Moving table MRI with frequency-encoding in the z-direction

Details Moving table MRI with frequency-encoding in the z-direction Moving table MRI with frequency-encoding in the z-direction

2001-10-05

2004-09-21

Gutierrez, Diego (Department: 2859)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S318000, C324S306000, C324S307000, C600S410000, C600S415000

Reexamination Certificate

active

06794869

ABSTRACT:

BACKGROUND OF INVENTION
The present invention relates generally to an improved method of medical imaging over large areas, and more particularly, to a method and apparatus of acquiring magnetic resonance (MR) images over an area that is greater than the optimal imaging area of an MR scanner using continuous table movement through the MR scanner without incurring slab-boundary artifacts.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M
z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
. A signal is emitted by the excited spins after the excitation signal B
1
is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G
x
G
y
and G
z
) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In such MRI systems, the volume for acquiring MR data with optimal gradient linearity, having a uniform magnetic field B
o
, and uniform radio frequency (RF) homogeneity is of limited extent. Desired fields-of-view (FOV) that exceed this limited volume are traditionally acquired in sections, with table motion between scans. The resulting concatenated images often exhibit discontinuities at the slab junctions. These slab-boundary artifacts result in non-ideal images. When these artifacts are either severe or occur in a critical region-of-interest, complete re-acquisition of data may be needed for a thorough analysis. In addition, the time for table motion extends scan time beyond that required for data acquisition.
Known methods designed to eliminate slab boundary artifacts in angiograms and 3D fast spin echo acquisitions include Sliding Interleaved k
y
Acquisition (SLINKY) and Shifted Interleaved Multi-Volume Acquisition (SIMVA). These methods however, move the slab-selective excitation while keeping the table stationary rather than moving the table and keeping the slab position stationary. As a result, these methods are limited by the inherent optimal imaging volume of the MR scanner. These methods employ phase encoding in the z-direction and do not account for those situations where the z matrix is not equal to the number of k
x
-k
y
subsets. Similarly, other known methods designed to eliminate slab boundary artifacts have likewise moved the imaging slab rather than the patient table. Other known techniques implement a continuously moving table in both the phase encoding and frequency direction. However, prior techniques that move the patient table in the frequency-encode direction encode the data as if they were acquiring an image of the entire FOV and data is combined prior to any Fourier transform.
Other known systems employ stepped table and/or moving table approach with an array of receiver coils that move with the imaged object. Data is collected from each coil independently as it moves through the homogeneous volume of the scanner. None of these known systems however, collect data from a slab thickness that is fixed relative to the magnet, place the frequency encoding axis in the direction of table motion, and combine the data after Fourier transforming in the direction of table motion.
It would therefore be desirable to have a new method and apparatus that allows coverage of large FOV without slab-boundary artifacts in the resulting concatenated images.
BRIEF DESCRIPTION OF INVENTION
The present invention relates to a system and method of acquiring large FOV MR images using continuous table motion for increased volume coverage that results in reconstructed images without discontinuities.
Slab-boundary artifacts are eliminated and scan time is reduced in an acquisition sequence by continuously moving an imaging object with respect to the optimal volume of the imaging apparatus, or vice versa. The thickness of the slab, which is smaller than the desired FOV, is selected to remain within the optimal volume of the MR system. The selected slab position remains fixed relative to the magnet of the MR scanner during the scan, and the table is moved continuously during scanning of the entire FOV. MR data is acquired by applying an excitation that excites spins and applying magnetic field gradient waveforms to encode the volume of interest The volume of interest is restricted in the direction of table motion. The magnetic field gradients traverse k-space following a trajectory that is uniform in the k-space dimension that is in the direction of table motion. The magnetic field gradient waveforms that encode the k-space directions perpendicular to table motion are divided into subsets. During each acquisition, all the k-space data in the direction of table motion are acquired for a subset of at least one other k-space dimension. After acquisition, data is transformed in the direction of table motion, sorted, and aligned to match anatomical locations in the direction of table motion. This procedure is repeated to fill the entire matrix. A final image is reconstructed by transforming the data in the remaining dimension(s) perpendicular to table motion. This approach provides reconstructed images absent of slab-boundary artifacts over a large FOV.
In accordance with one aspect of the invention, a method of imaging large volumes without resulting slab-boundary artifacts includes defining a desired FOV larger than an optimal imaging volume of an MR scanner and selecting a slab thickness in a first direction that is smaller than the desired FOV but that is within the optimal imaging volume of the MR scanner. MR data is then acquired by exciting and encoding spins to acquire data that is restricted to the selected slab thickness. The imaging object is then moved continuously with respect to the imaging area, or vice versa. This process is repeated until the desired FOV is fully encoded using a series of cyclically repeated magnetic field gradient waveforms. The selected trajectory need only be uniform in the direction of motion.
In accordance with another aspect of the invention, an MRI apparatus is disclosed to acquire multiple sets of MR data with a moving table and reconstruct MR images without slab-boundary artifacts that includes a magnetic resonance imaging system having an RF transceiver system and a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF switch is controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR data. A patient table is moveable fore and aft in the MR system about the magnetic bore to translate the patient so that an FOV larger than the optimal scanning area of the MRI system can be scanned. A computer is programmed to receive input defining a desired FOV larger than the optimal imaging volume of the MR system. The computer is also capable of defining a fixed slab with respect to the magnet to acquire the MR data therein. The computer then acquires full MR data using frequency encoding in the direction of table motion for a subset of the data in the direction(s) perpendicular to table motion while repositioning the patient table and maintaining the position of the fixed slab. The algorithm is repeated, collecting the necessary MR data across the defined FOV. The patient table can be tracked using the acquired MR data. To reconstruct the image, the MR data is first transformed in the direction of table m

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