Digestive disorders

I./3.5.: Transient Hepatic Attenuation Differences (CT) – THAD; Transient Hepatic Intensity Differencies (MRI) – THID

I./3.5.1.: THAD/THID-phenomenon, morphologic causes

There is a passing local (transient) enhancement in early arterial phase ont h couse of bolus-injecting dynamic CT/MRI examinations, which becomes invisible in portal venous phase (due to equilibration). This phenomenon is visible in quick (multislice, spiral-CT)/fast, dynamic MRI sequences with multiphase examinations and has a predictive value. Larger lesions, which are in background of the phenomenon, are also visible in portal phase.

Causes which are in the background of fast wash-out phenomenon can be classified morphologically as:

• - wedge-shaped, often peripheral, may extend to a whole liver segment or to one segment (e.g. portal vein thrombosis); or

• - technically circular, thus a form appearing in the periphery of the lesion which plays role in the background of this phenomenon; furthermore,

• - mosaic-like, i.e. diffuse liver lesion –such as e.g. cirrhosis, or right sided congestive heart failure developing due to an inhibited backfilling.

Please remember that liver has a dual blood supply: 75% of the whole blood amount arrives from the portal vein, and 25% arrives from the hepatic artery (in an earlier phase as the portal vein).

I./3.5.2.: Appearence of THAD/THID phenomenon in each phases of dynamic imaging

This phenomenon appears in dynamic imaging as the following: (i) in early hepatic arterial phase (early HAP) the whole coloration of hepatic artery is seen (only the branches of hepatic artery are visible, which are hyperdense on CT and have hyperintense signal in MRI). In this phase, contrast enhancement of the liver parenchyma is only minimal; (ii) later in the late hepatic arterial phase (late HAP) portal vein starts to fill and by this time slight liver parenchymal enhancement can be observed; (iii) in portal venous phase (PVP) portal vein fills totally, maximal enhancement of the liver parenchyma is seen. Finally (iv) in the equilibrium phase (EqP), contrast conditions equalize among arteries, veins and liver parenchyma.

I./3.5.3.: Physiological causes of THAD/THID phenomenon

There are different physiological causes of this phenomenon. (i) Changes of inflow may serve as triggers of THAD/THID phenomenon, such as e.g. decreased portal venous inflow, arterio-portal (venous) shunts, increased arterial flow and decreased arterial inflow. (ii) Hemodynamic changes caused by outflow changes may lead to this early arterial transient filling, such as e.g. decreased venous flow (decreased hepatic venous flow). (iii) Additional causes include e.g. anatomical variations etc.

I./3.5.4.: Decrease of portal venous flow

Causes of decrease of portal venous flow may be multiple, including portal vein obstruction due to various reasons as well as portal vein thrombosis. Important to note that pylephlebitis (infective suppurative thrombosis of the portal vein [v. portae]) must be recognised which leads to pylethrombosis (thrombosis of the portal vein [v. portae]) and infected thrombosis of intrahepatic small portal vein branches (septic thrombosis). Mind it in case of e.g. diverticulitis, appendicitis.

Cause of portal vein stenosis include vascular invasion of hepatocellular carcinoma (HCC), or extraordinary portal vein invasion by cholangiocellular carcinoma. These space occupying lesions in liver may lead to portal vein or smaller vein compression in liver hilum: tumor/metastasis, infection (abscess). Large subcapsular haematoma or fluid collection can compress the liver parenchyma. Additional cause of THAD/THID phenomenon include surgical ligation. Nevertheless, THID/THAD caused by biliar obstruction is rare.

I./3.5.5.: Arterio-portal (AV) shunt

Arterio-portal (venous) shunt (AV shunt) arises from e.g. circulation change due to liver cirrhosis, because tiny shunts develop based on a pseudolobar transformation in this case. Its appearence varies, might be mosaic-like, often peripherial. Iatrogenic shunt might occur e.g. after biopsy due to AV-fistula: branches of portal vein fill up in early phase in this case, which leads to enhancement. Among all tumors, HCC causes the early transient contrast filling most often. It is not rare that the tumour becomes visible only after several months of contrast filling phenomenon, thus THID/THAD-phenomenons may have even thoughtful predictive value. Accordingly, if an unknown local transient contrast filling is visible or its cause is unexplainable, follow-up of the patient is essential. Rapidly enhancing hemangioma or trauma can also cause arterioportal shunt.

I./3.5.6.: Increase of hepatic arterial flow

Causes of increase in hepatic arterial flow are usually local. Liver tumors are usually supplied by the hepatic artery. Tumors may be hyper- and hypovascular. Increased arterial flow due to tumors leads to early enhancement of the sorrounding liver parenchyma. Steal phenomenon may also appear. Increased flow pretension of focal/local inflammations (cholecystitis, abscess) may be an additional cause of early transient filling.

I./3.5.7.: Decrease of hepatic arterial flow

Most often iatrogenic cause of hepatic arterial inflow reduction is the surgical ligation, which usually leads to a larger ischemia involving the entire lobe or segment. Unwillful ligation of the vessel may occur on the course of surgical intervention of the region, e.g. during laparoscopic cholecystectomy. Thrombosis, embolism may also occur. Hepatic arterial flow reduction occurs rarely due to atherosclerosis or vasculitis as well.

I./3.5.8.: Decrease in hepatic venous flow

Causes of hepatic veinous flow reduction are different in their nature. Thrombosis of hepatic veins/inferior vena cava (Budd-Chiari-syndrome) leads to congestiv hepatopathy, hepatomegaly as well as to congestive right heart failure, severe tricuspidal regurgitation, pericardial effusion, or adhesive pericarditis (concretio pericardii). Postsinusiodal blood congestion occurs in the liver (in long-term: nutmeg liver).

Due to the congestion apparent in hepatic veins and sinusoids, the blood does not follow its normal route, „escapes”, thus it will be visible in arterial phase on the course of the dynamic imaging. There is no seeded, discrete, early contrast filling area in the heterogenously filled liver parenchyma, however, the entire liver looks „moth-holed” or „mosaic-like”. Important to recognise the postsinusoidal filling (hepatic veins, inferior vena cava itself) in this case in order to exclude their thrombosis/obstructions. Inhomogeneous filling may also occur due to Budd-Chiari-syndrome (thrombosis of hepatic vein branches).

I./3.5.9.: Other causes

Anatomic variations can also contribute to the phenomenon, including: aberrant arterial inflow, collateral vessels, obstruction caused by superior vena cava syndrome (mind the drainage towards the collaterals, e.g. periumbilical, oesophagus, splenic hilum, which help to understand the explanation of the phenomenon). Additional occurent anatomic variabilities e.g. accessory veins (capsular or aberrant) or right gastric vein. Further cause of THAD/THID can be the so-called „third-inflow tract” which develops due to the following causes: (1) aberrant venous outflow from the direction of the stomach leading towards the fourth liver segment and (2) drainage of the gallbladder towards the liver. Additional, undetected/unknown reasons can play a role as well, thus e.g. the phenomenon was noticed following radiation therapy, too.

I./3.5.10.: Conclusions

Important to know that focal liver lesions do not always cause THAD, and the cause of THAD phenomenon cannot be always discovered. Sometimes the cause related to the morphologic change becomes visible in the course of time only. The importance of the observation of THAD/THID phenomenon lies in that it can be the first sign of a morphologically yet not visible but later a lesion which becomes important (e.g. „premontory sign” of HCC). If we look for the cause of the observed phenomenon, a remarkable relationship can be discovered (e.g. septic thrombosis of a small vessel of portal vein caused by diverticulitis, or superior vena cava syndrome). Important to note that in case of evaluation of CT/MRI image generated by a dynamic imaging, THAD/THID phenomenon should not be measured together with the tumor e.g. if the tumor size is determinated (i.e. portal phase should be performed for the appropriate evaluation). Furthermore, the phenomenon per se should not be confused with the tumor.