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Chapter 9
Conclusions and future
perspectives
9.1 Summary and conclusions
A set of numerical simulations was designed and implemented to forecast the behaviour of MR signal coming from volumes containing venous blood. With the aid of this tool, it was possible to investigate in depth the characteristics of venous MR signal in order to exploit at its best the potentialities of MRI. It was evidenced how the venous signal has a complicate dependence on vessel configuration, magnetic field intensity, blood oxygenation and so on, so that it looks reasonable to rethink the standard techniques of angiographic and functional MRI in order to improve them.
In particular, in chapter 6 it was shown how it is possible to infer about the validity of a specific imaging methodology using the results provided by the numerical simulations. In that case, we showed the limit of vessel visibility in terms of minimum vessel size, which is an important characterization of the technique. Of course, other imaging methods can be evaluated with this approach, since the data provided depict the evolution of a physical phenomenon, and as such do not refer to a specific imaging technique.
In chapter 7 we illustrated how an imaging method considered standardised in the current literature, such as high-resolution BOLD venography, can be still significantly improved if one takes into account the specific properties of venous signal. The method we proposed is a very flexible tool since it is capable to adapt to the intrinsic signal-to-noise ratio of the datasets and to make full benefit of the phase signal.
Finally, in chapter 8 our work took into consideration an important issue related to functional imaging, i.e. the validity of brain functional maps obtained from BOLD signal analysis. We developed an experimental scanning procedure to analyze and compare the functional signal with brain microvasculature, which is the primary source of functional signal generation. This study has therefore a double importance since it represents at the same time the design of an experimental technique for functional signal analysis, and an
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experimental characterization of the functional response intensity referred to the presence of venous blood in the activated cortical areas.
In conclusion, the studies described in this thesis try to focus and clarify some important issues of current MRI methodologies. Despite the variety of analyses done, theoretical and experimental, methodological and investigational, this work constitutes an organic work, which faces the intrinsic complexity of BOLD MRI through a variety of methods.
9.2 Future developments
The validity of the results provided by numerical simulations is related to the ability of the analytical models to describe as much as possible a realistic situation in which MR signal is generated. Therefore, it is possible to complicate and enlarge the models in order to account for other characteristics, such as water diffusion, vessel curving and so on.
An important enhancement of this model would be to include time varying properties, in order to allow the model to describe transient phenomena. The main application of this could be the mathematical representation of the functional activation, which appears as a temporal modification of the local hemodynamics.
Another point of further investigation could be the validation of the vessel segmentation technique based on phase contrast with other current angiographic MRI techniques. This comparison would give an idea on the efficacy of the method itself and provide indications on how well blood signal is exploited by each of these techniques.
Finally, the results of the investigation on the vascular nature of the BOLD functional signal suggest that it would be important and feasible to develop techniques of quantitative correlation between BOLD functional signal and BOLD anatomic-venographic signal. This kind of study would further clarify how BOLD signal is generated in either applications, and each of them can be used to understand the physiological phenomena occurring in human brain.
