Define

In[34]:= Clear[\[CapitalOmega]]
\[CapitalOmega] =  ImplicitRegion[2 <= (x^2 + y^2 + z^2) <= 4, {x, y, z}]
Out[35]= ImplicitRegion[2 <= x^2 + y^2 + z^2 <= 4, {x, y, z}]

plot it to see the thickness of the shell

In[36]:= RegionPlot3D[\[CapitalOmega], {x, -2, 2}, {y, -2, 2}, {z, -2,1/2}]
During evaluation of In[36]:= RegionPlot3D::boolf: \[CapitalOmega] must be a Boolean function. >>
Out[36]= RegionPlot3D[\[CapitalOmega], {x, -2, 2}, {y, -2, 2}, {z, -2, 1/2}]

to see nothing but an error message ... now

RegionPlot3D[\[CapitalOmega], AspectRatio -> Automatic]

the good old sphere in 3D is shown and now \[CapitalOmega] is a Boolean function. The following works too

In[38]:= Clear[\[CapitalOmega]]
\[CapitalOmega] = ImplicitRegion[2 <= (x^2 + y^2 + z^2) <= 4, {{x, -2, 2}, {y, -2, 2}, {z, -2, 1/2}}]
Out[39]= ImplicitRegion[2 <= x^2 + y^2 + z^2 <= 4 && -2 <= x <= 2 && -2 <= y <= 2 && -2 <= z <= 1/2, {x, y, z}]

In[40]:= RegionPlot3D[\[CapitalOmega], AspectRatio -> Automatic]