Electronic calibration of single photon emission computed tomography cameras

• US 5,481,115 A
• Filed: 12/20/1993
• Issued: 01/02/1996
• Est. Priority Date: 06/10/1991
• Status: Expired due to Term

• Current Assignee: University of Utah (State of Utah)
• Original Assignee: University of Utah (State of Utah)
• Inventors: Hsieh, Yu-Lung, Zeng, Gengsheng L., Gullberg, Grant T.
• Primary Examiner(s): Hannaher, Constantine
• Application Number:
  US08/170,811
• Time in Patent Office: 743 Days
• Field of Search: 250/363.09, 250/363.07, 250/363.04, 250/252.1 R
• US Class Current: 250/363.04
• CPC Class Codes: A61B 5/1077 Measuring of profiles
• CPC Class Codes: A61B 6/032 Transmission computed tomography [CT]
• CPC Class Codes: A61B 6/037 Emission tomography
• CPC Class Codes: A61B 6/107 Protection against radiation, e.g. shielding
• CPC Class Codes: A61B 6/4014 arranged in multiple source-detector units
• CPC Class Codes: A61B 6/4291 the detector being combined with a grid or grating
• CPC Class Codes: A61B 6/4429 related to the mounting of source units and detector units
• CPC Class Codes: A61B 6/482 involving multiple energy imaging
• CPC Class Codes: A61B 6/583 using calibration phantoms
• CPC Class Codes: A61B 6/589 Setting distance between source unit and patient
• CPC Class Codes: A61B 8/0858 involving measuring tissue layers, e.g. skin, interfaces
• CPC Class Codes: A61B 8/4416 related to combined acquisition of different diagnostic modalities, e.g. combination of ultrasound and X-ray acquisitions
• CPC Class Codes: G01T 1/1615 using both transmission and emission sources simultaneously SPECT imaging G01T1/1642; PET imaging G01T1/2985; detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects G01V5/20
• CPC Class Codes: G01T 1/1642 using a scintillation crystal and position sensing photodetector arrays, e.g. ANGER cameras
• CPC Class Codes: G01T 1/1648 Ancillary equipment for scintillation cameras, e.g. reference markers, devices for removing motion artifacts, calibration devices adapted for flow studies G01T1/1647
• CPC Class Codes: G01T 1/2985 In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis); using external radiation sources A61B6/02
• CPC Class Codes: G06T 11/005 Specific pre-processing for tomographic reconstruction, e.g. calibration, source positioning, rebinning, scatter correction, retrospective gating
• CPC Class Codes: G06T 2211/432 Truncation
• CPC Class Codes: G21K 1/025 using multiple collimators, e.g. Bucky screens; other devices for eliminating undesired or dispersed radiation

Abstract

A SPECT camera (A) includes a plurality of detector heads (12a, 12b, 12c) which rotate around an examination region (10). A reconstruction processor (50) reconstructs output projection data from the detector heads into a three-dimensional image representation which is stored in an image memory (62). Selected data from the image memory (62) is converted to a human-readable display on a video monitor (66). To adjust automatically for detector head misalignment, a calibration phantom (B) which has two orthogonal lines of scintillators (20₁, 20₂, 20₃, 20₄, 20₅) arranged orthogonal to each other with a common scintillator in both lines is positioned in the examination region. As the detector heads rotate around the phantom, a misalignment parameter generator (40) generates the displacement distance of the actual rotation from the theoretical rotation axis (ζ), the actual rotation radius (R), a projection of the location of the focal point on a detector plane of the detector head in the transaxial direction (C.sub.ξ), and a focal length of the collimator (F). A correction processor (42) corrects the reconstruction processor (50) in accordance with the generated misalignment parameters. In the preferred embodiment, the correction processor provides a revised weighting function to the reconstruction processor and modifies its backprojection algorithm.

Claims

1. A nuclear camera system comprising:

at least one radiation detector head for detecting radiation in an examination region, the radiation detection head including a collimator;

a means for moving the detector head relative to the examination region;

an image reconstruction means for reconstructing image data from the detector head into a diagnostic image representation;

an image memory for storing the image representation;

a display means for converting at least selected portions of data in the image memory into a human-readable display;

a calibration phantom removably inserted in the examination region, the phantom including first and second lines of radiation sources, each line including at least three radiation sources and the first and second lines having a preselected, fixed orientation relative to each other;

a means for analyzing the data from the detector head as the detector head is moved relative to the phantom to generate detector head misalignment parameter values;

an electronic correcting means for correcting the image reconstruction means in accordance with the misalignment parameter values.

2. The system as set forth in claim 1 wherein the collimator is one of a fan-beam collimator and a conebeam collimator.

3. A radiographic camera system comprising:

at least one radiation detector assembly for detecting radiation exiting from an examination region along a fan-shaped array of paths;

an image reconstruction processor for reconstructing image data from the detector assembly into a diagnostic image representation;

an image memory for storing the image representation;

a monitor for converting at least selected portions of data in the image memory into a human-readable display;

a calibration phantom including first and second lines of radiation sources, each line including at least three radiation sources and the first and second lines having a preselected, fixed orientation relative to each other, the phantom being removably inserted in the examination region;

a means for analyzing the data from the detector head as the detector head is moved relative to the phantom to generate detector assembly misalignment parameters values;

an electronic correcting means for correcting the image reconstruction means in accordance with the misalignment parameter values.

4. The system as set forth in claim 3 wherein the first and second lines intersect with a preselected angle of orientation.

5. The system as set forth in claim 4 wherein the angle of intersection is orthogonal.

6. The system as set forth in claim 4 wherein the first and second lines share a common radiation source disposed at the intersection of the first and second lines.

7. The system as set forth in claim 3 wherein the reconstruction processor includes a weighting means for weighting data from the detector head with a weighting function, a filter for filtering the weighted data, and a backprojector for backprojecting the filtered, weighted data into the image representation for storage in the image memory and wherein:

the correcting means includes a means for adjusting the backprojecting means.

8. The system as set forth in claim 7 wherein the correcting means further includes a means for adjusting the weighting function means.

9. A method of automatically recalibrating a nuclear camera which includes at least one:

radiation detector head for detecting radiation in an examination region, a convergent ray collimator focuses at a focal point attached to the radiation detection head, a means for rotating the detector head around the examination region, and an image reconstruction processor for reconstructing image data from the detector head into the diagnostic image representation, the method comprising;

positioning a phantom in the examination region, which phantom includes first and second lines of radiation sources, each tine including at least three radiation sources and the first and second lines having a preselected, fixed orientation relative to each other;

rotating the detector head around the examination region;

from output data from the detector head generated while rotating around the phantom, generating values indicative of misalignment of the detector head;

electronically adjusting the image reconstruction processor in accordance with the misalignment values.

10. The method as set forth in claim 9 wherein the first and second lines intersect with a preselected angle of orientation.

11. The method as set forth in claim 10 wherein the angle of intersection is orthogonal.

12. The method as set forth in claim 10 wherein the first and second lines share a common radiation source disposed at the intersection of the first and second lines.

13. The method as set forth in claim 9 wherein the reconstruction means (i) weights data from the detector head with a weighting function, (ii) filters the weighted data, and (iii) backprojects the filtered, weighted data into the image representation with a backprojection function and wherein:

the adjusting step includes adjusting the backprojection function.

14. The method as set forth in claim 13 wherein the adjusting step further includes adjusting the weighting function.

15. The method as set forth in claim 9 wherein the misalignment values include an offset of an actual axis of rotation relative to a target axis of rotation, an actual distance between the detector head and the actual axis of rotation, a projected location of the focal point on the detector head, and an actual focal length of the collimator.

16. The method as set forth in claim 15 wherein the step of generating the misalignment values includes a χ

² minimization process.

17. The method as set forth in claim 15 wherein the step of generating the misalignment values includes varying smoothly between conjugate gradient and steepest descent minimization methods in accordance with Marquardt'"'"'s algorithm.

18. A phantom for a radiographic diagnostic imaging system in which radiation traverses an examination region along divergent paths, a radiation detection assembly which detects the radiation which has traversed the examination region along the divergent paths and producing electronic data indicative thereof, and a reconstruction processor for reconstructing the electronic data into a diagnostic image representation, the phantom comprising:

first and second lines of radiation sources removably disposed in the examination region, each line including at least three radiation sources and the first and second lines having a preselected, fixed orientation relative to each other.