Abstract:
An extraoral dental radiographic apparatus which performs panoramic radiographies, cone beam volumetric radiographies of the facial skeleton and cranial teleradiographies. An alternating mechanism brings x-ray sensors toward the x-ray radiation source, such that the sensor axes intersect at an angle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention refers to the technical field of extraoral dental radiography, and particularly to a device alternatively performing panoramic radiographies, cone beam volumetric radiographies of facial skeleton and cranial teleradiographies. All these types of radiographies are well known in the art. 
     Panoramic radiography (also known as orthopantomography) produces a radiographic image of a curved plan approximating patient jaws, with blurring of the anatomical structures laying outside a narrow layer around the predesigned curved plane. 
     Cone beam volumetric radiography (also known as CBCT) is the acquisition, from different projection angles, of a series of bidimensional radiographic images which will be processed post-acquisition to reconstruct tridimensional volumes. 
     Teleradiography is a projective radiographic technique, producing radiographic images of the skull or of other anatomical areas from different projections, with minimum magnification and geometrical distortion. Usually two perspectives are represented, latero-lateral and anteroposterior. 
     Generally on the market two types of equipment are available: a first type is panoramic equipment, alternatively performing panoramic radiographies and teleradiographies, and a second type dedicated to CBCT volumetric radiography. These equipments are expensive and cumbersome. 
     BRIEF SUMMARY OF THE INVENTION 
     Aim of the present invention is providing an X-ray imager alternatively performing, after appropriate setting up of the X-ray imager, said three types of radiologic examination, and which can be easily and cheaply produced. 
     The X-ray imager according to the present invention has an X-ray source and different X-ray sensors, of the type appropriate for the examination to be performed, arranged at an appropriate distance from the X-ray source so that good-quality images can be obtained. The X-ray source is the same for the three types of radiographic acquisition. 
     In particular, the invention consists in the respective disposition of the panoramic (PAN) and the cone beam volumetric radiography (CBCT) sensors, which are respectively arranged to form an angle. 
     In the choice of the angle between PAN sensor and CBCT sensor different issues have to be accounted for: X-ray imager set up rapidity, temperature, and protection from non-collimated X-rays. 
     The shortest X-ray imager set up time for the specific acquisition is desirable: a small angle between the two sensors would accelerate their exchange. 
     When the sensor is active, and especially when it has to acquire and transmit a great number of signals, as it is needed in this type of application, heat is produced: this would lead to the longest possible distance between PAN sensor and CBCT sensor, with the aim of an efficient heat removal. 
     When X-rays are emitted from the X-ray source, a small quantity of rays might hit outside the sensor&#39;s sensitive area, potentially degrading the gathered signal. The respective disposition of the sensors should be able to avoid this risk. 
     From all the above-cited considerations, the optimal respective position for the two sensors is an angle comprised between 40° and 140°, preferably between 80° and 100°, most preferably 90°. This particular disposition allows to set the X-ray imager rapidly and conveniently for both X-ray operator and patient. 
     Moreover, the present disposition allows to insert a further element, e.g. an optional collimator or a camera taking pictures of the patient undergoing the radiographic acquisition. 
     The positions of PAN and CBCT sensors on the alternating mechanism, and therefore the distance of the specific sensor from the X-ray source, are opportunely chosen so as to obtain the best radiographic result. The distance between X-ray source and sensor varies with the specific sensor used. 
     The X-ray imager comprises a base from which a post raises, the post supporting a device capable of vertically moving a C-arm, the C-arm carrying on one side the X-ray source and a primary collimator immediately downstream the X-rays exit, and on the other side two different types of sensors. 
     In particular, the PAN sensor and the CBCT sensor are arranged on a mechanism alternating the two sensors, bringing the sensor needed for the desired acquisition in the position where it can be hit by X-rays, and at the same time removing from the X-ray path the undesired sensor. When a teleradiography has to be acquired, the alternating mechanism excludes from the X-ray path both PAN and CBCT sensors. 
     Two different configurations of the X-ray imager are possible: a minimal configuration with a PAN sensor and a CBCT sensor, without teleradiography support. A second optional configuration is a complete X-ray imager, allowing to alternatively obtain panoramic radiographies, CBCT volumetric radiographies images and teleradiography images, and therefore having a support for the teleradiography, too. 
     Moreover, two distinct embodiments of the X-ray imager performing teleradiography, too, are possible: one simpler and cheaper embodiment wherein one PAN sensor only is present, and this PAN sensor is detached from the PAN support and positioned on the teleradiography support when a teleradiography is to be acquired. In a second, more expensive embodiment, two sensors are present, one PAN sensor and a teleradiography sensor. In this second embodiment, the teleradiography sensor is bigger than the PAN sensor. 
     The set up of the X-ray imager can be performed before or after the positioning of the patient in the X-ray imager. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
       The present invention will be now described in more detail with the help of the following figures, showing: 
         FIG. 1  a prospective view of the complete X-ray imager; 
         FIG. 2   a  a schematic view of the position of PAN, CBCT and CEPH sensors during a panoramic image acquisition; 
         FIG. 2   b  a schematic view of the position of PAN, CBCT and CEPH sensors during a CBCT volumetric radiography acquisition; 
         FIG. 2   c  a schematic view of the position of PAN, CBCT and CEPH sensors during a teleradiography image acquisition; 
         FIG. 3   a  a perspective view of the mechanism alternating PAN and CBCT sensor; 
         FIG. 3   b  horizontal section of the mechanism alternating PAN and CBCT sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows in its entirety an X-ray imager  1 , which can alternatively produce panoramic radiographies, CBCT volumetric radiographies and teleradiography images. The imager  1  comprises a base  2  and a post  3  supporting an extension  4 , which is provided with a vertical section sliding on the post  3  and a horizontal section. The vertical section of the extension  4  allows the vertical movement of a C-arm  6 , which is attached to the horizontal section of the extension  4  and which in its turn supports an X-ray source  7 , and a alternating mechanism  8 , for alternating the positions of a PAN sensor  9  and a CBCT sensor  10 . Moreover, a device  5  for the positioning of the patient is present. On a further optional arm  11 , arranged on support  4 , a support  12  for CEPH sensor  15  for teleradiography and a further device  13  for positioning the patient are present. 
     In all acquisition modalities, the X-rays must hit the specific sensor: for radiography acquisition the sensor must be brought in the position allowing the hitting. C-arm  6  has at one of its ends the X-ray source  7 , and at its other end the alternating mechanism  8 , on which PAN sensor  9  and CBCT sensor  10  are arranged. The alternating mechanism  8  is fixed to C-arm  6  so that it can be moved either manually or automatically exposing the specific sensor needed. 
     The alternating mechanism  8  has at least two stops, schematically shown in  FIGS. 2   a ,  2   b ,  2   c : one bringing the PAN sensor  9  in the position needed for the acquisition of panoramic images, and one bringing the CBCT sensor  10  in the position needed for the acquisition of CBCT volumetric radiographies. The acquisition can start only when the specific sensor is in the proper stop position. The sensor remains in its stop position for all the acquisition process, and can be moved only after acquisition end. 
     The positions of PAN sensor  9  and CBCT sensor  10  on alternating mechanism  8 , and therefore the distance of the specific sensor from the X-ray source  7  are chosen to get the best radiographic result. The distance between X-ray source  7  and specific sensor varies with the type of sensor used. Generally, the distance between PAN sensor  9  and Xray source  7  is shorter than the distance between CBCT sensor  10  and X-ray source  7 . 
     Compatibly with the magnification degree and the consequent spatial distortion which are held optimal for the kind of acquisition, alternating mechanism  8  is made so that it can bring PAN sensor  9  for the panoramic acquisition at a distance usually comprised between 520 and 580 mm from Xray source  7 , while CBCT sensor  10  in CBCT volumetric radiography is positioned at distances usually comprised between 600 and 700 mm from X-ray source  7 . 
     In cases where the machine comprises an optional secondary collimator  16  mounted on the alternating mechanism  8  for beam collimation during the teleradiographic acquisition, for a good alignment, the distance between X-ray source  7  and CEPH sensor  15  is comprised between 1400 and 1800 mm. Keeping in mind these dimensions, the preferred embodiment of the elements in the alternating mechanism  8 , according to the aims of the present invention, and with the aim of optimizing bulk and set up speed, is shown in  FIGS. 3   a  and  3   b .  FIG. 3   a  is a perspective view of the alternating mechanism  8 , by which the PAN sensor  9 , the CBCT sensor  10 , and the secondary collimator  16  can be pivoted around a rotation axis  17 .  FIGS. 3   a  and  3   b  further show a PAN sensor axis  18  and CBCT sensor axis  19 . The PAN sensor axis  18  and the CBCT sensor axis  19  are centered normals to a sensitive area  20  of the PAN sensor  9  and to a sensitive area  21  of the CBCT sensor  10 . In the embodiment depicted in  FIGS. 3   a  and  3   b , the PAN sensor axis  18  and the CBCT sensor axis  19  are positioned at an angle a of 90°, respectively; the distance of the photosensitive area  20  of PAN sensor  9  ranges between 220 and 180 mm from the rotation centre  17  of alternating mechanism  8 , the distance between the photosensitive area  21  of CBCT sensor  10  and rotation centre  17  ranges between 50 and 130 mm; finally, secondary collimator  16  is at a distance between 80 and 120 mm from the rotation centre  17  of mechanism  8  and positioned at an angle _ ranging between 15° and 25° with respect to the PAN sensor axis  18  of PAN sensor  9 . The figures show a preferred embodiment wherein the secondary collimator  16  is placed near the CBCT sensor  10 , but this placement is only one of the many embodiments that are possible. 
     The alternating mechanism  8  may comprises a plate  22 , which can be fixed to the arc  6 . The plate  22  is provided with a central opening  23 , in which a reduced section  24  of a stepper motor  25  is fixed. If the alternating mechanism  8  is mounted on the arc  6 , a drive shaft  26  is oriented in a vertical position. At an other end  27  of the drive shaft  26 , a plate  28  with a supporting mechanism  29  of the various elements (PAN sensor  9 , CBCT sensor  10  and secondary collimator  16 ) is mounted. Motor actuation can be electronically driven by a microcontroller, and driven when the set up command (for PAN, CBCT or CEPH acquisitions) is sent to the drive circuit. Stop positions can be detected with photoelectric cells, one for each stop position, so that the passage from one configuration to the other can occur without passing from an “initial” position, and counting motor steps necessary to get the desired positions. 
     On the plate  28 , the mechanisms for sensors mounting/demounting can also be present (in the case of an X-ray imager with removable sensors). The plate  28  can also be provided with a mechanism for vertical positioning of the secondary collimator  16  for a correct centring of X-ray beam on CEPH sensor  15 . 
     Alternating mechanism  8  is better shown in  FIG. 3   b , which represents a horizontal section of the mechanism itself supporting PAN sensor  9 , CBCT sensor  10  and optional secondary collimator  16 , respectively. This  FIG. 3   b  allows to appreciate the spatial relationship between the two sensors.  FIG. 3   b  shows the rotation axis  17  of the alternating mechanism  8 . 
     When a panoramic acquisition has to be performed, the PAN sensor  9  must be brought on the X-ray path and the primary collimator must be opportunely set. The PAN sensor  9  can be a linear sensor (monodimensional), well known in the art, but also an area sensor (bidimensional). The patient is positioned within the X-ray imager  1  using patient positioning device  5 . Once the patient is properly positioned in the X-ray imager  1 , the acquisition can start with the emission of X-rays from X-ray source  7 . 
     When a CBCT volumetric radiography has to be performed, the CBCT sensor  10  must be brought on the X-ray path and the primary collimator must be opportunely set. The alternating mechanism  8  rotates, manually or automatically, and the CBCT sensor  10  is brought to intercepts X-rays. 
     In a particular embodiment, the X-rays emission modality can be modified according to the radiographic acquisition: panoramic acquisitions might be performed with a X-ray continuous emission, while CBCT volumetric radiography might be performed with pulsed X-ray emission, in order to get the best radiographic result with the minimal X-ray dose to the patient. 
     When a teleradiography acquisition must be performed, the anatomic portion of patient to be imaged must be positioned within support  13 , and, if the machine is endowed with a removable sensor only, PAN sensor  9  must be removed from alternating mechanism  8  and positioned on teleradiography support  12 , while the primary collimator and the optional secondary collimator  16  must be opportunely set. In a more complete embodiment, wherein the X-ray imager has two sensors  9  and  15 , the X-ray imager is ready for the teleradiography acquisition right after patient positioning. 
     CEPH sensor  15  can be a linear sensor (monodimensional) or an area sensor (bidimensional). 
     In both cases, alternating mechanism  8  must be positioned so that both PAN sensor  9  and CBCT sensor  10  are excluded from the X-ray path. 
     On alternating mechanism  8 , moreover, a secondary collimator  16  may be present, which must be arranged so as to be crossed by X-rays, therefore stopping the alternating mechanism  8  in a third position. In this case, too, the acquisition can start only when alternating mechanism  8  is in the proper position. This movement can be performed either manually or automatically. Secondary collimator  16  collimates X-rays during teleradiography acquisition, so that the X-ray hits the most precisely possible the sensitive part of the CEPH sensor  15 . 
     Once the X-ray imager  1  is properly set up, teleradiography acquisition can start.