Separation and identification of water and fat MR images at...

Details Separation and identification of water and fat MR images at... Separation and identification of water and fat MR images at...

2001-08-10

2003-08-05

Lateef, Marvin M. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C600S419000, C382S128000, C382S130000, C324S307000, C324S309000

Reexamination Certificate

active

06603990

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to magnetic resonance (MR) imaging (MRI) techniques. In particular, it relates to methods for generating water/fat separated images and, more particularly, to a method for generating and identifying water/fat component MR images with reduced T
2
/T
2
*
weighting.
BACKGROUND AND SUMMARY OF THE INVENTION
In broad summary, the present invention relates to MRI techniques for generating images in which fat and water regions can be readily discerned. MRI is often used to non-invasively generate images of the internal organs and other body parts of human patients. It is desirable to distinguish regions of water and fat in an MR image of a patient. Distinguishing water and fat regions using MR imaging techniques is difficult. At mid-field strength the Three-Point Dixon method is the method of choice for separating water and fat images. The original Dixon method for separation of water and fat images depends upon the ability to accurately compensate for inhomogeneities in a polarizing static magnetic field B
O
.
A problem in using the original Dixon method occurs because of inhomogeneities in the static B
O
field. Prior approaches to this problem have utilized additional information obtained during single or multiple scans (each having some associated disadvantages). One such prior approach is disclosed in U.S. Pat. No. 5,909,119 (the '119 patent) entitled “Method and Apparatus for Providing Separate Fat and Water MRI Images In A Single Acquisition Scan”.
Prior approaches for separating images of water and fat have not been entirely successful in generating MR images that provide good contrast between fat and water regions. Some approaches are slow and require at least three MR scans. These approaches with long acquisition times are susceptible to motion problems that arise if the patient moves during the scan period. Approaches that use single scans (and have fast scan periods) have long minimum echo periods, which result in heavy T
2
/T
2
*
weighted image contrasts. Heavy T
2
/T
2
*
weighting is undesirable when strong T
1
weighting is desired. Some single scan approaches, such as that disclosed in the '119 patent, do not uniquely identify water and fat nuclei images after separation. Instead, user intervention relying on anatomical information is needed for the identification. Accordingly, there are long-felt needs for an MRI technique able to uniquely identify water and fat regions, and that provides water and fat images with reduced T
2
/T
2
*
weighting. The present invention fulfills these needs.
By way of general background, MRI systems use the nuclear magnetic resonance (NMR) effects that RF transmissions at the nuclei Lamor frequency have on atomic nuclei having a net magnetic moment such as those in hydrogen. Applying RF transmissions to a patient affects the nuclear spin moments of the atomic nuclei in the body of the patient. The net magnetic moment of the nuclei in the patient are first magnetically aligned by a strong static magnetic field B
0
(e.g., typically created by magnetic poles on opposite sides of the MRI imaging volume or inside a solenoidal cryogenic superconducting electromagnet). The static field B
0
is altered by gradient magnetic fields created in the X, Y, and Z directions of the imaging volume. Selected nuclei, which are in spin alignment with the B
0
field, are nutated by a perpendicular magnetic field of a NMR RF transmission at the Lamor frequency. The nutation causes a population of such nuclei to tip from the direction of the magnetic field B
0
.
As shown in
FIG. 1
, certain nuclei (designated by magnetic moment M
0
) are aligned with the “Z′” axis by the static B
0
field and then rotated to the X′-Y′ plane as a result of an RF signal being imposed on the nuclei. The nuclei then precess in the X′-Y′ plane as shown by the circulating arrow in
FIG. 1
(which is a reference frame rotating at the nominal Lamor resonance frequency around the Z′ axis).
The NMR RF spin-nutating signal tips more than one nuclei species in the area targeted by the RF signal. Immediately after the nutating RF signal tips the nuclei, the spinning nuclei of all species are in-phase with each other. The rotating magnetic moments of all NMR species initially all rotate across the ‘Y’ axis all at approximately the same time. However, after the NMR nutating RF pulse ends, each species of nuclei begin to freely precess at its own characteristic speed around the Z′ axis.
As these nuclei precess, the phases of each of the rotating nuclei species will differ as a result of such parameters as the physical or chemical environment in which the nuclei are located. Nuclei in fat, for example, precess at a different rate than do nuclei in water. This difference in phase between water and fat nuclei is detected and used to distinguish water and fat in an MR image. In an MRI imaging pulse sequence there are also magnetic field gradients which dephase the moments due to their local resonance frequency varying in space. These phase differences in the nuclei spin moments are detected by an RF receiver and are used to determine the location and type of the source nuclei.
Once the nuclei spins are disturbed from their equilibrium, “relaxation” processes cause the phase-coherent component of magnetic moments in the X′-Y′ plane to decay and the Z′-component to recover to its equilibrium magnitude, M
0
. These processes are usually characterized by exponentials whose time constants are called T
2
and T
1
decay times, respectively. When magnetic resonance signals are observed through flux oscillation in a plane coexistent with the X′-Y′ plane, both of these processes decrease the signal strength as a function of time.
The relative phase of components of the magnetic moments in the X′-Y′ plane of
FIG. 1
begin aligned on the Y′-axis, but over time they spread out and disperse to fill the full rotational area in the X′-Y′ plane. The nuclei of moment M
2
, for example, which initially crossed the Y′-axis at the same time as M
0
, gradually moves during the dephasing period to the position shown in
FIG. 1
as it spins faster than M
0
. M
1
, by contrast, spins slower than both M
0
and M
2
, and thus begins to lag them during the dephasing period. The strength of the detectable NMR response signal decays as the relative phases of the magnetic moments disperse (i.e., lose phase coherence) in the X′-Y′ plane, a process often referred to as T
2
*
relaxation.
Information about NMR hydrogen nuclei can be obtained, in part, by measuring their T
2
and T
1
decay times. In addition, before the nuclei become completely dephased, another RF signal (e.g., a 180° signal) can tip the magnetic moments (e.g., to a 180° inverted position). This RF signal inverts the spinning magnetic moments M
0
, M
1
and M
2
of the three species of nuclei so the fastest moment M
2
now lags (instead of leading) moment M
0
, which in turn also now lags the slowest moment M
1
. Eventually, the faster moment M
2
will again catch up with and pass the slowest moment M
1
during which, a so-called “spin-echo” NMR RF response can be detected from the changes in net magnetic moment as the various magnetic moments come back into phase coherence. The whole procedure must be completed before T
1
or T
2
relaxation processes destroy the detectable X′-Y′ components of the magnetic moments.
Detectable NMR RF response echoes can also be formed by application of a field gradient and the subsequent reversal of the gradient, provided that the reversal is done before T
1
or T
2
relaxation decays destroy M
X′Y′
. This is commonly called a field echo, gradient echo or racetrack echo.
The differences in the phase relationships between the species of nuclei in one tissue versus another can be used as information to separate MRI images of fat components of tissue from fluids or water-based tissue (for these purposes, “water-based t

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