5.1.1 Basics

Magnetic resonance imaging (MRI) is a modern, noninvasive technique for obtaining cross-sectional images that is also very valuable for diagnosing congenital heart defects.

Advantages and disadvantages

One major advantage of the method is that it does not require the use of ionizing radiation or iodinated contrast medium. The disadvantage is the long time needed for the examination, which may be over an hour for typical pediatric cardiology problems. Anesthesia is therefore generally needed for uncooperative younger children. Since many images must be made using the breath-hold technique, endotracheal anesthesia is usually the method of choice for uncooperative patients.

Functional principle

The principle of MRI can be briefly summarized as follows. The protons in the nucleus of an atom have intrinsic angular momentum (spin) that gives the nuclei a magnetic moment. Due to the strong magnetic field of the MRI, the nuclei are aligned in a certain direction. Radio waves are used to deflect the nuclei from their normal alignment. The nuclei begin to wobble. After the radio waves are switched off, the nuclei return to the aligned positions. The atomic nuclei display different behavior depending on their surroundings. In this process, they emit weak magnetic signals that can be measured by the MRI unit, from which cross-sectional images can be made.

Rapidly switching the magnetic field on and off generates electromagnetic fields that pull on the coils of the MRI unit and cause the typical thumping sound during the examination.

Gadolinium (Gd), a paramagnetic substance, is used as a contrast medium for a cardiac MRI. Gadolinium is toxic and must be bound in a stable chelate molecule (e.g., in the form of Gd-DTPA).

Frequently, 3D datasets are made that can later be reconstructed in various ways.

Imaging process

MRI examinations were originally performed using primarily spin-echo sequences. Now considerably faster sequences are commonly used. More technical details are available in the specific literature.

Contraindications

Due to the very strong magnetic field, patients must remove all metal items (watch, wallet, piercings, etc.) before an MRI scan. The magnetic strips on chip cards will also be permanently erased by the magnetic field. Patients with ferromagnetic implants (cardiac pacemakers, AICDs, cochlear implants), or large tattoos (metallic dyes) can generally undergo an MRI scan only if special measures are taken. There is a relative contraindication for this examination for such patients. However, occlusion systems implanted in a catheter-based procedure such as ASD occluders or coils and artificial heart valves are usually not a problem. The wire cerclage used after a sternotomy also poses no problem, but the metallic parts can cause artifacts in the surrounding area.

Indications

MRI is particularly suited for displaying non-bony structures such as soft tissue, brain, internal organs, and cartilage. Only lung tissue and bones are difficult to assess. The disadvantages compared with cardiac catheterization are mainly that the pressures in the heart chambers and great vessels cannot be measured directly and that no intervention is possible.

Cardiac MRI is especially useful for the following examinations:

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  Determining cardiac anatomy (in addition to the other imaging methods)

  
  
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  Assessing global and regional cardiac function

  
  
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  Volumetric analysis of the different cardiac chambers

  
  
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  Measuring and analyzing blood flow (e.g., determining systemic and pulmonary blood flow)

  
  
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  Quantifying shunts

  
  
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  Characterizing tissue (e.g., more precise classification of a cardiac tumor)

  
  
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  Assessing myocardial perfusion

  
  
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  Assessing myocardial vitality (using the delayed enhancement technique)

  
  
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  Visualizing the great vessels using MR angiography

The most frequent clinical questions for cardiac MRI in childhood are listed in Table 5.1.

Cardiac anatomy

The examination process is based on standard planes, similar to echocardiography, which can, however, be freely varied depending on the clinical question. The individual sections can be selected in all spatial planes. Examples of the most common standard sections are presented in Fig. 5.1.

Global and regional myocardial function

Global and regional myocardial function is generally displayed using cine sequences. These sequences, which are usually executed as breath-hold, ECG-triggered images, yield a kind of short film of several cardiac cycles.

Volumetric analysis

A complete series of short-axis slices from the base of the heart to the apex is used to quantify the volume of the ventricles. Ventricular volume is calculated using the planimetric data of ventricular surface areas combined with slice thickness and spacing (single-plane Simpson formula). Stroke volume, ejection fraction, and cardiac output are calculated using the ventricular volume measurements.

Measurement and analysis of blood flow

Vascular blood flow is usually determined using phase contrast flow measurement. Similar to the method used in Doppler ultrasound, the approximate estimated flow velocity must be set to achieve the best possible results. Flow is optimally measured in a direction orthogonal to the respective blood vessel.

Quantifying shunts

An intracardiac shunt is easy to quantify using the ratio of pulmonary blood flow (Q _p) to systemic blood flow (Q _s). To do this, flow is measured in the pulmonary artery and aorta and the ratio is calculated (Q _p/Q _s). As in cardiac catheterization, a ratio of more than 1 is a sign of a left-to-right shunt and a ratio less than 1 indicates a right-to-left shunt. If the ratio of Q _p/Q _s is 1, there is no shunt.

Myocardial perfusion

Contrast medium (normally Gd-DTPA) can be administered to assess myocardial perfusion. Depending on how the contrast medium travels in the heart, myocardial perfusion can be assessed both visually-qualitatively and quantitatively.

Myocardial vitality

The vitality of myocardial tissue can be assessed using the delayed enhancement technique. After intravenous administration of contrast medium, delayed images are made in which the nonviable myocardial tissue shows contrast medium enhancement. This technique is used to visualize myocardial scars after infarctions or from coronary artery stenosis as well as after myocyte damage due to other causes (myocarditis, cardiomyopathy). Even compared with the gold standard diagnostic measure for vitality, the PET scan, the sensitivity and specificity for detecting nonviable myocardial tissue using this method are very high.

MR angiography

In MR angiography, the thoracic vessels can be readily visualized after contrast medium is administered. These images are generally made while the breath is held. Modern devices generate a complete 3D dataset within 30 seconds. Indications for an MR angiography are primarily stenoses in the thoracic aorta (e.g., coarctation), peripheral pulmonary stenoses, and anomalous pulmonary venous connections.