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I./3.6.: Magnetic resonance imaging (cardiac MRI)
Approximately 1% of coronary angiographies performed in patients with suspected ST segment elevation myocardial infarction produce a negative result. This may be due to several causes. In a part of the cases the patient actually has a myocardial infarction, but there may be either a rupture of a minor plaque which caused no significant stenosis, possibly with spontaneous recanalization, and the plaque cannot be visualized by coronary angiography, or the infarction is caused by a prolonged spasm which developed without any organic stenosis. Another possibility that the patient has no infarction, but some other disease. If the global clinical picture suggests an infarction (chest pain, ST segment elevation) underlying the symptoms there is most frequently peri-myocarditis, occasionally special cardiomyopathy (HOCM, takotsubo cardiomyopathy).
Takotsubo cardiomyopathy is a special disease; it can be observed mainly in female patients, a necrosis of the myocardium is caused by prolonged vasospasm which developed in a severe stress situation. Coronary angiography is negative, at the same time echocardiography often shows an extended disorder of wall movement with a characteristic apical balloon-like bulging. In the majority of cases there is a significant transient stunning, when no definitive large necrosis occurs and the left ventricular function shows a significant spontaneous recovery in a few days. It is very important to differentiate between the two groups (infarction or other diseases affecting the myocardium), as it essentially determines the patients’ further therapy.
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In the differential diagnosis of these two groups of patients neither the ECG nor the laboratory tests offer any help (elevated biomarker levels are expected in both groups), and the dilemma cannot be resolved reliably also by echocardiography.
Acute cardiac MRI, performed after a negative coronary angiography, helps to decide for one of the two groups. In patients with an infarction a segmental edema and subendocardial/transmural necrosis can be detected corresponding to the area supplied by the given coronary; while in the other patients these abnormities show a diffuse (including nodular) localization.
Cardiac MRI provides further help in the care of patients with acute myocardial infarction. Partly due to its even higher resolution as compared to the echocardiography, it is suitable for even more accurate calculations of left and right ventricular function; a regional wall motion disorder can be visualized well, and an accurate diagnosis may be obtained by detecting the occasional complications, e.g. by exact clarification of the layers of the pseudo-aneurysm and making the lack of the ventricular wall unequivocal in patients with the extremely rare covered free ventricular wall rupture,
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A further important field of examination is the assessment of the post-infarction status, detection of the viable myocardial areas. After the application of a special contrast agent the necrotic, non-viable area shows accumulation that appears on the late recordings as a white stripe. If the late accumulation reaches 75% of the ventricular wall’s thickness, the area is unfit for life (white = necrosis). In a given patient a potential coronary revascularization depends on the presence of viability. If the viability of the actually akinetic, afunctional myocardium, supplied by the given coronary, can be demonstrated, a revascularization can restore the contractility and by this way it may result in an improvement of left or right ventricular function of the given patient.
Disadvantages of the MRI examination include its limited availability, the relatively long duration of the examination, the intolerance of some patients to the feeling of being closed in (claustrophobia) and to the harsh effects of sound (clicking noise of the machine), and a significant requirement of the work of specialists who are competent at the highest level. The latter relates not so much to the performance of the examination itself, but to the evaluation and analysis which surpasses the time of the actual examination even several times.
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