AIM: Ch 11: X-ray Dosimetry in Projection Imaging and Computed Tomography Flashcards
It is that value that most closely matches the attenuation profile of a polyenergetic x-ray spectrum
Effective linear attenuation coefficient
In diagnostic radiology, x-rays interact by a combination of complete absorption events (____) and scattering events (Compton and Rayleigh scattering).
Photoelectric absorption
T/F: Only the photoelectric effect and Compton scatter lead to dose deposited in tissue
True
It is defined as:
Absorbed dose
For a 25-cm-thick patient, with a 2-cm separation between the patient and the detector, the SSD is computed as ____
SID: 100 cm (typical) or 183 for upright chest PA
100 - [25 + 2] = 73
These measurements are also known as “without backscatter.”
Free-in-air measurements
If one were to actually measure the kerma by placing an ionization chamber in front of a tissueequivalent phantom, the probe would measure both the incident primary beam and a considerable amount of scattered radiation from the phantom; because this scatter is directed back toward the x-ray source, it is called backscatter. When backscatter is included, depending on the beam energy and geometry, the measurement would be about 15% to 30% higher than a free-in-air measurement at the same position and with the same x-ray technique factors.
It is a surrogate measure of the dose used in projection radiography
Entrance Skin Kerma (ESK)
ESK is not an accurate estimate of tissue dose but has evolved as an informal metric for comparing doses among radiographic procedures.
In fluoroscopy, the ____ is a function of the mA and kV, and the mA fluctuates during the procedure depending on the positioning of the fluoroscopic system relative to the patient’s body and on the mode settings of the fluoroscopy system.
Kerma rate
Modern fluoroscopy systems have the ability to automatically record the total fluoroscopy time used (in minutes) for a procedure, and the total fluoroscopy time is converted to seconds and multiplied by the kerma rate to determine the total ESK.
Seen on all modern fluoroscopy systems modulates the mA based on the radiation intensity striking the fluoroscopic detector
Automatic brightness control system
The kerma rate is measured as a function of the thickness of the PMMA over a range of possible patient thicknesses, and these measurements can be used to estimate the entrance kerma rate to the patient
If the chamber is positioned close to the scatter material, then the measurements will be “with backscatter.” Alternatively, the radiation meter can be positioned well away from the attenuator (i.e., closer to the x-ray tube), which would reduce the contribution of scatter to the measurement such that it is “without backscatter.” In this later case, the inverse square law is then used to calculate the measured kerma rates at the entrant surface of the “patient.”
It is based on tracking the trajectory and energy deposition of x-ray photons through a mathematical phantom with a defined geometry and composition
Monte Carlo dosimetry
It is measured using either a 16-cm or 32-cmdiameter polymethyl methacrylate (PMMA) phantom. The dosimeter is placed serially in the center hole and the peripheral hole, and the measurements are combined to produce the weighted ____
Computed tomography dose index (CTDI)
The most important limitation of CTDIvol is that it is a dose index, and was not initially meant to be a measurement of dose per se
