Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-09-25
2003-12-30
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S419000, C600S420000, C128S898000, C128S922000, C382S128000, C382S130000, C382S276000, C382S280000, C324S306000, C324S307000, C324S309000
Reexamination Certificate
active
06671536
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is magnetic resonance imaging (“MRI”), and particularly, studies which extend over a field of view which is larger than the static field of view of the MRI system. One such study is magnetic resonance angiography of human vasculature using contrast agents.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. 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, and 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. Each measurement is referred to in the art as a “view” and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, or k-space samples, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. With conventional techniques, the total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
The most prevalent method for acquiring an NMR data set from which an image can be reconstructed is referred to as the “Fourier transform” imaging technique or “spin-warp” technique. This technique is discussed in an article entitled “Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging”, by W. A. Edelstein et al.,
Physics in Medicine and Biology,
Vol. 25, p. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G
y
) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (G
x
) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G
y
is incremented (&Dgr;G
y
) in the sequence of views that are acquired during the scan. In a three-dimensional implementation (3DFT) a third gradient (G
z
) is applied before each signal readout to phase encode along the third axis. The magnitude of this second phase encoding gradient pulse G
z
is also stepped through values during the scan. These 2DFT and 3DFT methods sample k-space in a rectilinear pattern.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick with this contrast enhanced (CE) MRA method is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CEMRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds.
As indicated above, the acquisition of MRA data is timed such that the central region of k-space is acquired as the bolus of contrast agent arrives in the arteries of interest. The ability to time the arrival of contrast varies considerably and it is helpful in many applications to acquire a series of MRA images in what is referred to as a dynamic study which depicts the separate enhancement of arteries and veins. The temporal series of images from such a dynamic study is also useful for observing delayed vessel filling patterns caused by disease. This requirement has been partially addressed by acquiring a series of time resolved images using a 3D “Fourier” acquisition as described by Korosec F., Frayne R, Grist T., Mistretta C., “Time-Resolved Contrast-Enhanced 3D MR Angiography”,
Magn. Reson. Med.
1996; 36:345-351 and in U.S. Pat. No. 5,713,358. However, with this method, the increased sampling rate of the center of k-space reduces the spatial resolution of the individual images in the time resolved series to about 75% of the resolution obtained when a single timed image is acquired during the passage of contrast.
There has been recent work using projection reconstruction methods for acquiring MRA data. Projection reconstruction methods have been known since the inception of magnetic resonance imaging. Rather than sampling k-space in a rectilinear scan pattern as is done in Fourier imaging and shown in
FIG. 2
, projection reconstruction methods sample k-space with a series of views that sample radial lines extending outward from the center of k-space as shown in FIG.
3
. The number of views needed to sample k-space determines the length of the scan and if an insufficient number of views are acquired, streak artifacts are produced in the reconstructed image.
Efforts have been made to acquire CEMRA images in shorter scan times using undersampled projection reconstruction scanning methods. A method for reducing the number of projections in a 3D acquisition by a factor of two has been reported by F. Boada, J. Christensen, J. Gillen, and K. Thulborn, “Three-Dimensional Projection Imaging With Half The Number Of Projections”,
MRM
37:470-477 (1997). Other methods are described in co-pending U.S. patent application Ser. No. 09/767,757 filed on Jan. 23, 2001 and entitled “Magnetic Resonance Angiography Using Undersampled 3D Projection Imaging”.
The non-invasiveness of MRA makes it a valuable screening tool for cardiovascular diseases. Screening typically requires imaging vessels in a large volume. This is particularly true for diseases in the runoff vessels of the lower extremity. The field of view (FOV) in MR imaging is limited by the volume of the B
0
field homogeneity and the receiver coil size (typically, the FOV<48 cm on current commercial MR scanners). The anatomic region of interest in the lower extremity, for example, is about 100 cm and this requires several scanner FOVs, or stations, for a complete study. This requires that the patient be repositioned inside the bore of the magnet, the patient be re-landmarked, scout images be acquired and a preparation scan be performed for each FOV. All of these additional steps take time and, therefore, are expensive. When contrast enhanced MRA is performed, the repositioning also necessitates additional contrast injections.
Recently gadolinium-enhanced bolus chase techniques have been reported which overcome this difficulty, K. Y. Ho, T. Leiner, M. H. de Hann, J. M. A. van Engleshoven, “Gadolinium optimized tracking technique: a new MRA technique for imaging the peripheral vascular tree from aorta to the foot using one bolus of gadolinium (abs).”
Proc.
Lateef Marvin M.
Lin Jeoyuh
Quarles & Brady LLP
Wisconsin Alumni Research Foundation
LandOfFree
Magnetic resonance angiography using floating table... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Magnetic resonance angiography using floating table..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic resonance angiography using floating table... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3166354