Good question.
For me intimal thickening has the nice white line intact of the intimate with a thicker dark area immediately below it, so the Intima / Media thickness is increased.
When that white line is lost and there is a hypo echoic bulge in the vessel wall without the crisp white line then I call it early plaque development.
It is a spectrum really where intimal thickening progresses to plaque.
Steve.