Real-time MRI catches beating fetal heart

Researchers from The Children's Hospital of Philadelphia have performed real-time functional cardiac MRI in fetuses. Theirs is the first report of this technique, which may represent an advance over the current gold standard of fetal echocardiography.

The literature supports the success of fetal echocardiography to diagnose problems and monitor therapy. But the technique is not a true 3D imaging modality, which limits assessment of ventricular volumes and mass, and it can be constrained by acoustic windows, according to the study published in the September/October issue of Fetal Diagnosis and Therapy.

Fetal echocardiography relies on geometric assumptions to determine ventricular volume. This approach can be problematic with odd-shaped hearts found in congenital heart disease patients, said lead investigator Dr. Mark A. Fogel, director of cardiac MRI at CHOP.

"MRI produces 3D images and can directly measure ventricular volume," he said.

While fetal MRI of extracardiac structures has gained ground, cardiac triggering difficulty and fetal motion artifacts have kept functional fetal cardiac MRI from being performed. Recent advances in functional MRI, however, have made this pursuit easier.

Fogel and colleagues credit three factors for their success in using real-time TrueFISP cine MRI to evaluate the hearts of two fetuses:

• Sedation (of one fetus) was delivered transplacentally to minimize fetal movement.

• Real-time TrueFISP imaging is a single-shot technique (entire image is obtained in less than one heartbeat) in which the images are created continuously. The temporal resolution can be as great as 75 msec. Since fetal imaging requires a small field-of-view, researchers needed to compromise temporal resolution (126 msec).

• TrueFISP imaging has a high signal-to-noise ratio.

Using axial T2-weighted HASTE images as localizers, researchers performed real-time TrueFISP cine MRI on the heart and on the ductus arteriosus and the descending aorta of the two fetuses (35 and 32 gestational weeks). They validated the MRI volume measures with echocardiography and cardiac index by Doppler.

Cardiac MRI was able to visualize the beating heart and assess ventricular volumes. Cardiac index and assessment of right ventricular hypertrophy and dilation by echocardiography were consistent with the ventricular volumes and right ventricular hypertrophy assessment obtained by cardiac MRI, according to the study.

Cardiologists had serially followed one fetus with a left diaphragmatic hernia and cardiac compression for 17 weeks. Fetal echocardiography could not differentiate between true extracardiac compression of a normal heart and hypoplastic left heart syndrome.

The distinction is important as it influences medical management. Extracardiac compression of a normal heart would necessitate ex utero treatment. A hypoplastic diagnosis would result in supportive care.

With data from both modalities, physicians diagnosed the fetus as having hypoplastic left heart syndrome. The baby was born two days later and died within 45 minutes.

The second fetus had ductus arteriosus constriction and right ventricular dilation and hypertrophy. Measurement of the ductus arteriosus and descending aorta by MRI (5 mm) was within 1 mm of the value obtained by fetal echocardiography (6 mm), the researchers said.

Postnatal echocardiography on the ?rst day of life demonstrated mild tricuspid regurgitation, mild right ventricular dysfunction, marked right ventricular hypertrophy, and a small patent ductus arteriosus.

Fetal echocardiography yielded a cardiac index of 153 cm³/kg/min. MRI yielded a cardiac index of 127 cm³/kg/min. MRI functional data (cardiac output, derived from ventricular volume measurements) came within 15% of the value calculated by Doppler echocardiography in the great arteries. The lower MRI cardiac index could be due in part to the sedation of one fetus during MRI but not at echocardiography.

The fetus with ductal constriction was not sedated and thus the right ventricular volumes could not be calculated reliably. The researchers pointed out, however, that this imaging proved that functional fetal cardiac MRI can be performed in the unsedated fetus and fetal movement is not an impediment to cardiac MRI.

They concluded that several congenital heart diseases would bene?t from understanding ventricular volumes and mass in utero. They also said that both temporal and spatial resolution will only improve with time, making this technique even more clinically valuable.

For more information from the Diagnostic Imaging archives:

MR provides safe and effective fetal imaging with appropriate use

MR imaging contrast agents and pregnant patients

Fast imaging techniques boost viability of MR fetography

Real-time 3D shows more than just pretty faces