Outpacing movement — ultrafast volume coverage in neuropediatric magnetic resonance imaging

Background Conventional MRI sequences are often affected in neuropediatric imaging by unavoidable movements. Therefore, children younger than 6 years usually have to be examined under sedation/anesthesia. A new real-time MRI technique with automatic slice advancement allows for motion-robust T2-weighted volume coverage of the whole brain within a few seconds in adults. Objective To evaluate to which extent the new volume coverage method can be used to visualize cerebrospinal fluid and reduce the need for anesthesia in children. Materials and methods We assessed 30 children ages 6 years and younger with suspected or proven hydrocephalus, hygroma or macrocephalus using volume coverage sequences with 20 slices per second in three planes. If necessary, a parent was placed in the bore together with the child for calming and gentle immobilization. We compared visualization of cerebrospinal fluid spaces and course of the shunt catheter in volume coverage sequences vs. fast spin-echo sequences. Results The clinical issue could be sufficiently assessed in all children with use of volume coverage sequences, whereas conventional fast spin-echo sequences performed moderately to poorly. Visualization of the tip of a shunt failed in 16% of volume coverage scans and 27% of turbo spin-echo scans. A subsequent examination under anesthesia was never necessary. None of the examinations had to be stopped prematurely. Conclusion The motion-robust volume coverage sequences with T2-type contrast can be used to avoid sedation of children in the evaluation of cerebrospinal fluid spaces, even in the presence of vigorous motion. For other indications and contrasts, the technique must still be evaluated.

MR angiography

The appearance of blood on magnetic resonance (MR) images is directly linked to its flowing nature. The contrast mechanism relies on the time-of-flight mechanism. In spin echo sequences, the excited blood flows out before the echo is created, resulting in black blood images, whereas in gradient echo images, the rapid succession of radiofrequency pulses saturates stationary signals, while fresh blood continuously flows in, leading to bright blood images. This phenomenon can be exploited to create inflow or time-of-flight angiography. It is also possible to encode the movement by using gradients that create phase differences between stationary and moving tissues. This technique, known as phase contrast angiography, can be used to image the venous and arterial phases separately. It also forms the basics of blood flow quantification. Finally, it is possible to use gadolinium-based agents to acquire contrast-enhanced angiographies.