A large number of pulse sequences have been developed since the introduction of MRI into clinical practice [10, 11, 12, 13]. There are very few studies that estimate the optimal use of sequences for the evaluation of complications in neonates with CoA requiring surgical correction.
In our study, two techniques were used for brain protection: deep hypothermic circulatory arrest with a rectal temperature of 20 degrees [5] and full-flow perfusion by double cannulation of the aorta [6]. We were able to detect CVEs in both groups of patients. However, we received unexpected results: in group 1 (DHCA) the frequency of neurological complications was much higher in group 1 (DHCA) than in group 2. In another study, the same results were obtained, and similar outcomes were frequently observed in children with DHCA [7]. However, according to actual clinical data, both perfusion techniques have been used in clinical practice based on medical team choice [8], which depends on clinical experience, previous results, and policy of the given department.
Regardless of the method for organ protection, the medical staff is required to perform cerebral monitoring during surgery and administer diagnostic procedures to determine certain complications after surgery. The wide spectrum of diagnostic methods and techniques available in routine clinical practice is greatly advanced in recent years. We have to take into account that neonates require different MRI sequences to detect pathology, according to their developmental features. When choosing a method to diagnose intracerebral ischemic complications, it is noteworthy remembering that the use of CT is inappropriate due to the low diagnostic efficacy of the method and the presence of radiation exposure [14]. By far, there is a relentless tendency in the western world for reduction of using CT in clinical practice in infants and children because of well-known high radiosensitivity among patients of this group and possible harmful effects.
On the contrary, the most advisable and widely used method to diagnose pathologic changes in the brain structures in infants is neuro-sonography [14]. However, lack of distinct parameters to assess the myelinization process and availability of limited sonographic window make this technique restricted to apply among infants and newborns. Besides, the artifacts and distortions due to anisotropic effect produced by transcranial sonography can simulate lesions of brain matter, which may result in false-positive results. Nonetheless, evaluation of periventricular hemorrhage provides a clue to wide range of pathology.
The cases of ischemic cerebral events during the early postoperative period after aortic arch surgery are not uncommon despite administration of any technique of cerebral protection [15]. One study found that certain complications often remain asymptomatic during span of time [9]. Thus, despite the availability of neuro-sonographic examination, MRI is the main method to diagnose anatomic lesions both of brain tissue and cerebral vessels after aortic arch surgery. In turn, state-of-the-art MRI equipment, deep anesthetic paralysis, and profound monitoring of vital signs are always necessary, which makes this procedure highly challenging in critically ill patients. Nonetheless, diagnostic value of MRI with high-resolution images exceeds the risks and limitations. Additionally, the aim of conducting MRI examination is to visualize ischemic lesions and perfusion voids in the brain, since they may dramatically worsen the outcome [9, 16].
Our study showed that 35.7% of patients developed embolic events, presumably due to emboli of air bubbles or blood clots causing the occlusion of cerebral arteries. This leads to distortion and signal void artifacts of perfusion and necrosis. Another consequence of ischemia is watershed cerebral infarctions localized in the vulnerable zones between the cerebral vascular regions where the tissues are supplied by the anterior, posterior, and middle cerebral arteries [17, 18, 19]. Watershed infarctions occur in the areas of the brain located between the non-anastomosing arterial vessels i.e. in the areas supplied by the terminal branches. Due to insufficient collateral circulation, the watershed areas are more vulnerable to hypoperfusion and highly depend on adequate perfusion pressure so that a decrease in blood flow leads to the occurrence of perfusion voids and ischemia in these areas. Therefore, ischemia localized in the watershed areas is a hallmark of hemodynamic stroke [17, 18]. In our study, 20% of the patients developed this complication. Other studies reported the rates of similar events ranging from 19–64% [16–18], which is comparable with our results.
The detection of stroke is crucial because it can cause a long-term neurodevelopmental handicap in children. The main neurodevelopmental dysfunctions may include motor dysfunction, oral apraxia, cognitive impairment, paroxysmal disorders (epilepsy), emotional/behavioral disorders, pain syndrome, and asthenia [16]. While assessing the flow, we should be aware that blood flow to the white matter in the premature human infant is particularly low and reaches only 17% of the blood flow to the grey matter [9, 17, 18, 19]. To identify perfusion abnormalities, DW-MRI mode was applied in the axial plane with a minimum of two b-factors (maximum 1,000 s/mm2) showing excellent sensitivity for detecting perfusion voids. DWI is suitable for any age including preterm infants since DWI is widely used to diagnose lesions of acute ischemia even in a fetus in the second trimester of pregnancy [11]. When interpreting DWI data, it is necessary to evaluate the maps of the apparent diffusion coefficient (ADC), which allows not only to exclude T2 shine-through but also, as in adults, to differentiate the stage of hypoxic-ischemic brain damage. Thus, in neonates, DWI with ADC maps plays an important role in detecting hypoperfusion abnormalities [11, 12].
In our study, MRI allowed to detect cortical laminar necrosis in three patients (7.5%) in both groups. This observation is comparable with the other studies [20]. The basic causes of laminar cortical necrosis lie in the higher metabolic activity of the cerebral cortex and, as a consequence, in the greater vulnerability of the cortex to hypoxia. In case of circulatory arrest, the cortex, as the most vulnerable structure, has a high risk of damage due to a decrease in the blood supply to the brain. Pathophysiological changes lead to coagulation necrosis in the cortex, which has a characteristic reflection on MRI sequences [14, 20].
Typical MRI feature of cortical laminar infarction is a hyper-intensity on T1-W images due to the reactive tissue alteration of the glia and deposition of fat-laden macrophages [20]. Thus, the third was the T1-3D sequence in the sagittal plane followed by reconstruction of the coronal and axial images.
The other common pathologic finding is an intracerebral hematoma. In our study, 20% of patients had this complication. Preoperatively, these patients had no pathologic cerebrovascular findings. According to other studies, these complications may occur as a consequence of using anticoagulants in the perioperative period [5, 21]. A balance between the risk of thromboembolism and prevention of excessive bleeding should be reached [5].
Therefore, all patients underwent SWI or T2 MRI modes, which are very sensitive to the local changes in the uniform magnetic field and allow to verify the presence of extravascular blood in the brain structures. Images weighted by magnetic susceptibility make it possible to differentiate de-oxyhemoglobin, iron, and calcium [10]. The T2* sequence is less specific in terms of differential diagnosis. However, the sensitivity of this technique for detecting de-oxyhemoglobin allows to rely on it with confidence in daily clinical practice in determining hemorrhages.
Myelination of the neonatal brain is far from complete and does not reach maturity until two years. By far, myelin maturation from birth to six months of age is best assessed using T1-W imaging. It is worth bearing in mind that the myelination progresses from central to periphery, caudal to cephalad, and dorsal to ventral. After completion of myelination to the age of about two years, dual-echo short-tau sequence should be used, and T2-W images are more useful over two years of age [13]. It is very important to use a dual-echo short-tau inversion recovery sequence that has improved contrast resolution in neonates instead of the T2-W sequence. With all neonates, one should evaluate the myelinization of the brain structures with age and differentiate from atrophy lesions [13, 16]. Since CoA with AAH is a critical defect, neonates should undergo surgical treatment during the first months of life [8]. Under these circumstances, the characteristics of white matter in neonates differ significantly from the MRI pattern in older children.
Therefore, for differential diagnosis of dysmyelination and atrophic changes in the white matter of the brain, axial images weighted by dual-echo short-tau inversion recovery sequence should be performed. It consists of one repetition time (TR) and two different echo times (TEs), which corresponds to two sequences [22]. Our study did not aim at finding any type of dysplasia, but five (12.5%) of patients from both groups had myelination disorders or other congenital problems which we were able to find first.
In the presence of massive lesions of acute ischemia due to embolism, it is advisable to perform non-contrast MR-angiography to determine the occlusion of large blood vessels by air embolism.
In complicated cases of subarachnoid hemorrhage, fluid attenuated inversion recovery (FLAIR) should be used when intracranial hemorrhage is suspected. It is efficient in identifying it with high sensitivity, and T2 sequences are more successful in children older than six months [14].