BEST MODE FOR CARRYING OUT THE INVENTION

[0074] FIG. 1 a is a conceptual diagram explaining a procedure of a display method of X-ray projection images for medical use according to the present invention, FIGS. 1 b, 1 c and 1 d show an example of X-ray projection images of a tooth shown according to the method.

[0075] FIG. 1 a shows an image wherein three dimensional X-ray absorption coefficient data obtained by an X-ray CT method in which an X-ray generator 1 and an X-ray detector 2 are opposed and rotated around a dental arch S, being an object to be examined, are seen from an image layer CP having a predetermined thickness in a direction perpendicular to a radiated plane obtained by rotation of X-ray radiation, namely shows an image when three dimensional X-ray absorption coefficient data are seen from an axial direction of a rotation center 3 a of X-ray radiation.

[0076] Circular area around the center of the image layer CP is called as a virtual local region Q which is always locally radiated with X-rays in a radiography wherein a local X-ray CT method is applied to panoramic images as mentioned hereinafter. When an object is a dental arch S, the region Q is generally selected around a median line inside of the dental arch. Here in this specification, the virtual local region Q and the areas around four corners of the figure are eliminated from the area to calculate three dimensional X-ray absorption coefficient data to be left as more density parts.

[0077] According to the display method of X-ray projection images for medical use of the present invention, a projection interested area PA to construct the X-ray projection images of the object, namely a dental arch S in this specification, is set under the following procedures.

[0078] <Setting of Projection Interested Area PA>

[0079] 1. As shown in FIG. 1 a, the image of the image layer CP is displayed.

[0080] 2. Border lines of the dental arch S and a jawbone Sg are appeared on the image layer CP, although unclear, in such a manner their position on a radiated plane can be specified. A dental arch area to extract three dimensional X-ray absorption coefficient data is defined aiming at the border lines so as to include the dental arch S and its supporting alveolar therearound.

[0081] 3. Each specific point on the border lines of the projection interested area PA in the figure is specified, for example, on the image of the image layer CP by means of a pointing device such as a mouse (not shown) and the points are combined with a line so as to be compensated, thereby achieving setting. Dental arch area pattern calculated from the past statistical data is applied per the kinds of the object, for example age and sex, thereby achieving setting.

[0082] 4. In this case, as shown in the figure, such parts as a neck bone being an obstacle shade Ob in the projection interested area PA are designed to be eliminated.

[0083] Thus the projection interested area PA is set in advance, three dimensional X-ray absorption coefficient data only in this area PA are used for projection, and X-ray projection images without obstacle shades Ob can be obtained.

[0084] Then, the projection interested area PA is divided into plural projection interested layers Pal and Pa 2 which are adjacent each other in a direction of an X-ray radiating direction RD. Here the dental arch area is set as a projection interested area PA and the area PA is divided into two parts Pal which is a cheek side and Pa 2 which is a tongue side by a center line SC of the dental arch. The setting method of the center line SC of the dental arch is the same as that of the projection interested area PA. If a shape of the projection interested area PA is once determined, sometimes all required is to specify the thickness of the produced projection interested layer in order to divide the area PA into plural projection interested layers adjacent each other in the X-ray radiating direction RD at each area PA.

[0085] As shown in the figure, for obtaining the X-ray projection image of a tooth S 7 of the dental arch S, a projection plane TP intersecting the X-ray radiating direction RD from which the tooth S 7 is radiated is set. The projection plane TP is thus set and only effective three dimensional X-ray absorption coefficient data aligning along the X-ray radiating direction RD are projected on the projection plane TP, thus obtaining clear X-ray projection images.

[0086] After the projection plane TP is set, the three dimensional X-ray absorption coefficient data in the projection interested area PA are projected on the projection plane TP only from the projection plate side, in this case from the opposite side of the X-ray radiating direction RD against the projection plane TP. Then the X-ray projection image TI of the tooth S 7 as shown in FIG. 1 c can be obtained.

[0087] Although it can be understood there exist three dental roots on the X-ray projection image TI of the tooth S 7 , it is difficult to determine whether those roots exist in a cheek side or in a tongue side. In this case, the X-ray projection image using the projection interested layers Pa 1 and Pa 2 is helpful.

[0088] When the three dimensional X-ray absorption coefficient data of the projection interested layer Pa 1 at cheek side is projected on the projection plane TP, the X-ray projection image TI of the tooth S 7 shown in FIG. 1 b can be obtained. When the three dimensional X-ray absorption coefficient data of the projection interested layer Pa 2 at tongue side is projected on the projection plane TP, the X-ray projection image TI of the tooth S 7 shown in FIG. 1 d can be obtained. From the X-ray projection images TI shown in FIGS. 1 b and 1 d, it is understood that there is one root at cheek side and other two roots are at tongue side.

[0089] Exemplifying a dental arch as mentioned above, what exists in which image layer can be understood as an X-ray projection image by dividing the projection interested area PA into plural projection interested layers Pa, that is for example a dental root exists where in the cheek side to the tongue side.

[0090] More detailed positional detection is possible by appropriately selecting a projection interested area PA at first. Further it is also possible by dividing the area into a larger number of projection interested layers.

[0091] FIG. 2 is an example of displays shown by a display method of X-ray projection images for medical use according to the present invention.

[0092] This figure is a display arranging showing in array the projection interested area PA explained in FIG. 1 , an image of the image layer CP on which a projection interested area PA, and projection interested layers Pa 1 and Pa 2 are set, and an X-ray projection images TI of tongue side, entire area and cheek side produced for each part of the dental arch according to the method mentioned in FIG. 1 .

[0093] The X-ray projection image TI is obtained in such a manner that a rotation center Ta of the projection plane TP is aligned with a rotation center 3 a of X-ray radiation to be fixed, a tooth of which X-ray projection image is required is moved in sequence, and the projection plane TP is correspondingly rotated. When the center Ta of the projection plane TP is fixed according to radiography conditions, radiation conditions and radiography conditions are conformed so that clear X-ray projection images can be obtained and further projection procedures are simplified.

[0094] Partial tooth images of the entire dental arch S are arranged on a display in array and they can be seen contrasting the tongue side, the entire area and the cheek side, therefore treatment parts are easily specified so as to be useful for medical care. The projection interested area and the projection interested layer are shown at one time so that their corresponding relation can be easily understood. Further, if names of the selected projection interested area and the projection interested layer are also shown beside the X-ray projection images, more accurate diagnosis is possible.

[0095] FIG. 3 - FIG. 7 show an example of X-ray projection images shown by a display method of X-ray projection images for medical use according to the present invention. They are X-ray projection images constructed for broader area than the image shown in FIG. 2 . FIG. 3 is an X-ray projection image TI at a cheek side when a dental arch is seen from left, FIG. 4 is an X-ray projection image TI at a tongue side seen from the same direction as FIG. 3 , FIG. 5 is an X-ray projection image TI of the entire area seen from the same direction as FIG. 3 , FIG. 6 is an X-ray projection image TI of the entire area when a dental arch is seen from the front and FIG. 7 is an X-ray projection image TI seen from right.

[0096] From the X-ray projection images TI in FIG. 3 , FIG. 4 and FIG. 5 , the dental arch S seen from a specific direction can be understood, namely a dental arch S is perspectively displayed as a normal view. Comparing with a panoramic image shown in FIG. 8 , those images TI are easily understood by intuition and available for determination material for diagnosis.

[0097] Those images may be shown on a display respectively or may be shown in array like FIG. 2 such as cheek side, tongue side and entire area per each part. If they are arranged in array, each image is easily compared and the position of dental root from the cheek side to the tongue side is obtained.

[0098] From the X-ray projection images TI in FIG. 5 , FIG. 6 , and FIG. 7 , X-ray projection images are shown by rotating the projecting direction to the dental arch, such as left, front and right. When the image of the dental arch being an object is displayed in rotation by sequentially and continuously showing plural X-ray projection images, even if a display area is limited, X-ray projection images seen from different directions can be continuously displayed, thus enabling selection of an image required for diagnosis and achieving convenience.

[0099] Specifically, in case of a dental arch, about 512 X-ray projection images taken from different projection angles are sequentially and continuously displayed at intervals from 1/30 sec. to 1/15 sec., thereby displaying in rotation a maxillo facial area.

[0100] Combining a display in rotation and a display in array, X-ray projection images of cheek side, tongue side and entire area are displayed in array and each X-ray projection image is displayed in rotation by sequentially changing the projecting direction.

[0101] FIGS. 8 a, 8 b and 8 c show an example of X-ray panoramic images shown by a display method of X-ray projection images for medical use according to the present invention.

[0102] Generally in case of X-ray panoramic images, a projection plane to obtain panoramic images isn't flat like the X-ray projection image in FIG. 1 but is curved and its projecting direction is as a rule perpendicular to a part of each curved projection plane. Therefore, the method of the present invention isn't applied as it is.

[0103] However, application of a projection interested area and a projection interested layer is useful for constructing X-ray panoramic images. The three dimensional X-ray absorption coefficient data to be projected on the curved projection plane in order to obtain X-ray panoramic images can be limited to those in the projection interested area or the projection interested layer.

[0104] FIGS. 8 a, 8 b and 8 c show X-ray panoramic images V obtained by setting the projection interested area PA and the projection interested layers Pa 1 and Pa 2 as shown in FIG. 1 against the dental arch S. FIG. 8 a is a panoramic image V at cheek side, FIG. 8 b shows a panoramic image V of the entire area and FIG. 8 c shows a panoramic image V at tongue side. If each name of the setting area or setting layer such as cheek side, entire area and tongue side is described beside the image, mistakes are avoided and accurate diagnosis can be executed.

[0105] As mentioned above, if the projection interested area and projection interested layer are applied to X-ray panoramic images, an image which has little obstacle shade and can show the position of a dental root from cheek side to tongue side can be obtained.

[0106] FIG. 9 is a flow chart showing one example of procedures of a display method of X-ray projection images for medical use according to the present invention. The display procedure of the present invention will be detailed hereinafter. The procedure for obtaining three dimensional X-ray absorption coefficient data in order to apply the display method of the present invention will be explained here, however, the object of the present invention is to provide a display method and the invention isn't limited to a production method of three dimensional X-ray absorption coefficient data.

[0107] At first three dimensional X-ray absorption coefficient data are required to be obtained by radiating X-rays on an object to be examined. To this end, an imaging mode of X-ray CT is selected (S 1 ). The reason is that this invention applies as a pre-stage a local X-ray CT method, namely a radiography in which the effects of the display method of the present invention are interactively achieved, to obtain X-ray panoramic images. In such a case, an imaging mode is required to be selected whether a general local CT mode or a CT mode for panoramic images. In this case, a panoramic CT is selected.

[0108] Next, the object sits on a chair for picturing shown as the reference numeral 4 in FIG. 20 . His head is fixed with a holding means 4 a shown in FIG. 20 to set X, Y and Z positions (S 2 ), a rotation center 3 a of a rotary arm 3 in FIG. 20 , namely a rotation center of X-ray radiation, is set to be a center Qa of a virtual local region Q in case of panoramic CT mode (S 3 ).

[0109] Then conical X-ray beams are locally radiated according to a projection mode while the rotary arm is turned in a fixed angle area corresponding to a projection mode (S 4 ), an image processing including backprojection is executed based on the obtained X-ray transmitted data according to a projection mode to obtain three dimensional X-ray absorption coefficient data (S 5 ), and the data are stored (S 6 ).

[0110] Thus, after obtaining three dimensional X-ray absorption coefficient data, a projection interested area PA is set as mentioned above for the obtained three dimensional X-ray absorption coefficient data (S 11 ), a projection interested layer Pa is selected if necessary (S 12 ), a projection plane TP from an X-ray radiating direction on a point where projection is desired is determined and a projection figure is produced by projecting three dimensional X-ray absorption coefficient data on the projection interested area PA or the projection interested layer Pa depending on the selection (S 13 ), then the figure is stored (S 14 ). These procedures are repeated for a desired range in the projection interested area PA (S 15 ).

[0111] After producing and storing the projection figure, a display method is selected from a display in array, a rotary display and their combination (S 16 ), the figures are shown on a display means according to the selected display method (S 17 ), and the displayed images are stored if necessary (S 18 ). These procedures are repeated in a required area and finished (S 19 ).

[0112] According to the above-mentioned procedures, advantages of the local X-ray CT method and apparatus and three dimensional X-ray absorption coefficient data are effectively used, thereby enabling display of X-ray projection images which has little obstacle shades, is easily understandable by intuition for medical diagnosis, and can show the position of dental root from a cheek side to a tongue side.

[0113] FIG. 10 is a conceptual view explaining a radiography method wherein a local X-ray CT method used in a display method of X-ray projection images for medical use according to the present invention is applied to panoramic images. The reference numerals already explained have the same numerals and their explanations are omitted hereinafter.

[0114] In FIG. 10, a rotary arm 3 having an X-ray generator 1 and a two dimensional X-ray image sensor 2 , which is an X-ray detector, at both ends thereof is turned at a constant velocity so as to keep the width between the rotation center 3 a of the rotary arm 3 and a conical X-ray beam 1 a so as to form a virtual local region Q. The X-ray generator 1 radiates a conical X-ray beam 1 a with a fixed width in a scanning direction accompanying with the movement of the rotary arm 3 and sequentially produces X-ray transmitted images of a dental arch S on the two dimensional X-ray image sensor 2 by the conical X-ray beam 1 a. Only a partial X-ray transmitted images produced by an ortho-conical X-ray beams 1 b substantially orthogonal to the dental arch S among the beam bundles of the conical X-ray beam 1 a is extracted for the X-ray transmitted images sequentially produced on the two dimensional X-ray image sensor 2 . The extracted partial X-ray transmitted images are arithmetically processed to obtain three dimensional X-ray absorption coefficient data of the dental arch area including the dental arch S.

[0115] Thus applying a basic local X-ray CT method wherein the rotary arm 3 is turned with its center 3 a fixed during radiation and a conical X-ray beam 1 a with a predetermined width is locally radiated, three dimensional X-ray absorption coefficient data for X-ray panoramic images can be produced. Further, the X-ray exposure duration is short and the exposed dose amount is reduced into 1/50 of the prior CT method, in addition a sufficient three dimensional X-ray absorption coefficient data can be obtained like the prior art.

[0116] FIG. 11 is a conceptual diagram explaining one example of setting methods of a rotation center of a rotary arm when a local X-ray CT method is applied for panoramic images.

[0117] According to a local X-ray CT method, the rotation center 3 a of the rotary arm 3 is aligned with the center of a virtual local region on an axis of symmetry Lo at the center of the dental arch area SA, the rotary arm is turned at a constant velocity or a variable speed in a rotary angle area according to radiography conditions, and a conical X-ray beam with a fixed width is locally radiated, thus obtained X-ray transmitted images of the dental arch area SA.

[0118] Generally in case of prior film-type panoramic imaging, it is required to execute radiography while an X-ray beam bundle is moved by transferring the rotation center of the rotary arm in such a manner that X-rays become orthogonal to the dental arch for each tooth in the curved sectional area SA. In FIG. 11 , such X-ray beam bundle is shown with the reference numeral L. When X-ray beam bundles L . . . substantially orthogonal to each tooth are shown for the dental arch area SA, an envelope line La of each X-ray beam bundle L . . . is produced. Assuming an incircle G which touches internally the envelope line La, all the X-ray beams for the dental arch area SA passes through the incircle G.

[0119] In this local X-ray CT, a center Ga of the incircle G is aligned with the center 3 a of the rotary arm 3 and radiation is executed by turning the rotary arm without moving the rotation center 3 a thereof. Conical X-ray beam 1 a with a fixed width so as to include the incircle G is locally radiated on the object from its surround. The conical X-ray beam 1 a always includes X-ray beam bundles substantially orthogonal to the dental arch area SA (called as an ortho-conical X-ray beam hereinafter).

[0120] In this example, the incircle G becomes a virtual local region as shown in FIG. 10 , which is also referred to the reference numeral Q. The X-ray beam bundles orthogonal to each tooth are the ortho-conical X-ray beams as mentioned above, which are shown as the reference numeral 1 b.

[0121] When the X-ray conical beam 1 a is locally radiated so as to form the virtual local region Q, partial X-ray transmitted images obtained by the ortho-conical X-ray beams lb substantially orthogonal to the dental arch area SA are extracted from the X-ray transmitted images of the dental arch area SA sequentially produced on the two dimensional X-ray image sensor 2 and are arithmetically processed to obtain the three dimensional X-ray absorption coefficient data of the dental arch area SA.

[0122] A projection method wherein the local X-ray CT method used in the X-ray CT apparatus of the present invention is applied to panoramic images is based on the above-mentioned concept. The position of the rotation center 3 a of the rotary arm 3 and the width of the conical X-ray beam 1 a, namely the position and size of the specific region Q, are appropriately set according to the images to be produced finally. In summary, ortho-conical X-ray beams according to the images are necessary to be included in conical X-ray beams.

[0123] For example, the center 3 a of the rotation center 3 which is set at the same time of radiation and the width of conical X-ray beam, that is the virtual local region Q, aren't limited to the above-mentioned incircle G internally touching the envelope line La. It may be an incircle G′ or G″ shown in FIG. 11 . If such a circle is defined as the region Q, the center of the circle is always on the axis of symmetry Lo inside of the dental arch S. The position of the center 3 a of the rotary arm 3 when the region Q is the incircle G′ or G″ is shown as the reference numerals 3 a ′ or 3 a″.

[0124] X-ray panoramic images are not limited to ortho-radial X-ray panoramic images in which X-ray beam bundles are substantially orthogonal to the tooth. There are standard X-ray panoramic images, X-ray panoramic images of a jawbone and X-ray panoramic images on either one side of right or left. In such images, ortho-conical X-ray beams aren't always orthogonal to the dental arch S. If X-ray panoramic images by such a radiography method are produced, the position of the center 3 a of the rotary arm 3 on the axis of symmetry Lo of the dental arch S and the width of conical X-ray beam 1 a, namely the local region Q, are required to be determined so as to include all the ortho-conical X-ray beams 1 b. Such examples are the above-mentioned incircles G′ and G″. For producing X-ray panoramic images on either side of right or left, the center 3 a of the rotary arm 3 may be inside of the dental arch area SA or may not be on the axis of symmetry Lo.

[0125] The local region Q to produce the X-ray panoramic images is determined corresponding to the X-ray panoramic images to be produced. Smaller area is better considering the reduction of the X-ray exposure amount.

[0126] As understood from FIG. 11 , the rotary arm 3 is required to be turned over 180 degrees, however it isn't required to be turned 360 degrees. Namely radiation is executed by turning the rotary arm 3 from 180 degree to 360 degree. If the angle is small, the X-ray exposed dose can be reduced accordingly.

[0127] In this invention an improved radiography method when the local X-ray CT method is applied to panoramic images is also proposed such that the rotation center 3 a is moved by the minute during X-ray radiation without fixing the center 3 a. For example, the center 3 a may be moved along an envelope line La like a film-type radiography.

[0128] In such a case, while keeping the effect of reduction of the local X-ray exposed amount and the effect of reduction of imaging time which are characteristic of the local X-ray CT, only more narrow ortho-conical X-ray beams are radiated although a movement control of the rotary arm 3 becomes complicated. Further, a radiated tooth is disposed near the rotation center so that transmitted images with a constant size can be always radiated on the X-ray detector 2 . Therefore, the detector area is effectively used, images with constant resolution and accuracy can be obtained. This method is also generally used other than panoramic images.

[0129] When the rotation center 3 a is moved during X-ray radiation, the rotation center Ta of the projection plane TP is correspondingly moved as explained referring to FIG. 1 , then the projection conditions conform with the radiation conditions to obtain clearer X-ray projection images.

[0130] Further, if an X-ray tube voltage and/or an X-ray tube current are changed during X-ray radiation, or if X-rays are radiated while varying the rotation speed of the rotary arm 3 , density compensation depending on the radiated tooth can be executed, thus obtaining better X-ray projection images.

[0131] FIG. 12 is a conceptual diagram explaining other example of setting methods of a rotation center of a rotary arm when a local X-ray CT method is applied to panoramic images.

[0132] In this embodiment, there exists metal M such as crown at plural teeth. If X-rays are radiated from the conical X-ray beam 1 a (1) to 1 a (2) while fixing the center 3 a of the conical X-ray beam 1 a, X-rays are radiated on both metals M of a forward teeth and of a rear teeth at the conical X-ray beam 1 a (2). In such a case a metal artifact is generated so that correct X-ray transmitted images can't be obtained and hence correct three dimensional X-ray absorption coefficient data can't be obtained.

[0133] In order to avoid such simultaneous radiation on plural metals M, X-rays are radiated by moving the rotation center 3 a in such a manner that the conical X-ray beam la is positioned like 1 a (3) after 1 a (1).

[0134] The effect of moving the rotation center 3 a is to avoid the metal artifact as mentioned above, thereby obtaining better three dimensional X-ray absorption coefficient data and better X-ray projection images.

[0135] FIGS. 13 a and 13 b are conceptual diagrams showing one example of X-ray radiation method in an X-ray CT used for an X-ray CT apparatus for medical use according to the present invention. FIGS. 13 c and 13 d show one example of X-ray projection images obtained by the method. FIGS. 13 e and 13 f show one example of X-ray projection images obtained by a prior art.

[0136] FIG. 13 a shows the dental arch area SA in a form of a vertical section of a front jaw. In the figure, an upper front tooth S 1 U and a lower front tooth S 1 L are shown. FIGS. 13 a, 13 c and 13 e show the upper front tooth S 1 U and FIGS. 13 b, 13 d and 13 f show the lower front tooth S 1 L.

[0137] In this method, X-rays are radiated on a normal line γ against a rising direction T 1 U of the upper front tooth S 1 U. The projection plane TP perpendicular to the normal line γ is determined and three dimensional X-ray absorption coefficient data on the dental arch area SA which is also a projection interested area are projected on the plane TP, thus obtaining X-ray projection images.

[0138] This method is applied to the lower front tooth S 1 L in the same manner. The reference numeral T 1 L in the figure shows a rising direction of the lower front tooth S 1 L. The length of the upper front tooth S 1 U and the lower front tooth S 1 L in a direction of a panoramic sectional area SB of the dental arch S is shown as WU and WL respectively, and the length perpendicular to the nominal line γ is shown as VU and VL respectively. The reference numerals H 3 and H 4 show a foreign material hidden in the upper front tooth S 1 U and in the lower front tooth S 1 L.

[0139] FIG. 13 c is a partial view of the upper front tooth S 1 U of the produced X-ray projection image TI and FIG. 13 d is a partial view of the lower front tooth S 1 L. FIGS. 13 e and 13 f show partial views of the X-ray projection image TI projected on a projection plane TP including a central perpendicular line SB of the dental arch area SA regardless a rising direction.

[0140] Comparing those figures, it is understood that the length VU and VL which is an actual entire length of the upper front tooth S 1 U and the lower front tooth S 1 L and an accurate position of the foreign material H 3 and H 4 for the length VU and VL are obtained by FIGS. 13 c and 13 d. The lengths WU and WL obtained by FIGS. 13 e and 13 f were varied by the angle of a rising direction T 1 U and T 1 L of the upper front tooth S 1 U and the lower front tooth S 1 L, therefore accurate length were not obtained.

[0141] According to this method, the X-ray projection images in which each tooth is clearly appeared, not the X-ray projection images parallel to the central perpendicular line SB of the dental arch SA, can be obtained.

[0142] The normal line γ is changed as the rising direction is varied according to the tooth and is determined corresponding to each tooth.

[0143] In FIG. 14 the X-ray radiation method in FIG. 13 is applied to other teeth. FIG. 14 a is a conceptual diagram when the teeth are seen from a direction of a general normal line, FIG. 14 b shows thus obtained X-ray projection image, FIG. 14 c is a conceptual diagram when the teeth are seen from a normal line into a rising direction of the teeth, and FIG. 14 d shows thus obtained X-ray projection image.

[0144] Comparing each figure in FIG. 14 , X-rays are radiated in such a manner that an upper X-ray radiating direction, a projecting direction γU, and a lower X-ray radiating direction, a projecting direction γL become a nominal line direction into a rising direction of the upper tooth SU and the lower tooth SL respectively. X-ray projection images are produced based on thus obtained three dimensional X-ray absorption coefficient data, the length of each tooth is shown in approximately the same as an actual size and if the upper tooth and the lower tooth are overlapped, the overlapped part is eliminated on the image, achieving convenience for diagnosis. Otherwise, a radiography is executed from a direction orthogonal to a rotation axis of X-ray radiation and the X-ray projection image is shown by setting a projection plane parallel to an inclination of the tooth only for display.

[0145] FIGS. 15 a and 15 b are conceptual diagrams showing other example of X-ray radiation method in an X-ray CT method used for an X-ray CT apparatus for medical use according to the present invention.

[0146] As seen from the figures, if a rotation center 3 a of the rotary arm 3 , namely an axial direction of the rotation axis, is inclined for the object O like FIG. 15 a, or if the object O is inclined for the axial direction of the rotation axis 3 a of the rotary arm 3 which is a vertical direction, affect of obstacle shadows such as a jawbone is eliminated. In case of FIG. 15 b, the object O which doesn't rotate is inclined so that the construction of an object holding means (described later) which is an inclination means is simplified.

[0147] Thus the radiating direction is inclined, for projecting the obtained three dimensional X-ray absorption coefficient data, the projecting direction is aligned with the inclined direction, obtaining clearer X-ray projection images.

[0148] The flat plane perpendicular to the rotation center 3 a and produced by radiating and rotating conical X-ray beams 1 a as a central axis of the rotation center 3 a is called as a radiation plane.

[0149] FIG. 16 shows a conceptual diagram showing other example of X-ray radiation method in an X-ray CT used in an X-ray CT apparatus for medical use according to the present invention.

[0150] As a method for inclining a radiating direction, other than inclining the axial direction of the rotation center 3 a and inclining the object, the X-ray generator 1 and the X-ray detector 2 are inclined against the rotary arm 3 as shown in this figure. The same effect as mentioned above can be achieved.

[0151] FIG. 17 shows a conceptual diagram showing other example of X-ray radiation method in an X-ray CT used in an X-ray CT apparatus for medical use according to the present invention. In this method a rotation axis of a rotary arm executes precession.

[0152] As seen from FIG. 17 while the rotation axis, the rotation center 3 a of the rotary arm 3 executes precession, namely a grinding operation in such a manner that the center 3 a is rotated around a specific point against a specific rotation axis 3 b keeping a predetermined angle into a direction shown as an arrow, conical X-ray beams 1 a are radiated by turning the rotary arm 3 . Comparing with a vertical radiation or with a case a rotation axis is simply inclined, alternatives of a projection method of X-ray conical beams 1 a so as to avoid obstacle shades are increased so that the method can more suitably correspond to several kinds of obstacle shades.

[0153] In case of X-ray panoramic radiography, not a normal CT, X-rays are radiated with about 5 degrees of angle of elevation in order to alleviate the obstacle shades. Executing the same method for CT, X-ray transmitted images with less obstacle shades can be obtained and three dimensional X-ray absorption coefficient data with less obstacle shades can be obtained by backprojection based on the X-ray transmitted images.

[0154] Also in this case, when the projecting direction is aligned with the radiating direction executing precession, better X-ray projection images can be obtained.

[0155] FIG. 18 shows a basic construction of one example of an X-ray CT apparatus according to the present invention.

[0156] X-ray CT apparatus 20 in FIG. 18 is provided with an X-ray imaging means A, an X-ray beam width restriction means B, a rotary arm driving control means C, an arithmetic processing means D, a display means E, an object holding means 4 , a main frame 10 , an operation console 11 and an operation panel 12 .

[0157] The X-ray imaging means A has a rotary arm 3 from which an X-ray generator 1 and a two dimensional image sensor 2 opposing each other are suspended.

[0158] The X-ray generator 1 has an X-ray beam width restriction means B with a radiation control slit 8 and an X-ray beam controller 8 b in such a manner that X-ray beams radiated from an X-ray tube are controlled by the X-ray beam width restriction means B so as to emit conical X-ray beams 1 a or ortho-conical X-ray beams