What is half value layer in radiology?
In radiography, technologists use the half value layer (HVL) to measure the quality or intensity of the beam. The HVL of an x-ray beam is defined as the amount of absorbing material that is needed to reduce the beam to half of its original potential. HVL is an indirect measure of photon energy or beam hardness.
What does HVL mean in radiology?
Therefore, determination of the half-value layer (HVL) is often used to describe the X-ray beam quality. The HVL of a beam is the thickness of material required to reduce the intensity of an X-ray or gamma-ray beam to one-half of its initial value (2,3).
How is x-ray quantity measured?
X-ray photon quantity refers to the number of photons produced during an exposure. Factors influencing x-ray quantity includes: peak voltage (kVp): beam quantity is approximately proportional to the square of the tube potential. generator type/voltage waveform: reducing ripple increases beam quantity.
What effect does adding more half value layer have on the quality of the x-ray beam?
The half value layer is indirectly measuring the beam quality since a harder beam will have a higher half value layer (HVL) and a softer beam will have a lower half value layer. Harder x-ray spectrum will require more thickness of material as low energy x-rays are preferentially attenuated.
How do you calculate half value?
HVL is measured in millimeters of aluminum. After filtration by one HVL, the subsequent HVL will be higher as the filtered photons have higher energy (thicker material is required to attenuate half of the penetrating beams). It is related to linear attenuation coefficient (μ) with following formula: HVL = 0.693 / μ
How much Gy is an X-ray?
The average radiation dose from an abdominal X-ray is 0.7 milligrays (0.0007 Gy), that from an abdominal CT scan is 8 mGy, that from a pelvic CT scan is 6 mGy, and that from a selective CT scan of the abdomen and the pelvis is 14 mGy.
What is the unit of measurement for X-ray tube current?
The x-ray tube current, measured in milliamperes (mA), represents the number of electrons per second flowing from the cathode to the anode, where 1 mA = 6.24 × 1015 electrons/s. For projection radiography, the tube current typically ranges from 100 to 1,000 mA with exposure times less than 100 ms for most examinations.
How is half value layer measured?
What is the role of the half value layer in getting the accurate amount of the shielding for radiation protection?
A low (or thin) half value layer for a given x-ray beam indicates that the x-ray beam contains more low energy and less penetrating radiation. Sufficient half value layers of shielding are calculated to reduce the radiation exposure outside the room to reasonable levels.
What is the half value layer of steel?
Shielding Layer Examples
| Half-Value Layer (cm) | ||
|---|---|---|
| Radioisotope | Gamma (MeV) | Steel |
| Cesium-137 | 0.66 | 1.6 |
| Cobalt-60 | 1.17, 1.33 | 2.1 |
What is the relationship between the half value layer and a tenth value layer?
Thus, the half value layer, reduces dose rate to one-half of the initial dose, the tenth value layer, reduces dose rate to one-tenth of the initial dose rate [2].