Grenz Rays

The quality of x rays is defined as their penetrating ability The most frequently used definition for various x ray qualities is the half-value layer (HVL).

 

It is defined as that thickness of a given filter material (in dermatology usually aluminum [Al]) that reduces the intensity to 50% of the original incident radiation. For grenz rays, they will be referred to as soft (HVL up to 0.02 mm Al), medium (HVL 0.023-0.029 mm Al), and hard (HVL0.030-0.036 mm Al).

 

The HVL is influenced by multiple factors but for practical purposes only two of them are important: kilovoltage and additional filtration. An x-ray beam produced by higher kilovoltage has shorter wavelengths and greater penetrating power. By placing a filter in the x-ray beam the quality is changed in such a way that the higher the atomic number of the filter, the greater the reduction in beam intensity. 

 

The intensity or dose rate of radiation is influenced by kilovoltage (kV), milliamperage (mA), filter, exposure time, and target skin distance (TSD). It increases when the kV and mA are increased. It decreases as the distance is increased, approximately in inverse square proportion, and it is also reduced as the thickness and atomic number of the filter are increased. The radiation dose is directly proportional to the exposure time if all other factors remain constant. The x-ray dose in roentgen (R) specifies the exposure to a certain quantity of radiation, based on its ability to ionize air. It is not identical with the observed dose in the tissue, which is expressed in rads. The unit of the absorbed dose (tissue dose) used today is Gray (Gy) : according to the International System of Units (SI) standards, 1 Gy equals 100 rads.