Ring suppression filter for use in computed tomography systems

• US 5,745,542 A
• Filed: 04/29/1997
• Issued: 04/28/1998
• Est. Priority Date: 03/13/1996
• Status: Expired due to Fees

• Current Assignee: Analogic Corporation
• Original Assignee: Analogic Corporation
• Inventors: Ching-Ming, Lai, Gordon, Bernard M.
• Primary Examiner(s): Porta, David P.
• Assistant Examiner(s): Bruce, David Vernon
• Application Number:
  US08/840,367
• Time in Patent Office: 364 Days
• Field of Search: 378/4, 378/15, 378/901, 364/413.15, 364/413.16
• US Class Current: 378/4
• CPC Class Codes: A61B 6/583 using calibration phantoms
• CPC Class Codes: G06T 11/003 Reconstruction from projections, e.g. tomography
• CPC Class Codes: Y10S 378/901 Computer tomography program or processor

Abstract

A computed tomography (CT) system utilizes a ring suppression filter for suppressing ring artifacts in a CT image. The system generates projections at a plurality of projection angles, and each projection includes one measurement of a plurality of projection data signals. The ring suppression filter includes a high pass filter, a histogram generator, and a combiner. The high pass filter generates a measurement of an error signal corresponding to each of the measurements of one of the projection data signals, and each of the error signal measurements is representative of the high frequency components in a portion of one of the projections. The histogram generator generates a histogram signal representative of a relationship between the measurements of the error signal and the measurements of the projection data signal. The combiner combines the measurements of the error signal and the histogram signal to generate a plurality of measurements of a ring corrected signal.

Claims

1. A computed tomography system for creating a computed tomographic image of a patient and including (a) a source of X-rays and (b) channel defining means for defining a plurality of signal processing channels having corresponding transfer functions and comprising means, including a corresponding plurality of detectors of a detector array for detecting X-rays emitted from said source, for generating a plurality of image data signals as a function of the X-rays detected by the corresponding detectors at each of a plurality of projection angles of a tomographic scan and representative of the tomographic image of the patient, said system further comprising:

means for generating error correction data in response to image data signals generated during an entire scan of a patient; and

means for automatically calibrating the transfer functions of said channels as a function of said error correction data so as to minimize ring artifacts in computed tomographic images produced from said image data signals.

4. A computed tomography system according to claim 1, wherein said means for automatically calibrating the transfer function of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel and the amplitudes of the data signals processed through channels associated with detectors of said array adjacent to the corresponding detector.

5. A computed tomography system according to claim 1, wherein said means for automatically calibrating the transfer function of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel and the amplitudes of the data signals processed through channels associated with detectors of said array adjacent to the corresponding detectors for preselected ones of said projection angles.

6. A computed tomography system according to claim 1, wherein said means for automatically calibrating the transfer functions of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel for each of a predetermined number of projection angles.

7. A computed tomography system according to claim 1, wherein said means for automatically calibrating the transfer functions of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of (a) the amplitude of the data signals provided by the corresponding detector and processed through that channel at each of said projection angles, and (b) the amplitude of the data signals of that detector at a preselected number of adjacent projection angles.

8. A computed tomography system according to claim 7, wherein said means for automatically calibrating the transfer functions of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel and the amplitudes of the data signals processed through channels associated with detectors of said array adjacent the corresponding detector.

2. A computed tomography system for creating a computed tomographic image of a patient and including (a) a source of X-rays and (b) channel defining means for defining a plurality of signal processing channels having corresponding transfer functions and comprising means, including a corresponding plurality of detectors of a detector array for detecting X-rays emitted from said source, for generating a plurality of image data signals as a function of the X-rays detected by the corresponding detectors at each of a plurality of projection angles of a tomographic scan and representative of the tomographic image of the patient, said system further comprising:

means for automatically calibrating the transfer functions of said channels as a function of error correction data obtained from image data signals generated during a scan of a patient so as to minimize ring artifacts in computed tomographic images produced from said image data signals, said means for automatically calibrating the transfer functions of said channels including means for determining the error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals processed through that channel.

3. A computed tomography system according to claim 2, wherein said means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals processed through that channel includes means for generating an estimate signal representative of the estimated error in the transfer function for each of said channels at each of said projection angles, means for comparing the value of the amplitude of each of said estimate signals with a predetermined threshold value, and means for generating error correction data for each of said estimate signals only when the value of the estimate signal is under-threshold.

9. A computed tomography system for scanning an object comprising:

(a) a plurality of signal processing means, including a corresponding plurality of detectors, for generating a respective plurality of projection data signals at a plurality of projection angles, each of the signal processing means being characterized by a respective one of a plurality of transfer functions, each of the projection data signals including a data component and an error component, the error components being representative of relative differences between the transfer functions of the signal processing means and also being representative of variations in the transfer functions independent of each other and occurring over time, the data components being representative of the densities of portions of the object, and

(b) a filter comprising means for generating a plurality of estimates of the error components for each of said detectors, each of the estimates being a function of the average amplitude of a predetermined number of the projection data signals acquired by the corresponding detector at a predetermined number of projection angles.

10. A computed tomography system according to claim 9, further comprising means for correcting the error components in each of the projection data signals independent of the other projection data signals.

11. A computed tomography system according to claim 9, wherein said filter includes a filter kernel which is constant for all detectors at all of the projection angles.

12. A computed tomography system according to claim 9, wherein said filter in a non-linear filter.

13. A computed tomography system according to claim 12, wherein said non-linear filter includes (a) threshold means for comparing each measurement of the projection data signals to a threshold and for designating each measurement of the projection data signal that is less than the threshold as being under-threshold, and (b) histogram means for averaging all the measurements of the measurement error signal that correspond to under-threshold measurements of the projection data signal.

14. A computed tomography system for generating a computed tomographic image of an object, the system including means for generating a plurality of projections, each of the projections being generated at a corresponding projection angle, each of the projections including a measurement of each of a plurality of projection data signals, each of the measurements of the projection data signals being representative of a density of a portion of the object, said system further including a ring suppression filter for suppressing ring artifacts in the tomographic image, said filter comprising:

(A) high pass filter means for generating a plurality of measurement error signals as a function of the corresponding measurements of the projection data signals for each of said projections, each measurement error signal being representative of a high frequency component associated with at least a portion of one of the projections;

(B) threshold means for comparing each of the measurements of the projection data signals to a threshold and for designating each measurement of the projection data signal that is less than the threshold as being under-threshold; and

(C) histogram means for averaging all the measurements of the measurement error signal that correspond to under-threshold measurements of the projection data signal.

15. In a computed tomography system for generating an image of an object, said system including means for generating a plurality of projections, each of the projections being generated at a corresponding projection angle, each of the projections including one measurement of each of a plurality of projection data signals, each of the measurements of the projection data signals being representative of a density of a portion of the object, a ring suppression filter comprising:

(A) high pass filter means for generating a plurality of measurements of an error signal, each of the measurements of the error signal being representative of the high frequency components associated with a portion of one of the projections;

(B) histogram means, responsive to the measurements of one of the projection data signals and to the measurements of the error signal, for generating a histogram signal, the histogram signal being representative of a relationship between the select measurements of the error signal and each of the corresponding projection data signals; and

(C) combining means, responsive to the measurements of the corresponding projection data signal and to the histogram signal, for generating therefrom a plurality of measurements of a ring corrected signal.

16. A ring suppression filter according to claim 15, wherein the histogram means includes quantization means for assigning one of a plurality of bin numbers to each of the measurements of the one projection data signal and the error signal so that each measurement of the one projection data signal and a corresponding measurement of the error signal are assigned the same bin number.

17. A ring suppression filter according to claim 16, wherein the quantization means assigns the same bin number to measurements of the one projection data signal having the same amplitude.

18. A ring suppression filter according to claim 17, wherein the histogram means generates the histogram signal so that the histogram signal has a value for each of the bin numbers.

19. A ring suppression filter according to claim 18, wherein the histogram means averages all the measurements of the error signal that are assigned to one bin number to generate the value of the histogram signal for that one bin number.

20. A ring suppression filter according to claim 18, wherein the histogram means averages all the measurements of the error signal that are assigned to one bin number and that have a magnitude less than a threshold to generate the value of the histogram signal for that one bin number.

21. A ring suppression filter according to claim 18, wherein the histogram means includes means for comparing each of the measurements of the error signal to a threshold and further includes means for designating each of the measurements of the error signal having a magnitude less than the threshold as an under-threshold measurement.

22. A ring suppression filter according to claim 21, wherein the histogram means averages all the under-threshold measurements that are assigned to one bin number to generate the value of the histogram signal for that one bin number.

23. A ring suppression filter according to claim 22, wherein the combining means generates each measurement of the ring corrected signal by determining which bin number has been assigned to one of the measurements of the one projection data signal and then subtracting from the one measurement of the one projection data signal the value of the histogram signal for that bin number.

24. A ring suppression filter according to claim 21, wherein the histogram means includes means for generating a first intermediate signal and a second intermediate signal, for each bin number the first intermediate signal having a value substantially equal to a sum of all the under-threshold measurements of the error signal assigned to that bin number, and for each bin number the second intermediate signal having a value substantially equal to a total number of all the under-threshold measurements of the error signal assigned to that bin number.

25. A ring suppression filter according to claim 21, wherein the histogram means includes first low pass filter means for receiving and low pass filtering the first intermediate signal and thereby generating a first filtered signal, and wherein the histogram means includes second low pass filter means for receiving and low pass filtering the second intermediate signal and thereby generating a second filtered signal.

26. A ring suppression filter according to claim 25, wherein the histogram means includes dividing means for dividing the first intermediate signal by the second intermediate signal and thereby generating the histogram signal.

27. A ring suppression filter according to claim 26, further including offset means for receiving the histogram signal and for generating therefrom an offset signal, the offset signal having a magnitude less than or equal to that of the histogram signal.

28. A ring suppression filter according to claim 15, further including low pass filter means for low pass filtering the each of the projections and for applying the low pass filtered projections to the high pass filter means.

29. A ring suppression filter according to claim 15, wherein each of the projections is a fan beam projection.

30. A ring suppression filter according to claim 15, wherein each of the projections is a parallel beam projection.

31. A ring suppression filter according to claim 15, wherein said histogram means is responsive to measurements of projection data signals and error signals generated during two or more scans.

32. A computed tomography system for creating a tomographic image of a scanned object and including a source of X-rays and channel defining means for defining a plurality of signal processing channels characterized by a corresponding plurality of transfer functions, said plurality of signal processing channels comprising means, including a corresponding plurality of detectors of a detector array for detecting X-rays emitted from said source, for generating a plurality of data signals as a function of the X-rays detected by the corresponding detectors at each of a plurality of projection angles of a tomographic scan through the corresponding signal processing channels, said system further including:

means for automatically calibrating the transfer functions of said channels as a function of error correction data obtained from data signals generated during at least one previous scan of a patient so as to minimize ring artifacts in tomographic images produced from said data signals.

33. A computed tomography system according to claim 32, wherein said means for automatically calibrating the transfer functions of said channels as a function of error correction data obtained from data signals generated during at least one previous scan of a patient and the present scan of a patient so as to minimize ring artifacts in tomographic images produced from said data signals.

34. A computed tomography system according to claim 32, wherein said means for automatically calibrating the transfer functions of said channels as a function of error correction data obtained from data signals generated during a plurality of previous scans of one or more patients so as to minimize ring artifacts in tomographic images produced from said data signals.

35. A computed tomography system according to claim 32, wherein said means for automatically calibrating the transfer functions of said channels includes means for determining the error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals processed through that channel.

36. A computed tomography system according to claim 32, wherein said means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals processed through that channel, means for generating an estimate signal representative of the estimated error in the transfer function for each of said channels at each of said projection angles, means for comparing the value of the amplitude of each of said estimate signals with a predetermined threshold value, and means for generating error correction data for each of said estimate signals only when the value of the estimate signal is under-threshold.

37. A computed tomography system according to claim 32, wherein said means for automatically calibrating the transfer function of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel and the amplitudes of the data signals processed through channels associated with detectors of said array adjacent to the corresponding detector.

38. A computed tomography system according to claim 32, wherein said means for automatically calibrating the transfer function of said channels includes means for determining error correction data from each of said image dam signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel and the amplitudes of the data signals processed through channels associated with detectors of said array adjacent to the corresponding detectors for preselected ones of said projection angles.

39. A computed tomography system according to claim 32, wherein said means for automatically calibrating the transfer functions of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel for each of a predetermined number of projection angles.

40. A computed tomography system according to claim 32, wherein said means for automatically calibrating the transfer functions of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of (a) the amplitude of the data signals provided by the corresponding detector and processed through that channel at each of said projection angles, and (b) the amplitude of the data signals of that detector at a preselected number of adjacent projection angles.

41. A computed tomography system according to claim 40, wherein said means for automatically calibrating the transfer functions of said channels includes means for determining error correction data from each of said image data signals processed through each channel as a function of the amplitude of the data signals provided by the corresponding detector and processed through that channel and the amplitudes of the data signals processed through channels associated with detectors of said array adjacent the corresponding detector.

42. A computed tomography system for scanning objects through a plurality of projection angles, the system including a plurality of signal processing means including respectively a plurality of detectors of a detector array, each of the processing means for generating a respective one of a plurality of projection data signals and each of the processing means being characterized by a respective one of a plurality of signal transfer functions, each of the projection data signals including a data component and an error component, the error components being representative of relative differences between the signal transfer functions of the processing means, the plurality of processing means generating their respective projection data signals for each scan so that the data components are representative of the densities of portions of an object being scanned, the system further comprising:

(A) storage means for storing a set of prior estimates of the error components collected from at least one prior scan for each of said detectors at each of said projection angles, said storage means including means for receiving a new set, of estimates of the error components from a current scan and for generating an updated set of estimates as a function of the prior and new sets of estimates; and

(B) ring suppression filter means for receiving the projection data signals and for generating therefrom the new set of estimates of the error components and for providing the new set of estimates to the storage means.

43. A computed tomography system for scanning objects, the system including a plurality of signal processing means, each of the processing means for generating a respective one of a plurality of projection data signals and each of the signal processing means being characterized by a respective one of a plurality of transfer functions, each of the projection data signals including a data component and an error component, the error components being representative of relative differences between the transfer functions of the plurality of processing means, the plurality of processing means generating their respective projection data signals for each scan so that the data components are representative of the densities of portions of an object being scanned, the system further comprising:

(A) storage means for storing a prior set of estimates of the error components obtained during at least one prior scan, said storage means including means for receiving a new set of estimates of the error components and for updating the set of estimates as a function of the prior set of estimates and the new sets of estimates;

(B) combining means for receiving the projection data signals of a scan and the old set of estimates and for generating a plurality of modified projection data signals as a function of the projection data signals and the old set of estimates; and

(C) ring suppression filter means for receiving the modified projection data signals and for generating therefrom the new set of estimates of the error components and for applying the new set of estimates to the storage means.