Method and system for determination of instantaneous and average blood flow rates from digital angiograms

• US 5,150,292 A
• Filed: 10/27/1989
• Issued: 09/22/1992
• Est. Priority Date: 10/27/1989
• Status: Expired due to Term

• Current Assignee: ARCH Development Corporation (University of Chicago)
• Original Assignee: ARCH Development Corporation (University of Chicago)
• Inventors: Hoffmann, Kenneth R., Doi, Kunio
• Primary Examiner(s): Shaw, Dale M.
• Assistant Examiner(s): Brutman, Laura
• Application Number:
  US07/428,059
• Time in Patent Office: 1,061 Days
• Field of Search: 128/653 R, 128/653 AF, 128/654, 128/691, 250/303, 364/413.07, 364/413.23, 378/62
• US Class Current: 600/431
• CPC Class Codes: A61B 6/481 involving the use of contrast agents
• CPC Class Codes: A61B 6/504 for diagnosis of blood vessels, e.g. by angiography
• CPC Class Codes: A61B 6/507 for determination of haemodynamic parameters, e.g. perfusion CT
• CPC Class Codes: G06T 2207/30101 Blood vessel; Artery; Vein; Vascular
• CPC Class Codes: G06T 7/0016 involving temporal comparison
• CPC Class Codes: G06T 7/20 Analysis of motion motion estimation for coding, decoding, compressing or decompressing digital video signals H04N19/43, H04N19/51

Abstract

A method and system for quantitation of blood flow rates by using digital subtraction angiographic (DSA) images, wherein the spatial shift of the distribution of contrast material injected into an opacified vessel in the acquired angiographic images is analyzed as a bolus of the contrast material proceeds through the vessel. In order to determine the distance that the bolus travels between image acquisitions, there is obtained from the DSA images the distribution of vessel contrast along the length of the vessel, called and "distance-density" curve. The distance that the contrast material travels during the time between two images acquisitions is determined by means of cross correlation of the two respective distance-density curves. The flow rate between the image acquisitions is calculated by multiplying this distance by the frame rate and the vessel cross-sectional area which is estimated from the vessel size assuming a circular cross section. Thus, for high frame-rate acquistions, instantaneous blood flow rates can be determined. The method and system are particularly useful for measurement of pulsatile blood flow rates.

Claims

1. A method for automated measurement of instantaneous and average blood flow rates using a digital X-ray image, comprising the steps of:

generating a sequence of digital X-ray images of an object as a contrast material is injected into the object;

storing said images;

identifying opacified vessels in each of said images;

measuring selected vessel parameters, including vessel size, contrast, and position, of the opacified vessels identified in said identifying step;

identifying the same vessel in different of said images;

deriving vessel longitudinal density curves representing vessel contrast as a function of one of longitudinal distance or volume along the length of said vessel based on the measured vessel contrast;

correlating said density curves from different images to measure one of distances or volumes traversed by said contrast material as a function of time; and

calculating at least one of instantaneous and average flow velocities and flow rates in said vessels from the measured one of distances or volumes and said measured vessel parameters.

2. The method as defined by claim 1, wherein said step of identifying opacified vessels in said images comprises:

selecting a starting point in an opacified vessel;

initiating tracking of said selected opacified vessel at said selected starting point;

measuring said vessel parameters, including local vessel size, for said selected opacified vessel at said selected starting point;

comparing values of the measured vessel parameters with preset threshold values;

identifying a next tracking point in said opacified vessel and identifying starting points of branches of said opacified vessels if said measured vessel parameters exceed said preset threshold values;

repeating said steps of measuring, comparing and identifying if measured vessel parameters exceed the said preset threshold values; and

terminating tracking of said opacified vessel if said measured vessel parameters do not exceed said preset threshold values.

3. The method according to claim 2, wherein said step of identifying starting points of branches of said opacified vessels comprises:

determining if one of said starting points of branch vessels has not been employed in said step of initiating tracking if vessel tracking of said opacified vessel has been terminated;

selecting said one of said starting points of branch vessels as a next starting point of a respective branch vessel to be next tracked if said respective branch vessel has not been previously tracked;

if said respective branch vessel has not been previously tracked, initiating tracking of said respective branch vessel at said next starting point,

repeating said steps of measuring, comparing and identifying if said values of said vessel parameters of said respective vessel exceed said preset threshold values, and

repeating said steps of terminating, determining, and initiating if said values of said vessel information do not exceed said present threshold values; and

halting tracking if all of said branch vessels have been tracked.

4. The method according to claim 2, wherein the double-square-box region-of-search step comprises:

centering on one said tracking points a first square box of width equal to twice said vessel size;

comparing the pixel values of pixels on the perimeter of said first square box to find local maxima on said perimeter;

identifying the maximum of said local maxima as a prospective next tracking point;

identifying others of said local maxima as starting points of branches;

calculating tracking direction as a vector from said one said tracking point to said prospective next tracking point;

ascertaining the existence of a tracking point immediately previous to said one said tracking point;

selecting as the next tracking point said prospective next tracking point if said tracking point immediately previous does not exist;

determining a previous tracking direction as a vector from the tracking point immediately previous to said one said tracking point to said one said tracking point if said tracking point immediately previous does exist;

comparing said tracking direction with said previous tracking direction if said tracking point immediately previous does exist;

selecting as a next tracking point said prospective next tracking point if said tracking point immediately previous does exist and change in tracking direction determined in said step of comparing does not exceed a preset threshold value; and

selecting a next tracking point using a second square box if said tracking point immediately previous does exist and change in tracking direction determined in said step of comparing does exceed a preset threshold value.

5. The method defined by claim 4, wherein said step of determining a next tracking point using a second square box on said tracking point comprises:

centering on said one of said tracking points a second square box of width equal to the said vessel size;

comparing the pixel values of pixels on the perimeter of said second square box to find a maximum pixel value on the perimeter of said second square box; and

selecting said maximum pixel value on the perimeter of the second square box as the next tracking point.

6. The method defined by claim 1, wherein said vessel parameter measuring step includes a step of measuring the size of the vessel by means of an iterative deconvolution step for vessels with sizes comparable to or less than a width of a line spread function of an imaging system from which said digital X-ray images were obtained.

7. The method defined in claim 6, wherein said iterative deconvolution step comprises:

obtaining actual vessel profiles from pixels lying along lines oriented perpendicular to the vessel axis in said images;

generating ideal vessel profiles by convolution of X-ray patterns of circular vessels with the line spread function of said imaging system;

comparing said ideal and actual vessel profiles using by determining root-mean-square differences therebetween;

changing the vessel size of said circular vessel if said root-mean-square difference is not minimum; and

selecting the vessel size of said circular vessel which yields the minimum of said root-mean-square differences to be the true vessel size.

8. The method defined by claim 1, wherein said step of deriving density curves comprises:

interpolating additional density curves from the density curves derived from said images to simulate higher frame rates than that used during acquisition of said images.

9. The method defined by claim 1, wherein the step of correlating said density curves correlates distance-density curves from different images to measure distances traversed by said contrast material as a function of time and comprises:

comparing the difference between a pair of said distance-density curves;

shifting one of said pair of said distance-density curves by selected distances until said difference is minimized;

selecting said distance for which said difference is minimized to be the distance that said contrast material traversed between acquisitions of said pair of images; and

representing said selected distance as a function of time.

10. The method defined by claim 1, wherein the step of correlating said density curves produces volume-density curves representing the volume traversed by said contrast material as a function of time and comprises:

comparing the difference between a pair of said distance-density curves;

shifting one of said pair of volume-density curves by selected volumes until said difference is minimized;

selecting said volume for which said difference is minimized to be the volume that said contrast material traversed between acquisitions of said pair of images; and

representing said selected volume as a function of time.

11. The method defined by claim 1, wherein the step of correlating measures distances traversed by said contrast material as a function of time, and said step of calculating comprises:

obtaining cumulative-distance curves as a function of time by summing said distance traversed by the contrast medium from the first image onward as a function of time;

calculating instantaneous velocities from the slopes of said cumulative-distance curves;

calculating the average cross-sectional area of said vessel from said vessel parameters;

calculating instantaneous flow rates by multiplying said instantaneous velocities by said average cross-sectional area of said vessel;

calculating average velocities from the slopes of lines fitted to the said cumulative-distance curves; and

calculating average flow rates by multiplying said average velocities by said average cross-sectional area of said vessel.

12. The method defined by claim 1, wherein the step of correlating measures volumes traversed by said contrast material as a function of time, and said step of calculating comprises:

obtaining cumulative-volume curves as a function of time by summing said volume traversed from the first image onward as a function of time;

calculating instantaneous flow rates from the slopes of said cumulative-volume curves;

calculating the average cross-sectional area of said vessel from said vessel parameters;

calculating instantaneous flow velocities by dividing said instantaneous flow rates by said average cross-sectional area of said vessel;

calculating average flow rates from the slopes of lines fitted to said cumulative-volume curves; and

calculating average flow velocities by dividing said average flow rates by said average cross-sectional area of said vessel.

13. A system for automated measurement of instantaneous and average blood flow rates using a digital X-ray image, comprising the steps of:

means for generating a sequence of digital X-ray images of an object as a contrast material is injected into the object;

means for storing said images;

means for identifying opacified vessels in each of said images;

means for measuring selected vessel parameters, including vessel size, contrast, and position, of the opacified vessels identified in said identifying means;

means for identifying the same vessel in different of said images;

means for deriving vessel longitudinal density curves representing vessel contrast as a function of one of longitudinal distance or volume along the length of said vessel based on the measured vessel contrast;

means for correlating said density curves from different images to measure one of distances or volumes traversed by said contrast material as a function of time; and

means for calculating at least one of instantaneous and average flow velocities and flow rates in said vessels from the measured one of distances or volumes and said measured vessel parameters.

14. The system as defined by claim 13, wherein said means for identifying opacified vessels in said images comprises:

means for selecting a starting point in an opacified vessel;

means for initiating tracking of said selected opacified vessel at said selected starting point;

means for measuring said vessel parameters, including local vessel size, for said selected opacified vessel at said selected starting point;

means for comparing values of the measured vessel parameters with preset threshold values;

means for identifying a next tracking point in said opacified vessel and identifying starting points of branches of said opacified vessels if said measured vessel parameters exceed said preset threshold values;

means for repeating said steps of measuring, comparing and identifying if said measured vessel parameters exceed said present threshold values; and

means for terminating tracking of said opacified vessel if said measured vessel parameters do not exceed said present threshold values.

15. The system according to claim 14, wherein said vessel tracking means further comprises:

means for determining if one of said starting points of branch vessels has not been employed in said step of initiating tracking if vessel tracking of said opacified vessel has been terminated;

means for selecting said one of said starting points of branch vessels as a next starting point of a respective branch vessel to be next tracked if said respective branch vessel has not been previously tracked;

means, if said respective branch vessel has not been previously tracked, for performing the functions of,

initiating tracking of said respective branch vessel at said next starting point,

repeating measuring, comparing and identifying if said values of said vessel parameters of said respective vessel exceed said preset threshold values, and

repeating terminating, determining, and initiating if said values of said vessel information do not exceed said preset threshold values; and

means for halting tracking if all of said branch vessels have been tracked.

16. The system as defined by claim 14, wherein the means for identifying a next tracking point further comprises:

means for centering on one said tracking points a first square box of width equal to twice said vessel size;

means for comparing the pixel values of pixels on the perimeter of said first square box to find local maxima on said perimeter;

means for identifying the maximum of said local maxima as a prospective next tracking point;

means for identifying others of said local maxima as starting points of branches;

means for calculating tracking direction as a vector from said one said tracking point to said prospective next tracking point;

means for ascertaining the existence of a tracking point immediately previous to said one said tracking point;

means for selecting as the next tracking point said prospective next tracking point if said tracking point immediately previous does not exist;

means for determining a previous tracking direction as a vector from the tracking point immediately previous to said one said tracking point to said one said tracking point if said tracking point immediately previous does exist;

means for comparing said tracking direction with said previous tracking direction if said tracking point immediately previous does exist;

means for selecting as a next tracking point said prospective next tracking point if said tracking point immediately previous does exist and change in tracking direction determined by said means for comparing does not exceed a preset threshold value; and

means for selecting a next tracking point using a second square box if said tracking point immediately previous does exist and change in tracking direction determined by said means for comparing does exceed a preset threshold value.

17. The system defined by claim 16, wherein said means for determining a next tracking point rising a second square box comprises:

means for centering on said one of said tracking points a second square box of width equal to said vessel size;

means for comparing the pixel values of pixels on the perimeter of said second square box to find a maximum pixel value on the perimeter of said second square box; and

means for selecting said maximum pixel value on the perimeter of the second square box as the next tracking point.

18. A system defined by claim 13, wherein said vessel parameter measuring means comprises:

means for measuring the size of the vessel by means of an iterative deconvolution for vessels with sizes comparable to or less than a width of a line spread function of said image generating means.

19. The system defined in claim 18, wherein said means for measuring by means of said iterative deconvolution step comprises:

means for obtaining actual vessel profiles from pixels lying along lines oriented perpendicular to the vessel axis in said images;

means for generating ideal vessel profiles by convolution of X-ray patterns of circular vessels with the line spread function of the image generating means;

means for comparing said ideal and actual vessel profiles by determining root-mean-square differences therebetween;

means for changing the vessel size of said circular vessel if said root-means-square difference is not a minimum; and

means for selecting the vessel size of the circular vessel which yields the minimum of said root-mean-square differences to be the true vessel size.

20. The system defined by claim 13, wherein said means for deriving density curves comprises:

means for interpolating additional density curves from the density curves derived from said images to simulate higher frame rates than that used during acquisition of said images.

21. The system defined by claim 13, wherein said means for correlating said density curves correlates distance-density curves to measure distances traversed by said contrast material as a function of time and comprises:

means for comparing the difference between a pair of said distance-density curves;

means for shifting one of said pair of said distance-density curves by selected distances until said difference is minimized;

means for selecting the distance for which said difference is minimized to be the distance that said contrast material traversed between acquisitions of said pair of images; and

means for representing said selected distance as a function of time.

22. The system defined by claim 13, wherein said means for correlating said density curves produces volume-density curves representing the volume traversed by said contrast material as a function of time and comprises:

means for comparing the difference between a pair of said volume-density curves;

means for shifting one of said pair of volume-density curves by selected volumes until said difference is minimized;

means for selecting the volume for which said difference is minimized to be the volume that said contrast material traversed between acquisitions of said pair of images; and

means for representing said selected volume as a function of time.

23. The system defined by claim 13, wherein said means for correlating measures distances traversed by said contrast material as a function of time, and said means for calculating comprises:

means for obtaining cumulative-distance curves as a function of time by summing the distance traversed by the contrast medium from the first image onward as a function of time;

means for calculating instantaneous velocities from the slopes of said cumulative-distance curves;

means for calculating the average cross-sectional area of said vessel from said vessel parameters;

means for calculating instantaneous flow rates by multiplying said instantaneous velocities by said average cross-sectional area of said vessel;

means for calculating average velocities from the slopes of lines fitted to the cumulative-distance curves; and

means for calculating average flow rates by multiplying said average velocities by said average cross-sectional area of said vessel.

24. The system defined by claim 13, wherein said means for correlating measures volumes traversed by said contrast material as a function of time, and said means for calculating comprises:

means for obtaining cumulative-volume curves as a function of time by summing volume traversed from the first image onward as a function of time;

means for calculating instantaneous flow rates from the slopes of said cumulative-volume curves;

means for calculating the average cross-sectional area of said vessel from said vessel parameters;

means for calculating instantaneous flow velocities by dividing said instantaneous flow rates by said average cross-sectional area of said vessel;

means for calculating average flow rates from the slopes of lines fitted to said cumulative-volume curves; and

means for calculating average flow velocities by dividing said average flow rates by said average cross-sectional area of said vessel.