Self calibrating ring suppression filter for use in computed tomography systems

• US 5,841,828 A
• Filed: 04/29/1997
• Issued: 11/24/1998
• Est. Priority Date: 03/13/1996
• Status: Expired due to Fees

• Current Assignee: Analogic Corporation
• Original Assignee: Analogic Corporation
• Inventors: Gordon, Bernard M., Ching-Ming, Lai
• Primary Examiner(s): Porta, David P.
• Assistant Examiner(s): Bruce, David Vernon
• Application Number:
  US08/840,681
• Time in Patent Office: 574 Days
• Field of Search: 364/413.19, 378/901, 378/4, 378/8
• 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 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 with an error correction histogram characterizing the error in each channel as a function of projection amplitude 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.

2. A computed tomography system according to claim 1, wherein said means for automatically calibrating the transfer functions of said channels is 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.

3. A computed tomography system according to claim 1, wherein said means for automatically calibrating the transfer functions of said channels is 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.

4. 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.

5. A computed tomography system according to claim 1, 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.

6. 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.

7. 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.

8. 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.

9. A computed tomography system according to claim 8, 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.

10. 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; and

means for determining said 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.

11. 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.

12. 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 comprising a histogram characterizing the error in each channel as a function of projection amplitude and 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.

13. 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; and

means for determining said 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.

14. 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; and

means for said 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.