Potential pitfalls of functional MRI using conventional gradient‐recalled echo techniques

Abstract

The conventional gradient-recalled echo technique, FLASH, has widely been used for functional MRI. FLASH results at 4 T with short TEs of 10-20 ms mimic those at 1.5 T with TEs of 25-50 ms or longer. Under these conditions, large venous vessels dominate the activated area; however, the use of longer TEs at 4 T reveals activation in gray matter areas as well as large vessels. Inflow effects of large vessels can be greatly reduced with centric-reordering of phase-encoding steps and inter-image delay. Finger and toe movement paradigms show that functional activation maps are consistent with classical somatotopic maps, and are specific to the tasks. Navigator-based motion correction generates functional maps with larger activation areas by reducing physiological noise.

The neural basis of the blood–oxygen–level–dependent functional magnetic resonance imaging signal

“…tissues whose longitudinal magnetization was not yet fully recovered, and the unsaturated blood flowing into this plane (Axel 1984(Axel , 1986 lead to signal enhancement in the image of a selected slice if the time between consecutive RF excitations is insufficient for the signal within the slice to reach full relaxation. Such inflow effects can be considerably stronger than the BOLD signal itself (Segebarth et al 1994;Frahm et al 1994;Kim et al 1994;Belle et al 1995) and much less tissue specific. The shorter the repetition times, the stronger the inflow effect will be (Glover & Lee 1995;Haacke et al 1995).…”

Localized cerebral blood flow response at submillimeter columnar resolution

“…The delayed optical signals associated with elevated oxyhemoglobin level and the delayed positive BOLD mapping signals were particularly sensitive to large draining vessels (16)(17)(18)(19)(20), which naturally lack spatial specificity. Therefore, it remains unclear whether the poor spatial specificity in the delayed optical and positive BOLD signals is caused by intrinsic properties of coupling between neuronal activity and blood flow, or alternatively, contamination from large draining veins as the stimulusinduced oxy-and͞or deoxyhemoglobin changes propagate ''downstream'' because of blood flow.…”

MRI detection of weak magnetic fields due to an extended current dipole in a conducting sphere: A model for direct detection of neuronal currents in the brain

“…fMRI and intrinsic optical imaging studies have shown that whereas the increase in oxygen consumption is localized to active columns of neurons, the changes in blood flow and blood volume have a spatial extent that is significantly larger than the actual area of activation (1,2). Furthermore, BOLD effects may originate from both large and small vessels, and involve changes in both the intra-and extravascular signals (3)(4)(5)(6)(7). Large vessel in-flow effects can also cause changes in the signal in images when the magnetization does not fully recover between consecutive image acquisitions (4).…”

“…Furthermore, BOLD effects may originate from both large and small vessels, and involve changes in both the intra-and extravascular signals (3)(4)(5)(6)(7). Large vessel in-flow effects can also cause changes in the signal in images when the magnetization does not fully recover between consecutive image acquisitions (4). The temporal resolution of BOLD-based fMRI is limited by the temporal characteristics of the hemodynamic response, which has a time constant of several seconds (8).…”