Imaging of the Heart and Great Vessels

Chest X-ray

Overview

• Chest X-ray: most common imaging test to evaluate the heart and great vessels; usually the initial test before using more expensive diagnostic tools:
  • Shadows that form the cardiac silhouette allow for assessment of the size and shape of the heart and major vessels. 
  • Lung parenchymal findings also give information regarding pulmonary pathologies secondary to cardiac abnormalities (e.g., pulmonary edema). 
  • Also used in the evaluation of the mediastinum 
• Positioning for specific views:
  • Posteroanterior (PA):
    • Board is against the anterior chest.
    • X-ray beams posterior → anterior direction through patient
  • Anteroposterior (AP):
    • Board is against the back. 
    • X-ray beams anterior → posterior direction through the patient
  • Lateral (from side):
    • Patient is upright.
    • Board is against the left or right side.
  • Lateral decubitus (from side while laying down): 
    • Patient is supine.
    • Board is against the side.
    • Usually left side down on table 
• Penetration: degree to which radiation has passed through body, resulting in darker or lighter image
  • Good penetration:
    • Ribs are only faintly visible behind the heart. 
    • Vascular markings are nonprominent but clear. 
  • Underpenetration: film appears whiter and features are less apparent.
  • Overpenetration: film appears darker and features are less apparent. 

Anatomy

The following structures must be identified and checked for abnormalities in size or shape:

• PA projection:
  • Structures:
    • Trachea: should be on the midline
    • Superior vena cava (SVC)
    • Ascending and descending aorta (aortic arch)
    • Pulmonary hilum (right and left)
    • Pulmonary artery
    • Right atrium (right heart border)
    • Left ventricle (left heart border)
  • To easily identify the structures on the left border in a cephalocaudal order, remember: 
    • 1st bump: aorta
    • 2nd bump: pulmonary artery
    • 3rd (the largest) bump: left ventricle
• Lateral projection:
  • Retrosternal space
  • Right ventricle
  • Right hemidiaphragm
  • Aortic arch
  • Pulmonary hilum
  • Left ventricle
  • Left hemidiaphragm
  • Posterior cardiac space

Heart

• On a plain chest film, heart size is quickly assessed calculating the cardiothoracic index (CI): the percentage of the chest diameter occupied by the heart. 
  • A normal CI is < 50%. 
  • However, this is reliable only on a PA projection taken upright.
• An enlarged heart can be due to:
  • Cardiomegaly
  • Pericardial effusion (water-bottle heart): pericardial effusions must be ≥ 250 mL to be detectable on a chest X-ray.
  • Valvular disease
  • Congenital heart disease
  • Masses

Great vessels

• In the PA and AP projections, the great vessels can be identified at either side of the mediastinal shadow. 
  • Right mediastinal margin (cephalocaudal):
    • SVC
    • Right pulmonary artery coinciding with the right pulmonary hilum
  • Left mediastinal margin (cephalocaudal):
    • Aorta
    • Left pulmonary artery coinciding with the left pulmonary hilum
• In a lateral projection, only the shadow of the aorta can be distinguished.

Echocardiography

Overview

• Ultrasound principles are used in echocardiography to produce anatomical images of the heart and great vessels. 
• Common indications:
  • Evaluation of ventricular function
  • Cardiomyopathy
  • Congenital heart disease
  • Valvular heart disease
  • Pericardial effusion
  • Cardiac masses
  • Proximal aortic disease
• Echocardiography is likewise an essential tool in assessing the dynamic function of the heart in stress tests (exercise or dobutamine stress echocardiography).

Evaluation of the heart and great vessels

• Transthoracic echocardiography (TTE):
  • More common; noninvasive
  • Uses a handheld transducer positioned on the chest to scan the heart
  • Provides the clinician with the ability to quickly assess the heart, especially in life-threatening situations
  • Allows for the rapid distinction between cardiac (e.g., pericardial effusion) and extracardiac (e.g., pulmonary embolus) causes of illness
• Transesophageal echocardiography (TEE):
  • Improved image quality (especially posterior cardiac structures) because of the proximity of the esophagus to the heart
  • Because of TEE’s invasive nature, TTE often precedes TEE.
  • Commonly used in:
    • Evaluation for a cardiac source of embolus
    • Valvular assessment in cases of endocarditis 
    • Determination of left atrial appendage (LAA) thrombi in patients with atrial fibrillation 
• Modalities:
  • 2-Dimensional: by undergoing complex processes (e.g., filtering, signal amplification), the signal received forms a cardiac image displayed on the monitor. 
  • M-mode: 
    • M = motion
    • Movement of the structures is graphically imaged over time by directing a linear beam of ultrasound at a single structure. 
  • Doppler:
    • Provides information regarding blood flow (hemodynamics)
    • Especially helpful in valvular and cardiac function assessment, as well as in evaluating intracardiac shunts
• Interpretation includes:
  • Reason for the test
  • Quality of image
  • Rate and rhythm
  • Chamber sizes
  • Hypertrophy
  • Right ventricular function
  • Left ventricular systolic function:
    • Ejection fraction = (stroke volume [end diastolic volume – end systolic volume]/end diastolic volume) × 100
    • Wall motion
  • Left ventricular diastolic function
  • Valves
  • Mass or thrombus
  • Atrial or ventricular septal defect
  • Pericardium
  • Other findings can include aneurysms, pleural effusion, and incidental findings.

Cardiac scan

Overview

• Cardiac scan: 
  • Also called heart scan or myocardial perfusion imaging
  • Detects variation in blood flow and myocardial extraction of radiotracers 
  • Materials: 
    • Isotopes: technetium-99m, thallium-201 
    • Radiopharmaceutical (radioisotope + organic molecule): sestamibi, teboroxime
• Procedure:
  • In normal coronary arteries, significant arterial dilatation is seen in response to exercise/stress.
  • Stenotic areas do not show dilatation; thus, ischemia and ECG changes occur. 
  • In this test, stress is either:
    • Exercise-induced by running on a treadmill
    • Pharmacologically induced by injecting adenosine or dobutamine for those who cannot run 
• Imaging is obtained both during stress and at rest.
• Radiopharmaceuticals are injected when 85% of the maximum predicted heart rate (MPHR) is reached.
• Indications:
  • Myocardial ischemia or MI evaluation
  • Wall-motion abnormalities: done by ECG-gated SPECT scan
  • Calculate left ventricular ejection fraction
• Normal cardiac scan obtained in 3 different planes:
  • The top row of each set is performed under cardiac stress
  • Bottom row of each set is performed at rest
  • Normal flow to all aspects of the heart both at rest and during stress

CT

Overview

• Thoracic CT is very useful for studying the anatomy of the heart, great vessels, and mediastinum. 
• The use of IV contrast allows for adequate visualization of the heart chambers and the venous and arterial anatomy. 
• Even though a CT scan is usually displayed as slices, the image data can be manipulated to obtain a 3-dimensional reconstruction of the anatomy.
• Cardiac CT, though, is associated with high dose of ionizing radiation, and use of contrast material carries a risk for renal failure.

Heart

• Coronary CTA (CCTA):
  • For evaluation of the coronary arteries in cases of indeterminate stress tests
  • Characterizes coronary or cardiac anomalies, including location and patency of bypass grafts
  • Triple rule-out in emergency situations of atypical chest pain:
    • Atherosclerotic disease
    • Pulmonary embolism
    • Aortic dissection
• Coronary artery calcium (CAC) scoring:
  • Noninvasive test that detects and quantifies CAC.
  • Generally recommended in asymptomatic adults ≥ 40 years of age with intermediate atherosclerotic cardiovascular disease risk
  • Requires a relatively low radiation dose (comparable to that for a screening mammography)
  • The amount of calcified plaque present in the epicardial coronary arteries, reported as the total calcium score (Agatston score):
    • No identifiable disease: 0 Agatston units
    • Mild disease: 1–99 Agatston units
    • Moderate disease: 100–399 Agatston units
    • Severe disease: ≥ 400 Agatston units 
  • Scoring provides guidance on further management (e.g., recommending aspirin and lipid-lowering agents).

Aorta

Aortic CTA:

• Contrast-enhanced CT can provide information on the thickness and composition of the vessel wall. 
• Most common initial imaging for aortic dissection evaluation
• Allows for:
  • Documentation of size and shape of aneurysms
  • Visualization of the true and false lumen in aortic dissection
  • Characterization of calcification and stenoses 
  • Visualization of the anatomy of congenital anomalies
  • Evaluation of vasculitis
• Also useful to detect postoperative complications: 
  • Restenosis
  • Thrombosis
  • Endoleak after aneurysm repair

Pulmonary arteries

CT pulmonary angiography (CTPA):

• Also called chest CT angiography with contrast
• Most frequently used imaging to evaluate the pulmonary arteries
• High sensitivity and specificity for pulmonary embolism

Chest MRI

Overview

• A thoracic MRI is for studying the heart and great vessels; it has greater definition of anatomy than CT. 
• The function of MRI is based on the utilization of magnetic fields and radiofrequencies to generate 3-dimensional, high-fidelity images. 
• Use is restricted to large medical centers because of its great cost and required operational expertise. 
• MRI is often reserved for evaluation of complex thoracic pathology initially assessed with more accessible imaging methods (e.g., CT).
• Contraindications: patients with metallic or electrical implants, devices, or foreign bodies that are not MRI-compatible. 

Heart

• Cardiac MRI (CMR): 
  • Single-plane 2-dimensional (2D) or multiple-plane 2D or 3-dimensional (3D) images can be obtained.
  • Can provide images of the heart going through a cardiac cycle 
  • Usual indications:
    • Congenital heart disease
    • Valvular dysfunction
    • Suspected intracardiac masses
    • Myocardial disease, including cardiomyopathy, ischemic heart disease, myocarditis
    • Pericardial disease (e.g., noncalcified constrictive pericarditis, pericardial tumor invasion)
    • Aortic abnormality (e.g., aneurysm, aortic dissection)
• MRA:
  • Contrast-enhanced angiography
  • Can be used to identify anomalous coronary arteries as well as other blood vessels (aorta) and to monitor coronary artery aneurysms

Cardiac Catheterization and Angiography

Overview

• Among the most commonly performed cardiovascular imaging tests
• A catheter is introduced into a peripheral vessel such as the femoral or axillary vein or artery (catheterization) and is positioned in the region of interest:
  • Performed under fluoroscopic guidance
  • Contrast material is injected to confirm the location of the catheter and to highlight the cardiac structures for diagnostic evaluation (angiography).
• Images are digitally recorded and stored and can be videotaped.
• While complications are rare, the following can occur:
  • Bleeding/hematoma
  • Infection
  • MI
  • Stroke
  • Arrhythmia
  • Allergic reaction (to contrast agent)
  • Renal failure (due to contrast agent)

Evaluation of the heart and great vessels

• Cardiac catheterization:
  • Diagnostic:
    • Defines the cardiac and coronary artery anatomy, cardiac function, and pulmonary arterial hemodynamics
    • Also used for endomyocardial biopsy and intravascular ultrasonography
  • Therapeutic:
    • For coronary artery disease therapy (percutaneous coronary intervention (PCI))
    • For valvuloplasty and valve replacement
• Left heart catheterization: 
  • Coronary angiography to assess coronary arteries (gold standard)
  • Assessment includes the ventricular outflow tracts, left ventricular motion, ejection fraction, mitral valve, and aortic valve function. 
  • Aortic angiography evaluates the aortic structure and function (e.g., aortic dissection).
• Right heart catheterization:
  • Checks right atrial and ventricular pressure
  • Assesses pulmonary artery pressure and pulmonary artery occlusion pressure
  • Pulmonary angiography can evaluate for pulmonary embolism (though this has been largely replaced by CTPA).

Clinical Correlation

Congestive heart failure

• Congestive heart failure (CHF) refers to the inability of the heart to supply the body with normal cardiac output to meet metabolic needs.
• Multiple causes, including ischemia/coronary artery disease, hypertension, valvular heart disease, and cardiomyopathy.
• Types:
  • Heart failure with reduced ejection fraction (HFrEF) or systolic failure: left ventricle unable to contract normally
  • Heart failure with preserved ejection fraction (HFpEF) or diastolic failure (or diastolic dysfunction): left ventricle unable to normally relax to fill the ventricle with blood
• Tests:
  • X-rays:
    • Initial test used in evaluating patients with symptoms of CHF (e.g., shortness of breath, orthopnea)
    • Gives information regarding any chamber enlargement or associated pulmonary congestion
  • Echocardiography:
    • Can confirm the diagnosis
    • Results include EF, associated structural and hemodynamic abnormalities which depend on the type and etiology of the heart failure (HF). 

Cardiomyopathy (CM)

• Disorder in which the heart muscle is structurally and functionally impaired and is not due to coronary artery, valvular, or congenital heart disease or hypertension.
• Types:
  • Dilated CM: dilated ventricle(s) associated with impaired contraction
  • Hypertrophic CM: hypertrophy of the left (and, occasionally, the right) ventricle (seen as increased wall thickness)
  • Restrictive CM: ventricles not dilated or hypertrophied but have impaired ventricular filling
  • Arrhythmogenic right ventricular CM: 
    • Genetic heart muscle disease 
    • Ventricular arrhythmias with fibrofatty tissue in the myocardium of the right ventricle
  • Unclassified
• Tests (findings depend on the CM type):
  • Echocardiography: 1 of the initial tests for suspected cardiomyopathy, giving information on cardiac structure and function
  • Cardiac MRI provides assessment of:
    • Ventricular volumes and systolic function (ejection fraction)
    • Myocardial mass and wall thickness
    • Wall motion (in cine sequences)

Pulmonary embolism

• A potentially fatal condition resulting from the mechanical obstruction of the pulmonary artery or its branches
• Can be due to any material (such as thrombus, air, or fat) from anywhere in the body. 
• CTPA:
  • 1st-choice diagnostic imaging method, especially in emergent settings
  • Highly sensitive in detecting pulmonary emboli (seen as filling defects)
  • Alternative diagnoses can be ruled out.

Aortic dissection

• A condition in which the inner coat (tunica intima) of the aortic wall develops a fissure allowing blood to enter the media layer to form a secondary lumen.
• CTA:
  • Most common initial test
  • Findings include:
    • Intimal flap with a false lumen (which is often larger than the true lumen)
    • Possible thrombus
    • Enlarged aorta 
  • An alternative is MRA.
• TEE: 
  • For those with suspected ascending aortic involvement or for hemodynamically unstable patients
  • Requires skilled operators and patient sedation
• Aortography: when initial tests are inconclusive.

Coronary artery disease

• The most common type of heart disease, occurring when the coronary arteries narrow, usually due to the buildup of plaque (atherosclerosis)
• As narrowing worsens, less blood can flow through the arteries, resulting in ischemia and cell death → MI
• Multiple methods used:
  • Stress echocardiography: can be used with exercise or pharmacologic stress test
  • Cardiac scan (stress): 
    • Myocardial ischemia: areas of photopenia (i.e., decreased uptake) under stress that improves at rest
    • MI: persistent photopenia (i.e., decreased uptake) despite rest state
  • CT:
    • CAC scoring: screening for patients with moderate or intermediate risk
    • CTA: screening for those with indeterminate stress tests and/or ECGs
  • Cardiac catheterization and coronary angiography:
    • Both for diagnostic and therapeutic management of acute coronary syndrome (ST-segment elevation MI)
    • For evaluation of patients with chronic debilitating angina despite maximal medical therapy
    • When noninvasive testing suggests severe ischemic disease

Endocarditis

• Inflammatory disease of the interior cardiac lining that can be due to either infectious or noninfectious causes; primarily and most notably affects the valves
• Echocardiography:
  • All patients with suspected infective endocarditis should undergo echocardiography.
  • Findings can include:
    • Vegetation
    • Abscess
    • New dehiscence of a prosthetic valve
  • TTE:
    • 1st diagnostic test
    • If images are inadequate, or complications are suspected (abscess, valvular abnormality needing cardiac surgery), TEE is performed.

Pericardial conditions

• Pericardial effusion:
  • Accumulation of fluid in the pericardial cavity of the heart
  • Since the pericardium does not expand, after enough fluid accumulates under pressure, the cardiac filling becomes restricted → cardiac tamponade
• Pericarditis:
  • Inflammation of the pericardium
  • Can be idiopathic or caused by a variety of conditions, including infection, previous MI, or systemic diseases
• Tests:
  • Chest X-ray: initial test done showing enlargement of the heart (in effusion)
  • Echocardiography: 
    • Often normal in pericarditis
    • In pericardial effusion, pericardial fluid is seen as an echolucent space between the epicardium and the pericardium.
  • CMR and cardiac CT: help in quantification of pericardial fluid