Slice of tissue X-rays is:

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 (1/16 inch) is most common way of protectionQ. Other protective agents include concrete (4-6 inch standard density i.e. 147 pounds/cubic 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.