Identification of vascular structures as a major source of signal contrast in high resolution 2D and 3D functional activation imaging of the motor cortex at l.5T preliminary results

Abstract

We have measured the T2* signal response associated with cortical activation due to finger motion at 1.5 Tesla. Both thin slice 2D and 3D images show signal intensity changes which vary from 2% to 32% depending on volunteer, echo time, slice thickness, and in-plane resolution. The largest signal change occurred for the thinnest slices and highest resolution (2 mm3). This is consistent with reducing partial volume effects and a simple difference in phase between the intravascular signal and surrounding parenchyma. No inflow enhancement was seen on the 2D or 3D scans, confirming the nature of the signal difference for this approach was due to local field inhomogeneity effects. Using 3D imaging, multiple effects can be seen simultaneously. With a 3D MRA method, it was possible to locate the vessel that was the source of the T2* behavior; it was in each case a vein on the surface of the cortical parenchyma.

Postacquisition suppression of large‐vessel BOLD signals in high‐resolution fMRI

“…Notably, the frequency pertubation, , caused by deoxyhemoglobin containing capillaries and veins can extend beyond the boundary of the vessel (see Eqs. [1] and [2]) (9). Typically, models of the BOLD effect begin with a consideration of the magnetic field or frequency offset inside and outside of a single vessel oriented at some angle with respect to the static magnetic field and occupying some volume fraction of a prototypical imaging voxel (10).…”

“…[1] and [2] (2,13), the NMR signal from a voxel in a BOLD fMRI experiment includes T 2 relaxation-modulated contributions from the intra-(S iv ) and extravascular (S ev ) water magnetization in vessels. Thus, any model that considers the effects of intravoxel dephasing must also include the effects of T 2 relaxation in the two compartments.…”

“…Because of the symmetry of the extravascular magnetic field perturbation (cos2 term in Eq. [2]), the extravascular spins in a voxel tend to have a near-zero net phase, while the intravascular spins have a coherent phase shift relative to the rotating frame (Eq. and Ref.…”

“…This time-dependent intravascular phase term (⌿ iv ) will cause a beat between the intravascular NMR signal (S iv ) and the extravascular NMR signal (S ev ), causing the magnetization in the voxel to vary in amplitude (S) and phase (⌽). Previous reports have considered the MRI signal properties of the intravascular compartment as a significant part of the voxel volume (2,13). One can go beyond this by looking for very subtle effects which cannot be seen in single images, but which can be revealed because of the statistical power realized through the repetitive nature of the fMRI technique.…”

“…Even at lower resolutions, larger veins can give rise to uncertainty in localization. For example, the presence of a draining vein in the central sulcus (2,8) makes distinguishing pre-from postcentral gyrus activity in the cortex extremely difficult, with obvious implications for other applications, such as presurgical planning in the motor cortex. While many clever techniques, such as spin-echo EPI, spiral, diffusion-weighted EPI, and perfusion-weighted methodologies, have been proposed to suppress the macrovascular BOLD effect, the use of gradient-echo EPI has been virtually ubiquitous in BOLD applications over the past decade because of its speed, multislice capability, ease of acquisition, and robust signal.…”