Mapping the MRI voxel volume in which thermal noise matches physiological noise--implications for fMRI

Abstract

This work addresses the choice of the imaging voxel volume in blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI). Noise of physiological origin that is present in the voxel time course is a prohibitive factor in the detection of small activation-induced BOLD signal changes. If the physiological noise contribution dominates over the temporal fluctuation contribution in the imaging voxel, further increases in the voxel signal-to-noise ratio (SNR) will have diminished corresponding increases in temporal signal-to-noise (TSNR), resulting in reduced corresponding increases in the ability to detect activation induced signal changes. On the other hand, if the thermal and system noise dominate (suggesting a relatively low SNR) further decreases in SNR can prohibit detection of activation-induced signal changes. Here we have proposed and called the "suggested" voxel volume for fMRI the volume where thermal plus system-related and physiological noise variances are equal. Based on this condition we have created maps of fMRI suggested voxel volume from our experimental data at 3T, since this value will spatially vary depending on the contribution of physiologic noise in each voxel. Based on our fast EPI segmentation technique we have found that for gray matter (GM), white matter (WM), and cerebral spinal fluid (CSF) brain compartments the mean suggested cubical voxel volume is: (1.8 mm)3, (2.1 mm)3 and (1.4 mm)3, respectively. Serendipitously, (1.8 mm)3 cubical voxel volume for GM approximately matches the cortical thickness, thus optimizing BOLD contrast by minimizing partial volume averaging. The introduced suggested fMRI voxel volume can be a useful parameter for choice of imaging volume for functional studies.

Figure 1

A simulation of the TSNR versus SNR relationship (Equation 1) for three different brain tissue components and a phantom is shown. The lines represent the human brain white matter (green), gray matter (red), and cerbro-spinal fluid (black). The dark dotted line represents the expected result for a phantom (no physiological noise present). The locations of the suggested voxels for specific brain compartments where Equation 2 holds are marked using large color dots. The dashed vertical lines indicate TSNR and SNR coordinates for OVV and the horizontal dotted/dashed lines limit for TSNR. The black horizontal lines below the horizontal axis show the SNR range available with the system standard transmit/receive birdcage coil and the 16 channel receive-only surface coil brain array are used.

Figure 2

A plot of the suggested cubical voxel size versus SNR₀ for three different brain tissues (Equation [5] simulation) is shown. The imaging voxel volume V₀ matches the volume used in experiments, and the TSNR_L or 1/λ values obtained in experiments were used (GM: 80, WM 130, CSF 45). The suggested voxel volumes for two different coils: a standard system provided birdcage head coil- coil1 (birdcage) and a 16 channel receive-only surface coil brain array- coil2 (array) are shown in gray matter (coil1 marked as a red oval, coil2 marked as a red square). Coil2 (array) has 2.8 times better SNR over the whole brain than coil1 (birdcage).

Figure 3

An example of the echo planar image segmentation is shown. a) A histogram of the T₁ values from all voxels in all subjects (n=111726) is shown. The colored horizontal lines represent the T₁ ranges used for segmentation; b) An example of single slice T₁ map is shown. Grey scale display range: 0s (black) to 4.5s (white). Examples of the resulting binary segmentation masks are also shown: c) white matter (WM); d) gray matter (GM); e) cerebrospinal fluid (CSF).

Figure 4

An example of single subject suggested fMRI cubical voxel maps is shown (display range: 0 mm³ (black) to 24 mm³ (white)). SVV mean value plus standard deviation for different tissue compartments are displayed.

Figure 5

Computed from the first image in Figure 4 (marked by a yellow box) maps of: a) SNR, b) TSNR, c) SNR_S=TSNR_Limit and, d) SVV map (display range: 0 mm³ (black) -24 mm³ (white)) are shown.