Introduction to Medical Imaging

Question 1

What is spatial resolution?

Answer 1

Ability to separate different object details or features in an image

Question 2

How can spatial resolution be quantified?

Answer 2

1. Width (FWHM) of the PSF
2. Modulation transfer function
3. Number of details per unit distance (lp/mm)

Question 3

What is contrast?

Answer 3

Measure of signal above ‘background’

Ability to differentiate an object from its surroundings

Represents differences in materials (density, Z)

Question 4

What is the difference between systematic and random noise?

Answer 4

Systematic: Structural interference
Non-uniformity (additive/multiplicative)
Predictable

Random: Quantum processes
Governed by Poisson statistics
root(signal)
Not random at all scales (autocorrelated)

Question 5

How can poor spatial resolution limit the detectability of a small, isolated feature of interest?

Answer 5

Poor spatial resolution results in a more spread out PSF which means more blurring in the image (information in one pixel is from a larger area meaning the signal from a small feature is more drowned out/averaged out)
A larger PSF or poorer spatial resolution results in a loss of contrast (contrast is more smeared)

Question 6

What are the advantages of multimodality imaging?

Answer 6

Combining multiple imaging modalities allows you to:
Utilise advantages of two different modalities
Get functional (physiological) information and map it to an anatomical location in the body
Aid in diagnosis and confidence
Attenuation correction

Question 7

What is anatomical imaging?

Answer 7

Structural imaging of the body (what is/isn’t there). How organs vary in their composition/location/size/shape

Question 8

What is a functional image?

Answer 8

How organs function within the body, how biological pathways function, uptake of pharmaceuticals within organs and lesions

Question 9

What are the disadvantages of multimodality imaging?

Answer 9

Often giving an additional dose
Possibility for misregistration
Higher costs

Question 10

What is a medical image?

Answer 10

A (usually 2D) representation of a 3D object which can be stacked in stacked (in third dimension or in time)
It is a map of the distribution of a property of the object (e.g. brightness, colour, emission, transimission, electrical impedanne, magnetic moment)

Question 11

What are the key features of an XR radiograph?

Answer 11

2-dimensional image using x-ray photons to measure variations in attenuation coefficients

Can be structural or functional (if attn co change e.g. for a barium meal)

Question 12

What are the key features of a CT image?

Answer 12

3D (stack of 2D slices which can be 3D rendered)
Uses photons to measure differences in attenuation coefficients, displaying different anatomy
Can be structural or functional if attn co change
Higher dose than XR

Question 13

What are the key features of NM imaging?

Answer 13

Can be planar or 3D (SPECT/PET)
Functional (can be structural if enitre organ has uptake)
Can measure amouint of uptake or the location
Radionuclide attached to a pharmaceutical specific to pathway of interest
Measuring isotope not patient
High dose

Question 14

What are the key feature of MRI?

Answer 14

3D (stack of 2D)
Fuctional and structural
Measures variation of magnetisation or magnetic moment of protons in different environments

Question 15

What are the key features of US imaging?

Answer 15

2D or 3D
Real-time
Uses reflective acoustic boundary between materials
Functional (Doppler) or structural (lesion detection)

Question 16

What is a line pair test?

Answer 16

A basic test of spatial resolution using a test object with a discrete bar pattern of different spatial frequencies

Question 17

What are the benefits of a line pair test?

Answer 17

Assesses limiting resolution
Consistency checking
Quick and easy

Question 18

What is a disadvantage to the line pair test?

Answer 18

Subjective - operator and conditions dependent

Question 20

How is a digital image obtained?

Answer 20

A digital image is acquired by taking discrete samples of an analogue signal

Question 21

What is the Nyquist frequency?

Answer 21

The maximum frequency in the image is N/2 where N is the number of sampling points

Sampling points is related to the size of pixels etc

Question 22

What is aliasing?

Answer 22

Sampling frequency too low to obtain digital data that is representative of the analogue signal

Question 23

What is the minimum detectable contrast?

Answer 23

The level at which an object can be distinguished from noise

Question 24

What is the consequence of a high contrast image?

Answer 24

Easy to detect the feature but it may reduce the dynamic range (saturation)

Question 25

How is contrast defined? (equations)

Answer 25

For periodic contrast Michelson: C= (|Imax - Imin|)/(|Imax - Imin|) For localised contrast against uniform background Weber: C= (|Imax - Imin|)/Imin

Question 26

What reduces contrast?

Answer 26

Background information e.g. film-fog, scatter, background radiation, overlying tissue

Question 27

What is noise?

Answer 27

Variation in measured signal for a constant input signal

Question 28

What are the types of noise?

Answer 28

Systemic Random

Question 30

What is image quality interdependence?

Answer 30

Resolution, contrast and noise are interrelated Resolution is defined in terms of relative contrast Contrast for small details is degraded by poor resolution (poor contrast -> hard to resolve Contrast detectability is limited by noise

Question 31

What is geometric linearity?

Answer 31

The ability to image without spatial distortion i.e. all points in the object are relatively in the same place in the image

Question 32

How is geometric linearity measures?

Answer 32

Using a regular grid of lines or points in a test object

Question 33

What can affect geometric linearity?

Answer 33

Object being imaged (e.g. in MRI and US)