CT and MRI of Heart

Cardiac computed tomography (CT) and cardiac magnetic resonance imaging (MRI) are advanced cross-sectional imaging techniques used to visualize the heart, coronary arteries, great vessels, pericardium, and surrounding thoracic structures. Both techniques can provide detailed anatomical information, but they rely on different physical principles and have different strengths.

Cardiac CT is particularly useful when high spatial resolution and rapid acquisition are required. It is widely used for imaging the coronary arteries, detecting coronary calcification, evaluating the aorta, and defining structural relationships before selected cardiac procedures.

Cardiac MRI provides excellent soft-tissue contrast and can evaluate cardiac anatomy, ventricular function, blood flow, myocardial tissue characteristics, congenital abnormalities, and many forms of myocardial disease without exposing the patient to ionizing radiation.

Cross-Sectional Anatomy of the Heart

CT and MRI display the heart as a three-dimensional structure that can be reconstructed or viewed in multiple anatomical planes.

Unlike conventional projection radiography, cross-sectional imaging allows individual chambers, walls, vessels, and adjacent structures to be separated spatially.

Structures Evaluated With Cardiac CT and MRI

• Right atrium
• Right ventricle
• Left atrium
• Left ventricle
• Interatrial and interventricular septa
• Cardiac valves
• Coronary arteries
• Coronary veins
• Ascending aorta and aortic root
• Pulmonary trunk and pulmonary arteries
• Pulmonary veins
• Pericardium
• Myocardium

Cardiac Imaging Planes

Although images may initially be acquired in standard axial planes, cardiac datasets are commonly reformatted into planes aligned with the heart itself.

This is important because the long axis of the heart does not correspond exactly to the standard anatomical axes of the thorax.

Short-Axis View

The short-axis view cuts across the ventricles perpendicular to the long axis of the left ventricle.

It is particularly useful for evaluating ventricular wall thickness, regional wall motion, chamber volumes, and myocardial anatomy from the base toward the apex.

Two-Chamber View

The two-chamber view primarily displays the left atrium and left ventricle.

It allows evaluation of the anterior and inferior left ventricular walls and the mitral valve relationship.

Four-Chamber View

The four-chamber view demonstrates both atria and both ventricles in a single plane.

It is useful for comparing chamber dimensions, evaluating the atrial and ventricular septa, and examining the atrioventricular valves.

Three-Chamber View

The three-chamber view, also called the left ventricular outflow tract view, demonstrates the left atrium, left ventricle, aortic root, mitral valve, and aortic valve.

This plane is useful for examining the relationship between left ventricular inflow and outflow structures.

Cardiac CT

Cardiac CT uses X-rays and computer reconstruction to produce high-resolution cross-sectional images of the heart and vessels.

Modern multidetector CT scanners acquire thin image sections rapidly, allowing detailed three-dimensional reconstruction of cardiovascular anatomy.

CT Attenuation

CT distinguishes tissues according to their attenuation of X-rays.

Air, fat, soft tissue, calcium, and iodinated contrast have different attenuation characteristics and therefore appear with different image densities.

ECG Synchronization

Cardiac motion can blur CT images if data are acquired without accounting for the cardiac cycle.

ECG synchronization coordinates image acquisition or reconstruction with specific phases of the cardiac cycle to reduce motion artifact.

Prospective ECG Triggering

With prospective ECG triggering, CT data are acquired during selected portions of the cardiac cycle based on the ECG signal.

This approach can reduce radiation exposure when appropriate imaging conditions are present.

Retrospective ECG Gating

Retrospective ECG gating records CT data over multiple phases of the cardiac cycle while simultaneously recording the ECG.

Images can then be reconstructed at different cardiac phases, although radiation exposure may be greater than with appropriately performed prospective techniques.

Cardiac CT With Contrast

Intravenous iodinated contrast material is used to opacify the cardiac chambers and blood vessels.

Precise timing of image acquisition allows the desired vascular structures to be maximally enhanced.

Coronary CT Angiography

Coronary CT angiography (CCTA) is a noninvasive method of visualizing the coronary arterial lumen and wall.

It can demonstrate the origins, course, branching patterns, stenoses, and atherosclerotic plaques of the coronary arteries.

Coronary Artery Anatomy on CT

CCTA can display the left main coronary artery, anterior interventricular artery, circumflex artery, right coronary artery, and their major branches.

Three-dimensional and curved multiplanar reconstructions allow tortuous coronary vessels to be followed along their course.

Coronary Artery Calcium

Calcium within coronary atherosclerotic plaques can be detected using noncontrast CT.

Coronary artery calcium scoring quantifies calcified coronary plaque burden and can contribute to cardiovascular risk assessment in selected individuals.

Calcified and Noncalcified Plaque

CCTA can demonstrate both calcified and noncalcified components of coronary atherosclerotic plaques.

This differs from noncontrast calcium scoring, which specifically measures calcified plaque.

Coronary Stenosis

A coronary stenosis appears as narrowing of the contrast-filled arterial lumen.

CT can evaluate the location and extent of narrowing and the morphology of associated plaque.

Cardiac Chambers on CT

Contrast-enhanced CT can clearly delineate the cardiac chambers.

Chamber enlargement, abnormal communication between chambers, intracardiac masses, thrombi, and structural relationships can be assessed in appropriate protocols.

Left Atrium

The left atrium lies posteriorly within the heart and receives the pulmonary veins.

CT is particularly useful for mapping left atrial and pulmonary venous anatomy before procedures such as selected catheter ablation techniques.

Left Atrial Appendage

The left atrial appendage projects from the left atrium and is an important site of thrombus formation, particularly in atrial fibrillation.

CT can define its anatomy and is used in planning selected left atrial appendage procedures.

Left Ventricle

The left ventricle forms much of the left cardiac border and apex and has a substantially thicker myocardial wall than the right ventricle.

CT can demonstrate ventricular dimensions, wall thickness, chamber morphology, and, with suitable gated protocols, aspects of ventricular function.

Right Ventricle

The right ventricle lies predominantly anterior to the left ventricle and has a more complex crescent-shaped geometry.

CT can evaluate right ventricular size, morphology, outflow tract anatomy, and relationships to the pulmonary trunk.

Cardiac Valves on CT

CT can demonstrate the anatomical location and morphology of cardiac valves, particularly when calcification or structural planning is important.

High spatial resolution is especially useful for evaluating the aortic valve and surrounding structures.

Aortic Valve

The aortic valve lies between the left ventricle and ascending aorta.

CT can demonstrate cusp morphology, calcification, the aortic annulus, sinuses of Valsalva, coronary ostia, and adjacent aortic root anatomy.

Transcatheter Aortic Valve Planning

CT is extensively used before transcatheter aortic valve replacement (TAVR).

Measurements can include aortic annular dimensions, coronary ostial height, aortic root geometry, calcification, and the caliber and course of potential vascular access routes.

Thoracic Aorta

CT provides detailed imaging of the ascending aorta, aortic arch, descending thoracic aorta, and major branch vessels.

It is particularly valuable for evaluating aneurysms, dissection, traumatic injury, and postoperative or endovascular repairs.

Pericardium on CT

The pericardium can often be identified as a thin structure surrounding the heart.

CT can demonstrate pericardial thickening, calcification, fluid collections, and relationships between the pericardium and adjacent thoracic structures.

Advantages of Cardiac CT

• High spatial resolution
• Rapid image acquisition
• Excellent visualization of coronary arteries
• Sensitive detection of calcification
• Detailed three-dimensional reconstruction
• Simultaneous evaluation of cardiac and extracardiac anatomy

Limitations of Cardiac CT

Cardiac CT uses ionizing radiation, although dose-reduction techniques can substantially limit exposure.

Contrast-enhanced studies commonly require iodinated contrast, and image quality can be affected by rapid or irregular heart rhythms, motion, heavy coronary calcification, and other technical factors.

Cardiac MRI

Cardiac magnetic resonance imaging (MRI), also called cardiovascular magnetic resonance, uses magnetic fields and radiofrequency signals to produce detailed images of cardiovascular structures.

It does not use ionizing radiation and provides excellent contrast between blood, myocardium, fat, scar, edema, and other tissues when appropriate sequences are used.

Multiplanar Imaging

MRI can acquire images directly in planes aligned with cardiac anatomy.

Short-axis, two-chamber, three-chamber, four-chamber, and specialized outflow tract views can be obtained according to the clinical question.

Cine MRI

Cine MRI produces a sequence of images across the cardiac cycle, creating a moving representation of cardiac contraction.

It allows direct assessment of chamber motion, valve movement, ventricular contraction, and regional wall motion.

Ventricular Volumes

Cardiac MRI can accurately measure end-diastolic volume and end-systolic volume.

These measurements allow calculation of stroke volume and ejection fraction.

Ejection Fraction

Ejection fraction can be calculated using:

Ejection Fraction = (End-Diastolic Volume − End-Systolic Volume) / End-Diastolic Volume × 100

MRI is widely used as a reference technique for quantitative assessment of ventricular volumes and systolic function.

Left Ventricular Mass

Cardiac MRI can measure myocardial volume and calculate left ventricular mass.

This is useful for evaluating ventricular hypertrophy and remodeling.

Right Ventricular Assessment

The complex shape of the right ventricle makes geometric assumptions difficult with some imaging methods.

MRI can directly measure right ventricular volumes and function and is particularly useful in congenital heart disease and disorders affecting the right ventricle.

Myocardial Tissue Characterization

One of the major strengths of cardiac MRI is its ability to characterize myocardial tissue.

Different MRI sequences can detect edema, fibrosis, scar, fat, iron deposition, and other alterations in myocardial composition.

Late Gadolinium Enhancement

Late gadolinium enhancement (LGE) is an MRI technique used to identify areas with increased extracellular space, commonly associated with myocardial scar or fibrosis.

The distribution of enhancement can provide important information about the underlying pattern of myocardial injury.

Myocardial Infarction on MRI

After myocardial infarction, necrotic and subsequently scarred myocardium can demonstrate characteristic late gadolinium enhancement.

Ischemic injury generally follows a coronary vascular territory and commonly involves the subendocardium, with more extensive infarctions extending toward the epicardial surface.

Myocardial Viability

MRI can help distinguish viable myocardium from regions extensively replaced by scar.

The transmural extent of scar within a myocardial segment can contribute to assessment of the likelihood of functional recovery after restoration of blood flow.

Myocardial Edema

Specialized MRI sequences can detect increased myocardial water content associated with acute tissue injury and inflammation.

This can help characterize acute myocardial abnormalities in appropriate clinical settings.

Myocarditis

Cardiac MRI is commonly used in the evaluation of suspected myocarditis.

Tissue characterization techniques can demonstrate patterns of edema and myocardial injury that differ from the typical coronary distribution of ischemic infarction.

Cardiomyopathy

MRI can evaluate ventricular morphology, wall thickness, chamber size, systolic function, and myocardial fibrosis in cardiomyopathies.

The distribution of structural abnormalities and tissue changes can help characterize different forms of myocardial disease.

Hypertrophic Cardiomyopathy

In hypertrophic cardiomyopathy, MRI can accurately define the distribution and degree of myocardial hypertrophy.

It can also assess left ventricular outflow tract anatomy, papillary muscles, mitral valve relationships, and myocardial fibrosis.

Dilated Cardiomyopathy

In dilated cardiomyopathy, MRI can demonstrate ventricular enlargement, reduced systolic function, altered chamber geometry, and patterns of myocardial fibrosis.

Arrhythmogenic Cardiomyopathy

MRI can evaluate right and left ventricular morphology and function in patients with suspected arrhythmogenic cardiomyopathy.

Regional wall motion abnormalities and structural remodeling can be assessed as part of a broader diagnostic evaluation.

Cardiac Masses

Cardiac MRI can characterize intracardiac and pericardial masses using their location, morphology, signal characteristics, perfusion, and enhancement patterns.

This can help distinguish thrombus from some tumors and define relationships with adjacent cardiac structures.

Cardiac Thrombus

Intracardiac thrombi may form in regions of blood stasis, particularly within poorly contracting ventricular segments or the atria.

MRI tissue characterization and contrast-enhanced techniques can help distinguish thrombus from vascularized tissue in selected cases.

Valvular Disease on MRI

MRI can demonstrate valve motion and the effects of valve disease on chamber size and ventricular function.

Flow-sensitive sequences can also quantify blood moving through selected planes.

Phase-Contrast MRI

Phase-contrast MRI measures the velocity and direction of moving blood.

It can be used to calculate flow volumes across vessels or valves and to evaluate shunts and regurgitant lesions.

Congenital Heart Disease

Both CT and MRI are important in congenital cardiovascular imaging.

They can demonstrate abnormal chamber connections, septal defects, great-vessel relationships, anomalous pulmonary or systemic veins, vascular rings, and postoperative anatomy.

MRI in Congenital Heart Disease

MRI is particularly useful when repeated assessment of ventricular function and blood flow is needed because it avoids ionizing radiation.

It can quantify shunt flow and evaluate complex three-dimensional cardiovascular anatomy.

CT in Congenital Heart Disease

CT is valuable when very high spatial resolution or rapid acquisition is required.

It can clearly depict small vessels, coronary anatomy, airway relationships, calcification, and postoperative conduits or devices.

Pericardial Disease on MRI

MRI can evaluate pericardial thickness, inflammation, effusion, and the effects of pericardial disease on ventricular filling.

Real-time and cine imaging can demonstrate abnormal interactions between the ventricles in selected forms of constrictive physiology.

Pericardial Effusion

Both CT and MRI can identify fluid within the pericardial cavity.

Imaging can demonstrate the amount and distribution of fluid and associated effects on cardiac structures, although echocardiography is often the primary technique for immediate hemodynamic assessment.

Aortic Imaging With MRI

MRI can evaluate the thoracic aorta without ionizing radiation and can provide both anatomical and flow information.

It is useful for selected patients requiring repeated surveillance of aortic disease.

Pulmonary Arteries and Veins

CT and MRI can both define pulmonary vascular anatomy.

CT is widely used for rapid pulmonary arterial imaging, while MRI can provide additional functional flow information in selected cardiovascular disorders.

Cardiac CT Versus Cardiac MRI

Feature	Cardiac CT	Cardiac MRI
Ionizing radiation	Yes	No
Spatial resolution	Very high	High
Coronary artery imaging	Major strength	More limited for routine distal coronary assessment
Calcification	Excellent	Limited
Ventricular function	Can be assessed with gated imaging	Major strength
Myocardial tissue characterization	Possible in selected applications	Major strength
Blood flow quantification	Limited routine role	Available with flow-sensitive techniques
Acquisition speed	Very rapid	Generally longer

Contrast Agents

Cardiac CT commonly uses iodinated contrast material for vascular and chamber enhancement.

Cardiac MRI may use gadolinium-based contrast agents for perfusion and tissue characterization, although many useful MRI sequences do not require contrast.

CT Contrast Considerations

Iodinated contrast administration requires consideration of previous contrast reactions, renal function, and other patient-specific factors.

The amount and timing of contrast depend on the cardiac structure or vascular territory being evaluated.

MRI Contrast Considerations

Gadolinium-based contrast agents have different safety considerations from iodinated CT contrast.

Renal function and the specific contrast agent are considered when contrast-enhanced MRI is planned.

MRI Safety

The strong magnetic field used in MRI requires careful screening for implanted devices, metallic foreign bodies, and other potential hazards.

Many modern cardiovascular devices are MR conditional, meaning they can be scanned only under specified conditions.

Motion Artifacts

Cardiac imaging is technically challenging because the heart moves continuously and respiratory motion changes its position within the thorax.

ECG synchronization, rapid acquisition, breath holding, respiratory navigation, and image reconstruction techniques are used to reduce motion artifacts.

Clinical Applications of Cardiac CT

Application	What CT Provides
Coronary artery disease	Coronary lumen, plaque and stenosis assessment
Coronary calcium	Quantification of calcified plaque burden
Aortic disease	Detailed aortic and branch vessel anatomy
Valve procedure planning	Annular, root, coronary and vascular access measurements
Pulmonary veins	Detailed venous anatomy before selected procedures
Congenital disease	High-resolution structural and vascular anatomy

Clinical Applications of Cardiac MRI

Application	What MRI Provides
Ventricular function	Volumes, ejection fraction and regional wall motion
Cardiomyopathy	Morphology and myocardial tissue characterization
Myocardial infarction	Scar distribution and extent
Myocarditis	Assessment of myocardial edema and injury patterns
Congenital heart disease	Complex anatomy, ventricular function and flow quantification
Valvular and shunt lesions	Flow measurement and chamber consequences
Cardiac masses	Anatomical and tissue characterization

Relationship to Echocardiography

Echocardiography is often the first-line imaging method for cardiac structure and function because it is widely available, portable, and provides real-time imaging without radiation.

CT and MRI provide complementary information when more detailed anatomical mapping, coronary imaging, tissue characterization, flow quantification, or evaluation of structures difficult to visualize with ultrasound is required.

Relationship to Invasive Angiography

CCTA can noninvasively demonstrate coronary anatomy and stenosis, but invasive coronary angiography remains important when direct luminal assessment and immediate catheter-based treatment are required.

CT, MRI, echocardiography, and invasive angiography therefore provide different but complementary views of cardiovascular anatomy and function.

Key Features of Cardiac CT

Feature	Key Point
Imaging principle	X-ray attenuation and computer reconstruction
Major anatomical strength	High-resolution coronary and vascular imaging
Calcification	Very well demonstrated
Contrast	Usually iodinated for angiographic studies
Cardiac motion control	ECG synchronization
Radiation	Uses ionizing radiation

Key Features of Cardiac MRI

Feature	Key Point
Imaging principle	Magnetic fields and radiofrequency signals
Major anatomical strength	Cardiac morphology and soft-tissue contrast
Major functional strength	Ventricular volumes, function and blood flow
Major tissue strength	Characterization of myocardial injury, fibrosis and edema
Contrast	Gadolinium-based agents for selected applications
Radiation	No ionizing radiation

Anatomical and Clinical Importance

Cardiac CT and MRI allow the heart to be examined as a three-dimensional organ rather than only as a silhouette or projection. Chambers, myocardium, valves, coronary arteries, great vessels, and the pericardium can be viewed from planes selected specifically for cardiac anatomy.

The two techniques provide complementary information. CT excels at rapid, high-resolution visualization of coronary arteries, calcification, vascular structures, and detailed spatial relationships. MRI excels at assessment of ventricular function, blood flow, myocardial composition, scar, fibrosis, and many forms of structural myocardial disease.

Together, CT and MRI connect cardiovascular anatomy with physiology and pathology. They can show not only where an abnormality is located, but also how it affects chamber geometry, myocardial contraction, blood flow, tissue integrity, and relationships among the heart and great vessels.