Pediatric Interventional Cardiovascular Magnetic Resonance

Improved Visualization of Cardiac Anatomy

A problem with x-ray-guided cardiac catheterization is the inherent poor contrast of soft tissues, such as the heart and great vessels. This makes it difficult for the cardiologist to manipulate or position guidewires, catheters, balloons, or interventional devices within the heart and surrounding vessels. A skilled operator usually relies on recognizing anatomic structures from previous experience or on contrast angiographic images acquired earlier in the procedure. The lack of adequate visualization increases the risk of perforating the heart or great vessels, especially when performing complex interventional procedures.

Certain interventional cardiac procedures involve selection of an appropriate cardiac device and its successful deployment within the heart, which requires accurate measurement of the size of defects and nearby anatomic structures. Such measurements are possible under x-ray fluoroscopy (XRF), but can be difficult.

A successful interventional cardiac procedure therefore relies heavily on adequate visualization of the heart or vessel. This implies the need for superior imaging methods that provide excellent visualization without increasing the risk to the patient. CMR fits this role very well because it provides exceptional structural delineation of both the heart and its surrounding vasculature and therefore allows safe guidance of interventional procedures.

Reduced Ionizing Radiation

There is a strong case for pediatric cardiac catheterization procedures to be made safer, especially in terms of ionizing radiation. According to the UK National Radiation Protection Board, the mean risk that a solid tumor will develop as a result of a single cardiac catheterization procedure is approximately 1 in 2500 in adults. This risk increases to 1 in 1000 in children if exposure occurs at 5 years of age. Also, the proportion of the body that is irradiated increases as the size of the patient decreases, and some procedures in patients with congenital heart disease often require much longer x-ray exposure. These risks are multiplied in children in particular, because they often undergo multiple cardiac catheter procedures. In addition to the patients, there is also a significant risk from ionizing radiation to the staff in the catheter laboratory during these procedures, despite the use of protective shields.

Physiologic Information

Conventional cardiac catheterization is used not only to provide anatomic information and perform intervention but also to obtain functional information. Invasive pressures and blood gases are commonly used to calculate systemic and pulmonary blood flow and resistance with the Fick principle. XRF angiography is also used to assess global ventricular function as well as regional wall motion abnormalities. The functional information obtained at cardiac catheterization is used alongside anatomic information to assess patient suitability for surgery or interventional cardiac catheterization or the need for long-term vasodilator therapy in patients with pulmonary vascular disease.

The Fick principle to quantify flow is dependent on multiple measurements (hemoglobin, aortic/pulmonary artery oxygen saturation, partial pressure, oxygen consumption), which can be a considerable source of inaccuracy. In addition, in patients with large intracardiac shunts and high pulmonary blood flow, accuracy is further reduced. Therefore there is a need for a method of flow quantification that allows accurate and reproducible measurement of pulmonary vascular resistance (PVR). Velocity encoded phase contrast CMR enables noninvasive quantification of blood flow in major vessels. Cardiac output and the pulmonary-to-systemic flow ratio (Qp:Qs) measured using this technique have been shown to be accurate. In addition, phase CMR has been validated in numerous phantom experiments, allowing for a novel method of quantification of PVR in patients with pulmonary hypertension by using invasive pressure measurements and CMR flow data. This method has since been used in a large clinical case series with good results and forms part of routine clinical practice in institutions where this is available.

Assessment of global and regional ventricular function can also be carried out much more accurately with cine steady-state free precession (SSFP) CMR than with x-ray angiography. When using CMR for assessing global ventricular function, there is no need to make assumptions about cardiac geometry, unlike with XRF or even echocardiography. This is particularly important when assessing right ventricular (RV) function and regional wall motion in the normal or systemic ventricle in patients with functionally single ventricle physiology.

Finally, combining invasive pressure measurements with CMR-derived blood flow and ventricular volumes also opens up interesting new ways of looking at pathophysiology. It allows for the study of pulmonary vascular compliance, derived ventricular pressure–volume loops, and assessment of load-independent ventricular function. Pharmacological stress studies have also been applied in CMR/XMR catheter studies to assess hemodynamic responses.

Passive Catheter Tracking and Visualization

The passive tracking technique is commonly based on visualization of susceptibility artifacts or signal voids caused by the interventional device under CMR imaging. This is a well-studied technique and to date it is the most clinically feasible ( Figs. 48.2 and 48.3 ). Passive visualization often does not require any special hardware or software and therefore it can be performed on any commercial CMR system.

The ideal passive tracking catheter or guidewire must be made of a material that provides adequate physical properties such as torque and steerability, and allows tracking without obscuring the underlying anatomy. Ferromagnetic materials cause large susceptibility artifacts and therefore are not generally suitable for CMR-guided procedures. This rules out most metals used for making cardiac devices. However, certain alloys, such as nitinol (nickel and titanium), have magnetic susceptibility close to that of tissue. Therefore they are best suited for making guidewires and braided catheters that are MR compatible but not necessarily CMR safe.

The polymeric materials used for making catheters typically have low magnetic susceptibility and therefore cannot be easily localized on CMR images. This implies that, if materials with higher susceptibility can be incorporated into the wall of the catheters or sheaths or the lumen filled with a suitable contrast agent, then improved visualization can be achieved.

One approach to generating susceptibility artifacts is locally impregnating the catheter wall with gadolinium (Gd)-like compounds, such as dysprosium oxide, in the form of rings or along the length of the catheter during the extrusion process (see Fig. 48.2C ). Another approach is to use Gd contrast agents in varying concentrations within catheter lumens or impregnated into catheter walls to create either a positive or negative signal on CMR imaging.

Metallic devices and guidewires produce susceptibility artifacts, which aid visualization by way of the artifacts, but different metals behave differently under CMR. Titanium alloys produce narrower artifacts compared with ferromagnetic, or even certain other nonferromagnetic alloys such as nickel–chromium, which can produce large RF and susceptibility artifacts. However, commercial guidewires can heat up during CMR because of standing wave formation along the conductive parts longer than a quarter wavelength at the resonant frequency, which corresponds to approximately 12 cm in humans at 1.5 T, which is relevant for cardiac catheterization where wires are inserted to at least that length and nearly always much further.

Guidewires with a fiberglass core and nonmetallic guidewires made of resin microparticle compound covered by polytetrafluoroethylene have been used for MR-guided interventions in animals. More recent wire developments include the use of fiberglass MR safe guidewires, which were used in preclinical and clinical trials and led to successful interventions in congenital heart disease, but these were difficult to steer and proved to be fragile. There continues to be a drive for the development of an optimal guidewire and techniques to measure heating risks in vivo have been employed. Consequently, a newer nitinol based guidewire with iron oxide markers along the length to impart visibility has been developed with good preclinical results.

In the case of balloon angiographic catheters, if the balloon is inflated with carbon dioxide, as is done conventionally with x-ray, then the inflated balloon creates a signal void in the CMR image, thus enabling visualization (see Fig. 48.2A and B ). This method has been used successfully to guide catheters in patients under CMR (see Fig. 48.3 ). Although this technique allows easy visualization of the tip, the length is impossible to visualize because the signal void from the catheter length is masked by volume averaging and dephasing effects of thicker slices. A similar approach is to inflate the balloon of the angiographic catheter with a 1% concentration of Gd contrast agent and the balloon appears as a white ball because of the signal from the contrast-filled balloon.

The success of passive visualization also relies on dedicated scan techniques. A dynamic gradient echo sequence, such as SSFP, has been shown to be ideal for passive catheter tracking, especially when signal voids or susceptibility artifacts are used for visualization. Cardiac catheterization under XRF guidance is usually performed at imaging speeds of 25 to 30 frames per second. The frame rates available for CMR-guided interventions are not comparable because of the postprocessing of CMR images and their subsequent display, allowing a maximum of 10 to 14 frames routinely. Some of the proposed passive catheter tracking techniques require image subtraction or positive contrast to improve visualization of markers on the catheter, which means that, along with faster scan techniques, faster image processing algorithms are required.