Week 3 Flashcards
What are the properties of xrays
- not seen, heard, or felt
- travels in straight lines
- not influenced by magnetic or electrical fields
- ability to ionize or destroy cells
A long wave length = what
Low energy
What are the main two interactions with matter in CT?
Compton and photoelectric effect
What is the relationship between frequency and wavelength?
Inversely proportional
Describe frequency
The rise and fall of cycle in a period measured in hertz (Hz)
Ionizing radiation is said to have a high or low frequency?
High frequency
Describe wavelength
The distance between crest to crest measured in meters
Ionizing radiation is said to have what kind of wavelength?
Short wavelength
What are the five forms of interaction with matter?
- coherent scatter
- Compton effect
- photoelectric effect
- pair production
- photodisintergration
At what kVp does photoelectric effect occur?
At diagnostic energy ranges 20-120 kVp
Which interaction with matter is the greatest risk for patient dose?
Photoelectric effect
Describe photoelectric effect
- incident X-ray with a greater or equal to energy than the inner shell binding energy
- X-ray fully absorbed with atoms inner shell electron, electron ejected (photoelectron)
- vacancy is filled by adjacent shell e-
- characteristic cascade occurs producing secondary low energy X-rays
What is the name of the ejected electron in the photoelectric effect?
Photoelectron
Removing an electron from an atom results in what?
Ionization of atom
The probability of Compton effect occurring decreases with what X-ray energy?
High X-ray energy
Describe Compton interactions
- incident X-ray hits outer shell electron
- electron is ejected in diff direction
- excess energy released as scatter photon in diff direction
- both scatter X-ray and ejected electron can produce new interactions (mainly photoelectric)
Which photon interaction with matter adds to diagnostic value to the image?
Compton interaction only adds fog to image
What is the definition of attenuation
The reduction of radiation intensity as it passes through tissue resulting in absorption and scatter
What factors affect attenuation?
Atomic #: higher # greater attenuation
Mass density of tissue: higher density, greater attenuation
Energy of X-rays: higher energy, less attenuation
What is Linear energy transfer?
Average amt of energy transferred into matter per unit distance travelled
What scan method is a single projection image of superimposed tissue with a large FOV?
Localizer scan aka scout
When is a localizer scan done in a ct exam?
At the beginning of exam
What type of localizer scan is used for perfectly transverse slices such as chest, abdomen, and pelvic studies?
AP localizer X-ray tube @ 0 degrees
What localizer scan is used if the gantry must be tilted for oblique slices? ex. Some Head and spine exams
Lateral localizers (X-ray tube at 90 degrees)
