Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-08-27
2001-07-17
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S410000, C128S899000, C324S307000, C324S309000, C324S312000, C324S300000, C324S320000
Reexamination Certificate
active
06263228
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetic resonance (MR) imaging (MRI) techniques. More particularly, it relates to methods for separating water and fat imaging information from single-scan single-point MRI Dixon sequences.
BACKGROUND OF THE INVENTION
Magnetic Resonance Imaging (MRI) has become a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structure of objects (such as the human body) having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR) phenomena. In MRI nuclei in a body to be imaged are polarized by imposing a strong main magnetic field Ho on the nuclei. Selected nuclei are excited by imposing a radio frequency (RF) signal at a particular NMR frequency. By spatially distributing the localized magnetic fields, and then suitably analyzing the resulting RF responses from the nuclei, a map or image of relative NMR responses as a function of the location of the nuclei can be determined. Following a Fourier analysis, data representing the NMR responses in space can be displayed on a CRT.
As shown in
FIG. 1
, the NMR imaging system typically includes a magnet
10
to impose the static magnetic field, gradient coils
12
for imposing spatially distributed magnetic fields in the three orthogonal coordinates, and RF coils
14
to transmit and receive RF signals to and from the selected nuclei. The NMR signal received by the coil
14
is transmitted to a computer
16
which processes the data into an image displayed on the display
18
. The magnetic resonance image is composed of picture elements called “pixels.” The intensity of a pixel is proportional to the NMR signal intensity of the contents of a corresponding volume element or “voxel” of the object being imaged. The computer
16
also controls the operation of the RF coils
14
and gradient coils
12
through the RF amplifier/detector
20
and gradient amplifiers
22
, respectively.
Each voxel of an image of the human body contains one or more tissues. The tissues of the human body are comprised primarily of fat and water. Fat and water have many hydrogen atoms (the human body is approximately
63
% hydrogen atoms). Since hydrogen nuclei have a readily discernible NMR signal, magnetic resonance imaging of the human body primarily images the NMR signal from the hydrogen nuclei.
In NMR, a strong static magnetic field is employed to align atoms whose nuclei have an odd number of protons and/or neutrons, such that the nuclei have a spin angular momentum and a magnetic dipole movement. A second RF magnetic field, applied as a single pulse transverse to the first field, pumps energy into the nuclei, which causes the angular orientation of the nuclei to flip by, for example, 90° or 180°. After this excitation, the nuclei precess and gradually relax into alignment with the static field. As they relax and precess, the nuclei emit energy in the form of weak but detectable free induction decay (FID). These FID signals and/or RF or magnetic gradient refocused “echoes” thereof (collectively referred to herein as MR signals) sensed by an NMR imaging system are used by a computer to produce images of the body.
The excitation frequency and the FID frequency are related by the Larmor relationship. This relationship states that the angular frequency, &ohgr;
0
, of the precession of the nuclei is the product of the magnetic field, B
0
, and the so-called magnetogyric ratio, &ggr;, a fundamental physical constant for each nuclear species:
&ohgr;
0
=B
0
·&ggr;
By superimposing a linear gradient magnetic field, B
Z
=Z·G
Z
on the static uniform field, B
0
, which is typically defined as the Z axis, for example, nuclei in a selected X-Y plane may be excited by proper choice of the frequency of the transverse excitation field applied along the X or Y axis. Similarly, a gradient magnetic field can be applied in the X-Y plane during detection of the MR signals to spatially localize emitted MR signals from the selected X-Y plane according to their frequency (and/or phase).
An example of an operation whereby the various coils produce the MR signal in a 3-D MRI is shown in
FIG. 2A
, which is a graphical representation of an example MRI acquisition sequence called a “field-echo” sequence. First, a gradient field, G
slice
, is superimposed along the main field to sensitize a “slab” of nuclei in the body to be imaged to a particular RF resonance frequency. An RF “nutation” pulse is then applied at the particular frequency to force some of the nuclei within the slab to precess in a perpendicular direction with respect to the main field. Thereafter, pulsed magnetic gradient fields of changing magnitudes, G
pe
and G
slice
, are used to phase encode the nuclei by inducing frequency differences, and hence phase differences, between nuclei in different locations along a direction within the slab. A gradient field applied perpendicular to the pe direction, in a “ro” (or “read out”) direction, G
ro
, first de-phases and then rephases the nuclei to form “field-echo” MR signals. During the field-echo, the applied gradient field, G
ro
, frequency encodes the selected slab of nuclei in the readout direction. The resultant MR signal (also called “raw data” or “k-space data”) is then read and analyzed by Fourier analysis. The frequency (domain) plot of that analysis is then scaled to render information about the nuclei population in Fourier space, which corresponds to an X-Y-Z position.
FIG. 2B
shows another example of a 2-D MRI surface. In this case, a slice of nuclei are selected by a 90° followed by a 180° RF to generate a spin-echo. When the time between the 180°-center and the echo-center is the same as that between the 90°-center and the 180°-center, a symmetric spin-echo is generated. However, the echo-center can be shifted by adjusting the field gradients so that an asymmetric spin-echo is produced instead.
In addition to using the frequency information content of an MR signal to generate images, the phase of an MR signal in frequency domain can be utilized to provide information indicative of some physical quantity. For example, depending on the type of pulse sequence used, the MR phase can represent a main B
0
field inhomogeneity or can be proportional to the velocity of the moving spins.
As mentioned above, the main magnetic field can be altered by gradient magnetic fields created in the X, Y, and Z directions of the imaging volume. For the purpose of simplifying the descriptive mathematics, a rotating reference coordinate system X′-Y′-Z′, that rotates at the nominal Larmor frequency about the Z′ axis, is often used to describe nuclear phenomenon in NMR. Selected nuclei, which precess in alignment with the B
0
field, are influenced (nutated) by the perpendicular magnetic field of an RF pulse at the Lamor frequency, causing a population of such nuclei to tip from the direction of the magnetic field B
0
. Thus, certain nuclei start aligned with the “Z′” axis by the static B
0
field and then are rotated to the X′-Y′ plane as a result of the RF signal being imposed on them. The nuclei then precess in the X′-Y′ plane.
The RF spin-nutating signal will, of course, tip more than one species of the target isotope in a particular area. For the purpose of simplifying the description in analyzing an MRI sequence, each RF pulse can be characterized by its center where nuclei can be considered all in-phase and whereafter each species of nuclei precess at their own characteristic speed. The phase of the processing nuclei species will gradually differ (de-phase) as a result of such parameters as the physical or chemical environment that the nuclei are located in. Nuclei in fat, for example, precess at a different rate than do nuclei in water due to their difference in chemical shift. Field gradients and inhomogenieties also contribute to this de-phasing.
MRI sequences are almost always designed to have the field gradients fully refocused at the echo-center,
Kramer David M.
Li Andrew
Zhang Weiguo
Lateef Marvin M.
Lin Jeoyuh
Nixon & Vanderhye P.C.
Toshiba America MRI Inc.
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