This talk describes the challenges associated with human diffusion imaging at UHF, and the developments from the literature which have attempted to address these difficulties to produce high-quality data.
The continued drive towards MRI scanners operating at increasingly higher main magnetic fields is primarily motivated by the maxim that more teslas mean more signal and lead to better images. This promise of increased signal, which cannot easily be achieved in other ways, encourages efforts to overcome the inextricable technical challenges which accompany this endeavor. Unlike for many applications, however, diffusion imaging is not currently able to directly reap these potential signal gains – currently it seems fair to say that, for matched gradient and RF hardware, the majority of diffusion images acquired at 7 T, while comparable in quality to those achievable at 3 T, do not demonstrate a clear advantage over what can be obtained at lower field. This does not mean that diffusion imaging at UHF is not a worthwhile pursuit – but more a reflection of the fact that the associated challenges are manifold – and converting the potential of higher field strengths into ‘better’ diffusion imaging is by no means a straightforward task.
This talk attempts to summarize the specific reasons that make diffusion imaging at UHF more complicated than one might expect, and to highlight the range of developments that have already been made which have enabled diffusion images of excellent quality to be acquired.
There are 3 principal categories of challenges associated with diffusion imaging at 7T:
SNR. Although the intrinsic SNR of the MR signal increases approximately linearly with field strength, the shorter T2 values can cancel out this advantage – especially for diffusion imaging where long echo times are typically used.
B1. Increased field strength leads to greater B1-inhomogeneity (due to the shorter RF wavelength) and tighter restrictions due to specific absorption rate (SAR) limits (due to the higher RF energy required to achieve the same flip angle). Both of these changes pose a particular problem for diffusion imaging as a typical diffusion sequence is spin-echo based – where a homogeneous 180 ° (i.e. high energy) refocusing pulse is highly desirable.
B0. B0-inhomogeneity is also increased at higher field strengths – which is a particular problem for diffusion imaging as the readout is typically using EPI, which can be severely distorted in the phase- encoding direction at 7 T. Shorter T2* values also lead to increased T2-related blurring due to decay occurring during the readout itself.
The talk will describe approaches from the literature which have been developed to address each of the challenges identified above, as well as additional advances which fall outside of these specific categories.
In terms of sequences which are already widely available, it is generally possible to produce images of a similar quality to that which users will be familiar with at 3 T – but these will typically take longer to acquire due to the increased SAR constraints. This means that 7 T would not currently be the field strength of choice for performing a ‘standard’ DTI study, for example. However, users wishing to run a multi-modal study at UHF should be able to include a diffusion imaging component without needing to have their subjects rescanned at lower field.
Generally speaking, the regime where 7T diffusion imaging is likely to be able to outperform lower field magnets is for lower b-values (allowing shorter TE and less T2-decay) and at high spatial resolution (where the increased parallel acceleration capability keeps the readout within a feasible duration).
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