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MRI of fetal heart and vessels using balanced sequences

Best Practice
Prof. Saleem, Sahar, MBBCh, MSc, MD Kasr Al-Ainy-Cairo University Egypt

MR imaging of the cardiovascular system has not been thoroughly investigated in-utero because of the technical difficulties caused by fetal cardiac motion; fetal body motion; maternal inevitable motion such as peristalsis and respiration; and blood flow effects.

 

In the most widely used sequence in fetal MRI, single-shot turbo spin echo (SSh TSE), the cardiovascular system appears flow void with limited structural details. Moreover, the SSh TSE sequence has potential problems in degradation of spatial resolution.

 

As a consequence, there remains room for other sequences. In Cairo University, we use a steady-state free precession sequence (B-FFE or B-TFE) for visualization of the fetal cardiovascular system. B-FFE uses balanced-gradient waveforms which act on any stationary spins on resonance between two consecutive RF pulses and return them to the same phase they had before the gradients were applied. As balanced gradients maintain the longitudinal and transverse magnetizations, both T1 and T2 contrasts are represented in the resultant image. B-FFE produces images with increased signal from fluid (like T2- weighted sequences) as well as retaining T1-weighted tissue contrast.

 

With B-FFE the vessels and blood pool in-utero are visualized as hyperintense structures, the cardiac wall is visualized as hypointense structure - without fetal sedation or paralysis. Compared to SSh TSE sequences this leads to much better delineation of cardiac structures, as illustrated in figure 1.

 

 A B
A
B

Figure 1. Axial MR images of fetal thorax at 24 weeks of

gestation using SSh/TSE (A) and B-TFE (B). B-TFE is superb

in visualizing cardiac morphology, interventricular septum

(arrowhead) and thoracic aorta (arrows). Cardiac size and axis

can be determined more accurately in axial b-TFE.

 

 

Controlled maternal apnea was useful in decreasing motion artifacts in some cases. However, B-FFE was acquired during free maternal breathing in most cases. A spatial resolution of 1.36 x 1.17 x 4 mm was found sufficient for reliable visualization of the major fetal cardiovascular structures (figures 2, 3).

 

Another advantage of B-FFE is the three times lower RF heat deposition rate compared to conventional SSh TSE.

 

In conclusion, we believe that B-FFE could have a potential role in fetal cardiovascular MR imaging in multiple planes. It may help us to study the development of the (ab)normal cardiovascular system in utero in more detail.

 

 A B
A
B

Figure 2. MRI of normal Umbilical vessels at 23 weeks of

gestation using B-FFE

A. Sagittal image of fetal abdomen shows the umbilical arteries (arrowhead)

run alongside the fetal bladder to the umbilicus insertion. The umbilical vein

(arrow) courses cephalad from the its cord insertion and enters the liver.

B. Sagittal image of fetal thorax and abdomen: the umbilical vein (white

arrowhead) anastomoses with the portal vein (black arrowhead) which ends

in the inferior vena cava (short white arrow). The black arrow points to the

right atrium which receives the systemic inflow veins: superior vena cava

(long white arrow) and inferior vena cava (short white arrow).

 

 

 A B
A
B

Figure 3. Normal fetal cardiac MR imaging using B FFE.

A. Sagittal image of fetal heart at 24 weeks of gestation shows the right

ventricle (short white arrow) and its outflow tract (arrowhead). The low

signal interventricular septum (black arrowhead) separates the right

and left ventricles (long white arrow). The image shows that the origin

of the left ventricular outflow tract (black arrow) is perpendicular to

that of the right ventricular outflow tract.

B. Coronal image of fetal heart at 22 weeks of gestation shows the left

ventricle (long arrow) and its outflow tract (short arrow). Notice the

superior and inferior venae cavae (arrowheads) entering the right atrium.

 

 

Dr. Saleem acknowledges Eng. Khaled Siraj (Philips-Egypt) and Miss Amal Mahmoud (MRI technician-Cairo University) for their sincere efforts.

 

Note: Dr. Sahar Saleem was Cum Laude Award winner at RSNA 2007.



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