**11. Direct method labeling without bi-functional chelating agent**

In this type of labeling process, there is no need of bi-functional chelating agents or metal cheater. These are discussed below.

#### **11.1 Isotopes exchange labeling**

In this method, some atoms from the compound which is to be labeled is replaced by isotope of the same atom of the element having different atomic mass (more or less) such as I-123, I-124, I-125, I-127, and I-131. the compound is labeled with isotope of the same element so the compound to be labeled and radiolabeled

### *Single-Photon Emission Computed Tomography (SPECT) Radiopharmaceuticals DOI: http://dx.doi.org/10.5772/intechopen.93449*

are similar in biological properties, except for the energy emitted from different isotopes of the same element which is used for labeling [14]. This method used for in vitro study. Examples of isotope exchange labeling reactions are labeling of the triiodothyronine (T3) with I-125, labeling of thyroxine with I-125, and labeling with C-14, S-35 and H-3 labeled compounds [15].
