Optimization and initial experience of a multisection balanced steady-state free precession cine sequence for the assessment of fetal behavior in utero

Abstract

Background and purpose: The assessment of motor function is an essential component of neurologic examinations, which imaging studies have extended to the fetus. US assessment is hampered by a limited FOV, whereas MR imaging has the potential to be an alternative. Our objectives were to optimize a cine MR imaging sequence for capturing fetal movements and to perform a pilot analysis of the relationship between the frequency of movements and uterine spatial constrictions in healthy fetuses.

Materials and methods: Initially, a bSSFP cine sequence was selected for optimization, and various compromises were explored in all acquisition parameters to achieve an effective balance between anatomic coverage of the fetus and the temporal resolution of cine data, with the aim of maximizing both. Subsequently, cross-sectional qualitative and quantitative analyses of fetal movements were performed prospectively by using a cohort of 37 healthy fetuses (median GA, 29 weeks; range, 20-37 weeks) with the optimized cine protocol. Two smaller subgroups were selected for representative sampling of overall behavior patterns by using cine data of longer duration and for volumetric quantification of free intrauterine space.

Results: The optimized cine sequence, with TR/TE of 3.21/1.59 ms, coupled with parallel imaging and partial-Fourier imaging, resulted in a section-acquisition time of 0.303 seconds. Anatomic coverage was enhanced by using a combination of thick sagittal sections (30-40 mm) and multisection acquisitions to display movements in all fetal limbs, head, and trunk simultaneously. All expected motor patterns were observed throughout this gestational period, and a significant decreasing trend in overall movement frequency with age was demonstrated (r = -0.514, P = .0011). Also a significant negative correlation was found between overall movement frequency and the total intrauterine free space (r = -0.703, P = .0001). Furthermore, a significant decrease in the frequency of leg movements was shown in fetuses older then 30 weeks' GA compared with those younger than that (P = .015).

Conclusions: Cine MR imaging is effective for observing fetal movements from midgestation with near full-body coverage. Also, reductions in free space with increasing GA appear to be a factor in the gradual reductions in overall levels of fetal activity as well as in restrictions in movement within specific regions of the fetal anatomy.

Fig 1.

Example of a cine acquisition at GA 36 weeks. A–C, Successive frames from a multisection cine image sequence (section thickness, 30 mm). D–F, Piloting procedure used to set up the scan. Upper and lower limbs, trunk, and head are all visible within the FOV. D, Fetal localizer scan, which is used to generate sagittal-oriented cine data; E and F, Maternal pilots in orthogonal planes used to ensure that the prescribed sections have FOVs that cover the full extent of maternal tissue.

Fig 2.

Examples of the fetal movement repertoire. A–C, Time points 0, 4, and 8 seconds of cine data from a GA 22 weeks' fetus. D–F, A GA 30 weeks' fetus at time points 0, 6, and 10 seconds. The younger fetus is engaging in GM, demonstrating a complete flexion at its knees and extension in its trunk, followed by trunk relaxation and knee extension. D–F, The older fetus also shows GM consisting of flexion at the knee joint (dashed line) as well as at the elbow (dotted line). Arrows indicate hands, 1 of which is rotating in the older fetus. The overall variability in amplitude of movements that older fetuses perform is significantly reduced and appears associated with uterine restrictions giving a cramped appearance to movements. This is in contrast to younger fetuses, that tend to make full use of their surrounding space.

Fig 3.

Facilitation of flexed posture by the uterine wall. A, T2-weighted coronal section through a GA 22 weeks' fetus. The uterine wall is in close proximity to the fetus's lateral aspects (arrows). B, Still image from cine data of the same fetus. The fetal arm is flexed and located adjacent to the fetal head (thick arrow). C, Image of a GA 35 weeks' fetus demonstrates the redistribution of amniotic fluid volume near term, anterior to the thorax of the fetus (dashed line), due to its posture and the uterine limits; this collection of amniotic fluid may facilitate upper limb movements and trunk extensions.

Fig 4.

Graph shows that the proportion of imaging time spent moving by all fetuses decreases significantly with increasing GA (r = −0.514, P = .001).

Fig 5.

Graph shows the median frequency of movements in fetuses grouped by age (± interquartile range). There is a significant decrease in the rate of leg movements between fetuses younger than GA 25 weeks and those older than GA 31 weeks (P = .015). There are no statistically significant differences in the median rates of head, arm, or trunk movements between age groups (P = .746, P = .310, and P = .397, respectively).

Fig 6.

Graph shows overall activity levels in long-duration cine acquisitions compared with pooled shorter duration data from the corresponding age groups. The results show that there are similar proportions of 60-second epochs that contain each level of motility between each age group and the corresponding long acquisition.

Fig 7.

Graph shows activity levels during 20 minutes. Data from the long cine acquisitions from the fetuses at GA 22 (dashed line) and GA 36 (solid line) are shown. There are notable differences in the maximum frequency of movements. Also the behavior of the GA 36 weeks' fetus appears more ordered with more clearly defined periods of activity and inactivity than the GA 22 weeks' fetus.

Fig 8.

Graph of movement frequency in a subset of 24 healthy fetuses shows a significant reduction as the proportion of uterine volume that the fetus occupies increases (r = −0.703, P = .0001).