Hemorrhagic stroke in SCD is relatively more common in adults than in children. Although it is rare, primary hemorrhagic stroke, such as subarachnoid and parenchymal hemorrhage, is frequently fatal in SCD patients. Aneurysms are the most common identified cause of subarachnoid hemorrhage in adult patients with SCD [27], although their exact prevalence is unknown. Compared with patients without SCD, aneurysms in SCD patients are often multiple, have an increased propensity for the posterior cerebral circulation, and may be prone to rupture at smaller sizes [38]. Moyamoya syndrome is a common cause of parenchymal hemorrhage in SCD in children [8], which will be discussed later in more detail.

Non-contrast CT is the first imaging modality of choice in the diagnosis of acute hemorrhagic stroke (Figs. 7 and 8). In the current clinical practice, MRI may occasionally be obtained prior to CT when there is high suspicion for acute stroke based on clinical symptoms. Radiologists should be familiar with the MR imaging features of acute hemorrhage that are the same as non-SCD patients, to avoid delayed diagnosis of hemorrhagic strokes. The accuracy of MRI in detecting acute parenchymal hemorrhage is similar to CT, especially when gradient echo sequences, such as T2*-WI imaging and SWI, are used [39]. Catheter angiogram using three-dimensional (3D) digital subtraction angiography is still the gold standard for the diagnosis and preoperative delineation of intracranial aneurysms. Noninvasive techniques such as MRA and CTA are more commonly used, and recently their accuracy is considered satisfactory [40, 41]. CTA is often the first-line modality to search for underlying aneurysms in the setting of acute subarachnoid hemorrhage, while MRA is commonly used in the non-acute setting (Figs. 9 and 10).

SCI is the most common neurovascular complication in both pediatric and adult patients with SCD. SCIs have been seen in children under the age of 6 years [42]. They may represent a different pathogenesis from either ischemic or hemorrhagic strokes as they involve small vessels in watershed distributions instead of the larger cerebral vessels [43]. Measurement of arterial velocities with transcranial Doppler (TCD) ultrasound are not predictive of SCI risk, unlike with overt ischemic stroke, which supports a different pathophysiology between overt stroke and SCI [44]. Several studies showed that SCIs are more frequent in patients with arterial stenosis and intracranial arterial tortuosity, suggesting that large vessel disease may play a role in the pathophysiology of these lesions [42, 45, 46]. Children with arterial stenosis are at higher risk of brain parenchymal injury as they have more SCIs [47] (Fig. 10).

The presence of SCI is considered a predictor of future strokes [17, 48, 49]. SCIs have been shown to progress in both number and size over time [49]. Associated neurocognitive abnormalities also may be progressive [50]. Small foci of acute SCI may be found in asymptomatic children with SCD [42]. SCIs may continue to progress in spite of initiation of chronic transfusion therapy after a stroke [51]; however, a recent study showed that in patients without a stroke and not at high risk of a stroke, chronic transfusions reduced the occurrence of new SCIs by 56 % over a median observation period of 3 years [23].

SCIs are defined as T2 high signal intensity lesions of at least 3 mm in the greatest linear dimension in children and 5 mm in adults, visible on at least two planes of T2-WI images [20, 52] (Figs. 11 and 12). Because of their small size, detection of SCI depends largely on the MR imaging technique, such as the use of FLAIR, slice thickness, multiplanar acquisitions, and the magnetic field strength [53]. With further advances in imaging and more utilization of 3.0-T or higher magnet field units, it is likely that more cases of SCI will be detected in individuals with SCD [49].

Fat embolism syndrome is a very rare but potentially lethal complication of SCD that is not widely recognized [54]. In non-SCD patients, fat embolism syndrome is usually a complication of long bone fractures. The pathogenesis of fat embolism syndrome in SCD remains controversial. It is thought to be caused by bone marrow infarcts and necrosis, with subsequent embolization through osseous venous channels into the venous circulation. Neurologic dysfunction in the setting of fat embolism syndrome may result directly from vessel occlusion by fat emboli, from disruption of the blood–brain barrier by toxic free fatty acids, or both [54, 55].

Typical MR imaging findings of cerebral fat embolism include tiny foci of restricted diffusion and corresponding T2 high signal intensities representing microinfarcts. Lesions can involve the cerebral white matter, basal ganglia, thalamus, brain stem, and cerebellum. This MR imaging pattern, sometimes called a “starfield” pattern, is not very specific and can also be seen in cardiogenic or septic emboli, vasculitis, and tiny hemorrhagic metastases [54]. T2*-WI images and SWI frequently demonstrate microhemorrhages in cerebral fat embolism [55].

Vascular pathology in SCD includes arterial tortuosity, stenosis, occlusion, and aneurysm formation. A special form of cerebrovascular manifestation in SCD patients is “moyamoya syndrome,” a progressive stenosis of major intracranial arteries with formation of numerous collateral circulations, typically via lenticulostriate and thalamoperforating arteries (Fig. 13). Moyamoya syndrome typically involves the distal ICA (internal carotid artery) and proximal ACA (anterior cerebral artery) and MCA, with relative sparing of the posterior circulation. The name moyamoya syndrome is derived from moyamoya disease, a rare, presumed genetic, progressive cerebrovascular disorder. The term “moyamoya” means “puff of smoke” in Japanese and refers to the appearance of collateral vessels on cerebral angiogram [56]. The pathophysiology of moyamoya syndrome in SCD is poorly understood. It is hypothesized that sickle-shaped RBCs occlude the vasa vasorum of the carotid arteries [31], leading to intimal hyperplasia [57]. The prevalence of moyamoya syndrome in SCD patients has been reported for 35 % of conventional catheter angiogram studies [7] and in 20–40 % of MRA studies [8, 58]. It was shown that 75 % of patients with moyamoya syndrome demonstrated radiographic progression of arteriopathy and 65 % of patients showed clinical progression during follow-up [59] (Fig. 14).

Moyamoya syndrome can be detected on MRA as progressive large vessel occlusions and major collateral flow patterns, the so-called moyamoya vessels. On T2-WI images, moyamoya syndrome presents with loss of flow voids in the circle of Willis (Fig. 15) and on time-of-flight MRA as a loss of flow-related signal (Fig. 16). Collaterals typically present as multiple flow voids on T2-WI and T1-WI images in the thalamoperforate and lenticulostriate territories. High signal intensity vessels within the cortical sulci on FLAIR images can indicate a decreased flow of cortical arteries [37] (Fig. 6).

Incidentally discovered asymptomatic moyamoya syndrome in children has the potential to progress, both radiographically and clinically. Monitoring and screening with imaging in high-risk patients facilitates early referral to neurosurgery and allows for timely revascularization therapy [59]. Patients with moyamoya syndrome may have a particularly poor prognosis and may benefit from revascularization of ischemic brain parenchyma by establishing collaterals from the external carotid artery (ECA) branches to the ICA territories [60]. Superficial temporal artery (STA) to middle cerebral artery (MCA) bypass is a direct revascularization procedure, which is technically difficult. Indirect revascularization procedures such as encephaloduroarteriosynangiosis (EDAS) and pial synangiosis are becoming popular for the surgical treatment of moyamoya syndrome [61]. Pial synangiosis is a technique in which the dura and the arachnoid are opened and the STA adventitia is sutured directly to the pia [62], and it has a relatively low risk of complications [63]. Pial synangiosis typically results in an increase in collaterals from the STA or middle meningeal artery (MMA) to the brain [62]. MRA or CTA can be used to monitor the patency of synangiosis between STA branches and pial arterial branches after cerebral revascularization (Fig. 14) .

Extracranial ICA stenosis is rare, but it has been reported that there is an association between the presence of extracranial internal carotid arteriopathy and stroke risk in SCD [64–66]. Doppler ultrasound screening studies found stenosis or occlusion of the cervical ICA in 3–6 % of the children, and the majority of these also had strokes [65]. One MRA study reported that patients with an acute neurologic event, TCD abnormalities, and previous history of stroke also showed cervical ICA stenosis and/or occlusion. The most commonly involved artery is the proximal ICA, within 1 cm from its origin from the CCA [66] (Fig. 17). In addition, a recent study showed that the high velocity of extracranial ICA detected with submandibular Doppler ultrasound is highly predictive of intracranial arterial stenosis seen on MRA [67]. The differential diagnosis of extracranial ICA stenosis and/or occlusion includes atherosclerosis, arteritis, and fibromuscular dysplasia. Age, location, and lack of systemic inflammation or hypercoagulability help differentiate SCD-related extracranial internal carotid arteriopathy from other conditions [68].

Arterial tortuosity is considered to be an adaptive response to chronic anemia in patients with SCD. Chronic anemia is associated with increased cardiac output and high blood flow velocity due to the increased need for blood supply, which can lead to arterial tortuosity and can occur in all cervical arteries [68]. Cerebrovascular tortuosity may serve as a predictor of chronic brain hypoxia [53]. On imaging, tortuosity can be diagnosed when the following features are present: dilatation or ectasia of an artery segment, abnormal increase in length of an arterial segment, and obvious bowing of an artery [53, 69] (Figs. 17, 18, and 19) .

There have been several case reports of posterior reversible encephalopathy syndrome (PRES), also known as reversible posterior leukoencephalopathy syndrome (RPLS), in patients with SCD [70–74]. The vast majority of the patients are in the pediatric population [71]. It is believed that there are two major pathomechanisms in PRES: a cytotoxic event after an increase in blood pressure that causes vasoconstriction and cerebral ischemia or a vasogenic etiology where hypertension in combination with endothelial dysfunction and failure of cerebral autoregulation causes vasodilation [70]. Studies have identified PRES as a common neurological complication of SCD, and this entity needs to be considered when evaluating a child with acute neurological deterioration, especially if the patient is hypertensive, has history of ACS, and has received systemic steroid therapy or blood transfusion [72–74]. A diagnosis of PRES may predict a high risk for morbidity and mortality [73]. On MR imaging, PRES usually presents as symmetrical areas of high signal intensities on T2-WI and FLAIR images without associated restricted diffusion, suggesting vasogenic edema, which is the same as non-SCD patients [73] (Fig. 20) .

There are several osseous complications related to SCD. These include chronic bone marrow changes in response to long-standing anemia and acute bone infarcts as well as osteomyelitis. Osteoporosis and iron deposition in the bone marrow due to repeated transfusions can also be seen [13, 15, 68, 75, 76].