Physics of Imaging Flashcards
Give 5 differences of in structures transversing throughout the body
Shape/Size, Radiosensitivity, Thickness, Atomic Number, Density
What 3 things can happen when a photon enters the body
Transmission, Absorption and Scattering
The photons which manage to pass through the patient and hit the detector create ….
a ‘grey scale’ image of the body which spatially replicates the pattern of absorption of the photon beam as it passes through the body
The white parts of the image …
many photons have been absorbed by the patient, therefore few photons remain to strike the detector and therefore the intensity is now low = appears white
The black parts of the image …
few photons have been absorbed by the patient therefore many photons remain to strike the detector and the intensity is high = appears black
The shades of grey of the image …
where the number of photons absorbed lies between the 2 extremes, therefore some photons remain to strike the detector and its intensity is neither high nor low
Nam the 2 types of contrast
Subject Contrast & Radiographic Contrast
Describe Subject Contrast
the ratio of radiation intensities transmitted through different areas of the object being imaged - dependent upon differences in absorption properties of the parts being x-rayed
Give examples where high subject contrast could be found and where low subject contrast could be found
High Subject Contrast = between bone (High Z) and tissue (Low Z)
Low Subject Contrast = between kidneys (Low Z) and abdominal organs (Low Z)
Give the 3 different absorption properties which the parts of the bodies under examination are dependent on
Differences in absorption coefficient of the areas being X-Rayed - dependent on the beam energy (Which is dependent on kV selected) and the atomic number of the part
Thickness of parts
Density of parts
The mass attenuation coefficient rapidly …
reduces as the energy of photons increases
Mass Attenuation Coefficient is proportional to …
1/(photon energy) cubed and to (atomic number) cubed, however compton scatter is also present - competing processes
Describe the consequence of photoelectric absorption and compton scattering being competing processes and the ultimate problem
they rely heavily on differences in absorption to produce contrast in our images therefore scatter is a problem
Which factor is independent of scatter
Atomic Number
What do we need to do if we want to see contrast
need to use photons in range where photoelectric absorption is the dominant process - achieved by using energies towards the left of a graph (e.g. low photon energies - between 30-120 KeV)
the more contrast we want, the lower KeV we use
What would be the consequences of using higher photon energies than 120 KeV
create proportionally more of the incident X-Ray photons being scattered (doesn’t differentiate between areas of different Z) and fewer photoelectrically being absorbed (does differentiate between different Z)
contrast goes down at higher energies where scatter dominates
What do differences in tissue density and tissue thickness mean
that theres more atoms with which X-Ray photons can undergo aforementioned processes - greater thickness = absorb more photons, higher density = absorb more photons
Describe Radiographic Contrast
High subject contrast (e.g. bone) leads to high radiographic contrast and low subject contrast (e.g. kidneys) leads to low radiographic contrast
Give 4 ways to alter radiographic contrast
Raise/Lower kV providing you achieve the desired penetration of the part being imaged
Use secondary radiation (‘anti-scatter’) grid to improve contrast
Introduce a radiographic contrast agent
Contrast can be electronically increased and decreased
Describe raising/lowering kV to alter contrast
low kV maximises the dominance of photoelectric absorption - highly dependent on differences in (Z) cubed of materials being imaged, contrast increases with lower kV
high kV reduces dominance of photoelectric effect and scatter becomes more dominant - differences in Z make little difference in attenuation, contrast decreases with high kV
Describe using secondary radiation (‘anti-scatter’) grid to improve contrast
grids = thin, flat sheet made of alternating slats of alternate radiolucent/radio-opaque materials - designed to absorb obliquely travelling scattered photons before they reach the detector whilst still permitting the more forward travelling primary photons to reach the detector to produce a useful image, therefore placed between patient and detector
no grid = scatter reaches detector and degrades contrasat
with grid = much scatter is absorbed and less reaches detector therefore contrast improves
Descibe the introduction of a radiographic contrast agent to improve contrast
this is where something is placed into the body to improve contrast, POSITIVE contrast agents (High Z) e.g. iodine/barium sulphate - introduced into lumen of otherwise low atomic structure (e.g. kidneys/arteries) to increase the photoelectric absorption of X-Rays - appear lighter than surrounding structures
NEGATIVE contrast agents (Low Z) e.g. air/CO2 - introduced into lumen of otherwise low atomic structures (e.g. lungs/colon) - to distend them and decrease the photoelectric absorption of X-Rays - appear darker than surrounding structures
Describe how to alter contrast electronically
using suitable ‘post-processing’ software available from the operators console
Name the 4 concepts used to describe image quality
contrast
resolution
sharpness
image noise
Define Resolution
the ability of an imaging system to faithfully record and demonstrate fine differences in the structure of an object
Define Sharpness
a property of the image which occurs independently of the spatial resolution of the imaging system which produces it, influences by 3 things
Name the 3 things sharpness is influenced by
Voluntary/Involuntary patient movement - relative to the detector
Sub-Optimal Geometry - of the image formation (patient needs to be as close to the receptor as poss)
Image Manipulation - post processing
Describe geometric unsharpness
X-Rays can’t be produced by a point source and can’t be focused like visible light and therefore there is always a penumbra - images always have inherent unsharpness - due to focal spot size being also affected by SDD and ODD
What does SDD and ODD stand for
SDD = Source to detector distance ODD = object to detector distance
Describe Image Noise
the result of random variation in pixel number due to the random distribution of X-Ray photons and imperfections in the imaging system - causes a reduction in image quality
noise reduces as intensity increases but this comes at the cost of higher patient dose
Noise a 1/ (number of photons) square rooted
Define concept of optimisation
where doses should be as low as reasonably practicable (ALARP)
