Liver transplantation

Transplantation has become an established treatment for many hepatic conditions and indeed is often the last resort for patient survival. The number of transplantations is increasing as improved surgical techniques, immunosuppression, and postoperative care have increased success rates [1, 2]. The common types of liver transplant are discussed in Table 1. With the increasing number of liver transplantations have come new challenges for the radiologist in terms of pre- and posttransplant imaging. Preoperative imaging is often done to evaluate the potential donor to exclude any significant pathology and to obtain anatomical information for operative planning. Following transplantation imaging is mainly performed to evaluate complications, which may be acute or chronic [3].

Table 1 Types of liver transplant
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The principles of pre- and posttransplant imaging of the liver in children are outlined here along with posttransplantation lymphoproliferative disorder (PTLD). Rejection of the transplanted organ or tissue occurs when the recipient’s immune system attacks it and causes damage to the organ; this can have systemic effects. Although transplant rejection is one of the more common complications, it does not have specific radiological features [4].

Pretransplant imaging

Pretransplant imaging is often performed to evaluate the donor as well as recipient [5]. Live donor organ imaging may be performed to evaluate hepatic size and vasculature ensuring suitability for transplant. Length, caliber and anatomic variations of both vessels and the biliary system are important for surgical planning. The usual modalities used include US, CT and MR.

Common indications for liver transplantation in children include biliary diseases, such as biliary atresia, sclerosing cholangitis, cystic fibrosis, primary biliary cirrhosis; along with metabolic diseases, such as alpha-1 antitrypsin deficiency, glycogen storage disease, Wilson disease, hemochromatosis; and cirrhosis from any cause.

Presently used grafts include pediatric cadaveric whole-organ graft, segmental or split adult cadaveric grafts, and living related adult segmental graft (segments II and III or II–IV). Use of split cadaveric graft and living donor segmental graft has increased the donor pool. Usually the donor hepatic artery, portal vein, suprahepatic IVC and infraheaptic IVC are anastomosed end-to-end to the recipient respective vessels. If the vascular pedicle is short, especially in split and segmental grafts, autologous iliac artery conduit or donor conduit from the infrarenal aorta can be used. However, this increases the risk of vascular complications. When recipient vena cava is kept as it is and donor hepatic vein is attached end-to-side to it, the technique is called ‘piggyback’ technique. Biliary anastomosis is typically performed in an end-to-end fashion except in biliary atresia where hepatojejunostomy is performed.

Posttransplant imaging

Vascular complications usually occur in the early postoperative period (Table 2). As the hepatic artery is the sole supply to the biliary epithelium of transplanted liver its patency is vital for graft survival. Caliber difference between donor and recipient vessels may be normally seen. Hemodynamically significant stenosis is diagnosed when a three-to fourfold increase in velocity is seen at the site of narrowing (Figs. 1, 2 and 3). Intrahepatic parvus tardus pattern may be normally seen in the first 72 h after transplant because of edema at the anastomotic site; hence, this finding should be interpreted with caution.

Table 2 Vascular complications of liver transplant [10–20]
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Fig. 1
figure 1

Doppler images (a–c) in 8-yrs-old child imaged on day 5 after orthotopic liver transplant show aliasing suggestive of turbulent high flow in the region of anastomosis of the hepatic artery (arrow on a). Spectral waveform shows high velocity of 4.4 m/s at the anastomosis (b)

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Fig. 2
figure 2

a Portal vein in a child with segmental liver transplant shows caliber difference in recipient (arrows) and donor (arrowheads) portal veins. b Percutaneous portal venogram in another child performed 6 years after liver transplant for biliary atresia shows stricture of the portal vein at the porta (arrow)

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Fig. 3
figure 3

Doppler image in 10- yr-old child with segmental liver transplant show high velocity up to 2.7 m/s at the anastomosis of hepatic vein and IVC suggestive of anastomotic narrowing, confirmed on subsequent percutaneous transhepatic venography

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Biliary complications are the most common complications seen after pediatric liver transplant (in up to 27% of cases) and the majority of them occur in first 3 months following surgery (Fig. 4) [6–9]. They include anastomotic leakage and stenosis with proximal dilatation, bile duct stones, sludge, bilioma and rare mucocele of cystic duct remnant. Bile duct leak or stricture can lead to cholangitis, sepsis and abscess. Nonanastomotic strictures are probably related to hepatic arterial insufficiency. Posttransplantation biliary leak should prompt a search for hepatic artery thrombosis [21].

Fig. 4
figure 4

Bile leak with biloma in a 2 month-old-baby with segmental liver transplant for biliary atresia. Ultrasound showed marked dilatation of intrahepatic bile duct and a complex collection on inferior aspect of the liver (a). Percutaneous transhepatic cholangiogram confirmed the bile leak communicating with the collection compatible with bilioma (b)

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Liver parenchymal abnormalities such as infarction, biliomas or abscess can complicate transplantation. Infarction can be seen as a round or geographical hypoechoic solid lesion on ultrasound. It is seen as an irregular, peripherally located wedge-shaped hypoattenuating lesion on CT images. A periportal area of low echogenicity on US and hypodensity on CT, called ‘periportal collar sign’, can be seen normally and is thought to result from dilatation of lymphatic channels. It usually resolves within few weeks. Organ rejection is a common complication; however, it does not have specific imaging features.

Other complications include localized collections, extrahepatic biliomas and post transplant lymphoproliferative disorder (PTLD).

Posttransplantation lymphoproliferative disorder

PTLD is a spectrum of unregulated lymphoid expansion that can occur in the transplant patients and range from polyclonal hyperplasia to monoclonal malignant lymphoma [10, 22–27]. It is related to chronic immunosuppression and in most cases, results from Epstein-Barr virus (EBV)-induced B cell lymphoproliferation. Overall the frequency of PTLD is around 2% but is seen with higher frequency (approximately 8%) in children. Three major risk factors include allograft type, EBV infection or reactivation and intense immunosuppressive regimens. About 85% of PTLD cases are of B cell origin and contain EBV. Most of the PTLD is seen in the first year after transplantation and presents with variable clinical manifestations. PTLD can involve any organ system including the allograft. In descending order of frequency it involves the abdomen, chest, head and neck, and brain. Histologically, three forms are seen: hyperplastic (early lesion), polymorphic and monomorphic (lymphomas). Polymorphic form has better prognosis and is more likely to respond to reduction in immunosuppressive therapy.

CT is the main modality for evaluation of PTLD in terms of presence, extent and biopsy guidance. Hypodense nodular masses or diffuse infiltration and enlargement can be seen in the liver, spleen and kidneys. Circumferential wall thickening, dilatation, ulceration and intussusception can be seen in the bowel with small bowel being most frequently involved. Abdominal lymphadenopathy, omental and mesenteric involvement are other manifestations. PTLD in the chest can manifest as discrete nodular masses, air-space consolidation that do not respond to antibiotics therapy and mediastenal lymphadenopathy. Diffuse enlargement of the pharyngeal and palatine tonsils and cervical lymphadenopathy is seen in head and neck PTLD. Sinonasal involvement by PTLD cannot be distinguished from an infective process radiologically. Solitary brain lesion is the most frequent manifestation of brain PTLD [28].

The radiologist plays a key role in evaluation of acute and chronic complications of pediatric liver transplantation. As these patients live longer, we must be vigilant in assessing for other long-term complications including pulmonary hypertension [29].