Tc-99m-sestamibi and Tc-99m-tetrafosmin are lipophilic, bind to plasma proteins and are taken up to a variable extent by all tissues because they target mitochondria in which glucose is used to synthesise ATP. They can be thought of as the single-photon equivalents of FDG. The main clinical indication of Tc-99m-sestamibi is localisation of parathyroid adenomas, while that of Tc-99m-tetrafosmin is to image myocardial perfusion. Both are substrates for multidrug resistance (MDR) proteins and therefore fail to accumulate in or are eliminated from tissues that express these proteins, especially brain, liver, kidney and thyroid. In contrast MDR proteins are not expressed in myocardium or 90% of parathyroid adenomas. The blood vessels in a zone of myocardium served by a diseased coronary artery undergo dilatation in response to ischaemia and therefore, in contrast to healthy myocardium, fail to dilate in response to a vasodilator such as i.v. adenosine. The zone appears normal at rest after i.v. injection of Tc-99m-tetrafosmin but as a defect after adenosine as a result of 'steal' by healthy myocardium.