<prompt> Slice of tissue X-rays is:TomographyMammographyContrast studiesAll of the above  <response>: A i.e. Tomography Hounsfield units CT number represent average linear attenuation coefficientQ. The Hounsfield scale is an arbitray one with air at-1000 and water at 0 units as fixed pointsQ. Fat which is less dense than water also has a negative value - 20 to - 100 range. CT has been designated a high dose procedure as it exposes 50-500 times higher radiation doses than a conventional radiograph. Lead shielding or coating 116 inch is most common way of protectionQ. Other protective agents include concrete 4-6 inch standard density i.e. 147 poundscubic foot, brick blocks and barium plaster. Narrow beam collimation, high spatial reconstruction algorhythm like bone algorhythm and small field of view improve spatial resolution of HRCTQ. 