Automatic alignment of focus in CT X-ray tubes
Original Publication Date: 2002-Apr-01
Included in the Prior Art Database: 2003-Jul-23
Prof. Dr. Stefan Popescu: AUTHOR [+2]
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.
Automatic alignment of focus in CTX-ray tubes
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!