Decreased brain stiffness in Alzheimer's disease determined by magnetic resonance elastography

Abstract

Purpose: To test patient acceptance and reproducibility of the 3D magnetic resonance elastography (MRE) brain exam using a soft vibration source, and to determine if MRE could noninvasively measure a change in the elastic properties of the brain parenchyma due to Alzheimer's disease (AD).

Materials and methods: MRE exams were performed using an accelerated spin-echo echo planar imaging (EPI) pulse sequence and stiffness was calculated with a 3D direct inversion algorithm. Reproducibility of the technique was assessed in 10 male volunteers, who each underwent four MRE exams separated into two imaging sessions. The effect of AD on brain stiffness was assessed in 28 volunteers, 7 with probable AD, 14 age- and gender-matched PIB-negative (Pittsburgh Compound B, a PET amyloid imaging ligand) cognitively normal controls (CN-), and 7 age- and gender-matched PIB-positive cognitively normal controls (CN+).

Results: The median stiffness of the 10 volunteers was 3.07 kPa with a range of 0.40 kPa. The median and maximum coefficients of variation for these volunteers were 1.71% and 3.07%. The median stiffness of the 14 CN- subjects was 2.37 kPa (0.44 kPa range) compared to 2.32 kPa (0.49 kPa range) within the CN+ group and 2.20 kPa (0.33 kPa range) within the AD group. A significant difference was found between the three groups (P = 0.0055, Kruskal-Wallis one-way analysis of variance). Both the CN+ and CN- groups were significantly different from the AD group.

Conclusion: 3D MRE of the brain can be performed reproducibly and demonstrates significantly reduced brain tissue stiffness in patients with AD.

Figure 1

Summary plot of reproducibility results. Each marker represents the median stiffness for a separate MRE exam. Each subject underwent 4 exams over 2 days. Each subject displays a tight distribution of stiffness measurements, all with a coefficient of variation less than 3.1%.

Figure 2

Plot of the median stiffness for each volunteer. Lines indicate the median stiffness of each group. The AD group had a significantly lower median stiffness than the CN- group (**p=0.0015) and the CN+ group (*p=0.026).

Figure 3

Example brain MRE images collected with a spin-echo EPI pulse sequence (axial plane, TR/TE=1500/61.3 ms). Images in the top row show the results from an 89-year-old male CN- subject and the bottom row shows results from an 93-year-old male subject with AD. EPI magnitude images for each subject are shown in the first column. The second column shows the first time offset of the z component of the curl of the MRE wave images. The resulting elastograms are shown in the last column.