S. Arvela, R. Hari, J. Hiltunen, Y. Hlushchuk, R. Joensuu, V. Jousmäki, S. Malinen, P. Pöyhönen, V. Renvall, M. Schürmann, M. Seppä, T. Suortti, O. Tanzer, and A. Tarkiainen
FMRI reveals changes in blood oxygen level dependent (BOLD) signal after considerable processing. The whole acquisition and analysis chain of fMRI has remained unexplored with functional phantoms because no such devices have been available. We have designed, implemented and tested an fMRI phantom where electric current applied to a thin wire within proton-rich medium substituted BOLD distortion of the magnetic field; the scanner detects these two distortions as practically identical signal changes. The phantom has a number of possible applications. Signal changes across sessions, days, instruments, and individuals could be monitored. Placing the phantom close to a subject during an fMRI experiment could allow differentiating signal changes due to instrumentation from changes in the subject’s state and performance during the experiment. The spatial extent of brain activations and effects of various changes in the chain of image formation could be analyzed utilizing current-induced “activations”. Furthermore, the phantom could expedite fMRI sequence development by reducing the need to scan human subjects, who, besides, introduce uncertainty to the signal. Thus, this fMRI phantom could be useful for both cognitive fMRI studies and scanner calibration.
Diffusion tensor imaging (DTI) and tractography have been widely applied to study fiber tracts of white matter in human brain, and more recently also of other human anatomical structures. We have obtained the first DTI and tractography results of human distal peripheral nerves (median, ulnar, and radial nerves in the upper limb and tibial and peroneal nerves in the lower limb). We first quantified the apparent diffusion coefficient and the fractional anisotropy index, and then visualized the nerves in 3D with tractography, which nicely illustrated the 3D course of the nerves and distinguished then from surrounding muscle tissue and ligaments. Further studies will show whether DTI of distal peripheral nerves is useful in the diagnosis and follow-up of nerve lesions, entrapments, and regeneration. Peripheral nerves, as well-lineated structures that also contain abundant branching into bundles of different diameters, could be used as “living phantoms” for testing and validating different tractography methods.
FMRI can reveal human brain activations with high precision which may, however, be impaired by movement of cerebrospinal fluid and deformation of brain tissue associated with cardiac pulsations. We have shown that correcting for such artifacts by time-locking the fMRI data acquisition to the cardiac cycle improves signal detection both in cortex and thalamus in studies of somatosensory processing. Variance of the BOLD signal decreased on average by 23% in thalamus and by 32% in SII during cardiac triggering compared with conventional imaging. Consequently, both thalamic and SII responses were seen in a larger number of subjects. Group analysis of cardiac-triggered data revealed somatotopical organization for activations in the ventroposterior thalamus for the three stimulus sites. At the cortical level, two distinct activation areas were observed to both finger and toe stimuli, one in the SII cortex and the other deeper in the insula.