7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results

Magnetic resonance imaging (MRI) plays a key role in the investigation of cerebrovascular diseases. Compared with computed tomography (CT) and digital subtraction angiography (DSA), its advantages in diagnosing cerebrovascular pathology include its superior tissue contrast, its ability to visualize blood vessels without the use of a contrast agent, and its use of magnetic fields and radiofrequency pulses instead of ionizing radiation. In recent years, ultrahigh field MRI at 7 tesla (7 T) has shown promise in the diagnosis of many cerebrovascular diseases. The increased signal-to-noise ratio (SNR; 2.3x and 4.7x increase compared with 3 and 1.5 T, respectively) and contrast-to-noise ratio (CNR) at this higher field strength can be exploited to obtain a higher spatial resolution and higher lesion conspicuousness, enabling assessment of smaller brain structures and lesions. Cerebrovascular diseases can be assessed at different tissue levels; for instance, changes of the arteries feeding the brain can be visualized to determine the cause of ischemic stroke, regional changes in brain perfusion can be mapped to predict outcome after revascularization, and tissue damage, including old and recent ischemic infarcts, can be evaluated as a marker of ischemic burden. For the purpose of this review, we will discriminate 3 levels of assessment of cerebrovascular diseases using MRI: Pipes, Perfusion, and Parenchyma (3 Ps). The term Pipes refers to the brain-feeding arteries from the heart and aortic arch, upwards to the carotid arteries, vertebral arteries, circle of Willis, and smaller intracranial arterial branches. Perfusion is the amount of blood arriving at the brain tissue level, and includes the vascular reserve and perfusion territories. Parenchyma refers to the acute and chronic burden of brain tissue damage, which includes larger infarcts, smaller microinfarcts, and small vessel disease manifestations such as white matter lesions, lacunar infarcts, and microbleeds. In this review, we will describe the key developments in the last decade of 7-T MRI of cerebrovascular diseases, subdivided for these 3 levels of assessment.