PMOR-XMiIC X-RAY APPARATUS AND POSITIONING OF A LAYER TO BE IMAGED FOR PANORAMIC IMAGING

FIELD OF THE INVENTION

The invention concerns a panoramic x-ray apparatus according to patent claim 1 and a method according to patent claim 7 for positioning a layer to be imaged for panoramic x-ray imaging.

PRIOR ART TECHNOLOGY

Panoramic x-ray imaging is layer imaging, tomographic imaging, in which during an imaging scan a radiation source and an image information receiver are turned during an exposure with respect to dental arch in such a way that in the image being formed, essentially the dental arch becomes imaged sharp while other anatomies of the skull get blurred. Typically, the imaging scan and the related turning movement of the imaging means creating the tomographic effect are performed on a horizontal plane, in some special imaging modes one may produce tomographic effect by also moving partly on some other than horizontal plane. A dental panoramic x-ray image shows the dental arch as spread out on a plane. The panoramic image gives a good overview of the whole set of teeth of a patient, including jaw joints.

The most essential diagnostic value of a panoramic x-ray image relates to imaging the roots of the teeth. Due to the shape of the anatomy to be imaged, the imaging geometry of the panoramic x-ray imaging typically produces a tomographic image in which, regarding the frontal area of the dental arch, the layer becoming imaged sharp is relatively thin, typically only ca. 10 mm. As, due to the way the imaging scan is realized, the layer that becomes imaged sharp is wider in the molar area, several tens of mm even, positioning the dental arch at the centre of the layer becoming imaged is not too critical in that area.

As the size and the shape of anatomies to be imaged vary from one object to the another, optimal realization of a panoramic imaging scan with respect to the anatomy to be imaged is most challenging, particularly regarding imaging the incisors. Thus, typically one tries

to position the anatomy for panoramic imaging particularly in such a way that at least the roots of incisors will be located in the layer of the frontal area of the dental arch which becomes imaged sharp. Unsuccessful positioning of the incisor area is supposedly the greatest individual reason for failed panoramic images.

When using the most traditional panoramic x-ray apparatus, the operator must estimate the location of the incisor roots externally on the grounds of the side profile of patient's face, and of the crown surfaces of the teeth that are visible. This estimation is particularly difficult when extremely buck or withdrawn incisors are concerned, but as the incisors are somewhat tilted even in the case of normal occlusion, estimating location of their roots is always difficult. In practice, less failed positionings are typically made by operators having sound professional skills and experience.

It is known to arrange various kinds of positioning lights to panoramic imaging apparatus, by which one may project light beams on patient's face to assist positioning. By means of the lights it is possible e.g. to ensure optimal orientation of the skull .

Positioning lights have also been used to assist positioning of the layer to be imaged by e.g. driving a vertical, layer-positioning light beam illuminating the patient from the side to a position in which the incisor roots presumingly are located and by arranging the imaging apparatus to realize the imaging movement according to the location of this positioning light beam. However, also this kind of positioning is only based on "an educated guess" of the location of the incisor roots, which the operator makes based on external features. Thus, even an experienced operator can also fail in the positioning.

Prior art includes even other positioning techniques based on analysis of external features of anatomy, such as solutions based on use of distance sensors and cameras. However, even their function is mostly based on examination of external facial features of a patient and as a consequence, they don't necessarily give a correct picture of the actual structure of the set of teeth. Further, these kinds of solutions may require, inter alia, integration of various accessories to the

imaging apparatus and other possible special arrangements, which make the imaging apparatus more complicated and may even make difficult or slow down the implementation of the imaging process.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to develop positioning of an anatomy in connection with dental panoramic x-ray imaging, particularly in view of how a desired vertical tomographic layer containing the incisor roots may be positioned into the layer the panoramic x-ray apparatus images as sharp. In other words, an object of the invention is to reduce the need for retake imagings caused by faulty positionings .

An object is also embodiments, which enable realization of the abovementioned object in such a way that the imaging apparatus can still be realized as relatively simple without a need to arrange thereto a considerable amount of new devices, components and/or functionalities .

Further, an object of the invention is embodiments, which enable the desired layer to become automatically imaged sharp in a panoramic image after the anatomy has been positioned to a patient support means of the imaging apparatus.

Objects of the invention are reached by a solution whose essential features are presented in the attached patent claims. Thus, according to the invention, a side profile x-ray image (a transillumination image orienting in the direction of the mid-sagittal-plane) is taken of the frontal area of a dental arch being positioned to be imaged, such a vertical tomographic layer is determined from said x-ray image which is optimal considering the layer which upon panoramic x-ray imaging gets imaged sharp from the incisor area of the dental arch, after which imaging means of the panoramic apparatus are driven such that particularly this layer gets imaged sharp in the panoramic x-ray image. Thus, the invention enables imaging of especially the desired tomographic layer based on, instead of presumption, the true structure of the anatomy being imaged.

For realizing the solution according to the invention a panoramic x-ray- apparatus is used, a horizontal degree of freedom of movement of the imaging means of which is arranged to enable, in addition to the actual panoramic imaging, taking a side profile x-ray image according to the invention as well.

Setting of the layer desired to become imaged sharp can be done e.g. by an operator by pointing it e.g. from a digital x-ray image shown on a display arranged in the x-ray apparatus, or location of incisor roots can also be determined according to the invention automatically with the help of a pattern recognition algorithm, e.g. by means of image processing and feature extracting known as such. According to one preferable embodiment of the invention, pattern recognition based on a neural network is used in which location of the desired layer is made into function's approximation problem and location of the desired layer is determined from the preview image with the help of the neuron network as a function of features which are fundamentally easy to define and which represent the structure of the anatomy. Use of neuron network according to the invention enables use of pattern recognition also when the anatomy to be imaged is such that not necessarily each of the reference points of the anatomy defined to be a source of information for pattern recognition is programmatically recognizable. The neuron network can be arranged not only to learn to find the correct layer, but it can also be taught to recognize the anatomy of the patient.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention and some of its embodiments and advantages achievable by them are described more closely by also referring to the enclosed figures, of which figures

Fig. 1 presents a typical dental panoramic x-ray apparatus, Fig. 2 presents a patient positioned at a patient support means for panoramic imaging,

Fig. 3 presents variation of width of a typical tomographic layer formed upon panoramic imaging along various sections of a dental arch and

Fig. 2 presents a patient positioned at a patient support means for panoramic imaging,

Fig. 3 presents variation of width, of a typical tomographic layer formed upon panoramic imaging along various sections of a dental arch and

Fig. 4 presents a side profile x-ray image used in the invention and, therein, location of the tomographic layer desired to become imaged sharp in a panoramic image from the frontal area of a dental arch.

DESCRIPTION OF THE PREFERABLE EMBODIMENTS OF THE INVENTION

Fig. 1 presents a basic construction of a typical panoramic x-ray apparatus. An apparatus according to Fig. 1 comprises a column-shaped body part (11) and a horizontal support arm (12) attached to the body part. Essentially at the other end of the support arm (12), there is arranged an imaging arm (13) , a so-called C-arm, which supports an image information receiver (14) and a radiation source (15) . Further, a patient support means (16) is arranged to the body part (11) .

An imaging arm (13) is arranged to be rotated in relation to the support arm (12) . Additionally, the apparatus construction suitable to be used in the invention has to enable movement of the imaging arm (13) on a horizontal plane in such a way that the imaging means can be positioned for taking a side profile image of the anatomy positioned in the patient support means (16) . This can be realized e.g. by arranging an x, y - degree of freedom of movement to a construction (17) with the help of which the imaging arm (13) is attached to the support arm (12) . On the other hand, also e.g. such a panoramic x-ray apparatus known as such can be used in which between the imaging arm (13) and the support arm (12) there has been arranged an element, which has been arranged turnable in relation to both of these arms and around two vertical rotational axes locating at a distance from each other.

Fig. 2 presents a patient positioned at the patient support means (16) for panoramic imaging. Prior art knows a number of kinds of patient support means the purpose of which is to help the patient to remain stationary for the duration of the imaging event, which lasts considerably long. The solution presented in Fig. 2 comprises a patient

Figure 3 demonstrates how the width of the tomographic layer becoming imaged in panoramic imaging changes in different areas of the dental arch when the imaging means are driven according to the geometry of an imaging scan typically used in panoramic imaging. The width of the tomographic layer formed in the imaging is thus typically of the order of magnitude of 10 mm in the frontal area of the dental arch and considerably wider in the molar area. Realizing the panoramic imaging in such a way that the tomographic layer would be significantly wider in the frontal area is quite difficult in practise, at least without causing other problems to the imaging / without at the same time weakening the diagnostic value of the panoramic image from the viewpoint of some other criteria. Thus, positioning of particularly the incisor area at a correct place for panoramic imaging, with respect to the imaging scan movement the imaging means make during imaging, is considerably more critical than positioning of the molar area.

In Fig. 4, on top of the anatomy there is defined by an unbroken line an exemplary area, which in view of the invention covers a large enough part of the anatomy considering positioning of the location of the tomographic layer desired to become imaged sharp. The width of this area can be e.g. ca. 30 mm.

Additionally, one tomographic layer of ca. 10 mm is demonstrated in Fig. 4 with dashed lines, which layer in the case of the anatomy presented in that Fig. might be desired to be chosen to be the layer that gets imaged sharp in panoramic imaging. The patient of Fig. 4 has a relatively normal bite, thereby it is possible to get the roots of both upper and lower teeth visible.

Differing from the methods measuring external shapes of an anatomy, according to the invention, thus, first an x-ray image of the side profile of the anatomy is taken, with the help of which choosing of the layer to get imaged sharp in the panoramic image may be realized based on the actual structure of the teeth. As there is no need to determine anything else based on this side profile x-ray image than the location of the incisor roots, its properties need not to correspond to those of an x-ray image to be taken for actual diagnostic use. Thus, this x-ray

image which is taken only for the purpose of positioning can be taken by generating a relatively small radiation dose - preferably, as there is no specific need to get the soft tissue, among other things, visible in this image, by using x-radiation generated with a relatively high voltage. Preferably the voltage and the current of the x-ray tube are chosen in such a way that one gets as high a contrast of the image and as small a dose absorbed into the patient as possible. When the current is kept low, one gets with high voltages as high a contrast as possible between the soft tissue and the bone. If e.g. a voltage of 80 kV and a current of 1 mA are used and the area from which the side profile image is taken is properly limited, it is possible to reach a radiation dose which is e.g. below 3 %, or even less, of the dose produced by the actual panoramic imaging scan.

The x-ray image taken from the side of the anatomy forming an essential part of the invention may in principle be taken in many ways, if necessary by arranging to a panoramic x-ray apparatus a separate imaging sensor particularly for this purpose. However, in a preferable embodiment of the invention, one uses the imaging sensor already existing in a digital panoramic x-ray apparatus. Aligning and possible scanning movement of the beam needed for the imaging can be realized in many ways obvious to a man skilled in the art. One possibility to produce a side profile image is to linearly drive the radiation source

(15) producing a narrow beam and the image information receiver (14) on the right side and the left side of the anatomy to produce a transillumination image, or in fact a side profile x-ray image essentially corresponding to a transillumination image.

In a solution according to the invention, a panoramic imaging apparatus is used in which one has arranged for the imaging means the degree of freedom of movement needed for taking both the panoramic image and the side profile x-ray image. Further, control system of the apparatus is arranged to recognize in a suitable way the location, in relation to the imaging means, of the coordinates that will be delivered to it concerning location of the layer desired to become imaged sharp in the panoramic x-ray image. Thus, one embodiment of the invention comprises a control system of a panoramic x-ray apparatus, which is at all times aware of, or which as a response to an impulse received begins to

follow or to save to a memory, where the imaging means are or were located in the set of coordinates the area of their degree of freedom of movement covers .

The layer desired to become imaged can be set e.g. by pointing it manually from a side profile image to the imaging apparatus or the layer can be arranged to become automatically set by a suitable solution based on computer vision. When acting interactively, the side profile x-ray image of the incisor area of the patient is taken before taking the actual panoramic image. This image may be arranged to be shown on a separate computer display or on a display arranged to the actual x-ray apparatus. After location of the desired layer has been determined from the image, this information is provided to the imaging apparatus- e.g. by directly pointing it from the image shown on the display. As according to the invention the x-ray apparatus already holds information of those coordinates of the horizontal plane from which the side profile image for determining location of the tomographic layer has been taken, and as the actual panoramic image will be taken in the same set of coordinates, the geometry may be quite simply calculated according to which the panoramic image should be taken so that the layer of the incisor area becoming imaged sharp would hit the desired location of the anatomy positioned at the patient support means.

However, the place through which the layer becoming imaged sharp should travel in the incisor area may also be determined from the side profile x-ray image by means of methods based on computer vision. In that case, an algorithm to be used for controlling the panoramic apparatus may comprise e.g. the following measures: - Taking a digital side profile x-ray image of the anatomy in the set of coordinates covered by the area of freedom of movement of the imaging means recognized by the imaging apparatus

- Pre-processing the image by image processing means

- Searching predefined features of the anatomy from the image - Determining location of the layer in the incisor area desired to become imaged based on the recognized features

- Determining, based on the location information in question, the geometry of movement of the imaging means in said set of coordinates by following of which said desired layer would become imaged sharp

- Taking a panoramic image according to the imaging geometry in question.

In one embodiment of the invention a method is used in which such features are searched from the side profile x-ray image based on which it is possible e.g. with the help of neuron networks to conclude location of the layer desired to become imaged sharp. Preferably, a similar pre-processing is always done on the side profile image prior to using the actual feature searching algorithm in order to have images taken of different anatomies to be of the same greyscale value with each other.

Naturally, features of the anatomy searched for by the algorithm should represent location of an incisor and its roots as precisely as possible. When choosing a layer interactively, the operator would set the desired layer to the preview image by estimating location of the tip of the root from the image and would place the center of the layer e.g. near the base of the incisor root of the maxilla. However, when choosing the layer automatically, the automatic localization of the tip of the root, the apex, may be difficult. As the points to be recognized should be easily and faultlessly recognizable, one acts according to one preferable embodiment of the invention such that a group of more easily recognizable points are searched from the side profile image which represent the size and the position of the anatomy being imaged. E.g. the tip of a tooth is one apparent such a point as it is easily recognizable for its wide contrast. On the other hand, the inclination angle of the frontal edge of the jawbone can be considered as a significant feature in view of position of the incisors. Even two values can be specified to the inclination angle from different parts of the anatomy and by using different methods. In addition to the position of the teeth, also their size tells something about the location of the root of a tooth. Regarding the maxilla, size of a tooth can be determined by finding from the image the bones of balate and as a straight extension thereof the anterior nasal spine. The length of a tooth may be presumed to be in relation to the distance between the tip

of the tooth and this spine. In the mandible, for its part, the size may be estimated e.g. by finding the tip of the jaw, the distance from which to the tip of the lower frontal incisor presumably being proportional to the length of the lower teeth. Generally speaking, it is preferable to choose the features to be searched for based on the fact that they are not dependent on each other but represent well the position and the size of the incisors.

In a preferable embodiment of the invention a neuron network is used as an approximator for a function, which approximates location of the desired layer as a function of the features identified from the side profile image. With the help of the neuron networks, the features that have been found can be combined and their interdependences sorted out. With the help of the neuron networks, location of the layer can be made into a function whose value depends on the features that have been found. By using different kinds of patients for teaching the neuron network one may assist the algorithm to be able to find the right result regardless of the anatomy of the patient.