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Phrenicomediastinal recess

The phrenicomediastinal recess is a narrow pleural space located at the junction of the mediastinal pleura and diaphragmatic pleura. It is found along the inferior margin of the mediastinum where the pericardium and diaphragm meet. Like other pleural recesses, it represents an area where pleural reflections form a potential space, allowing lung expansion during deep inspiration.

Although smaller and less clinically prominent than the costodiaphragmatic or costomediastinal recesses, the phrenicomediastinal recess has important surgical and radiological relevance. It can be a site of small pleural effusions, collections, or disease spread. It also serves as an anatomical landmark in cardiothoracic imaging and interventions.

Synonyms

  • Mediastinodiaphragmatic recess

  • Inferior mediastinal pleural recess

Function

  • Acts as a potential pleural space for lung expansion during deep inspiration

  • Serves as a reservoir for small pleural fluid collections

  • Provides an anatomical pathway for spread of pleural or mediastinal disease

  • Functions as a landmark in thoracic imaging and surgery

MRI Appearance

T1-weighted images:

  • Normally seen as a thin hypointense line between mediastinum and diaphragm

  • Small fluid collections appear intermediate to low signal

T2-weighted images:

  • Pleural fluid in the recess appears bright hyperintense, outlining the recess clearly

  • Useful for identifying effusions, cysts, or masses

STIR:

  • Fat suppression highlights recess fluid or inflammatory processes as bright hyperintense signal

  • Helps distinguish fluid from adjacent mediastinal or diaphragmatic fat

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal pleura shows thin rim enhancement

  • Abnormal thickening and heterogeneous enhancement suggest infection, pleural carcinomatosis, or mesothelioma

MRI Non-Contrast 3D Imaging:

  • Provides a volumetric map of the pleural space and recess extensions

  • Useful for surgical planning and evaluating pleural effusion distribution

CT Appearance

CT Pre-Contrast:

  • Normally appears as a slit-like potential space between mediastinal and diaphragmatic pleura

  • Fluid collections appear as soft-tissue attenuation pockets

CT Post-Contrast:

  • Pleural fluid remains non-enhancing, while enhancing pleura indicates infection, tumor, or inflammatory disease

  • Accentuates the recess in cases of effusion, pleural thickening, or masses

CT Venography (CTV):

  • Indirectly useful in cases where venous congestion leads to pleural effusion pooling in recesses

  • Provides multiplanar detail for surgical approaches

CT image

Phrenicomediastinal recess CT axial image anatomy image -img-00000-00000

MRI image

Phrenicomediastinal recess MRI axial image anatomy image -img-00000-00000