The MRI examinations enabled the visualisation of coup and contrecoup injuries (case 1) and microlesions (case 2) that were not identifiable on CT scans. Compared with 3-T MRI, 7-T MRI improved the assessability of the extent of the coup and contrecoup injuries and facilitated the detection of microlesions.
Overall, 3-T images appeared blurrier than the 7-T images, as expected. In fact, spin polarisation is increased for 7-T units, compared with that of the widely used 1.5-T and 3-T units, which in turn leads to a stronger signal and thus an increased signal-to-noise ratio . The increased signal-to-noise ratio directly improves the image quality and can be used to improve spatial resolution or to shorten the scan time for the same spatial resolution . Due to the roughly twofold lower signal-to-noise ratio at 3-T than at 7-T, the scan time would need to be at least four times longer to achieve similar image quality on 3-T MRI with the same spatial resolution. A drawback of 7-T MRI is the greater signal inhomogeneity due to the transmit B1 inhomogeneity. This phenomenon was illustrated by case 2. To improve the image quality in the presence of inhomogeneous magnetic fields, various approaches have been investigated regarding acquisition (e.g., the use of multichannel transmit arrays [17, 18]) or postprocessing (e.g., improved image reconstruction algorithms ).
The use of sharp thin-sliced 7-T images with high sensitivity to metals is appreciated for the radiologic diagnosis of subtle lesions and iron accumulation in specific brain diseases [20,21,22,23,24]. Especially for the early diagnosis of multiple sclerosis, 7-T MRI can be of particular importance, since 7-T MRI improves the ability to detect smaller and earlier multiple sclerosis lesions and 7-T MRI allows a more accurate characterisation of these lesions for discriminating multiple sclerosis from other brain diseases compared to 1.5-T and 3-T MRI .
According to our observations, 7-T MRI can facilitate the classification of the origin of cerebral haemorrhages as either traumatic or natural. In case 1, the detection of coup/contrecoup injuries allowed the description of the impact, leading to the diagnosis of an acceleration-deceleration injury. The exclusion of shearing injuries was facilitated on 7-T images, which can be considered an advantage over 3-T MRI and may be of special value in the assessment of child abuse . The use of a three-dimensional gradient-echo sequence at 7 T provided a detailed visualisation of the parietal bone fracture. As a consequence, this approach may be appropriate for delineating cranial fractures on MRI. For the same purpose, dedicated sequences can be applied at 3 T [27, 28]. An accurate depiction of cranial fractures on MRI can aid in the assessment of related soft tissue injuries.
In case 2, 7-T MRI showed a slight advantage over 3-T MRI in the classification of discrete lesions, in accordance with previous research . The 7-T T2-weighted images not only allowed a clear distinction of the microinfarct from the surrounding tissue but also depicted the rather precise extent of this microlesion, which is due to the small isotropic voxels of the 7-T sequence. On T2-weighted images, the microhaemorrhages were larger in diameter at 7 T than at 3 T, in accordance with the literature [30, 31]. On susceptibility-weighted images, microhaemorrhages were highlighted at both 3 T and 7 T, due to susceptibility effects of paramagnetic deoxygenated blood . A voxel size of 1.5 mm (7-T MRI) or 1.6 mm (3-T MRI) in the slice direction still enabled the clear visualisation of these subtle lesions on susceptibility-weighted images. An increase in the diameter of the microhaemorrhages at 7 T over 3 T, as observed on the T2-weighted images, was not observed on the susceptibility-weighted images. Likewise, only a small increase in the diameter of the microhaemorrhages has been reported between 1.5-T and 3-T MRI in the literature .
In conclusion, all distinctive and subtle findings were detectable on both 3-T and 7-T MRI, but the 7-T images provided higher detectability and assessability of traumatic brain injuries and microlesions. Certainly, the relevance of this advantage should be weighed against the practicability of 7-T MRI, which is dependent on several factors, in particular the accessibility to a 7-T scanner. Notwithstanding this limitation, further studies on 7-T MRI for various diagnostic purposes are appreciated.