Luis Gruberg, MD, FACC

Disclosures

July 20, 2006

Editorial Collaboration

Medscape &

Introduction

"Nowadays it takes longer time to order a caffe latte at the nearby Starbucks than to get a cardiac CT" -- Peter J. Fitzgerald, MD, PhD, Stanford University Medical Center (Stanford, California)

The utterance of this observation was how Dr. Fitzgerald initiated his talk during a fascinating session presented at the i2 Summit during the American College of Cardiology Scientific Session, which addressed advances in imaging techniques. The advent of multislice computed tomography (CT) angiography with high-resolution images obtained in seconds has completely revolutionized the way we look at, diagnose, and attempt to treat cardiovascular diseases. Patients can now request a simple, (relatively) cheap, noninvasive test that completely assesses the state of their coronary arteries and heart, a process that was at one point very time-consuming and not devoid of complications. Patients are strongly targeted by commercial companies, private enterprises, and hospitals to have their coronary arteries checked. Instead of doctors showing the patient the state of their vessels, patients are now approaching the doctor with their angiography to get a second (or third) opinion.

Since the introduction of selective coronary angiography and percutaneous coronary angioplasty, imaging has become increasingly important in the diagnosis and management of heart disease. New imaging techniques are constantly emerging that strive to improve and simplify the way we assess heart disease.

Currently, there are 4 areas of intense research in new imaging modalities, including:

  • Cardiac CT for the diagnosis of coronary artery disease. The widespread use of 16 and, more recently, 64-slice cardiac CT has shortened acquisition time and significantly improved temporal and spatial resolution, providing high-quality images of the heart and coronary vessels. New 254-slice machines are in advanced stages of research and will certainly further improve the quality of the images obtained. Cardiac CT is rapidly gaining its place in the emergency room and chest pain units for the diagnosis of the 3 main causes of chest pain: coronary heart disease, aortic dissection, and pulmonary embolism. Its use in patients with ambiguous stress tests or atypical chest pain in the ambulatory setting is also gaining widespread acceptance. Whether it should be performed in a routine manner to rule out coronary artery disease in completely asymptomatic patients is still a matter of debate. There is little doubt that cardiac CT may eventually replace diagnostic coronary angiography, such that the majority of patients will go to the cath lab only for the interventional procedure. Magnetic resonance imaging (MRI) of the heart is becoming an important tool in patients with acute myocardial infarction (MI), valvular heart disease, and myocardial and/or pericardial disease. It still has some limitations, but new indications for the percutaneous treatment of aortic stenosis will increase its use in the near future.

  • Cath lab guidance technologies include the use of robotic systems that will assist physicians during the performance of routine aspects of the diagnostic catheterization and the interventional procedure. These systems will provide the interventionalist with a safer, x-ray-free environment and at the same provide accurate measurement and allow for exact positioning of devices, without the need for x-ray or contrast material. Some of these devices use either magnetic fields or robotic technology to perform basic operations, such as placement of guidewires, balloons, and stents at the correct anatomic position, while other devices allow for accurate image acquisition and plaque composition and interpretation.

  • Intravascular ultrasound (IVUS), one of the initial intravascular tools for precise coronary imaging, has morphed into more complex systems that can produce accurate, almost histologic images of the plaque (virtual histology). Using this technology, we can now discern plaque and vessel wall characteristics, as well as temperature and elastic uniqueness. This new information will likely guide us to better understand and treat patients with coronary artery disease. New-generation IVUS devices will be incorporated into the basic cath lab in a nonintrusive way and become part of the interventional procedure. Optical coherence tomography (OCT) can provide impressive pictures of the anatomy inside the coronary vessels that correlate very well with histologic observations. These imaging devices can be used in conjunction with the x-ray images obtained during the procedure, not only to produce a 3-dimensional image of the coronary vessel, but also to allow accurate plaque investigation and device positioning. Improved angioscopy equipment can now provide live pictures of the heart and its chambers and help during therapeutic procedures.

  • Other new therapeutic imaging technologies are being created with transthoracic ultrasound that can help lyse lethal clots, whether they are in the heart or in the vessels of the lung. In addition, new imaging devices are under investigation for cardiothoracic surgeons that help assess the state of newly performed bypass grafts; this technology will provide surgeons with the opportunity to repair faulty grafts before chest closure.

These new technologies and their new applications were discussed in this i2 Summit session and provided attendees with an informative glimpse into the future of the cath lab.

Cardiac Imaging: A Landscape View Presenter: Peter J. Fitzgerald, MD, PhD, Stanford Medical Center (Stanford, California)
  • As noted by Dr. Fitzgerald's quote above, the widespread availability of cardiac CT enables patients to obtain these tests easily and quickly. Patients are directly targeted by easy-access magazines that offer cheap and fast ways to get a cardiac CT. Companies and hospitals are using direct consumer marketing to increase the number of patients referred to CT for the diagnosis of coronary disease. Dr. Fitzgerald observed that, in his practice, patients arrive to the clinics with their CTs in hand. This, he said, is not so bad, but it will change the way that patients get to the hospital and flow within the hospital. Safety is a critical issue, and physicians have a large responsibility to interpret the CT results correctly and to advise their patients appropriately.

The Next Generation of IVUS Presenter: Gary S. Mintz, MD, Cardiovascular Research Foundation (New York, NY)

Dr. Mintz described the next generation of IVUS. Although some of these new technologies are currently available, they have not been incorporated in a routine fashion and are only used by a minority of cath labs around the world. This new generation of IVUS systems, by virtue of their incorporation into the cath lab, will facilitate the acquisition and interpretation of these images. New technologies that will be available include:

  • Ultrahigh-frequency transducers, perhaps as high as 100 MHz or dual frequency

  • High-performance catheters that are similar to balloon catheters (or at least equivalent to stent delivery systems)

  • Forward-looking catheters

  • Wireless connection with the CPU and monitor (or at least minimization of the necessary cabling)

  • Consistent image quality with no need for image optimization (or at least automatic image optimization)

  • Enhanced border recognition

  • Virtual histology that can make accurate tissue characterization and define different plaque composition that will help define the presence of a thick or a thin-cap fibroatheroma

  • Integration of IVUS into the cath lab will make using the technology less expensive and easier to perform, and will save space. In addition, a fourth screen can be added to the cath lab and a small console at the table will help facilitate performing IVUS and also help to correlate the images of the IVUS with images captured by the coronary angiogram.

  • IVUS is not alone anymore and is now being incorporated with other technologies, such as virtual histology, palpography, back-scatter IVUS, and contrast injection.

  • IVUS should be simple, plug and play, and although much of this new technology is currently available, Dr. Mintz feels strongly that it still needs to be integrated into a single device that will enhance the way we perform IVUS.

Magnetic Assisted Intervention: The Stereotaxis Niobe Artis System Presenter: Albert E. Raizner, MD, Methodist DeBakey Heart Center (Houston, Texas)

Magnetic assisted navigation for the catheterization laboratory consists of 3 basic components:

  • Magnet system, which comprises the magnetic field and can move the vector in any direction;

  • Guidewires and catheters that have small magnets in their tips and are steered by the magnet; and

  • Automated catheter advancer, which is used to remotely advance and retract the catheter and other interventional devices.

The concept of this system is very simple: 2 large magnets sit in the cath lab at the level of the chest of the patient. By being oriented in a variety of positions, these magnets can create a magnetic field that can move the vector in any 3D position. When the direction of the vector is changed, the instruments change direction. Four options are available for use: a preset anatomy system that has the most common anatomic variation of the coronary arteries; an endovascular pathfinder; a 2-dimensional anatomic map, and a 3-dimensional anatomic map.

With the use of the magnetic force, almost any curve and tortuosity can be successfully and safely maneuvered using the guidewire and the device. Therefore, the opportunities provided by this system will likely merge imaging, navigation, and therapeutics. The device can use a preconstructed 3D image to navigate the devices through all these curves. CT-obtained images can also be used to navigate these systems and devices, even in completely occluded vessels. It is conceivable that, in the future, a series of modalities will serve to guide devices while the cardiologist sits in a remote position and advances the devices with a joystick.

Robotic Catheterization and Navigation in the Cath Lab Presenter: Luis Gruberg, MD, Rambam Medical Center (Haifa, Israel)

The results of the first-in-man study using a new remote navigation system were presented at this meeting. In this preliminary study, the investigators assessed the ability of the system to:

  • Safely navigate and deliver the guidewire inside the coronary arteries and across lesions;

  • Drive and accurately position the device (balloon and or stent) at the lesion site; and

  • Safely perform remote control stent-assisted percutaneous coronary intervention.

The system consists of a mechanical wire manipulator for advancing/retracting and rotating the guidewire, a delivery system manipulator for advancing/retracting the device, and a user control console unit which can control the device from a touch screen or a joystick. Basically, patients undergo the first part of the procedure in an identical manner as a standard angiography. After obtaining vascular access, the system is placed on top of the patient's groin, which will help deliver the guidewire to a prespecified position and the device to the lesion site.

Initial studies were performed in a glass model and later in animal models. The pilot clinical study assessed the system in 18 patients in Israel and Romania whose baseline clinical characteristics were similar to those of the average patient undergoing coronary intervention. The technical results were good, with successful wire delivery in 17 patients and successful device delivery in 15 patients. There were no complications with the device. Total catheterization time and total fluoroscopy time were rather similar to those obtained with conventional systems. Therefore, this system has the potential to offer radiation safety and a convenient environment for the operator. However, the system will need to undergo some refinements to broaden its use in clinical practice.

Devices to Image Plaque Characteristics Presenter: Ron Waksman, MD, Cardiovascular Research Institute (Washington, DC)

Coronary angiography is not a perfect method to assess coronary artery anatomy, especially if one only focuses on plaque anatomy. IVUS has really helped to assess the anatomic characteristics of the vulnerable plaque, particularly in calcified lesions, but its use is severely limited. Dr. Waksman highlighted other technologies that have tried to overcome these limitations.

Palpography, also referred to as IVUS elastography, measures differences in hardness and elasticity of tissues with different histopathologic compositions. At the same time, it can assess conformational changes in coronary plaques resulting from intracoronary pressure changes during the cardiac cycle. By measuring the local strain over the plaque, this technique can discriminate between soft and hard tissues.

Virtual histology uses the IVUS and color codes radiofrequency waves that are converted into color-coded figures: green for fibrotic, red for lipid core, yellow for fibro-lipidic, and white for calcium.

Intravascular MRI catheters , which consist of a small magnet, a coil, and 2 fields at the tip of a catheter, have the ability to differentiate fibrous tissue, lipid rich necrotic plaque, calcium, and even thrombus. The catheter needs to be stabilized with a balloon at the site of interest.

OCT uses near infrared light and is an optical analogue of IVUS. It has greater image clarity and resolution and is able to perform excellent tissue characterization with spectroscopic and polarization imaging. The image wire is 0.014" long and has a light source at the distal tip in a 0.006" micro-optic inner core wire, which is movable within the image wire. The images have far superior quality when compared with regular IVUS. Unfortunately blood flow needs to be interrupted and continuous saline has to be injected during the procedure.

Angioscopy is an old system that is being used again to assess plaque morphology.

Intravascular termography is used to detect the heat generated by the plaque and differences in temperature along the vessel wall.

Vaso vasorum imaging can provide valuable information on the vulnerable plaque, which is assessed using a microbubble injection.

Imaging Through Blood Using an Optical Catheter Presenter: Marc D. Feldman, MD, University of Texas Health Science Center (San Antonio, Texas)

The viewing capabilities of optical catheters have become very accurate. Molecular imaging with OCT is an especially important area of research. OCT can actually give a very accurate histologic picture from the lumen of the vessel and the vulnerable plaque. New methods can use metals such as iron oxide inside macrophages that can be visualized via OCT with magnets (the metal moves with the magnetic fields). Initial studies were performed in liver models and are currently under study in atherosclerosis models. Therefore, OCT can identify macrophages in a plaque, gradients of collagen in the fibrous cap, and, due to a resolution of 2-4 micrometers (mcm), the technology has the potential to perform molecular imaging in patients during heart catheterization.

In Vivo Evaluation of Thin-Cap Fibroatheroma: OCT vs Angioscopy vs IVUS Presenter: Yoshihiro Takeda, MD, Toyohashi Heart Center (Toyohashi, Japan)

Plaques that are prone to rupture are known as thin-cap fibroatheroma (TCFA), and are the most common lesions underlying acute coronary syndromes (ACS). The TCFA has a thin fibrous cap (per definition, < 65 mcm), a large lipid-rich pool, and active inflammation. Prior studies using angioscopy have shown that yellow plaques have these characteristics. The high resolution of OCT (10 mcm), which is 10 times higher than IVUS, may visualize a thin fibrous cap (< 65 mcm) overlying a lipid-rich plaque, known as TCFA. A comparison performed by Dr. Takeda and colleagues in 26 patients compared OCT vs angioscopy and IVUS. The minimum cap thickness and the area of the lipid pool were assessed. OCT successfully assessed the rupture of the TCFA and the presence of thrombus. ACS patients had a higher frequency of lipid-rich plaques compared with patients with stable angina. IVUS was not able to demonstrate thrombus or ruptured plaques as effectively as OCT did.

CT Angiography Presenter: Robert S. Schwartz, MD, Minneapolis Heart Institute Foundation (Minneapolis, Minnesota)

According to Dr. Schwartz, the advent of new "3-dimensional" imaging technology has made a big impact on our diagnostic and therapeutic capabilities, namely through its massive information processing capabilities. Multislice CT uses multiple rows of detectors ranging from 20 mm to 32 mm in width. The x-ray beam is divided into smaller slices, 2, 4, 8, and 16 slices with one rotation of the x-ray tube. Generating more slices per rotation with a wider beam can drastically reduce the scan time producing more motion-free scans, a benefit for patients who can't hold their breath for very long. These systems have achieved high grades of sensitivity (92%) and specificity (91%), although these rates vary according to the literature, with approximately 20% of scans deemed nonvaluable.

The goals of coronary CT imaging are to eliminate false-negative assessment, to guide future therapy, and to limit the risk of coronary angiography in the diagnosis of certain patient subsets (eg, those with atypical chest pain, low-risk patients, preoperative assessment for noncardiac surgery patients, and patients at high risk for cholesterol embolization). CT angiography can become an important tool for the triage of patients with ambiguous noninvasive testing, and will allow physicians to send the right patient to the cath lab with a knowledge of the anatomy.

As noted by Dr. Schwartz, CT angiography may be indicated in the following instances:

  • Uncertain or suspected normal or abnormal stress tests;

  • Initial evaluation of possible coronary artery disease symptoms;

  • Prior positive stress tests or known disease where a repeat stress test would remain abnormal;

  • Difficult clinical questions regarding prevention;

  • Risk assessment in the elderly or reluctant patient;

  • Follow-up of patients who have undergone coronary artery bypass surgery;

  • New left main stenting or early post-stent chest pain;

  • Total occlusion of coronary vessels to assess revascularization possibilities; and

  • Noninvasive assessment of valves when valvuloplasty is considered (calcium location and quantification).

He added that new machines with 254 slices are already being assessed that will improve temporal and spatial resolution, which will have an important role in patients admitted to the emergency room with chest pain. There is no doubt that the use of CT angiography will replace diagnostic invasive angiography and will help us refer patients to the cath lab for treatment, armed with the knowledge of the correct anatomy.

Cardiac MRI Presenter: Michael J. Gallagher, MD, William Beaumont Hospital (Royal Oak, Michigan)

MRI of the heart can offer important data when obtained in the acute phase of an MI. It can help assess infarct size, microvascular obstruction, and the presence of transmural infarction, all of which have been associated with adverse remodeling and poor long-term survival. After injection of the contrast (gadolinium), normal myocardium stains very fast, whereas infarcted myocardium has a slower peak with a slow washout period, making it useful for the defining function, flow, perfusion, and viability.

According to Dr. Gallagher, MRI has proven to be extremely useful in:

  • Diagnosis of perimyocarditis;

  • Diagnosis of left ventricular thrombus;

  • Visualization of valve anatomy;

  • Diagnosis of hypertrophic cardiomyopathy;

  • Localization of congenital defects; and

  • Quantification of shunts and sizing of defects for future repair.

Conclusion

Novel imaging technology, in conjunction with miniaturization of devices, is producing a new generation of systems that will help us assess the heart and its vessels in a more comprehensive manner. As described in this session, these advances will enable us to understand the pathologic changes and therefore provide a wider range of therapeutic alternatives to our patients.

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