Computer tomography apparatus which avoids image artifacts caused by periodical voltage variations

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a computer tomography apparatus of the type having a circular anode and a circular radiation detector surrounding a measuring opening in which a patient to be examined is disposed so that the patient is irradiated with x-radiation from different angular positions.

2. Description of the Prior Art

Computer tomography devices are known in the art wherein a patient is disposed inside of a circular anode and a circular radiation detector. An electron beam is generated and is deflected by a beam deflection system in a circular orbit, on the circular anode, around the patient, so that the patient is irradiated by a fan-shaped x-ray beam from different angular directions. The radiation attenuated by the patient is recorded by the radiation detector, consisting of an array of individual detector elements, and the measured values from the detector elements are supplied to a data acquisition system, from which the measured values are supplied to a computer which constructs an image of a slice of the examination subject from those values. The image is then visually displayed.

Other types of computer tomography devices are known wherein the x-ray source and the radiation detector are mechanically moved around the examination subject, so as to expose the patient to radiation from the different angular positions. In tomography devices of this type, if a tomogram of a beating heart is needed, measured values of the attenuated radiation during a plurality of heart cycles must be selected in order to achieve images of the beating heart which are low in artifacts. The measured values are always produced at the same heart phase, i.e., at the same time within the heart beat cycle. Fluctuations in the line voltage which supplies high-voltage generator for the x-ray source will result in variations of the intensity and mean energy of the x-radiation and in migrations of the x-ray beam position as it exits the x-ray source. Beam position monitors in this type of tomography apparatus can be mechanically mounted at the beam exit port of the x-ray source, and thus co-moved with the x-ray source around the patient. The detector then supplies a signal to the computer which is used to correct for voltage variations, so that the change in the intensity and mean energy of the x-ray beam caused by the voltage fluctuations can be taken into account in the computerized construction of the image, and image artifacts, which would otherwise be caused by this voltage fluctuation, can be avoided.

A computer tomography apparatus of the type first described above, i.e., having a circular anode on which an electron beam orbits around a patient, is described in U.S. Pat. No. 4,352,021. In this type of computer tomography apparatus, a very fast movement of the focus around the circular anode is possible, so that the registration of heart phases is possible during a single heart cycle. The focus of the electron beam is conducted around the circular anode using an electrical and/or magnetic beam deflection system. Therefore mechanical parts for moving the focus are not present in this type of tomography apparatus, and the aforementioned mechanical radiation monitors cannot be used, and thus image artifacts caused by fluctuations in the line voltage are present.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a computer tomography apparatus of the type having a circular anode on which an electron beam is caused to orbit an examination subject, wherein line voltage fluctuations do not lead to image artifacts.

The above object is achieved in accordance with the principles of the present invention in a computer tomography apparatus, and a method for operating such an apparatus, wherein the beam deflection system and the data acquisition system are synchronized so that, at each of the n revolutions of the focus around the patient, the focus movement and the read-out of the measured values ensue slightly phase-shifted compared to the preceding resolution relative to the ripple of the line voltage which is supplied to the deflection voltage generator which supplies the electron beam deflection unit. Such synchronization is based on two assumptions. The first assumption is that the fluctuation of the line voltage is of the type known as voltage ripple, and occurs substantially periodically, with the ripple frequency being a known multiple of the line frequency. The second assumption is that the focus must repeatedly sweep or orbit the circular anode in order to achieve low-noise images.

In the apparatus and method disclosed and claimed herein, it is thus irrelevant whether homologous measured data are combined before the image construction, or whether tomography images respectively associated with the individual revolutions of the focus are superimposed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a computer tomography apparatus constructed in accordance with the principles of the present invention.

FIG. 2 is a schematic block diagram for an exemplary embodiment of the synchronization unit in the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A computer tomography apparatus constructed in accordance with the principles of the present invention is shown in FIG. 1, and includes a vacuum vessel 1 of known construction which includes a circular anode 3. The volume surrounded by the anode 3 defines a measuring aperture 2. An electron gun 7 generates an electron beam which is focussed in a beam focusing unit 8. Both the electron gun 7 and the beam focusing unit 8 are supplied with high voltage from a high voltage generator 9. The focussed beam is deflected in a beam deflection unit 5 so as to enter into the vacuum vessel 1 in which further deflection plates 4 are disposed. The beam deflection unit 5 is fed by a deflection voltage generator 17. The focus of the electron beam on the circular anode 3 is caused to orbit around the measuring aperture 2 by the beam deflection unit 5. The electron gun 7, the beam focusing unit 8, the beam deflection unit 5, the vacuum vessel 1 with the deflection plates 4 therein, and the anode 3 thus constitute an x-ray source.

The focus rotating around the circular anode 3 generates a fan-shaped rotating x-ray beam, which may be gated by a diaphragm 10. The fan-shaped x-ray beam transirradiates a patient 11, lying on a patient support 12 in the measuring aperture 2, from different directions as the electron beam focus orbits or revolves around the aperture 2. The radiation attenuated by the patient 11 is incident on a circular radiation detector 13, also surrounding the measuring aperture 2, which consists of a circular row of individual detector elements. The x-ray beam proceeds at an angle relative to the axis A which deviates from 90.degree.. The detector elements measure the incident radiation, and each supplies an electrical signal corresponding to the incident radiation as a measured value to a data acquisition system 14 which conducts a read-out of each detector element. The signals from the data acquisition system 14 are supplied to an image construction computer 15, which calculates the attenuation values of predetermined points of the examined slice of the patient 11 in a known manner, and generates an image which can be displayed on a display and evaluation system 16.

Each of the high voltage generator 9 and the deflection voltage generator 6 are supplied by the line voltage on line 17. The line voltage may exhibit a fluctuation in the form of a ripple which, if present will be at a frequency (and thus have a period) which is a known multiple of the line frequency. The line voltage is also supplied to a synchronization unit 18, which identifies the frequency of the line voltage and supplies signals based thereon to each of the deflection system 6 and the data acquisition system 14 so that, at each of the n revolutions of the focus, the focus movement and the read-out of the measured values ensue slightly phase-shifted compared to the preceding revolution relative to the ripple period of the high-voltage of the x-ray source, which is derived from the line frequency. For this purpose, a control computer 19 is connected to the synchronization unit 18 and to the computer 15.

The phase offset is selected as the p-fold multiple of the n.sup.th part of the ripple period. The number n denotes the plurality of focus revolutions which are used, and p denotes an arbitrary integer. The number p is preferably selected such that p and n are relatively prime. The effectiveness of this method can be recognized in the following way.

Let r1 (ft) reference the ripple function supplied by the high-voltage generator 9 having a ripple frequency f and the chronological variable t. The Fourier development is as follows: ##EQU1## By averaging r1 (ft) over n sampling points equidistantly distributed in the chronological spacing

.DELTA.t=p/(n.multidot.f),

the reduced ripple function rn (ft) is derived as follows: ##EQU2## In the above, n'=n/q, with q being the highest common divisor of p and n. Only those harmonics having an index which is a whole multiple of n' are not suppressed by the averaging. The most favorable relationships are present when n=n', i.e., when n and p are relatively prime.

The revolution time tu of the focus, the number n of revolutions, and the integer p are selected such that

tu =(l+p/n)tr

is valid with l being an integer.

As an example, for a low-frequency high-voltage generator 9, tr=10 ms can apply. If l=5 and p=1 are selected, and n is selected between 10 and 20, it follows that

tu=(5+1/n)tr.apprxeq.50 ms.

The condition

tu=(l+p/n)tr

can be observed by frequency synchronization. With f'=1/tu as the revolution frequency,

f'=f/(l+p/n)

is to be set.

The rated phase which is characterized by the integers l, p and n is prescribed by the central control computer 15. The synchronization 18 supplies the frequency f' which is required for guiding the electron beam on the specified circle and for the read-out clock at the detector elements. All voltages and currents are subject to fluctuations having the frequency f, as identified in FIG. 1.

A block circuit diagram for an embodiment of the synchronization unit 18 is shown in FIG. 2. This embodiment includes a frequency divider 20, an oscillator 21, a phase-to-voltage converter 22, and a comparator 23 having an input 24 to which information identifying the rated phase is supplied. The phase-to-voltage converter 22 has an input 25 to which the line frequency is supplied, and an output 26.

Continuing with the above example, if the line frequency is selected as 50 Hz, and if the frequency divider 20 has a division ratio of 1024:1, the oscillator frequency will be fixed at 51.2 kHz. A line frequency of 60 Hz, and other division ratios, are also possible. The synchronization unit 18 supplied the frequency f' for the deflection voltage generator 6 and for the data acquisition system 14.

Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.

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