Method and apparatus for adjusting technical exposure factors during radiographic acquisition

Abstract

A method and apparatus for acquiring a radiographic image of a patient includes using an X-ray source and at least a X-ray detector supported in opposed positions, the area under investigation being positioned between the X-ray source and detector in the propagation bundle of the radiation emitted by the X-ray source, the X-ray source and detector being movable along a pre-set trajectory due to at least two movements. The X-ray dose administered to a patient can be adjusted automatically, dispensing the minimum dose necessary to achieve a radiographic image of good quality. The method and apparatus provide for performing a scout acquisition, preceding the acquisition of a real panoramic image, in a particularly efficient way. During the scout acquisition, data are collected for adjusting the X-ray dose for the following radiographic image acquisition.

Claims

1. An apparatus for panoramic radiography comprising: an X-ray source projecting a collimated X-ray bundle through a patient; a bi-dimensional X-ray detector positioned to measure intensity of radiation after the radiation has crossed the patient; a C-arm on which said X-ray source and X-ray detector are fixed at opposed ends, a position of the C-arm being adjusted according to patient height with a mobile post; a mechanical system providing rotation and translation of the C-arm around the patient, such to acquire radiographic images from different positions; a device for positioning the patient; and an electronic system to control and synchronize operation of apparatus components, wherein the apparatus is configured to perform a method of patient specific, real time automatic adjustment of a technical exposure factor selected from the group consisting of X-ray tube power (kV), X-ray tube current (mA), exposure duration (sec), and film speed to acquire a radiographic image, the method comprising the following steps: performing a first (scout) acquisition in a specific anatomical area of a patient using pre defined parameters; extracting a region of interest (ROI) from the scout acquisition; developing a grey level histogram of image pixels from the ROI, segmenting the grey level histogram into a plurality of sectors, each sector corresponding to a gray level of a tissue typically found in a panoramic radiographic image; assigning a coefficient to each sector; assigning a weight to each coefficient; computing an exposure parameter according to the weight assigned to each sector; and performing an exposure of a real acquisition using the exposure parameters.

2. The apparatus according to claim 1, wherein the plurality of sectors are five sectors, and wherein the exposure of the real acquisition is computed as follows: [kV, mA, sec, filmspeed]=f(K.sub.1*W.sub.1, K.sub.2*W.sub.2, K.sub.3*W.sub.3, K.sub.4*W.sub.4, K.sub.5*W.sub.5), and wherein: K =the coefficient of each sector W =the weight of each coefficient.

3. The apparatus according to claim 2, wherein: W1=Num(Seg/Num(totROI), and wherein: Num(Segi) =number of pixels in sector i Num(totRoI) =number of pixels in the region of interest.

4. A method of acquiring a radiographic image of a patient with an apparatus comprising a X-ray source and a X-ray detector, the X-ray source and the detector being disposes disposed in opposed positions, a region of interest of the patient being positioned in an intermediate position between the X-ray source and the detector and within a propagation bundle of a radiation emitted by the X-ray source, wherein the X-ray source and the detector are movable along a pre-set trajectory due to at least two movements: a rotational movement of the X-ray source and the detector around a rotation axis, and a translational movement of the rotation axis along a Y-direction in a horizontal plane perpendicular to the rotation axis, or a rotatory movement of the rotation axis around a second axis of rotation perpendicular to a plane parallel to the rotation axis, wherein, due to a combination of the at least two movements, a virtual rotation center which is an instant rotation center of the X-ray source and the detector moves during each acquisition, the method comprising: acquiring a scout image by acquiring a pre-exam image; and subsequently acquiring a real image, wherein, for acquiring the scout image, the rotational movement of the X-ray source and the detector around the rotation axis occurs in opposite directions with respect to a rotation direction of the X-ray source and the detector around the rotation axis during exposure for acquiring the real image.

5. The method according to claim 4, wherein the X-ray source and the detector perform three combined movements, namely, the rotational movement around the rotation axis, a translational movement in the plane perpendicular to the rotation axis along an X-direction, the translational movement in the plane perpendicular to the rotation axis along the Y-direction different from the X-direction, or a combined translational movement according to two different directions along a curved line due to a rotational movement of the rotation axis around the second axis of rotation parallel to the rotation axis.

6. The method according to claim 4, wherein the step of acquiring the real image occurs immediately after the step of acquiring the scout image without repositioning of the patient.

7. The method according to claim 4, wherein image acquisition utilizes full-frame technology.

8. The method according to claim 4, wherein image acquisition utilizes Time-Delay Integration (TDI) technology.

9. The method according to claim 4, wherein the real image is a bi-dimensional image or a volumetric three-dimensional image of a patient's maxilla and mandible.

10. The method according to claim 4, wherein: the step of acquiring a scout image comprises, using pre-defined parameters; extracting a region of interest (ROI) from the scout image; developing a grey level histogram of image pixels from the ROI, segmenting the grey level histogram into a plurality of sectors; assigning a coefficient to each sector; assigning a weight to each coefficient; computing an exposure parameter according to the weight assigned to each sector; and the step of acquiring the real image is performed using the exposure parameters.

11. A method of acquiring a radiographic image of a patient with an apparatus comprising a X-ray source and a X-ray detector, the X-ray source and the detector being disposes in opposed positions, a region of interest of the patient being positioned in an intermediate position between the X-ray source and the detector and within a propagation bundle of a radiation emitted by the X-ray source, wherein the X-ray source and the detector are movable along a pre-set trajectory due to at least two movements: a rotational movement of the X-ray source and the detector around a rotation axis, and a translational movement of the rotation axis along a Y-direction in a horizontal plane perpendicular to the rotation axis, or a rotatory movement of the rotation axis around a second axis of rotation perpendicular to a plane parallel to the rotation axis, wherein, due to a combination of the at least two movements, a virtual rotation center which is an instant rotation center of the X-ray source and the detector moves during each acquisition, the method comprising: acquiring a scout image by acquiring pre-exam image; and subsequently acquiring a real image, wherein, for acquiring the scout image, the rotational movement of the X-ray source and the detector around the rotation axis occurs in opposite directions with respect to a rotation direction of the X-ray source and the detector around the rotation axis during exposure for acquiring the real image, and wherein the steps of acquiring a scout image and a real image are each performed with the virtual rotation center) in a different position with respect to the X-ray source, the detector, and the patient under investigation.

12. The method according to claim 11, wherein, during the step of acquiring a scout image, the virtual rotation center is beyond the detector on a side opposed to the X-ray source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and properties of the present invention are disclosed in the following description, in which exemplary embodiments of the present invention are explained in detail based on the drawings:

(2) FIG. 1: Exemplary panoramic radiographic apparatus;

(3) FIG. 2: Histogram of grey levels of a typical panoramic radiographic image;

(4) FIG. 3: Histogram of grey levels of a radiographic image to perform the method of the present invention;

(5) FIG. 4: Flow diagram of the known art;

(6) FIG. 5: Flow diagram of the present invention;

(7) FIG. 6: Anatomic area for performing a scout acquisition.

(8) FIGS. 7A-7D: Movement steps of the rotating arm supporting X-ray source and detector during the acquisition of an overall scout+panoramic image according to the present invention, wherein FIG. 7A depicts the rotating arm at reset position, FIG. 7B depicts the rotating arm at scout position, FIG. 7C depicts the rotating arm at exam start position, and FIG. 7D depicts the rotating arm at exam position;

(9) FIGS. 8A-8B: Movements of said rotating arm in a full-frame acquisition system, wherein FIG. 8A depicts a small portion of an acquisition and FIG. 8B depicts a small portion of an acquisition with inverted direction of detector reading;

(10) FIGS. 9A-9B: Movements of said rotating arm in a TDI acquisition system, showing in FIG. 9A that one reading direction is possible and in FIG. 9B that the opposite direction is not possible;

(11) FIGS. 10A-10C: Movements of said rotating arm which are possible by shifting the virtual center of rotation. wherein in FIG. 10A the virtual center of rotation is at a first point, in FIG. 10B is at a second point, and in FIG. 10C is at a third point;

(12) FIGS. 11A-11D: Movement steps of rotating arm during the acquisition of an overall scout+panoramic image in a TDI acquisition system according to the present invention, wherein in FIG. 11A the virtual center of rotation is at reset position, in FIG. 11B is at a first point, in FIG. 11C is at a second point, and in FIG. 11D is at a third point.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(13) FIG. 1 shows a typical panoramic radiographic apparatus 1, comprising an X-ray source 2 projecting a collimated X-ray bundle across a patient (not shown), a bi-dimensional X-ray detector 3 positioned to measure the intensity of radiation after it has crossed the patient, a C-arm 4 on which said X-ray source 2 and detector 3 are fixed at opposed ends, a mechanical system 5 rotating and translating said support around the patient, in order to acquire radiographic images from different positions; and an electronic system (not shown) to control and synchronize the working of the various apparatus components. Panoramic apparatus 1 comprises moreover a device 6 for positioning the patient, in FIG. 1 consisting of a bite and supports for positioning the patient's skull. The position of C-arm 4 can be adjusted according to patient height due to vertically mobile post 7.

(14) FIG. 2 shows a histogram of grey levels of a typical panoramic radiographic image. The tissues crossed by X-rays attenuate X-rays in a way proportional to the atomic density of the tissue. As a consequence, given an input X-ray radiation, the denser tissue will absorb a higher quantity of radiation with respect to the less dense tissue. Therefore, in the first case a lesser quantity of X-ray radiation will impinge on the X-ray sensor with respect to less dense tissue. In practice, a black-and-white image will be obtained, ranging from pure white to pure black, passing through a gradation of intermediate grey's. The number of intermediate grey levels depends on the quality of the acquisition system. Typically, present day digital acquisition system are designed with 16 bits and therefore can show 2.sup.16 grey levels.

(15) Any digital radiographic image consists of a pixel matrix, each of which has a grey level determined by the quantity of X-rays attenuated by the passage through the patient's tissues corresponding to the image pixels. Starting from grey levels, other kinds of more or less complex attenuation measure, such as e.g. Hounsfield Unit (HU), or conversion of grey scale to a polychromatic scale, can be derived. A method according to the present invention can anyway be analogously applied to all cases, in which the image is codified as a set of small units, each characterized by one or more values describing radio opacity. A person of skill in the art can use and combine the features of this invention in any different way that technology and knowledge allow. In general, for each radiographic image a histogram like that of FIG. 2 can be represented, bearing grey levels on an X-axis and the number of pixels having that grey level on a Y-axis.

(16) In the exemplary histogram shown in FIG. 2, 10,000 pixels with a grey level of 3,500 are present. The histogram of the image is a way to show the quality of the radiographic image. If the pixels were present only in a fraction of grey levels, an image having insufficient contrast would result, where distinguishing among the different kinds of tissue would be difficult. Vice versa, a histogram showing a wide distribution of grey levels corresponds to a radiographic image where different grey levels are assigned to the different kinds of tissue, and, therefore, tissues are easily distinguishable from each other.

(17) In FIG. 3 a histogram prepared for use in a method according to the present invention is shown.

(18) The grey levels (0 to 16,400) in the present example are subdivided into five sectors, each corresponding to the grey levels of a tissue typically represented in a panoramic image. For example, the values attributable to each sector of FIG. 3, assuming that the intensity of grey levels is a way to represent the radio-opacity of tissues, are:

(19) Sector 1 [0-800]: metal and prosthesis

(20) Sector 2 [800-3000]: hard bone

(21) Sector 3 [3000-6000]: spongy bone

(22) Sector 4 [6000-10000]: soft tissues

(23) Sector 5 [10000-16400]: airways.

(24) The ends identifying a sector can be both fixed (static segmentation) and variable (dynamic segmentation). For example, the segmentation can be correlated to particular trend lines of the histogram, or to minimum/maximum of a polynomial approximation of the histogram, etc.

(25) The same sectors can have overlapping regions and not necessarily be adjoining to each other. The known types of mathematical/numerical analysis to achieve histogram segmentation can be different and of different complexity. In the present embodiment a static segmentation in sectors without overlapping was chosen.

(26) In FIG. 4 the flow diagram of user operations, i.e. the material actions that the operator must perform to acquire a panoramic image, and the flow diagram of the operations performed by an apparatus in the known art, are shown.

(27) In FIG. 5 the flow diagram of user operations, the flow diagram of the operations performed by the apparatus, and the flow of the algorithm steps for computing the technical exposure factors according to the present invention are shown. Such diagram highlights, with respect to FIG. 4:

(28) The absence of additional steps for operator/patient in the acquisition flow with respect to the known art;

(29) The integration of the method in the operative flow of the apparatus;

(30) The description of algorithm steps.

(31) The method comprises the following steps:

(32) A. Acquisition of a patient's scout image:

(33) The positioning of the patient is the standard positioning for a panoramic acquisition, so that the patient remains in the same position even during the real acquisition (meaning the acquisition of the panoramic image).

(34) B. Extraction of a region of interest (ROI) from the scout image.

(35) C. Construction of a histogram of grey levels of image pixels, similar to that of FIG. 2.

(36) D. Segmentation of the image according to a number of sectors:

(37) In the preferred embodiment shown in FIG. 3, the number of sectors corresponds to five pre-set kinds of tissue, see above. Performing a segmentation according to different criteria is nonetheless possible. For example, the histogram can be cut only for the values that are typical for metal, in order to study this portion of the image. Alternatively, it is possible to detect the outline of the tissues having a very different radio opacity, thereby identifying the sectors of the histogram where a high gradient of grey levels is present.

(38) E. Assignment of a coefficient to each sector:

(39) The coefficient is a number representative of its sector. The value of such coefficient can be pre-set or depend on the information present in scout acquisition. In the present case, referring to the histogram of FIG. 3, the coefficient of metal portion is a number given by the average of the values belonging to metal sector, and hard bone is the average of the values of hard bone sector; the same applies to the other sectors.

(40) Example: Ki=arithmetic average of grey levels of the pixels belonging to each sector of interest. The values computed for each sector are:

(41) K1:400; K2:2150; K3:5300; K4:8600; K5:15150.

(42) F. Assignment of a weight to each coefficient:

(43) The weight can be assigned to the coefficient by a human operator before performing the scout acquisition, or at the installation of the apparatus, according to diagnostic needs. In the preferred embodiment, the coefficient weight is pre-set by the manufacturer, based on the fact that the kinds of tissue which can be found in a panoramic image are known. Always referring to FIG. 3 as a histogram example, we can set: W1=W5=0, because optimizing the exposure for metal or airways is useless. Typically, in a panoramic image hard bone is important, but not overexposing other tissues is also important.

(44) In the preferred embodiment, the definition of weights is based on how many pixels belong to each kind of tissue in the entire scout image, assuming that the scout acquisition is a representative sample of the group of tissues to be evaluated.

(45) In the example of FIG. 3, sectors 2, 3, 4 corresponding to hard bone, spongy bone and soft tissues will be considered only. For each sector weights W2, W3, W4 are computed, to be assigned to coefficient K2, K3, K4, comparing the number of pixels belonging to each sector and the total number of scout pixels.

(46) For instance:
Wi=Num(Segi)/Num(totROI);

(47) NB:Num(A)=number of pixels A; totROI is the overall number of ROI (Region of Interest) pixels;

(48) W2=0.45, W3=0.3. W4=0.25.

(49) Nothing prevents assigning weights according to other criteria, even independent from ROI informative content, but e.g. based on statistical or clinical elements (for edentulous patients assigning a higher weight to soft tissues might be advantageous, so as to make gum line more apparent).

(50) G. Computation of technical exposure factors of the real acquisition:

(51) The computing of real exposure is a result of a function of product K.sub.i*W.sub.i of each sector, i.e. mathematically:
[kV, mA, sec, filmspeed]=f(K1*W1, K2*W2, K3*W3, K4*W4, K5*W5).

(52) In the preferred embodiment, only kV are modulated with a weighted average of coefficients according to their weight. Defining more complex functions modifying the other technical exposure factors is nonetheless possible (mA, sec):

(53) $f\left(x\right)=\{\begin{array}{cc}\mathrm{mA}=5& \left(\mathrm{fixed}\right)\\ \sec =9& \left(\mathrm{fixed}\right)\\ \mathrm{filmspeed}=10000& \left(\mathrm{fixed}\right)\\ \mathrm{kV}=\left(a*K2*W2+b*K3W3+c*K4*W4\right)/& \\ \left(K2*W2+K3*W3+K4*W4\right)=74.7\mathrm{kV}& \end{array}$

(54) In the example of FIG. 3, a, b, c are values of kV suitably chosen based on the tissue to be irradiated. In a typical X ray tube in a panoramic apparatus, the value of kV can vary in a range of 60-80 kV.

(55) The creation of the ROI histogram is a non-onerous operation, in that finishing the scout acquisition is not necessary to perform the extraction itself. The extraction can, as a matter of fact, occur while the sensor pixels are read. The method is based essentially on simple arithmetic calculations, which can be performed by the electronic system of the apparatus in the time interval between the end of scout acquisition and the beginning of the real panoramic acquisition, without the need of big memories or computing power, beyond those anyway necessary for the normal managing of panoramic images.

(56) It is apparent that the present method of adjustment of technical exposure factors can be usefully employed in any kind of radiologic apparatus.

(57) The preferred embodiment, as said above, relates to a panoramic radiographic apparatus. In this specific case, the area where the scout image is acquired is very important. The best results were obtained by acquiring the scout image in the area highlighted by the square in FIG. 6: an area comprising a portion of the ascending ramus of the mandible, including as well distal molars (seventh, eighth teeth), comprising as well a portion of the maxilla. In this way the three kinds of tissue (hard bone, spongy bone and soft tissues) are present in the scout and, as a consequence, in the histogram, and therefore differentiating the histogram in the three meaningful sectors for the calculation of technical exposure factors is possible.

(58) An important advantage of the present invention is that the calculation and the consequent adjustment of technical exposure factors occur completely automatically, without any action by the operator, who must only suitably position the patient in the positioning device.

(59) With regard to scout acquisition, some clarifications are needed. The scout exposes a limited portion of the patient. Supposing that the time necessary for acquiring the patient's whole dental arch is 100, the scout is performed exposing the patient for a time range of 1-8. All this occurs with the same technical exposure factors, therefore, the X-ray dose administered to the patient for the scout is a small fraction of the total administered dose. This dose increase is justified because the total dose is then tailored on that specific patient, with a suitable cost/benefit ratio, in addition to preventing the risk of re-takes. In this way an X-ray dose with the technical exposure factors typical of the acquisition of real panoramic images is administered for a limited time. This is advantageous also for the entire acquisition time, which is only slightly prolonged.

(60) In an alternative embodiment, administering an equivalent X-ray dose using a reduced current for a more prolonged time is possible, with equal X-ray dose.

(61) In the preferred embodiment, scout acquisition can be considered as a fraction of the acquisition of the real panoramic image under every point of view: patient positioning, X-ray dose, time. It is nonetheless possible acquiring a scout image with a specific collimation and trajectory.

(62) Because ROI extraction occurs at the same time as scout acquisition, and scout acquisition covers a small portion (1-8%) of the time of real panoramic acquisition, the acquisition of the real panoramic image is prolonged of a very small fraction of time. Moreover, the acquisition occurs by positioning the patient in panoramic apparatus 1 through positioning devices 6, while scout and real panoramic acquisition occur one immediately after the other. This makes the acquisition comfortable both for patient and operator.

(63) A further aspect of the present invention, which might be provided separately or in combination with the above described features in relation to the embodiment of FIGS. 1 to 6, is described in FIGS. 7 to 12.

(64) In the present description, the words detector or sensor are equivalent, meaning a unit receiving the radiation transmitted through the patient, and transforming the intensity of said radiation into electric signals corresponding to the intensity of said radiation. The mechanical system 5 is provided with at least two, possibly three, different degrees of freedom of movement, which allow the rotation of C-arm 4 around an axis R perpendicular to the longitudinal extension of the arm, and intermediate between X-ray source 2 and detector 3, and the translation of said axis of rotation R according to at least one, possibly two, different directions X, Y in a plane perpendicular to said axis of rotation R of C-arm 4.

(65) Generally, in present day digital panoramic apparatuses, before exposure, the rotating group, consisting of C-arm 4 and X-ray source 2 and detector 3, is positioned in the so-called angular patient entry position, i.e. the position wherein, according to the mechanical structure of the apparatus, patient entry into the compartment below the arm between X-ray source and detector, and the access into patient positioning device 6, are allowed.

(66) In the present exemplary embodiment, this position is reached by positioning C-arm 4 with its longitudinal axis perpendicular to patient positioning device 6, i.e. to the anteroposterior axis of patient's skull positioned in said patient positioning device 6, in order to allow a comfortable access of the patient to the positioning device.

(67) From this position, which can be called RESET, C-arm 4 must move to EXAM START position, i.e. to the point wherein X-ray emission starts, with the consequent acquisition of patient radiographic image. In most exams these two positions (RESET and EXAM START) do not coincide.

(68) In a panoramic acquisition, in particular, image acquisition and therefore exposure starts while the rotating arm 4 is positioned so that X-ray sensor 3 is behind the patient's condyle, and the X-ray tube, i.e. X-ray generator 2, is angularly displaced of about 30-55 with respect to RESET position.

(69) For good ergonomics, the scout acquisition should be performed so that the trajectory of rotating arm 4 during scout acquisition starts from RESET point and ends in the EXAM START position.

(70) FIG. 7 shows in a very stylized way X-ray source 2 and digital bi-dimensional X-ray detector 3 united by C-arm 4; the system rotates around patient 11.

(71) FIG. 7A shows the typical RESET position. From here the assembly X-ray source 2, detector 3 and C-arm 4 of apparatus 11 must go to the position EXAM START shown in FIG. 7C, passing through the position shown in FIG. 7B.

(72) In FIG. 7B, the sense of rotation of C-arm 4 is clockwise, as shown by the curved arrow. C-arm 4 rotates angularly displacing X-ray source 2 and detector 3 around patient 11 to bring the C-arm 4 and said X-ray source 2 and detector 3 from the position as in FIG. 7A to the position as in FIG. 7C. A key feature of the invention relates to exploiting the movement shown in FIG. 8B to acquire a scout image.

(73) FIG. 7D shows the acquisition of the real panoramic image, performed in a direction opposed to that of the scout acquisition. In the present embodiment, the real acquisition is performed anti-clockwise, as shown by the curved arrow. In FIG. 7B the dotted line indicates the position of C-arm 4 in RESET position, and therefore allows visualizing the movement that C-arm 4 has performed during scout acquisition. In FIG. 7D the dotted line indicates the position of C-arm 4 at exam start, and allows therefore visualizing the movement that C-arm 4 performs during the acquisition of the real panoramic image.

(74) Any type of bi-dimensional digital detector can be represented as an ordered set of columns, each formed by X-ray photosensitive elements.

(75) The two main acquisition technologies known in the art are:

(76) Full-frame systems, typical, but not exclusive, of C-MOS detectors. In full-frame systems, all detector columns are exposed at the same time, and at pre-set periods of time are read all together, according to a given order. Once they have been read, their information content is cancelled. The final image is obtained reconstructing the information read in each frame;

(77) Time-Delay Integration (TDI), typical, but not exclusive, of CCD detectors. In TDI systems all detector columns are exposed at the same time, and at pre-set period of times only one of the two columns at the ends of the detector is read, which is called read-out column. The information content of all non-read columns is shifted to the adjacent column on the right or on the left, depending on detector setting, summing it up to information content already present in that column. In this way, detector columns, during the shifting between the first position and read-out position, gather information content minimizing the quantity of X-ray dose administered to patient. The final image is obtained reconstructing the information read in each column.

(78) The following description relates to two different embodiments, each using a different bi-dimensional digital detector, C-MOS and CCD, the implementation of which poses different challenges.

(79) The first embodiment makes use of full-frame technology, coupled with a C-MOS detector. Although this methodology of acquisition is more onerous under the point of view of both hardware and final image reconstruction, it does not pose limitations neither in the read-out direction of columns, and therefore nor in the rotation direction of the system around patient.

(80) In FIG. 8A a small portion of an acquisition is shown. As known, a panoramic is formed by successive acquisitions performed by the digital detector, which are then processed to provide the image of a whole patient's arch. In FIG. 8A, the dotted line indicates the starting position of C-arm 4 at time to, while the continuous line indicates the successive position of C-arm 4 at time t.sub.1. Between to and ti the acquisition of a small anatomic portion occurs, represented by segment AB, which is projected on sensor 3 in the positions A and B, respectively.

(81) The curved arrow indicates the rotation direction of C-arm 4, while the bold linear arrow indicates the reading direction of detector, which must be coherent with the rotation direction of exposure.

(82) Since C-arm 4 of the present invention has three degrees of freedom of movement, i.e. rotation around axis R and translation in the two directions X, Y, the virtual center of rotation, represented in the Figure with CVR, does not necessarily correspond to the axis of rotation R of C-arm 4, but the center of rotation R can be translated during its rotation in the plane perpendicular to rotation axis in the directions X, Y. This leads to the formation of a virtual center of rotation CVR for all or part of overall acquisition trajectory. System controlling two or more axes can generally move CVR during acquisition so as to obtain complex trajectories.

(83) Here the term virtual center of rotation (CVR) has the meaning of instant center of rotation as already defined above.

(84) FIG. 8B shows the small portion of acquisition of FIG. 7A, wherein the direction of rotation and direction of detector reading are inverted with respect to FIG. 8A, as shown by the curved arrow.

(85) Full-frame technology for C-MOS systems allows to read detectors indifferently in both directions, and therefore this embodiment does not have particular difficulties in its implementation.

(86) The second embodiment makes use of TDI technology coupled with a CCD X-ray detector.

(87) With respect to the first embodiment using a C-MOS detector, the read-out direction of a CCD detector is pre-set by the manufacturer of the detector. In order to acquire the same AB object by inverting the direction of trajectory, artifices must be used for maintaining the same read-out direction of detector, while performing the rotation for scout acquisition in the direction opposite to pre-set reading direction of detector. FIGS. 9A and 9B show that one reading direction is possible, the opposite is not.

(88) The artifice consists in using trajectories of movement of the assembly comprising C-arm 4, X-ray source 2 and detector 3 wherein the virtual center of rotation CVR shifts with respect to the position shown in FIG. 9A. Different configurations are possible, which are shown in FIGS. 10A, 10B, 10C.

(89) FIG. 10A shows the shifting of position of the virtual center of rotation (CVR) beyond object (AB), but in a point situated between the object and X-ray source 2.

(90) FIG. 10B shows the shifting of the position of the virtual center of rotation (CVR) beyond the detector (AB).

(91) FIG. 10C shows the shifting of the position of the virtual center of rotation (CVR) beyond object (AB), out of the interval X-ray detector-source.

(92) The configurations shown in FIG. 10A and 10C are of purely theoretical interest, in that the reading direction of detector is still forbidden.

(93) Instead, the configuration shown in FIG. 10B is usable in that the read-out direction of detector continues to be the in pre-set direction of the detector. Moreover, in the configuration shown in FIG. 10B, the final arm 4 position is the most compatible with the position of exam start.

(94) The acquisition of a whole panoramic image is shown in FIG. 11, with FIG. 11A depicting a reset position.

(95) During scout acquisition a complex trajectory is performed, wherein the virtual center of rotation (as defined above) moves in the neighborhood of point CVR shown in FIG. 11B, while during the acquisition of the real panoramic image the virtual center of rotation describes a complex trajectory in the neighborhood of point CVR shown in FIG. 11D. As it is apparent in comparing FIGS. 11B and 11D, during scout acquisition CVR is beyond X-ray sensor 3, on the opposite side to X-ray source 2, while during real panoramic acquisition CVR is between the object to be acquired and X-ray sensor 3.

(96) It is apparent that the definition of the virtual center of rotation is not free from consequences on the magnifying factor of the image, and therefore the different magnifying factors will have to be taken into account in the scout with respect to the real acquisition, or positions of CVR will have to be chosen so as to have the same magnification.

(97) The setting of the virtual center of rotation (CVR) according to the configuration of FIG. 10B allows bringing back the acquisition of scout and real image to what is shown in FIG. 7, i.e. the direction of scout acquisition is opposed with respect to acquisition direction of real panoramic image.

(98) Basically, FIG. 11 corresponds to FIG. 7, wherein the acquisition is partitioned in four stages t.sub.0, t.sub.1, t.sub.2, t.sub.3 and therefore the object to be acquired is not the whole dental arch indicated in FIG. 7 with 11, but the successive small anatomic portions AB acquired at each time by detector 3.

(99) In FIG. 11 it is apparent that each final point of C-arm 4 in a given step, which in the figures is indicated with dotted line, corresponds to the starting point of the successive step. As a consequence, all the steps (RESET, SCOUT, EXAM START and EXAM) can be performed one after the other, minimizing the acquisition time of the whole panoramic image, formed by scout plus real acquisition.

(100) The present description always referred to the preferred embodiment, for the acquisition of a panoramic image of a patient; it is however apparent to the skilled person that this method for acquiring a scout before the acquisition of a real panoramic image can be advantageously used also for acquisitions according to different trajectories, e.g. performing acquisitions on condyles, maxillary sinuses, small arch portions.

(101) Moreover, the same method can be used in cone beam apparatuses for the acquisition of volumetric three-dimensional images (Cone-Beam Computerized Tomography, CBCT).