Magnetic resonance imaging using blood flow navigation
An fMRI study is conducted using a 2D EPI pulse sequence. A separate blood flow navigator pulse sequence is interleaved with the fMRI data acquisition to monitor blood flow in arteries that feed the brain tissue. Information in the blood flow navigator data is used to reduce errors in the fMRI data due to tissue motion caused by pulsatile blood flow. In one embodiment the blood flow navigator information is used to gate the fMRI acquisition and in another embodiment the information is used to retrospectively correct acquired fMRI data.
1. A method for acquiring MRI data with a magnetic resonance imaging (MRI) system from tissues supplied by pulsatile blood flow through an artery, the steps comprising:
a) performing a blood flow navigator pulse sequence with the MRI system to acquire navigator data from said artery that indicates blood flow, the navigator pulse sequence including a magnetization maintenance sequence that produces an RF excitation pulse having a flip-angle selected to maintain a steady-state spin magnetization in said tissues;
b) acquiring tissue data from said tissues using the MRI system; and
c) using information in the navigator data acquired with the blood flow navigator pulse sequence to reduce the effect of pulsatile blood flow on an image of the tissues reconstructed with the tissue data acquired in step b).
2. The method as recited in claim 1 in which step c) includes using the navigator data acquired in step a) to produce a gating signal, and using the gating signal to initiate the acquisition of the tissue data in step b).
3. The method as recited in claim 1 in which step c) includes using the navigator data acquired in step a) to calculate corrections; and correcting the tissue data acquired in step b) using the calculated corrections.
4. The method as recited in claim 1 in which the tissues are located in the brain of a subject and the artery is located in the neck of the subject.
5. The method as recited in claim 4 in which steps a) and b) are repeated a plurality of times to form a plurality of images from the tissue data.
6. The method as recited in claim 5 in which the plurality of images form a time course fMRI study.
7. The method as recited in claim 1 in which the blood flow navigator pulse sequence also includes a velocity encoding gradient pulse, and step c) further includes reconstructing a phase image from the navigator data acquired in step a).
8. The method as recited in claim 7 in which the blood flow navigator pulse sequence acquires the navigator data using an echo planar image (EPI) pulse sequence.
9. The method as recited in claim 8 in which the tissue data is acquired in step b) using an EPI pulse sequence.
10. The method as recited in claim 9 in which the EPI pulse sequence employed in step b) is a two dimensional EPI pulse sequence that acquires image data from a slice of said tissues, and the two-dimensional EPI pulse sequence is performed a plurality of times.
11. The method as recited in claim 1 in which the RF excitation pulse in the magnetization maintenance sequence is spatially selective and the magnetization maintenance sequence includes:
producing a slab select gradient that operates in combination with the spatially selective RF excitation pulse to excite spins in a slab that includes the tissues.
12. The method as recited in claim 11 in which the magnetization maintenance sequence also includes:
producing a spoiler gradient that substantially dephases transverse spin magnetization produced in said slab by said RF excitation pulse.