Absorptiometric measurement
C M Langton, C F Njeh in The Physical Measurement of Bone, 2016
X-rays for diagnostic uses are produced when a stream of electrons accelerates through a potential difference ranging from 20 to 120 kV and strikes a target (figure 8.5). The typical X-ray tube consists of a cathode, where the electrons are generated, and an anode, where the electrons are stopped and X-rays generated. The X-rays originate principally from rapid deceleration of the electrons when they strike the material. The X-rays generated by the deceleration of electrons (versus photoelectric absorption) are known as Bremsstrahlung. The cathode electrons are produced from a tungsten filament by thermionic emission. They are accelerated in the vacuum of the tube by the electric field force between the electrodes, then strike the tungsten target. Most medical X-ray tube anodes are made from tungsten (symbol W). Tungsten is typically used for X-ray accelerating voltages above about 35kVp because of its high stopping power and good heat dissipation. Special purpose X-ray tubes exist for lower energy X-ray production. For example, molybdenum is commonly used as an anode for mammography tubes because of its strong k-edge emission lines at 17 and 23 keV. The X-rays are emitted in all directions from the anode. Most X-rays are stopped by internal shielding in the X-ray tube. Only a small fraction of the X-rays are emitted through a collimator, escape from the tube head, and are available to the user.
Facility, Equipment, and Radiation Protection
Michael J. Thali M.D., Mark D. Viner, B. G. Brogdon in Brogdon's Forensic Radiology, 2010
X-ray tube: The function of the x-ray tube is to produce x-rays. The principal components of an x-ray tube are the filament and anode. The anode is the area of the tube where x-rays are produced. The anode is frequently referred to as the focus of the tube, and it plays an important role in producing fine detail in the radiographic image. Most x-ray tubes have two filaments designated as large and small. The small filament causes the anode (focus) to produce fine-detail images. If available, the small filament should be selected whenever fine detail is desirable. High-voltage electricity to produce x-rays is obtained from the standard power supply by means of a step-up transformer. In fixed units, the generator is usually separate and sits on the floor or is mounted on the wall or ceiling. In mobile x-ray units it is self-contained.
X-ray Vision: Diagnostic X-rays and CT Scans
Suzanne Amador Kane, Boris A. Gelman in Introduction to Physics in Modern Medicine, 2020
Less than 1% of the kinetic energy of electrons emitted from the filament actually goes toward x-ray production, with the remaining energy going toward heating the anode. Not only does this heat serve no purpose in x-ray production, but it requires cooling the anode to avoid damaging the tube. The advantage of rotating anode sources over fixed target x-ray tubes is that they allow the point at which electrons strike the anode to be constantly moved as the anode rotates rapidly. This spreads the heat to be dissipated over a greater area, enabling more electrons to strike the anode, and hence produce more x-rays, without melting the target. We will see later how this can be advantageous for some types of imaging (e.g., breast imaging to follow-up on an abnormal screening mammogram).
Monte Carlo dosimetry using Fluka code and experimental dosimetry with Gafchromic EBT2 and XR-RV3 of self-built experimental setup for radiobiological studies with low-energy X-rays
Published in International Journal of Radiation Biology, 2020
Joanna Czub, Janusz Braziewicz, Adam Wasilewski, Anna Wysocka-Rabin, Paweł Wołowiec, Andrzej Wójcik
This relatively unconventional experimental setup for radiobiological research required a number of experimental solutions and simulation calculations. The experimental system was created at Jan Kochanowski University, Kielce, Poland and used an X-ray beam with a maximum energy of 60 keV, filtered through an Al filter of thickness 1 mm. The X-ray tube was adapted from the field of X-ray diffraction science to radiobiological studies. The consequence of this step is the heterogeneous distribution of X-rays in the vertical beam profile and the need for cell irradiation in a vertical position. Hence, to ensure the uniformity of the radiation distribution on the irradiated surface, a rotation system was introduced to provide a uniform distribution of radiation with deviation equal to ±3.5%. A specially constructed Petri dish was introduced to enable vertical irradiation of living cells. The component of this Petri dish on which cells were located during irradiation was the coverglass, which was made from borosilicate glass. MC simulations show that this component caused an increase in the dose absorbed by the cells, due to the emission of characteristic radiation in the backscattered radiation spectrum. These simulations also show that the absorbed dose rate was equal to 0.9 Gy/min. At this point it should be emphasized that the construction of the Petri dish which is used in radiobiological research is a very important component in the process of determining the dose absorbed by living cells.
Radiation therapy techniques in the treatment of skin cancer: an overview of the current status and outlook
Published in Journal of Dermatological Treatment, 2019
Ali Pashazadeh, Axel Boese, Michael Friebe
While useful in the management of skin cancers, it should be noticed that electron beam therapy has its challenges. It has complicated dosimetry. In the treatment of small lesions, which is the case in most of NMSCs, it is associated with some degree of uncertainty in dose calculation (26). Percent depth dose (PDD) and output factors can change significantly in small-field treatments, typically less than 10 mm in diameter, which should be considered during electron dosimetry (33). Compared to the X-ray therapy that has a sharp edge of the radiation field, the edge of the electron beam field is blurry. The lead cutouts used for dose collimation and better dose coverage on the skin are usually messy in terms of construction and may be uncomfortable for patients (33). There is also uncertainty in the amount of bolus needed for each patient. In electron beam therapy of skin tumor, a relatively large safety margin of 10–20 mm is usually required (34). In contrast to X-ray photons that can be produced with Co-60, X-ray tubes and linear accelerators, high-energy electrons used in electron beam therapy are mainly produced by a linear accelerator. Therefore, production of the electron beam is always expensive and the treatments are costly (30). For tumors located in the anatomically challenging areas and irregular anatomies, dosimetry of electron beam therapy will be difficult and subject to error in calculation.
Does the combination of hyperthermia with low LET (linear energy transfer) radiation induce anti-tumor effects equivalent to those seen with high LET radiation alone?
Published in International Journal of Hyperthermia, 2021
Pernille B. Elming, Brita S. Sørensen, Harald Spejlborg, Jens Overgaard, Michael R. Horsman
Single dose tumor irradiations were given as described previously [20,21]. Initially the radiation source was a conventional therapeutic Philips X-ray machine (240 kV ortho-voltage X-rays, 10 mA, dose rate of 2.3 Gy/min). With this apparatus, the radiation dose was determined using an integrating chamber. This involved using a modified mouse restraining jig in which an ionizing chamber could be held in exactly the same position as the mouse foot tumor as described later in this section. However, during this study, this old X-ray tube broke and since it could not be replaced, additional studies were performed using a clinical Linear Accelerator (Varian Clinac iX) with 6 MV mage-voltage X-rays (dose rate of 6 Gy/min). The Linac dose output was isocentrically calibrated in 5 cm depth according to IAEA TRS398 (Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water, IAEA TRS-398) and is stable to within ± 1%. Doses to the mouse legs were determined by use of Varian Eclipse Treatment Planning System (AAA-algorithm) from CT-scan of the mouse/water tank. Dose variation caused by uncertainty in mouse leg position laterally and in-depth was approx. ± 3%. All irradiations to the tumor-bearing feet were given locally to the tumors of non-anesthetized mice, which were restrained in specially constructed Lucite jigs; the tumor-bearing legs being exposed and loosely attached to the jig with tape, without impairing the blood supply to the foot [21]. To secure homogeneity of the radiation dose, the tumors were immersed in a circulating water bath (type TE 623; Heto, Birkerød, Denmark) set at 25 °C with about 5 cm of water between the X-ray source and the tumor. The water-bath was covered with a Lucite plate with holes allowing immersion of the foot approximately 1 cm below the water surface. In order to irradiate only the tumors, the remainder of the mouse was shielded by 1 cm of lead.
Related Knowledge Centers
- Angiography
- Copper
- Crazing
- Ionizing Radiation
- Molybdenum
- Radiography
- X-Ray
- CT Scan
- X-Ray Crystallography
- Crookes Tube