Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-06-16
2004-05-25
Sykes, Angela D. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S407000, C600S410000, C600S420000, C382S128000, C324S300000, C324S307000
Reexamination Certificate
active
06741880
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the art of locating a blood vessel lesion in a human subject, and more particularly, to an apparatus and method to efficiently identify a lesion and grade the stenosis using magnetic resonance imaging (MRI) technology.
The narrowing or constriction of vessels carrying blood to the heart is a well-known cause of heart attacks, and gone untreated, can lead to sudden death. In such stenotic vessels, it is known that the flow in the vessel at the point of narrowing and immediately after the narrowing is characterized by rapid flow velocities and/or complex flow patterns. In general, narrowing of blood carrying vessels supplying an organ will ultimately lead to compromised function of the organ in question, at best, and organ failure at worst. Quantitative flow-velocity data can readily aid in the diagnosis and management of patients and also help in the basic understanding of disease processes. There are many techniques available for the measurement of regional blood flow to a specific region of the anatomy, including imaging based methods using radiographic imaging of contrast agents, both in projection and computed tomography (CT), ultrasound, and nuclear medicine techniques. Radiographic and nuclear medicine techniques require the use of ionizing radiation and/or contrast agents. However, none of these techniques provide instantaneous flow-velocity measurements at a specific spatial location and/or specific time in the cardiac cycle. Two methods that are in current use are doppler ultrasound using an external transducer or the more invasive method of an intra-vascular doppler ultrasound guide-wire/probe.
The functional significance of a stenosis is conventionally determined using Doppler ultrasound to measure the velocity/pressure gradient across the vessel constriction along the axis of flow. The higher the gradient, the more significant the stenosis. However, using Doppler ultrasound is dependent on having an acoustic window allowing the ultrasound beam to insonify the vessel of interest at an angle of incidence as close to zero (i.e., parallel to the vessel) as possible. Furthermore, Doppler ultrasound does not provide the quality of images that are produced using MR technology. Further, ultrasound techniques are difficult to apply in certain situations because of intervening tissues such as bone, excessive fat or air. The use of an intra-vascular doppler ultrasound probe avoids some of these pitfalls but the procedure is quite invasive and has an associated risk of patient morbidity.
Phase contrast magnetic resonance angiography (MRA) is a practical and clinically applicable technique for imaging blood flow-velocities. MRI utilizes radio frequency pulses and magnetic field gradients applied to a subject in a strong magnetic field to produce viewable images. When a substance containing nuclei with net nuclear magnetic moment, such as the protons in 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 (assumed to be in the z-direction), but precess about the direction of this magnetic field at a characteristic frequency known as the Larmor frequency. If the substance, or tissue, is subjected to a time-varying magnetic field (excitation field B
1
) applied at a frequency equal to the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M
Z
, may be nutated, 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 (as the excited spins decays to the ground state) 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
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 MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Phase contrast MRA makes use of flow encoding gradient pulses which impart a velocity-dependent phase shift to the transverse magnetization of moving spins while leaving stationary spins unaffected (Moran P. R. A Flow Velocity Zeugmatographic Interlace for NMR Imaging in Humans. Magnetic Resonance Imaging 1982; 1: 197-203). Each phase contrast acquisition generates two images: a magnitude image that is proportional to the proton density of the object and may also be T
1
-weighted, and an image representing the phase of the object. The phase image produced has information only from the moving spins and the signal from stationary tissue is suppressed. Images representing both the average flow-velocity over the entire cardiac cycle and at a series of individual points in the cycle have been generated using this technique. The phase contrast MR method produces phase images with intensities that represent the magnitude of the flow velocity and also the direction of flow. Therefore, such images may be used for both qualitative observation of blood flow and quantitative measurement. The practical application of phase contrast MR angiography and venography to the quantitative determination of flow velocity is therefore evident.
It would also be advantageous to use magnetic resonance imaging technology to efficiently locate and identify a stenosis in a blood vessel and use this MR technology to grade the stenosis for patient management decisions. Previous attempts at using MR technology to improve the ability to detect and grade coronary artery stenosis, for example, have relied primarily on using a single scan and decreasing the intra-voxel flow dephasing effects by decreasing pixel size, together with using first moment gradient nulling for flow compensation, and decreasing echo time (TE). It would be desirable to improve on this prior art by accomplishing the converse. That is, it would be advantageous to increase the intra-voxel flow dephasing effects to exacerbate flow voids, and therefore increase the conspicuity of lesions on the coronary artery that result in a stenosis in a quick screening exam. It would also be advantageous to have a method and apparatus for efficient visualization of a stenosis using MR technology followed with a more thorough exam if a stenosis is detected initially.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for efficient stenosis identification and assessment using MR technology, that solves the aforementioned problems.
The present invention includes a two step approach to accurately identify a blood vessel lesion and specify the degree of stenosis. In the initial step, an examination for lesion identification is disclosed using a low spatial resolution MR image. Preferably, the MR image is acquired using a gradient echo imaging pulse sequence with a flow sensitive bi-polar gradient waveform. The bi-polar gradients generate a broad distribution of velocities in a large voxel. Since a stenosis present in a given voxel will result in intra-voxel flow dephasing in voxels immediate to and distal to the stenosis, the stenosis can be quickly and efficiently localized using the initial step. After the stenosis is identified, a second step is performed in which a high spatial resolution MR image is acquired for more accurate and specific grading of the stenosis in the targeted area.
According to one aspect of the invention, a method of identifying a stenotic vessel using MR imaging is disclosed which includes performing a screening study by acquiring a first MR image having a low resolution to scan a suspected stenosis region. The method next includes analyzing the first MR image to identify a suspected stenosis within the suspected stenosis region, then performing a detailed study by acquiring a second MR image having a higher resolution than the first MR image, to sc
Foo Thomas K. F.
Ho Vincent B.
Saranathan Manojkumar
Della Penna Michael A.
General Electric Company
Horton Carl B.
Jung William C.
Sykes Angela D.
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