Introduction

Amide proton transfer (APT) imaging is a type of chemical exchange saturation transfer (CEST) that can detect intracranial metabolic changes at the molecular level. APT has been applied in studies of brain injury, brain tumors, and neurodegenerative diseases.^1,2. The incidence of middle cerebral arterial (MCA) stenosis increases yearly, molecular biomarkers for this condition are needed. APT benefits from increased field strength, which improves signal-to-noise ratio, and widens frequency distance between compounds^3,4. Currently, few studies have applied CEST to investigate the changes of APT in MCA stenosis patients. This study aims to explore the value of APT in patients with unilateral MCA stenosis, by analyzing both the affected side and the unaffected sides of the brain.

Methods

Eight patients (aged 59.50±10.35 years, five men and three women), diagnosed with unilateral MCA stenosis or occlusion, were enrolled in this study. All the patients underwent routine MR examinations including T2w, T1w, time-of-flight MR angiography (TOF-MRA), and CEST. The examination was performed at a 7.0T whole-body scanner (MAGNETOM Terra, Siemens Healthcare, Erlangen, Germany) equipped with an 8Tx/32Rx head coil. 3D fast spoiled gradient echo was used in CEST imaging, with the following parameters: field of view 220 mm, acquisition matrix 128x128, slice thickness 5 mm, number of slices 12, flip angle 5-degree, echo time of 3.09 ms and repetition time of 5 ms. Fifty-six offset frequencies were sampled, using the recommended frequency table listed on the website [https://www.cest-sources.org]. Two B1 amplitude, 0.6uT and 0.9uT were acquired with an extra B1 map acquired to correct for B1 inhomogeneity⁵. The APT map was analyzed from the original CEST images and corrected for B1 inhomogeneity. The MCA supply region was outlined in ITK-SNAP and the mean APT value of the corresponding region was calculated. Statistical analyses were performed using SPSS version 26.0 (IBM Corporation, Armonk, NY, USA). Paired t-tests were utilized to compare changes in APT values between vessels on the affected and normal sides. Interobserver agreement of the MRI readings was assessed using the intraclass correlation coefficient^6,7.

Results

The APT values of the affected MCA blood-supplying brain regions in the eight patients with unilateral MCA stenosis or occlusion were 0.028±0.006 %, and those of the healthy MCA blood-supplying brain regions were 0.024±0.002 %. Figure 1 shows a comparison of the APT values. Figure 2 shows CEST and conventional MRI images of a patient with MCA stenosis occlusion. Although t-tests revealed no statistically significant differences in APT values (P=0.108), the results show that the APT values were higher for the affected MCA supplied brain area than for the healthy MCA-supplied brain area.

Discussion and Conclusion

We quantitatively evaluated the APT of the affected and healthy hemisphere in patients with unilateral MCA stenosis. Our results indicate that the APT value of the affected side is markedly higher than that of the healthy side, indicating that metabolic changes occur intracranially in patients with MCA stenosis, thus providing a new imaging marker for patients with MCA stenosis. Whether these abnormalities precede the development of intracranial complications requires further exploration.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No.81825012, No. 82151309 to X.L. and No. 82271952 to J.H.L)

References

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