DE3817148C2 - Computer tomograph with periodic focus deflection - Google Patents

Description

The invention relates to a computer tomograph with a Rotating frame enclosing the measuring opening, on which one of a Row of detector elements and a radiation detector X-ray tube for a fan-shaped, on the radiation detection tor impinging X-ray beam are arranged and which is used to scan the object under ver different directions around a perpendicular to the fan plane de, the axis passing through the measuring opening is rotatable, in which a Computer for generating a cross-sectional image of the subsu object from the with the help of a data acquisition system generated measured values of the radiation detector is present and at which a deflection unit for periodically deflecting the focus of the X-ray emitter in the fan plane perpendicular to the central sink right of the radiation detector is provided. Such one Computer tomograph is known from DE 25 38 517 A1.

In a computer tomograph of the type mentioned, the measured values are described by the following two parameters:
α indicates the angular position of the focus of gravity against a fixed axis.
β is the angle between the central beam of the X-ray beam and the connecting line between the respective focus and the detector element under consideration.

If the focus has a fixed position in the rotating frame, that is is not distracted, there are so many different β values like detector elements. When realizing a periodic Ab directing the focus by a point on the perpendicular of the Detector in the direction perpendicular to this perpendicular bisector can be measured values for with a certain detector element  gain several β values, i.e. the total number of measured values and thus increasing the image resolution.

Let S (t) be the periodic deflection of the focus over time t with S (t + T) = S (t) and

if T the Period of focus deflection is. The phase of focus deflection is like this chosen that

Of the Focus in the interval

is different from the focus in the interval

With the help of integrators, one measured value can be obtained for each detector element in the first interval and another measured value in the second interval be formed. With a suitable choice of function S (t) can be achieved that the β values of the second interval between the β values of the first interval.

If all measured values of an interval are formed at the same time, storage means must be provided in which the end of an interval of available measured values up to the derivation from the respective Measured values for proportional digital values saved will. For this, e.g. B. each a double integrator for a detec gate element can be used, which at times n · with n = 1, 2,. . . is switched (n numbers the measurement data sentences). Such a data acquisition system is complex.

The invention has for its object a computer tomo graphs of the type mentioned in such a way that a simple data acquisition system results.

This object is achieved in that the Da tener registration system is designed so that the measured value formation it is offset in time from detector element to detector element follows.

The invention is based on one in the drawing illustrated embodiment explained in more detail. Show it:  

Fig. 1 shows a computer tomograph according to the invention, and

FIG. 2 shows a detail of the computer tomograph according to FIG. 1.

In FIG. 1, a rotating frame 1 is shown, the opening of a measuring 2 encloses and supports a elements 3 a, etc. existing ray detector 3 and a Rönt genstrahler 4 for a fan-shaped from a number of Detektorele, incident on the radiation detector 3 X-ray beam 5. The rotating frame 1 is for irradiating a measuring field 6 in which there is a test object, for. B. a patient on a couch, is rotatable under different directions about an axis 19 which perpendicular to the fan plane of the X-ray beam 5 passes through the measurement opening 2 and thus the measurement field 6 . A computer 7 he creates a cross-sectional image of the examination object from the measured values of the radiation detector 3 obtained with the aid of a data acquisition system 8 . This is shown on a display device 9 .

The focus 10 of the X-ray source 4 is periodically to increase the number of the measured values with the aid of a deflection unit 11 in an X-ray generator 12, directed in the direction of the double arrow 13, ie perpendicular to the perpendicular bisector 14 of the radiation detector 3 from.

In Fig. 1, the angle α for a focus of gravity and for the detector element 3, the angle β n located.

Fig. 2 shows the structure of the X-ray source 4 to the rule periodi deflection of the x-ray beam. 5 A cathode 15 sends an electron beam 16 to an anode 17 , from which the X-ray beam 5 extends so that the fan plane is perpendicular to the plane of the drawing. With the aid of deflection coils 18 , which are connected to the deflection unit 11 , the periodic focus deflection takes place perpendicular to the plane of the drawing.

Not all measured values are generated at the same time.  

In a first embodiment each detector element 3 are formed a two measured values, etc. within the time of a Fokusablenkperiode. For simplification, it is assumed that in the data acquisition system 8 there is only a single analog / digital converter before, to which the analog signals of the individual measuring channels are supplied in sequence. The nth measured value of the detector element with the number v is then in the time domain

formed with v = 1, 2,. . . N _D , where N _{D is} the number of detector elements. T is the period of focus deflection, t is time. The time interval Δt between two successive A / D conversions must be so small that N _D · Δt does not significantly exceed the time T.

The phases of the measurement value formation relating to the focus movement S (t) belonging to the different v are different, but measurement values arise for 2 N _D different β values: a set of measurement values for each detector element in which the focus of the focus is deflected in one direction (e.g. measured values with even n) and a second set of measured values in which the focus is deflected in the other direction (measured values with odd n). A suitable interpolation between these values can be used to generate projections (these are measured value sets for a fixed value α and for many equidistantly lying β values) which contain more than N _D values (e.g. 2 N _D values) . This results in fewer scanning errors than are given with projections that contain N _D measured values.

An essential feature is therefore the combination of a periodic deflection of the focus 10 with a (Δt) measurement value formation which is staggered in time from detector element to detector element.

In a second embodiment, measured values are formed for each detector element with a frequency that is at least equal to four times the focus deflection frequency (in the first embodiment, the frequency of the measurement value formation is exactly equal to the double focus deflection frequency). The times tvn at which the analog measured values of a measuring channel v are fed to one of the existing A / D converters need not have a fixed phase position for focus deflection. By measuring the temporal position of tvn with respect to the periodic function S (t), it is determined how large the mean focus deflection is for the measurement interval ended at the time tvn. By a suitable combination of the data belonging to the same half period of the focus deflection of a measuring channel and, if necessary, an interpolation of the data thus generated in the β direction, projections with more than N _D (z. B. 2 N _D ) values can be generated per projection.

Claims (4)

1. Computer tomograph with a measuring opening ( 2 ) enclose the rotating frame ( 1 ), on which one of a number of detector elements ( 3 a... 3 n...) Radiated detector ( 3 ) and an X-ray source ( 4 ) for a fan-shaped, on the radiation detector ( 3 ) impinging X-ray beam ( 5 ) are arranged and which for irradiating the examination object under different directions about a perpendicular to the fan plane, the measuring opening ( 2 ) penetrating axis ( 19 ) is rotatable, in which a computer ( 7 ) for generating a cross-sectional image of the examination object from the measured values of the beam detector ( 3 ) generated with the aid of a data acquisition system ( 8 ) is present and in which a deflection unit ( 11 ) for periodically deflecting the focus ( 10 ) of the x-ray emitter ( 4 ) in the fan plane perpendicular to the perpendicular perpendicular ( 14 ) of the radiation detector ( 3 ) are provided, characterized in that the data acquisition system ( 8 ) is designed such that the measured value formation from detector element to detector element takes place at different times. 2. Computer tomograph according to claim 1, characterized in that the measurement value formation in the mutually offset time ranges takes place with v = 1, 2. . . N _D , the focus being deflected in one direction for the measured values with even n and in the other direction for the measured values with odd n.N _D = number of detector elements
v = number of the detector element
n = number of the respective measured value
T = period of focus deflection
t = time
Δt = time interval between two successive A / D conversions. 3. Computer tomograph according to claim 1 or 2, characterized characterized in that during the time of A / D conversions no focus deflection. 4. Computer tomograph according to claim 1, characterized in that each detector element ( 3 a... 3 n...) Measured values are formed with a frequency which is at least four times as large as the focus deflection frequency, the times tvn, in to which the analog measured values of the vth measuring channel are fed to one of the A / D converters present in the data acquisition system ( 8 ), need not have a fixed phase position for focus deflection, that by measuring the temporal position of tvn with respect to the periodic function S (t ) it is determined how large the average focus deflection is for the measurement interval ended at the time tvn, and that by a suitable summary of the data belonging to the same half period of the focus deflection of a measurement channel and, if appropriate, an interpolation of the data thus generated, projections with more than N _D values generated per projection, where N _{D is} the number of detector elements.