Introduction
Recent randomized controlled trials have confirmed the effective of reperfusion therapy in patients with acute ischemic stroke (AIS) and large-vessel occlusion (LVO)¹. Perfusion imaging is commonly used to assess the tissue-level reperfusion status. However, perfusion parameters can only provide an indirect estimate of whether tissue is viable and are not directly translatable to events at the cellular level². Amide proton transfer (APT) imaging can clarify the metabolic microenvironment of ischemic tissue³. However, few clinical studies to date have examined its use in assessing the tissue microenvironment in AIS patients after reperfusion therapy. In the current study, we used a numerical fitting of extrapolated semi-solid magnetization transfer reference (NEMR) method, which has been introduced to improve the detection of ischemic stroke in our previous study. We hypothesis that APT signals can indicate the effects of tissue-level reperfusion after therapy, thus could provide complementary information to perfusion for predicting neurological outcomes.

Methods
We prospectively recruited 63 patients with AIS and LVO, who received reperfusion therapy. Multiparametric MRI including diffusion-weighted imaging (DWI), arterial spin labeling (ASL) and APT imaging were performed within 1 week after reperfusion therapies.

Single-slice APT images were acquired at a far off-resonant frequency (>1500 ppm) for signal normalization, and 62 saturated frequency offsets with a saturation power of 1μT. Quantitative values of APTw, APT^#, NOE^# and the percentage change (PC) of APT^# and NOE^# between infarction and contralateral normal-appearing white matter were calculated. The mismatch pattern between APT and DWI were qualitatively analyzed and classified as either APT < DWI or APT ≥ DWI.

The association between APT signals and status of tissue reperfusion were investigated. Univariable analyses followed by logistic regression analysis were performed to investigate independent variables associated with favorable functional outcomes (mRS ≤ 2) at 90 days.

Results
Patients with successful reperfusion showed higher APT^# (5.29±0.66 versus 4.73±0.77, P = 0.005), higher PC of APT^# (-0.14±0.11% vs. -0.22±0.15%, P = 0.018), and a higher proportion presenting the mismatch pattern of APT < DWI (77.3% versus 47.4%, P = 0.037), compared to those with poor reperfusion.

Patients with favorable outcomes at 90 days had a lower NIHSS_24h (median, 3 versus 10, P < 0.001), a higher APT^# (5.30±0.67 versus 4.58±0.67, P < 0.001), a higher PC of APT^# (-0.14±0.13% versus -0.23±0.11%, P = 0.017) and a higher proportion presenting the mismatch pattern of APT < DWI (80.9% versus 31.3%, P < 0.001), compared to those with poor outcomes.

In a subgroup analysis of patients with successful reperfusion, a higher APT^# (5.50±0.60 versus 5.01±0.64, P = 0.012) and a higher proportion presenting the mismatch pattern of APT < DWI (92.0% vs. 63.2%, P = 0.027) could be observed in patients with excellent outcome at 90 days (90d-mRS ≤ 1), compared to those with 90d-mRS＞1.

APT^# (odds ratio [OR], 11.6; [95% CI, 1.1-119.8], P = 0.040) and a mismatch pattern of APT < DWI (OR, 5.5; [95% CI,1.2-24.6], P = 0.024) could independently predict good neurological outcome besides NIHSS_24h and successful reperfusion.

Discussion
Perfusion parameters cannot reflect tissue metabolism⁴. After reperfusion therapy, part of tissue may remain hypoxia due to heterogeneous oxygen distribution, leading to lactate accumulation⁵. Our results showed that quantitative APT^# value could be an effective imaging biomarker to characterize the tissue microenvironment after reperfusion. A higher APT signal reflected an increase in pH and an alleviation of tissue acidosis after successful reperfusion⁶.

An APT deficit lager than DWI deficit is commonly observed after ischemia, defined as APT-DWI mismatch, and is considered as metabolic penumbra—tissue at risk to infarction but can be still viable^7,8. We found a higher proportion of patients presenting with a mismatch pattern of APT < DWI after successful reperfusion, indicating a reversable metabolic penumbra after effective therapies. We suspected that part of the DWI lesions captured by APT/DWI mismatch may also be viable and reversible.

Both APT^# value and a mismatch pattern of APT<DWI independently associated with 90-day functional outcomes besides perfusion and clinical parameters. In particular, a higher APT^# and a mismatch pattern of APT<DWI in patients with successful reperfusion may indicate excellent functional outcomes at 90 days. These findings suggested that APT imaging could be potentially used to complement perfusion or diffusion imaging, as APT could clarify tissue metabolism after reperfusion therapy.

Conclusion
APT^# and mismatch pattern between APT and DWI (APT < DWI) could be effective imaging marker to reflect tissue-level reperfusion status, thus providing complementary information to cerebral perfusion for predicting 90-day neurological outcomes in patients with AIS and LVO.

Acknowledgements

No acknowledgement found.

References

1. Nogueira RG, Jadhav AP, Haussen DC, et al. Thrombectomy 6 to 24 Hours after Stroke with a Mismatch between Deficit and Infarct. New England Journal of Medicine 2018;378(1):11-21.

2. Leigh R, Knutsson L, Zhou J, van Zijl PCM. Imaging the physiological evolution of the ischemic penumbra in acute ischemic stroke. Journal of Cerebral Blood Flow & Metabolism 2017;38(9):1500-1516.

3. Zhou J, Payen JF, Wilson DA, Traystman RJ, van Zijl PC. Using the amide proton signals of intracellular proteins and peptides to detect pH effects in MRI. Nat Med 2003;9(8):1085-1090.

4. Cheung J, Doerr M, Hu R, Sun PZ. Refined Ischemic Penumbra Imaging with Tissue pH and Diffusion Kurtosis Magnetic Resonance Imaging. Transl Stroke Res 2021;12(5):742-753.

5. Weiss HR, Grayson J, Liu X, Barsoum S, Shah H, Chi OZ. Cerebral Ischemia and Reperfusion Increases the Heterogeneity of Local Oxygen Supply/Consumption Balance. Stroke 2013;44(9):2553-2558.

6. Park JE, Jung SC, Kim HS, et al. Amide proton transfer-weighted MRI can detect tissue acidosis and monitor recovery in a transient middle cerebral artery occlusion model compared with a permanent occlusion model in rats. Eur Radiol 2019;29(8):4096-4104.

7. Harston GW, Tee YK, Blockley N, et al. Identifying the ischaemic penumbra using pH-weighted magnetic resonance imaging. Brain 2015;138(Pt 1):36-42.

8. Heo HY, Zhang Y, Burton TM, et al. Improving the detection sensitivity of pH-weighted amide proton transfer MRI in acute stroke patients using extrapolated semisolid magnetization transfer reference signals. Magn Reson Med 2017;78(3):871-880.