Magnetic Resonance Imaging of fetal brain: study technique and correlation between gestional age and the exam duration

Abstract

Introduction

In the fetal area, MRI was first described in 1980 for the study of placenta, but it was assumed that this method could be used for the study of the fetal brain only in the 90s. The main obstacles were both long acquisition times and fetal movements, which created too many artifacts. But it was not until the implementation of ultra-fast Single Shot T2 sequences, that it started to be electively used in clinical practice.

Objective

Check if there is a relationship between the total study time of an MRI fetal brain examination and the patient's gestational age in order to optimize exam planning, to reduce acquisition times and to increase safety for both the mother and the fetus. The FMR is a free radiations technique used to study the fetus and, starting from 19-20 weeks of gestation, it allows for important information about anatomy and fetal development; nowadays, it must be considered as a III level exam that requires a proper, targeted clinical question, and that has to be performed after a II level ultrasound.

Material and methods

This retrospective study was conducted at the Pediatric Radiology and Neuroradiology Unit of the “Vittore Buzzi” Children’s Hospital in Milan.

The examination times of 484 fetal brain MRI, performed between January 2015 and December 2017, were collected and analyzed through the statistical functions of the RIS & PACS system. The information related to the patients’ gestational age was collected from the medical reports of each exam.

Results

The data analysis shows that, out of 484 MR, 30,37% was performed in patients at 21 weeks of gestation, 8,68% at 20 weeks and 8,26% at 22 weeks; patients at 19 weeks and at 35 weeks were less than 1%. From 31 weeks of gestation onwards, the use of this method has greatly decreased, considering that the execution of the MRI is no longer appropriate for prognosis; moreover, MRI performed at this gestational age usually gives the same diagnostic information as an ultrasound examination.

For every MRI exam, we collected and analyzed the data:

• Total study time, divided by gestational age. The analysis shows how the duration of the exam reduces as gestational weeks progress.
• “Dead time” that occurs between the different sequences acquired during the exam. From the data analysis it’s clear that it tends to decrease with the gestational age progression. This is because greater movements are present in smaller fetuses; these movements require greater technical ability and more experience in the setting of the scanning sequence plans.
• “Live time”, corrisponding to the transmission time of radiofrequencies to the fetus. This data has an intrinsic calculated error because it is an approximate time and it varies in accordance to the size of the fetal brain. A larger fetus will lead to an increase live time because of the greater number of acquisition “slices”.
• Total number of acquired sequences: the data shows an almost non-existent correlation between gestational age and number of sequences. However, it is worth considering the limit of this analysis because non-diagnostic sequences -as they present too many artifacts- are often not stored in the PACS system.

Conclusion

From the results obtained, we can conclude that the examination times for the study of the fetal brain tend to decrease with the gestational age progression. However, it would be useful to analyze other variables, not taken into consideration in this study, in order to obtain more reliable results. These variables emerged during the data analysis and include the clinical question and the Radiographer’s experience and competence. In this way the hypothesis that the exam acquisition times decrease with an increase of the Radiographer’s experience and practice, could be tested; this would apply especially to 19-22 weeks of gestation women and represents the biggest challenge for the exam management.