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MRI at the Leading Edge: High Speed and High Resolution
12:54 pm Tissue Engineering: Living Implants for Cardiovascular Applications Gail K Naughton, PhD Advanced Tissue Sciences, Inc. 1£1 jolla, California 1:18 pm MRI at the Leading Edge: High Speed and High Resolution Paul A . Bottomley, Ergin Aralar, Henry Halperin,
Albert Lardo, Tom Fool and Xiaoming Yang johns Hopkins University Baltimore, Maryland
Proton magnetic resonance imaging (MRI) is an established clinical diagnostic lOol in radiology. The fac t that the MRl signal is based on transitions of a nJy about one in a million TH nuclei represents a fundamental obstacle that limits sensitivity) and consequently spatial resolution and scan time, in MRI. Nevertheless, 25 years of steady progress have reduced MRI scan times of an hour or more to less than 1 second. MRI fluoroscopy is now in sight! The tremendous gains in sensitivity that have made this possible derive from increased magnetic field strength, advanced MRI pulse sequences, advanced MRI detector coil design, and, to a lesser extent, improvements in electronics that reduce system no ise. Magnetic field strength provides an increase in signal-to-noise ratio (SNR) that is, with ideal detectors, approximately linear. Pulse sequences increase SNR by maximizing the amount of MRI signal that can be sampled per unit time, although ga ins are constrained by the tissue relaxation times T1 and T2. MRI detectors, such as sma ll su rface coils placed over the field of view (FOV) of interest in the body, dramatica lly increase the SNR by virtue of their prox.imiry to the signal sources, and their insensiti vity to the integrated noise detected from remote locations. The limited range of the single surface coil has been ove rcome by the development of arrays of detector COils, phased-arrays, from which the MRI signals are processed in multiple channels to provide the high sensitivi[), and resolution of a single surface coil with the extended FOV of the entire array. The ability to reconstru ct and display images acquired from multichannel phased-arrays in real-time at refresh rates of ?7 frames per second with interactive image plane and parameter adjustment represents the leading edge of no ninvasive high-speed MRI today. It opens windows to dynarrtic studies such as stress-testing of patients with coronary artery disease and real-time monitoring of interventions, including those that produce changes in MRI signal intenSity such as hyperthermia. An example of this is the use of MRl to monitor lesions produced by radio frequency catheter ablation as a treatment of atrial fibrillation.
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To maximize SNR at higher resolution still, coils ca n be reduced to catheter-sized receiver coils that may be introduced into the body, such as via the esophagus in humans. Intravascular coils are currently under evaluation in animal slUdies in o ur laboratories on instrumentation compatible with human MRl studies, and MRlguided angiopJasty is possible. These coils have the potential for hjgh-resolulio n MRI of athe roscle rotic plaques, differentiating the cap from unde rlying flu idfilled cores, for example, as shown in studies of human autopsy specimens. The coils may be introduced under MRl fluoroscopic gUidance, by overlaying or combining a large FOV image from an external coil with that from the internal coiL Although such techniques are yet to be proven effective in the clinic, the work in progress demonstrates that the traditional MRI advantages of sensitivity to soft tissue and disease-and new advantages of an ability to mon ~ itor interventions directly-can be combined with high speed and high resolution. This places us at the threshold of a whole new range of potential clinical applications of real-time MRI. Acknowledgments The author thanks the NIH, SurgiVision Inc. and GE for research support. 1:42 pm Percutaneous Perladventitial Heparin Delivery via CrossUnked liquid Polymer Matthew S. johnson, MD Indiana University Hospital Indianapolis, Indiana Learning objectives: Upon completion of this presentation, the attendee should be able to: 1) Discuss the rationale for periadventilial drug de/iv€1y in the prevention of restenosis; 2) DeSCribe the techniques used to accurately deliver the polymer to the periadvenlitial space; 3) Discuss potential applications ofpolymer-based drug delivery. Percutaneous balloon angioplasty has become an accepted treatment of stenotic arteries and veins. This technique, while achieving excellent technical and early clinical success rates, has a high rare of early failure. In the coronary arteries, for example, 30%-400/0 of stenoses treated with angioplasty recur within one year of the procedure 0-3). Likewise, angioplasty of the peripheral arteries is frequently complicated by early recurrence of the stenosis, or "restenosis." The rate of restenosis in these vessels is inversely related to the size of the art.ery, with higher rates in the femoral and infra popliteal a.rteries than in the aorta and iliac arteries (4 -6). Restenosis in any artery is tho ught to result mainly from a combination of vessel recoil, remodeling and intimal hyperplasia. Recoil, which occurs ove r minutes, and remodeling , which occurs over days, are due to contracting m ec ha n~ ical forces of the vessel wall. These forces may be over-
Albert Lardo, Tom Fool and Xiaoming Yang johns Hopkins University Baltimore, Maryland
Proton magnetic resonance imaging (MRI) is an established clinical diagnostic lOol in radiology. The fac t that the MRl signal is based on transitions of a nJy about one in a million TH nuclei represents a fundamental obstacle that limits sensitivity) and consequently spatial resolution and scan time, in MRI. Nevertheless, 25 years of steady progress have reduced MRI scan times of an hour or more to less than 1 second. MRI fluoroscopy is now in sight! The tremendous gains in sensitivity that have made this possible derive from increased magnetic field strength, advanced MRI pulse sequences, advanced MRI detector coil design, and, to a lesser extent, improvements in electronics that reduce system no ise. Magnetic field strength provides an increase in signal-to-noise ratio (SNR) that is, with ideal detectors, approximately linear. Pulse sequences increase SNR by maximizing the amount of MRI signal that can be sampled per unit time, although ga ins are constrained by the tissue relaxation times T1 and T2. MRI detectors, such as sma ll su rface coils placed over the field of view (FOV) of interest in the body, dramatica lly increase the SNR by virtue of their prox.imiry to the signal sources, and their insensiti vity to the integrated noise detected from remote locations. The limited range of the single surface coil has been ove rcome by the development of arrays of detector COils, phased-arrays, from which the MRI signals are processed in multiple channels to provide the high sensitivi[), and resolution of a single surface coil with the extended FOV of the entire array. The ability to reconstru ct and display images acquired from multichannel phased-arrays in real-time at refresh rates of ?7 frames per second with interactive image plane and parameter adjustment represents the leading edge of no ninvasive high-speed MRI today. It opens windows to dynarrtic studies such as stress-testing of patients with coronary artery disease and real-time monitoring of interventions, including those that produce changes in MRI signal intenSity such as hyperthermia. An example of this is the use of MRl to monitor lesions produced by radio frequency catheter ablation as a treatment of atrial fibrillation.
348
To maximize SNR at higher resolution still, coils ca n be reduced to catheter-sized receiver coils that may be introduced into the body, such as via the esophagus in humans. Intravascular coils are currently under evaluation in animal slUdies in o ur laboratories on instrumentation compatible with human MRl studies, and MRlguided angiopJasty is possible. These coils have the potential for hjgh-resolulio n MRI of athe roscle rotic plaques, differentiating the cap from unde rlying flu idfilled cores, for example, as shown in studies of human autopsy specimens. The coils may be introduced under MRl fluoroscopic gUidance, by overlaying or combining a large FOV image from an external coil with that from the internal coiL Although such techniques are yet to be proven effective in the clinic, the work in progress demonstrates that the traditional MRI advantages of sensitivity to soft tissue and disease-and new advantages of an ability to mon ~ itor interventions directly-can be combined with high speed and high resolution. This places us at the threshold of a whole new range of potential clinical applications of real-time MRI. Acknowledgments The author thanks the NIH, SurgiVision Inc. and GE for research support. 1:42 pm Percutaneous Perladventitial Heparin Delivery via CrossUnked liquid Polymer Matthew S. johnson, MD Indiana University Hospital Indianapolis, Indiana Learning objectives: Upon completion of this presentation, the attendee should be able to: 1) Discuss the rationale for periadventilial drug de/iv€1y in the prevention of restenosis; 2) DeSCribe the techniques used to accurately deliver the polymer to the periadvenlitial space; 3) Discuss potential applications ofpolymer-based drug delivery. Percutaneous balloon angioplasty has become an accepted treatment of stenotic arteries and veins. This technique, while achieving excellent technical and early clinical success rates, has a high rare of early failure. In the coronary arteries, for example, 30%-400/0 of stenoses treated with angioplasty recur within one year of the procedure 0-3). Likewise, angioplasty of the peripheral arteries is frequently complicated by early recurrence of the stenosis, or "restenosis." The rate of restenosis in these vessels is inversely related to the size of the art.ery, with higher rates in the femoral and infra popliteal a.rteries than in the aorta and iliac arteries (4 -6). Restenosis in any artery is tho ught to result mainly from a combination of vessel recoil, remodeling and intimal hyperplasia. Recoil, which occurs ove r minutes, and remodeling , which occurs over days, are due to contracting m ec ha n~ ical forces of the vessel wall. These forces may be over-