Variable bandwidth MRI data collection

Details Variable bandwidth MRI data collection Variable bandwidth MRI data collection

1999-03-08

2001-07-03

Oda, Christine (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S307000

Reexamination Certificate

active

06255820

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In MRI, a substantially uniform temporally constant main magnetic field, B
0
, is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the magnetic field by transmitting radio frequency (RF) excitation signals into the examination region. Specifically, RF pulses transmitted via an RF coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding RF magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The RF magnetic resonance signal is received by the RF coil assembly, and from received signals, an image representation is reconstructed for display on a human viewable display.
The appropriate frequency for exciting resonance in selected dipoles is governed by the Larmor equation. That is to say, the precession frequency of a dipole in a magnetic field, and hence the appropriate frequency for exciting resonance in that dipole, is a product of the gyromagnetic ratio &ggr; of the dipole and the strength of the magnetic field. In a 1.5 T magnetic field, hydrogen (
1
H) dipoles have a resonance frequency of approximately 64 MHz. Generally in MRI, the hydrogen species is excited because of its abundance and because it yields a strong MR signal. As a result, typical MRI apparatus are equipped with built-in whole-body RF coils tuned to the resonant frequency for hydrogen.
One obstacle to overcome in MRI is potential degradation in the image reconstruction due a low signal to noise ratio (SNR) in the acquired MR signals or echos.
Previously developed methods employed to address the SNR problem have had generally limited success due to various drawbacks. Some multi-echo sequences decrease the bandwidth of data collection for each echo in succession such that the later echos, which are of a lower amplitude due to MR signal decay, are collected at a lower bandwidth to preserve SNR as best as possible. One example is multi-echo spin echo sequences which provide different contrasts in a set of images built each in turn from one of the echos. However, these techniques do not vary the bandwidth of data collection within a single image. Additionally, the SNR remains degraded by the noise accompanying data from the corners of k-space which are not fully visualized in the image.
Normally, a number of rows or horizontal lines with a predetermined number of sample points are collected for an MR image. This raw data often fills k-space in a rectangular or square shape. Employment of a circular Fermi filter applied to the raw MR data to chop off the corners of k-space has been shown to improve SNR by as much as 13%. However, the disadvantage associated with this technique is that resources are wasted on the collection of data which is ultimately discarded.
There is also the variable encoding time (VET) method. Recognizing that central lines of k-space generally employ less phase encoding as compared to the outer lines, the VET method shortens the inter-echo spacing of a sequence while varying the data sampling window. By shortening the data sampling window time and preserving the same sampling bandwidth, time is made available for larger phase encode lobes, or the time is made available for more data samples when the desired phase encode amounts are low. This allows for the trimming of k-space corners.
The present invention contemplates a new and improved data collection technique which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a method of MRI data collection with multiple data acquisitions for a single image reconstruction is provided. It includes initiating an MRI pulse sequence and collecting MRI data from a first resulting echo at a first sample rate and bandwidth. Thereafter, the sample rate and bandwidth are varied such that they are set to a new sample rate and bandwidth. MRI data is then collected from a next resulting echo at the new sample rate and bandwidth. The steps of varying the bandwidth and sample rate and collection of resulting echos are repeated for consecutive echos until a desired amount of MRI data is collected. Ultimately, an image representation is reconstructed from the collected MRI data.
In accordance with a more limited aspect of the present invention, variations in the sample rate and bandwidth are such that a total collection time for each echo remains substantially constant.
In accordance with a more limited aspect of the present invention, the collection of MRI data is conducted with a fixed duration window such that varying the sample rate varies a number of data points sampled.
In accordance with a more limited aspect of the present invention, collected MRI data from the first echo is mapped to a central line of k-space.
In accordance with a more limited aspect of the present invention, later collected MRI data from subsequent echos are progressively mapped to outer lines of k-space.
In accordance with a more limited aspect of the present invention, the bandwidth is varied such that it is progressively made narrower.
In accordance with a more limited aspect of the present invention, the number of data points sampled decreases as progressively outer lines of k-space are mapped such that a circular shaped area of k-space is filled.
In accordance with a more limited aspect of the present invention, the MRI pulse sequence is selected from a group consisting of a GSE sequence, a FSE sequence, and a single shot FSE sequence.
In accordance with a more limited aspect of the present invention, the bandwidth is varied by changing an amplitude of a read gradient so that a predetermined FOV is maintained.
In accordance with a more limited aspect of the present invention, the bandwidth selected for each echo is determined based on the echo's relative signal strength such that lower signal strengths correspond to lower selected bandwidths.
In accordance with another aspect of the present invention, an MRI apparatus is provided. It includes a main magnet that generates a substantially uniform temporally constant main magnetic field through an examination region wherein an object being imaged is positioned. A magnetic gradient generator produces magnetic gradients in the main magnetic field across the examination region. A transmission system includes an RF transmitter that drives an RF coil which is proximate to the examination region. A sequence control manipulates the magnetic gradient generator and the transmission system to produce an MRI pulse sequence. The MRI pulse sequence induces magnetic resonance echos from the object. A reception system includes a receiver that receives and demodulates the echos at varying sample rates and varying bandwidths. A reconstruction processor reconstructs a single image from data collected via the reception system, and an output device produces a human viewable rendering of the image.
In accordance with a more limited aspect of the present invention, variations in the sample rate and variations in the bandwidth are such that a total collection time for each echo remains substantially constant.
In accordance with a more limited aspect of the present invention, reception of each echo is conducted with a fixed duration window such that variations in sample rate translate to variations in number of data points sampled.
In accordance with a more limited aspect of the present invention, multiple acquisitions

Affiliated with

Also associated with

Rating

Variable bandwidth MRI data collection does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Variable bandwidth MRI data collection, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Variable bandwidth MRI data collection will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2477810