Evaluation of Cardiac Volumetry Using MRI - An Outline

Introduction

MRI (magnetic resonance imaging) is highly accurate and reliable for measuring cardiac (heart) function, particularly for assessing the volumes of the left and right ventricles. Because of this, MRI is often considered the best method for evaluating heart performance. It also helps assess how well the heart pumps blood and the condition of the heart muscle. New MRI techniques can also help assess heart muscle strength. Although traditional MRI scans take time, new, faster methods allow heart function to be estimated in just a few seconds. These improvements help make MRI more useful for routine heart check-ups.

What Is Cardiac Volumetry?

The process of measuring the volumes of the heart chambers, mainly the ventricles, to assess cardiac efficiency and functions is known as cardiac volumetry. It involves quantifying the amount of blood in the heart during different phases of the cardiac cycle. This quantification is particularly done towards the end of diastole (when the heart is pumped with blood) and the end of systole (after the heart has contracted and pumped out blood).

Key Components of Cardiac Volumetry:

1. End-Diastolic Volume (EDV): It isthe total volume of blood present in a ventricle at the end of diastole, just before the heart's contraction.

2. End-Systolic Volume (ESV): The volume of blood in a ventricle at the end of the systole after the heart has contracted and pumped out blood is known as end-systolic volume.

3. Stroke Volume (SV): Stroke volume is the amount of blood pumped out by a ventricle during a single heartbeat. It is calculated as the difference between EDV and ESV.

4. Ejection Fraction (EF): The percentage of blood ejected from the ventricle with each contraction.

What Are the Imaging Techniques Used to Measure Cardiac Volumetry?

Cardiac volumetry is a critical tool for evaluating cardiac efficiency, monitoring other heart diseases, and determining early diagnosis and treatment. Various imaging techniques can be used to assess cardiac volumetry, including:

• Cardiac Magnetic Resonance Imaging (MRI): MRI isthe gold standard for cardiac volumetry due to its high accuracy and reproducibility.

• Echocardiography: Echocardiography is an ultrasound-based technique that is commonly used for volumetric assessment. However, it is less accurate than MRI.

• Computed Tomography (CT): CT scan is another imaging modality that can be used. However, it is typically used in cases where MRI is contraindicated.

How Cardiac Volumetry Is Assessed With MRI?

1. MRI Technique:

• Type of MRI Used: Typically, cardiac MRI (CMR) captures detailed images of the heart by employing specialized imaging sequences.

• Contrast Agents: Gadolinium-based contrast agents are sometimes used to enhance image quality and highlight specific areas, such as scar tissue or areas of fibrosis.

2. Retrieval of Image:

• Precise Planning: To acquire accurate images, precise planning is required to capture heart images from several angles and phases of the cardiac cycle. The usual standard cardiac imaging planes include the long-axis view and short-axis view.

• Sequence Types:

  • Cine Magnetic Resonance Imaging: It is a novel technique for capturing motion under physiological conditions. It provides dynamic images of the heart throughout the cardiac cycle allowing for assessment of motion and cardiac function.

  • Steady-State Free Precession (SSFP) MRI: It is used for high resolution sequencing and good contrast between blood and cardiac muscle (myocardium).

  • Gradient Echo Sequences MRI: This technique is used for detailed structural imaging of the heart chambers.

3. Image Processing:

• Segmentation: The process involves segmenting the heart chambers and myocardium (cardiac muscle) from the scanned images. This includes carefully delineating the endocardial (inner heart wall) and epicardial (outer heart wall) boundaries in each frame throughout the cardiac cycle phases.

• Volumetric Analysis: Volumes are determined by adding up the areas of the segmented heart regions throughout the cardiac cycle. This typically involves calculating two key volumes: the End-Diastolic Volume (EDV), which is the amount of blood in the ventricles when the heart is fully relaxed, and the End-Systolic Volume (ESV), which is the amount of blood remaining in the ventricles after the heart has contracted.

4. Functional Parameters: They are the key indicators of heart health.

• The Ejection Fraction (EF): This is an important measure of heart function. It is calculated by dividing the difference between the End-Diastolic Volume (EDV) and End-Systolic Volume (ESV) by the EDV. This helps determine the percentage of blood the heart pumps out with each beat.

• Myocardial Mass: This is determined by measuring the thickness of the heart muscle. It helps in diagnosing conditions like hypertrophy of the cardiac muscles.

5. Modernized Heart Assessment Techniques:

• Strain and Strain Rate Imaging: It helps measure how the heart muscle stretches and contracts giving more detailed information about how well the heart functions.

• Late Gadolinium Enhancement (LGE): It detects scar tissue or fibrosis in the heart. This helps assess the impact of how the heart works and its overall health implications.

6. Clinical Implications:

• Cardiac Failure: This helps monitor changes in the size and function of the heart’s chambers, which is crucial for diagnosing and treating cardiac (heart) failure.

• Cardiomyopathies: This helps check for alterations in heart size and diagnose different types of heart muscle diseases, including dilated or hypertrophic cardiomyopathy.

• Before and After Treatment: These assessments by MRI are also crucial for seeing how well surgeries or treatments have worked to improve heart function.

How Does MRI Help in the Evaluation of Cardiac Volumetry?

• Detailed Imaging: MRI provides high-contrast, high-resolution images of the heart and its surrounding structures.

• Measurement of the Cardiac Volumes: MRI allows for precise measurement of the volumes of the left and right heart chambers (ventricles in particular). This is crucial to identify any cardiac abnormalities.

• Evaluation of Cardiac Functions and Performance: MRI can provide insights into the heart's systolic and diastolic function to evaluate its performance.

• Evaluation of Cardiomyopathies: MRI helps identify pathological conditions like hypertrophy (enlargement of the component cells) and cardiomyopathy.

• 3-Dimensional Imaging Capabilities: MRI can develop three-dimensional images of the heart. This 3D-MRI imaging helps in comprehending complex anatomical relationships.

• Minimal or No Ionizing Radiation: Unlike CT scans, MRI does not use ionizing radiation, making it a safer option for follow-up assessments.

Conclusion

Cardiac volumetry using MRI (magnetic resonance imaging) is noninvasive and helps assess detailed cardiac structure and function. It is a gold-standard technique for assessing ventricular volumes, cardiomyopathies, abnormal myocardial mass, and ejection fraction. It is highly accurate, proficient, and reproducible. Ongoing research continues to push for new advancements in this field, aiming to enhance patient care.