Reconstruction method for images of the beating heart
Projection views of the moving heart and stationary background tissues are acquired and processed to provide corresponding moving tissue projection views. An average image is reconstructed in a conventional manner and a moving tissue image is reconstructed using a highly constrained backprojection method and a composite image formed from selected moving tissue projection views. The average image is then combined with the moving tissue image. The method is disclosed in a cardiac gated MRI scan.
1. A method for producing an image of a moving subject positioned in a field of view (FOV) of a medical imaging system, the steps comprising:
a) acquiring with the medical imaging system a set of projection views of the subject for each of a series of image frames;
b) producing from the projection views acquired in step a) corresponding moving tissue projection views in which substantially only moving tissues are depicted;
c) producing from more than one set of the projection views acquired in step a) an average image;
d) reconstructing a composite image for one of said image frames using moving tissue projection views corresponding to the projection views acquired for said image frame plus other selected moving tissue projection views;
e) reconstructing a moving tissue image for said image frame by:
e)i) back projecting the moving tissue projection views corresponding to the projection views acquired for said image frame and weighting the value backprojected into each image pixel by the value of the corresponding pixel in the composite image; and
e)ii) summing the backprojected values for each image pixel; and
f) combining the moving tissue image and the average image.
2. The method as recited in claim 1 in which step c) includes reconstructing the average image from a sufficient number of projection views acquired in step a) that static tissues in the FOV are depicted with few artifacts.
3. The method as recited in claim 1 in which step b) includes: b)i) reprojecting the average image to produce average image reprojections; and b)ii) subtracting average image reprojections from corresponding acquired projection views to produce corresponding moving tissue projection views.
4. The method as recited in claim 3 is which substantially all the projection views acquired in step a) are used to reconstruct the average image in step c).
5. The method as recited in claim 4 in which the moving tissue projection views are produced in step b) by reprojecting the average image at each of the view angles of the projection views acquired in step a).
6. The method as recited in claim 1 in which the acquisitions in step a) are cardiac gated and the series of image frames depict the moving subject at a succession of cardiac phases.
7. The method as recited in claim 6 in which the projection views acquired in step a) are acquired over a plurality of heartbeats.
8. The method as recited in claim 7 in which the medical imaging system is a magnetic resonance imaging system.
9. The method as recited in claim 1 in which said other moving tissue projection views are selected in step d) by:
d)i) determining how much the subject has moved while acquiring each set of projection-views relative to the subject's position when the projection views for said image frame were acquired; and
d)ii) selecting moving tissue projection views that were acquired when the subject was within a predetermined distance from said image frame position.
10. The method as recited in claim 1 in which steps d), e) and f) are repeated to produce additional images from the other sets of image frame projection views.
11. The method as recited in claim 1 in which each weighted image pixel backprojected value S.sub.n is calculated in step e) as S n=(P .times. C n)/n=1 N .times. C n where P=the projection view being backprojected; C.sub.n=corresponding pixel value in the contour image; S.sub.n=the value of the no pixel along the backprojecting path; and N=total number of pixels along the backprojection path.
12. The method as recited in claim 1 in which the projection views acquired in step a) are interleaved.
13. The method as recited in claim 1 in which the moving tissue projection views used in step d) are interleaved.
14. The method as recited in claim 1 in which sets of projection views acquired in step a) correspond to a slice location acquired at each of a plurality of cardiac phases.
15. The method as recited in claim 14 in which the projection views acquired for said slice location are interleaved.
16. The method as recited in claim 15 in which the composite image is formed using moving tissue projection views corresponding to interleaved projection views acquired for said slice.
17. The method as recited in claim 16 in which steps d), e) and f) are repeated to produce an image depicting the moving subject at each of the cardiac phases.
18. The method as recited in claim 16 in which sets of projection views acquired in step a) correspond to additional slice locations acquired at each of the plurality of cardiac phases and steps d), e) and f) are repeated to produce an image depicting the moving subject at each of the slice locations.
19. The method as recited in claim 18 in which steps d), e) and f) are repeated to produce an image depicting the moving subject at each slice location at each of the cardiac phases.