TRIAL NOTES

Recognizing Normal Cardiac Anatomy

Introduction to Cardiac Imaging

  • We will assess heart size with conventional radiography, describe normal and abnormal contours on frontal radiographs, and discuss heart anatomy using
    computed tomography (CT) and magnetic resonance imaging (MRI).

Evaluating the Heart on Chest Radiographs

Recognizing a Normal-Sized Heart
Important Points

  • Cardiothoracic Ratio:

    • Measurement of the cardiac silhouette on the frontal chest radiograph.

    • Defined as the widest transverse diameter of the heart compared with the widest internal diameter of the rib cage (measured from inside right rib to inside left rib at the diaphragm).

    • Figure 4.1: Illustrates how to estimate the cardiothoracic ratio.

    • Normal value: Less than 50% in most adults at full inspiration (about nine posterior ribs visible).

Normal Cardiac Contours

  • The contours of the heart seen on frontal projection include:

    • Right Heart Contours:

    • Ascending aorta (first contour on the right) meets with right atrium’s contour.

    • Indentation at the junction indicates possible left atrial enlargement (referred to as the double-density sign).

    • Right heart border is formed by the right atrium.

    • Left Heart Contours:

    • Aortic knob as the first contour (foreshortened aortic arch).

    • Main pulmonary artery segment (visible below the aortic knob).

    • An indentation below this suggests possible left atrial enlargement.

    • Left ventricle forms the last contour on the left.

    • The descending aorta is usually hidden by the spine shadow.

General Principles

  • Regardless of the imaging modality:

    • Ventricles respond to outflow obstruction with hypertrophy before dilatation.

Appearances of the Aorta (Figure 4.3)

  • Normal (A):

    • Ascending aorta appears low-density, not projecting beyond the right heart border.

    • Aortic knob is not enlarged, and descending aorta fades into the thoracic shadow.

  • Aortic Stenosis (B):

    • Ascending aorta projects convexly outward due to post-stenotic dilatation, whereas the rest remains normal.

  • Systemic Hypertension (C):

    • Ascending and descending aorta project excessively laterally, with an enlarged aortic knob.

Key Points About Cardiac Contours

  • The ascending aorta should not exceed the right heart border.

  • The normal aortic knob measures under 35 mm, often shifting the trachea slightly to the right.

  • The pulmonary artery is generally flat or concave; however, it can appear convex in younger females.

  • The left atrium does not shape the cardiac border in non-rotated frontal views but may “fill-in” when enlarged.

  • The descending aorta should be nearly parallel to the spine and hardly visible in frontal views.

Recognizing Cardiomegaly

  • Cardiomegaly indicates ventricular enlargement, while isolated atrial enlargements do not affect silhouette size.

  • Enlargement due to volume overload is more pronounced than from pressure overload. E.g., Aortic regurgitation enlarges the heart more than aortic stenosis.

Evaluating the Heart on Cardiac CT

  • Cardiac CT utilizes a fast, multislice scanner, typically involving intravenous contrast for imaging and ECG-gated acquisition to minimize motion artifacts.

  • It assesses coronary arteries, valves, masses, and analyzes wall motion, ejection fraction, and myocardial perfusion.

  • Standard Imaging Planes: Provide perspectives in three orientations:

    • Axial

    • Sagittal

    • Coronal

Normal Cardiac CT Anatomy

  • Focus on several major anatomical landmarks across six key levels, using contrast-enhanced scans.

Five-Vessel Level (Figure 4.5)

  • Identify:

    • Lungs

    • Trachea (about 2 cm in diameter, oval shape)

    • Esophagus (collapsed but may contain air)

    • Observable great vessels: right and left brachiocephalic veins, innominate, left common carotid, and left subclavian arteries.

Aortic Arch Level (Figure 4.6)

  • Identify:

    • Aortic arch (U-shaped structure)

    • Superior vena cava

    • Azygos vein

Aortopulmonary Window Level (Figure 4.7)

  • Identify:

    • Ascending and descending aorta, superior vena cava, and upper segment of left pulmonary artery.

  • Ascending aorta measures between 2.5 cm and 3.5 cm, while the descending aorta is between 2 cm and 3 cm.

Main Pulmonary Artery Level (Figure 4.8)

  • Identify:

    • Main, right, and left pulmonary arteries; right and left main bronchi; bronchus intermedius.

High Cardiac Level (Figure 4.9)

  • Identify:

    • Left atrium occupies the posterior and central heart area, exhibiting pulmonary veins.

    • Right atrium forms the right heart border.

Low Cardiac Level (Figure 4.10)

  • Identify:

    • Right atrium, right ventricle, left ventricle, and interventricular septum (noteworthy thickness differences).

Uses of Cardiac CT

  • Evaluation of:

    • Coronary arteries

    • Cardiac masses

    • Aortic abnormalities (including aortic dissection)

    • Pericardial diseases

  • Three-dimensional reconstructions and calcium scoring are noteworthy capabilities of cardiac CT.

Calcium Scoring

  • Based on calcium amounts in the coronary arteries, which correlate to atherosclerosis degree and predict future cardiac events (such as heart attacks).

  • Correlation between calcium scores and risk:

    • Score of 0: Low likelihood of cardiac events in the near future.

    • Scores 100 - 300: Mild to moderate risk over 3-5 years.

    • Scores greater than 300: Higher risk of severe disease.

Coronary CT Angiography (CCTA)

  • Evaluates coronary anatomy with good accuracy against the traditional angiography (i.e., catheter-based).

  • Variations in anatomy exist, including:

    • Left coronary artery (LCA) divides into circumflex and left anterior descending (LAD) arteries.

    • Right coronary artery (RCA) generally supplies the posterior descending artery (PDA).

Coronary Artery Dominance

  • Determines by the artery supplying PDA:

    • Right Dominant: PDA from RCA.

    • Left Dominant: PDA from circumflex artery.

    • Codominant: PDA supplied by both arteries.

Cardiac MRI

Overview

  • MRI provides detailed anatomical and functional views without high radiation doses.

  • Offers insights into scarring from myocardial infarction and functionality of cardiac chambers.

  • Normal cardiac MRI images can be captured in various planes.

Specific Views Used in Cardiac MRI:

  1. Horizontal Long Axis (Four-Chamber View)

    • Best for evaluating left ventricle's walls and size, visualizing mitral and tricuspid valves.

  2. Vertical Long Axis View

    • Useful for visualizing anterior and inferior walls of the left ventricle.

  3. Short Axis View

    • Useful for volumetric assessments.

  4. Three-Chamber View

    • Assesses left atrial size, mitral and aortic valves, and left ventricle walls.

MRI Imaging Techniques

  • Blood imaging can be either:

    • Bright (for functional assessment)

    • Black (for depicting anatomy without interference)

Take Home Points

  • Quick heart size assessment via cardiothoracic ratio (<50% typical).

  • Normal heart contours described.

  • Understand cardiac response to outflow obstruction (hypertrophy before dilatation).

  • Familiarize with CT anatomy at six levels, including the significant implications for diagnosis and treatment.