Question

• Give some examples of objects or substances mentioned by Dr. Weissman which have negative Hounsfield units in CT images?
• What does it mean to have a negative slice spacing in a CT scan? How does it impact the radiation dose?
• What can you do if a metal object causes streaks in a CT image?

Answer 1

Answer1. Hounsfield units are the universal dimensional units for CT scan. They are obtained from linear transformation of the measured value. The density of tissues is measured with respect to water, which is zero. Tissues denser than water have positive values and tissues lighter than negative values. In case of brain CT scan, the areas with full black image are those having fats or that of air will show negative readings because fats and air have lesser density than water.

Answer 2.  In a typical CT image a slice through the body is observed and during the image reconstruction phase that slice is divided into a matrix of voxels and these voxels have a major impact on image blurring, noise and on radiation dosage given to the patient.

Positive slicing means that other slices are stacked behind the first main slice, which is a good sign and is easy to study. But in negative slicing, the stack of other slices is in front of main slice, which gives negative impact.

As a result of this negative slicing, the radiation dose given to the patient needs to be increased so as to get more clear picture and to study that particular slice which is lying behind others.

Answer 3. The presence of any metal piece on the scan can lead to severe artifacts because they are far denser than the scanning ability of the equipment, thereby gives improper results. To avoid its effects, the kVp, that is, thepotential applied to the x-ray tube, is increased and mAs, that is, milli ampere second for radiation projection is reduced. This will lead to increased average proton energy and and low energy protons will be attenuated by metal. This issue can also be resolved by usin the metal deletion technique(MDT).