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Reduction of CSF and Blood Flow Artifacts on FLAIR Images of the Brain with k-Space Reordered by Inversion Time at each Slice Position (KRISP)

Amy H. Herlihya, Joseph V. Hajnala, Walter L. Curatia, Nazma Virjia, Angela Oatridgea, Basant K. Puria and Graeme M. BydderGo,a

a From the Robert Steiner Magnetic Resonance Unit, MRC Clinical Sciences Centre, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Rd, London W12 0HS.



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  		FIG 1. Illustration of the KRISP FLAIR pulse sequence.

A, Three transverse slices through the brain.

B, Diagram shows T1 recovery curves for brain and CSF and illustrates sampling (with a 90° pulse at each data collection) at three progressively increasing TIs: {alpha}, ß, and {gamma}, with CSF nulled at time ß.

C, Diagram shows mapping of k-space for each of the three slices. In TR1, lines are mapped as shown with the first sample at TI = {alpha} to the first strip of k-space in slice 1; sample 2, at TI = ß to the second strip of slice 2; and sample 3 at TI = {gamma} to the third strip of k-space in slice 3. For TR2, the sample at TI = {alpha} is mapped to the first strip of slice 3, the sample at TI = ß to the second strip of slice 1, and so on.

D, Diagram shows the end result for each slice with the first strip of k-space mapped with TI = {alpha}, the middle strip with TI = ß, and the third strip with TI = {gamma}. Since the signals from tissues and fluids vary with TI as a function of their T1 value, this applies a tissue- and fluid-specific T1 filter across k-space, which is the same for each slice



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  		FIG 2. Diagrams show associations between regions of k-space and TE for a three-echo FSE sequence (A), as well as TI for a three-slice KRISP FLAIR sequence (B). The center of k-space controls image contrast, and the periphery of k-space contains information about fine structures, such as edges. With the FSE sequence, the TE used for the center of k-space is known as the effective echo time (TEeff). In A, the third echo, which has a long TE, provides the data for the center of k-space, and echoes 1 and 2, which have shorter TEs, are used to fill the periphery of k-space. A heavily T2-weighted image appearance with good edge definition would result from this type of acquisition. With the KRISP sequence, image contrast is determined by the TIeff, which is the TI of the data at the center of k-space. In B, a three-slice example is shown with TIs corresponding to the slice excitation timings shown in figure 1. Data with TIeff = ß are shown in the center of k-space and have been chosen to null CSF. The high-amplitude signals from brain and CSF at TI = {alpha} and TI = {gamma} provide good edge definition



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  		FIG 3. Comparison of conventional (A) and KRISP (B) FLAIR images at level of third ventricle obtained with identical parameters 8142/135/1; TI = 2250. The CSF signal is higher in A than in B at the foramen of Munro and in the third ventricle. The meninges are more clearly seen in B.



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  		FIG 4. Comparison of conventional (A) and KRISP (B) FLAIR images (8142/135/1; TI = 2250) at the level of the pons. In A there is high signal anterior to the pons as well as in the fourth ventricle and across the pons. This has been controlled in B, where the pons, cerebellum, and meninges are clearly seen



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  		FIG 5. Comparison of conventional (A) and KRISP (B) FLAIR images (8142/135/1; TI = 2250) at the level of the medulla. High levels of artifact are seen from the vertebral arteries and the CSF on A. These are not seen on B.



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  		FIG 6. Comparison of conventional (A) and KRISP (B) FLAIR sequences (8142/135/1; TI = 2250). High signal is seen in a sylvian branch of the middle cerebral artery on the right in A (arrow) but not in B.



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  		FIG 7. Extraaxial tumor (probably a meningioma) on conventional (A) and KRISP (B) FLAIR sequences (8142/135/1; TI = 2250). The tumor is difficult to see because of CSF artifacts on A, but is obvious on B (arrow)



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  		FIG 8. Postoperative cyst on conventional (A) and KRISP (B) FLAIR sequences (8142/135/1; TI = 2250). The cyst is of slightly increased signal in A but is of low signal in B (long arrow). The brain stem and partially resected meningioma (short arrow) are also better seen in B.



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  		FIG 9. False-positive finding on conventional (A) but not on KRISP (B) FLAIR sequence (8142/135/1; TI = 2250) in the suprasellar cistern. A high-signal lesion is seen on A (arrow) but not on B. It was not present on other sequence (SE 2500/20–80) or on transverse or sagittal T1-weighted sequences (SE 720/20). The left cerebral peduncle shows evidence of wallerian degeneration





Copyright © 2009 by the American Society of Neuroradiology. Print ISSN: 0195-6108 Online ISSN: 1936-959X