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Lesson 1 of 13
Cardiac topography
How the heart sits in the chest, what the cardiac axes are, and how the LV walls and 17 segments are named.
Lesson contents
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).
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- Thinking of the heart as a pyramid gives a fixed map for any image: the face resting on the diaphragm is always inferior, and the one facing the sternum is anterior, even on an oblique slice.
- The base is the posterior face, towards the pulmonary veins and oesophagus. The tip of the pyramid, the apex, lies at the opposite end, forwards and downwards.
- Because this pyramid is tilted within the chest, the strict axial, coronal and sagittal planes of the localisers cut it diagonally. That is why CMR then uses the heart’s own axes.
Sources: CMR Essential (J. Vega) · Imagen Cardíaca Esencial ICE course 2024 (J. Vega) · Kramer et al., JCMR 2020
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.
Tap each level to see its slice.
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- The AHA divides the LV into equal thirds along its long axis. The papillary muscles set the limits: the basal third reaches their tips, the mid third contains their full length and the apical third continues until the cavity ends.
- Only slices where myocardium surrounds the whole cavity (360°) count. So at the base, the valve plane and the outflow tract are not segmented, even if they appear in the study.
- The levels are used in practice: first-pass perfusion acquires at least three short-axis slices per heartbeat, typically one basal, one mid and one apical.
Sources: Cerqueira et al., Circulation 2002 · Kramer et al., JCMR 2020
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°.
4CSAX2C3C
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- All axes are referenced to the interventricular septum: long axes run parallel to it and the short axis crosses it at 90°. Each view then shows the same section of the heart in any patient, however it is tilted.
- The short axis is the most important cine for measurement: LVEF, volumes, wall thickness and wall motion. That is why it is built on two orthogonal long axes, ideally all three (2C, 4C and 3C).
- In the SCMR guideline, the short axis is planned on the 4C and 2C, parallel to the mitral valve plane, and the stack runs through the LV from base to apex.
Sources: Imagen Cardíaca Esencial ICE course 2024 (J. Vega) · Kramer et al., JCMR 2020
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.
Right 3C
Left 3C
Right 2C
Left 2C
Right OT
Left OT
Tap each axis to see the right and the left one.
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- All long axes follow the same rule: they run parallel to the interventricular septum. The 4C cuts both ventricles and both atria at once, which is why it is the only one serving both sides.
- Right-sided axes are harder: the RV is a thinner, less familiar structure. They are learnt after the left-sided ones, which also serve as references to plan them.
- Each side has its outflow tract: the LV’s leads to the aorta and the RVOT to the pulmonary valve. For the RV, the SCMR guideline asks for a long axis aligned with tricuspid inflow and an RVOT view.
Sources: CMR Essential (J. Vega) · Imagen Cardíaca Esencial ICE course 2024 (J. Vega) · Kramer et al., JCMR 2020
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.
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- Ischaemia advances like a wavefront: necrosis starts in the subendocardium and progresses towards the epicardium. So every infarct involves the subendocardium: involving up to 50 % of wall thickness it is subendocardial; more than 50 %, transmural.
- The subendocardium is the layer most sensitive to ischaemia and contributes most to systolic thickening: over 50 %.
- In non-ischaemic damage, layer and location guide you: subepicardial in myocarditis (also in Anderson-Fabry) and basal septal mid-wall in idiopathic dilated cardiomyopathy. These are suggestive patterns with variable yield.
Sources: CMR Essential (J. Vega)
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.
📝 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).
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- The insertion points are an anatomical landmark visible on every basal and mid slice, in any patient. That is why the AHA uses them to separate the septum from the anterior and inferior walls.
- At basal and mid level, the LV ring is split into 6 segments of 60° each, so every segment covers a comparable share of the wall.
- The insertion points also matter clinically: the RV pulling on the LV concentrates shear forces there, and pressure or volume overload can cause focal enhancement at these points.
Sources: Cerqueira et al., Circulation 2002 · CMR Essential (J. Vega)
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.
📝 Why a "virtual" 17th segment? The original model had 16 segments; segment 17, the apical cap, was added to align segmentation with nuclear medicine.
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- Towards the tip the LV diameter decreases, so the apical level is split into 4 segments of 90° rather than 6 of 60°.
- Numbering starts at the anterior wall and moves towards the septum: 1 anterior, 2 anteroseptal, 3 inferoseptal, 4 inferior, 5 inferolateral and 6 anterolateral. The mid level repeats the order, 7 to 12.
- At apical level: 13 anterior, 14 septal, 15 inferior and 16 lateral. The anterior and inferior walls run from base to tip; the septal and lateral walls are split only at basal and mid level.
Sources: Cerqueira et al., Circulation 2002 · Imagen Cardíaca Esencial ICE course 2024 (J. Vega)
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.
💡 In the next lesson you will see which segments each long axis cuts.
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- The short axis shows each segment within its ring; the long axes show it from base to apex. The SCMR guideline asks to review all cines and validate what is seen in one plane with the others, ruling out artefacts.
- That is why late enhancement is acquired in the same short and long axes as the cines, with the same slice thickness: each segment can then be compared directly across sequences.
- The 17-segment model is the common language: the guideline asks to locate perfusion defects and late enhancement with it, and to name segments by coronary territory when wall motion is abnormal.
Sources: Schulz-Menger et al., JCMR 2020 · Kramer et al., JCMR 2020
Quick question
Which plane shows the segments in slices across the heart’s axis?
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Lesson complete
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📌 Remember from this lesson
- •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
Content based on CMR Essential (J. Vega) and the SCMR standardised protocols (Kramer et al., JCMR 2020).