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Automatic alignment of focus in CT X-ray tubes

IP.com Disclosure Number: IPCOM000018248D
Original Publication Date: 2002-Apr-01
Included in the Prior Art Database: 2003-Jul-23
Document File: 2 page(s) / 157K

Publishing Venue

Siemens

Related People

Prof. Dr. Stefan Popescu: AUTHOR [+2]

Abstract

The X-ray tubes used actual by CT systems suffer from the instability of the focus position that impinge negatively on image quality and necessitate compli- cate and generally pour reliable software correction algorithms. Systems without detector collimator are especially sensitive to focus errors. The same is valid for the outer slices of a multislice CT system.

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Automatic alignment of focus in CTX-ray tubes

Gesundheit

Idee: Prof. Dr. Stefan Popescu, Forchheim;

Mathias Berger, Forchheim

The X-ray tubes used actual by CT systems sufferfrom the instability of the focus position that impingenegatively on image quality and necessitate compli-cate and generally pour reliable software correctionalgorithms. Systems without detector collimator areespecially sensitive to focus errors. The same is validfor the outer slices of a multislice CT system.

Thermal effects as well as the mechanical oscillationsand geometrical inaccuracies of the anode plate im-pact surface  procedure  focus  position  shifting  de-pending on tube temperature or on the plate angularposition (synchronous to anode rotation). Moreover,the new power tube “Drehkolben-Röhre” that is lesssusceptible  to temperature  induced  focus  shift  (atleast if exploited up to actual standards) may mani-fest additional mechanical  focus  instabilities.  Al-though it rotates the emitting filament synchronous tothe anode plate, the impact point is still moving onplate surface, therefore the geometrical imprecisionof  anode  plate  surface  produces the same focusshifting effects as for a “normal” tube. Additionally,as  the  emitting  filament  of the power tube rotatesaround the rotation axis, if the main hot-spot of thefilament is not exactly centred in the rotation centrethen the electron beam will oscillate around the axisand will induce additional focus shifting (see fig.1).

FOCUS ERROR CORRECTION

Actually software algorithms correct the focus drifteffects but not the focus drift itself. These methodsuse the Signal form the outer detectors of the detectorbank to get a measure of focus drift and to correctapproximately the error. In order for the method towork it is required that the outer detectors to be indirect view with the  x-ray  tube.  However,  whenscanning thick patients for some or most exposureangles  the  outer  detectors  are  covered  by  patientbody and these methods are unreliable and unpre-dictable.

AUTOMATIC FOCUS ALIGNMENT

The  argumentation  above  justifies  some  kind ofautomation intended to adjust the focus position in zand

j    directions dynamically in order to correct forvarious sources of errors. The proposed structure isdepicted in fig.2. A special focus position detectorfinds the actual focus positions in z and

j    directionsand  supplies  two  error  signals.  Two  closed  loopfeedback regulators correct the focus shift by drivingtwo deflection coils that deflect the electron beamsuch that the error signals are minimised.

The system is easy to implement as the deflectioncoil in x /

j    direction is already available being usedfor the flying focus modes. To correct for focus er-rors in  z  direction  with  usual  tubes  an  additionaldeflexion coil and  the  implied  electronics  are  re-quired to deflect the electron beam in y-direction. Forthe power tube both  deflection  coils  are  alreadyavailable!

The...