The utility of high b-value DWI in evaluation of ischemic stroke at 3 T

https://doi.org/10.1016/j.ejrad.2009.10.011Get rights and content

Abstract

Purpose

The utility of DWI with high b-value in ischemic stroke is still unsettled. The purpose of this study is to compare high b-value (3000) and standard b-value (1000) diffusion-weighted images in patients with ischemic stroke at 3 T.

Materials and methods

27 patients with acute stroke who were admitted to the hospital during the first 24 h after symptom onset were included in this study. All patients had a brain MRI study with stroke protocol including standard (b = 1000) DWI and high b-value (b = 3000) DWI sequences at 3 T MR scanner.

Number and localization of the lesions were assessed MR signal intensities (SI), signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), contrast ratio (CR) and apparent diffusion coefficient (ADC) values of the lesions and normal parenchyma on DWI with b = 1000 and b = 3000 sequences were measured.

Results

All patients with acute stroke revealed hyperintense lesions due to restricted diffusion on DWI with both b-values. However, lesions of restricted diffusion were more conspicuous in b = 3000 value DWI than b = 1000, and additional 4 ischemic lesions were detected on b = 3000 DWI. SNR, CNR, SI and also ADC values in both stroke area and normal parenchyma were lower at b = 3000 than the value at b = 1000. At b = 3000, CR was significantly greater than b = 1000 images.

Conclusions

Although quantitative analysis shows higher SI, SNR and CNR values with standard b-value (b = 1000) diffusion-weighted imaging, using higher b-value may still be beneficial in detecting additional subtle lesions in patients whose clinical findings are not correlated with standard b-value DWI in stroke.

Introduction

Early and accurate detection of ischemic stroke is very crucial since the time window for treatment with TPA is narrow. Conventional T1-and T2-weighted imaging has high false negative results during the first 6 h after stroke onset [1]. Diffusion-weighted imaging (DWI) is still the most reliable method to diagnose acute ischemic stroke [1], [2], [3]. However there are limitations for DWI: the sensitivity of DWI is low in small infarctions, brain stem locations, or imaging performed very early after symptom onset [1], [2], [3], [4], [5].

There have been studies focused on improving the sensitivity of DWI and overcoming limitations in ischemic stroke. Some of the studies have evaluated higher magnetic field strengths and different b-values investigated to decrease the limitations of DWI [5], [6]. DWI at higher-field MR might improve the detection of small lesions in ischemic stroke [7]. Current MR gradient systems permit the use of higher b-values at short TEs. The commonly used b-values applied in the stroke studies with DWI are between 1500 and 5000 s/mm2. Previous studies focused on high b-value for acute or subacute stroke show no additional diagnostic advantages in comparison with standard b-value. However, it was shown that high b-value might have impact on diagnosis of hyperacute stroke or global ischemia [5], [8].

It is still debatable whether there is a role for high b-value in stroke imaging. There has been small number of studies investigating the effect of high b-value at 3 T [9], [10]. In this study we compared the high b-value (b = 3000) diffusion-weighted images (DWI) with standard b-value (b = 1000) at 3 T MR system in patients with hyperacute and acute ischemic stroke.

Section snippets

Subjects

Local institutional review board approved this study and informed consent was obtained for MR imaging from all subjects. All patients admitted to the emergency department and were examined by the staff neurologist. The patients were clinically evaluated with the National Institutes of Health Stroke Scale (NIHSS) score and Modified Rankin Scale prior to MR examination and the time between symptom onset and MR imaging was recorded.

In this prospective study, 88 consecutive patients were evaluated

Results

After using the exclusion criteria, 27 patients (19 men and 8 women, mean age 75, range 29–92) were included in the analysis. All patients were admitted to hospital due to acute stroke between 1 and 24 h (mean: 13 h) after symptoms onset. 15 patients were classified as hyperacute, 12 cases acute ischemic stroke. Majority of patients had additional chronic ischemic lesions.

The cause of ischemic stroke was classified according to the TOAST criteria [12], [13]. 11 patients (40%) had large-artery

Discussion

DWI has become the most useful sequence of the brain MRI in evaluating acute ischemic stroke, and diagnosis of cerebral infarction. DWI is more likely to be positive within minutes of onset in several animal models and more reliable than other conventional MR sequences [5], [14], [15].

By changing the duration, strength of diffusion gradients and diffusion sensitivity, the image contrast can be altered. Recently, MR gradient technology has improved and DWI with b-values up to 3000 can be

Conclusion

Although quantitative analysis showed higher SI, SNR and CNR values with standard b-value (b = 1000) diffusion weighted imaging, using of higher b-value may be beneficial in detecting additional subtle lesions in patients whose clinical findings are not correlated with standard b-value DWI in stroke.

Conflict of interest statement

There is no conflict of interest.

Acknowledgment

We thank Ali Demir, MS, for his expert statistical assistance.

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    So early on, b-values of 2500 and 3000 s/mm² were identified to have a better conspicuity, although no more lesions were detected compared with a b-value of 1000 s/mm² in a sample of 30 patients with supratentorial infarctions.10 A further study has shown additional 4 stroke lesions on DWI with high b-value of 3000 s/mm² in comparison with the lower one at 1000 s/mm² in a sample of 27 patients.24 In another small series of 6 patients with vertebrobasilar territory infarction, ischemic lesions were better identified with b-values of both 3000 s/mm² and 5000 s/mm² compared to 1000 s/mm².

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