Disease and injury affecting the cardiovascular system in animals, and particularly humans, are commonplace in today's society. One such disease is atherosclerosis. This disease is characterized by partial blockage (stenosis) of a blood vessel, typically by a narrowing of one or more arteries. In its most severe form, the vessel narrows to the point that it becomes completely blocked (occluded). In coronary arteries, stenosis and occlusion often manifest themselves in the form of severe chest pains and, potentially, myocardial infarction (heart attack). Not limited to coronary arteries, atherosclerosis can also affect the peripheral vasculature, i.e., arteries (and veins) that circulate blood throughout the arms and legs, the carotid arteries, i.e., arteries that carry blood to the brain, and intracranial arteries, i.e., arteries that distribute blood within the brain.
One therapy commonly employed in an effort to overcome the effects of atherosclerosis in coronary and peripheral vessels is bypass graft surgery. During this procedure, a vascular graft, e.g., a vein or artery or, alternatively, a flexible artificial tube, is surgically inserted in a manner that permits blood to bypass the stenotic or occluded portion of a native vessel. Perhaps the best-known example of bypass graft surgery is coronary artery bypass graft (CABG) surgery. In CABG, a graft, commonly a saphenous vein or internal mammary artery, is harvested or dissected from the patient, respectively, and then located within the patient to permit blood flow to bypass the stenotic or occluded vessel portion. Alternatively, or in addition thereto, a graft may be used to permit blood to flow directly from the aorta to a location downstream of a stenotic or occluded portion of an artery.
The success of bypass grafts, at least in terms of clinical improvement, depends in significant part upon the ability of the treated vessel to remain free of occlusions over both the short- and long-term. This freedom from occlusions is commonly referred to as vessel patency. Poor patency in the first few months after surgery is thought to be the result of various factors, with the following believed to be the most significant: poor blood circulation, poor coronary arterial runoff, injury to the graft during preparation or faulty surgical technique.
While cardiac surgery in recent years has focused on strategies to minimize trauma to the myocardium, these strategies may increase the likelihood of problems if used during vessel grafting procedures. For example, while surgical techniques now permit CABG to be performed on a beating heart to minimize trauma, there exists a concern relating to the quality of the resulting graft. The use of limited access incisions during CABG procedures has been developed for, at least, the revascularization of the left anterior descending artery using a left internal mammary artery, with the hope of faster recovery, a shorter hospital stay and reduction in cost. However, this method has also raised concerns relating to graft quality. Indeed, there exist reports of early failure in grafts completed using limited access incisions.
Other issues affecting CABG procedures are diagnostic in nature, and include relatively slow and inaccurate identification of stenotic and occluded vessels during the initial phase of CABG procedures (as some of these vessels lie within the heart tissue which inhibits visual identification), and an inability to quickly and accurately determine the extent of blood flow through the relatively smaller downstream vessels (and, more generally, whether the graft was successful in restoring blood flow to affected tissue) after the graft is completed.
Arterial patency issues may arise in therapies that do not include grafts. For example, patency evaluation is desirable in carotid arteries during and after an endarterectomy, in cranial vessels during and after neurosurgery, and in the context of kidney hemodialysis, wherein an assessment of AV fistula patency is desirable. While vessel patency information in these contexts may be obtained using X-ray technology, the disadvantages mentioned previously remain.
The extent of blood flow within a particular tissue or portion thereof, commonly referred to as perfusion, is important in connection with the diagnosis and treatment of a variety of ailments. For example, a perfusion analysis would be desirable in the context of a treatment designed to reduce undesired blood flow into tissue, e.g., halting blood flow into a tumor. At present, MRI may be used to obtain perfusion information, but this information is imprecise and only available after treatment is completed. This lessens the probability that a physician will be able to identify and remedy problems during that same procedure, thereby precluding the need for a subsequent remedial procedure.
Another affliction that requires treatment of the circulatory system is renal failure. In many cases of renal failure, it is desirable to create an AV fistula to provide vascular access for hemodialysis. The fistula is created by joining an artery and vein by a surgical procedure, providing a vessel having a relatively high rate of blood flow. While X-ray technology can be used to assist the physician in determining whether the creation of a properly functioning fistula is possible, and the type of fistula that should be created, the technology suffers from the previously mentioned limitations.
In view of the foregoing, a need exists for a diagnostic procedure that permits a physician to evaluate the patency of a particular vessel, and particularly vessels that have undergone an invasive procedure such as a bypass graft procedure. A further need exists for a method of quickly and accurately locating a particular stenotic or occluded vessel, such as a coronary artery during the initial phase of CABG surgery. In addition, improved methods for evaluating the extent of blood flow downstream of a graft are needed, e.g., in coronary arteries and peripheral vasculature, as are more accurate methods for determining the extent of blood perfusion in selected body tissue. A need also exists for an improved means of identifying candidate vessels for AV fistulas, and of obtaining information relevant to a determination of the type of fistula that should be created in a patient with renal impairment.