Patent Number: 
Section: description

The computed tomography apparatus shown in FIG. 1 includes a gantry 1 which is capable of rotation about an axis of rotation 14; to this end, the gantry 1 is driven by a motor 2. A radiation source S, for example, an X-ray source, is mounted on the gantry 1. Using a first diaphragm device 32, first a conical radiation beam 40 is generated, said beam being incident on a second diaphragm arrangement 31, being a so-called slit diaphragm, which forms a fan-shaped radiation beam 41, that is, a so-called primary fan beam, therefrom. The primary fan beam 41 extends perpendicularly to the axis of rotation 14 and, because of the small slit width, it has only small dimensions in the direction of the axis of rotation 14 (for example, 1 mm). The primary fan beam 41 is then incident on a modulation unit 33 which temporally and spatially modulates the radiation, thus yielding a modulated primary fan beam 42. The modulated primary fan beam 42 penetrates a cylindrical examination zone 13 in which, for example, a patient on a patient table (both not shown) or a technical object may be present so as to be examined. After having traversed the examination zone 13, the fan beam 42 is incident on a two-dimensional detector array D which is mounted on the gantry 1 and includes a plurality of detector elements which are arranged in the form of a matrix. The detector elements are arranged in rows and columns, the columns extending parallel to the axis of rotation 14 while the rows extend in planes perpendicular to the axis of rotation 14, for example, along an arc of a circle around the radiation source S. Generally speaking, the detector rows comprise significantly more detector elements (for example, 1000) than the detector columns (for example, 16). The primary fan beam 42 is oriented in such a manner that it is incident on the central detector row of the detector array D which is denoted by shading in FIG. 1. A linear movement of the examination zone 13 along the axis of rotation 14 under the influence of the motor 5 can be superposed on the rotary movement of the gantry 1, resulting in a helical scanning motion of the radiation source S and the detector array D. When a technical object is to be examined, the gantry 1 may also be stationary and the object may be rotated around the axis of rotation 14. The measuring data acquired by the detector array D is applied to an image processing computer 10 which reconstructs the desired images or evaluates the measuring data in another manner. The reconstructed images or other data determined can be displayed on a display screen 11. The image processing computer 10 is controlled by a control unit 7, like the motors 2 and 5. FIG. 2 shows a detail of the computed tomography apparatus shown in FIG. 1. Starting from the X-ray source S, the first diaphragm 32 forms a conical X-ray beam 40 which is subsequently incident on the slit diaphragm 31. From this diaphragm primary radiation 41 emerges in the form of a fan beam which is also incident on the modulation unit 33 so that the desired modulated primary fan beam 42 is generated; this beam is subsequently incident on the examination zone which is not shown herein. The slit of the slit diaphragm 31 has a small dimension of, for example, only 1 mm in the w direction. The slit, however, is significantly wider in the v direction. This dimension can also be characterized by an angle "PHgr" which characterizes the angle between the direct connecting line 20 between the focal point of the X-ray source S and the center of the detector D or the center of rotation of the computed tomography apparatus and a primary beam 21 of the primary radiation beam 41. The angle variable "PHgr" satisfies the condition xe2x88x92"PHgr"fan/2xe2x89xa6"PHgr"xe2x89xa6"PHgr"fan/2, where "PHgr"fan/2 corresponds to half the angle of the primary fan beam 41. The modulation unit 33 is configured in such a manner that its transmission factor T("PHgr"t) varies periodically as a function of the angle "PHgr" and with the time t, be it that the condition 0xe2x89xa6T("PHgr",t)xe2x89xa61 is always satisfied. The primary fan beam 41 is thus spatially and temporally modulated; this offers special advantages for the evaluation of the scattered radiation measured by the detector array D as will be described in detail hereinafter. For example, the transmission factor T of the modulation unit 33 may be chosen in conformity with the following rule: T("PHgr",t)=A0 sin(A1xc2x7"PHgr"+A2xc2x7t).  As can be clearly seen from this rule, in the case of a fixed angle "PHgr" the transmission varies periodically as a function of time at a frequency A2/(2xcfx80). The phase of this transmission rule is repeated even at points of the v axis along the slit of the slit diaphragm 31, that is, each time after an interval xcex94"PHgr"=2xcfx80/A1. The variable A1 should, therefore, have values of 2xcfx80n/"PHgr"fan, where n represents a positive integer value. The significance of the choice of the parameter n will be explained in detail hereinafter. The FIGS. 3 and 4 show a feasible embodiment of the modulation unit 33 which satisfies the above conditions. FIG. 3 is a side elevation of such a modulation unit 33 from the same perspective as FIG. 2. In a radiation transparent housing 34 there is shown a shaft 37 which extends along a modulation axis 16 extending in the v direction. On the shaft 37 two diaphragm elements 35, 36 are arranged so as to be diametrically oppositely situated, said elements extending helically around the shaft 37 in the v direction. These diaphragm elements 35, 36 may be compared, for example, with two threads of a screw which are arranged so as to be rotated 180xc2x0 relative to one another while commencing from the same point along the v axis. FIG. 4 shows cross-sections of the modulation unit 33 along the planes E1, E2, E3, E4 shown in FIG. 3, that is, planes extending perpendicularly to the v axis. The primary beam 43 is represented by a dotted line therein and extends through the modulation axis 16. Inside the housing 34 the diaphragm elements 35, 36 are shown in a cross-sectional view, the Figure showing different cross-sections in an adjacent fashion. The shaded position of the diaphragm elements 351, 361 represents the cross-section of the diaphragm elements 35, 36 in the plane E1. The further positions 352, 353, 354 and 362, 363, 364 represent the respective cross-section of the diaphragm elements 35, 36 in the planes E2, E3, E4. Thus, a linear relationship exists between the illustrated cross-sections of the diaphragm elements 35, 36 and the angle "PHgr" as shown in FIG. 2; this means that, in dependence on the angle "PHgr" considered, the diaphragm elements 35, 36 occupy a different angular position in a cross-sectional view as shown in FIG. 4. The diaphragm elements 35, 36 are preferably made of a material such as aluminium. Using a motor (not shown), the diaphragm elements 35, 36 or the shaft 37 can be driven in such a manner that the diaphragm elements 35, 36 rotate around the modulation axis 16 with a known phase and a constant angular speed in such a manner that the primary radiation is temporally encoded at a fixed angle "PHgr", the phase of the modulation being known and linearly proportional to the angle "PHgr". It is to be noted that the embodiment of the modulation unit 33 shown could be compared with the cutting device of a hand-operated lawn mower provided with two diametrically arranged cutting blades extending helically around the axis of rotation of the cutting device. The thickness of the diaphragm elements 35, 36 is chosen to be such that a sinusoidal transmission is obtained when they are rotated. For example, for the variable A0 of the above transmission rule the value 0.495 could be chosen and the value 0.505 for the variable A2. The modulation unit shown in the FIGS. 3 and 4 realizes a temporal modulation of the intensity of the primary radiation 41 where the phase of the modulation is linked to the position along the fan beam 41. The gantry 1 rotates for the acquisition of measuring values, so that the detector elements of the detector array D detect the primary radiation and the scattered radiation from a plurality of angular positions. The detector element or elements at the center of each detector column detects (detect) the primary radiation whereas the scattered radiation (secondary radiation) is detected by the detector elements which are situated further outwards in each column. The momentum transfer, whose spectrum is to be reconstructed as a function of the location u, v, is known to be the product of the energy of the scattered X-ray quanta and the sine of half the scatter angle. In order to enable the momentum transfer to be determined, on the one hand the scatter angle must be known and on the other hand the energy of the scattered X-ray quantum. The scatter angle is given by the position of the detector element and the position of the point in the primary fan beam in which the scatter process has taken place. The energy of the scattered X-ray quanta must either be measured, implying that the detector elements should be capable of energy-resolved measurement, or use must be made of X-rays with quantum energies from an as small as possible range (monochromatic X-rays in the ideal case). In order to reconstruct the location-dependent momentum transfer spectrum, it is first necessary to carry out a phase-sensitive detection as described, for example, in D. C. Champeney xe2x80x9cFourier transforms and their physical applicationsxe2x80x9d, Academic Press, 1973, for the scattered signal arriving at the detector. To this end, a cross-correlation is performed between the scatter signal detected by a given detector element, that is, the scattered radiation measured by a given detector element, and the sinusoidal modulation signal generated by the modulation unit for the corresponding segment of the primary fan beam. This segment of the primary fan beam is situated in a plane which is determined by the focus of the X-ray source and the detector column in which the relevant detector element is situated. Because the scatter signal is always in phase with the primary beam wherefrom the associated scatter has originated by scattering on an object, the cross-correlation always produces a positive result, whereas a cross-correlation of two signals which are not in phase tends towards zero. The coherent scatter of a pencil-shaped beam has the highest intensity value in the case of a small scatter angle and tends towards zero as the scatter angle increases. When a given detector element is considered, therefore, the contributions of scatter from segments of the primary fan beam decrease when the scatter angle of scatter incident on the relevant detector element increases. For scatter angles larger than 10xc2x0, therefore, no coherent scatter can reach a detector element from neighboring segments of the primary fan beam. Therefore, the primary fan beam can be subdivided into a given number of, for example, five sections (as shown in FIG. 2), each section having an identical phase characteristic. In this case the above parameter A1 of the transmission rule would have the value 2xcfx80n/"PHgr"fan, where n=5. In addition to the described possibilities for modulation, other possibilities are feasible. It is to be ensured merely that the temporal modulation regularly varies at each fixed point of the primary fan beam and that the phase of the temporal modulation varies continuously along the primary fan beam. FIG. 5 shows a further embodiment of a modulation unit 33. The modulation unit therein has a housing 70 which is made of a material having a thickness such that the incident X-rays are practically completely absorbed. One of the hollow shafts 71 is connected to a motor in order to enable rotation of the modulation unit 33 around the modulation axis 16. In the housing there are provided two helical slits 72, 73 which are mutually offset, extend around the axis of modulation 16 and allow passage of the X-rays. The inclinations of the slits 72, 73, the number of turns, their axial length and width as well as their relative positions are adapted to the desired modulation and are shown merely by way of example herein. Such a modulation unit is also capable of achieving the desired modulation of the primary fan beam. The primary fan beam is thus decomposed into a kind of xe2x80x9ccombxe2x80x9d function as shown in FIG. 6 where the xe2x80x9cteethxe2x80x9d P of the xe2x80x9ccombxe2x80x9d extend along the axis v in a regularly recurrent fashion. It is also to be noted that one or more lamellas of a collimator array as described in the European patent application 01200652.4 may also be provided between the object to be examined and the detector array. The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.