|OBGY RADIOLOGY: MRI GRAVID UTERUS|
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Conjoined Twins (Janiceps type)
Magnetic resonance imaging of a moving fetus was always considered difficult because of the longer scanning time. Ultrafast magnetic resonance imaging with faster gradients and new software enables rapid imaging of the fetus. This enables imaging quality superior to two-dimensional ultrasonography and should be considered an adjunct to ultrasonography for characterization of some fetal abnormalities (1). MRI has emerged as a useful imaging technique because of high resolution, ability to characterize tissues and inherent capability to generate information in three planes (2).
Conjoined (Siamese) twins are rare with an incidence of 1 in 30,000 to 1 in 1,00,000 births (3). The frequency is independent of maternal age or parity and it occurs sporadically. Conjoined twins result from monozygotic twinning when the embryo does not split until after the second week of gestation (4).
They are classified according to the location of the tissue that links the twins as craniopagus (joined at cranium), thoracopagus (joined at the thorax), omphalopagus (joined at the anterior abdominal wall), pyopagus (joined at buttocks and lower spine and lying back to back), and ischiopagus (joined at sacrum and coccyx and lying side to side). Early antenatal diagnosis and assessment of the rare entity of conjoined twins helps in determination of postnatal viability, which depends on the organs shared and whether surgical correction is possible. Many anatomical abnormalities are common among conjoined twins. Congenital heart disease ( most commonly a ventricular septal defect or Tetralogy of Fallot ) is present in 75% of thoracopagus twins. The liver is shared in 81% of omphalopagus twins. In craniopagus twins, the prognosis depends on the degree of connection between the brains (5).
Janiceps type of cephalo-thoraco-abdominopagus twins is even more rare. Its incidence is 1 in 3 million conjoined twins (6). It derives its name from Janus, the god of doors and gateways, his statue with two faces, facing east and west for the beginning and ending of the day (6,7). Survival of conjoined twins depends on the presence or absence of associated anomalies. The surgical challenges include separation of the important shared organs and closure of defects in bones and soft tissues (2).
Complex congenital anomalies are optimally evaluated with sonography and magnetic resonance imaging. With the advent of ultrafast imaging sequences, magnetic resonance imaging of the fetus is now easier. It allows demonstration of anatomical details in a three dimensional perspective. Magnetic resonance imaging is considered to be superior to other modalities in illustrating neurosurgical abnormalities in craniopagus twins (8). Magnetic resonance imaging simplifies the work-up of conjoined twins, without the risks of ionizing radiation or injection of radio-opaque contrast. To assist in planning the separation, a three-dimensional computer model of the conjoined twins can be constructed on the basis of the prenatal magnetic resonance imaging studies, as in our case. This helps the surgeons and the parents to have a better visualization of the anomaly. Because of the ability of magnetic resonance imaging to enhance the visualization of abnormal anatomy, the surgeon can deal with greater confidence and can plan potentially safer operations for all types of conjoined twins (6).
Magnetic resonance imaging can be conveniently performed in cases of conjoined twins. It complements and enhances the diagnosis made on ultrasonography. High resolution whole body images are useful in counseling the parents on the degree of abnormalities and the multiplanar images are well understood even by the non-radiologists. The accurate identification of the extent of fusion helps rational decision making and planning for the surgery, if possible.