This difference in information also parallels the techniques of percutaneous transhepatic cholangiography (PTC) and endoscopic retrograde cholangiopancreatography (ERCP) where, as the names imply, both can visualize the biliary system but ERCP provides easier images of the exocrine pancreatic ducts to detect pancreatic pathologies . Additionally, like canalograms and trabeculotomies, PTC could be considered ab-externo, as the tracer is introduced external through the surface skin inserting a needle into the gall bladder. ERCP, like aqueous angiography and goniotomies, takes an internal approach through the gastrointestinal lumen.