Learning Radiology Chapter 3 - Recognizing Normal Cardiac Anatomy

Heart Size Assessment on Frontal Chest X-Ray

• Cardiothoracic Ratio (CTR)
  • CTR = \frac{\text{widest heart diameter}}{\text{widest internal thoracic diameter at diaphragm level}}
  • Normal adult (full inspiration): CTR < 0.50 (heart < half rib-cage width)
• Measurement landmarks
  • Heart: outermost right–left borders
  • Thorax: inside rib to inside rib at hemidiaphragm

Normal Cardiac Contours (Frontal View)

• Right-sided silhouettes (cranial ➜ caudal)
  • Ascending aorta
  • Small indentation (potential double-density when LA enlarges)
  • Right atrium (forms majority of right border)
• Left-sided silhouettes (cranial ➜ caudal)
  • Aortic knob (< 35\,\text{mm} from tracheal edge; pushes trachea slightly right)
  • Main pulmonary artery (concave/flat; can be convex in young females)
  • Concavity for LA / LA appendage (fills in if LA enlarges)
  • Left ventricle (forms lower left border)
  • Descending thoracic aorta (normally parallels spine, barely seen)

Key quantitative/qualitative checkpoints

• Ascending aorta should not project to right of right atrium
• Tortuous or "uncoiled" descending aorta curves leftward with age/HTN

General Principles of Chamber Response

• Ventricular outflow obstruction ➜ concentric hypertrophy first (size may look normal until failure)
  ➜ Examples: aortic stenosis, pulmonic stenosis, coarctation, systemic HTN
• "Cardiomegaly" on CXR = ventricular enlargement; isolated atrial enlargement often invisible to CTR
• Volume overload (regurgitant lesions) produces larger chambers than pressure load (stenosis)
  • Aortic regurgitation > aortic stenosis in LV size
  • Mitral regurgitation > mitral stenosis in LA size

Radiographic Appearances of the Aorta (Fig 3.3)

• Normal: straight ascending margin, small knob, descending aorta obscured by spine
• Aortic stenosis: post-stenotic dilatation ➜ convex ascending aorta; knob/desc aorta normal
• Systemic HTN: generalized dilatation ➜ both ascending & descending project laterally; enlarged knob

Stenotic vs Regurgitant Valves (Fig 3.4)

• Aortic stenosis: LV hypertrophy without silhouette enlargement; post-stenotic dilatation of AA
• Aortic regurgitation: marked LV dilatation ➜ enlarged cardiac silhouette

Cardiac CT – Technical Fundamentals

• Performed on multi-detector scanners with IV iodinated contrast
• ECG-gated (prospective or retrospective) ➜ images during diastole (least motion)
• Reconstruct multiple phases ➜ wall motion, EF, perfusion analysis
• Standard display planes: axial, coronal, sagittal + reformats/3-D

Systematic CT Anatomy – Six Landmark Levels

1. Five-Vessel Level (Fig 3.5)

• Structures: trachea (air, oval, ~2 cm), esophagus (collapsed; +/- air)
• Vessels (ant ➜ post): R & L brachiocephalic (innominate) veins; arteries – innominate, L common carotid, L subclavian

2. Aortic Arch Level (Fig 3.6)

• Inverted "U" aortic arch (comma shape if skimmed)
• SVC right of trachea; azygos vein drains into SVC

3. Aortopulmonary (AP) Window (Fig 3.7)

• Ascending aorta anteriorly (2.5–3.5 cm dia); descending aorta posterior-left (2–3 cm)
• AP window: fat space beneath arch, above L pulmonary artery – common site for lymphadenopathy
• Tracheal bifurcation (carina) at/just below this level

4. Main Pulmonary Artery Level (Fig 3.8)

• Main PA, LPA (anterior to descending aorta, higher), RPA (posterior to ascending aorta, 90° angle)
• Bronchi: R main ➜ RUL bronchus ➜ bronchus intermedius (posterior wall 2–3 mm; lung only posterior) ; L main bronchus circular air density

5. High Cardiac Level (Fig 3.9)

• Left atrium central-posterior; pulmonary veins enter
• Right atrium anterior-right border
• Aortic root; Right ventricular outflow tract (RVOT) is Anterior-Lateral-Superior to aortic root (mnemonic PALS)

6. Low Cardiac Level (Fig 3.10)

• Right atrium (right border), right ventricle anterior (thin wall, coarse trabeculations)
• Left ventricle posterior-left (thicker wall)
• Interventricular septum visible with contrast
• Pericardium: ~2 mm thick between mediastinal & epicardial fat

Clinical Uses of Cardiac CT

• Coronary artery imaging (CCTA) – lumen, plaque, stents, grafts
• Calcium scoring (non-contrast)
  • Higher Agatston score ➜ ↑ future cardiac event risk; zero score ➜ high NPV for obstructive CAD
• Cardiac masses, pericardial disease, great-vessel pathology (dissection)
• "Triple rule-out" emergent scan ➜ simultaneous CAD, PE, aortic dissection evaluation
• Radiation dose now reduced to < annual background levels via dose-modulation techniques

Calcium Scoring Details (Fig 3.11)

• Algorithm combines area × density of calcium voxels
• Score ranges:
  • 0 = no detectable calcium ➜ very low near-term risk
  • 100{-}300 = mild–moderate risk over 3–5 yrs
  • >300 = high likelihood of severe CAD & MI risk

Coronary CT Angiography – Normal Anatomy

• Main trunks: Left Main (LM) & Right Coronary Artery (RCA)

Left Coronary System (Figs 3.13–3.14)

• LM arises from left coronary cusp ➜ bifurcates into:
  • Left Anterior Descending (LAD): in anterior IV groove to apex; branches – septals (IV septum), diagonals (anterior LV wall)
  • Circumflex (Cx): in AV groove between LA & LV; gives obtuse marginal (OM) branches to lateral LV

Right Coronary System (Fig 3.15)

• RCA from right coronary cusp ➜ courses in AV groove, supplies:
  • Conus branch (RVOT)
  • SA node artery (often)
  • Acute marginal (AM) branch – RV free wall
  • AV nodal branch
  • Posterior Descending Artery (PDA) in most patients

Coronary Dominance

• Defined by supply to PDA
  • Right dominant (≈ 85–90%) – PDA from RCA
  • Left dominant (≈ 10%) – PDA from Cx ➜ ↑ non-fatal MI & mortality
  • Co-dominant – PDA from both

Cardiac MRI – Fundamentals

• Combines ECG-gating + rapid sequences; breath-hold to reduce respiratory motion
• Can be non-contrast or use IV Gadolinium
• Applications: scar imaging (delayed enhancement), perfusion, morphology, valvular and chamber function, congenital heart disease (esp. paediatrics)
• Blood signal depends on pulse sequence:
  • Spin-echo (black blood) ➜ anatomic/morphologic assessment
  • Gradient-echo (bright blood) ➜ functional cine evaluation

MRI Planes & Standard Cardiac Views

• Axial, Coronal, Sagittal (similar to CT) – Fig 3.16
• Additional dedicated planes (Fig 3.17):
  • Horizontal Long-Axis (HLA) / 4-Chamber
  • Visualises all 4 chambers; evaluates septal & lateral LV walls, RV free wall, chamber size; mitral & tricuspid valves (Fig 3.18)
  • Vertical Long-Axis (VLA) / 2-Chamber
  • Sagittal-like; assesses anterior/inferior LV walls & apex; LA & mitral valve (Fig 3.19)
  • Short-Axis (SAX)
  • Stack from base ➜ apex; best for volumetrics (EDV, ESV, stroke volume, EF) (Fig 3.20)
  • Three-Chamber (3CH) / LVOT view
  • Coronal-like; shows LV, LVOT, aortic & mitral valves, LA size, anteroseptal & inferolateral LV walls (Fig 3.21)

Quantitative Functional Analysis

• From 3-D SAX cine stack at end-systole & end-diastole:
  • EDV, ESV, \text{Mass}
  • Stroke\ Volume = EDV - ESV
  • Ejection\ Fraction = \frac{Stroke\ Volume}{EDV}

Bright vs Black Blood Imaging (Fig 3.22)

• Bright blood: cine loops for wall motion, valve excursion, flow
• Black blood: static images eliminating high-signal blood ➜ delineate myocardium, pericardium, masses

Case Quiz 3 Summary

• Finding: Ascending aorta projects beyond right heart border ➜ enlarged
• Interpretation: Post-stenotic dilatation characteristic of aortic valvular stenosis (confirmed by echo)

High-Yield Take-Home Messages (condensed)

• CTR < 0.5 indicates normal heart size on frontal CXR
• Ventricular hypertrophy precedes dilatation with outflow obstruction; volume overload enlarges chambers more than pressure load
• Six CT chest levels provide systematic roadmap – recall key structures at each
• Cardiac CT: coronary anatomy, calcium scoring, triple rule-out; ECG gating minimises motion
• Coronary dominance governed by PDA supply – majority right dominant
• Cardiac MRI offers multiparametric assessment; standard views (HLA, VLA, SAX, 3CH) target specific walls & valves
• Bright blood MRI for function; black blood for detailed morphology