Note the similarity between the left diagram of the neutral hydrogen (HI) distribution within the Milky Way and the radio image of the HI distribution within the galaxy M33 (blue color superimposed over its visible light image).  The HI distribution of the Milky Way was determined by using a technique in which the radial velocity of HI clouds is determined by radio spectroscopy of the 21 cm radio emission line of neutral hydrogen.  These radial velocities combined with the known rotation curve of the Milky Way allow the positions of these HI clouds to be plotted relative to the Sun.  This technique fails within a small angle directly towards and away from the galactic center where all the clouds would have essentially the same radial velocity of about zero and so their distances from the Sun can't be determined.   (Note the absence of data in the plot within a small angle towards the center of the galaxy in the left diagram.  Some features in this illustration such as the central ring, nearby small spiral structures, and the spiral structure in the Solar neighborhood have been filled in using other techniques.)  This technique is not needed to determine the HI distribution in external galaxies such as M33.  The radio image of M33 shows its HI distribution directly.  Note also that the M33 HI spiral structure, while overlapping some of the visible spiral structure, does not correlate perfectly with the visible structure and is somewhat more "scattered" than the "grand design" spiral arms of M33 as seen in the visible.  This is probably the case with the Milky Way such that the visible structure of the Milky Way's spiral arms mostly likely would also have a more "grand design" structure than its HI distribution.