CIRCLE OF WILLIS - MORPHOLOGIC VARIATION ON 3-DIMENSIONAL TIME-OF-FLIGHT MR ANGIOGRAMS

PURPOSE: To establish normal reference values for the presence. of the anatomic variants of the circle of Willis and average diameters for i ts component vessels by using three-dimensional time-of-fight magnetic resonance(MR) angiography and to determine whether age- or sex-relate d differences exist in the circle's anatomy. MATERIALS AND METHODS: On e hundred fifty volunteers were grouped according to age: those aged 2 0-25 years (n = 50) and those aged 60-88 years (n - 100). All subjects underwent three-dimensional time-of-flight MR angiography of the arte rial circle at 1.5 T. The anatomic variants of the anterior and poster ior parts of the circle were determined separately, the completeness o f the entire circle was assessed, and the diameters of all component v essels were measured. RESULTS: On MR angiograms, 111 (74%) subjects de monstrated a complete anterior part of the circle, 78 (52%) demonstrat ed a complete posterior part of the circle, and 63 (42%) demonstrated an entirely complete circle of Willis (complete anterior and posterior parts of the circle combined). The presence of an entirely complete c ircle of Willis was slightly higher in younger persons and in women. M ost vessel diameters were smaller in women, except for the diameter of the posterior communicating artery. Statistically significant differe nces were found in vessel diameters between the younger and the older age groups. CONCLUSION: The authors determined normal reference values for morphologic variants and diameter measurements of the circle of W illis specific to three-dimensional time-of-flight MR angiography.