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Costal part of diaphragm

The costal part of the diaphragm consists of muscular fibers that originate from the inner surfaces of the lower six ribs and their costal cartilages. These fibers curve inward and upward, converging to insert into the central tendon of the diaphragm. Along with the sternal and lumbar parts, it forms the dome-shaped diaphragm, the primary muscle of respiration. The costal fibers are broad and powerful, providing the largest muscular contribution to the diaphragm and forming the lateral domes that separate the thoracic and abdominal cavities.

Synonyms

  • Muscular costal diaphragm

  • Costal diaphragmatic fibers

  • Lateral muscular diaphragm

Function

  • Provides the main contractile force for respiration

  • During contraction, it pulls downward to increase thoracic cavity volume and promote inspiration

  • Helps maintain intra-abdominal and intrathoracic pressure balance

  • Contributes to functions such as coughing, sneezing, vomiting, and defecation

  • Assists in protecting thoracoabdominal viscera by forming a muscular partition

Arterial Supply

  • Musculophrenic artery (branch of internal thoracic artery)

  • Lower posterior intercostal arteries

  • Contributions from the phrenic arteries

Venous Drainage

  • Drains into the musculophrenic and internal thoracic veins

  • Additional drainage into the intercostal veins and phrenic veins

Nerve Supply

  • Innervated by the phrenic nerve (C3–C5), which provides motor supply

  • Peripheral parts (including costal fibers) also receive sensory innervation from intercostal nerves (T5–T11) and subcostal nerve (T12)

MRI Appearance

T1-weighted images:

  • Costal diaphragm appears as a low signal intensity muscular structure lining the lower thoracic cage

  • Surrounded by hyperintense fat planes, aiding visualization

  • Useful for assessing diaphragmatic position, thickness, and muscular integrity

T2-weighted images:

  • Muscular fibers appear intermediate to low signal intensity

  • Pathology such as edema, tear, or denervation changes appear hyperintense

  • Fluid collections in pleural or peritoneal spaces appear bright hyperintense and contrast sharply with diaphragm

STIR (Short Tau Inversion Recovery):

  • Suppresses fat, allowing clear visualization of muscle edema, inflammation, or infiltrative disease

  • Injured or inflamed diaphragm appears bright hyperintense, normal fibers remain dark

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal costal diaphragm shows mild, uniform enhancement

  • Abnormal areas (e.g., tumors, inflammatory infiltration, or post-surgical changes) demonstrate focal or heterogeneous enhancement

CT Appearance

Non-contrast CT:

  • Appears as a thin muscular sheet arising from the inner lower ribs and arching toward the central tendon

  • Difficult to delineate without adjacent fat or air contrast

  • Hernias, ruptures, or eventrations are best seen on axial and coronal views

Contrast-enhanced CT (CECT):

  • Provides clear definition of the diaphragm against liver, spleen, and lungs

  • Useful for detecting diaphragmatic hernias, tumors, traumatic ruptures, or paralysis

  • Multiplanar reformats (MPR) highlight costal fiber origin and continuity with central tendon

MRI image

Costal part of diaphragm MRI axial image -img-00000-00000

CT image

Costal part of diaphragm ct axial image -img-00000-00000