Diffusion-weighted imaging of cerebral infarctions: Are higher b values better?

Purpose: The purpose of this work is to determine whether high b-value (b = 3,000 s/mm²) diffusion-weighted (DW) imaging is superior to low b-value (b = 1,000 s/mm²) DW imaging for the detection of cerebral infarctions older than 6 h. Method: Echo planar DW imaging was performed at 1.5 T in 26 consecutive patients (mean age 66 years) referred for clinical diagnosis of definite acute/subacute cerebral infarction (6 h to 14 days old). The DW imaging sequences were performed using matched parameters (TR = 10,000 ms, TE_eff = 97 ms, FOV = 24 cm, 128 × 192 matrix, slice = 5 mm, NEX = 2) with b values of 1,000 and 3,000 s/mm². Areas of infarction were compared visually by two experienced neuroradiologists. Quantitative measures of MR signal and noise levels in the infarcted areas compared with contralateral normal brain were also obtained. Results: The median time after infarction was 2.5 days (range 10 h to 14 days). By visual inspection, all infarctions were reliably identified on both the b = 1,000 and the b = 3,000 images. The gross signal ratio (infarct/normal brain) was approximately 33% higher in the b = 3,000 images, but the b = 3,000 images were rated as noticeably "noisier" by both observers in every case. This visual observation was confirmed quantitatively: The signal-to-noise (SNR) and contrast-to-noise (CNR) ratios were 70% and 51% higher in the b = 1,000 than the b = 3,000 images (p < 0.0005 for both). Conclusion: For the evaluation of late acute/subacute cerebral infarctions, high b-value (b = 3,000 s/mm²) DW imaging offers no apparent diagnostic advantages compared with b = 1,000 images and is significantly inferior in terms of SNR and CNR.