Cardiac Imaging In Myocardial Fibrosis
While myocardial fibrosis is accepted as a significant factor in cardiac disease development, it isn’t typically assessed routinely in clinical practice. This is in part due to the challenge posed by assessing fibrosis accurately and non-invasively, with imaging techniques such as magnetic resonance imaging (MRI), offering a lower resolution than invasive assessment using microscopes. But with the development of cardiac magnetic resonance (CMR) technology, is there now more opportunity than ever to diagnose and accurately assess myocardial fibrosis non-invasively?
In this article, we will discuss myocardial fibrosis and its causes, before providing an overview of imaging in myocardial fibrosis, highlighting the role of CMR in diagnosing and assessing the condition and comparing this technique with other imaging modalities.
What is myocardial fibrosis?
Myocardial fibrosis – also known as cardiac fibrosis - is a feature of cardiac remodeling following heart failure or injury. The condition involves the hardening or scarring of tissue due to collagen overproduction, which impairs the myocytes; the heart’s muscle cells. In this way, myocardial fibrosis hardens the heart and makes it inflexible, causing symptoms such as chest pain, fatigue, nausea, and abdominal swelling, and leading to numerous heart problems. Fibrosis, the class of disease to which myocardial fibrosis belongs, also affects other organs like the liver and lungs.
A pathological process in cardiac disease development, myocardial fibrosis contributes to conditions such as:
- Aortic stenosis
- Myocardial ischemia
- Aortic regurgitation
Myocardial fibrosis causes
Myocardial fibrosis can be caused by several conditions. Coronary artery disease, which involves the blocking of the heart’s blood supply due to a build-up in the coronary arteries, often causes fibrosis in the heart’s midwall, particularly in patients with a poor prognosis.
In the early stages of aortic stenosis, the narrowing of the heart’s aortic valve is often predominated by fibrosis. Hypertension, when the blood pressure is higher than normal, involves limited fibrosis, but the extent increases with concomitant hypertrophy or chronic kidney disease.
Imaging in myocardial fibrosis
There are various imaging techniques for the diagnosis and assessment of myocardial fibrosis. These include:
- Echocardiography – echocardiograms use sound waves to create images of the heart and surrounding blood vessels. They are usually the first step for myocardial function and structure assessment, allowing information about the myocardial scar to be obtained non-invasively, at a low cost.
- CT – computed tomography (CT) scans use a series of x-ray images taken from various angles to produce cross-sectional images of soft tissues, blood vessels, and bones. Although CT can be used for soft tissue characterization, it offers limited value in the diagnosis and assessment of myocardial fibrosis.
- MRI – MRI scans use strong magnetic fields and radio waves to create images of the anatomy. The development of cardiac MRI over the years has led to widely available tools for the identification and quantification of myocardial fibrosis, such as late gadolinium enhancement cardiac magnetic resonance (LGE-CMR).
The role of CMR in myocardial fibrosis
CMR has developed to become the main imaging technique for myocardial fibrosis. The robust technology offers the ability to represent changes in the injured myocardium at a macroscopic level.
Myocardial fibrosis results in the extracellular matrix expanding, and gadolinium, an extracellular agent, accumulates in areas of interstitial expansion. LGE-CMR uses a contrast agent to produce images that show diseased myocardium. The cost-effective tool enables myocardial volume, mass, and function to be quantified non-invasively, identifying myocardial fibrosis.
T1 mapping, another CMR technique, is effective in distinguishing all myocarditis fibrosis types, as well as identifying the causes of cardiomyopathy. T1 maps depict T1 relaxation times specific to various tissue compositions. This can be done after gadolinium administration or using no contrast, also known as native T1. The technique is effective in assessing myocardial ischemia severity and portraying areas at most risk following reperfusion interventions.
Compared to other imaging techniques, such as CT and echocardiography, CMR appears to provide a higher value in diagnosing and assessing myocardial fibrosis. While research has shown that contrast-enhanced cardiac CT holds potential in myocardial fibrosis assessment, compared to CMR, its quantification ability is in question. Echocardiography can be used as a qualitative diagnostic tool, but it is not able to directly identify and quantify myocardial fibrosis type or extent.
CT and myocardial fibrosis
CT had previously demonstrated a limited ability to depict myocardial fibrosis, however, in recent years, the development of contrast-enhanced cardiac CT has shown potential in myocardial fibrosis. A study showed that the technique could enable myocardial extracellular volume (ECV) quantification to determine the extent of myocardial fibrosis. This ability shows that CT is a more effective assessment tool than echocardiography for myocardial fibrosis, but compared to CMR, research suggests CT’s diagnostic accuracy is limited.
Why is MRI better than CT for cardiac fibrosis?
While CT has demonstrated a degree of potential, cardiac MRI is considered superior for soft tissue characterization. Techniques such as LGE-CMR and T1 mapping make MRI the first choice image modality for myocardial fibrosis assessment, offering the detection and quantification of both replacement fibrosis and interstitial fibrosis. In this respect, CMR is unmatched in its ability to offer a comprehensive assessment of myocardial disease, facilitating the most accurate clinical interpretation.
For more information, we suggest reading our article ‘Can Cardiac Fibrosis Imaging Be Inserted Into Regular Practice?’.
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As we have outlined in this article, CMR is currently recognized as the gold standard modality for myocardial fibrosis. CMR offers the highest diagnostic and prognostic value of any imaging modality in the assessment of myocardial fibrosis, with techniques such as LGE-CMR and T1 mapping providing a detailed characterization of the injured myocardium.
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