WO2008071835A1

WO2008071835A1 - X-ray imaging unit and method for imaging the jawbone region

Info

Publication number: WO2008071835A1
Application number: PCT/FI2007/050646
Authority: WO
Inventors: Henri SETÄLÄ
Original assignee: Palodex Group Oy
Priority date: 2006-12-12
Filing date: 2007-11-29
Publication date: 2008-06-19
Also published as: FI20065793A0; FI20065793L

Abstract

The invention relates to an X-ray imaging unit and method for imaging the jawbone region for providing slice images of the region of interest. The unit includes a carrier element which comprises an X-ray source (20) disposed on one side of the object and a receiving means (21) disposed on the side of the object opposite from the X- ray source for receiving the radiation transmitted through the object for providing projection images. The carrier element is adapted to rotate around its rotation axis such that the X-ray source and the receiving means revolve around the object. The unit is adapted to effect a spinning motion of the carrier element around an object substantially stationary relative to the unit such that, during the spinning motion, the carrier element's rotation axis advances along a path which substantially follows the jawbone anatomy. During said spinning motion, projection images of the jawbone over a limited angular range are produced such that areas inside the jawbone arch remain substantially unexposed to radiation. The unit comprises computing means for working out reconstructed volume models by effecting the computation with at least one angular range reconstruction process on the basis of said projection images of a limited angular range, and computing means for working out the required slice images of the region of interest on the basis of said volume models.

Description

X-ray imaging unit and method for imaging the jawbone region

The present invention relates to an X-ray imaging unit for imaging the jawbone region for providing slice images of a region of interest, said unit including a carrier element which comprises an X-ray source disposed on one side of an object and a receiving means disposed on the side of an object opposite from the X-ray source for receiving the radiation transmitted through the object for providing projection images, and said carrier element is adapted to rotate around its rotation axis such that the X-ray source and the receiving means revolve around the object. The invention relates also to a method for X-ray imaging the jawbone region for providing slice images of a region of interest.

Three-dimensional radiography is based on capturing one-dimensional or two- dimensional projection images of a three-dimensional object from various angles. If one-dimensional projection images are obtained around a two-dimensional slice of an object, with high-density angular sampling, it is possible to determine an internal structure of the slice. This is known as computerized tomography (CT) imaging technology, which is applied extensively in today's medicine.

US 6,434,214 describes a CT radiographic apparatus and method. Fig. 1 shows a general construction for a conventional cone-beam radiographic CT imaging apparatus. The conventional cone-beam radiographic CT imaging apparatus is divided into an imaging unit 1 for effecting the radiography and an image processing unit 2 for processing the detected image data. A control unit 3 executes a total control for the imaging unit 1 and the image processing unit 2. The imaging unit 1 is provided with an X-ray source 5 and a radiation receiving means 6 in an opposite relationship with each other across the object. Both the X-ray source 5 and the two-dimensional radiation receiving means 6 are disposed on a scanning device, which spins around an object 7 with a rotation axis 9 functioning as the centre of rotation. A scanning device 4 is rotated to each predetermined angle and the receiving means 6 executes a measurement for the intensity of X-ray beams 8 emitted from the X-ray source, said X-ray beams having propagated through the object 7 at each of the predetermined angles. In the receiving means, the captured X-ray image is converted into digital image data to be transmitted further to the image processing unit 2. In the image processing unit, pre-processing is effected by devices 10, for example a gamma correction of the receiving means 6, a distortion correction, a logarithmic conversion and a correction of inconsistency. Next, after completing the pre- processing, a three-dimensional reconstructed image is reconstructed by reconstruction devices 11 on the basis of transmitted radiation images (all projected images), said image being a space distribution of the X-radiation absorption coefficient representing the imaged portion of an object.

Regarding this reconstruction calculating method, there is prior known Feldkamp's cone-beam reconstruction calculating method or the like described in the article L. A. Feldkamp et al.: PRACTICAL CONE-BEAM ALGORITHM, Journal of Optical Society of America, A. Vol. 1, No. 6, pp. 612 to 619 (1984) (article 1).

Finally, the three-dimensional reconstructed image is exposed by imaging means 12 to image processing, such as e.g. volume-rendering or MIP projection processing (maximum-intensity-projection processing), for presenting the resulting image as a two-dimensional image on a display 13. Thus, the imaging means 12 execute image processing on the basis of a viewing angle, a region under observation, and the like parameters, which have been input by instruction equipment such as a keyboard, a mouse, and a control ball.

In conventional CT radiography, a scanning device 4, provided with an imaging system which contains the X-ray source 5 and the two-dimensional radiation receiving means 6, is set to revolve around an object 7, preferably in the angular range of about 360°. The received transmitted radiation is imaged and the reconstruction devices 11 produce a space distribution of the X-radiation absorption coefficient about the object 7 placed in a stationary set of coordinates disposed on a frame of the apparatus. The stationary set of coordinates is defined by means of the imaging system, i.e. the z-axis as the centre of rotation 9 for the scanning device and the orthogonal x- and y-axes in a plane coincident with the orbit of a focus 14 of the X-radiation emitted by the X-ray source 5.

Especially in medical application, it is important to employ a total radiation dose as low as possible. Accordingly, just a limited number of images are generally captured and an attempt is made to only focus the radiation on a desired area and an attempt is made to provide a three-dimensional model from a small amount of initial data. When using a small number of projection directions for providing a three- dimensional image, the tomography is referred to as sparse projection data tomography. In particular, if projection images are not obtained from everywhere around an object, but just from a limited angle of opening, the tomography is referred to as limited-angle tomography.

An object of the present invention is to provide a unit and method, enabling the implementation of imaging the jawbone region, especially for providing lateral slices, with a radiation dose as low as possible and also capabie of more precise focusing on the object of interest. In order to achieve this objective, a unit according to the invention is characterized in that the unit is adapted to effect a spinning motion of the carrier element around an object substantially stationary relative to the unit such that, during the spinning motion, the carrier element's rotation axis advances along a path which substantially follows the jawbone anatomy and, during said spinning motion, projection images of the jawbone over a limited angular range are produced such that areas inside the jawbone arch remain substantially unexposed to radiation, and in that the unit comprises computing means for working out reconstructed volume models, the computation being effected with at least one angular range reconstruction process on the basis of said projection images of a limited angular range, and computing means for working out the required slice images of a region of interest on the basis of said volume models.

On the other hand, a method according to the invention is characterized in that method comprises providing limited-angle projection images of various parts of the jawbone substantially over the entire jawbone region, such that areas inside the jawbone arch remain substantially unexposed to radiation, and in that the method comprises working out, on the basis of obtained projection images, a reconstructed volume model by applying at least one limited-angle reconstruction process, said volume models being used for further working out the required slice images of the regions of interest.

The invention will now be described more closely with reference to the accompanying drawings, in which: Hg. 1 shows schematically a configuration for a conventional cone-beam CT radiographic apparatus, and

Fig. 2 shows schematically an imaging arrangement of the invention.

In fig. 2, reference numeral 20 represents the focus of an X-ray source and reference numeral 21 represents a detector, which is preferably equal in terms of its height and width, e.g. within the range of 50 mm x 50 mm - 80 mm x 80 mm, but can also come in another size. The X-ray source and the detector are preferably mounted on the opposite arms of a C-shaped carrier element (not shown), said carrier element being adapted to rotate around a rotation axis for making the X-ray source and the detector to revolve around the head of a patient for capturing the required projection images. Depicted by way of example in fig. 2 are a few angular positions for the focus 20 and the detector 21 while revolving around the head of a patient. Revolution of the X-ray source and the detector around a patient's head proceeds preferably at a constant speed. The centre of rotation for the carrier element is indicated in fig. 2 by reference numeral 22 and it is adapted to proceed along a path indicated by reference numeral 23 during a spinning motion of the X- ray source and the detector around the head. The path 23 is substantially follows the anatomy of a patient's head. The unit is operated to capture individual limited- angle projection images of various parts over the jawbone region. The number of these projection images can be e.g. 80 for one full circle around the head of a patient. For each projection image, the X-ray source is switched on, e.g. for the period of 1/5 s.

These limited-angle projection images are used as a basis for computing a reconstructed volume model by means of limited-angle reconstruction methods, said volume models being further used for computing necessary slice images of regions of interest by means of interpolation. The method applied in reconstruction computation can be e.g. ART (Algebraic Reconstruction Technique), SI (Statistical Inversion) or TACT (Tuned Aperture Computed Tomography) or two or three of said reconstruction methods in combination. One of the benefits gained by a solution of the invention is the fact that the use of limited-angle projection imaging, which is effected from various parts of the jawbone over the entire jawbone region in such a way that areas inside the jawbone arch remain substantially unexposed to radiation and non-reconstructed, enables avoiding the unnecessary irradiation of regions outside the object of interest, such as e.g. the tongue and nasal cavities.

Claims

1. An X-ray imaging unit for imaging the jawbone region for providing layer images of a region of interest, said unit including a carrier element which comprises an X- ray source disposed on one side of an object and receiving means disposed on the side of an object opposite from the X-ray source for receiving the radiation transmitted through the object for providing projection images, and said carrier element is adapted to rotate around its rotation axis such that the X-ray source and the receiving means revolve around the object, characterized in that the unit is adapted to effect a spinning motion of the carrier element around an object substantially stationary relative to the unit such that, during the spinning motion, the carrier element's rotation axis advances along a path which substantially follows the jawbone anatomy and, during said spinning motion, projection images of the jawbone over a limited angular range are produced such that areas inside the jawbone arch remain substantially unexposed to radiation, and in that the unit comprises computing means for working out reconstructed volume models, the computation being effected with at least one limited range reconstruction process on the basis of said projection images of a limited angular range, and computing means for working out the required layer images of a region of interest on the basis of said volume models.

2. An X-ray imaging unit as set forth in claim 1, characterized in that the unit comprises computing means whereby computation is effected by applying an ART, SI or TACT method as the reconstruction process or two or three of said computation methods in combination.

3. A method for X-ray imaging the jawbone region for providing layer images of a region of interest, said method employing a unit which includes an X-ray source to be disposed on one side of an object and receiving means to be disposed on the side of an object opposite from the X-ray source for receiving the radiation transmitted through the object for providing projection images, characterized in that the method comprises providing limited-angle projection images of various parts of the jawbone substantially over the entire jawbone region, such that areas inside the jawbone arch remain substantially unexposed to radiation, and in that the method comprises working out, on the basis of obtained projection images, a reconstructed volume model by applying at least one limited-angle reconstruction process, said volume models being used for further working out the required layer images of the regions of interest.

4. A method as set forth in claim 3, characterized in that the method employs a unit, including a carrier element which comprises an X-ray source disposed on one side of an object and a receiving means disposed on the side of an object opposite from the X-ray source for receiving the radiation transmitted through the object for providing projection images, and said carrier element is adapted to rotate around its rotation axis such that the X-ray source and the receiving means revolve around the object, said method comprising providing limited-angle projection images of various parts of the jawbone substantially over the entire jawbone region, such that the carrier element is spun around a stationary object such that, during the spinning motion, the carrier element's rotation axis advances along a path which substantially follows the object's anatomy.

5. A method as set forth in claim 3 or 4, characterized in that the applied reconstruction process comprises an ART, SI or TACT computation method or two or three of said computation methods in combination.