housing 16 in the region of the beam exit window. Identical parts in this connection are denoted by identical reference numerals and will not be elucidated again in any detail in the following.

The measuring device 32, which is shown in Figure 8 in a front view, includes a frame 50, on the upper cross-beam 52 of which a slide 54 is capable of being moved along a direc- tion indicated by a double-headed arrow P3. Attached to the slide 54 via a thick lead wire 56 is a plumb bob 58.

In the course of the operation of the X-ray apparatus 10 the slide 54 is moved in such a way along the upper cross-beam 52 that the lead wire 56 hanging down vertically extends through the chosen image section and is consequently subjected to the X-radiation 22.

Figure 9 shows schematically an X-ray image that was generated with the aid of the X-ray apparatus 10 shown in Figures 7 and 8. The lead wire 56, which is opaque to X- radiation, appears on the X-ray image as a narrow dark streak 60. The orientation of the streak 60 indicates to the surgeon, when observing the X-ray image, how the vertebrae Wl to W3 are oriented relative to the gravitational direction. Figure 10 shows, in a top view, an adhesive element denoted overall by 62, which is likewise a subject of the invention. The adhesive element 62 exhibits a thin fabric-like carrier element 64 which has an approximately rectangular shape and is designed in the manner of a sticking plaster. On its side facing away from the observer the carrier element 64 is pro^¬ vided with a layer of adhesive 66 with which the adhesive element 62 can be temporarily attached to the skin of a patient. On the front side of the carrier element 64 pointing towards the observer a measuring device 32 is appended which is designed in exactly the same way as the measuring device represented in Figures 3 and 4 but optionally has somewhat smaller dimensions. The entire adhesive element 62 may, for example, have dimensions of 3 cm x 5 cm. Prior to implementation of the X-ray exposure the adhesive element 62 with the measuring device 32 is attached to a place on the skin of the patient where the X-radiation will pass through the patient. More modern X-ray apparatuses frequently indicate this region by lines of light. The adhesive element 62 should in this case be placed in such a way that the measuring device 32 does not cover the tissue structures that are actually of interest but is somewhat laterally offset relative thereto.

Figure 11 show schematically an X-ray image such as is ob- tained after appending the adhesive element 62 to the patient 30. The mercury 42, which is opaque to the X-ray light, is discernible on the X-ray image as a bright doubly angled streak 68. For the treating physician the ends of the streak 68 indicate immediately the position of the horizontal, so that he/she can discern the orientation of the vertebrae Wl to 3 relative to the gravitational direction without additional aids.

Claims

Medical X-ray system for examining body tissue, comprising : a) an X-ray source (18) configured to generate X- radiation (22 ) , b) a detector (28) configured to detect X-radiation

(22) that has passed through the body tissue, and c) a measuring device (32) configured to determine a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived, wherein the measuring device comprises display means (40, 42; 56) that indicate the measured reference direction, wherein the display means (40, 42; 56) are arranged in the beam path between the X-ray source (18) and the detector (28) in such a manner that upon transillumination of the body tissue with X- radiation (22), the display means (42; 56) are, together with the body tissue, projected onto the detector (28) and that the reference direction can be read off visually on an X-ray image formed on the detector (28).

X-ray system according to Claim 1, wherein the display means (40, 42; 56) include a component (42; 56) which has a transmissivity for X-radiation (22) which is so small that the component is discernible on the X-ray image, and is relocated spatially in an external fixed reference system if the orientation of the measuring device (32) changes.

X-ray system according to Claim 2, wherein the component is a freely suspended elongated element (56) having a free end at which a plumb bob (58) is attached.

X-ray system of claim 3, characterized in that the element is a cord, a chain or a bar.

X-ray system according to Claim 2 or 3, wherein the measuring device (32) includes a system of communicating tubes (40) , and wherein the component is a liquid (42) which is located in the system of communicating tubes (40) .

X-ray system according to one of the preceding claims, comprising an adjustable diaphragm (20) being associated with the X-ray source (18) , wherein the adjustable diaphragm (20) is configured to variably determine an image section (46) of the X-ray image and is rotatable about an axis of rotation (24) which runs at least substantially perpendicular to a plane in which a diaphragm aperture (45) of the diaphragm (20) extends.

X-ray system according to Claim 6, wherein the measuring device (32) is integrated into the diaphragm (20) in such a manner that it is, jointly with the diaphragm (20), rotatable about the axis of rotation (24), and wherein the display means (40, 42) are located in the region of the diaphragm aperture (45). X-ray system according to one of Claims 1 to 5, wherein the measuring device (32) is a sealed unit which is arranged in the beam path between the X-ray source (18) and the detector (28) but outside a housing (16) of the X-ray source (18) and of the detector (28).

X-ray system according to Claim 8, wherein the measuring device (32) is appended to a carrier element (64) that is configured to be detachably attached to a patient (30) .

Adhesive element (62) for attaching to a patient (30), comprising : a carrier element (64) that configured to be detachably attached to the skin of the patient (30) or to an article of clothing worn by the patient, and a measuring device (32) which is connected to the carrier element (64) and which is configured to determine a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived, wherein the measuring device comprises display means (40, 42) that are configured to indicate the measured reference direction and are designed in such a manner that, when the display means (40, 42) are arranged in a beam path between an X-ray source (18) and a detector (28) and upon transillumination of the body tissue, the display means (40, 42) are, together with the body tissue, projected onto the detector and the reference direction can be read off visually on an X- ray image formed on the detector (28) . Method for determining a reference direction, in particular the gravitational direction or a direction from which the gravitational direction can be derived, in a medical X-ray system (10) for examining body tissue, comprising the following steps: a) providing an X-ray source (18) configured to generate X-radiation (22); b) providing a detector (28) configured to detect X- radiation (22) that has passed through the body tissue; c) providing a measuring device (32) configured to determine the reference direction, wherein the measuring device comprises display means (40, 42; 56) that are configured to indicate the measured reference direction; d) arranging the display means (40, 42; 56) in a beam path between the X-ray source (18) and the detector (28) ; e) transilluminating the body tissue and the display means (40, 42; 56) with X-radiation (22) ; f) generating an X-ray image of the body tissue and of the display means (40, 42; 56), wherein the reference direction can be read off visually on the X- ray image.

Method according to Claim 11, wherein the X-ray system is designed in accordance with one of Claims 1 to 9.

Method according to Claim 11 or 12, wherein the reference direction is read off on the X-ray image by image- processing software.