European Nuclear Medicine Guide
While the terms physiology or pathophysiology are seldomly mentioned in the field of nuclear medicine, they provide the mechanistic fundaments for tracer distribution and accumulation. More specifically, these features define the availability of the tracer to the target tissue as well as the ability of the target tissue to accumulate tracer in higher abundances than the background. The resulting nuclear medicine images, however, merely provide an interpretation of the disease status. Pathological validation in the form of e.g. immunohistochemistry is key to establish a direct relation between imaging findings and actual disease-related phenomena such as receptor over-expression. Below, the individual disciplines are briefly discussed in relation to imaging.
The field of physiology studies the mechanisms that organisms, organ systems, organs, cells, and biomolecules use to carry out their biochemical, mechanical and/or physical functions or processes. A straightforward physiologic phenomenon is the accumulation of glucose to maintain cell metabolism. Hence when cell growth rate increases, an increase in metabolism enhances glucose consumption. This physiological phenomenon is successfully explored in PET, where the most metabolically active tissues (e.g. the active brain, active muscles (e.g. the heart), brown fat, or malignant tumours) become visible due to their accumulation of the glucose analogue 2-[18F]FDG[13]. Other physiological features that are directly related to the accumulation of radionuclides/tracers are e.g. the uptake of iodine in the thyroid (uptake by electrochemical gradient and transporter-mediated mechanism)[14]. and the uptake of radium (uptake as calcium analogue)[15] and bisphosphonates (uptake as hydroxyapatite binding sites) in bone[16]. These physiological aspects to nuclear medicine are well understood and are even exploited for the design and development of novel tissue-specific tracers.
Physiology may also affect the biological distribution of tracers. For example, the vascular physiology, meaning the degree and type of vascularization of target tissue defines whether or not imaging tracers are presented to their biological targets in sufficient quantity to allow for the visualization of the tissue. This effect clearly has an interplay with the above-mentioned glucose uptake, namely if/when the tissue does not present enough blood vessels to supply the tissue with glucose, the 2-[18F]FDG uptake will be minimal or even non-existent. Alternatively, disruptions in the brain vasculature may result in a leaky blood brain barrier (BBB). This in turn may drive local blood components spillage into the brain and as such may drive tracer accumulation.
A physiological topic of recent studies in the field of nuclear medicine is the clearance of tracers e.g. by influencing the renal excretion and/or the re-absorption of tracers in the kidneys or by driving hepatic clearance/bile excretion. Alternatively some tracers may accumulate in (salivary) glands[17,18]. Despite that the resulting background signals may limit a tracer’s potential to identify diseases located in the clearance tissues e.g. a liver lesion for a hepatically cleared tracer. Getting rid of background accumulations may help to prevent unwanted side effects in therapeutic tracer applications e.g. damage to salivary glands in therapeutic PSMA therapies. Unfortunately, tracer clearance mechanisms are not yet fully controlled. It is likely that further understanding of the relation between physiology and the chemical design of tracers may substantially influence the future in tracer design.
The definition of pathology is “the study of the essential nature of diseases and especially of the structural and functional changes produced by them” (https://en.wikipedia.org/wiki/Pathophysiology). Pathology also refers to the medical discipline that describes tissue changes that are observed during a disease state e.g. via the microscopic analysis of tissue. In the clinic, pathological investigations can be considered the “gate-keeper” or “ground truth” of diagnosis and therapy. While initially structural differentiations in tissue were used to identify abnormalities, functional molecular phenotyping is increasingly being used as a tool to assess tissue characteristics. Immunohistochemistry – the use of specific antibodies to determine (protein) expression and localization – has proven itself to be a key technology in determining the functional expression levels of biomarkers (so-called molecular pathology) e.g. the chemokine receptor 4 or the somatostatin receptor[19]. As such, PET imaging findings, e.g. using [68Ga]Ga-pentixafor or [68Ga]Ga-DOTA-TOC, can be related directly to target expression levels in diseased cells. Diagnosis via immunohistochemistry often uses a colour-based staining protocol, where either brown or fluorescent coloration of tissue is microscopically assessed or is assessed via flow-cytometry[20]. Modern alternatives use mass-spec technologies to increase the number of features that may be investigated simultaneously[21].
The discipline of pathology has also embraced technologies such as gene profiling and proteomics to investigate biomarker expression levels at the genetic/protein level. While relevant, in nuclear medicine generally only receptors in contact with the extracellular matrix can be targeted. As the extracellular accessibility (functional expression level) may be substantially different from the expression levels identified using profiling efforts (non-functional expression levels), the relation with imaging data may not be straightforward.
Pathophysiology is a convergence of the two previously mentioned disciplines and is defined as the functional changes that accompany a particular syndrome or disease. This can best be explained based on the above-mentioned example of vascular physiology. The disease related expression of integrins may promote angiogenesis, which in turn influences the local vascular anatomy. As a result, at these angiogenic sites, imaging tracers may leak out of the vasculature and, because reabsorption is less efficient in these impaired vascular anatomies, exposure of tissue to the tracer is enhanced. This locally increased tracer availability may increase tracer uptake. In extreme cases of angiogenesis or vascular damage, blood may leak out into the tissue. Here the enhanced permeability and retention (EPR) effect may result in the non-specific accumulation of larger molecular entities e.g. monoclonal antibodies, proteins or nanoparticles[22]. While the last physiological effect could potentially be used as a non-receptor mediated targeting tool, it may also mislead researchers that assume tracer uptake is caused by receptor expression. This is one of the many reasons why immunohistochemical validation of receptor expression and tissue morphology is a key validation step in any imaging experiment.
While physiology and pathology are commonly forgotten in the routine interpretation of nuclear medicine images, one should always remember that these aspects play a crucial role in tracer uptake. As such, these components are directly linked to the patients’ disease status.