Role and capabilities of the method
Cardiovascular magnetic resonance (CMR) is rapidly becoming one of the most sought-after tools in the diagnosis of non-ischemic heart disease. Although portable echocardiography traditionally serves as the first-line investigation, MRI significantly expands physicians' capabilities.
Key clinical advantages of the scan:
- Large field of view and exceptionally high tissue contrast resolution.
- Complete independence of results from operator skill and the presence of a so-called "poor acoustic window" in the patient.
- High-precision planimetric and volumetric measurements of the heart chambers.
- Capability for dynamic serial monitoring of ventricular function in patients with cardiomyopathies (here, MRI significantly outperforms ultrasound in reproducibility).
- Objective quantitative assessment of valve function (severity of regurgitation and stenosis).
- Detailed visualization of cardiac tumor morphology and precise determination of the extent of their invasion into surrounding structures.
Tissue characterization and late gadolinium enhancement
A crucial feature of magnetic resonance imaging is its ability to analyze the myocardial tissue itself for pathological inclusions. For this purpose, cardiologists widely use the phenomenon of late gadolinium enhancement (Late Gadolinium Enhancement, LGE).
The accumulation of contrast agent in the late phases helps confirm the diagnosis and determine prognosis in such serious conditions as:
- Hypertrophic cardiomyopathy (HCM).
- Dilated cardiomyopathy (DCM).
- Infiltrative lesions (e.g., sarcoidosis or myocarditis).
For comparison: intramural perfusion defects can be observed in the first-pass contrast frames, while delayed scans in the same zone reveal enhancement caused by the formation of a dense scar or fibrosis.
Parametric mapping
Modern mapping sequences utilize various inversion times. They generate recovery curves, creating pixel-by-pixel parametric images. This allows for quantitative evaluation of the myocardium at the cellular and extracellular levels.
Assessment of native T1 time (Native T1)
- T1 prolongation occurs with an increase in water content (inflammation, local edema in acute infarction) or expansion of the extracellular space (protein deposition in amyloidosis, collagen in fibrosis).
- T1 shortening is characteristic of intracellular lipid accumulation (e.g., in Anderson-Fabry disease) or iron overload of the heart muscle.
Extracellular volume fraction (Extracellular volume, ECV) This parameter is calculated using T1 mapping before and after the administration of a specific extracellular contrast agent (which shortens T1).
- ECV increases in edema, marked fibrosis, and amyloidosis.
- ECV decreases in thrombosis or fatty (lipomatous) metaplasia of the myocardium.
T2 and T2* mapping
- T2 maps objectively and quantitatively reflect local inflammation and edema. This method lacks the subjectivity inherent in standard qualitative T2-weighted images.
- T2* maps, based on gradient echo, are critically important for the precise quantitative assessment of myocardial iron overload. This is vital for patients suffering from transfusion-dependent hemoglobinopathies (e.g., major β-thalassemia).
CMR in cardiomyopathies
In the hypertrophic phenotype (including familial mutations of sarcomere proteins with characteristic myofibrillar disarray), MRI helps examine the structure of the excessively thickened myocardium in detail. Native T1 maps and ECV maps record high values in hypertrophied segments (especially frequently in the interventricular septum), which directly correlate with the development of interstitial fibrosis. It is important to note that these microscopic changes can be detected even when classical late gadolinium enhancement does not yet demonstrate bright foci.
In the dilated phenotype (which may be caused by cytoskeletal gene mutations, toxic exposure, or the sequelae of past myocarditis), MRI is indispensable for high-precision measurements. While chest radiography only allows for indirect estimation of end-diastolic pressure via the pulmonary vasculature, and echocardiography calculates ejection fraction and sphericity index, MRI provides the most objective volumetric picture, especially when ultrasound visualization is challenging.
Briefly: answers to topical questions
What absolute contraindications exist for cardiac MRI?
Absolute contraindications for cardiac MRI include:
- MRI-incompatible pacemakers;
- implantable cardioverter-defibrillators (ICDs);
- cardiac resynchronization therapy (CRT) devices;
- metallic devices on cerebral vessels;
- presence of metal in the orbits, including ferromagnetic foreign bodies inside or near the eyeball.
At the same time, modern models of certain electronic devices may be MRI-conditional, so their presence is not always an absolute contraindication.
Which quantitative ventricular function parameters are measured during cardiac MRI?
During cardiac MRI, basic volumetric and functional parameters are measured to assess ventricular function. These include:
- End-diastolic volume (end-diastolic volume) — maximum ventricular filling.
- End-systolic volume (end-systolic volume) — maximum ventricular emptying.
- Stroke volume (stroke volume): the difference between end-diastolic and end-systolic volumes.
- Ejection fraction (ejection fraction) — the ratio of stroke volume to end-diastolic volume.
- Cardiac output (cardiac output) — the product of stroke volume and heart rate.
Additionally, left ventricular mass is calculated during the study of the left ventricle.
What specific MRI features are characteristic of hypertrophic cardiomyopathy (HCM)?
Hypertrophic cardiomyopathy is characterized by specific changes in myocardial structure and the presence of interstitial fibrosis. The following MRI features are distinguished:
- Localized hypertrophy — myocardial thickening, frequently in the basal segments of the interventricular septum.
- Myocardial texture — "bright", granular myocardium and the presence of myocardial crypts.
- Late gadolinium enhancement (delayed enhancement / Late gadolinium enhancement) — hyperintense intramural foci of fibrous tissue ("bright" myocardium against a "dark" background).
- Changes on parametric maps — elevation of native T1 map values and extracellular volume (extracellular volume) map values, which is also associated with interstitial fibrosis.
To remember shifts in T1 time: T1 PROLONGS if extra "volume" appears in the tissue (water in edema, protein in amyloidosis, collagen in fibrosis). T1 SHORTENS if "dense/heavy" elements or fat accumulate in cells (iron, lipids in Fabry disease).
Class and exam questions
What is the main advantage of cardiac MRI over conventional echocardiography?
Unlike ultrasound, MRI is independent of the patient's acoustic window and operator skill, provides a larger field of view, and possesses a unique capacity for myocardial tissue characterization (detection of edema, fibrosis, iron).
What do cardiologists use T2* mapping for?
This method allows for the precise quantitative assessment of myocardial iron overload. This is critically important for determining the prognosis and management of patients with transfusion-dependent hemoglobinopathies (e.g., thalassemia).
What does an increase in the extracellular volume (ECV) fraction on MRI signify?
An increase in this parameter indicates expansion of the extracellular space in the myocardium. Most frequently, this is due to the development of fibrosis, pronounced inflammatory edema, or the deposition of pathological proteins (amyloidosis).
What else to study on this topic
- Radiography in the assessment of pulmonary hemodynamics and end-diastolic pressure
- Echocardiographic criteria for dilated cardiomyopathy (sphericity index, ejection and shortening fractions)
- Phenotypic and etiological classifications of cardiomyopathies according to ESC and AHA guidelines
- Genetic features and pathomorphology of classical hypertrophic cardiomyopathy
- The role of ECG-gated multislice computed tomography (MSCT) in cardiology
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