HIGH VALUE LAYER AND X-RAY ENERGY

• Although x-rays are attenuated exponentially, high-energy x-rays are more penetrating than low-energy x-rays.
• Although 100-keV x-rays are attenuated at the rate of approximately 3%/cm of soft tissue, 10-keV x-rays are attenuated at approximately 15%/cm of soft tissue.
• X-rays of any given energy are more penetrating in material of low atomic number than in material of high atomic number.
• In radiography, the energy of x-rays is usually measured by the HVL because such measurement is relatively easy.
• The HVL is an energy characteristic of the useful x-ray beam.
• The HVL of an x-ray beam is the thickness of absorbing material necessary to reduce the x-ray intensity to half of its original value.
• The absorbing material used to measure the HVL of a diagnostic x-ray beam is Al.
• A diagnostic x-ray beam usually has an HVL in the range of 3 to 5 mm Al.
• The HVL in soft tissue is 3 to 6 cm.
• The setup for radiography consists of three principal parts: the x-ray tube; a radiation detector; and graded thicknesses of filters, usually Al.
• In radiography, the first step is to make a radiation measurement with no filter between the x-ray tube and the radiation detector.
• In the next step of radiography, measurements of radiation intensity are made for successively thicker sections of filter.
• X-ray beam penetrability changes in a complex way with variations in kVp and filtration.
• The thickness of filtration that reduces the x-ray intensity to half of its original value is the HVL.
• Different combinations of added filtration and kVp can result in the same x-ray beam HVL.
• Several methods can be used to determine the HVL of an x-ray beam.
• X-ray beam energy can be identified by kVp or filtration.
• In this case, x-ray penetrability remains constant, as does the HVL.
• For example, measurements may show that a single x-ray imaging system has the same HVL when operated at 90 kVp with 2-mm Al total filtration as when operated at 70 kVp with 4-mm Al total filtration.
• HVL is most appropriate for identifying x-ray beam energy.
• The most straightforward way to determine the HVL of an x-ray beam is to graph the results of x-ray intensity measurements made with an experimental setup.