Interventional

To date, X-ray-based fluoroscopic techniques have been the gold standard for most invasive diagnostic and therapeutic applications for the heart. With the advent of ultrafast MRI and the development of MRI-compatible catheters and guidewires,

Fig. 17. (A) T2* map of an isolated beating rat heart in the short-axis view. Indicated is the epicardial myocardium (EP), the endocardial myocardium (EN), the left ventricle (LV) with a balloon inside, the collapsed right ventricle (RV), papillary muscle (PM), and vessel (V). Imaging parameters were as follows: field of view = 20 x 20 mm, matrix = 256 x 256, and spatial resolution = 78 ^m in plane. The slice thickness was varied between 250 ^m and 1 mm. (B) Corresponding histological section. To minimize motion artifacts from the beating heart, data were collected in middiastole. Image provided by Dr. Sascha Köhler and Dr. Peter M. Jakob, Physikalisches Institut, Universität Würzburg, Germany.

Fig. 17. (A) T2* map of an isolated beating rat heart in the short-axis view. Indicated is the epicardial myocardium (EP), the endocardial myocardium (EN), the left ventricle (LV) with a balloon inside, the collapsed right ventricle (RV), papillary muscle (PM), and vessel (V). Imaging parameters were as follows: field of view = 20 x 20 mm, matrix = 256 x 256, and spatial resolution = 78 ^m in plane. The slice thickness was varied between 250 ^m and 1 mm. (B) Corresponding histological section. To minimize motion artifacts from the beating heart, data were collected in middiastole. Image provided by Dr. Sascha Köhler and Dr. Peter M. Jakob, Physikalisches Institut, Universität Würzburg, Germany.

the goal of achieving real-time guidance by MRI for cardiovascular interventions is emerging as a new alternative (25). The use of MRI for guided interventions would eliminate the reliance on ionizing radiation and iodinated contrast agents, an important advantage particularly for pediatric patients.

To date, continuous improvements of MRI techniques and MRI scanner hardware have rendered it feasible to achieve fluoroscopic image rates of 5-15 images/s (26). Thus, it is possible to use MRI for guiding interventional cardiovascular procedures, such as coronary catherization (27) or gene therapy delivery (28), with close to real-time image refresh rates. Initial interventional studies with MRI guidance have demonstrated the advantages of MRI, including the ability to image arbitrarily oriented cross-sections, interactive steering of the image plane, and excellent soft tissue contrast for the detection and visualization of lesions (29).

Several other technical advances have also been crucial for performing interventional procedures under MRI guidance, including: (1) development of 1.5-T magnets with short bores that allow access to the groin area for catheter-based procedures; (2) liquid crystal image displays that can be exposed to high magnetic fields to allow the operator to perform an intervention and control MRI scan parameters from a position right next to the magnet; and (3) development of catheter-based MRI antennae (30) for localized intravascular signal reception and high-resolution imaging.

4.1. MR-Compatible Devices

MR-guided interventions, such as the pulmonary artery dilation shown in Fig. 18, can only be performed with devices free of ferromagnetic components because severe magnetic field and image distortions would be encountered, not to men tion the physical forces exerted on the device itself by the static magnetic field (31). Even without the use of ferromagnetic materials near the MRI-compatible devices, image artifacts are not entirely unavoidable. On the other hand, this can be used as an advantage to differentiate the device from surrounding tissue or blood, thereby providing a means to track the position of the device; this method of device monitoring is termed passive tracking.

For example, in our laboratory we have used a customized, MRI-compatible variation of the Amplatzer® Septal Occluder (AGA Medical, Golden Valley, MN), which consists of a niti-nol wire mesh that produces minimal artifacts (32). This device is visible on MR images by causing a relatively small signal void in its proximity. Experiments have been conducted in our laboratory for closure of an atrial septal defect with the Amplatz device under MRI guidance (32). (For more information on such devices, see Chapter 29.)

In addition to passive tracking, we have taken advantage of active tracking devices in the form of miniature radio-frequency-receiving antennae mounted on a catheter tip (Surgi-Vision Inc., Gaithersburg, MD). With these miniature antennae, it is possible to acquire images from very small fields of view, that is, to achieve high spatial resolution if the external receiving antennae have been switched off. Such miniature antennas also enable imaging of vessel walls (plaque and aneu-rysm imaging), surrounding soft tissue (tumor, hematoma), or atrial septal anatomy, or obtaining a signal-to-noise ratio in the images not achievable with external antennae. Furthermore, the image generated by a coil on the tip of a device can be superimposed on other MR images, obtained simultaneously with external antennas, for device tracking during an inter-ventional procedure.

Outflow Tract m :

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Fig. 18. Axial snapshot obtained with a gradient echo sequence with steady-state free precession and optimized for real-time imaging at frame rates up to 8 images/s. The image was acquired for a slice at the level of the aortic root and the main pulmonary artery in a porcine model of pulmonary stenosis. The balloon was filled with gadolinium contrast agent and saline (1:20) for optimal contrast. In the center of the image is the aorta with the left ventricle trigger catheter in place. The balloon is fully inflated (4-5 bar) in the main pulmonary artery. The operatively created stenosis was successfully dilated during the procedure guided by magnetic resonance imaging.

5. MRI AND BIOMEDICAL DEVICES 5.1. MRI Safety and Compatibility

Most currently used implantable devices contain metallic parts that would both seriously interfere with MR cardiac imaging, or pose a safety risk for the patient. Implanted devices with ferromagnetic parts are considered a strict contraindication for cardiac MRI because of the potential hazards caused by movement, dislodgement, or heating effects. Although cardiac pacemakers are implanted in large numbers of patients, little has been accomplished to date to make pacemakers MRI compatible or MRI safe.

A notable exception is a study that employed fiber-optic cardiac pacing leads; the study reported that the use of such leads resulted in an absence of heating effects and only minimal magnetic deflection forces (33). Fiber-optic catheters attached to a wearable temporary external photonic pulse generator may therefore provide in the future a means for safely performing MRI studies on patients with implanted pacemakers.

MRI compatibility can be defined as the property of a device not to interfere with imaging, such as by causing distortions of the magnetic field that cause signal loss. A device may be MR compatible, but that does not necessarily mean it is MR safe. The latter requires that the exposure to a strong static magnetic field, pulsing of the magnetic field gradients, and applications of radiofrequency pulses do not cause adverse effects. For example, the pulsing of the magnetic field gradients produces a changing magnetic flux that can induce current flow in lead wires, which in turn may lead to tissue heating. The presence of metallic parts can also cause an inhomogeneous deposition of radiofrequency power in the vicinity of the device, which could lead to radiofrequency heating of tissue or blood. Implanted cardiac pacemakers are therefore not only a contraindication for cardiac exams, but for MRI exams in general. Investigators have reported that MRI imaging caused temperature elevations as high as 23°C at 0.5 T and 60°C at 1.5 T at pacing lead tips (34,35).

It is foreseen that completely new safety concerns will be raised when MRI is performed using intravascular coils (36,37). Such intravascular coils may, for example, be used to examine vulnerable plaque on vessel walls; therefore, localized heating in the vicinity of the coil could disrupt the plaque, with catastrophic consequences. Heating strongly depends on the wavelength (MRI frequency), geometry of the body and the device, and placement of the body and device with respect to each other and within the MR system.

Our group conducted temperature measurements on the potential heating effects within a gel phantom to study the interaction when employing a miniature, loopless intravas-cular antenna; a 1-m loopless, coaxial (dipole) antenna was connected to a decoupling, matching, and tuning (DMT) interface. The DMT contained a radiofrequency trap circuit and an active detuning circuit with a radiofrequency PIN diode. During the radiofrequency transmit phase, the PIN diode was activated and presented a short circuit for signals traveling on the inner conductor of the loopless antenna.

The effectiveness of the decoupling mechanism was tested in an 8-in diameter and 20-in long cylindrical gel phantom with a dielectric constant of 81 and conductivity of 0.7 Siemens/m. Temperature changes in the vicinity of the device were measured while a conventional spin-echo sequence was continuously running. We found that the exact temperature rise varied as a function of insertion depth and position of the coil in the phantom and the magnet bore. In the connected state, the temperature increase never exceeded 4°C and, in most cases, remained below 1 °C. However, in the absence of a DMT or with malfunction of the DMT, a temperature rise in excess of 30°C could be produced for the identical scan parameters; this underlines the need for careful safety mechanisms in coils used for interventional MRI.

5.2. Testing of Biomedical Devices With MRI

Cardiac MRI also provides a unique opportunity to test, in vivo, the performance of implanted devices such as prosthetic heart valves and heart pacing devices (those that would be MRI compatible). In patients with artificial aortic valves, the flow downstream from the implanted valve may be severely altered. These changes have been associated with an increased risk of thrombus formation and mechanical hemolysis. MRI velocity mapping is considered very useful for the noninvasive evaluation of the flow profiles in patients with a mechanical valve prosthesis (38,39). For example, Botnar et al. (40) found that peak flow velocity in the aorta was significantly higher in patients with a valvular prosthesis than in normal controls. In

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