Power injection of contrast through an intravenous line during MRI is frequently used to assess cerebral perfusion in adults, but use of this technique in neonates is problematic for a number of reasons, according to Dr. Ellen Grant, director of pediatric radiology at Massachusetts General Hospital in Boston.
Many contrast agents used for MRI are not specifically indicated for very young children. The IV line also may be placed in an awkward or precarious position and may not be suitable for bolus injection.
“The other problem with these bolus injections of contrast for perfusion used on adults is that they emphasize all of the [cerebral] vessels. Neonates have relatively large veins on the cortical surface so that's all you see—cortical veins. I don't get much information about [deeper] cortical perfusion,” said Dr. Grant.
Arterial spin labeling MRI (ASL-MRI) eliminates the need for the injection of a contrast bolus by magnetically labeling blood that flows from the heart past an external radiofrequency pulse sequencer placed at the infant's neck. The magnetically labeled blood perfuses into the brain tissue, altering the tissue's magnetization. By making assumptions about a few parameters, such as the distance between the point of blood magnetization at the neck and any given point in the brain, the technique can be used to produce quantitative maps of cerebral blood flow, such as the image seen at right.
“I think the most important thing is that it gives you more reproducible data than the bolus perfusion method. It's easier to compare subject to subject or one subject over several time points,” said Dr. Grant.
Another advantage of this method over a contrast bolus is that the blood is tagged right as it goes into the brain, instead of having to go through the heart and lungs, which spreads out the bolus.
ASL-MRI is “kind of like a poor man's look into brain function because it's blood flow, which is linked to metabolism,” said Dr. Grant.
The technique reveals injury in areas that looked normal on diffusion-weighted imaging,” said Dr. Grant. “We want to know not only what's happening to the area that shows up as necrotic on a diffusion scan, we want to know what's happening to the rest of the brain.”
This information may be particularly important in the neonatal brain because animal models of neonatal brain injury indicate that apoptotic cell death is often the predominant form of cell death. “We know that diffusion-weighted imaging picks up necrotic cell death but we're probably not picking up that other possibly dominant proportion that is apoptotic,” said Dr. Grant.
Dr. Grant uses a 1.5-T scanner to image infants, who have been fed and swaddled tightly so that they fall asleep. “Having 3 tesla does give you increased signal to noise but with the ASL sequence we have from Dr. David Alsop [of Harvard University in Boston], we get good signal at 1.5 T,” said Dr. Grant. One advantage in imaging babies is that blood flow is much faster than in adults. “Even in a newborn I'm getting decent images that I can start to see different patterns of blood flow with different types of injuries,” she said. The sequence, written by Dr. Alsop, is fast-spin echo-based, and it provides whole-brain coverage rather than a few slices. The process takes 5–6 minutes.
ASL-MRI will help researchers better understand the health of neurons not identified as injured using conventional scanning techniques. Dr. Grant and her colleagues are starting to couple blood-flow imaging with quantitative optical imaging to come up with estimates of the cerebral metabolic rate of oxygen, which can reveal the functional status of brain tissue.
The technique may prove to be useful in the early stratification of neonates and infants with developmental delays without exposing them to radiation, said Dr. Grant.