Many consider a maximum thickness ~2 mm (as measured on CT/MRI) as the upper limit of normal, with a thickness greater than 4 mm as abnormal pericardial thickening.
The pericardial thickness varies over different parts of the heart and is reported to be thinnest over the left ventricle.
The "visualized thickness" of pericardium may also depend on image resolution. Bull et al, reported that when using 1 mm high-resolution CT, the upper limit of the thinnest part of the pericardium was 0.7 mm and when using 10 mm CT slices was 1.2 mm.
Pericardial thickening can occur in various forms (i.e.uniform/nodular) and may or may not be accompanied by a pericardial effusion.