The technology relates to the first radioligands that can be used to image and quantify the enzyme phosphodiesterase subtype D (PDE4D). The PDE4D proteins have a role in carrying out signal transduction pathways in several cell types and is thought to be the key target of various antidepressants. Current work with imaging the radioligands in monkey brains using positron emission tomography (PET) has been successful, and further work with humans is needed. An effective radioligand that can visualize PDE4D levels in human using PET can be useful in evaluation of health human subjects and in the development of PDE4D inhibitors.