Lesson contents

Lesson 1 of 80 XPBeta

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Lesson 1 of 8

Cardiac topography

How the heart sits in the chest, what the cardiac axes are, and how the LV walls and 17 segments are named.

Topography

An inverted pyramid

•The heart has the shape of an inverted pyramid within the chest. Each face of the pyramid relates to an anatomical structure.
•Posterior: the base, facing the pulmonary veins and the oesophagus.
•Inferior: resting on the diaphragm.
•Anterior: in contact with the sternum.
•Lateral: facing the lungs (left and right).

✦ Key points

Heart shaped like an inverted pyramid
Inferior face on the diaphragm; anterior face against the sternum
Posterior base facing pulmonary veins and oesophagus

Quick question

Which face of the heart rests on the diaphragm?

Cardiac segmentation

Three levels: basal, mid and apical

•From base to tip, the left ventricle is divided into three levels: basal, mid and apical.
•The base is at valve level, where the pulmonary, aortic, tricuspid and mitral valves sit.
•The papillary muscles insert at the mid level. The apical level is the tip of the heart.

Slices of the heart at valve, basal, mid and apical level

✦ Key points

LV divided into three levels: basal, mid and apical
Base at valve level: pulmonary, aortic, tricuspid and mitral
Papillary muscles inserting at the mid level

Quick question

At which level are the cardiac valves?

Cardiac segmentation

What are the cardiac axes?

•Cardiac imaging uses planes that are oblique to the strict anatomical axes of the body (axial, coronal and sagittal).
•This is because the heart has its own orientation within the chest, and the views must align with its true axis.
•These planes are the basis of CMR planning and let us apply the AHA 17-segment model from different angles.
•Long axis: runs through the heart from base to apex, aligned with the long axis of the LV and following the interventricular septum. It includes the 2C, 3C and 4C.
•Short axis: perpendicular to the long axis; it slices the ventricle from base to apex and meets the septum at 90°.

Long-axis plane (red) and short-axis plane (green) on the heart

CMR views: 4-chamber, short axis, 2-chamber and 3-chamber — How they look on CMR: 4C, short axis (EC in Spanish), 2C and 3C.

✦ Key points

Oblique planes aligned with the heart's true axis
Long axis from base to apex: 2C, 3C and 4C
Short axis perpendicular, slicing the LV at 90° to the septum

Quick question

Why does cardiac MR use oblique planes instead of strict axial, coronal and sagittal?

Quick question

How does the short axis relate to the long axis?

Long axes

Left and right long axes

•Like the left side, the right heart has its own long axes: 3C, 2C and outflow tract.
•The 4C is the only long axis shared by the left and right sides.
•This course starts with the left-sided axes.

From the 4C: right long axes (RV 3C, 2C and outflow tract) and left long axes (LV 3C, 2C and outflow tract) — TSV: ventricular outflow tract. Labels in Spanish (derecha = right, izquierda = left).

✦ Key points

Right heart with its own long axes: 3C, 2C and outflow tract
4C, the only long axis shared by both sides
Course starting with the left-sided axes

Quick question

Which long axis is shared by the left and right sides?

Cardiac segmentation

The three myocardial layers

•Heart muscle is organised in three concentric layers:
•Endocardium: the innermost, in contact with the ventricular cavity.
•Mesocardium: the middle layer, where contractile fibres concentrate.
•Subepicardium: the outer layer, next to the pericardium.
•On CMR, the layer involved helps tell diseases apart: the subendocardium is typically affected in infarction (ischaemic pattern), the subepicardium is characteristic of myocarditis and the mesocardium is common in non-ischaemic cardiomyopathy.

Endocardium, mesocardium and subepicardium

Late enhancement types: subendocardial, insertion point, transmural, mid-wall and subepicardial — Late gadolinium enhancement by layer involved.

✦ Key points

Three layers: endocardium, mesocardium and subepicardium
Subendocardium in infarction (ischaemic pattern)
Subepicardium in myocarditis; mesocardium in non-ischaemic cardiomyopathy

Quick question

Subendocardial late enhancement typically suggests…

LV walls

Four walls and six segments

•The right ventricle (RV) attaches to the left ventricle (LV) at the superior and inferior interventricular insertion points.
•Lines drawn from each insertion point through the middle of the LV cavity (in diastole) divide the LV into 4 walls: anterior (A), septal (S), inferior (I) and lateral (L). The septum is so called because it separates both ventricles.
•Only the septal and lateral walls, being larger, are split again in two, each half named after the nearest wall: anteroseptal and inferoseptal, anterolateral and inferolateral.

Lines from the insertion points dividing the LV into anterior, septal, inferior and lateral

The six segments (Spanish labels: A, SA, SI, I, LI, LA)

📝 The figures use Spanish abbreviations, which name the divided wall first: SA septal-anterior (anteroseptal), SI septal-inferior (inferoseptal), LA lateral-anterior (anterolateral), LI lateral-inferior (inferolateral).

✦ Key points

RV insertion points dividing the LV into 4 walls
Anterior, septal, inferior and lateral
Only septal and lateral split: anteroseptal, inferoseptal, anterolateral, inferolateral

Quick question

Which LV walls are split into two segments?

AHA model

The 17 segments

•The AHA (American Heart Association) standard divides the LV myocardium into 17 segments, grouped into three levels from base to apex:
•Basal (1–6): closest to the AV valves. It ends just before the papillary muscles.
•Mid (7–12): the middle portion, where the papillary muscles insert.
•Apical (13–16): the distal end, the tip. The cavity narrows here and the septal and lateral walls are not split.
•True apex (17): the most distal point of the LV, a virtual slice that does not touch the cavity.

Basal, mid, apical and apical cap levels

📝 Why a "virtual" 17th segment? The original model had 16 segments; segment 17, the apical cap, was added to align segmentation with nuclear medicine.

✦ Key points

AHA model of 17 segments in three levels
Basal 1–6, mid 7–12 with papillary muscles, apical 13–16
Segment 17: true apex or apical cap

Quick question

At which level do the papillary muscles insert?

Quick question

How many segments does the apical level have?

AHA model

Segments from different axes

•The 17 segments can be seen from several imaging planes:
•Short axis (SAX): shows the segments in slices across the long axis of the heart.
•Long axes (2C, 3C, 4C): show longitudinal projections. Each view cuts a different combination of segments.
•This multiplanar correlation is key to interpreting cine, perfusion, parametric mapping and late enhancement findings correctly.

Segments seen in short axis and in the 2C, 4C and 3C long axes

💡 In the next lesson you will see which segments each long axis cuts.

✦ Key points

Segments seen in short axis and long axes
Each long axis cutting a different combination of segments
Multiplanar correlation key for cine, perfusion, mapping and late enhancement

Quick question

Which plane shows the segments in slices across the heart’s axis?

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Lesson complete

Right on first try

📌 Remember

•Heart as an inverted pyramid: inferior face on the diaphragm
•Oblique cardiac axes: long axis (2C, 3C, 4C) and perpendicular short axis
•4C, the only long axis shared by left and right sides
•Layer involved guides diagnosis: subendocardium infarction, subepicardium myocarditis, mid-wall non-ischaemic
•RV insertion points define 4 walls; septal and lateral split in two
•17 AHA segments: basal 1–6, mid 7–12, apical 13–16, apex 17

Next lesson: Left ventricular axes →Back to course

Content based on CMR Essential (J. Vega) and the SCMR standardised protocols (Kramer et al., JCMR 2020).