Artifacts due to residual magnetization in three-dimensional magnetic resonance imaging.

An artifact is identified in magnetic resonance images produced by the three-dimensional FLASH technique, which features a short repetition time TR. The artifact is caused by differential spoiling of transverse magnetization by the phase-encoding gradients. The image intensity in different slices becomes altered, especially for short TR and large flip angle, which are conditions for achieving strong T1-weighted contrast. The effectiveness of spoiler gradient and rephasing gradients in suppressing the artifact is evaluated experimentally in images of a uniform phantom. Spoiler gradients that are incremented in amplitude cause even more slices to deviate in intensity, and are therefore less effective than in two-dimensional techniques. Rephasing gradients make the slices uniformly intense, but also enhance the intensity of tissues that have longer T2. The further addition of constant spoiler gradients has reduced this intensity increase by one-half and allowed for an intensity difference between white matter and gray matter comparable to without a rephasing gradient.