Functional mapping in the human brain using high magnetic fields

Abstract

An avidly pursued new dimension in magnetic resonance imaging (MRI) research is the acquisition of physiological and biochemical information non-invasively using the nuclear spins of the water molecules in the human body. In this trial, a recent and unique accomplishment was the introduction of the ability to map human brain function non-invasively. Today, functional images with subcentimetre resolution of the entire human brain can be generated in single subjects and in data acquisition times of several minutes using 1.5 tesla (T) MRI scanners that are often used in hospitals for clinical purposes. However, there have been accomplishments beyond this type of imaging using significantly higher magnetic fields such as 4 T. Efforts for developing high magnetic field human brain imaging and functional mapping using MRI (fMRI) were undertaken at about the same time. It has been demonstrated that high magnetic fields result in improved contrast and, more importantly, in elevated sensitivity to capillary level changes coupled to neuronal activity in the blood oxygenation level dependent (BOLD) contrast mechanism used in fMRI. These advantages have been used to generate, for example, high resolution functional maps of ocular dominance columns, retinotopy within the small lateral geniculate nucleus, true single-trial fMRI and early negative signal changes in the temporal evolution of the BOLD signal. So far these have not been duplicated or have been observed as significantly weaker effects at much lower field strengths. Some of these high-field advantages and accomplishments are reviewed in this paper.

Imaging brain function in humans at 7 Tesla

“…The studies presented here demonstrate that bloodrelated mechanisms that contribute to the BOLD effect (see discussion in [17][18][19][20][21] are virtually inoperative at 7 T for TEs equal to or exceeding the optimum TE of 25 msec, while they are still significant at the optimum TE of 35 msec at 4 T (see Fig. 2, 34 msec and 22 msec images for 4T and 7T, respectively).…”

“…This is most likely due to the fact that there is more signal fluctuation in the vascular area from cardiac pulsation. In addition, pixels that contain both tissue and relatively large venous blood volume fraction (e.g., due to the presence of large blood vessels) may also exhibit an oscillatory behavior (17)(18)(19)(20). This oscillatory behavior arises because the deoxyhemoglobin containing blood has a different resonance frequency compared to surrounding tissue (17)(18)(19)(20).…”

“…In animal models, higher magnetic fields (4.7 T and 9.4 T) have been widely used (9 -11). Theoretical considerations (12)(13)(14) and experimental data (10,11,15,16) have consistently revealed that the specificity, sensitivity, and contrast of the BOLD response to neural activity increases with the field strength (also see [17][18][19][20][21]. This field strength dependence has led to a recent surge in the number of high field (3 and 4 T) MRI systems available.…”

How accurate is magnetic resonance imaging of brain function?

“…faithful to the borders of the perfusion increase can be obtained with Hahn spin-echo (HSE) BOLD at high but not low magnetic fields. HSE fMRI responds to apparent changes in T 2 (as opposed to T p 2 ) originating from the diffusion of water in the presence of magnetic-field gradients generated in the extravascular space around microvasculature [36,37], as well as from the exchange of water into and out of red blood cells in the blood itself [38][39][40]. The former provides spatial specificity of ,100 mm because capillaries are separated on average by 25 mm [41].…”

Macrovascular Contribution in Activation Patterns of Working Memory

“…The intra-, and extravascular components of the BOLD signal reflect local magnetic field variations produced by changes in the fractional oxygen saturation of hemoglobin during the fMRI tasks (Boxerman et al, 1995b;Menon et al, 1995;Ugurbil et al, 1999); however, the intravascular pool accounts for the majority of the BOLD-fMRI dependent signal (Boxerman et al, 1995a).…”