X Ray Flashcards
What is the basic principle behind how an Xray produces an image?
Different structures stop Xrays at different degrees depending on the structure and atomic number. This relates to the attenuation.
what type of radiation is X rays?
electromagnetic radiation that is ionising and penetrating
What are the different effects of the ionising radiation from X ray?
- direct action
2. indirect action
what is direct action of the Xray ionising radiation?
The Xrays hit inside the actual cells and break one or both of the sugar-phosphate DNA backbone or base pairs of DNA
What is indirect action of Xrays?
the radiation acts on water and forms free radicals. These are highly reactive and damage cells
what effect of X ray radiation is most common?
indirect. As Xray radiation is penetrating so rarely hits directly inside of the cells
what is Xray radiation dosage measured in?
milliseverts (mSv)
what does the Xray radiation dosage depend on?
the energy of the imaging method
the sensitivity of the organ
what is the typical annual background radiation?
2.2mSv
what is the radiation dosage of a chest xray?
0.02mSv
what is the radiation dosage of a CT pelvis?
7mSv
what is the imaging chain of an X ray?
X ray tube | Detector | Processor | Film
where in the imaging chain does the patient go?
between the X ray tube and the detector
what is the role of the Xray tube?
together with a generator produces X rays.
what are types of detectors for linear 2D XRays?
- Film hardcopy
- Computer Phosphor plate
- Digital radiology
what is the film hardcopy detector for linear Xray?
A film processor is used with a tank of chemicals
Allows for high spatial resolution but digital is more commonly used now
what is the computer phosphor plate detector for linear xray?
A laser scanner or CR reader is used. It’s electronically processed without chemicals but a phosphor plate is still needed.
what is digit radiology detector for xray?
A flat panel detector is used and a digital system automatically produces the image
what is the analogue image type?
The characteristics of the object are represented by brightness of colour. No digital representation is provided
what is the digital image type?
Image information is stored as digital numbers. This is processed to provide an image. Can also store and copy.
what values are involved in formatting a digital image?
N bits= provides the bit depth
Format= x by y by N per pixel
what are advantages of digital image?
ability to store and retrieve
able to analyse
can be processed
can provide an interactive display during procedures
what is involved in assessing performance of the system?
Efficiency
Spatial resolution
Contrast Resolution
Dynamic range
what is efficiency?
How much output signal you get for a given radiation dose
A high efficiency means more input energy is turned into the actual image.
Also referred to as speed as assesses safety.
what is spatial resolution?
ability to see images close together
what is dynamic range?
the range between the lowest and highest signal. Tells you how well the system copes with high values compared to low values.
the bigger the dynamic range the better
what is exponential attenuation?
the thicker the material being imaged the more attenuation there will be. The intensity curve never goes to zero so regardless of how thick the tissue is some x rays will always pass through
what causes high attenuation?
high atomic number: the more electrons the more x rays won’t pass through
high density
Xray basically gives an attenuation map
how do you measure spatial resolution?
number of lines per mm
high resolution: 20 lines per mm
standard: 8 lines per mm
what is contrast resolution?
ability to see objects of low contrast described by the smallest size of the object visible as the stated contrast level threshold.
what is the relationship between contrast resolution, noise and signal?
The signal has to be large enough to see above noise. Image noise affects how we see low contrast objects
what is object contrast?
difference in signal/contrast due to the object
what is image contrast?
difference between image characteristic between two locations on the image
what is image noise?
random variations in signal
what are disadvantages of planar Xray?
shadow imaging: reduced contrast in the image due to overlapping structures
hard to get 3D localisation
Poor geometric accuracy
why does Xray give poor geometric accuracy?
The Xrays come from a joint focal point and then fan out around the object. As the detector object distances increases there is more magnification due to this fanning out from the focal point
what is fluoroscopy with X ray?
allows real time x ray imaging.A catheter is fed inside of an artery and a dye injected. The dye is dense allowing you to see flow inside the vessel.
why is Xray tomography different to normal Xray?
in tomography the image corresponds to a cut transaxial cross section which allows you to see areas seperated instead of overlapping allowing for better contrast.
how does tomography work?
The detectors are on a mobile gantry which moves upto 4 times a second. This produces data which is used to form an image. Just like X rays measures attenuation.
what numbers are used in tomography?
CT numbers. This is the density of the tissue in reference to the density of water
Density of tissue above water: positive number
density of tissue below water: negative number
what is helical scanning for tomography?
A larger bank of detectors so more coverage is achieved per rotation. This is much faster.
why is hybrid scanning with PET:CT good?
PET provides functional information and CT provides the anatomical location
