Assessment Parameters for Radiotracers Used in Cardiac Molecular Imaging
Published on 08 Sep, 2016
Capable of improving diagnostic accuracy and prognostic evaluation, radiopharmaceutical agents represent some important developments among life-saving technologies in the nuclear age.
Developed to aid in the visualization of cardiac functioning, cardiac molecular imaging provides non-invasive, cheaper and in-vivo quantitative modality to study cardiac physiology. Cardiac molecular imaging techniques include cell molecular biology, radiotracers, and imaging principles to visualize cardiac functioning.
Radiotracers are primarily used for the assessment of neural dysfunction, post-transplant cellular response, atherosclerotic pathophysiological progress, metabolism, and viability assessment of tissue.
Since the inception of the Single Photon Emission Computed Tomography (SPECT) myocardial imaging, technetium-99m or thallium-201 has been used extensively. Recent novel radiopharmaceuticals agents targeted toward specific subcellular process are capable of significantly improving diagnostic accuracy and prognostic evaluation.
Radiotracers are mainly used for the following assessment parameters in cardiac disease.
Myocardial Perfusion Imaging
201TI and 99mTc are the most commonly used radiotracers for cardiac perfusion monitoring. 99mTc-Sestamibi, 99mTc-Tetrofosmin and 99mTc-Teboroxime are FDA approved salts used for clinical use. They provide benefits such as nonspecific passive diffusion across myocardial membrane and non-specific localization in cytosolic or mitochondrial subcellular components. Major limitations includes rapid washout at high flow in 99mTc-Teboroxime which requires the images to be taken in less than 2 minutes, whereas 201TI creates attenuation artifacts specifically in the inferior wall. Apart from that, 99mTc-nitrido complexes provide high cardiac uptake and retention for more than 2 hours, showing promise as novel agents for the future.
Currently 123I-Rotenone, 18F-Flurpiridaz, 18F-Labeled p-fluorobenzyl triphenyl phosphonium cation and 13NH3 and 82Rb+salts are being explored for the perfusion imaging of cardiac tissue.
Myocardial Metabolism and Viability
Disruption in the blood flow to the myocardium disturbs metabolic functions of cells, which might lead to reversible damage, necrosis, or remodeling. Update of radiotracers depends on the vitality of tissue, and metabolic function is proportional to nutrient consumption. Radiotracers used for metabolic assessment are usually done using molecules that are sugar intermediates. 18F-FDG has replaced 201TI, which was used in initial days to assess vitality. 18F-FDG has been recently used to assess cardiac myocyte metabolism after Cardiac Resynchronization Therapy (CRT). 123I-BMIPP is another complex which includes fatty acids, and is predominantly used as metabolic fuel for myocytes under rest conditions.
Imaging Atherosclerotic Plaques
Atherosclerosis is a chronic progressive condition involving the inflammation of the vascular intimal layer. Recent radiotracers used in PET scans such as 18F-FDG images inflammation in the plaque, showing more promising outcome than prior apoptosis imaging agents targeted towards annexin-V cellular subcomponent. 18F-FDG is primarily concentrated in leukocytes, especially in macrophages, and can hence be effective in early stages. Advanced plaque formation on the other hand involves calcium deposition in atherosclerotic layers. Sodium 18F-Fluorid molecules could be useful to assess osteoblastic activity.
Autonomic Dysfunction, Apoptosis, and Myocardial Infarct Repair
Nuclear imaging of autonomic dysfunction is primarily targeted towards the norepinephrine analogues; 123I-meta-iodobenzyl guanidine (123I-MIBG) is a widely used molecule for this purpose to detect the hyper-adrenergic state. 11Cmeta-Hydroxyephedrine is a novel alternative to 123I-MIBG, which is under clinical consideration to assess sympathetic functions.
Myocyte apoptosis plays pathophysiological role in development of atherosclerotic plaques, ischemia, chronic heart failure, myocarditis, and graft rejection. The radiotracers used for apoptosis purposes overall quantifies the extent of disease process. Molecules such as 18F-labeled 2-(5-fluoropentyl)- 2-methyl malonic acid, 18F-(S)-1-((1-(2-fluoroethyl)-1H-(1,2,3)-triazol-4-yl)methyl)-5-(2(2,4- difluorophenoxymethyl)-pyrrolidine-1-sulfonyl)isatin, and 18F-fluorobenzyl triphenyl phosphonium are currently used radiotracers.
In hybrid techniques the combination of (DTPA-Gd) with the above molecules provides prognostic details.
Check out our report on Cardiac Nuclear Imaging and Radiotracers for more on developments in the field of radiotracers used for cardiac imaging.