MRA

Hi, I am a newer tech and have often heard people say that MRA brain/head orders that are w/wo contrast are pointless and don’t need it where as it’s very normal for an MRA neck to be w/wo contrast. Can anyone explain to me why an MRA head w/wo contrast is virtually useless and why an MRA neck w/wo contrast is beneficial. Hope this makes sense TIA!!! I love learning new things every day but get intimidated to ask my coworkers these questions now that I’m not a student anymore.