PDE10A and Huntington disease: Striatal and pallidal PDE10A availability was lower in HD subjects than in healthy control subjectsStriatal and pallidal [18F]MNI‐659 uptake progressively declined in the premanifest stages but plateaued between stages 1 and 2The percentage decline of [18F]MNI‐659 uptake was greater than that of D2/3 receptor availability and of volumetric changes