Longitudinal relaxographic imaging of white matter hyperintensities in the elderly

Background

Incidental white matter hyperintensities (WMHs) are common findings on T₂-weighted magnetic resonance images of the aged brain and have been associated with cognitive decline. While a variety of pathogenic mechanisms have been proposed, the origin of WMHs and the extent to which lesions in the deep and periventricular white matter reflect distinct etiologies remains unclear. Our aim was to quantify the fractional blood volume (v_b) of small WMHs in vivo using a novel magnetic resonance imaging (MRI) approach and examine the contribution of blood–brain barrier disturbances to WMH formation in the deep and periventricular white matter.

Methods

Twenty-three elderly volunteers (aged 59–82 years) underwent 7 Tesla relaxographic imaging and fluid-attenuated inversion recovery (FLAIR) MRI. Maps of longitudinal relaxation rate constant (R₁) were prepared before contrast reagent (CR) injection and throughout CR washout. Voxelwise estimates of v_b were determined by fitting temporal changes in R₁ values to a two-site model that incorporates the effects of transendothelial water exchange. Average v_b values in deep and periventricular WMHs were determined after semi-automated segmentation of FLAIR images. Ventricular permeability was estimated from the change in CSF R₁ values during CR washout.

Results

In the absence of CR, the total water fraction in both deep and periventricular WMHs was increased compared to normal appearing white matter (NAWM). The v_b of deep WMHs was 1.8 ± 0.6 mL/100 g and was significantly reduced compared to NAWM (2.4 ± 0.8 mL/100 g). In contrast, the v_b of periventricular WMHs was unchanged compared to NAWM, decreased with ventricular volume and showed a positive association with ventricular permeability.

Conclusions

Hyperintensities in the deep WM appear to be driven by vascular compromise, while those in the periventricular WM are most likely the result of a compromised ependyma in which the small vessels remain relatively intact. These findings support varying contributions of blood–brain barrier and brain-CSF interface disturbances in the pathophysiology of deep and periventricular WMHs in the aged human brain.

Frontal lobe changes after severe diffuse closed head injury in children: a volumetric study of magnetic resonance imaging

In view of the pathophysiology and biomechanics of severe closed head injury (CHI) in children, we postulated that the frontal lobes sustain diffuse injury, even in the absence of focal brain lesions detected by magnetic resonance imaging (MRI). This study quantitated the morphological effects of CHI on the frontal lobes in children who sustained head trauma of varying severity. The MRI findings of 14 children who had sustained severe CHIs (Glasgow Coma Scale score of < or = 8) were compared with the findings in a matched group of 14 children having sustained mild head injuries (Glasgow Coma Scale score of 13-15). The patients ranged in age from 5 to 15 years at the time of their MRIs, which were acquired at least 3 months postinjury. MRI findings revealed no focal areas of abnormal signal in the frontal lobes. Volumetric analysis disclosed that the total prefrontal cerebrospinal fluid increased and the gray matter volume decreased in the patients with severe CHI, relative to the mildly injured comparison group. Gray matter volume was also reduced in the orbitofrontal and dorsolateral regions of the brains of children with severe CHI, relative to the children who sustained mild head trauma. These volumetric findings indicate that prefrontal tissue loss occurs after severe CHI in children, even in the absence of focal brain lesions in this area. Nearly two-thirds of the children who sustained severe CHIs were moderately disabled after an average postinjury interval of 3 years or more, whereas 12 of the 14 patients with mild CHIs attained a good recovery (2 were moderately disabled) by the time of study.(ABSTRACT TRUNCATED AT 250 WORDS)