Method and apparatus for controlling dose in computed X-ray imaging
The present invention pertains to a system and method for X-ray imaging wherein a targeted fluence at the detector for projection images can be achieved at a plurality of projection angles around the imaging subject by control of exposure times implemented during image acquisition. Exposure time for a second projection image may be determined by the fluence in a first projection image, and in a third projection image by the fluence in a second projection image, where projection images are acquired within two degrees of one another. An acquisition parameter calculation can be configured to calculate acquisition parameters, such as said exposure times, to achieve the targeted fluence in projection images and can be coupled to a rotation controller that implements the acquisition parameters by controlling a relative angle between the imaging subject and X-ray image acquisition device.
1. A method of acquiring an X-ray image of an object comprising:
positioning said object in field of view of an X-ray image acquisition device;
selecting a set of at least 9 unique projection angles between said X-ray image acquisition device and an axis through said object, wherein said set spans at least 45 degrees around said object;
selecting a targeted noise level at a detector for projection images to be acquired at each of said projection angles;
determining a plurality of targeted total exposure times for each of said set of projection angles corresponding to said targeted noise level;
implementing said plurality of targeted total exposure times in said X-ray image acquisition device over said set of projection angles to acquire an image data set; and
reconstructing an X-ray image from said image data set.
2. The method of claim 1 further comprising:
controlling total exposure times by controlling duration of time rotation is paused.
3. The method of claim 1 further comprising:
controlling total exposure times by controlling number of exposures taken at each of said projection angles.
4. The method of claim 1 further comprising:
controlling total exposure times by modulating individual exposure time of each exposure.
5. The method of claim 1 further comprising:
controlling total exposure times by controlling rotational velocity at each of said projection angles.
6. The method of claim 1 further comprising:
controlling total exposure times based on attenuation at each of said projection angles.
7. The method of claim 1 further comprising:
using a preliminary scan to determine attenuation at each of said projection angles.
8. The method of claim 7 further comprising:
collimating an X-ray beam to a smaller portion of a solid angle of a full beam during said preliminary scan.
9. The method of claim 1 further comprising:
re-scanning at said projection angles where noise performance is below a predetermined threshold.
10. The method of claim 1 further comprising:
acquiring a preliminary image data set with less than 50% of X-ray source power utilized to acquire said image data set; and
determining said targeted total exposure times meeting said targeted noise level using said preliminary image data set.
11. The method of claim 1 further comprising:
selecting a mean transmitted fluence value over full area of said detector at each of said projection angles; and
controlling total exposure times at each of said projection angles based on said mean transmitted fluence value.
12. The method of claim 1 further comprising:
selecting a minimum transmitted fluence value over all beam paths reaching said detector at each of said projection angles; and
controlling total exposure times at each of said projection angles based on said minimum transmitted fluence value.
13. The method of claim 1 further comprising:
selecting a minimum transmitted fluence value in a predetermined region of said detector at each of said projection angles; and
controlling total exposure times at each of said projection angles based on said minimum transmitted fluence value.
14. The method of claim 1 further comprising:
selecting a mean transmitted fluence value over a predetermined region of said detector at each of said projection angles; and
controlling total exposure times at each of said projection angles based on said mean transmitted fluence value.
15. The method of claim 1 further comprising:
selecting a mean transmitted fluence value over a region of interest in said X-ray image at each of said projection angles; and
controlling total exposure times at each of said projection angles based on said mean transmitted fluence value.
16. The method of claim 1 further comprising:
determining minimum fluence at said detector.
17. The method of claim 1 further comprising:
determining mean fluence at said detector.
18. The method of claim 1 further comprising:
determining minimum fluence in a predetermined region.
19. The method of claim 1 further comprising:
determining mean fluence in a predetermined region.