Physics of Imaging

Question 1

Give 5 differences of in structures transversing throughout the body

Answer 1

Shape/Size, Radiosensitivity, Thickness, Atomic Number, Density

Question 2

What 3 things can happen when a photon enters the body

Answer 2

Transmission, Absorption and Scattering

Question 3

The photons which manage to pass through the patient and hit the detector create ….

Answer 3

a ‘grey scale’ image of the body which spatially replicates the pattern of absorption of the photon beam as it passes through the body

Question 4

The white parts of the image …

Answer 4

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

Question 5

The black parts of the image …

Answer 5

few photons have been absorbed by the patient therefore many photons remain to strike the detector and the intensity is high = appears black

Question 6

The shades of grey of the image …

Answer 6

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

Question 7

Nam the 2 types of contrast

Answer 7

Subject Contrast & Radiographic Contrast

Question 8

Describe Subject Contrast

Answer 8

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

Question 9

Give examples where high subject contrast could be found and where low subject contrast could be found

Answer 9

High Subject Contrast = between bone (High Z) and tissue (Low Z)
Low Subject Contrast = between kidneys (Low Z) and abdominal organs (Low Z)

Question 10

Give the 3 different absorption properties which the parts of the bodies under examination are dependent on

Answer 10

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

Question 11

The mass attenuation coefficient rapidly …

Answer 11

reduces as the energy of photons increases

Question 12

Mass Attenuation Coefficient is proportional to …

Answer 12

1/(photon energy) cubed and to (atomic number) cubed, however compton scatter is also present - competing processes

Question 13

Describe the consequence of photoelectric absorption and compton scattering being competing processes and the ultimate problem

Answer 13

they rely heavily on differences in absorption to produce contrast in our images therefore scatter is a problem

Question 14

Which factor is independent of scatter

Answer 14

Atomic Number

Question 15

What do we need to do if we want to see contrast

Answer 15

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

Question 16

What would be the consequences of using higher photon energies than 120 KeV

Answer 16

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

Question 17

What do differences in tissue density and tissue thickness mean

Answer 17

that theres more atoms with which X-Ray photons can undergo aforementioned processes - greater thickness = absorb more photons, higher density = absorb more photons

Question 18

Describe Radiographic Contrast

Answer 18

High subject contrast (e.g. bone) leads to high radiographic contrast and low subject contrast (e.g. kidneys) leads to low radiographic contrast

Question 19

Give 4 ways to alter radiographic contrast

Answer 19

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

Question 20

Describe raising/lowering kV to alter contrast

Answer 20

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

Question 21

Describe using secondary radiation (‘anti-scatter’) grid to improve contrast

Answer 21

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

Question 22

Descibe the introduction of a radiographic contrast agent to improve contrast

Answer 22

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

Question 23

Describe how to alter contrast electronically

Answer 23

using suitable ‘post-processing’ software available from the operators console

Question 24

Name the 4 concepts used to describe image quality

Answer 24

contrast
resolution
sharpness
image noise

Question 25

Define Resolution

Answer 25

the ability of an imaging system to faithfully record and demonstrate fine differences in the structure of an object

Question 26

Define Sharpness

Answer 26

a property of the image which occurs independently of the spatial resolution of the imaging system which produces it, influences by 3 things

Question 27

Name the 3 things sharpness is influenced by

Answer 27

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

Question 28

Describe geometric unsharpness

Answer 28

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

Question 29

What does SDD and ODD stand for

Answer 29

SDD = Source to detector distance
ODD = object to detector distance

Question 30

Describe Image Noise

Answer 30

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

Question 31

Define concept of optimisation

Answer 31

where doses should be as low as reasonably practicable (ALARP)