Means deoxygenated hemoglobin. Oxygen associates with haemoglobin by means of weak chemical bonds. However, once oxyhemoglobin encounters an environment with a low partial pressure of oxygen, the oxygen molecule releases itself from the oxyhemoglobin molecule to move into the low partial pressure environment. With this transition, oxyhemoglobin becomes dHB.
See Blood Oxygenation Level Dependent Contrast (BOLD signal), Carboxyhemoglobin (CoHb), Hemoglobin, Optical imaging, Oxyhemoglobin