Radiology
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To establish normal reference values for the presence of the anatomic variants of the circle of Willis and average diameters for its component vessels by using three-dimensional time-of-flight magnetic resonance (MR) angiography and to determine whether age- or sex-related differences exist in the circle's anatomy. ⋯ The authors determined normal reference values for morphologic variants and diameter measurements of the circle of Willis specific to three dimensional time-of-flight MR angiography.