Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-02-08
2001-03-06
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S419000, C600S509000, C324S307000, C324S309000
Reexamination Certificate
active
06198959
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is magnetic resonance angiography (“MRA”), and particularly, studies of the human vasculature using contrast agents which enhance the NMR signals.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography (“DSA”) have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. One of the advantages of these x-ray techniques is that image data can be acquired at a high rate (i.e. high temporal resolution) so that a sequence of images may be acquired during injection of the contrast agent. Such “dynamic studies” enable one to select the image in which the bolus of contrast agent is flowing through the vasculature of interest. Images showing the circulation of blood in the arteries and veins of the kidneys, the neck and head, the extremities and other organs have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged. There is also the issue of increased nephro-toxicity and allergic reactions to iodinated contrast agents used in conventional x-ray angiography.
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, or “views”, 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.
MR angiography (MRA) has been an active area of research. Two basic techniques have been proposed and evaluated. The first class, time-of-flight (TOF) techniques, consists of methods which use the motion of the blood relative to the surrounding tissue. The most common approach is to exploit the differences in magnetization saturation that exist between flowing blood and stationary tissue. Flowing blood, which is moving through the excited section, is continually refreshed by spins experiencing fewer excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal, moving blood and the low-signal, stationary tissues.
MRA methods have also been developed that encode motion into the phase of the acquired signal as disclosed in U.S. Pat. No. Re. 32,701. These form the second class of MRA techniques and are known as phase contrast (PC) methods. Currently, most PC MRA techniques acquire two images, with each image having a different sensitivity to the same velocity component. Angiographic images are then obtained by forming either the phase difference or complex difference between the pair of velocity-encoded images.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Excellent diagnostic images may be acquired using contrast-enhanced MRA if the data acquisition is properly timed with the bolus passage.
There are a number of techniques for detecting the arrival of the contrast agent in the arteries being imaged. The prevailing thinking is that once the presence of the bolus is detected, the data acquisition should be acquired in a centric view order in which the central region of k-space is acquired first. This approach works well for most vasculature, but this method cannot be used to image the coronary arteries, due to respiratory and cardiac motion.
SUMMARY OF THE INVENTION
The present invention is a method for acquiring MRA data when motion is an important factor in determining image quality. The method consists of two parts. In the first part scout images are acquired of the region of interest during a cardiac cycle to measure movement of the arteries of interest at different cardiac phases. Information acquired from the scout images is used to determine a “quiescent” time interval during mid-diastole when artery motion is minimal, and this is used in the subsequent MRA scan. During the cardiac gated MRA acquisition and after bolus arrival is detected, k-space is sampled during the diastolic portion of the successive cardiac cycles, with the center of k-space being sampled during the quiescent time period and the periphery of k-space being sampled on each side of the quiescent period. Similarly, the acquisition of NMR data may be tailored to other cyclic movements such as that produced by respiration.
A general object of the invention is to reliably image arteries that are subject to motion during the cardiac cycle. By measuring the quiescent period with scout acquisitions and tailoring the subsequent MRA acquisition to the particular patient, the variability in image quality is substantially reduced. It has been discovered that the quiescent period varies considerably between patients and that reliable image quality can be achieved by adapting the MRA acquisition accordingly.
Another object of the invention is to reduce total scan time so that an image can be acquired in a single breath hold. By sampling the periphery of k-space before and after the quiescent period, more views can be acquired during each cardiac cycle without significantly affecting image quality. This is because the peripheral views may be acquired while there is artery motion and the acquisition can be extended over a larger portion of the cardiac cycle without adversely affecting image quality.
Another aspect of the present invention is to tailor the data acquisition to the combined motion produced by the heart and respiration. A scout image is acquired to measure movement of arteries as a function of cardiac phase indicated by a cardiac trigger signal, and a scout image is acquired to measure movement as a function of respiratory phase. A two-dimensional motion phase diagram is produced which is indicative of artery motion during the respiratory gating window and the cardiac acquisition window. This is mapped to segments of k-space to produce a look-up table of views that are to be acquired for any combination of respiratory and cardiac phase.
REFERENCES:
patent: 5277182 (1994-01-01), Koizumi et al.
patent: 5830143 (1998-11-01), Mistretta et al.
patent: 5897496 (1999-04-01), Watanabe
patent: 5987348 (1999-12-01), Fischer et al.
patent: 6009341 (1999-12-01), Edelman
Navigator-Echo-based Real-Time Respiratory Gating and Triggering for Reduction of Respiration Effects in Three-dimensional Coronary MR Angiography, Radiology 1996; 198:55-60, Wang, et al.
Respiratory Motion of the Heart: Kinematics and the Implications for the Spatial Resolution in Coronary Imaging, MRM 33:713-719 (1995), Wang, et al.
Cornell Research Foundation Inc.
Jaworski Francis J.
Mantis Mercader Eleni
Quarles & Brady LLP
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