Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-05-23
2004-01-20
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S314000, C324S309000
Reexamination Certificate
active
06680610
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to an apparatus and methods for magnetic resonance imaging, also known as magnetic resonance imaging (MRI) and, in particular, to an apparatus and methods for nuclear magnetic resonance imaging (NMRI) for decreasing magnetic resonance (MR) data acquisition times, wherein magnetic resonance data is acquired in parallel using an array of receiver coils at least partially surrounding the object of interest, and the desired MRI image is then reconstructed in parallel.
BACKGROUND OF THE INVENTION
In dynamic MRI applications, such as functional imaging, interventional imaging and cardiac imaging, there has long been a need in the art for methods and apparatus that provide high quality (e.g., high-resolution and signal-to-noise ratio) images. Conventional MRI imaging apparatus and methods, however, operate at speeds that are an order of magnitude slower than those which are currently deemed to be desirable. Some of these conventional methods are described in the Background section of U.S. Pat. No. 5,365,172 to Hrovat et al. for “Methods and Apparatus for MRI”, the disclosure of which is hereby incorporated by reference herein.
In an attempt to attain faster operating speeds, several so-called “parallel” encoding apparatus combinations and/or methods have been developed. These apparatus combinations and/or methods rely on the use of multiple receiver coils for the acquisition of magnetic resonance data and high-speed data processors for the reconstruction of the field of view with significantly smaller data sets.
Among the parallel imaging techniques described in the literature, the work of Kwiat et al. (“A Decoupled Coil Detector Array for Fast Image Acquisition in Magnetic Resonance Imaging”,
Medical Physics,
18:251, 1991, the disclosure of which is hereby incorporated by reference herein) is significant. This work involved the investigation of methods for solving the inverse source problem on magnetic resonance signals received in multiple RF receiver coils. The technique proposed required the use of a number of RF coils equal to the number of pixels in the desired image. It also required that the sensitivity of the coils used be increased by an order of magnitude. Since these requirements are quite impractical in conventional magnetic resonance imaging (wherein the usual number of pixels in the image is on the order of 256×256), this technique has never been used successfully in a biological imaging experiment.
Other so-called “parallel” imaging techniques that use one dimensional sensitivity profiles of RF coils to encode space in a MRI context also have been proposed. For example, Ra, et al. (“Fast Imaging Using Sub-encoding Data Sets From Multiple Detectors”,
Magn. Reson. Med.,
30:142, 1993, the disclosure of which is hereby incorporated by reference herein), describes a method that uses sets of equally spaced k-space lines from multiple receiver coils in a line, and combines them with the one dimensional sensitivity profile information to remove the aliasing that occurs due to undersampling. A four-fold decrease in the image acquisition time of a water phantom was postulated to be possible by using an array of four coils.
Nevertheless, no biological images were shown in this article. It is believed that this may be indicative of a possible lack of robustness of the alaising removal algorithm in practical situations.
A method called SMASH proposed by Sodickson et al. (“Simultaneous Acquisition Of Spatial Harmonics (SMASH): Fast Imaging With Radio Frequency Coil Arrays”,
Magn. Reson. Med.,
38:591-603, 1997, the disclosure of which is hereby incorporated by reference herein) has been found to be practical, and yielded good results. SMASH enhances imaging speed by using multiple RF receiver coils. More specifically, it uses linear combinations of the 1D sensitivity profiles of receiver coils (weighted so as to form sinusoidal harmonics) of a one dimensional array to generate all k-space lines from a small subset of collected magnetic resonance data.
The SMASH method, however, is somewhat limited. It has an inherent inflexibility in the choice of the imaging plane to be viewed. Also, it has a demonstrable limitation in depth penetration. Further, there is a practical, physical limit on the number of coils that can be placed along one direction in a magnetic resonance imaging apparatus—particularly if the coils are to be de-coupled from one another.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and a method that significantly decrease both magnetic resonance data acquisition time, and image reconstruction time, in magnetic resonance imaging.
Also, the present invention provides an apparatus and a method wherein sets of magnetic resonance information are acquired simultaneously, in parallel, with one another, and the elements of those sets preferably are subsequently processed in parallel with one another to reconstruct images.
These, and other, features and advantages of the present invention constitute a generalization of, and an improvement upon, the SMASH apparatus and method. More specifically, the present invention contemplates the placement of an array of RF coils, comprising substantially any number of RF receiver coils, at least partially around an object of interest located in the imaging volume of a magnetic imaging device. The present invention also contemplates the provision of a so-called “parallel” imaging capability wherein the output image may be taken in any plane transverse to the imaging volume of the apparatus.
In the present invention, parallel encoding in MRI is achieved by using the sensitivity profiles of an array of RF receiver coils at least partially surrounding the object of interest. Given this fact, the equation describing the MR signal seen by the i
th
coil of a coil array surrounding the imaging volume of a magnetic resonance imaging device can be written as:
S
i
(
t
)=∫∫&rgr;(
x,y
).
W
i
(
x,y
).
e
j&ggr;(G
x
xt+G
y
y&tgr;)
dx.dy
Where W
i
(x,y) represents the 2D sensitivity profile of the i
th
coil of the array, and &rgr;(x, y) is an image slice in a selected (x, y) plane. Then, taking the Fourier transform of that signal with respect to x yields:
F
i
(
x
)=
FT[S
i
(
t
)]=∫&rgr;(
x,y
).
W
i
(
x,y
).
e
j&ggr;G
y
&tgr;
.dy
The latter equation represents a projection of the phase modulated image &rgr;(x,y) onto the x-axis. Further, this signal can be represented in discrete form by the following matrix product:
F
i
(
x
)=&Sgr;
y
[W
i
(
x,y
).
e
j&ggr;G
y
&tgr;
].[&rgr;(
x,y
)]
If the number of receiver coils used simultaneously is N, and the 2D sensitivity profile W
i
(x,y) of each one of them is known, it is possible to reconstruct the image from only one N
th
of the total number of k-space lines that would normally be required. Accordingly, the apparatus and method of the present invention use phase modulated projections of the received magnetic resonance data onto the frequency encoded (x-) axis, weighted by the 2D sensitivity profiles of the coils in the array, in order to reconstruct &rgr;(x,y) column by column (i.e., orthogonal to the x-axis).
The 2D sensitivity profiles are calculated first by acquiring a baseline image using the RF body coil of the conventional magnetic resonance imaging apparatus, whereby the sensitivity profile may be considered to be constant (W
B
(x,y)=1) and a slice of the image in the (x, y) plane may be written as: &rgr;(x,y). An image is then acquired with each coil in the array. In the i
th
coil, this image can be written as:
W
i
(
x,y
)&rgr;(
x,y
)
W
i
(x,y) is then computed by forming a point-by-point ratio between magnetic resonance data from the i
th
coil and that from the body coil.
It, therefore, will be seen that a preferred embodiment of the invention is a method for generating a magnetic resonance image of an object of interest composed of a plurality of adjacent image lines. The method generally includes the following steps. Fir
Kacher Daniel F.
Kyriakos Walid E.
Lefkowitz Edward
Porter Edward W.
Shrivastav Brij B.
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