Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-12
2001-12-18
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S307000, C324S309000, C324S310000, C324S311000, C324S312000, C324S314000, C324S318000
Reexamination Certificate
active
06332088
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to interventional magnetic resonance imaging (IMRI) techniques. In particular, it relates to methods and apparatus for the imaging of paramagnetic, diamagnetic or ferromagnetic instruments within tissues during interventional procedures and, more particularly, to a method of generating of an asymmetric spin-echo imaging sequence whereby the apparent size of such an instrument can be varied.
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
14
for imposing spatially distributed magnetic fields along three orthogonal coordinates, and RF coils
15
and
16
to transmit and receive RF signals to and from the selected nuclei. The NMR signal received by the coil
16
is transmitted to a computer
19
which processes the data into an image displayed on display
24
. 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
19
also controls the operation of RF coils
15
and gradient coils
14
through the RF amplifier/detector
22
and gradient amplifiers
20
, respectively.
Each voxel of an image of the human body contains information representative of 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.
Basically, in NMR a strong static magnetic field is employed to commonly align atoms whose nuclei have a spin angular momentum and a magnetic dipole movement, i.e., atoms whose nuclei have an odd number of protons and/or neutrons. A second magnetic field, applied transverse to the first field as a single RF pulse, 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 back into alignment with the static field. As the nuclei precess and relax, they will induce a weak but detectable electrical energy in the surrounding coils that is known as free induction decay (FID). These FID signals, as well as magnetic gradient or RF refocused “echoes” thereof, which are collectively referred to herein as MR signals, are analyzed by a computer to produce images of fat and water containing tissue; 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 RF excitation field pulse applied along the X or Y axis. In addition, gradient magnetic fields are 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 phases.
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 pulse being imposed on them. The nuclei then precess about the Z′ axis in the X′-Y′ plane.
The RF nutating pulse will, of course, tip more than one species of the target isotope in a particular area. For the purpose of simplifying the description and analysis of an MRI acquisition sequence, each RF pulse used in the sequence is characterized by its center—which is representative of the time at which the nutated precessing nuclei can be considered as all being in-phase (synchronized) and after which each species of nuclei will begin to precess at their own characteristic speed. The phase of the precessing nuclei species will gradually differ (de-phase) as a result of such parameters as the physical or chemical environment in which the nuclei are located.
Once the magnetic moments of the nuclei are disturbed from their equilibrium, processes known as “relaxation” causes the Z′-component of the spinning (precessing) magnetic moments to recover to an equilibrium magnitude, M
0
, in alignment with the background B
0
field, and the phase-coherent component in the X′-Y′ plane to decay. The duration of these relaxation processes are termed the “spin-lattice relaxation” time and the “spin-spin relaxation” time and are characterized by exponentials whose defined time constants are labeled as T
1
and T
2
, respectively. As magnetic resonance signals are exhibited through the oscillation of magnetic flux from nuclei in a plane coexistent with the X′-Y′ plane, both of these relaxation processes cause the signal strength to decrease as a function of time. In the interests of accuracy, an “apparent relaxation” time constant, T
2
*, is often defined as characterizing transverse signal decay due to spin-spin relaxation in the presence of B
0
field inhomogeneities.
An operation whereby the various coils produces RF excitation pulses and gradient fields to result in an MR signal is called an MRI “acquisition sequence.” A graphical representation of an example MRI acquisition sequence is shown in FIG.
2
. In this example, the particular timing of applied RF pulses and magnetic fields is known as a “spin-echo” sequence since the MR signals appear as an “echo” of a 180° spin rotation RF pulse. First, a gradient field, G
slice
, is superimposed along the main field to sensitize a “slice” or “slab” (for 3D imaging) of nuclei in the patient's body tissue to a particular RF resonance frequency. A 90° RF excitation or “nutation” pulse, &bgr;, is then applied at the particular frequency to force some of the nuclei within the slab to precess in a direction perpendicular with respect to the main field. Thereafter, pulsed magnetic gradient fields of changing magnitudes, G
pe
(and G
slice
for 3D imaging), are used to phase encode the nuclei by inducing a temporary freque
Avram Hector
Kaufman Leon
Zhang Weigno
Lateef Marvin M.
Lin Jeoyuh
Nixon & Vanderhye P.C.
Toshiba America MRI Inc.
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