Time resolved computed tomography angiography
A contrast enhanced dynamic study of a subject is performed with a CT system. A series of undersampled image data sets are acquired during the study with successive data sets acquired at interleaved projection angles. More fully sampled image data sets are formed by transforming the x-ray attenuation projection data to k-space and then sharing peripheral k-space data between undersampled k-space data sets. Artifacts due to undersampling are thus reduced without significantly affecting the time resolution of a series of reconstructed images.
1. A method for producing an image with a computed tomography (CT) system, the steps comprising:
a) positioning a subject in the CT system;
b) acquiring a first undersampled image data set from a selected region of interest in the subject, the undersampled image data set comprising a plurality of projections acquired at a corresponding plurality of projection angles;
c) transforming the undersampled image data set to form a corresponding first undersampled k-space data set;
d) repeating steps b) and c) to produce a second undersampled k-space data set from a plurality of projections acquired at projection angles interleaved with the projection angles used to acquire the first undersampled image data set;
e) combining k-space data from one of said undersampled k-space data sets with peripheral k-space data from the other undersampled k-space data set; and
f) reconstructing an image of the region of interest by transforming the combined k-space data.
2. The method as recited in claim 1 in which the region of interest includes vasculature and the method further includes:
g) injecting a contrast agent which flows into the vasculature;
h) repeating steps b), c), d), e) and f) to produce a contrast enhanced image of the region of interest; and
i) subtracting the image of the region of interest from the contrast enhanced image of the region of interest to produce an angiogram.
3. The method as recited in claim 2 in which steps h) and i) are repeated to produce a time series of angiograms that depict the vasculature as the contrast agent flows into the vasculature.
4. The method as recited in claim 1 which includes repeating steps b) and c) to produce a third undersampled k-space data set from a plurality of projections acquired at projection angles interleaved with the projection angles used to acquire the first and second undersampled image data sets; and
step e) is performed by combining substantially all the k-space data from the second undersampled k-space data set with peripheral k-space data from the first and third undersampled k-space data sets.
5. The method as recited in claim 1 in which step c) is performed by Fourier transforming each acquired projection.
6. The method as recited in claim 1 in which step f) includes:
regridding the combined k-space and;
Fourier transforming the regridded k-space data.
7. A method for producing an image of a subject with a computed tomography x-ray imaging system, the steps comprising:
a) acquiring a set of projection data with the imaging system from a selected view angle with respect to the subject;
b) changing the view angle by a first selected amount;
c) repeating steps a) and b) to acquire a first plurality of sets of projection data; d) transforming the first plurality of sets of projection data to form a first undersampled k-space data set;
e) changing the view angle by a second selected amount which is less than said first selected amount;
f) repeating steps a) and b) to acquire a second plurality of sets of projection data that are interleaved with the first plurality of sets of projection data;
g) transforming the second plurality of sets of projection data to form a second undersampled k-space data set;
h) combining the k-space data from one of said first or second undersampled k-space data sets with peripheral k-space data from the other of said first or second undersampled k-space data sets to form a more fully sampled k-space data set; and
i) reconstructing an image from the more fully sampled k-space data set.
8. The method as recited in claim 7 in which the imaging system acquires x-ray attenuation projection data and the undersampled k-space data sets are formed in steps d) and g) by Fourier transforming each set of projection data in the respective first and second plurality of sets of projection data.
9. The method as recited in claim 8 in which each set of acquired projection data is a two-dimensional array of x-ray attenuation data and steps d) and g) are performed by Fourier transforming each set of projection data along one axis of the two-dimensional array of x-ray attenuation data.
10. The method as recited in claim 9 in which the undersampled k-space data sets and the more fully sampled k-space data sets are three-dimensional data sets and the reconstructed image is a three-dimensional image.
11. The method as recited in claim 10 which further includes:
j) producing a two-dimensional projection image from the reconstructed three-dimensional image.
12. The method as recited in claim 7 in which step i) includes:
regridding the more fully sampled k-space data set; and
Fourier transforming the regridded k-space data set.
13. A method for producing an image of a subject with a computed tomography x-ray imaging system, the steps comprising:
a) acquiring a series of undersampled image data sets with the imaging system, each successive undersampled image data set being a set of projections acquired at respective projection angles which are interleaved with projection angles used to acquire temporally adjacent undersampled image data sets;
b) transforming the series of undersampled image data sets into a corresponding series of undersampled k-space data sets;
c) selecting a plurality of the undersampled k-space data sets in said series;
d) combining substantially all the k-space data from one of the selected undersampled k-space data set with peripheral k-space data from the other of the plurality of selected undersampled k-space data sets; and
e) reconstructing an image from the combined k-space data.
14. The method as recited in claim 13 in which a time series of images are produced by repeating steps c), d) and e) a plurality of times and selecting a different plurality of undersampled k-space data sets in said series during each repetition.
15. The method as recited in claim 13 in which step a) is performed by acquiring the undersampled image data sets using three different sets of projection angles which are interleaved with each other.
16. The method as recited in claim 15 in which step c) is performed by selecting three successive undersampled k-space data sets in said series.
17. The method as recited in claim 16 in which step d) is performed by combining substantially all the k-space data from the second of the three selected successive undersampled k-space data sets with peripheral k-space data from the first and the third of the three selected successive undersampled k-space data sets.
18. The method as recited in claim 13 in which step b) is performed by Fourier transforming each projection.
19. The method as recited in claim 13 in which step d) includes:
regridding the combined k-space data; and
Fourier transforming the regridded k-space data.
20. The method as recited in claim 13 in which the imaging system acquires projections as a two-dimensional array of X-ray attenuation data and step b) is performed by Fourier transforming each projection along one axis of the two-dimensional array of x-ray attenuation data.
21. The method as recited in claim 13 in which a contrast agent is added to the subject during the performance of step a), and the undersampled k-space data sets selected in step c) are acquired after the addition of the contrast agent, the method further comprising:
f) producing a mask image by repeating steps c), d) and e) with undersampled k-space data sets acquired before addition of the contrast agent; and
g) subtracting the mask image from said image.