<prompt> CT number of water and bones respectively100,00,-10000, 10001000,-100  <response>: Answer: c 0, 1000CT- scan or CAT scanInvented by God Frey Hounsfield in 1963, awarded Nobel Prize.Basic principle of CT is linear attenuation of x-rays.Incident x-rays are linearly attenuated by their interaction with orbital electrons of tissues.Measurement of attenuation of emerging detected beam - gives density of intervening tissues and this density forms basis of signal intensity variation obtained in x-ray tomograms.Two type of contrast agents are usedIonic - water soluble iodide dyes e.g. Na- diatrizoate, megliumine, conray, urograffin, angiografin - may cause anaphylaxisNon ionic- Safer but expensive e.g. iohexol Omnipaque, lopamiroElectron density of tissues in numbered as Hounsfield number H.N. or Hounsfield units.Air have a value of -1000 HUFat:-120 to-200 HUWater-0 HUSoft tissue: 20-60 HUBlood: 50-60 HUBone:1000 HU 