Attenuation correction remains a challenge in simultaneous PETMR, as MR signal is not directly related to attenuation. This is particularly problematic in PETMR studies of the brain, where accurate radiotracer quantification is extremely important. Here, we present a method for generating individual-specific transmission data for attenuation correction from an input MPRAGE MR volume. The method uses an atlas of matched MPRAGE and transmission images in order to achieve this. Our method does not add to scan time, as do common MR-based methods, and directly yields attenuation data for PET energies, unlike CT-based methods.