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Cardiac MRI

MRI is a method that uses magnets and radio waves to map the state of hydrogen nuclei in the body. MRI requires equipment equipped with magnets to generate a uniform magnetic field (Figure 20A). Generating an effective magnetic field requires very high technology, that is why MRI machines are expensive. In addition, in order to receive the radio frequency signals generated by the human body, each body part needs a receiving coil. For the heart RF signal will be a dedicated receiver coil of the heart (Figure 20B). Place the body in an electromagnetic tube with a uniform magnetic field and radiate radio waves in a short time. When the radio waves stop, the radio waves emitted by the body are received to create an image ((Figure 20C). When you look in more detail, you will find Rotational imaging of hydrogen nuclei in the body (Figure 20D). In MRI, the vibration of the machine is generated every time radio waves are applied, and there is noise. This is the reason why loud noise can be heard during MRI examination. The advantage of MRI is that it does not use radiation, so there is no need to worry about "exposure radiation".

Figure 20. Principle of cardiac MRI

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In cardiac MRI, the coronary arteries can be visualized without radiation and without using contrast agents. Also the function of the heart can be evaluated. The evaluation by cardiac MRI is highly reliable, so it has been used in clinical studies of various cardiac functions. In addition, The contrast-agent cardiac MRI has high potential, and all cardiac imaging diagnoses such as echocardiography, myocardial scintigraphy, and coronary angiography can be obtained in1 hour. Therefore, it is called a "one-stop shop" abroad.

1. Cardiac MRI

The cardiac MRI full study examination recommended by the Society for Cardiovascular Magnetic Resonance (SCMR) mainly focuses on contrast examinations: ① cine MRI examination → ② T2 weighted image photography → ③ coronary artery MRA photography (Whole heart coronary MRA) → ④ Perfusion imaging → ⑤ Delayed contrast examination is performed in 40 to 50 minutes (Fig. 21). Among them, the process of (1) → (2) → (3) is a complete non-contrast examination, so it can be performed even for relatively low-risk patients. In fact, our facility uses it for cardiac docks, which are often asymptomatic. Images of cardiac function taken with high tissue contrast are very useful for more accurate wall motion evaluation, heart size evaluation, and cardiac hypertrophy evaluation.  Contrast-enhanced MRI examination is useful for accurate diagnosis for problems with  reduced sensitivity in myocardial scintigraphy such as multivascular small vessel disease, right coronary artery area, and left branch branch area etc.

Figure 21. Cardiac MRI examination protocol

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2. Coronary MRA examination

In coronary MRA, we can be clearly visualized without using a contrast agent (Fig. 22). This is an imaging method that can solve the problems of contrast agent and radiation exposure in cardiac CT. Coronary MRA does not require respiratory arrest as in cardiac CT examinations, and performed using both diaphragm and ECG synchronization under normal breathing conditions. Therefore, it can be easily implemented even for those who have difficulty breathing or deafness. In addition, In addition, it is less invasive than CT, and can depict blood white without being affected by coronary artery calcification. Apart from the advantage that coronary artery calcification is used as a calcification score, it also has the disadvantage that if it is too advanced, it will prevent accurate CT diagnosis of coronary artery disease. Coronary MRA imaging technology is a new MRI imaging technology developed in 2003, and the number of medical institutions that can be implemented worldwide is still limited. Since the technique of coronary MRA imaging is suitable for the Japanese people, Japan is one of the countries in the world where the technique of coronary MRA is advanced.

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3. Delayed contrast MRI examination

Using a gadolinium contrast agent for MRI can observe the state of myocardium in detail. An important technique in cardiac MRI imaging is called delayed contrast MRI. When the myocardium is injured by myocardial infarction or cardiomyopathy, the myocardial cells in this area will be destroyed. The contrast agent that has penetrated into the myocardium can be photographed with white light using a special imaging technique, and the area where myocardial infarction has occurred can be clearly imaged with high resolution (Fig. 23).This can accurately determine the size and severity of myocardial injury. It is also used as an indicator of myocardial viability based on the depth of delayed angiography extending to the heart muscle, and is reported to be associated with recovery of cardiac function by coronary revascularization and patient prognosis.

Figure 23. Myocardial viability by delayed contrast MRI

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4. Loaded myocardial blood flow MRI examination

Figure 24. Drug-loaded myocardial blood flow MRI

Stress myocardial perfusion MRI and myocardial scintigraphy is very similar. This is a drug called adenosine, which increases blood flow in the heart muscle and checks for sufficient blood flow increase through MRI.　If the coronary arteries are thin, there is damage to the myocardium, and if myocardial blood flow does not increase sufficiently, detected as abnormal. Compared with myocardial scintigraphy, it is possible to perform inspection with higher resolution (Fig. 24).

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