Elucidating Mechanisms That Underlie the Symptomatology of Functional Dyspepsia Using Novel Techniques and Its Therapeutic Validation Using Neuromodulators

The goal of this study is to investigate the mechanisms of impaired gastric accommodation and emptying, dysfunctional duodenum, and micro-inflammation using novel imaging techniques of SPECT/CT, gastric emptying scintigraphy, MRI, high-resolution manometry, and inflammatory biomarkers, as well as to validate these mechanisms using a therapeutic trial of neuromodulator (mirtazapine) in functional dyspepsia (FD) and health. The main objective\[s\] it aims to answer are: * to investigate impaired gastric accommodation through SPECT/CT imaging and high-resolution manometry findings of the stomach fundus. * to investigate impaired gastric emptying through gastric emptying scintigraphy * to investigate for a dysfunctional duodenum through MRI imaging of the duodenum. * to investigate micro-inflammation through SPECT/CT standard uptake value (SUV), inflammatory biomarkers (eosinophils, mast cells, IL-6, IL-10) and mucosal barrier marker (E-cadherin). * to investigate if a therapeutic trial of a neuromodulator agent, mirtazapine, ameliorates symptoms of FD through improvement in impaired gastric accommodation. For objectives 1-4, FD patients and healthy volunteers will be consecutively recruited, and all will undergo SPECT/CT, MRI, high-resolution manometry and biomarkers, and data acquired from these tests will be analyzed. For objective 5, the enrolled participants who did all baseline tests/markers are given mirtazapine for four weeks, and all tests/markers, except biomarkers and MRI, are repeated at the end of the trial