Development of radiolabelled white cell scanning

The use of gamma emitting radioisotope labelled white cells for imaging inflammation was introduced in 1976. It was possible to study the whole body distribution of granulocytes using this method. Cells were labelled in plasma using tropolone as chelate to avoid cell activation which causes non-physiological persistent lung uptake. Such studies revealed labelled cell uptake in bone marrow, liver and spleen in normal subjects. Intense liver activity was found in cases of cell damage resulting in cell destruction by the liver. The large splenic pool of leucocytes could be mobilized rapidly to sites of inflammation and indeed the fall in splenic activity was a measure of the degree of inflammation. Measurements of the faecal excretion of 111In-labelled granulocytes were also used to assess Crohn's disease activity. Alternatively, the whole body retention of 111In was used. Later studies have involved the use of 99mTc, initially using labelled colloids, which resulted in cell activation and later using the lipophilic complex HMPAO to label leucocytes. The several advantages over 111In for imaging inflamed bowel are described. Early imaging is important to avoid non-specific bowel activity. Newer approaches include labelling immunoglobulins, monoclonal granulocyte antibodies and targeting of activated endothelium adhesion molecules. Such techniques remain experimental and the best current technique for localizing inflammation is to use radiolabelled leucocytes.