Question

Define the CT numbers and describe the effect of the CT acquisition parameters (kVp, mAs, pitch) on the CT numbers, and then briefly discuss why the CT number for a tissue can depend on the acquisition parameters.

Answer 1

CT Scanners use a set of software algorithms to calculate the amount of X Ray Radiation absorbed by every element of a tissue slice. Each of these elements is repreensented by a pixel or voxel. Each pixel or voxel assigned a numerical value called a CT Number. This number is compared to the attenuation value of water and is displayed on a scale of arbitary units named Hounsfield Units or HU. The HU scale is applied to the CT data by scaling all measured attenuation coefficients from a tissue slice to the attenuation of water using the equation

HU=1000×(μ − μH2O)/μH2O,

From the equation the CT number from a pixel now in the HU scale is proportional to the measured attenuation coefficient in this pixel scaled by the attenuation coefficient of water.

Here is an example of HU Scale applied to several bodies.

Properties of X Ray Beam

KVP: Kilovolt Peak Mount of voltage that is given across the X Ray Tube Voltage is directly proportional to energy of the beam. i.e, more the voltage higher is its beam energy. As energy is directly proportional to the penetrating power of beam KVP is proportional to penetrating power of beam. Increased KVP = Increased penetrance. Therefore image will be more whiter or contrast decreases. i.e, Increased KVP = Decreased Contrast

MAS: Milli Ampere Second Current passed times the exposure time MAS has no effect on energy, so no effect on penetrating power of the beam. Increased MAS = Increased in background blackness. Therefore image contrast increases. i.e, Increased MAS = Increased contrast

However the most important factor for contrast between KVP and MAS is KVP.

*Contrast - - Ability to see something in relation to background