Cardiothoracic Ratio

=  CT ratio = widest transverse cardiac diameter ÷ widest inside thoracic diameter

< 0.5 = normal in > 1 month old (45% sensitive, 85% specific, 59% accurate)
< 0.6 = normal in < 1 month old

Purpose:   measurement of LV dilatation

Dependent on:

•  lung volume: CT ratio enlarges in expiration

•  patient position: CT ratio increases on supine film


›  no change unless LV volume increases by > ²/³

›  no change in moderate enlargement of LA / RV


Vertical Long Axis

=  parasagittal plane along long axis of LV

Best assessment of:

1.   LA and LV (relationship)

2.   Inferior + anterior walls of LV myocardium

3.   Bicuspid MV (structure & function) versus LV

4.   LV (structure & function)

4.   LA appendage and coronary sinus

Horizontal Long Axis / 4-Chamber View

=  horizontal plane bisecting all 4 chambers

Best assessment of:

1.   Chamber size and valve position

2.   Septal + apical + lateral LV walls

3.   AV valve (subjective assessment in cine mode)

4.   Ventricular function (subjective assessment)

5.   LA size (quick measurement)

Three-Chamber View

=  oblique long-axis view optimizing visualization of LV, LA, aortic root, MV, aortic valve

Best assessment of:

1.   LV outflow tract, aortic valve, aortic root, proximal ascending thoracic aorta

2.   Posteromedial papillary muscles arising from LV free (lateral) wall

3.   Chordae tendineae of MV

Short Axis View

=  oblique coronal plane across barrel of LV lumen

1.   Basal, middle, apical portions of LV myocardium

2.   LV size (easy assessment)

3.   LV myocardial contractility (easy assessment)


1.   Coronary arteries: origin, course, segmental anatomy

2.   Dominance and size of LAD artery, LCx artery, and RCA

3.   Number of diagonal + marginal branches

4.   Size of heart + cardiac chambers

5.   LV function with ejection fraction and wall motion

6.   End-diastolic + end-systolic LV volume

7.   LV myocardial mass + thickness



semilunar valves develop simultaneously with formation of RV + right ventricular outflow tract (RVOT) from conotruncal endocardial cushions around distal part of conus

Heart Valve Positions


reference line = oblique line drawn from distal left mainstem bronchus to right cardiophrenic angle

√  aortic valve resides in profile superior to this line overlying the thoracic spine

√  pulmonic valve just inferior to left mainstem bronchus

√  mitral valve resides inferior to this line centrally located within cardiac silhouette

√  tricuspid valve inferior to this line more basilar and midline


reference line = oblique line drawn from carina / right pulmonary artery shadow to anterior cardiophrenic sulcus

√  aortic valve resides superior to this line

√  pulmonic valve anterior + superior to aortic valve

√  mitral valve resides inferoposteriorly to this line

√  tricuspid valve inferior to this line anteriorly

Aortic Valve

=  separates LV outflow tract from ascending aorta by a valve composed of annulus, commissures and usually 3 (range, 1 – 4) thin semilunar valve leaflets / cusps

(a)  right cusp: inferior to right coronary sinus + origin of RCA

(b)  left cusp: inferior to left coronary sinus + origin of LCA




(c)  posterior / noncoronary cusp

Nodules of Arantius = thickened fibrous bulge at center of each free cusp margin forming central coaptation area

Commissures = junctions of cusps at attachment to aortic wall at level of sinotubular junction

Area:     2.5 –4.0 cm2

Aortic valve planimetry is optimal during midsystole = 20% of R-R interval or 50–100 msec from R-wave peak!

Mitral Valve

=  bicuspid valve (= 2 leaflets) anchored on mitral valve annulus + connected to LV papillary muscles by chordae tendineae; MV and aortic valve share fibrous continuity

Area:     4–6 cm2

Circumference:   10 cm

Embryology:   develops during 5th–15th week of gestation

Papillary Muscles of Mitral Valve

Origin:   lateral wall of LV

(a)  anterolateral papillary muscle

(b)  posteromedial papillary muscle (single blood supply)

Prognosis:  vulnerable to ischemia + rupture

Mitral Valve Annulus

=  D-shaped ring within L atrioventricular groove; imbedded within myocardium as part of cardiac skeleton

(a)  straight border = anterior portion of annulus; in fibrous continuity with aortic valve + heart skeleton

(b)  curved border = posterior portion of annulus: attached to pliant endocardium

Function:   site of valve leaflet attachment

Border:   LCx artery + coronary sinus

Mitral Valve Leaflets

Normal thickness:   < 5 mm

(a)  semicircular anterior leaflet attaches to ⅓ of annulus + forms part of LVOT

Segments:  lateral A1, middle A2, medial A3

(b)  crescentic posterior leaflet

Segments:  lateral P1, middle P2, medial P3

Mitral valve components are best evaluated on reformatted 2-chamber long-axis images perpendicular to the valve during middiastole = 65% of R-R interval for open mitral valve and 5% of R-R for closed mitral valve!

Pulmonic / Pulmonary Valve

=  separates RV outflow tract from main pulmonary artery by a semilunar valve composed of 3 cusps (similar to aortic valve although separate from atrioventricular valve)

mnemonic:   A cusp on P and P cusp on A anterior cusp (short-axis view) ← pulmonic valve posterior (noncoronary) cusp ← aortic valve

  (a)  anterior cusp

  (b)  right cusp

  (b)  left cusp

Area:     2.0 cm2 / m2 of body surface area

Tricuspid Valve

=  right atrioventricular valve separating RA from RV; anchored on tricuspid valve annulus + connected to RV papillary muscles by chordae tendineae; composed of 3 (range, 2 to 4) leaflets

Crista supraventricularis:   muscular ridge that separates TV from pulmonary valve

(a)  septal leaflet

(b)  anterior leaflet

(b)  posterior leaflet


Right Ventricle

Trabeculae:   coarse

Papillary muscles:   attached to interventricular septum + free wall; apical moderator band

Differentiating features of RV (from LV):

1.   Heavily trabeculated apex

2.   Well-developed infundibulum

3.   Septal papillary muscles

4.   Lack of fibrous continuity of AV valve + outflow tract

Moderator Band

=  muscular band extending from interventricular septum to base of anterior papillary muscle

Function:   part of right bundle branch conduction system

Right Ventricular Outflow Tract (RVOT)

=  smooth muscular infundibulum / conus inferior to PV

In the right ventricle, trabeculae are coarse, and presence of an apical moderator band is CHARACTERISTIC!

Left Ventricle

Trabeculae:   thin, delicate, smooth septal surface

Papillary muscles:   attached to free wall only


Embryology of Atrial Septa

1.   Septum primum = thin partition dividing primitive atrium into right and left atria at 4th week; membrane grows from dorsal atrial wall toward endocardial cushion → ultimately forms thin flap valve of fossa ovalis

2.   Ostium primum = temporary orifice between septum primum + endocardial cushions close to AV valves; it becomes obliterated by 5th week

3.   Ostium secundum = multiple small coalescing fenestrations in center of septum primum

4.   Septum secundum = stiff membrane developing on right side of septum primum growing from right atrial roof → ultimately forms thicker limbus of fossa ovalis

5.   Foramen ovale = orifice limited by septum secundum + septum primum

6.   Foramen ovale flap = lower edge of septum primum (patent in 6%, probe-patent in 25%); not considered an ASD

Right Atrium (RA)




(1)  appendage

(2)  venous part

(3)  vestibule

Embryology:   originates from primitive trabeculated RA (persists as RA appendage) + from sinus venosus forming smooth-walled portion of RA

Right Atrial Appendage

=  triangular superior extension of RA that wraps around aortic root

√  pyramidal shape with narrow base

Rx:  right atrial lead tip is typically placed at right atrial appendage for pacemaker / internal cardiac defibrillator

Sinus Venosus

›  located in posterolateral wall of RA between orifices of SVC and IVC

(a)  right horn: gives rise to crista terminalis, eustachian ridge, thebesian valve

(b)  left horn: gives rise to coronary sinus

Vestibule of Right Atrium

=  smooth muscular rim surrounding tricuspid orifice

Crista Terminalis

=  prominent fibromuscular ridge separating smooth-walled venous part of posterior RA (= sinus venosus) from trabeculated muscle fibers of appendage anteriorly

Variations: in size + extent among individuals

Location:   junction of sinus venosus and primitive RA

Clinical significance:

thickening of crista terminalis → development of atrial flutter + focal right tachycardias (²/³ arise in crista terminalis) → target for catheter RF ablation

√  vertically oriented smooth muscular ridge varying in size + thickness amongst individuals:

√  small thin valvelike / broad-based structure

√  may be large ← fatty infiltration of crista terminalis in lipomatous hypertrophy of the atrial septum

√  superiorly arches anterior to the orifice of the SVC

√  extends to area of anterior interatrial groove

√  merges with interatrial bundle (= Bachmann bundle)

√  indistinct inferior border located near IVC orifice merging with small trabeculations of inferior portion of cavotricuspid isthmus

›  gives rise to thick muscle bundles

(a)  anterior pectinate muscles fanning out anteriorly

(b)  septum spurium = most prominent anterior pectinate muscle (in 80%) arising from crista terminalis

Mean thickness:   4.5 mm

Sinoatrial Node

=  subepicardial spindle-shaped structure as source of cardiac impulse (= dominant pacemaker)

Length:   20 ± 3 mm

Location:   in myocardium at superior cavoatrial junction between crista terminalis and SVC

√  surrounds sinoatrial nodal artery, which may course centrally (70%) / eccentrically within node

Koch Triangle

Location:   at orifice of coronary sinus


›  posteriorly: fibrous extension from eustachian valve (tendon of Todaro) = Todaro-eustachian ridge

›  anteriorly: attachment of septal leaflet of tricuspid valve

›  inferiorly: coronary sinus

›  at apex: central fibrous body of heart (= site of penetration of His bundle)

›  midportion: contains compact AV node (fast pathway)

›  base: bordered by coronary sinus ostium + septal isthmus (= area between edge of coronary sinus ostium + septal tricuspid valve); contains slow pathway

◊  Septal isthmus = frequently target for ablation of slow pathway in AV node reentrant tachycardia!

AV Node

Location:   within Koch triangle


›  coronary sinus ostium

›  septal leaflet of tricuspid valve

›  tendon of Todaro = fibrous band connecting eustachian and thebesian valves

[Francesco Todaro (1839–1918), Italian professor of anatomy at the Universities of Messina and Rome]

Importance:    electrophysiologists frequently modify this node in dual atrioventricular nodal pathways → AV nodal reentrant tachycardia

Terminal Groove / Sulcus Terminalis

=  fat-filled groove on epicardial side corresponding internally to crista terminalis close to cavoatrial junction

›  location of sinus node + terminal segment of sinoatrial nodal artery

Eustachian Valve

[Bartolomeo Eustachi (1500 or 1514–1574), one of the founders of the science of human anatomy in Rome, Italy]

=  valve of inferior vena cava (guarding entrance into IVC)

Function:   directs flow toward foramen ovale

Location:   junction of RA and IVC; inserts medially onto eustachian ridge (= border between oval fossa + coronary sinus)

›  directs blood from IVC to foramen ovale in fetus

›  free border continues as tendon of Todaro

√  thin linear structure, not routinely imaged

◊  Rarely an unusually large muscular valve may pose an obstacle to passage of a catheter!

Cavotricuspid Isthmus

=  area between IVC + tricuspid valve of highly variable isthmian anatomy

◊  Target of catheter ablation as treatment of choice for isthmus-dependent atrial flutter!

N.B.:   obstacles to successful ablation may be a large eustachian ridge, aneurysmal pouches, or a concave deformation of the entire isthmus

Thebesian Valve

=  valve of coronary sinus

[Adam Christian Thebesius (1686–1732), anatomist and municipal physician in Hirschberg, Silesia]

Prevalence:   in 80% of cadaveric specimens

Function:    prevents reflux from RA into coronary sinus

Location:   entry of coronary sinus into RA

Morphology:   complete circular (30%), crescentic (35%), absent (20%), threadlike (2%), fenestrated (10%)

√  thin semilunar fold at anteroinferior rim of coronary sinus ostium

Mechanical barrier:   large eustachian valve / ridge (25%), > 5 mm deep subthebesian recess (45%), hypoplastic coronary sinus ostium, large thebesian valve

Subthebesian Pouch

=  subeustachian sinus = sinus of Keith

=  pouchlike atrial wall inferior to orifice of coronary sinus

Depth:   4.3 ± 2.1 (range, 1.5–9.4) mm

◊  Substrate for reentrant circuit during atrial flutter

N.B.:   main source of RF procedural difficulty

Interatrial Septum

Septum = wall that can be removed without exposing heart cavity to extracardiac structures

Parts:     flap valve of foramen ovale (septum primum) + part of its anteroinferior margin

◊  Superior rim of fossa (septum secundum = infolded wall between SVC and right pulmonary veins) is not a true septum!

√  thin septum that is difficult to image

√  may contain small amount of fat sparing fossa ovalis

DDx:  Lipomatous hypertrophy of interatrial septum

√  characteristic dumbbell shape ← sparing of fossa ovalis

√  abnormal amount of fat in older / obese adults

Fossa Ovalis

=  circular indentation in interatrial septum

Variant:   patent fossa ovalis / foramen ovale



Prevalence:   15%

Function:   can result in R-to-L shunt

Associated with:

paradoxical emboli, cryptogenic stroke, hypoxemia in patients with obstructive sleep apnea, increased risk for decompression sickness, increased risk for atrial fibrillation after cardiac surgery

√  demonstrated by contrast-enhanced echocardiography

Left Atrium (LA)

Components:   venous component, vestibule, appendage

Embryology:   originates from primitive trabeculated LA (persists as LA appendage) + from pulmonary veins forming smooth-walled portion of LA

√  smooth walled venous + septal component + vestibule

√  ridge of smooth muscle (± bulbous tip) at junction of LA appendage and entrance of left superior pulmonary vein

Venous Component of Left Atrium

=  located posteriorly → pulmonary vein orifices at each corner

Left Atrial Vestibule

=  surrounds the mitral orifice

Left Atrial Appendage

=  arises from superolateral aspect of LA

√  rough trabeculated surface of tubular shape

√  projects anteriorly over proximal LCx artery

√  3.5–6.5 mm thick superior wall / dome

Cx:  narrow neck predisposes to thrombus deposition

Atrial Appendages

√  linear filling defects (R > L) = pectinate muscles fibers running parallel to each other measuring > 1 mm (in 97%)

DDx:  thrombus in LA


Anatomy of Left Coronary Artery (LCA)

arises from left (posterior) coronary sinus (= left sinus of Valsalva) near sinotubular ridge; passes to left and posterior to pulmonary trunk

1.   Left main coronary artery (LM)

Segments:   ostium to bifurcation = 5–20 mm short stem

›  bifurcates into LAD + LCx

›  trifurcates (in 15%) = ramus intermedius (RI) branch coursing laterally toward LV free wall similar to D1

2.   Left anterior descending (LAD)

Course:   within anterior interventricular groove toward apex

Supply:   majority of LV + anterolateral papillary muscle ± small branches to anterior RV wall


›  proximal: from left main bifurcation to origin of 1st septal branch

›  midportion: from 1st septal branch to an acute angle (may coincide with origin of 2nd septal perforator)

–  otherwise split halfway between 1st septal perforator and apex into:

›  middle LAD

›  distal LAD

›  apical segment = termination of LAD

(a)  Diagonal perforating branches (D1, D2, etc) arise from LAD, course over anterolateral wall of LV

Supply:   LV free wall

mnemonic:   Diagonals course downward from LAD

(b)  Septal perforating branches (S) course medially toward anterior interventricular septum

Supply:   majority of interventricular septum + AV bundle + proximal bundle branch

3.   Left circumflex artery (LCx) travels within left atrioventricular sulcus (groove);

terminates at obtuse (blunt / round) margin of heart

(a)  Obtuse marginal (lateral) branches (OM1, OM2, etc)

for lateral wall of LV

(b)  Left atrial circumflex artery (LACX) for atrium

(c) variably:    branches to posterolateral + posterior descending artery supplying diaphragmatic portion of LV (= left dominance)

Supply:   LV free wall + variable portion of anterolateral papillary muscle

Terminology from surface perspective of apical view:

›  rounded obtuse margin of heart formed mainly by LV

›  sharp angle = acute margin of heart formed mainly by RV

Segments:   proximal + distal (based on origin of large obtuse marginal branches)










Anatomy of Right Coronary Artery (RCA)

arises from right (anterior) coronary sinus → passes to right + posteriorly to pulmonary artery → travels downward within right atrioventricular sulcus (groove) → rounds the acute margin of the heart toward crux

Terminology based on posterior surface view of heart:

crux cordis = cross formed by

›  AV groove (transecting a line formed by)

›  posterior interventricular + interatrial sulci

[crux, Latin = cross, junction]


›  proximal: ostium to halfway to acute margin of heart

›  mid:

›  distal: acute angle of heart to origin of PDA

1.  Conus artery (CB)

=  1st branch of RCA (in 50–60% of patients); may originate directly from coronary sinus of aorta (in 30–35%)

√  forms circle of Vieussens = anastomosis with LAD

Supply:   RVOT = conus arteriosus

2.   Sinoatrial node artery (SANA)

=  2nd branch of RCA (in 60%) / from LCx (in 40%)

√  courses along anterior interatrial groove toward superior cavoatrial junction

√  at cavoatrial junction circling either anteriorly (precaval) / posteriorly (retrocaval) to enter node

3.  Marginal branches (M1, M2, etc) have an anterior course

›  acute marginal branch = at junction of middle + distal RCA

Supply:   RV

4.  Posterior descending artery (PDA)

=  origin of PDA determines coronary artery dominance

Origin:   usually RCA near crux (in 70%) / distal acute marginal branch (= right dominance)

Supply:   posterior third of ventricular septum + diaphragmatic segment of LV + posteromedial papillary muscle

5.  Atrioventricular node artery (AVNA)

=  small branch to AV node

Origin:   apex of U-turn of distal RCA (80–87%) / terminal portion of LCx (8–13%) / both RCA and LCx (2–10%)

Supply:   posterior interventricular septum, interatrial septum, AV node, His bundle

√  penetrates base of posterior interatrial septum at crux

√  may course beneath endocardium near ostium of coronary sinus + septal isthmus

◊  High risk for AV nodal artery coagulation during RF ablation!

6.  Posterolateral segment arteries (PLSA)

supplies posterolateral wall of LV

Coronary Artery Territory

septum = LAD
anterior wall = LAD
lateral wall = LCx
posterior wall = RCA
inferior / diaphragmatic wall = RCA
apex + inferolateral wall = watershed areas

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Jun 29, 2017 | Posted by in GENERAL RADIOLOGY | Comments Off on Anatomy

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