Introduction

Sickle cell disease (SCD) is an inherited hemoglobinopathy that leads to chronic hemolysis, vaso-occlusion, and endothelial injury. The central nervous system is a major target of disease-related morbidity. In SCD, the brain can be affected by overt ischemic stroke, silent cerebral infarction, hemorrhage, and progressive vasculopathy. MRI has become the reference standard for identifying these abnormalities, while MR angiography (MRA) provides an adjunctive view of the major intracranial arteries.

Why MRI Is Preferred

Computed tomography is often used in the acute setting to exclude hemorrhage, but MRI offers superior soft tissue resolution and sensitivity to ischemia. In children and adults with SCD, MRI can demonstrate infarcts that produce no clinical symptoms, known as silent cerebral infarcts. These lesions are associated with cognitive difficulties, poor school performance, and an increased risk of future stroke. Therefore, MRI screening is recommended in at-risk populations, even when neurologic examination findings are normal.

Key MRI Sequences Used in SCD

A dedicated SCD imaging protocol typically includes both structural and vascular sequences. The most important sequences are listed below.

• T1-weighted imaging: Useful for anatomy and for detecting subacute hemorrhage or fat-containing lesions. It also helps in assessing brain volume and atrophy.
• T2-weighted and FLAIR imaging: Fluid-attenuated inversion recovery (FLAIR) is the mainstay for detecting infarcts, especially small cortical or subcortical lesions. Acute and chronic ischemic changes appear as areas of abnormal signal in the parenchyma.
• Diffusion-weighted imaging (DWI): DWI is the most sensitive sequence for acute ischemia. Cytotoxic edema restricts water diffusion, producing bright signal on DWI with corresponding low values on the apparent diffusion coefficient (ADC) map.
• SWI or T2* sequences: Susceptibility-weighted imaging (SWI) detects hemosiderin, calcification, and venous structures. It is helpful for identifying prior hemorrhage, cavernous malformations, and microbleeds, which can occur in SCD patients receiving chronic transfusion therapy.
• MR angiography (MRA): Time-of-flight MRA of the circle of Willis evaluates for moyamoya syndrome, stenosis, and occlusion. In SCD, progressive narrowing of the distal internal carotid, middle cerebral, and anterior cerebral arteries can develop, often in a moyamoya pattern.

Imaging Findings in SCD

Silent Cerebral Infarction

Silent cerebral infarcts are the most common neurologic injury in SCD, seen in roughly one-third of children by age 14. They appear as focal areas of FLAIR hyperintensity, usually in the deep white matter of the frontal and parietal lobes, basal ganglia, and thalamus. They do not localize to a classic vascular territory in many cases. Over time, repeated silent infarcts can lead to progressive white matter burden and cognitive decline.

Acute Ischemic Stroke

Acute infarction in SCD is most often seen in the territory of the middle cerebral artery. On DWI, the infarct appears bright with restricted diffusion. Concurrent MRA may demonstrate stenosis or occlusion of the parent vessel. In children, acute stroke is frequently associated with a recent acute chest syndrome, infection, or dehydration, while in adults, hemorrhagic transformation and large-artery disease become more common.

Moyamoya Syndrome

Moyamoya, meaning puffy smoke, refers to the appearance of tiny collateral vessels that develop near the skull base when the major arteries are narrowed. In SCD, this is a severe complication that increases the risk of both ischemic and hemorrhagic events. MRA shows the narrowed vessels, while the collateral network may be visible as fine vascular channels near the basal ganglia.

Hemorrhage and Microbleeds

Intraparenchymal hemorrhage is a major cause of death in adults with SCD. SWI can reveal microbleeds that are invisible on other sequences, helping to stratify bleeding risk. Prior hemorrhages appear as areas of susceptibility artifact with blooming on SWI.

Role of MRI in Transfusion Therapy

Chronic red cell transfusion therapy reduces the risk of stroke in SCD patients with abnormal transcranial Doppler velocities. MRI and MRA are used to monitor for progression of cerebral vasculopathy and to assess for new silent infarcts while on transfusion. In patients who are candidates for hematopoietic stem cell transplantation, baseline MRI is performed to document pre-existing brain injury, and postoperative MRI helps evaluate for posterior reversible encephalopathy syndrome or other treatment-related complications.

Challenges and Artifacts

MRI in SCD patients can be technically challenging. Motion artifact is common in children, and sedation may be required in young patients. The presence of ferromagnetic dental hardware or other implants can create susceptibility artifacts. Additionally, chronic anemia alters blood flow, and the differentiation of acute versus chronic thrombus in the dural sinuses can be difficult. Newer techniques such as vessel wall imaging are being studied to better characterize arterial wall inflammation and remodeling in SCD.

Emerging Advanced MRI Techniques

Quantitative susceptibility mapping can estimate tissue oxygen extraction and iron deposition in the brain, offering insight into chronic hypoxia. Arterial spin labeling (ASL) provides a measure of cerebral blood flow without intravenous contrast. These advanced MRI techniques hold promise for identifying patients at highest risk of infarction, although they remain primarily research tools at present.

Conclusion

MRI is indispensable in the care of patients with sickle cell disease. It detects silent cerebral infarction, characterizes acute ischemic events, identifies hemorrhage and microbleeds, and assesses the intracranial vasculature. The combination of structural MRI, DWI, SWI, and MRA allows clinicians to monitor disease progression and to select appropriate preventative therapies such as chronic transfusion. As advanced imaging techniques continue to evolve, the role of MRI in guiding personalized treatment for SCD is expected to expand further.