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PHYS855: Medical Imaging > Introduction to Medical Imaging > Flashcards

Introduction to Medical Imaging Flashcards

(103 cards)

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1
Q

What are the 6 key purposes for imaging in medicine?

A

1) Screening of largely asymptomatic individuals
2) Diagnosis of largely symptomatic individuals
3) Staging of disease
4) Planning and delivery of treatment
5) Monitoring of disease/recurrence
6) Research

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2
Q

Name 5 imaging modalities that utilise ionising radiation

A

1) X-ray images
2) Computed tomography (CT)
3) Fluoroscopy
4) Nuclear medicine
4) Positron emission tomography (PET)

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3
Q

Name 2 imaging modalities that utilise non-ionising radiation

A

1) Ultrasound
2) Magnetic resonance imaging (MRI)

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4
Q

What is anatomical imaging?

A

Structural imaging of the body that depicts what is present and shows variations in organ composition, location, size, and shape.

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5
Q

What is functional/physiological imaging?

A

Imaging that shows:
- How organs within the body are functioning
- How biological pathways are functioning
- The uptake of pharmaceuticals within organs and lesions

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6
Q

What are X-ray radiographs?

A

A form of 2D imaging that can be either static or dynamic. It utilises X-ray photons to measure variations in the attentuation coefficient throughout the body.

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7
Q

Are X-ray radiographs anatomical or functional?

A

Anatomical/structural, but can be functional if contrast agents are used.

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8
Q

What is computed tomography (CT)?

A

A 3D imaging modality that either displays 2D slices from a 3D stack or renders a 3D model of the slices. It utilises photons to measure differences in attenuation coefficient throughout the body.

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9
Q

Are CT scans anatomical or functional?

A

Anatomical/structural, but can be functional if contrast agents are used.

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10
Q

CT scans have a much _______ dose than X-ray radiographs but provide better ________ images and have no __________ structures.

A

Higher
Contrast
Overlapping

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11
Q

What is nuclear medicine imaging?

A

A medical imaging technique that uses a radioactive tracer (a radioactive isotope attached to a pharmaceutical) to image biological pathways and physiology. For these scans, the tracer is imaged rather than the patient.

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12
Q

Do nuclear medicine scans produce 2D or 3D images?

A

Both: planar imaging produces 2D images whilst SPECT produces 3D images.

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13
Q

Are nuclear medicine scans anatomical or functional?

A

Functional

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14
Q

What is positron emission tomography (PET)?

A

An imaging technique that measures the uptake of a radioactive tracer within the body. The radionuclide emits positrons that are annihilated, emitting gamma ray pairs which are then detected by the scanner.

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15
Q

Do PET scans produce 2D or 3D images?

A

3D

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16
Q

Are PET scans anatomical or functional?

A

Functional

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17
Q

What is the most common radioactive tracer used in PET imaging?

A

FDG

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18
Q

What is magnetic resonance imaging (MRI)?

A

A form of 3D imaging that displays 2D slices through a 3D stack. It measures the response (the extent of magnetisation) of hydrogen atoms to varying magnetic fields, allowing for differentiation between different tissue types.

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19
Q

Are MRI scans anatomical or functional?

A

They are both anatomical/structural and functional

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20
Q

What is ultrasound?

A

An imaging modality that utilises high-frequency sound waves travelling through the body and the acoustic reflective boundaries between materials to provide information about the internal structures of a patient.

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21
Q

Does ultrasound produce 2D or 3D images?

A

They are typically 2D, real-time images but sometimes 3D foetal images can be obtained.

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22
Q

Are ultrasound scans anatomical or functional?

A

Both: different modes can create either functional or anatomical/structural images.

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23
Q

What type of imaging uses infrared radiation?

A

Thermography

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24
Q

What is electrical impedance tomography?

A

A noninvasive type of medical imaging in which the electrical conductivity, permittivity, and impedance of a part of the body is inferred from surface electrode measurements. This is used to form a tomographic image of that part of the body.

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25
State 3 types of optical imaging
- Naked eye - Endoscopy - Microscopy
26
Why are there so many types of medical image?
Each modality can provide different or complementary clinical information, meaning that they each have their own uses.
27
What is multi-modal fused imaging?
A technique that combines multiple imaging modalities to utilise the advantages of both. This is particularly useful when combining functional imaging data with anatomical imaging data.
28
Give 3 examples of multi-modal fusing
NM-CT PET-CT PET-MRI
29
To produce an image, information is _________ from the object to the image domain.
Transferred
30
What is the difference between emission and transmission when producing images?
Emission: information is transferred directly from the object to the detector. Transmission: information is transferred from the source, through the object, to the detector.
31
What are the 3 basical stages of forming an images?
1) Signal is detected 2) Processing 3) Image is displayed
32
Why are medical images not a perfect replica of patient anatomy?
Each point being imaged principally transfers data to its respective voxel in the imaging system, but also contributes a small amount of data to the voxels surrounding it. This reduces the resolution of the image.
33
Describe the difference between the signal profile in an ideal imaging system compared to a realistic imaging system
An ideal system has a 1:1 relationship between the object point and the detector point. A realistic system has a finite distribution of signal in the imaging domain, known as a point spread function
34
What is the superposition principle in imaging?
Assuming signals are additive, the net response of N stimuli is equal to the sum of N individual responses to each stimulus.
35
For a linear imaging system, the contribution to the image from any point is proportional to the ________ of the signal at that point.
Strength
36
For shift-invariant systems, the point spread function depends on the ________ _________ of the points in the image and object (not their absolute position).
Relative displacement
37
Images are produced by a ________ of the point spread function and the object.
Convolution
38
True or false: linear, shift-invariant systems image sinusoidal components of any spatial frequency independently of all others and without changing the frequency.
True
39
State 4 properties of images
1) Spatial resoltuin 2) Contrast 3) Noise 4) Geometric linearity
40
Define spatial resolution
The ability to separate object details within an image.
41
What are the 3 ways to determine spatial resolution?
- Number of details per unit distance - Width of PSF - Modulation transfer function
42
What test is carried out to determine spatial resolution? How?
The line pairs test: a discreet bar pattern of different sizes is imaged, providing a quick and easy way to determine the limiting resolution for a medical imaging device.
43
What are the pros and cons of the line pairs test?
+ Quick and easy to perform/measure - Subjective
44
What is a full width half maximum (FWHM)?
A statistical measure used to describe the width of a normal distribution or Gaussian distribution (such as a point spread function).
45
Two equal points need to be ________ _____ the FWHM apart to be seen as two peaks on an image.
Greater than
46
What is the modulation transfer function?
The ability of a detector to retain the contrast (signal amplitude) of an object. Each detector has its own limiting spatial resolution, which causes a loss in contrast on the resulting image.
47
How is the modulation transfer function of an image measured?
1) A line along the detector is scanned 2) An image is produced and a line spread function is generated 3) A normalised fourier transform of the LSF is produced to retrieve the imaging response to all spatial frequencies 4) This is plotted as modulation against spatial frequency (the MTF)
48
What is the MTF used for?
To compare the retention of contrast with increasing spatial frequency for different detectors/technologies.
49
The overall MTF of a detector is often the ______ of multiple ________ MTFs.
Product Component
50
Define aliasing
The misidentification of a signal frequency, introducing distortion or error. This is often caused by using digital methods to discreetly sample an analogue signal.
51
What is the Nyquist frequency?
The maximum frequency that can be accurately represented when sampling a continuous signal without introducing aliasing. This is equal to N/2 when N points are sampled.
52
Define contrast
The difference in image signal between a feature in the image and its neighbourhood. The minimum detectable contrast is the level which can be distinguished from noise.
53
State the 2 types of contrast and their applications
Michelson: useful for periodic contrast Weber: useful for localised contrast against a uniform background
54
Give the equation for Michelson contrast
C = contrast I = intensity
55
Give the equation for Weber contrast
C = contrast I = intensity
56
State 4 sources of background information that can reduce contrast
- Film fog - Scatter - Background radiation - Overlying tissue
57
Give the equation for the ratio of background to average signal
58
Give the equation for the change in contrast due to background signal
C' = change in contrast C = contrast R = ratio of background to average signal
59
Define noise
The variation in measured signal for a constant input signal
60
What are the systemic sources of noise?
- Unwanted structural interference - Non-uniformity (additive/multiplicative)
61
What are the random sources of noise?
- Quantum processes - Poisson statistical events
62
What is the noise power spectrum?
The spatial frequency dependence of noise in an image, described by its Wiener spectrum, which characterizes the distribution of noise across different frequencies.
63
How are Poisson noise and the number of detected photons (n) related?
Poisson noise = √n
64
Define signal to noise ratio
A measure of the strength of a signal relative to the amount of background noise. The higher the signal to noise, the easier the signal is to discern.
65
Describe how noise effects high contrast images compared to low contrast images
66
How are resolution, contrast, and noise related?
Resolution is defined in terms of relative contrast. Contrast for small details is degraded by poor resolution. Contrast is noise limited.
67
Define geometric linearity (spatial accuracy)
The ability to produce an image without spatial distortion (all points in the object are relatively in the same place in the image).
68
How is geometric linearity measured?
It is measured by imaging a regular grid of lines/points on a test object.
69
When reconstructing a 3D object as a 2D image, what are the 2 ways to present an image?
1) 3D projected onto 2D (e.g. X-ray) 2) Image in 2D and stack in the third dimension (e.g. CT)
70
What are the 2 most common ways to reconstruct 3D images?
1) Filtered back projection 2) Iterative reconstruction
71
What is a sinogram?
A 2D representation of projection data, essentially a collection of line integrals from different angles of the object being imaged.
72
What is back projection reconstruction?
A method used in medical imaging, like CT scans, to build an image from projection data. It essentially "smears back" the projection data across the image, creating a reconstructed image.
73
Why does simple back-projection produce poorly defined images?
Because there is additional blurring from the point spread function, which acts as 1/r (where r = distance between detector and object).
74
What is filtered back projection?
The filtering (modification) of an image prior to back projecting it as a form of edge-enhancement. This emphasises the difference between points which are changing rapidly.
75
Why are different filters used for filtered back projection?
Different filters are required for different purposes (e.g. noise reduction for resolution increase).
76
Why is filtering important when reconstructing medical images?
Filtering can improve the quality of an image, meaning that it can be used for clinical purposes like diagnostics.
77
Why do filters have to be specifically selected?
To appropriate increase resolution without amplifying too much noise.
78
What are the pros and cons of filtered back projection?
+ Fast + Uses real projection data - Prone to artefacts - Difficult to model physics principles (like attenuation/scatter)
79
What is iterative reconstruction?
An alternative to FBP that works by initially guessing what the object distribution could be, comparing the prediction to the physical projection, then adjusting the prediction iteratively until the two match.
80
State 2 types of iterative reconstruction
Maximum likelihood expectation maximisation (MLEM) Ordered subset expectation maximum (OSEM)
81
What is OSEM iterative reconstruction?
A method of iterative reconstruction that improves computation time without reducing image quality by dividing the projections into subsets and comparing one subset at a time.
82
What type of iterative reconstruction is most commonly used in clinical settings?
OSEM
83
State the pros and cons of iterative reconstruction
+ Produces better quality images + Can incorporate corrections for attenuation, scatter, etc. - Computationally expensive - Actual data isn't used in the final image - Increased projection = increased noise
84
Name 5 common techniques used in image processing
1) Restoration (undo degredation due to the imaging process) 2) Filtering 3) Segmentation (including defining regions of interest and extracting features) 4) Transformation 5) Measurements/calculations
85
Why is it difficult to restore images?
All images have an element of noise that can't be removed, and by amplifying high frequency details the noise is also amplified.
86
What is the difference between a uniform and an adaptive filter?
A uniform filter is applied to the whole image uniformly but an adaptive filter is adjusted for local image properties.
87
Why is segmentation important in medical imaging?
Medical images typically contain something that we are interested in (e.g. organs, lesions, blood vessels) that may be obscured by other information, so often it is helpful to identify and separate these features from the rest of the image for further processing/analysis.
88
How is transformation used in medical images?
Significant features can be enhanced via processing to make something clearer or examine functional processes. This can be done using techniques like Maximum Intensity Projection (MIP).
89
What are parametric images?
Images in which each pixel contains a calculated parameter (e.g. MRS).
90
What is image quantification?
The process of deriving quantiative information from images and regions of interest (ROIs). This can include measurement, calculation, or curve statistics.
91
What are the two main ways to display images?
1) Non-digital (e.g. film) 2) Digital
92
Why are high quality display and image windowing required when viewing images?
To maximise the quality of the images being displayed (otherwise they can become limiting factors in image quality)
93
What is image windowing?
A technique used in medical imaging to enhance the visibility of specific structures or tissues in the image. It involves adjusting the brightness and contrast of the image so that the visible information can be comprehended by humans (as monitors can display more shades of grey than people can comprehend).
94
How are settings chosen for image windowing?
A decision is made based on the clinical question. The most important tissues are selected (window level) and given a wide range of pixel values (window width) so that they can be accurately visualised. Pixel values outside of this range are left as either black or white.
95
Define window level
The center of the range of HU values you want to focus on (based on which tissue type is being visualised). The window level determines the brightness of the image.
96
Define window width
The range of HU values around the window level that will be displayed. The window width determines the contrast: a narrow width gives high contrast, and a wide width gives low contrast.
97
True or false: all look up tables for windowing are in greyscale.
FALSE, they can also be in colour
98
Which viewing conditions can impact the quality of an image?
- Ambient light/glare (reduced perception of low contrast) - Viewing distance (reduced perception of small details)
99
What functions do imaging systems required to optimally display images?
They must be versatile and be able to: - Display dynamic series - Display volumetric series - Compare multiple images - Measure objects on-screen
100
What does human perception and intepretation of a medical image depend on?
1) Information in the image 2) Display quality 3) The observers visual system 4) Revelant knowledge/training/experience of the observer
101
What is signal detection theory?
A framework that quantifies how well an observer can distinguish between signal and noise, accounting for both perceptual sensitivity and decision-making bias.
102
What are ROCs?
Receiver Operator Characteristics are the relative proportion of false positives and negatives. They depend on the visual stimulus chosen, the observer, and the level of confidence.
103
Describe the impact of noise and blur on an image

PHYS855: Medical Imaging (5 decks)

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