Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-03-31
2002-10-22
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06469505
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus to reduce ghosting artifacts, resulting from orthogonal perturbation fields, in MR images acquired using fast imaging techniques.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field in the z direction, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is at or near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M
z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
. A signal is emitted by the excited spins after the excitation signal B
1
is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G
x
G
y
and G
2
) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Imperfections in the linear magnetic field gradients (G
x
, G
y
and G
z
) produce artifacts in the reconstructed images. It is a well-known problem, for example, that eddy currents produced by gradient pulses will distort the gradient fields and produce image artifacts. Methods for compensating for such eddy current errors are also well known and are disclosed in U.S. Pat. Nos. 4,698,591; 4,950,994; and 5,226,418, for example. It is also known that the gradients may not be perfectly uniform over the entire imaging volume, which may lead to image distortion. Methods for compensating this non-uniformity are described, for example, in U.S. Pat. No. 4,591,789.
Other than uncompensated eddy current errors and gradient non-uniformity errors that escape correction, it is often assumed that the magnetic field gradients (G
x
, G
y
, and G
z
) produce linear magnetic fields exactly as programmed, thus spatially encoding the MR data accurately. With these gradients, the overall static magnetic field at location (x,y,z) is conventionally given as B
0
+G
x
(x)+G
y
(y)+G
z
(z), and the direction of the field is usually thought to be along the z-axis. This description, however, is not exactly correct. As long as a linear magnetic field gradient is applied, the overall magnetic field direction is changed from the z-axis and its amplitude exhibits higher-order spatial dependencies (x
2
, y
2
, z
2
, z
3
, . . . ). These phenomena are a direct consequence of the Maxwell equations which require that the overall magnetic field satisfy the following two condition: {right arrow over (∇)}·{right arrow over (B)}=0 and {right arrow over (∇)}×{right arrow over (B)}≈{right arrow over (0)}. The higher-order magnetic fields, referred to as “Maxwell terms” (or Maxwell fields), represent a fundamental physics effect, and are not related to eddy currents or imperfection in hardware design and manufacture.
Many MR scanners still in use to produce medical images require several minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, improves image quality by reducing motion artifacts and enables dynamic and functional studies. There is a class of pulse sequences which can acquire an image in seconds, or even sub-second, rather than minutes.
One of these fast imaging techniques is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al. in an article in
Magnetic Resonance in Medicine
3,823-833 (1986) entitled “RARE Imaging: A Fast Imaging Method for Clinical MR.” A slight variation of the RARE sequence produces a fast spin echo (FSE) sequence which is used for clinical diagnosis in many commercial scanners. Images acquired using an FSE sequence are very susceptible to artifacts caused by eddy currents induced by the rapidly changing magnetic field gradients. While eddy current compensation techniques are adequate for scans performed with conventional MRI pulse sequences, it has been observed that image artifacts caused by eddy currents are frequently present in FSE scans.
Echo-planar imaging (EPI) is another ultrafast MR imaging technique which is extremely susceptible to system imperfections, such as eddy currents, B
0
inhomogeneity, and gradient group delays. In the presence of eddy currents, for example, ghosting artifacts can considerably degrade the image quality and adversely affect EPI's diagnostic value.
To minimize the ghosts created by such fast imaging techniques using echo trains, such as FSE and EPI, a common approach is to employ a reference scan prior to the actual image acquisition. In these reference scans, signals from a full echo train are acquired in the absence of the phase-encoding gradient. Each echo in the echo train is Fourier transformed along the readout direction to obtain a set of projections. Spatially constant and linear phase errors, &phgr;
0
and &phgr;
1
, are then extracted from the projections, followed by phase corrections using &phgr;
0
and &phgr;
1
, either during image acquisition, as in the case of FSE, or in image reconstruction, as in the case of EPI.
This type of phase correction assumes that spatially varying magnetic fields along the phase-encoding direction are negligible during the reference scans. However, when the Maxwell terms and other perturbation fields are considered, this assumption does not hold, especially when a strong gradient is used at relatively low static magnetic fields. In addition to the Maxwell terms, other factors that can cause perturbations to the reference scans, that can result in incomplete or erroneous phase corrections, include linear eddy currents from any gradient to the phase-encoding axis, a magnetic field inhomogeneity in the phase-encoding direction, and/or magnetic hysteresis that creates phase encoding direction field variations. Together with the Maxwell terms, these perturbations are herein referred to as orthogonal perturbation fields (OPFs).
In the presence of these fields, signal dephasing along the phase-encoding direction can introduce substantial errors in the constant and linear phase calculations that can lead to incomplete or erroneous phase correction. The perturbation of these terms to the reference scans may be evidenced by the fact that the aforementioned phase correction method works markedly well for axial EPI scans performed on a horizontal superconducting magnet, but not as well for sagittal and coronal scans. In the former case, the EPI readout gradient does not produce a quadratic Maxwell term on the phase-encoding axis, whereas in the latter cases, substantial Maxwell terms can be introduced.
It would therefore be desirable to have a technique to minimize the effects of OPFs on reference scans to thereby reduce ghosting.
SUMMARY OF THE INVENTION
The present invention relates to a method and system to reduce the effects of orthogonal perturbation fields (OPFs) in MR images by restricting the region of interest when acquiring a reference scan that overcomes the aforementioned problems.
The technique of the present invention involves limiting the region of interest when acquiring the MR reference scan to a relatively narrow band within the imaging subject. Preferably, the narrow band is selected parallel to the readout direction, or readout axis, centered about the MR magnet's iso-center where the OPFs are minimal. Alternatively, the narrow band can also be restricted to a region of the field-of-view (FOV) where the OPFs are approximately const
Huff Steven J.
Maier Joseph K.
Zhou Xiaohong
Della Penna, Esq. Michael A.
Fetzner Tiffany A.
GE Medical Systems Global Technology Co. LLC
Horton Esq. Carl B.
Lefkowitz Edward
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