An exploration of task based fMRI in neonates using echo-shifting to allow acquisition at longer T without loss of temporal efficiency

Abstract

Optimal Contrast to Noise Ratio of the BOLD signal in neonatal and fetal fMRI has been hard to achieve because of the much longer T * 2 values in developing brain tissue in comparison to those in the mature adult brain. The conventional approach of optimizing fMRI sequences would suggest matching the echo time (T E ) and the T * 2 of the neonatal and fetal brain. However, the use of a long echo time would typically increase the minimum repetition time (T R ) resulting in inefficient sampling. Here we apply the concept of echo shifting to task based neonatal fMRI in order to achieve an improved Contrast to Noise Ratio and efficient data sampling at the same time. Echo shifted EPI (es-EPI) is a modification of a standard 2D-EPI sequence which enables echo times longer than the time * Corresponding author Email address: giulio.ferrazzi@kcl.ac.uk (Giulio Ferrazzi) 1 Contributed equally to the paper Preprint submitted to NeuroImage January 18, 2016, where N S is the number of acquired slices and T S the inter-slice repetition time). The proposed method was tested on neonatal subjects using a passive sensori-motor task paradigm. Dual echo EPI datasets with an identical readout structure to es-EPI were also acquired and used as control data to assess BOLD activation. From the results of the latter analysis, an average increase of 78 ± 41 % in Contrast to Noise Ratio was observable when comparing late to short echoes. Furthermore, es-EPI allowed the acquisition of data with an identical contrast to the late echo, but more efficiently since a higher number of slices could be acquired in the same amount of time.

Optimal echo time for functional MRI of the infant brain identified in response to noxious stimulation

“…The average value across the whole infant brain was approximately 100 ms, which is similar to the value recently reported at 3T in the motor cortex in a single infant . This is substantially longer than values reported in adults, where values from 41 to 66 ms have been observed in gray and white matter regions .…”

“…As the infant brain has higher water content and lower lipid concentrations due to immature myelination and synaptic formations compared with adults , the (and therefore TE) should theoretically be longer. Experimentally, values have been shown to be longer in the infant brain compared with adults: at 1.5T, values were reported as being two times longer in both gray and white matter regions compared with adults ; and at 3T, values of between 102 and 120 ms have been reported in the motor cortex in a single infant , which is again approximately double those reported in adults . These observations have led to recommendations that a longer TE should be used in infant fMRI .…”

Inner‐volume echo volumar imaging (IVEVI) for robust fetal brain imaging

“…To introduce weighting, the k‐space center was offset relative to the SE, which took place at approximately 15 ms, and the effective gradient echo occurred long after the SE. Effective TEs were within the range of 110 to 130 ms for single‐shot IVEVI, close to the values (of the order of 120 ms) reported for fetal cortical gray matter at 3 T . Using a TE of the order of may result in a higher sensitivity to the blood oxygen–level‐dependent effect.…”

“…Using a TE of the order of may result in a higher sensitivity to the blood oxygen–level‐dependent effect. For example, by following the procedure outlined in and by using a value of 120 ms, an increase in blood oxygen–level dependent to noise contrast of 34% is to be expected when comparing acquisitions run using conventional TEs of 50 ms, although this still needs to be verified. However, choosing a shorter TE, as in the tested multishot acquisitions (57 and 111 ms), could be beneficial, as there is less time for transverse magnetization to decay, reducing localized signal dropouts and improving the signal‐to‐noise ratio.…”

“…Quoting , values “of 154 ± 24 ms in the thalamic deep gray matter of healthy foetuses , and of 137 ± 13 ms for babies born prematurely ” are reported in the literature at 1.5 T. In contrast, ranges from 40 to 60 ms in the adult healthy brain at the same field strength . As far as we are aware, there is no study reporting values in the healthy fetal brain at 3 T, although values up to 120 ms were measured in the cortical gray matter in a single‐term infant at 41 weeks at that field strength . Furthermore, was found to linearly decrease as a function of gestational age ; therefore, higher values are expected to be found in the fetus.…”