Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-12-01
2002-11-05
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S318000
Reexamination Certificate
active
06476606
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates to apparatus and methods for magnetic resonance imaging (MRI), also known as nuclear magnetic resonance imaging (NMRI). More particularly the present invention relates to methods and apparatus for decreasing magnetic resonance data acquisition times including time for reconstructing the image wherein the data is acquired and the image is reconstructed in parallel. The present invention also relates to other methods and MRI systems and apparatus related thereto.
BACKGROUND OF THE INVENTION
Magnetic resonance imaging (MRI) is a technique that is capable of providing three-dimensional imaging of an object. A conventional MRI system typically includes a main or primary magnet(s) that provides the background magnetic field B
o
, gradient coils and radio frequency (RF) coils, which are used for spatial encoding, exciting and detecting the nuclei for imaging. Typically, the main or primary magnet(s) are designed to provide a homogeneous magnetic field in an internal region within the main magnet, for example, in the air space of a large central bore of a solenoid or in the air gap between the magnetic pole plates of a C-type magnet. The patient or object to be imaged is positioned in the homogeneous field region located in such air space. The gradient field and the RF coils are typically located external to the patient or object to be imaged and inside the geometry of the main or primary magnet(s) surrounding the air space. There is shown in U.S. Pat. Nos. 4,968,937 and 5,990,681, the teachings of which are incorporated herein by reference, some exemplary MRI systems.
In MRI, the uniform magnetic field B. generated by the main or primary magnet(s) is applied to an imaged object by convention along the z-axis of a Cartesian coordinate system, the origin of which is within the imaged object. The uniform magnetic field B
o
being applied has the effect of aligning the nuclear spins, a quantum mechanical property of macroscopic particles comprising the imaged object, along the z-axis. In response to RF pulses of the proper frequency, that are orientated within the XY plane, the nuclei resonate at their Larmor frequencies. In a typical imaging sequence, the RF signal centered about the desired Lamor frequency is applied to the imaged object at the same time a magnetic field gradient G
z
is being applied along the z-axis. This gradient field G
z
causes only the nuclei in a slice with a limited width through the object along the XY plane, to have the resonant frequency and to be excited into resonance.
After excitation of the nuclei in the slice, magnetic field gradients are applied along the x-axis and y-axis respectively. The gradient G
x
along the x-axis causes the nuclei to precess at different frequencies depending on their position along the x-axis, that is, G
x
spatially encodes the precessing nuclei by frequency (i.e., frequency encoding). The y-axis gradient G
y
is incremented through a series of values and encodes the Y position into the rate of change of the phase of the precessing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
The quality of the image produced by the MRI techniques is dependent, in part, upon the strength of the magnetic resonance (MR) signal received from the precessing nuclei. For this reason an independent RF coil is placed in close proximity to the region of interest of the imaged object in order to improve the strength of the received signal. Such RF coils are sometimes referred to as local coils or surface coils.
There is described in U.S. Pat. No. 4,825,162 a surface coil(s) for use in MRI/NMRI imaging and methods related thereto. In the preferred embodiment of that invention, each surface coil is connected to the input of an associated one of a like plurality of low-input-impedance preamplifiers, which minimizes the interaction between any surface coil and any other surface coils not immediately adjacent thereto. These surface coils can have square, circular and the like geometries. This yields an array of a plurality of closely spaced surface coils, each positioned so as to have substantially no interaction with all adjacent surface coils. A different MR response signal is received at each different one of the surface coils from an associated portion of the sample enclosed within the imaging volume defined by the array. Each different MR response signal is used to construct a different one of a like plurality of different images then being combined, on a point-by-point, basis to produce a single composite MR image of a total sample portion from which MR response signal distribution was received by any of the array of surface coils.
In the case of MRI phased-array coils, coils are de-coupled by two mechanisms; any adjacent pair of coils are de-coupled by overlapping and non-adjacent coils are de-coupled by combination of matching circuits and low impedance pre-amplifiers. The use of a phased array RF coils or surface coils with a tuned and matched circuit including low impedance pre-amplifiers have been used for de-coupling as well as a mechanism for improving the signal-to-noise ratio (SNR) and field of view (FOV). In this regard, it should be understood that the term “coupling” refers to the coupling of an MR signal in one coil to an adjacent coil such that the signal being outputted by the adjacent coil is a combination of the MR signal detected by the adjacent coil and the coupled MR signal. Consequently, the image from the adjacent coil would be distorted to some degree. Although the tuned and matched circuit including low impedance pre-amplifiers has been effective from the standpoint of improving SNR and FOV, such circuitry becomes ineffective when both the number of coils and the coil density is increased. In other words, as the spacing between adjacent coils and between adjacent portions of a coil is decreased signal coupling is increased irrespective of the tuned and matched circuits.
Although there are a variety of spatial encoding methodologies or techniques being implemented, the most popular method used in commercial MRI scanners is two dimensional Fourier transform (2DFT) encoding in which a two-dimensional spatial plane (e.g., XY plane) is encoded with both frequency and phase of the MR signals. Typically during one data acquisition, only a one dimensional time-domain signal is obtained and thus 2DFT encoding requires repeating the data acquisitions sequentially to achieve a pseudo second dimension of the time domain signals. The second dimension of the spatial information is encoded into the phase component by repeating the data acquisition with different phase encoding gradient strengths (i.e., varying G
y
to create the other pseudo-time dimension. In this technique, each slice of the imaged object is in effect divided into a multiplicity of layers in the y-direction or axis corresponding to the number of pixels in that direction (e.g., 128, 256). The number of pixels in turn is representative of the desired image resolution, in other words the higher the resolution the higher the number of pixels. In addition, the x- direction scanning process or the data acquisition is repeated by sequentially and repeatedly stepping through each of these y-direction layers. Because the resolution of the time-domain signals depends on the number of repetitions of the data acquisitions, and the repetition rate is limited by the MR relaxation times; a higher resolution image takes a longer time.
MR imaging has proven to be a valuable clinical diagnostic tool in a wide range of organ systems and pathophysiologic processes. Both anatomical and functional information can be gleamed from the MR data, and new applications continue to develop with each improvement in basic imaging technique and technology. For example, the ability to image and evaluate increasingly finer anatomical details have resulted with technological advances yielding improved achievable spatial resolution. Also, the technological advances allowing for fast imaging sequences
Corless Peter F.
Daley, Jr. William J.
Edwards & Angell LLP
Fetzner Tiffany A.
Johns Hopkins University
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