ABSTRACT
(Diagnostic Radiology) Some special x-ray tubes are produced with very small focal spot. These tubes are either called ultra-fine focus tubes or microfocus tubes. They are mainly used in mammography, angiography or macroradiography (rarely used method these days). The ultra-fine focal spot of these tubes is normally equal or smaller than 0.1 mm effective focal spot size (also called 0.1 nominal in the IEC documents). These tubes produce sharp images with extremely high spatial resolution, but are only used for low-power exposures (otherwise the small focus will be overloaded). Some contemporary x-ray tubes use electrical system that focuses the thermal electrons, thus producing variable focal spot size, including ultra-fine sizes.
