A number of positron emitting radiopharmaceuticals can be used as tracers of myocardial perfusion. The most common are Nitrogen-13 ammonia (¹³N-ammonia), Rubidium-82 (⁸²Rb), and Oxygen-15 labeled water (¹⁵O-water). ¹³N-ammonia enters the myocardial cells in proportion to blood flow by diffusion across the sarcolemma, followed by metabolic conversion to ¹³N-glutamine by glutamine synthetase, becoming trapped within the cell. ⁸²Rb is potassium analog, actively taken up by myocytes in proportion to blood flow via the Na⁺/K⁺-ATPase. ¹⁵O-water is a freely diffusible tracer and has been primarily utilized for absolute flow quantification research, but myocardial images are only obtained after application of kinetic modelling techniques (Table 22.1). At present, ¹³N-ammonia and ⁸²Rb are the only Food and Drug Administration (FDA) approved PET tracers for the evaluation of individuals with suspected or known coronary artery disease (CAD). Myocardial perfusion PET imaging is accomplished using flow tracers ¹³N-ammonia and ⁸²Rb at rest and during pharmacologic stress. For assessment of myocardial perfusion, PET is an excellent technique with sensitivity and specificity of about 90 % [5].¹⁸F-flurpiridaz is a newly developed PET myocardial perfusion agent targeted to Mitochondrial Complex –I (MC-I) in the heart [6]. It has improved biological and imaging characteristics compared with currently available perfusion imaging agents, including rapid uptake in the myocardium, prolonged retention, and superior extraction versus flow profiles. A first in man study has established the safety and dosimetry of ¹⁸F-flurpiridaz and confirmed high sustained cardiac uptake (Fig. 22.1). Subsequent studies performed in CAD patients established the dose and timing needed to detect perfusion deficits when the agent is administered under resting and stress conditions [7].

The primary tracer to image glucose metabolism is fluoro-deoxyglucose (FDG) labelled with fluorine-18 (¹⁸F-FDG) (Table 22.1).¹⁸F-FDG is a glucose analog taken up by myocardial cells via two carriers, the glucose transporters 1 and 4 (GLUT1 and GLUT 4). Similar to glucose, FDG is then phosphorylated by hexokinase to ¹⁸F-FDG-6-phosphate but unlike glucose-6-phosphate, subsequent metabolism of ¹⁸F-FDG-6-phosphate is minimal so that ¹⁸F-FDG becomes essentially trapped in the myocardium. ¹⁸F-FDG PET has been extensively used to assess myocardial viability and is still considered by some investigators as the gold standard for this assessment [8]. FDG is not a specific tracer, as it is also taken by activated macrophages and can be used to image inflammation.

Two tracers, palmitic acid and acetate have been labeled with ¹¹C. ¹¹C-palmitic acid can assess uptake and metabolism of long-chain fatty acids (FA). Their oxidation represents the major source of cardiac energy but complex kinetics and back-diffusion of the unmetabolized tracer by the myocytes allows only semi-quantitative measurements. ¹¹C-acetate, is a two-carbon-chain free FA which is extracted proportionally to myocardial blood flow, then rapidly enters the tricarboxylic acid cycle and is metabolized to carbon dioxide. It is considered a marker of overall oxidative metabolism without the complexity of substrate interaction between glucose and FA [9] (Table 22.1).

The main tracers to image myocardial innervation are hydroxyephedrine (HED) and epinephrine (EPI) labeled with ¹¹C. HED is a false transmitter analog of norepinephrine and highly resistant to intraneuronal metabolic degradation. HED is considered a marker of uptake-1 mechanism, which is the reuptake for storage or catabolic disposal of norepinephrine into the presynaptic terminal by an energy dependent transporter. EPI is a true catecholamine that, in contrast to HED, is sensitive to metabolic degradation under normal conditions and has also gained interest as a potential neuronal imaging tracer (Fig. 22.2). ¹¹C-EPI labeled compounds are not FDA approved, and it is unlikely that they will not receive a broader clinical application due to their short half-life and need for an on-site cyclotron. New innervation tracers, such as LMI1195, a benzylguanidine derivative similar to metaiodobenzylguanidine (MIBG), but labeled with ¹⁸F, are under investigation with the potential for a more widespread use from a logistical standpoint [10]. Post-synaptic receptor density can be assessed with a radiolabeled b-receptor antagonist CGP12177 and MQNB, a muscarinic receptor agonist, both labeled with ¹¹C (Table 22.1).

Beyond the tracers described above, a number of other agents have been tested mainly at a preclinical level and demonstrate potential for clinical translation. ¹⁸F-galacto–arginine-glycine-aspartate (RGD) binds to α_vβ₃integrin receptors that are expressed on the endothelial surface during angiogenesis. Another option for angiogenesis imaging is to use the recently developed ⁶⁸Ga labelled RGD agent (⁶⁸Ga-NOTA-RGD) [11]. ⁶⁸Ga labeled tracers are highly appealing for imaging particularly for centers with no direct access to cyclotron, because ⁶⁸Ga is produced from a commercially available ⁶⁸Ge generator system. Annexin-V, a molecule which specifically binds to phosphatidylserine that is expressed on the surface of cells entering the apoptotic cascade, has been extensively tested in preclinical studies to image apoptosis. Radiolabeling of annexin-V for PET imaging has also been performed. Because annexin-V harbors numerous functional groups requiring protection and deprotection steps, its direct radiolabeling with ¹⁸F is impractical. Recently, a set of novel small-molecule probes known as the Aposense compounds (a patented platform technology; Aposense Ltd.) has been designed to detect apoptotic membrane imprint [12]. Within this family, the PET tracer ¹⁸F-ML-10 shows selective uptake by apoptotic cells, in correlation with the apoptotic hallmarks of breakdown of mitochondrial membrane potential, caspase activation, or apoptotic DNA fragmentation. Signal is lost on membrane rupture, and thus ¹⁸F-ML-10 is capable of distinguishing between apoptotic and necrotic cells [12]. ¹⁸F-ML-10 is the first PET tracer for apoptosis that has been advanced into clinical trials, with promising results to date in biodistribution and dosimetry profiles. For imaging necrosis, the tracer hypericin labeled with Iodine-124 (¹²⁴I), has shown a specific and high affinity, although the exact mechanism of action is still unknown [13]. The ¹¹C-labelled PET tracer PK11195, is a selective ligand of the translocator protein, known as peripheral benzodiazepine receptor (PBR) that is expressed in high density in circulating human phagocyte populations, particularly in monocytes and neutrophils [14]. Matrix metalloproteinase (MMP) imaging is also feasible through a newly developed MMP inhibitor labeled with ¹⁸F [15]. Other probes targeting adhesion molecules (such as VCAM-1) labeled with ¹⁸F are under investigation in experimental models. Recently, ¹⁸F-sodium fluoride (¹⁸F-NaF), a radiopharmaceutical used for skeletal imaging, was tested in patients with coronary atherosclerosis and symptomatic carotid disease and proved to provide relevant information regarding active micro-deposition of calcium in atherosclerotic plaques, a process that is associated with plaque instability [16].

Several angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor 1 (AT₁R) antagonists have been radiolabeled for molecular imaging techniques. Initial attempts for developing specific ACE binding radiotracers were made by use of fluorine-18 labeled captopril (¹⁸F–CAP), the first clinically available ACE inhibitor. ¹⁸F–CAP, however, had a number of shortcomings that reduced its potential as a suitable tracer for examining ACE distribution. A new radiotracer, N-succinimidyl-4-[F-18]-fluorobenzoyllisinopril (¹⁸F–FBL), was synthetized to bind more specifically to ACE. Multiple PET radiotracers for AT₁R have been developed labeled with ¹¹C, but major limitations in pharmakokinetics and biodistribution, rendered these tracers unsuitable for clinical use. Another probe labeled with ¹¹C (¹¹C-KR31173) was found to bind selectively to AT₁R in various tissues including the heart and first experimental studies in animals and first-in-man application (in four healthy volunteers) proved the feasibility and safety of imaging cardiac AT₁R expression [17]. These results provide a rationale for broader clinical testing of AT₁R -targeted molecular imaging.