Horsch S, Skiold B, Hallberg B et al.  Cranial ultrasound and MRI at term age in extremely preterm infants.  Arch Dis Child Fetal Neonatal Ed. 2010 Apr 13. [Epub ahead of print].  Pubmed, Full text

Objectives Conventional MRI at term age has been reported to be superior to cranial ultrasound (cUS) in detecting white matter (WM) abnormalities and predicting outcome in preterm infants. However, in a previous study cUS was performed during the first 6 weeks only and not in parallel to MRI at term age. Therefore, the aim of the present work was to study brain injuries in preterm infants performing concomitant cUS and MRI at full-term age.

Methods In a population-based cohort of 72 extremely low gestational age infants paired cUS and conventional MRI were performed at term age. Abnormalities on MRI were graded according to a previously published scoring system. On cUS images the lateral ventricles, the corpus callosum, the interhemispheric fissure and the subarachnoidal spaces were measured and the presence of cysts, grey matter abnormalities and gyral folding were scored.

Results Moderate or severe WM abnormalities were detected on MRI in 17% of infants and abnormalities of the grey matter in 11% of infants. Among infants with normal ultrasound (n=28, 39%) none had moderate or severe WM abnormalities or abnormal grey matter on MRI. All infants with severe abnormalities (n=3, 4%) were identified as severe on MRI and cUS.

Conclusions All severe WM abnormalities identified on MRI at term age were also detected by cUS at term, providing the examinations were performed on the same day. Infants with normal cUS at term age were found to have a normal MRI or only mild WM abnormalities on MRI at term age.

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Comments

Ultrasound has become the preferred imaging modality for detecting CNS pathology in preterm infants due to portability and sensitivity in the detection of hemorrhagic lesions and ventriculomegaly.  However, recent reports have suggested that magnetic resonance imaging performed at term gestation can detect a variety of white and gray matter abnormalities that may not be apparent on routine ultrasound scanning.  The implications have been far-reaching with some suggesting that MRI at term should become routine for babies born very preterm in order to aid in prognostication. 

The authors of the above study compared the utility of cUS and MRI in the detection of CNS abnormalities in a group of infants who had been born very preterm.  In contrast to many of the previous studies comparing the two modalities in which the cUS scans were performed early on, cUS was performed in tandem with MRI at term gestation.  They found that infants with a normal cUS at term had either a normal MRI (64%) or mildly abnormal white matter abnormalities only (36%).  Severe abnormalities were picked up with both imaging methods.  There were a number of infants categorized as normal using MRI who had some abnormality on cUS and those with moderate abnormalities on MRI had a variety of cUS abnormalities that were not considered severe.  The authors conclude that, for the approximately 40% of very preterm infants who have normal cUS at term, conventional MRI adds marginally clinically relevant information.  MRI may be useful when cUS is mildly or moderately abnormal to refine estimates of white matter or grey matter injury.  Correlation with long-term outcome is also required.

How are you using MRI to assess this population in your NICU?  Have you found it helpful?  Is anyone using cUS at term as a means of assessing risk of long-term disability?  What value does neuroimaging have in individual prognostication?

I would also be very interested to hear from parents - is it helpful to know that your preemie who is now ready for discharge has some abnormality on CNS imaging that might increase the risk of long-term functional problems?  At this stage, there is really no change in management resulting from the acquisition of this information.  One might imagine that knowledge of these test results at a time when families are finally ready to celebrate their baby's release from hospital might actually be unwelcome. 

Michael Dunn