Diffusion MRI of Small Ischemic Brain Lesions: Technical aspects, clinical experiences and diagnostic criteria

Sammanfattning: Purpose: To compare the visualization of small ischemic brain lesions using diffusion using diffusion-weighted (DW) MRI and conventional imaging, with special regard to diagnostic efficiency. Imaging features of lesions of different ages, diagnostic limitations and technical principles were also investigated. Methods: All studies were performed with the same 1.5 T MR scanner. In the first study the diagnostic efficiencies of a conventional spin-echo (SE) PD and T2-weighted sequence, an echo-planar (EP) DW sequence and a DW conventional SE sequence in detecting acute stroke lesions in 27 patients were compared. In the second study imaging features of small ischemic lesions from 21 of the 27 patients in the first study were observed until the lesions became chronic. In the third study 18 preexisting chronic lacunar infarcts were selected from the EP DW images in the first study. The lowest value of b which led to them all being invisible in the DW images was determined. In the fourth study three EP DW acquisitions with and without ECG triggering were obtained in 6 healthy volunteers. Maps of the apparent diffusion coefficient (ADC) were calculated from all the acquisitions. The value of the ADC was measured in the same four positions on each map. The dependence of the measured values of ADC on ECG triggering was evaluated. In the fifth study 12 patients with brain metastases were examined with an EP DW sequence and a T2-weighted sequence. The features of the metastases were compared with the diagnostic criteria commonly used for acute and subacute small infarcts. Results and conclusions: An EP DW sequence is very efficient in detecting new ischemic lesions. A DW SE sequence performs well but it requires a perfectly cooperative patient. A conventional PD and T2-weighted sequence has low sensitivity and specificity. Ten of 21 small ischemic lesions exhibited persisting high signals in the DW images and 2 of them exhibited a decrease in ADC two months after the stroke. All these lesions became invisible and showed a high ADC at an examination after 1 year or more. A value of b = 1000•10E06 s/m2 was sufficient to make chronic lacunar infarcts invisible in DW images when the echo time was no longer than 120 ms. Some metastases have imaging features in common with small acute and subacute ischemic lesions when a standard MR protocol for acute stroke is used.