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

A computer tomography apparatus has a circular anode and a circular radiation detector in which a patient to be examined is disposed. An electron beam from an electron gun is deflected by a beam deflection system, fed by a line voltage, so that the focus of the electron beam on the anode orbits the patient, thereby irradiating the patient from different angular positions. The radiation attenuated by the patient is recorded by the radiation detector, and corresponding electrical signals are read-out from the radiation detector by a data acquisition system, from which an image of the patient is constructed. To avoid image artifacts due to fluctuations such as ripples in the line voltage which supplies the tomography apparatus, the beam deflection system and the data acquisition system are synchronized so that, at each of 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 line voltage.

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 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 
EQU .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 
EQU 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 
EQU tu=(5+1/n)tr.apprxeq.50 ms. 
The condition 
EQU tu=(l+p/n)tr 
can be observed by frequency synchronization. With f'=1/tu as the 
revolution frequency, 
EQU 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.