MR imaging detection of cerebral microbleeds: effect of susceptibility-weighted imaging, section thickness, and field strength

Abstract

Background and purpose: The emergence of cerebral microbleeds (CMB) as common MR imaging findings raises the question of how MR imaging parameters influence CMB detection. To evaluate the effects of modified gradient recalled-echo (GRE) MR imaging methods, we performed an analysis of sequence, section thickness, and field strength on CMB imaging properties and detection in subjects with cerebral amyloid angiopathy (CAA), a condition associated with microhemorrhage.

Materials and methods: Multiple MR images were obtained from subjects with probable CAA, with varying sequences (GRE versus susceptibility-weighted imaging [SWI]), section thicknesses (1.2-1.5 versus 5 mm), and magnetic field strengths (1.5T versus 3T). Individual CMB were manually identified and analyzed for contrast index (lesion intensity normalized to normal-appearing white matter signal intensity) and diameter. CMB counts were compared between 1.5T thick-section GRE and thin-section SWI for 3 subjects who underwent both protocols in the same scanning session.

Results: With other parameters constant, use of SWI, thinner sections, and a higher field strength yielded medium-to-large gains in CMB contrast index (CI; Cohen d 0.71-1.87). SWI was also associated with small increases in CMB diameter (Cohen d <0.3). Conventional thick-section GRE identified only 33% of CMB (103 of 310) seen on thin-section SWI. Lesions prospectively identified on GRE had significantly greater CI and diameter measured on the GRE image than those not prospectively identified.

Conclusions: The examined alternatives to conventional GRE MR imaging yield substantially improved CMB contrast and sensitivity for detection. Future studies based on these techniques will most likely yield even higher prevalence estimates for CMB.

Fig 1.

MR images from individual subjects illustrating increased contrast. The pairs of images illustrate comparisons of thick-section GRE (A) versus thin-section SWI (B, Table 3); thin-section GRE (C) versus thin-section SWI (D); thick-section GRE (E) versus thin-section GRE (F; all preceding images at 1.5T); and SWI at 1.5T (G) versus SWI at 3T (H). Image parameters are shown in Table 1. The black arrows in Fig 2A and B illustrate a CMB prospectively counted on both sequences, whereas lesions denoted by white arrows were initially identified only on the SWI image. The black arrows in the remaining images highlight lesions on the paired images for comparison.

Fig 2.

Detection of CMB as a function of lesion contrast and diameter. Scatterplots show CI (A), diameter (B), and a 2D plot of both parameters (C) of CMB prospectively identified (filled circles) or not prospectively identified (empty circles) by a blinded rater on clinical GRE MR images. C, The CMB in the lower right corner (solid arrow) were identified on GRE alone despite its lower CI, most likely because of its relatively large diameter. Conversely, the low-diameter CMB in the upper left corner of the figure (dashed arrow) were missed on GRE despite their relatively high CI. All measurements of CI and diameter were performed on the GRE images.