There are important indications for performing imaging procedures with Tc-99m labeled red cells, including MUGA studies, GI Bleeding Studies, and evaluation of patients for hepatic hemangioma. Special handling, careful selection of needle gauge, and anticoagulation techniques are required to achieve optimal labeling.

There are three generalized methods of labeling Red Blood Cells with Tc-99m. All are simple, rapid, inexpensive and reliable. It is important to compare the advantages and disadvantages of each, and to identify the labeling efficiencies obtained with each method.

GENERALIZED METHODS OF RBC LABELING:

* Invivo / Invivo Method

* Invivo / Invitro Method

* Invitro / Invitro Method

Stannous ion is used as the reducing agent to convert pertechnetate to Tc4 which is then able to radiolabel the red cells. This process is referred to as “tinning” the red cells. The term to the left of the slash refers to where the tinning of the red cells took place (inside or outside the body). The term to the right of the slash mark refers to where the radiolabeling took place (inside or outside the body).

When performing RBC Labeling with Tc-99m, the molecule labeled is hemoglobin; the portion labeled is the β-globin chain. The heme portion cannot be labeled since it already has an iron atom (Fe) in the center of a square planar array of nitrogen atoms. Since the Fe atom cannot be displaced by another metal under physiological conditions, the globin portion of the molecule binds the Tc-99m. Similar chemistry applies to Cr-51 labeling of RBCs: again, the radiometal binds to the β-globin chains.

To determine RBC labeling efficiency, a sample of blood containing radiolabeled RBCs is spun in a sealed microhematocrit tube. The tube is then scratched with a file and broken at the dividing line between the plasma and the packed cells and the two halves are counted. The labeling efficiency is equal to

Activity in red cells x 100%
____________________
activity in red cells + plasma

There are also important indications for performing in vivo non-imaging procedures with Cr-51 labeled red cells, including Red Cell Mass, Red Cell Survival Studies, and Splenic Sequestration Studies. Special handling and anticoagulation techniques are also required to achieve optimal labeling when labeling with Cr-51.

It is very easy to label red cells with Cr-51. One simply adds Cr-51 Na chromate to an anticoagulated sample of blood and incubates for 15 minutes. The reaction may then be terminated by adding ascorbic acid (Vitamin C) or by centrifuging the sample and removing the plasma. Isolation of RBCs is not required.

As in the case of Tc-99m red cells, Cr-51 binds to β-globin chains of hemoglobin.

Other labeling protocols involve preparation of In-111 White Blood Cells, Tc-99m White Blood Cells, and In-111 platelets. It is relatively easy to label White Blood Cells with either Tc-99m or In-111. Both preparations are quite labor-intensive and involve isolation of the WBCs and incubation with a lipophilic intermediate to effect radiolabeling.