CT Quadrate Lobe Hot Spot Sign.

A 14-year-old girl visited our hospital with pyrexia of unknown origin. In view of suspicion of tuberculosis, various haematological and radiological investigations were done. Further examinations revealed titres positive for lupus anticoagulant syndrome. Contrast-enhanced computed tomography (CT) of the chest and abdomen was also advised for further evaluation, which revealed type II superior vena cava obstruction. Contrast-enhanced CT of the abdomen revealed an interesting observation in the left lobe of the liver, which is known as the hot spot sign.

‘CT quadrate lobe hot spot sign’ was first described by Ishikawa in 1983. It manifests as an area of intense focal wedge-shaped enhancement of the quadrate lobe (functionally a part of the left lobe of the liver and designated segment IVb in the Bismuth-Couinaud classification system) of the liver in the arterial and venous phase (figure 1).[1] This hot spot sign was initially observed on 99mTc sulphur colloid scan of the liver as a focal area of increased radiopharmaceutical uptake and was diagnostic of superior vena cava syndrome.[2] Hot spot sign is caused by portosystemic venous shunting between the superior vena cava and the left portal vein via the internal mammary and paraumbilical veins along the ligamentum teres, secondary to superior vena cava obstruction.

Fig.1

(A) Axial contrast-enhanced tomogram of thorax showing filling defect in SVC (white arrow) suggestive of partial SVC obstruction with the reflux of contrast into the azygous vein (black arrow) and prominent internal mammary veins (arrow heads) and chest wall collaterals (B) Axial tomogram of the liver showing the hot spot sign (arrow) as peripheral wedge shaped abnormal increased enhancement in segment IV.

Stanford and colleagues classified SVC syndrome into four types - type I and type II referred as supra-azygous partial and near complete obstruction of SVC with antegrade flow in azygous vein. Type III is defined as complete obstruction of SVC with reversal of azygous blood flow and type IV as complete obstruction of SVC and azygous system with development of chest wall collaterals.[3] Two major collateral pathways are visualized on CT in SVC syndrome. One collateral pathway is the SVC–superficial epigastric vein– umbilical vein–left portal vein route. Another collateral pathway is the SVC–internal mammary vein–inferior phrenic vein-subcapsular vein– portal vein route.[3,4]

The importance of this sign is that it provides a clue to the diagnosis of thoracic SVC obstruction when enhanced CT of the abdomen is performed in clinically unapparent obstruction. The characteristic location in the quadrate lobe of the liver, and wedge shape enhancement in arterial and venous phase are useful in differentiating quadrate hot spot from focal hypervascular liver lesion.

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