Prenatal Imaging of Situs Anomalies: Utility of Fetal MRIPreeyacha Pacharn, MD; Leann Linam, MD; Maria Calvo Garcia, MD; Eva Rubio, MD; Beth Kline-Fath, MD Department of Radiology, Cincinnati Children’s Hospital Medical Center

IntroductionSitus anomalies are a diagnostic challenge because of the complex spectrum of findings. Prenatal ultrasound can support the diagnosis. However, fetal MRI can provide additional information to allow improved classification of this anomaly. Since prognosis is variable depending on associated malformations, knowledge of the entire spectrum is important for prenatal and postnatal management and counseling.

Discussion:The word “situs” means position. In the context of human anatomy, it refers to the position of internal organs in the human body.

Situs Solitus is the normal arrangement of organs and vessels in the body. The systemic atrium is on the right with a right-sided trilobed lung, liver, gallbladder and inferior vena cava. The pulmonary atrium is on the left with a left-sided bilobed lung, stomach, single spleen and aorta. The cardiac apex is on the left. The incidence of congenital heart disease in patients with situs solitus and levocardia is low, approximately 0.6-0.8%. (1)

Situs Inversus is the mirror image of situs solitus. The incidence of congenital heart disease in situs inversus is increased to 3-5%. (1)

Situs Ambiguous or Heterotaxy refers to visceral malposition and dysmorphism where the internal thoraco-abdominal organs demonstrate abnormal arrangement across the left-right axis of the body. These conditions have long been classified as right isomerism (asplenia syndrome) and left isomerism (polysplenia syndrome). The first description of asplenia associated with abnormal situs was reported by Ivemark in 1995. (2)

Prenatal assessment of the left and right sides of the fetus can sometimes be confusing. Several parameters should be considered including maternal position and fetal position. The display of the images on the PACS workstation is also important.

When heterotaxy is suspected, a careful search for associated anomalies should be performed. In particular, the position of the venous drainage under the diaphragm, presence of bowel loop malrotation and dilatation, and the anatomy of the gallbladder and spleen should be defined. Cardiac anomalies are also a primary concern but are best delineated by fetal echocardiography.

Heterotaxy Syndrome With AspleniaThe classic asplenia is also known as right isomerism with bilateral trilobed lungs, bilateral systemic atria, a centrally located liver and a stomach in an indeterminate position. The abdominal aorta and inferior vena cava are usually on the same side of the spine. These patients usually have severe congenital heart lesions including atrioventricular canal, single ventricle, pulmonary stenosis and atresia, transposition of the great arteries and anomalous pulmonary venous collection. (3)

Heterotaxy Syndrome With PolyspleniaThe classic left isomerism included bilateral bilobed lungs, bilateral pulmonary atria, centrally located liver, intermediate position of the stomach, multiple spleens and azygous continuation. Cardiac anomalies are usually less severe in polysplenia and include ventricular septal defect, double outlet right ventricle, left-sided obstructive lesions and anomalies of systemic venous return. (3)

Intestinal Rotation AbnormalitiesMalrotation of the bowel is a frequent finding in patients with heterotaxy syndrome. This entity is important to recognize because it can result in midgut volvulus. Ditchfield et al. noted that 19 of 27 children (70%) had malrotation and recommended upper GI contrast study in all heterotaxy patients. (4)

Fetal MRIBecause of the overlapping findings in heterotaxy, it has been suggested that a segmental approach should be utilized in order to describe the anatomy. Due to the complexity of the anomaly, this can be difficult with only a prenatal ultrasound. Fetal MRI, due to high soft tissue contrast, provides excellent organ anatomy, and therefore improves the classification of the heterotaxy. Fetal MRI is helpful in confirming presence or absence of spleen. With the large field of view, fetal MRI also provides improved venous anatomy. Although upper GI study is the best diagnostic tool for malrotation/midgut volvulus, findings of malrotation can be suggested by fetal MRI. Meconium demonstrates high T1 signal because of proteinaceous content and should be present in the rectum at approximately 20 weeks of gestation. (5) The possibility of malrotation can be suggested if the meconium-filled colon is seen in the left side of the abdomen. In this illustration, we will demonstrate our experience with fetal MRI in the diagnosis of heterotaxy syndromes.

ConclusionHeterotaxy syndromes are diangostically challenging due to complex spectrum of findings. A segmental approach aids in detailed anatomical evaluation of such patients, including evaluation of bowel position, presence or absence of the spleen and anomalous venous drainage. The spleen, vascular structures and bowel position are better delineated with fetal MRI than ultrasound due to a larger field of view and improved contrast resolution, as illustrated in our case series. Prenatal MRI, used in conjunction with ultrasound, is a valuable tool in the evaluation of the fetus with a suspected heterotaxy syndrome. With this information, appropriate management and counseling can be performed.

References1. Applegate KE, Goske MJ, Pierce G, et al. (1999) Situs revisited: imaging of the heterotaxy syndrome. Radiographics 19:837-852; discussion 853-834.

2. Ivemark B (1955) Implications of agenesis of the spleen on the pathogenesis of conotrunchus anomalies in Childhood:an analysis of the heart malformations in splenic agenesis syndrome, with fourteen new cases. Acta Pediatr 44:7-110.

3. Tonkin I, Tinkin A (1982) Visceral Situs Abnormalities: Sonographic and Computed Tomographic Appearance. AJR 138:509-515.

4. Ditchfield MH, JM. (1998) Intestinal rotational abnormalities in polysplenia and asplenia syndromes. Pediatr Radiol 28:303-306.

5. Zizka J EP, Hodik K (2006) Liver, meconium, haemorrhage : The value of T1-weighted images in fetal MRI. Pediatr Radiol 36:792-801.

Case 2Gestational age 28 weeks, referred to prenatal MRI because situs inversus was suspected from an outside ultrasound. Fetal MRI demonstrated left-sided cardiac apex, midline liver, right- sided stomach, small spleen, presence of the gallbladder, left-sided aortic arch and possible malrotation of bowel loops. Postnatal imaging confirmed the presence of spleen, position of the great vessels and small bowel loops in the right side of the abdomen consistent with malrotation.

Fig 2a

Fig 2h1 Fig 2h2 Fig 2h3

Fig 2b Fig 2c Fig 2d

Coronal FIESTA image of the fetus demonstrated midline liver, right-sided stomach. The cardiac apex is on the left side.

Coronal SSFSE showed gallbladder on the right side of the fetus (arrow).

Coronal SSFSE demonstrated the right-sided stomach and the left-sided thoracic aorta (arrow).

Coronal FIESTA demonstrated 2 hepatic veins (arrows) draining seperately into the heart

Fig 2e Fig 2f Fig 2g

Axial SSFSE through the body show a small spleen (arrow) posterior to the stomach (s)

Coronal T1 demonstrated colon in the left side of the abdomen, filled with meconium, which is bright on T1W sequence.

Plain radiograph of the chest and abdomen after the baby was born demonstrated left-sided cardiac apex and midline liver.

Ultrasound of the abdomen demonstrated midline liver and 2 small spleens in the right-sided abdomen (arrows). There are 2 hepatic veins (arrowheads).

Fig 2i Fig 2j1 Fig 2j2 Fig 2j3

Upper GI study showed stomach and duodenal sweep on the right side of the body.

Coronal reformatted CT images demonstrated midline liver, 2 spleens (arrowheads),left-sided aorta (AO) and left-sided IVC (arrow) and 2 hepatic veins (white arrows).

Case 3Gestational age 36 weeks, fetus diagnosed by prenatal ultrasound with situs inversus. Fetal echocardiogram suggested hypoplastic transverse arch at our institution. Ultrasound demonstrated spleen and stomach on the right side of the body. Fetal MRI show left-sided cardiac apex, midline liver, right sided stomach, multiple spleens, presence of the gallbladder, left-sided aorta, left-sided IVC, small bowel on the right and colon on the left, suggesting malrotation. Postnatal imaging showed midline liver and multiple spleens, consistent with polysplenia. Small bowels was abnormally rotated in the right side of the abdomen.

Fig 3a1

Fig 3a2

Fig 3b Fig 3c

Two transverse ultrasound images demonstrated stomach and spleen in the right side of the abdomen, concerning for situs inversus.

Coronal SSFSE demonstrated left-sided cardiac apex, midline liver and right-sided stomach.

Coronal FIESTA demonstrates small bowel loops on the right and colon on the left.

Fig 3d

Fig 3e Fig 3f1 Fig 3f2

Axial FIESTA demonstrated left sided aorta and IVC. Multiple soft tissue nodules, consistant with polysplenia (arrows)

Plain radiograph demonstrated a left-sided cardiac apex, midline liver. The feeding tube tip was seen in the right-sided stomach.

Upper GI study revealed right-sided stomach. The small bowel is present in the right side of the abdomen as seen on the fetal MRI.

Fig 3g1

Fig 3g2

Ultrasound of the upper abdomen demonstrated midline liver and presence of multiple spleens (arrows).

Case 4Gestational age is 32 weeks with suspected heterotaxy and abnormal kidneys. Fetal MRI demonstrated right-sided cardiac apex, midline liver, right-sided stomach, left-sided aorta. The spleen and stomach were not visualized. Both kidneys are abnormal with multiple cysts on the left and hydronephrosis on the right side. Postnatal imaging showed bilateral renal disease

Fig 4a Fig 4b Fig 4c Fig 4d

Coronal SSFSE demonstrated midline liver. The gallbladder was not seen.

Coronal SSFSE demonstrated aorta descending on the left side of the spine (arrow).

Axial SSFSE through the kidneys showed multicystic dysplastic left kidney (LK) . There is moderate right hydronephrosis (RK)

Case 1Gestational age 33 weeks , fetus with congenital heart disease and suspected heterotaxy syndrome. Ultrasound revealed small right-sided stomach, and gallbladder. Fetal MRI demonstrated midline liver, presence of the gallbladder on the right, right-sided aorta, right-sided stomach, no spleen and massive cariomegaly. Cardiac apex is on the left side.

Fig 1a1 Fig 1a2 Fig 1b Fig 1c

Ultrasound of the fetus through the abdomen. The stomach is small. The gallbladder is seen on the same side of the stomach. Longitudinal scan through the fetal chest and abdomen demonstrated cardiac chamber, (HT), stomach (ST) and urinary bladder (BL). The spleen was poorly visualized. The details of the bowel and vessels in the abdomen are not clearly seen.

Coronal SSFSE demonstrated cardiomegaly and midline liver.

Coronal FIESTA showed right-sided gallbladder (arrow).

Fig 1d Fig 1e Fig 1f

Coronal SSFSE demonstrated right sided aorta and left-sized IVCs.

Axial SSFSE revealed small, right-sided stomach (S) concerning for microgastria. No spleen is seen.

Coronal T1 showed high T1 signal meconium in the colon, mainly in the left side of the abdomen, concerning for. malrotation

Fig 4e

Fig 4f1Fig 4f2

Fig 4f3 Fig 4f4

Plain radiograph demonstrated right-sided cardiac apex, liver is midline. The stomach bubble is on the right.

Ultrasound abdomen showed midline liver and atretic gallbladder (arrow). The patient also had hyperbilirubinemia and liver biopsy confirmed the pathology of biliary atresia. Both kidneys were abnormal with multicystic dysplastic left kidney and moderate right hydronephrosis as seen on prenatal MR.

Case 5Gestational age 24 weeks, fetus with dextrocardia, right-sided stomach and right-sided gallbladder on prenatal ultrasound. Fetal MRI demonstrated left-sided cardiac apex, midline liver, right sided stomach, presence of the gallbladder, left-sided aorta and no spleen. Postnatal imaging confirmed the absence of the spleen consistent with asplenia.

Fig 5a1

Fig 5a2

Fig 5b Fig 5c

Longitudinal and transverse ultrasound images demonstrated stomach on the right side.

Coronal SSFSE demonstrated left-sided cardiac apex and midline liver.

Coronal SSFSE showed right-sided stomach.

Fig 5d

Fig 5e Fig 5f

Coronal SSFSE demonstrated aorta descending on the left side of the spine (arrow).

Axial FIESTA demonstrated gallbladder in the right side of the abdomen. The IVC was seen slightly anterior to the aorta.

Axial SSFSE showed no splenic tissue posterior to the stomach.

Fig 5i

Tc99m pertechnitate confirmed no splenic uptake

Fig 5g

Fig 2iPlain radiograph at day 1 of life demonstrated midline liver. Cardiac apex is on the left side.

Ultrasound abdomen demonstrated midline liver and a small gallbladder on the right side (arrow). The spleen was not seen.

Fig 5h

Coronal SSFSE demonstrated right-sided cardiac apex,right-sided stomach, hydronephrotic right kidney and a multicystic left kidney

S

AO IVCS

IVC

AO

RK LK

IVC

AO

AO