Noninvasive dynamic vascular imaging: arterial spin labeling-based noncontrast magnetic resonance digital subtraction angiography for cerebral disease diagnoses.

Digital subtraction angiography (DSA) is the gold standard for assessing cerebrovascular hemodynamics. DSA is predominantly utilized to evaluate the hemodynamic information of various cerebral diseases. However, DSA is relatively invasive and involves radiation exposure and risks of allergic reactions or renal dysfunction related to iodine-based contrast agents. Magnetic resonance DSA (MRDSA) with a gadolinium-based contrast agent is a popular method for assessing hemodynamics without radiation exposure. However, it has the disadvantage of the risk of allergy due to the contrast agent and lower temporal and spatial resolution than DSA. In recent years, arterial spin labeling-based noncontrast MRDSA (ASL-MRDSA), which utilizes blood labeled with radiofrequency pulses as an endogenous tracer, has been developed. ASL-MRDSA exhibits the same temporal resolution as DSA; therefore, it can yield dynamic information about various cerebral diseases. Pulsed ASL has been used as the major labeling method. However, the recent development of the pseudocontinuous ASL method has enabled high ASL signal maintenance even at delayed phases. This advancement allows for a more detailed assessment of hemodynamic information about vascular diseases, including major artery severe stenosis, occlusion, and moyamoya disease. Furthermore, combining it with superselective labeling pulses also allows for a more detailed hemodynamic assessment of complex vascular malformations such as DSA. In addition, ASL-MRDSA using ultrashort-echo time can acquire signals before phase dispersion occurs, which reduces artifacts induced by metallic materials and blood flow. This technique is useful for assessing post-clipping, post-coiling, and pseudostenosis. Furthermore, ASL-MRDSA using a wheel scan mode without full sampling achieves higher temporal resolution than DSA, which enables blood flow evaluation within giant cerebral aneurysms. Herein, we introduce the basic principles and various novel techniques of ASL-MRDSA and demonstrate its clinical applications, pitfalls, and limitations in cerebral disease diagnoses.