Magnetic field and tissue dependencies of human brain longitudinal ¹H₂O relaxation in vivo

MR angiography of the cerebral perforating arteries with magnetization prepared anatomical reference at 7T: Comparison with time‐of‐flight

“…It should be noted that the background suppression in MPRAGE is largely due to the inversion pulse before each shot, and not as much to saturation from the imaging pulses, like in the TOF sequence. Bright blood originates from the inflow of fresh blood between the inversion pulse and the first excitation pulse, since the inverted blood in the volume is suppressed like the background (the T 1 of blood at 7T is longer than that of gray and white matter (9)). The applied inversion pulse should therefore be spatially selective, inverting the imaged slab only.…”

“…The T 1 parameters for blood, gray matter (GM), white matter (WM), and CSF at 7T were taken from Ref. (9). The following parameters were varied: flip angle (1-40 degrees), inversion delay (700 -1400 msec), shot interval (total time between inversion pulses, varying from minimum (next inversion right after last excitation) until an extra delay of 3000 msec), and the TFE factor (number of excitations per shot, 50 -400).…”

Synthetic quantitative MRI through relaxometry modelling

“…This bias can arise because of components that have not been explicitly included in the model. However, it has previously been shown that macromolecular and iron components, which are rather well captured by MT and R 2 * respectively,31, 32 are the dominant contributors to R 1 11, 15, 16, 33, 34, 35. Any orientation‐dependent or higher‐order relationships that exist between the qMRI maps are not captured by the model either, and may also be a source of bias 36…”

“…e.g. 15, 38 Limitations in the validity of this assumption may be a source of bias. Models such as the one presented here are also a first step towards in vivo histology,2 in which quantitative maps are combined using biophysical models in order to extract descriptors of the underlying tissue such as myelin and iron levels3 or the degree of myelination of fibres 48.…”

“…In the absence of exogenous contrast agents, the measured R 1 is dominated by contributions from free water spins, bound water spins at macromolecular sites and a smaller, spatially varying contribution from iron sites 15, 16. Under conditions of fast exchange, the measured R 1 can be expressed as a weighted sum of the relaxivities of these compartments14: $R_1=R_{1\mathrm{normalf}}+f_{\mathrm{M}}{r}_{1\mathrm{normalM}}+f_{\mathrm{FE}}{r}_{1\mathrm{FE}}+true\underset{j}{true\sum }{f}_j{r}_{1j}.$ …”