The Latest Developments of Functional Molecular Imaging in the Diagnosis of Primary Aldosteronism

Differentiation of unilateral from bilateral aldosterone hypersecretion is the essential step in the clinical practice of primary aldosteronism (PA). Although adrenal venous sampling (AVS) has been established as the most standard test recommended by the guideline, its invasive and technically difficult nature has facilitated the approach to develop non-invasive functioning imaging as an alternative test. Compared to the conventional adrenocortical scintigraphy with cholesterol derivatives as tracer, the first-generation imaging, both of ¹¹C-MTO/PET and ¹²³I-IMTO/SPECT/CT, the second-generation imaging, bind with high specificity and affinity to CYP11B enzymes and have advantages in shortening the time for obtaining specific images, reducing the radiation exposure to the patient, and resolution of the images. Because of very short half-life of ¹¹C-MTO, ¹²³I-IMTO has a potential for a wider application than ¹¹C-MTO. Sensitivity of identifying an adenoma smaller than 1 cm in diameter is still a common limitation of these new functional imaging methods. The new functional imaging could be supplementary to AVS in lateralization of PA when the results of AVS are not conclusive. To be a substitute for AVS, however, it should fulfill various conditions including high selectivity and binding affinity to CYP11B2, high sensitivity in detecting small adenoma, high resolution image, reduction of radiation exposure, and general versatility. Considering the potential number of patients, cost-effectiveness of the subtype testing in the clinical practice of PA could be one of the issues of the medical expenses. Thus, development of a new non-invasive functional imaging will have a significant impact on the clinical practice of PA and hypertension.