Filters, Collimators And Grids Flashcards
What is collimation
The removal of X-rays from the X-ray beam to allow along a specific portion of the X-rays to reach the patient.
It consists of lead (high atomic number) sheets (reduce or expand field size) that attenuate the X-ray beam. The higher the atomic number, the more likely photoelectric effect is to occur
Has light source (allows us to see where we are collimating X-ray beam, as X-rays are invisible) that emits light which is reflected by a mirror at a 45 degree angle to emit light parallel to the X-ray beam towards the patient
- scatter gives us signal on the detector that is not congruent with the X-ray that was coming in. It adds noise to the image and reduces contrast - collimation allows narrower field of view - gets rid of scatter that occurs on peripheries - less noise, better spatial resolution and better contrast , reduces dose
- does not change energy of X-rays vs filtration that attenuates lower energy X-rays
- different collimators exist - square, circular, different for different types of X-rays
What is a filter
Filters are metal sheets placed in the x-ray beam between the window and the patient that are used to attenuate the low-energy (soft) x-ray photons from the spectrum.
Filtering is the removal of these low energy x-rays from the beam spectrum which would otherwise not contribute to image quality but would add to patient dose and scatter.
If unfiltered these low-energy x-ray photons are generally absorbed by superficial structures of the body and contribute to the entrance surface dose (ESD).
As they are absorbed by the superficial structures they contribute minimally to image formation. Using a filter reduces the ESD and to a lesser extent effective dose for the patient 1. The units of filtration are expressed in mm of aluminium equivalence (mm Al eq).
Filtration reduces x-ray intensity, but not the maximum energy of the x-ray beam spectrum. The change in the shape of the beam spectrum with filtration is referred to as beam hardening. This is due to the loss of lower energy photons from a polychromatic beam. The average x-ray energy is increased and becomes more penetrating
What are the common filter materials?
Beryllium (atomic number 4) - mammography
General radiography:
Aluminum (atomic number 13) - low energy radiation - lighter- easier handing of X-ray tube
Copper (atomic number 29) - high energy radiation
Tin (atomic number 50) - cuts out lower energies to reduce dose and optimize image quality at the interface between soft tissue and air. This has direct benefits in lung and colon imaging
How is k edge related to X-ray filters
A K-edge filter is used as an X-ray filter for separating energy spectrum of X-rays into high and low energy regions. The K-edge filter is made of a material that has a K-absorption edge in a target energy region of X-rays
Elements with larger K-edge values, that are within the useful portion of the x-ray spectrum, are of greater interest in radiology. The k-edge properties of certain materials can be specifically chosen for their use in contrast media, intensifying screens and beam filters.
For example:
iodine (Z = 53, K-edge = 33.2 keV) is commonly used as a contrast agent in radiography
molybdenum (Z = 42, K-edge = 20.0 keV) is commonly used as a beam filter in mammography
- The K-absorption edge (K-edge) refers to the abrupt increase in the photoelectric absorption of x-ray photons observed at an energy level just beyond the binding energy of the k-shell electrons of the absorbing atom
What are some techniques to reduce scatter and explain them and their implications on image quality and dose
- Collimation - most important
- Compression - decreases tissue thickness - better contrast and spatial resolution
- Grids - network of uniformly spaced horizontal and perpendicular lines
place antiscatter grid between patient and detector - series of highly attenuating lead septa parallel to the primary X-ray beam. Transmitted xrays will pass through septa as they are parallel. Most scattered xrays will be caught in the septa and be attenuated so few reach the detector
Increase grid height, decrease width of interspace material (aluminum or carbon fiber as air not good for structural integrity ) - less scatter will make it through
Grid ratio _determines scatter transmission factor Height of grid/width of inter space material H/W
Grid frequency _ 1/W + t
septal thickness
- Air gap technique - increase object to detector distance - so scattered photons in exit beam can no longer hit detector - magnifies object - as shadow cast on detector is much bigger, also causes geometric blurring/ unsharpness - due to focal spot having sides (in an ideal world it would be best as a point source) - where the shadow of the image is blurred - penumbra - calculate magnification by SID/SOD - redux anode angle - reduces effective focal spot size (closer it gets to a point source) - less geometric blurring
Equation : function of size of focal spot: f OID/SOD - can change multiple factors in primary X-ray beam , exit/remnant beam or in patient to reduce scatter
Explain linear energy transfer as it relates to scatter
The term “linear energy transfer (LET)” is used to indicate the average amount of energy that is lost per unit path-length as a charged particle travels through a given material
- Compton effect from scattered radiation. Photoelectron contributes to patient dose
Describe how anti-scatter grids work
It predominantly attenuates scattered photons. So primary transmission factor is greater than scatter transmission factor
We need to increase patient exposure and dose to use a grid - drawback
What are the different types of grids
Linear grid - doesn’t account for diversion in X-ray beam
Focused grid - outer septa angled to account for diversion in X-ray beam (isotopic emission) - septa match primary divergent beam _ set focal length - 40 inches (100 cm) - if not in length - cut of primary beam - types - off focal grid cut off, off center, off level (eg. when grid is perpendicular to X-ray beam or not parallel)
, upside down - large cut off on peripheries
Stationary grid - septa creates lines on grid
Buckeye grid - Reciprocating grid - moves
- as grid ratio increases so does Buckeye factor
What is Buckeye factor
Increase in intensity (to keep number of protons the same) that we need to apply to a patient in order for the X-ray detector to the patient to have the same exposure that it would have had without a grid
Dose with grid/Dose without grid
Don’t confuse with Buckeye grid
We can use our grid ratio (as grid ration increases - height of grid increases and interspace decreases) to determine the Buckeye factor that we require
