Inverse T₂ contrast at 1.5 Tesla between gray matter and white matter in the occipital lobe of normal adult human brain

Abstract

T 2 of cortical gray matter is generally assumed to be longer than that of white matter. It is shown here that this is not the case in the occipital lobe, but that this effect is often obscured at lower resolution and concealed in standard T 2 -weighted images. Using a high-resolution (1 ؋ 1.3 ؋ 2 mm 3 ) segmented EPI CarrPurcell-Meiboom-Gill sequence, T 2 relaxation times of the brain were measured at 1.5 T for eight healthy adult volunteers. The average T 2 values of cortical gray and white matter were found to be 88 ؎ 2 and 84 ؎ 3 msec in the frontal lobe, 84 ؎ 2 and 83 ؎ 3 msec in the parietal lobe, and 79 ؎ 1 and 87 ؎ 3 msec in the occipital lobe, respectively. This unexpected occipital T 2 contrast between gray and white matter is attributed to regional differences in iron concentration. The transverse relaxation time T 2 is one of the most commonly employed MRI contrast parameters. We have recently shown that absolute T 2 values can be used to quantify cerebral hemodynamics and oxygen extraction (1). As such, it is useful to accurately map T 2 of various normal and abnormal tissues. Several studies (2-11) have shown that T 2 varies across brain regions and during different stages of brain maturation and aging (5,10). Numerous studies in normal adults at multiple field strengths (2-4,9) have concluded that T 2 of cortical gray matter (GM) is longer than that of white matter (WM). On the contrary, gray matter T 2 was found to be shorter than that of WM in the brains of newborns (10) and in spinal cord (12). Several factors are likely to contribute to the heterogeneity of T 2 values. Among these may be regional variations in brain iron concentration, as well as variations in the microarchitecture of the cellular constituents and myelin structure (cytoarchitecture and myeloarchitecture, respectively). Recently, investigators examining the superior temporal gyrus and primary auditory cortex region have shown that the T 2 -weighted signal intensities can vary appreciably even between neighboring regions of the cortex (11). We found a similar pattern in our studies of the visual cortex, where typically an oblique slice through the calcarine fissure is studied. In order to better quantify the T 2 contrast between GM and WM in this location, we used very-high spatial resolution absolute T 2 measurements to minimize the likelihood of including WM and CSF (cerebrospinal fluid) in voxels nominally occupied by GM. MATERIALS AND METHODSMeasurements were performed on a 1.5 T ACS-NT clinical MRI scanner (Philips Medical Systems, Best, Netherlands). A body coil was used for RF transmission and a head coil was used for signal reception. Eight normal adult volunteers (four male, four female, age range 22-44 years, mean 32 Ϯ 8 SD years) were studied in accordance with institutional guidelines. A multiecho Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence with 13-shot segmented EPI acquisition was utilized at an echo-spacing of 25 msec. A single slice was acquired with 30 averages at the level of the dorsal-most aspects of the ...

The interaction between magnetization transfer and blood‐oxygen‐level‐dependent effects

“…Using the S par,sat /S par,unsat Ϫ P a CO 2 data (Fig. 3) and CBV norm ϭ 4.6 mL blood/100 g brain (17), the fitted (1 Ϫ MTR tissue ) is 0.418, 0.334, 0.326, and 0.387 at the offsets of 5, 3.5, Ϫ3.5, and Ϫ5 ppm, respectively. Based on these fitted (1 Ϫ MTR tissue ) values, CBV norm ϭ 3.9 mL blood/100 g brain is then obtained from the S par,sat /S par,sat norm Ϫ P a CO 2 data (Fig.…”

“…These voxel components each have individual tissue relaxation and MT parameters, and their relative contributions depend on the size and location of the voxel and the MRI acquisition parameters used. For homogeneously perfused tissue (parenchyma), the normalized imaging signal intensity as a function of echo time (TE) is a multiexponential decaying process (12,16,17),…”

“…We assume the tissue model used in Refs. (12,16,17), in which parenchyma consists of tissue, arterioles, venules, and capillaries. However, contrary to these papers, we now assume a constant total parenchymal water content (18), as shown to be the case in recent experiments (28).…”

“…We used the OER-CBV relationship for cylindrical vessels (12,16): CBV ϭ CBV norm ͑OER norm /OER͒ 0.5 , in which CBV norm ϭ 5.2 mL blood/100 g brain for GM and 2.8 mL blood/100 g brain for WM (17), OER norm ϭ 0.55 (16). The total water contents of GM and WM in the brain were previously measured to be 84.69 and 70.37 g water/100 g brain (40), respectively.…”

Proton T2 relaxation study of water, N‐acetylaspartate, and creatine in human brain using Hahn and Carr‐Purcell spin echoes at 4T and 7T

“…One of the factors that alters the T 2 † value of water is the presence of ferritin, a protein that stores the largest fractions of iron in the brain. Both loading of iron in the ferritin protein and the concentration of ferritin in solution alter the localized field gradients and create intertissue contrast in MRI (9,10).…”