Our study showed that the application of CS to high-resolution 3D T2WI has the potential to reduce scanning times without degrading image quality in paediatric brain imaging. With CS, scanning times were reduced by 9% which resulted in higher or comparable image quality. Quantitative CNR and SNR were higher with CS-SPACE compared to SPACE, a difference that was more prominent in the older age group that had full myelination (i.e., ≥ 24 months in age). CSF-related artifacts decreased with CS-SPACE, and this reduction was also more prominent in the older age group.
Reducing scanning time is particularly important in paediatric MRI studies19. Sedation may be necessary for children to undergo MRI scanning and by reducing sedation time, we can reduce the potential risks of sedation such as cardiac and respiratory depression or neurologic damage in this vulnerable patient population20. Even if a child is cooperative enough to undergo MRI scanning without sedation, it is not easy for them to hold still for the entire examination and this presents another problem as patient motion during scanning leads to degraded image quality. To resolve the above issues, various acceleration techniques have been developed for faster MR studies such as parallel MRI, radial sampling, spiral sampling, and recently, CS21. CS generates images from random or pseudo-random undersampled k-space data using an iterative nonlinear method21. As this allows high temporal resolution, CS has been applied to cardiovascular and abdominal imaging in paediatric patients21.
In this study, we applied CS to 3D volumetric T2-weighted brain imaging in children. Image quality improved using CS with 3D T2WI compared to conventional 3D T2WI and this was also observed in a previous study on adult brain imaging22. 3D volumetric imaging allows the retrospective analysis of brain structure along many axes which is an advantage over 2D images. It also allows quantitative imaging by enabling the calculation of cortical thickness or regional volume. For this reason, many research protocols for both paediatric and adult populations include 3D images rather than 2D images. In terms of T1WI, 3D imaging is considered to be the standard protocol for neonates, because it can identify more white matter punctate lesions that are frequently involved in neonates with ischemic insult23. In contrast, 3D T2WI imaging is not as well included in clinical practice, mainly because of its long scanning time. Nonetheless, T2WI is more suitable for anatomical segmentation24 or for assessing myelination degree in young populations that have not reached full myelination25. We think that 3D T2WI could be more broadly applied to children by using CS to shorten acquisition time as well as improving image quality.
In contrast to previous studies that showed substantially reduced scanning times ranging from 20–70% using CS12,26,27, our protocol resulted in approximately 9% reduction. This is mainly due to the longer TR set for CS-SPACE than conventional SPACE (5000 msec vs. 3500 msec). Further scan time reduction could be achieved with shorter repetition time; however, this reduction would be at the expense of image quality. Still, we achieved our initial goal to find an approach that could adapt 3D T2WI more reasonably to clinical settings since the scanning time of 2D T2-weighted spin echo imaging for paediatric brain MRI is approximately 2 min 30 seconds to 3 min 35 seconds and our scanning time was 3 min 13 seconds15. Further studies that discover ways to achieve additional time reduction are needed.
Interestingly, we found significant reduction of CSF-related artifacts after using CS-SPACE. CSF-related artifacts are categorized into time-of-flight effects and turbulent flow18. CSF-related artifacts at and around the third ventricle are explained with time-of-flight loss18. In a previous study that compared conventional and CS-applied 3D time-of-flight angiography, the use of CS resulted in better image quality28. Similar to this previous study, we postulate that the decrease in CSF-related artifacts is a result of undersampling and its reconstruction which leads to a denoising effect.
On the other hand, CS does not affect CSF-related artifacts identically when patients are of different ages and this is thought to be due to the different hemodynamic of CSF as younger patients show higher CSF velocities29. Unlike CSF-related artifacts, general artifacts decreased with the use of CS in the younger age group. We postulate that this is because of reductions in scanning time which may have led to less movement during scanning. Further scanning time reduction will result in less movement artifacts, but more study on this topic is necessary with a larger number of patients who are both sedated and not sedated.
There are several limitations to our study. Firstly, we compared the sequences in two different subjects for two different study periods. Secondly, there was large variation in age as we included patients with various degrees of brain maturation. As different results were found for each age group, large-scale studies that classify patients into more narrow age ranges could be needed. Thirdly, we did not evaluate specific artifacts that would be subject to the two sequences. A previous study applying CS to parallel imaging such as sensitivity encoding showed starry-sky or streaky-linear artifacts, which represented grainy image noise and horizontally oriented lines in the centre of the transverse reconstructed images, respectively30.
In conclusion, the application of compressed sensing to high-resolution 3D T2-weighted imaging in children has reduced scanning times without degrading image quality. Both quantitative and qualitative image quality were comparable when 3D T2-weighted imaging with and without compressed sensing were compared. This advanced technique has potential to increase routine use of high-resolution 3D T2-weighted imaging in children’s brain.