surface where it can help to cool a body through sweating and thermal dissipation. Many types of tumors, and especially certain types, have been described as "hot and bloody" because of their overperfusion relative to the body overall. Overperfusion and underperfusion should not be confused with hypoperfusion and hyperperfusion, which relate to the perfusion level relative to a tissue's current need to meet its metabolic needs. For example, hypoperfusion can be caused when an artery or arteriole that supplies blood to a volume of tissue becomes blocked by an embolus, causing either no blood or at least not enough blood to reach the tissue. Hyperperfusion can be caused by inflammation, producing hyperemia of a body part. Malperfusion, also called poor perfusion, is any type of incorrect perfusion. There is no official or formal dividing line between hypoperfusion and ischemia; sometimes the latter term refers to zero perfusion, but often it refers to any hypoperfusion that is bad enough to cause necrosis. Measurement In equations, the symbol Q is sometimes used to represent perfusion when referring to cardiac output. However, this terminology can be a source of confusion since both cardiac output and the symbol Q refer to flow (volume per unit time, for example, L/min), whereas perfusion is measured as flow per unit tissue mass (mL/(min·g)). Microspheres Microspheres that are labeled with radioactive isotopes have been widely used since the 1960s. Radioactively labeled particles are injected into the test subject and a radiation detector measures radioactivity in tissues of interest. Application of this process is used to develop radionuclide angiography, a method of diagnosing heart problems. In the 1990s, methods for using fluorescent microspheres became a common substitute for radioactive particles. Nuclear medicine Perfusion of various tissues can be readily measured in vivo with nuclear medicine methods which are mainly positron emission tomography (PET) and single photon emission computed tomography (SPECT). Various radiopharmaceuticals targeted at specific organs are also available, some of the most common are: 99mTc labelled HMPAO and ECD for brain perfusion (rCBF) studied with SPECT 99mTc labelled Tetrofosmin and Sestamibi for myocardial perfusion imaging with SPECT 133Xe-gas for absolute quantification of brain perfusion (rCBF) with SPECT 15O-labeled water for brain perfusion (rCBF) with PET (absolute quantification is possible when measuring arterial radioactivity concentration) 82Rb-chloride for measuring myocardial perfusion with PET (absolute quantification is possible) MRI Two main categories of magnetic resonance imaging (MRI) techniques can be used