Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-12-30
2003-06-17
Van, Quang T. (Department: 3742)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S420000
Reexamination Certificate
active
06580937
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates generally to magnetic resonance imaging (MRI) technology, and more particularly, to an apparatus and method to optimize imaging of the peripheral vasculature.
Arteries are the blood vessels emanating from the heart that supply the necessary nutrients to the organs and tissues of the human body. A narrowing or constriction of an artery reduces the delivery of nutrients, such as oxygen to the recipient tissue and has profound effects on tissue function. In general, significant narrowing of an artery leads to compromised function of the organ in question, at best, and organ failure or death at worst. Stenosis or narrowing at any number of locations along the course of the arteries from the abdominal aorta through the calf can result in compromise of arterial blood flow to the distal lower extremities. The evaluation of the peripheral vessels is further complicated by the high incidence of tandem or synchronous lesions, any one of which could be the underlying cause for diminished arterial blood flow. Furthermore, the surgical decisions for potential bypass procedures to improve distal blood flow are greatly affected by the ability to assess the arteries in the foot. As a result, the successful imaging of the lower extremities (i.e. the peripheral run-off study) requires not only the accurate assessment of the presence and functional significance of a narrowing, but also the ability to evaluate the entire arterial course of the peripheral arterial tree from abdominal aorta to the foot.
There are many techniques available for the assessment of the peripheral arteries that include traditional invasive catheter angiography and ultrasound. Because conventional x-ray angiography requires catheterization and the use of nephrotoxic iodinated contrast agents, it is reserved as the final option. Screening for peripheral arterial occlusive disease (PAOD) is typically performed using non-invasive methods such as ultrasound or plethysmography. However, neither of these techniques can provide angiographic illustration of the vessels and merely provides the assessment of individual segments of the intervening arterial anatomy. Both techniques are operator dependent and have confounding technical difficulties which make the imaging often tedious to perform. Moreover, neither technique can provide the comprehensive information required for surgical planning and traditional x-ray angiographic depiction is generally required as an adjunct for pre-operative management.
Magnetic resonance imaging is a method for the non-invasive assessment of arteries. 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.
The imaging of blood vessels using MRI, or magnetic resonance angiography (MRA), is an emerging method rapidly supplanting other non-invasive methods for arterial illustration. Until recently, the application of MRA has been tailored to individual smaller vascular territories (40-50 cm fields of view). With the ability now to translate the table and to image several overlapping fields-of-view or “stations” in rapid succession, MRA can now be used with a bolus chasing technique and result in the imaging of a much larger anatomic length, as necessary for evaluation of PAOD. The rapid performance of multiple MRA acquisitions in sequential and contiguous fashion during the arterial passage of a contrast agent down the lower extremities can yield the depiction of 1-1.2 meters of arterial anatomy. This technique called bolus chase peripheral MRA typically requires 1-2 minutes and utilizes an intravenously administered contrast agent, typically an extra-cellular Gadolinium (Gd)-chelate contrast agent.
This technique relies on the ability to coordinate the acquisition of image data (i.e. the MRA scan) with peak arterial concentration of the contrast bolus that typically occurs during the initial arterial phase of the contrast bolus (i.e., first pass acquisition). Poor coordination of image acquisition (i.e. poor timing) will result in insufficient arterial signal and poor arterial illustration. If imaging is performed late, there can be significant venous and background tissue enhancement that will also diminish the conspicuity of the arterial structures that are already faint secondary to lowered contrast agent concentration.
The typical bolus chase MRA starts in the mid-abdomen and includes the pelvis and extends to the ankle and feet. Timing is typically predicated by the contrast arrival in the initial station (i.e. abdominal aorta) with subsequent imaging performed in automatic sequential “rapid-fire” fashion. Thus, the primary imaging target is the abdominal aorta (i.e. proximal station) and this technique assumes that the speed of imaging alone will allow preferential arterial depiction of all subsequent imaging stations (e.g. thigh, calf and foot). This has been shown to work sufficiently for imaging the peripheral arterial tree above the knee using traditional extra-cellular contrast agents injected at a relatively slow rate, such as in the range of 0.3-1.0 mL/sec. The disadvantage of using a slow infusion rate is that the maximum achievable arterial concentration of contrast agent is markedly diminished and arterial enhancement is often insufficient for reliable depiction of smaller vessels of the infrapopliteal (below the knee) peripheral arterial tree.
Unlike traditional extracellular Gd-chelate contrast agents, intravascular contrast agents persist within the vasculature much longer secondary to their diminished leakage out of the vessels. These contrast agents, therefore, provide an improved opportunity to illustrate vascular structures by maintaining a reliably high concentration of contrast agent within the arteries for an extended period of time, thus providing a prolonged arterial phase or period of arterial enhancement. However, venous enhancement is also prolonged and can significantly diminish the conspicuity of adjacent arterial structures. Moreover, in using intra-vascular contrast agents, venous signal enhancement has an even higher likelihood as there is less dispersion of the contrast bolus into the peripheral tissue extra-cellular space. Therefore, in order to image the entire length of the peripheral vasculature using intravascular contrast agents, venous contamination is a much larger concern than when extra-cellular agents are used since the venous signal can be significant and more persistent. Furthermore, venous contamination is most noticeable in the distal extremities where the arteries are relatively much smaller and fewer in number than their associated venous structures. For example, arterial depictio
Foo Thomas K. F.
Ho Vincent B.
Della Penna Michael A.
GE Medical Systems Global Technology Co. LLC
Horton Carl B.
Van Quang T.
Ziolkowski Patent Solutions Group LLC
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