Hypoxic-ischemic encephalopathy (HIE) is a common reason for the early death of neonates. HIE occurs in 1 to 8 per 1000 live births in developed countries [1]. Some asphyxiated newborns have decreased cerebral blood flow (CBF) on the first day of life, which may be related to the development of their brains and hypoxic-ischemic injury, and hyperperfusion usually reaches the peak on days 2–3 of their lives [2, 3].

Magnetic resonance imaging (MRI) is the preferred method for HIE diagnosis. The water content in brain tissue increases after hypoxia, leading to prolonging T1 and T2 values on the MR image [4]. Studies have shown that T1-weighted (T1WI) and T2-weighted (T2WI) imaging is sensitive to the pathological processes of neuronal necrosis and water changes in neonates of 3–7 days and 6–10 days, respectively [4, 5]. In clinical practice, dynamic susceptibility contrast-enhanced perfusion MRI is commonly used to evaluate the microcirculation of brain tissue. With this technique, the hemodynamics, such as CBF, mean transit time, cerebral blood volume, and time to peak, can be evaluated through the time-signal intensity curve following the administration of contrast agents. However, its application in pediatrics is limited because of the administration of contrast agents, and few studies are using perfusion-weighted imaging in the very immature brain [6, 7].

Arterial spin labeling (ASL) is a new MR technique by which the regional CBF can be quantified using CBF images. It has been reported that ASL can evaluate abnormal brain perfusion in neonatal brain injury [8, 9]. Diffusion-weighted imaging (DWI) is to date the only non-invasive method to detect water molecule diffusion in vivo. Compared with the conventional MRI, DWI can detect the pathological process of early hypoxic-ischemic lesions and quantitatively measure the apparent diffusion coefficient (ADC) [10, 11, 12, 13], which is positively related to the diffusion rates of water molecules. In the case of restricted diffusion, the signal intensity exhibits an increase on the DWI image and a decrease on the ADC map.

We measured the CBF and ADC values of the full-term HIE neonates to determine the diagnostic performance of ASL and DWI for HIE.

ASL is an MR perfusion imaging method, first proposed and applied in animal experiments by Detre et al. in 1992 [15]. It was used in evaluating human brain perfusion for the first time in 1994 by Roberts et al. [16]. ASL employs magnetic marked arterial blood as an endogenous contrast medium and involves pulsed arterial spin labeling (PASL) or continuous arterial spin labeling (CASL) [17]. We adopted PASL technology, safe, non-invasive, convenient, no need for exogenous contrast agent, allowing repeated measurements, and less post-processing. However, PASL is relatively limited by the low signal-to-noise ratio (SNR) and poor spatial resolution. The CBF is lower, and the blood water content is higher in neonates than in adults, resulting in a higher T1 value and lower SNR [18, 19, 20]. Although CASL has been shown to generate a higher SNR and provide a better perfusion contrast than PASL, it may lead to neonatal radiofrequency burn because of the high specific absorption rate [21]. In addition, CASL may overestimate the perfusion because its longer labeled pulse could produce a magnetization transfer effect. Little is known for recently developed ASL methods such as pseudo-continuous arterial spin labeling for their application in pediatrics due to their complexity [22]. The PASL was used to measure the CBF, showing a sensitivity of 33.3% and a specificity of 90.9% in the diagnosis of HIE. The sensitivity of CBF was lower than that of conventional MRI and DWI (41.6% and 50%, respectively), which may be due to the poor resolution of PASL. The low sensitivity and specificity for DWI are likely due to the wide time range of the MRI.

Additionally, the sensitivity and specificity of ASL were different in our work, which was carried out by de Vis et al. [9]. The sensitivity was 85.7%, and the specificity was 100%. This may be due to the performance time of MRI. The MRI was performed within 15 days after birth, while, in de Vis et al. [9] results, the MRI was performed 2–7 days after birth.

The HIE lesions in 7 cases showed high signal intensity on DWI images and low signal intensity on ADC maps. In contrast, their signal intensity was normal in the conventional T1WI, T2WI and T2FLAIR sequences. These results indicated that DWI was more sensitive to the pathological changes in HIE than conventional MRI sequences, which is consistent with previous studies [12, 23]. Previous studies have reported that brain perfusion may be affected by developmental and postnatal age [16, 17, 18]. Reduced CBF and oxygen supply may result in a series of biochemical reactions and ultimately lead to cell damage in HIE [24]. For acute cell injury, the diffusion rate of water molecules in cells may reduce due to cytotoxic edema, thus showing decreased ADC values and high DWI signal intensity [25]. At the early stage of HIE, conventional MRI sequences may not detect the lesions because there is neither a significant change of the water molecule content nor necrosis in the damaged brain tissue.

However, 5 HIE cases showed no abnormalities on DWI images and ADC maps but the abnormal signal intensity in conventional MRI sequences. This result may be explained that the hypoxic-ischemic lesion may be in the subacute or early recovery stage. In this stage, selective necrosis of the cortex and proliferation of glial cells can cause the increase of water molecule diffusion and ADC values. Thus the high signal intensity in the infarction area may gradually develop into an isointensity signal on DWI images [12, 26]. Another possible explanation might be that the lesions may be in the period of ADC pseudonormalization. The cerebral vessels could automatically adjust the blood flow in 5 to 10 days, which may cause reperfusion injury and blood-brain barrier damage [27]. Therefore, water molecular diffusion increases and the ADC values return to normal [28]. Winter et al. [29] have demonstrated that ADC may show the “pseudonormalization” phenomenon in the HIE neonates 10 days from birth in the areas of frontal and occipital white matter, thalamus, lentiform nucleus, and posterior limb of the internal capsule. During this period, DWI may not exhibit abnormal signal intensity.

Comparative analysis showed that the CBF values in the thalamus and lentiform nucleus were higher. The ADC values in the thalamus, frontal white matter, and lentiform nucleus were lower in the HIE cases than those in the Controls. The CBF and ADC values changes in the thalamus and lentiform nucleus were more significant than in the other regions. There was a negative correlation between the CBF and ADC values. These results indicate that the CBF and ADC in the hypothalamus and lentiform nucleus are more sensitive to the pathological processes of ischemia and hypoxia. The hypothalamus and lentiform nucleus vessels may be dilated, accompanied by short-term high perfusion and restricted diffusion of water molecules [30]. Pienaar et al. [31] have also shown that the decreased ADC value was correlated with increased CBF value in the hypoxic-ischemic region, consistent with our results. These indicate that ASL and DWI can help early diagnosis of HIE.

In addition, the CBF value in HIE neonates aged 1–3 days was higher than those in elder HIE neonates, but there was no significant difference among neonates aged 4 days and older. The hypoxic-ischemic cerebral injury is acute in the HIE neonates aged 1–3 days, and the blood flow compensation increases [3]. However, three days later, the brain tissue is already at the stage of ischemia-reperfusion, and auto-regulation of cerebral blood vessels is decompensated, leading to reduced cerebral blood flow and CBF values [3]. Therefore, CBF might reflect the pathological process of brain injury in HIE, which is consistent with the previous report [31]. Aslan et al. [32] have demonstrated that regional CBF was more sensitive in detecting small CBF changes than absolute CBF. In addition, the ADC values in the HIE neonates aged 1–3 days were also lower than those in the other age groups. Barkovich et al. [11] have suggested that the DWI sequence was sensitive to cytotoxic edema induced by early cerebral ischemia and could reflect the severity and range of brain injury earlier than conventional MRI sequences. They also observed that some HIE cases showed decreased ADC values but no abnormal findings on the DWI images, indicating that the ADC value measurement is more accurate and sensitive than the DWI image. These indicate that CBF and ADC values are important in the 1st–3rd days of life. Liauw et al. [33] and Rutherford et al. [34] have obtained similar results, showing reduced ADC values but no abnormal signal intensity on DWI images in some hypoxic-ischemic lesions. Nevertheless, it was noticed that the scan range of ASL was small and the scan time was relatively long, so that the temporal and spatial resolution of pulse ASL is relatively low.

We encountered some limitations apart from the small sample size, and there was a low sensitivity and specificity for DWI and low sensitivity for ASL. This is likely due to the wide time range of performing the MRI. In addition, due to instrumental limitations, the slice thickness of the MRI scan used was thicker. Finally, only a few available follow-up data were acquired from these babies, and further analysis is needed to be performed.

Our findings show that ASL noninvasively reflects the CBF changes, while DWI demonstrates the early diffusion changes in brain tissues. These two techniques could provide more detailed information on the pathological processes of HIE.

ASL, arterial spin-labeled; DWI, diffusion-weighted imaging; T1WI, T1-weighted imaging; T2WI, T2-weighted imaging; HIE, hypoxic-ischemic encephalopathy; CBF, cerebral blood flow; ADC, apparent diffusion coefficient; MRI, Magnetic resonance imaging; PASL, pulse arterial spin labeling; CASL, continuous arterial spin labeling; SNR, signal-to-noise ratio.

LM, QW, YL, XXM and WL collected and analyzed the data. LM interpreted the data and drafted the article. QW designed the study and revised it critically for important intellectual content. All authors have read and approved the manuscript.

The Ethics Committee of Qilu Hospital of Shandong University approved this work (ethical approval number: 12184). In all cases, informed consent was obtained from the parents of all neonates.

Not applicable.

This research received no external funding.

The authors declare no conflict of interest.