Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-11-20
2002-11-26
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06486668
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the reduction of image artifacts caused by signals produced outside the field of view.
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, 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 near the Larmor frequency, the net aligned moment, 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, 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
z
) 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 set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
To accurately excite spins and resolve the locations of the resulting NMR signals the polarizing magnetic field B
0
must be highly homogeneous and the imaging gradient fields G
x
, G
y
and G
z
must be highly linear. Numerous structures and methods are known in the art to accomplish this in commercial MRI system, and the region where these fields meet the requirements is referred to as the designed spherical volume (“DSV”). The DSV may range for example, from a diameter of 40 to 48 cm. Outside the DSV, the polarizing magnetic field B
0
can become very inhomogeneous and the imaging gradients G
x
, G
y
and G
z
can become highly nonlinear. They are also very poorly controlled in these outer regions.
Referring particularly to
FIG. 2
, the DSV of a typical MRI system is indicated by dashed line
10
and a subject to be scanned
12
is placed in the DSV
10
. A field of view (FOV) from which accurate NMR data is acquired to reconstruct an image is indicated by dotted lines
14
. Portions of the subject
12
are outside the DSV
10
, and the spins therein are subject to the RF excitation fields and magnetic fields produced by the MRI system while imaging the FOV
14
. The NMR signals produced by spins located outside the DSV
10
can produce image artifacts. These image artifacts from outside the DSV
10
can be aliased into the reconstructed image because of the limited imaging FOV
14
, and they can be ghosted into the image because of the data inconsistency.
Methods and apparatus are known to reduce these artifacts. One solution is to increase the imaging FOV
14
to reduce aliasing. Hardware solutions include design of gradient coils with a larger linear region or RF transmit coils which significantly reduce RF excitation of spins outside the DSV
10
. These are costly solutions which require major system changes.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for reducing image artifacts caused by signals emanating from outside the system DSV. More particularly, the imaging pulse sequence performed by the MRI system is preceeded by a preparatory pulse sequence which excites spins in the regions outside the DSV and spoils the resulting NMR signals. The imaging pulse sequence follows the preparatory pulse sequence before the longitudinal magnetization of spins outside the DSV have time to recover.
Since the RF and magnetic fields outside the DSV produce unpredictable results, it has been discovered that the use of conventional spatial presaturation pulse sequences do not suppress the image artifacts. The present invention solves this problem by applying a first RF pulse that is selective to the FOV along one gradient axis, applying a second RF pulse having an equal, but opposite flip angle and that is selective to the FOV along another gradient axis, and spoiling the NMR signals produced by the two RF pulses. Spin longitudinal magnetization in the FOV is restored by the application of the two equal but opposite flip angle RF pulses and spin longitudinal magnetization outside the FOV and along the two gradient axes is suppressed.
REFERENCES:
patent: 4715383 (1987-12-01), Ehman et al.
patent: 5657757 (1997-08-01), Hurd et al.
patent: 6078175 (2000-06-01), Foo
patent: 6127824 (2000-10-01), Smith et al.
patent: 6310479 (2001-10-01), Zhu et al.
GE Medical Systems Global Technology Company LLC
Lefkowitz Edward
Quarles & Brady LLP
Vargas Dixomara
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