Choosing the right resolution for vessel visualization in MR angiography

In magnetic resonance imaging (MRI), the theoretically achievable spatial resolution is characterized by the extent of k-space used for image reconstruction, which is inversely proportional to the pixel size. Therefore, spatial resolution increases with the extent of k-space sampled. Whereas the visible resolution is characterized by the object size at which the object of interest is visually separable from the background. Since noise in MRI data is "white" (uniformly distributed across the k-space), sampling more k-space results in adding more noise to the image. This can result in the decrease of the object visibility with increase in the spatial resolution. Hence, it is important to choose the right spatial resolution to view the object of interest. In this paper, we present a theoretical relationship between the visibility of vessel detail in magnetic resonance angiography (MRA) and also the probability of projection of vessels in the minimum intensity projection (MinIP) and maximum intensity projection (MIP) images as a function of spatial resolution. This theory lays a foundation for determining the extent of k-space that should be used for image reconstruction to visually identify a particular vessel/anatomic detail of interest. The theory is validated using imaging studies and it is demonstrated that the vessel information displayed in MRA as well as projection images can be maximized for a particular anatomic detail of interest by optimal choice of spatial resolution.