Question 2965
{"accessible":false,"alternatives":[{"id":14432,"text":"25 cm/s"},{"id":14433,"text":"50 cm/s"},{"id":14434,"text":"100 cm/s"},{"id":14435,"text":"180 degrees"},{"id":14436,"text":"90 degrees"}],"archived":false,"correctAlternativeId":14433,"explanation":"\u003cp\u003eThe velocity encoding (VENC) value represents the maximum velocity (in cm/s) that can be accurately measured in a PC MRA without aliasing.\u003c/p\u003e\u003cp\u003eAliasing occurs at velocities higher than the VENC value because spins change phase more than 180 degrees. Therefore, the VENC value is the velocity at which a spin will change phase by 180 degrees.\u003c/p\u003e\u003cp\u003eBy manually setting the VENC value, the appropriate bipolar phase gradient strength and duration will be applied.\u003c/p\u003e\u003cp\u003eBecause, in this example, the spin had a phase change of 180 degrees, it must be travelling at the VENC value.\u003c/p\u003e","id":2965,"imageUrl":null,"imageAttribution":null,"imageAttributionCaseInfo":null,"firstQuestionPath":"/questions/2965","nextQuestionPath":"/articles/phase-contrast-imaging/questions/2964","relatedArticles":[{"id":18476,"title":"MR angiography","link":"/articles/mr-angiography-2?lang=gb"},{"id":22558,"title":"MRI physics","link":"/articles/mri-physics?lang=gb"},{"id":84142,"title":"Velocity encoding","link":"/articles/velocity-encoding?lang=gb"}],"alsoUsedIn":[{"id":1928,"kind":"Course","title":"Imaging Physics: MRI - page 1928","link":"https://radiopaedia.org/courses/imaging-physics-mri/pages/1928"}],"stem":"\u003cp\u003eIn phase contrast MR angiography (PC MRA), blood flowing at 50cm/s through a slice exhibits 180 degrees of phase change after being exposed to the bipolar phase gradients. What is the velocity encoding (VENC) value that has been used in this sequence?\u003c/p\u003e","menuLinks":[{"text":"Report problem with question","url":"https://docs.google.com/forms/d/e/1FAIpQLSfO3soWYhOjJ7yErSysyCe5V4A1CqW7WK3rDA7MtAkecMGqNw/viewform?entry.1624461248\u0026entry.553583435=https://radiopaedia.org/questions/2965"}],"attemptsPercentages":[{"alternativeId":"14432","percentage":13},{"alternativeId":"14434","percentage":23},{"alternativeId":"14435","percentage":10},{"alternativeId":"14433","percentage":45},{"alternativeId":"14436","percentage":9}],"promptToLogin":false,"questionManager":false,"articleId":"phase-contrast-imaging"}