Collimation apparatus for x-ray beam correction

• US 4,991,189 A
• Filed: 04/16/1990
• Issued: 02/05/1991
• Est. Priority Date: 04/16/1990
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Toth, Thomas L., Boomgaarden, Jonathan C., Englert, August O.
• Primary Examiner(s): Church, Craig E.
• Application Number:
  US07/509,598
• Time in Patent Office: 295 Days
• Field of Search: 378/151, 378/4, 378/150
• US Class Current: 378/4
• CPC Class Codes: A61B 6/032 Transmission computed tomography [CT]
• CPC Class Codes: A61B 6/06 Diaphragms
• CPC Class Codes: A61B 6/4021 involving movement of the focal spot
• CPC Class Codes: A61B 6/5258 involving detection or reduction of artifacts or noise
• CPC Class Codes: G21K 1/02 using diaphragms, collimators

Abstract

A computed tomography system corrects image errors resulting from misalignment of the fan beam as a result of placement of the x-ray tube, thermal drift of the x-ray tube anode of mechanical stresses of the gantry or x-ray tube. Misalignment is determined alternately by means of a z-axis offset detector which detects movement of the exposure area of the CT fan beam or a predictive model that calculates thermal drift of mechanical deflection of the focal spot of the x-ray tube based on previous use. The system includes a collimator with a controllable z-axis position which may be controlled to reduce z-axis offset of the fan beam exposure area or to make the fan beam more parallel with the imaging plane of the CT system.

Claims

1. A computed tomography system including an x-ray source for producing a fan beam of x-rays along a fan beam plane in response to an exposure technique, the fan beam plane being at deviation angle α

from an imaging plane and intersecting a detector array at position D_z, comprising;

a collimator means having position C_z for controlling the alignment of the fan beam plane;

a z-axis offset detector positioned to intercept the fan beam for producing a position signal dependant on D_z ;

an error calculation means for generating an error signal in response to the position signal; and

a collimator controller responsive to the error signal for changing the relative position C_z of the collimator with respect to the x-ray source and detector array so as to reduce the magnitude of the error signal.

2. The computed tomography system of claim 1 wherein the error signal is proportional to the difference in position between D_z and an alignment point D_O.

3. The computed tomography system of claim 1 wherein the error signal is proportional to the difference between D_z and C_z.

4. A computed tomography system including an x-ray source for producing a fan beam of x-rays emanating from a focal at a position F_z spot, along a fan beam plane in response to an exposure technique, the fan beam plane being at deviation angle α

from an imaging plane and intersecting a detector array at position D_z, comprising;

a collimator means having position C_z for controlling the alignment of the fan beam plane;

a memory for storing previous values of exposure techniques used with the x-ray source and the time of each exposure technique;

a thermodynamic/geometric model for calculating a current fan beam position D_zp from the values stored in the memory; and

a collimator controller responsive to the fan beam position D_zp for changing the collimator position C_z in response to F_z.

5. The computed tomography system Of claim 4 wherein the collimator controller changes the collimator position so as to reduce the difference between D_zp and C_z.

6. The computed tomography system of claim 4 wherein the collimator controller changes the collimator position so as to reduce the difference between D_zp and the center of the detector array D_O.

7. A computed tomography system including an x-ray source, mounted on a gantry for rotation about an axis at a rotational velocity and tilt angle, and for producing a fan beam of x-rays emanating from a focal at a position F_z spot, along a fan beam plane in response to an exposure technique, the fan beam plane being at deviation angle α

from an imaging plane and intersecting a detector array at position D_z, comprising;

a collimator means having position C_z for controlling the alignment of the fan beam plane;

a mechanical stress model for calculating a current fan beam position D_zp from the rotational velocity and the tilt angle of the gantry; and

a collimator controller responsive to the fan beam position D_zp for changing the collimator position C_z in response to F_z.

8. The computed tomography system of claim 7 wherein the collimator controller changes the collimator position so as to reduce the difference between D_zp and C_z.

9. The computed tomography system of claim 7 wherein the collimator controller changes the collimator position so as to reduce the difference between D_zp and the center of the detector array D_O.