Medical imaging system with a part which can be moved about a patient and a collision protection method

[0001] This application claims priority of German application No. 10 2005 023 165.9 filed May 19, 2005, which is incorporated by reference herein in its entirety.

[0002] The invention relates to a medical imaging system with a part which can be moved about a patient and in particular a system, with which the movement of the moveable part is controlled by a control module which stops or slows down the movement when the part enters a protective zone enclosing the patient. The invention likewise relates to a collision protection method for such an imaging system.

[0003] In the prior art, medical imaging systems, in particular x-ray systems are known, which are characterized by a great flexibility in the movements of the image recording devices about the patient. In the case of an x-ray system, these moveable parts are in particular the x-ray detector and the x-ray tube. The so-called C-arm x-ray systems particularly enjoy great popularity, in which the x-ray tube and the detector are fixed in each instance to opposing arms of a C-arm, which can be moved arbitrarily about the patient in order to allow x-ray recordings from any projection direction. By varying the distance between the x-ray detector and the patient, the enlargement can furthermore be changed and the image distorting scattered radiation can be minimized. Systems such as Siemens AG's AXIOM Artis for instance are used in particular as angiography systems. Quasi-tomographical 3D images are increasingly generated with such x-ray devices, with which the C-arm is moved about the patient through approximately 180°. With a cycle of this type, also referred to as DynaRun, some of the moveable parts are rotated about the patient at a considerable speed. One problem with these types of imaging systems with moveable parts is that the moveable parts may collide with the patient. Protective mechanisms must thus be introduced in order to rule out any risks to the patient.

[0004] With the above-cited AXION Artis system, a protective zone enclosing the patient is defined for instance. If a C-arm approaches this protective zone, it is clearly slowed down so as to avoid the risk of a collision. This protective zone is identical for all patients and has roughly the form of an ellipsoid arranged over the patient support. With thin patients in particular, this protective zone is thus often distanced far from the actual patient surface. It thus often takes an unnecessarily long time to start a specific angulation of the C-arm.

[0005] In the case of devices made by Philips, some of the capacitative sensors which detect the proximity of the patient and thereupon slow down the movement of the C-arm are positioned on the C-arms. However, these sensors only have a minimal coverage, so that the movement of the C-arm is also assumed to be slowed down here when it enters an accepted protective zone.

[0006] As a final safety feature, both systems have mechanical position indicators, which immediately stop the movement of the moveable parts when a patient is actually contacted.

[0007] The object addressed by the invention is thus to provide a medical imaging system with a faster, more accurate and simpler collision protection system, as well as a corresponding collision protection method. The invention achieves this object with the characterizing features of the claims. Preferred embodiments of the imaging system and collision protection method according to the invention are specified in each instance in the dependent claims.

[0008] In accordance with the invention, an optical sensor for detecting the surface of the patient is provided with a medical imaging system, with the imaging system being designed so as to calculate an individual protective zone for this patient from the detected surface of the patient. As known from the prior art, the movement of the moveable part can be stopped or slowed down by a control module when the part enters this protective zone. Since this protective zone takes account of the precise shape of the patient, which was determined by the optical surface detection, a considerably enlarged movement margin of the moveable part is thus enabled. The individual protective zone can begin for instance at a predetermined distance to the measured patient surface, e.g. at a distance of 2 to 6 cm. By way of example, an “envelope” is placed around the shape of the patient measured using the optical surface detection for instance, said “envelope” very accurately reproducing the necessary protective distance to a patient.

[0009] The optical sensor preferably comprises a light source and at least one camera. Optical sensors of this type are available from 3D-SHAPE GmbH and are based on the projection of striped patterns. The patterns are projected onto the object to be detected by means of a projector from a first direction and are viewed from another direction with a camera. The stripes appear more or less deformed to the camera, depending on the shape of the viewed object. The shape of the object can also be concluded from the deformation of the stripes. Preferably more than 3 striped patterns are projected, with the intensity of the stripes assuming a sinusoidal pattern. A second camera can be provided to detect 2 sides of the object simultaneously.

[0010] This method for optical surface detection, also known as shape detection, is described in DE 102 58 130 A1 for instance. This publication, the disclosure of which is herewith included in this patent application, also describes a method, which is referred to as “shape from shading”. With this method, the shape of the mapped object is concluded from the variation in the brightness in an image. If the photograph of a face is examined for instance, brightness variations are determined, although it can be assumed that the reflection coefficient of the skin hardly changes. Instead these brightness variations result from certain parts of the surface being oriented in such a way that they radiate more light to the camera than others. If the light from a light source strikes the surface at a right angle, the brightness is at a maximum, while with glancing incidence it is at a minimum. The contour can be determined from these differences.

[0011] Further methods for optical shape detection of objects are described in U.S. Pat. No. 6,590,669 B1 and EP 0 372 241 A1. Any suitable optical sensor system and/or method for optical surface detection can, in principle, be used for the present imaging system and collision protection method. The above-mentioned system from 3D-SHAPE GmbH, with which the object to be measured, in this instance the patient, is illuminated with a striped pattern, is particularly preferable. As only a relatively rough resolution of the measured surface is required for the purpose of the present application, the patient surface can be detected in a period of one or a few seconds up to approximately 1 minute.

[0012] A 3D-model of the patient is preferably created from the patient surface detected in this way. This model can for instance map the surface approximated from triangles.

[0013] This model is then preferably registered with the coordinate system of the moveable part of the imaging system so as to be able to correspondingly control the movement of the moveable part. To enable this registration, the coordinate system of the imaging system and that of the optical sensor are preferably calibrated spatially to one another. This means that the spatial position and orientation of the two devices are known to one another. In more precise terms, this means that the relation of the coordinate source of the coordinate system of the optical sensor and that of the coordinate source of the coordinate system of the imaging system are known to one another. The following exemplary embodiments exist in respect of the spatial arrangement of the optical sensor relative to the imaging system:

[0014] According to a first embodiment, the optical sensor is fixed permanently to the moveable part; in the case of a C-arm x-ray system thus with a C-arm. In this case, the calibration can already be carried out by the factory during the manufacture of the imaging system. If the imaging system is arranged in an examination room in a stationary manner, it is also possible to arrange the optical sensor at a fixed position (stationary) within this room, to mount it for instance on the ceiling. In this case, the spatial relationship between the sensor and the imaging system must only be determined once, e.g. during the assembly of the imaging system by means of calibration. Possible changes, e.g. rotating the optical sensor on its anchorage, can be acknowledged via rotating sensors, whereby an automatic update of the spatial relationship of the two coordinate systems is carried out.

[0015] According to a third embodiment, the optical sensor is arranged at the imaging system in a moveable manner. In this case, the optical sensor is arranged either in a mobile manner, e.g. on a stand which can be moved freely within the room, or similarly an infusion stand or the imaging system is mobile or both components are mobile. With this embodiment, the calibration of the spatial relationship of the two coordinate systems can be carried out by means of position/orientation sensors, which are mounted both on the optical sensor as well as on imaging system, in particular on the moveable part such as the C-arm, so as to determine the position of the two devices in respect of one another. Such position/orientation sensors use static or low frequency magnetic fields for instance.

[0016] A preferred application of the invention are C-arm x-ray systems, in which the moveable part is an x-ray detector and/or an x-ray tube, which is fixed in each instance to opposing arms of a C-arm. The optical surface detection and the calculation of the individual protective zone can be carried out in each instance prior to the recording of image data, in particular prior to an image-controlled diagnostic or surgical intervention and/or periodically during the intervention. The collision protection method is preferably repeated after a patient movement. The surface detection and calculation of the protective zone can also be carried out during a DynRun, so as to interrupt the scan during acute patient movements in order to reduce the dose. This presumes that the optical sensor operates rapidly enough and that the optical surface detection is completed within approximately 1 to 10 seconds, particularly within 5 seconds.

[0020] FIG. 1 shows a C-arm x-ray device 1 with a C-arm 2, at whose opposing arms an x-ray tube 4 and an x-ray detector 6 are fixed in each instance. The C-arm can be moved about a patient support 16 and is suspended on the stand 8 in a moveable fashion for this purpose. The electric motors effecting the movement of the C-arm are not shown. This movement is controlled by the control module 12, which can form part of a control and image processing computer 10. The x-ray images recorded using the imaging system 1 can be displayed on the screen 14. The patient 18 is shown on the support 16 as a hatched surface. The dashed line 19 circumscribes an ellipsoid, which represents the envelope for the protective zone around the patient in the case of imaging systems according to the prior art. As can be seen from the drawing, the envelope 19 is considerably wider and higher than the patient shown, as it is not tailored to this specific patient, but is instead intended to offer adequate protection for all conceivable and also all overweight patients.

[0021] A projector 24 and a camera 26 are further fixed to the C-arm 2, which together form the optical sensor. As described above, the projector 24 can illuminate the patient with a striped pattern, which is viewed by the camera 26 from a lateral direction. The surface shape of the patient 18 can be calculated for instance from the displacement of the stripes resulting from the contour of the patient, and likewise from the control computer 10 for instance.

[0022] This surface shape which was determined by the optical sensor with a resolution of approximately 1 mm to 4 cm, preferably with a resolution of approximately 1 to 2 cm, is then used to place an individual envelope 21 around the patient 18, shown in the drawing with a dashed line. This envelope has an average distance of 5 cm to the patient for instance, but must not follow the patient surface exactly as is also obvious from FIG. 2. The individual protective zone 22 for this patient is enclosed by this envelope 21. The envelope 21 is calibrated with the coordinate system of the C-arm, so that the control computer 10 can control the movements of the C-arm such that neither the x-ray tube 4 nor the detector 6 enter the protective zone 22, or such that the movement is significantly slowed down when the individual protective zone is crossed. If one of the moveable parts however crosses an area lying outside the protective zone 22, said area lying within the ellipsoid 19, the movement does not need to be slowed down.

[0023] The surface detection can be carried out once in each instance prior to an examination or an image-controlled intervention to the patient and/or repeated if necessary during the examination and/or the intervention.

[0024] FIG. 2 shows a schematic representation of a model 17 of a patient, which was created using the optical surface detection. Individual pixels are indicated with 28, from which the model is composed. Alternatively, the 3D model can also be spanned by a triangular surface. According to another alternative, the patient surface can also be detected in each instance as a silhouette, with the envelope 21 being placed in each instance about the silhouette. Patient silhouettes are preferably recorded here from several lines of vision. The envelope 21 reproduces the protective distance to the patient very accurately. It is also not bound to particular shape models and in contrast to ellipsoid models, thus takes account of a protruding arm or suchlike.

[0025] The imaging system and collision protection method according to the invention thus allow both an improved protection of the patient and also an improvement in the workflow. The latter results from fewer reductions in speed of the C-arm movement being necessary when the assumed protective zone is approximated, if the precise size and shape of the patient are known.