Comprehensive Cardiac Imaging and Cross-Sectional Anatomy

Assessment of Cardiac Size and Aortic Silhouettes

• Determining Cardiomegaly via Cardiothoracic Ratio: To determine if the heart is enlarged, the transverse width of the heart (measured from the right border to the left border) is compared to the transverse diameter of the thoracic cage.

  • The calculation involves dividing the heart diameter by the thoracic cage diameter.
  • A normal measurement is defined as anything less than $50\%$.
  • Numerical Threshold: $\text{Cardiac Diameter} / \text{Thoracic Diameter} < 0.50$.
  • Visual estimation often suffices; if the heart occupies less than half the width of the chest cavity, it is typically considered normal.

• Aortic Silhouette Characteristics:

  • Normal: The border of the aortic silhouette should be positioned away from the right atrial border.
  • Aortic Stenosis: In cases of pathology, the border of the aorta may align vertically with the right atrial border. This suggests stenosis, which is a narrowing of the aortic valve.
  • Fluid Dynamics of Stenosis: When blood passes through a narrowed valve, it creates turbulence and increased pressure, similar to placing a finger over the end of a hose. This creates a "fountain effect" where the liquid flow hits the walls of the ascending aorta, leading to post-stenotic dilation.
  • Hypertension: Persistent high blood pressure pushes against the walls of the aorta, leading to a general enlargement and stretching of the aortic walls.

Technical Principles of Cardiac Computed Tomography (CT)

• Contrast Media: A cardiac CT focused specifically on the heart must be performed with intravenous iodine-based contrast. Barium is not used for this purpose. The contrast is essential for highlighting heart chambers, walls, valves, and identifying pathologies.

• ECG Gated Acquisition: Because the heart is a moving structure, motion artifacts can degrade image quality, potentially mimicking or hiding pathologies. To resolve this, scanners utilize ECG-Gated Acquisition.

  • Electrodes are placed on the patient and connected to the CT machine.
  • The machine monitors the heart rhythm and triggers the "shutter" to take photos only during the phase of least movement.
  • Phase Selection: The images are captured during Diastole, the relaxation phase where chambers are filling, rather than systole (contraction).

• Clinical Applications:

  • Visualization of coronary arteries and heart valves.
  • Evaluation of cardiac masses and pericardial diseases.
  • Analysis of wall motion and ventricular contraction.
  • Ejection Fraction ($EF$): The calculation of the percentage of blood pumped out of the left ventricle per beat.
    • Normal Range: $50\%$ to $75\%$.
    • High (Hyperdynamic): Above $75\%$.
    • Low (Hypodynamic): Below $50\%$.
  • Myocardial Perfusion: Assessment of blood flow directed to the heart muscle itself.

Sectional Anatomy: The Six Levels of Cardiac CT

• Level 1: The Five Vessels: This is the most superior level.

  • Left Brachiocephalic Vein: Located immediately posterior to the sternum.
  • Right Brachiocephalic Vein: Located to the right of the patient.
  • The "Three Kings" (Aortic Arch Branches): These three vessels surround the trachea.
    1. Brachiocephalic (Innominate) Artery: The most rightward branch.
    2. Left Common Carotid Artery: The central branch.
    3. Left Subclavian Artery: The most posterior and leftward branch. This artery is closest to the lung pleura; pathologies in the lung can compress it, and vice versa.
  • Esophagus: Located posterior to the trachea, sandwhiched between the trachea and the vertebral body.

• Level 2: Aortic Arch:

  • The arch appears as an elongated, radiopaque "band" or smudge moving from the center/right toward the left.
  • Superior Vena Cava (SVC): Formed by the union of the two brachiocephalic veins; it is located on the right side of the patient.
  • Azygos Vein: Drains into the SVC, looping around the trachea.

• Level 3: Aortopulmonary Window ($AP$ Window):

  • The arch is no longer visible as a single band; instead, the Ascending Aorta (anterior) and Descending Aorta (posterior/left) appear as two separate circular structures.
  • The Pulmonary Artery appears anteriorly.
  • The space between the ascending aorta and the pulmonary artery is the Aortopulmonary Window.
  • Clinical Significance: This space is a common site for lymphadenopathy (lymph node inflammation/edema).
  • Measurement Threshold: The aorta should be no larger than $3\,cm$. Measurements exceeding $3\,cm$ suggest dilation or aneurysm.

• Level 4: Main Pulmonary Artery:

  • Features the Inverted Y Sign, represented by the bifurcating Pulmonary Trunk.
  • The trunk divides into the Right Pulmonary Artery and the Left Pulmonary Artery.
  • The Carina (tracheal bifurcation) is visible here, appearing in a "infant heart" shape or a "U" shape as the bronchi separate.
  • The esophagus is found posterior to the left bronchus. Mass effects in the esophagus at this level can compress the bronchus, leading to lung collapse.

• Level 5: High Cardiac Level:

  • The Left Atrium is identified as the most posterior chamber of the heart.
  • The Right Ventricular Outflow Tract ($RVOT$) is the most anterior part of this level, where blood exits the right ventricle.
  • The Aortic Root (the "hippy" or wide portion of the aorta) is visible in the center.

• Level 6: Low Cardiac Level:

  • The Left Ventricle is distinguished by its significantly thicker muscular wall compared to the Right Ventricle.
  • The Interventricular Septum separates the two.
  • The Azygos Vein remains visible anterior and to the right of the vertebral body, while the Descending Aorta is anterior and to the left.

Cardiac Magnetic Resonance Imaging (MRI)

• Contrast Agents: Cardiac MRI ($CMR$) uses Gadolinium ($Gd$), not iodine. It is used to evaluate myocardial scars from past infarcts, perfusion, and anatomical defects.

• Physics and Safety: MRI uses non-ionizing radiation via magnetic fields. While theoretically safer for pediatric and pregnant populations compared to ionizing radiation (CT/X-ray), the environment is restrictive and can trigger claustrophobia.

• Tissue Appearance:

  • Bone: In MRI, the bone cortex is hypointense (dark/black). In CT, bone is hyperdense (bright/white).
  • Fat: Appears hyperintense (bright/white) in MRI.

• Blood Signal Protocols:

  1. Black Blood (Spin Echo): Used to evaluate heart morphology and anatomy.
  2. Bright Blood (Gradient Echo): Used to evaluate cardiac function and blood flow.

Specialized Cardiac MRI Views

• Horizontal Long Axis (Four-Chamber View):

  • Visualizes both atria and both ventricles simultaneously.
  • The apex of the heart is formed by the left ventricle.
  • Ventricle identification: The Left Ventricle (LV) is normally much thicker than the Right Ventricle (RV).
  • Valves: Tricuspid Valve is between the right atrium and ventricle; the Mitral (Bicuspid) Valve is between the left atrium and ventricle.
  • Walls: Includes the free wall of the RV, the septal wall, and the lateral wall of the LV.

• Vertical Long Axis (Two-Chamber View):

  • Focuses primarily on the left side: Left Atrium and Left Ventricle.
  • Used to evaluate the apex, the anterior wall, and the inferior wall of the heart.

• Short Axis View:

  • Cross-sections of the ventricles (they appear as circles or "donuts").
  • Primary view for Volumetric Measurements: End-Diastolic Volume ($EDV$), End-Systolic Volume ($ESV$), Stroke Volume ($SV$), and Ejection Fraction ($EF$).

• Three-Chamber View:

  • Visualizes the Left Atrium, Left Ventricle, and the Left Ventricular Outflow Tract ($LVOT$) / Aorta.
  • Evaluates the Mitral and Aortic valves specifically.

Coronary Artery Anatomy and Dominance

• Left Coronary Artery (LCA):

  • Divides into the Left Anterior Descending (LAD) and the Circumflex Artery.
  • The LAD is the most common site for cardiac pathology/blockage.

• Right Coronary Artery (RCA):

  • Loops around the right side of the heart.

• Cardiac Dominance:

  • In approximately $90\%$ of the population, the Right Coronary Artery is dominant.
  • Dominance is determined by which artery gives rise to the Posterior Interventricular Artery.

Questions & Discussion

Question: What is the difference between an Open MRI and a Regular MRI? Response: Both can perform the same studies. The Open MRI offers more space and comfort for claustrophobic patients or those having panic attacks. However, because the machine is open, the magnetic field is less contained, leading to a degradation in image quality; the images are not as sharp or "beautiful" as those from a closed machine. Closed MRI is generally the first choice unless the patient has specific difficulties.

Question: Are the arrows seen in diagnostic images placed by radiologists? Response: Usually no. Arrows are typically added to textbooks and teaching slides to guide students. Radiologists only use arrows if a pathology is extremely difficult to identify or requires specific attention. In final clinical practice exams, images are provided without any arrows for identification.

Question: Why does the esophagus sometimes show air inside on a CT? Response: While the esophagus is usually collapsed when food is not passing, it is very common to find small amounts of air within it. This can sometimes lead students to confuse it with the trachea, although the trachea is much larger and positioned anteriorly.

Question: Can we see the coronary arteries without the rest of the heart muscle? Response: Yes, using specific software post-processing in CT Angiography, all muscular tissues can be "deleted" from the digital image, leaving only the arterial tree visible for detailed inspection.

Question: What is the importance of the AP Window in clinics? Response: It is a critical anatomical landmark for checking lymphadenopathy, inflammation, or specialized pathologies like those seen in COVID-19 or lymphatic system disorders.

Practice Image Identification Guide

• X-Ray Landmarks:

  • Heads of Clavicles: Used to determine if the patient is rotated. The distance to the spinous processes should be equal on both sides.
  • Inspiration Check: A good quality chest X-ray should allow the visualization of at least 10 posterior ribs.
  • Angles: Sharp, acute Costophrenic Angles (lateral) and Cardiophrenic Angles (medial) must be identified. Blunting suggests pleural effusion.
  • Retrosternal Space: A radiolucent area behind the sternum on a lateral view.

• Sectional Structures (CT/MRI):

  • 60: Transverse Process.
  • Spinous Process: Posterior extension of the vertebra.
  • Erector Spinae: The group of muscles found between the transverse and spinous processes.
  • Pectoralis Major/Minor: Muscles anterior to the ribs.
  • Lungs: Right and Left should be identified based on the patient's orientation (inverted in axial views).
  • Symmetry: The trachea is the central radiolucent (CT) / hypointense (MRI) tube; the esophagus is the smaller one posterior to it.