1
Q
What sized waves does MRI use?
A
Large wavelength, lower frequency and lower energy
2
Q
What are some concerns about while body MRI scans?
A
- Becoming commercialised
- Concers about abnormalities needing trained medical practitioners
- Removers radiographers from the NHS (already a shortage)
- Very expensive (around $4000)- certain people won’t be able to afford this
3
Q
What principles is MRI based on?
A
Electromagnetism from the 1800s from Nikola tesla’s rotating magnetic field
4
Q
What was an essential discovery, revolutionising MRI?
A
- The discovery of the spin of nuclei
- Understand how atoms react to a magnetic field, create and detect signals
- Rabi, Bloch and Purcell 1944
5
Q
Who first pioneered the use of MRI in medicine?
A
Raymond Damadian
6
Q
Who pioneered the implication of MRI methods?
A
Lauterber and Mansfield
7
Q
What are key advantages of MRI technology?
A
- Non-invasive- reduces the need for surgical or other invasive techniques
- No ionising radiation- safer for patients
- Visualisation of soft tissue in 3D
- Flexible contrast- to probe many aspects of the body e.g. vasculature
- High spatial resolution (uncertainty of this in functional)
- Structural and functional informaion
8
Q
What 2 primary things does MRI use?
A
- Strong magnetic firld; magnetic gradients, in order to create the signal
- Radiofrequency pulses (signal transmitter/receiver)
9
Q
Where does the MRI signal come from?
A
- Water molecules (or others) resonant in magnetic field
- Changes induced by the radiofrequency pulses
- Contrast in tissues from proton density, molecular environment and how protons influence each other
10
Q
How is the MRI signal transformed into images?
A
- Spatial encoding/decoding using magnetic gradients
- Mathematical reconstruction of MRI signal
11
Q
What is standard MRI strength?
A
1.5-3.0 tesla (car scrap yard magnet is 1 tesla)
12
Q
What is the strength of the strongest human scanners?
A
- 7.0-10.5tesla
- Have high resolution but are noisy as probe deeply in the resolution so have to make up for other sources of signal coming at these strengths
- People feel uncomfortable, nauseas and dizzy, are still safe but tend not to use
13
Q
What is a magnetic dipole?
A
Anything that consists of two opposite magnetic poles (north and south), like a small bar magnet and creates a magnetic field around it
14
Q
What is magnetic moment?
A
A measure of the strength and orientation of a magnetic dipole’s magnetic field, related to its tendency to align with an external magnetic field (how the object will react to the magnetic field)
15
Q
What is torque?
A
The force that causes an object to rotate
16
Q
What is resonance?
A
When an object or system vibrates in response to an external force or oscillation
17
Q
What is encoding/deconding?
A
- Encoding refers to converting information into a different form of storage or transmission
- Decoding is the process of interpreting or converting it back into its original form
18
Q
What are the magnetic dipoles in the body? Explain their base state.
A
- Hydrogen atom (abundant in the body as water makes up 60-70%)
- These protons and atom move around (odd atomic number) and have a north and south pole
- These will be oriented in a random direction when not subjected to external magnetic field (no net magnetisation)
19
Q
How do the magnetic fields and precession work in MRI?
A
- In the presence of a strong magnetic field (B₀), protons align with the field along the longitudinal plane.
- This creates a net magnetisation parallel to B₀.
- The magnetic moment of protons experiences torque in the B₀ field.
- Protons precess around the axis of B₀ at the Larmor frequency, which depends on the field strength and the gyromagnetic ratio.
- The gyromagnetic ratio is the relationship between a particle’s magnetic moment and its angular momentum.
20
Q
Outline how the resonance from the radiofrequency (RF) pulse is generated.
A
- The particles in the imaged object are aligned with the magnetic field they are within.
- However, the net magnetisation from protons cannot be detected because of the strong external magnetic field (B0)
- Thus there needs to be movement out of the main direction to detect the signal that we are creating without being overwhelmed by B0 (the z axis of magnetisation).
- The RF pulse excites the nuclei at 90 degrees (transverse plane) to their original axis.
- Protons will now shift the field towards this RF pulse and resonate in phase at this angle depending on the duration of pulse.
- From here, this is where the signal will be generated.
21
Q
Outline how resonance is generated in the transverse plane whilst the dipoles are being exposed to B0 to elicit a usable signal in MRI.
A
- The dipoles direction have been aligned to the main magnetic field B0.
- Now, the nuclei need to be pulled away from the main external magnetic field so that they can be detected.
- Thus, the RF transmitter creates an oscillating B1 field in the transverse plane.
- This leads to the dipoles orientation being pulled down/excited into the transverse plane at which the RF is firing.
- This excites the protons to resonate in phase in a new orientation i.e. it produces torque to pull the megnetisation into the new plane.
- It is then the return to the B0, whether it be of T1 or T2 measurment, that the signal is generated from.
22
Q
How is the MRI signal detected?
A
- The RF pulse shifts dipoles from the longitudinal to the transverse plane.
- After the RF pulse stops, nuclei relax back to alignment with the main magnetic field.
- Transverse magnetisation decreases over time while longitudinal alignment recovers.
- Oscillating protons induce a signal in the receiver coil as they pass by.
- The receiver coil detects the changing magnetisation in the transverse plane.
23
Q
How is the MR signal decoded?
A
- In RF receiver coil, the signal will oscillate over time due to the particles passing closer and further from the coil as they themselves oscillate (free induction decay)
- The signal is sent to a computer to be digitilised
- Signals can be complex and convoluted from different tissues with different frequencies
- The strength and distribution of the frequency components are mathematically determined using Fourier Transform (FT) which decodes the frequencies wihtin the MR signal
24
Q
What is free induction decay?
A
It is the signal generated by the decaying transverse magnetisation after the RF pulse is turned off.
It reflects the combined effects of T2 relaxation (dephasing) and the gradual recovery of longitudinal magnetisation and is the basis for MRI signal acquisition.
25
How is the image acquired from the signal?
- The RF receiver coil will detect a whole bunch of RF signal
- MRI signal is converted into an MRI signal spectrum in frequency domain
- The signal needs to be seperated to determine which voxel the signal is coming from
- Altering RF magnetic gradients so that each particle will resonate at a slightly different frequency because of the gradient of the field
- As we set the paramaters for the magnetic gradient, individual frequencies can be decoded from the MRI spectrum
- Postition is encoded by the frequency
26
What are the two common contrast mechanisms for MRI?
- T1 weighted (less sensitive to water content)
- T2 weighted (more sensitive to water content)
27
What do T1 and T2 relaxation times determine?
How quickly the water magnetisation returns to equilibrium following perturbation by RF pulse.
28
What are T1 and T2 relaxation times dependent on?
Nature of the tissue
29
Outline T1 relaxation
- Recovery in the longitudinal direction from the transverse plane to reach 63%
- Used to obtain brain structure and reconstruction of cortical surfaces
30
Outline T2 relaxation
- Is the decay of the signal (transverse plane) and is dependent on the spin-spin interaction
- Is faster than T1
- Information about the content of the tissue
31
What is specific T2 contrast relaxation?
Is the intrinsic dampening of the magnetic signal due to magentic field inhomogeneities, magnetic susceptibility, chemical shift effects
32
Who discovered the fMRI?
- 1948
- Kety and Schmidt measured oxygen metabolism and blood flow in their seminal paper to confirm that blood flow if regionally regulated by the brain itself
33
Explain how Kety and Schmidt found out about fMRI?
- When neuron use more oxygen, chemical signals cause nearby blood vessels to dilate. This increase in vascular volume leads to local increase in blood flow
- Administered ppts with nitrous oxide used as a freely diffusible tracer
- Arterial and venous blood sampling to determine the concentrations of nitrous oxide in the blood
- Global perfusion normalised to the mass of the brain
34
What was the first human fMRI study looking at?
- Cerebral blood volume changes using a contrast agent (radioactive tracer in the veins)
- Belliveau 1991
35
What other imaging techniques give functional readouts?
- PET
- NIRS
- MRI can be adapted to be sensitive to brain metabolism
36
What is fMRI?
- Is a method of measuring vascular response during change in brain activity
- Generally, the BOLD contrast is used for fMRI
- This is now the leading way to assess the function of the brain
37
How does the detection of functional activation work in MRI?
- Specific brain functions are located in specific anatomic regions
- Will be an initial drop in BOLD as the region uses up O2
- During functional activation, metabolic demand (oxygen consumption) increases in the activated region
- The activated regions demonstrates a local blood flow increase
- There is an over-supply of oxygentated blood to the activated region
- Activation results in a decrease in deoxyhaemoglobin content
38
Does deoxyhaemoglobin have paired or unpaired electrons and what does this mean?
Is paramagnetic
Unpaired, so has high magnetic susceptibility
Reacts a lot to the magnetic fields we are subjecting it to (and RF frequencies)
39
Does oxyhaemoglobin have paired or unpaired electrons and what does this mean?
Paired; it is diamagnetic.
Low magnetic susceptibility.
40
What does the magnetic susceptibility of the blood depend on?
The proportions of oxy and deoxy-haemoglobin
41
How can blood oxygention be detected?
- By its effect on T2* contrast
- Amount of time we wait after the initial pulse to measure our signal again.
- Oxygenation preserves T2 signals longer due to less inhomogeneities.
42
What is magnetic susceptibility?
Material acquire a degree of magnetisation when placed in a magnetic field
43
What is diamagnetic susceptability?
Materials that oppose the external field B0
44
What is paramagnetic susceptability?
Materials that increase the field
45
How does the T2* contrast work?
- fMRI uses sequences sensitive to T2* contrast, which occurs due to magentic field inhomogeneities that are caused by the paramagnetic particles
- Time after T RF pulse, will oscillate and gradually decrease intensity
- The higher the deoxyhaemoglobin, the bigger the increase in signal
- This can e tuned to the T2* valye which is maximum at particular magentic resonance parameter (echo time (TE))
46
Outline the fMRI biological signal cascade.
- Neuronal activity involved neurotransmitter release
- Neurotransmitters cause local vasodilation
- Increase in cerebral blood flow and volume
- Oxygen delivery exceeds metabolic need
- Increase in capillary and venous blood oxygenation
- Decrease in deoxyhaemoglobin
- Decrease in susceptibiity-related signal decay rate
- Increase in T2*- higher MRI signal
47
Outline the haemodynamic response signal (BOLD signal)
- Stimulus
- Have onset passing the initial dip (negative BOLD contrast- hard to measure)
- The time to rise (1-2 sec after stimulus)
- The the peak time (4-6 sec after stimulus)
- The amount of oxygenation decreases over time and there will be a post stimulus undershoot (signal supressed after stimulation ends)
48
What is dynamic causal modelling
- A method where the blood responses are removed and we make inferences directly on the neuronal activity (faster)
- Advanced and have to make a lot of assumptions about the data
49
Explain the biology behind BOLD contrast.
- Secondary to neural activity associated with peripheral stimulation, task performace or cognitive activity, there is transient increase in regional CBF
- In active brain regions, the concentration of the oxygen increases because of the over-supply of oxygenated blood (positive overshoot)
- The region of the overshoot is bigger than the region covered by the acctive neurons (poor resolution)- intrinsic limitation
- But, as activated areas are small and as fMRI has low resolution, this phenomenon does not affect the readout much
- The initial dip or negative BOLD has much better spatial resolution, but is small and hard to detect
50
What is the key limitation of studying the BOLD reesponse?
- Indirect measure of neuronal activation
- Responses we get are fairly large accross the brain (reduced spatial and temporal resolution)
51
How can we overcome the problem with resolution in measuring BOLD?
- Use other methodologies such as EEG of individual cell recordings
- However this comes with its own limitations such as working out where the signal is coming from
- The alternative methods may be more sensitive but have their own drawbacks
52
What are 5 limitations of studying BOLD?
- Indirect measure of neuronal activation
- Moderate spatial resolutions (physiological changes can occur away from the site of neuronal activation)
- Moderate temporal resolution
- Effect of various conditions/diseases/aging/lifestyle on vascular response is non-trivial
- Methematical methods to make inferences at the neuronal level but these make assumptions and carry unique drawbacks
53
What is the main difference between MRI and fMRI?
fMRI uses a dynamic series of images which are collected while a person performs a task in the scanner rather than a single image
54
What is the difference with domain general and domain specific brain areas?
- Domain general is when there is activity across a network of regions whose specific role is unclear such as the thalamus and attention or parsing of syntax
- Specific parts of the human brain are responsible for specific tasks
55
How is functional connectivity measured?
- Using BOLD correlations
- How spatially seperated across brain regions
- If there is coordinated activation, probably involved in doing the same task
56
What is an fMRI on-off study design used for?
- Used to find the region of the brain that are being functionally activated
- To consider whole brain network connectivity, a model independent approach is used such as independent component analysis
- Regions of correlated BOLD signal are found, considering maximally independent spatial networks
- Often used to analyse data collected 'at rest' and 'resting sate networks' have been well-characterised
- Van den Heuval et al 2010
56
What have been some significant advances of MRI in the past 50 years?
- Ultra high field scanners (7T clinically; >15T in research)
- Low-field portable scanners (0.05T)
- Hardware/software improvements for faster and higher resolution image acuiqision
57
What physiological parameters have MRI advances allowed for the measurement of?
- Blood flow
- Permeability
- Oxygen saturation
58
What is Bownian motion?
Water molecules undergo random thermal motion
59
What can the bownian motion be characterised by?
What does it depend on?
- Diffusion coefficient (D)
- Depend only on the size (molecule), the temperature and the viscocity of the medium
- Diffusion can be restricted in closed spaces such as cells, hindered by tortuous pathways and slow down due to exchange between compartments
60
How is a diffusion image acquired?
* Magnetic field gradients with different b-factors (e.g., b0 and b1000 s/mm²) are applied to measure the random motion of water molecules (diffusion).
* As the b-value increases, water motion causes a loss of signal intensity due to phase dispersion.
* This attenuation is mathematically modeled to calculate the Apparent Diffusion Coefficient (ADC), which reflects the combined water diffusivity within the voxel, providing insight into tissue microstructure.
61
What happens when b value is increased in diffusion imaging?
When the b-value increases, overall signal decreases but regions of restricted diffusion appear brighter relative to free diffusion areas.
62
What are the principles of the techniques for measuring dMRI?
A magnetic field gradient with a specific b-factor is applied, causing random motion (diffusion) to alter phase distribution.
This leads to signal attenuation, which reflects the degree and direction of water diffusion.
- Le Bihan D 2003
63
What is diffusion tractography imaging?
- Diffusivity can only be captured in the direction of the magnetic field
- However, there are cases where the molecules are restricted in one direction and free to move in another (anisotrophic diffusion) and when it can move around all at the same time (this is isotropic)
- This has proven powerful for imaging of white matter tracts for brain connectivity and microstructure.
- Le Bihan D 2003
64
What is perfusion as a biological process?
The process of delivery of O2 and nutrients to a particular tissue
65
What type of diseases is perfusion compromised in? (6)
- Stroke
- Tumours
- Vessel stenosis (narrowing)
- Dementia
- Migrane
- Depression
66
What can permeability measure?
The breakdwon in membranes that can be important for tumour and neurological conditions such as dynamic contrast enhanced MRI
67
Outline dynamic contrast enhanced MRI
- Uses a contrast agent- magnetic dye such as gadolinium which is injected into the bloodstream
- The contrast agent will leak into permeable regions of the extravascular tissue and can be quantified to measure microvascular permeability
- The method acquired a series of T1 images in rapid succession
68
What is dynamic contrast enhanced MRI often used in?
- Oncology
- Mesurement of tumour microvasculature and the grading of tumours
69
Explain how the contrast agent gadolinium acts on T1
- As soon as the exogenous contast agent is injected and it reaches the tissue to be imaged
- It causes the T1 shortening effects which means the signal increases in the tissues with the contrast agent.
- The shortening effects reduces the longitudinal T1 relaxation time in quick succession
- This means high diffusion contrast imaging can be obtained
70
What does a dynamic contrast enhanced MRI look like for a stroke?
- local permeability where can see lots of blood flow and the contrast agent accumulated in one area of the brain
- Means that permeability is high in these particular tissues so the agent propagates in the tissue of the stroke
71
Is permeability higher in stroke, tumours or ND diseases?
Is higher in stroke and tumours than in ND diseases
72
How is the rate of leakage in dynamic contrast enhanced MRI captured?
By acquiring a time course of images in quick succession (often temporal resolution around 10s)
73
When was arterial spin labelling (ASL) invented?
1990
74
Why is ASL completely non-invasive?
Is sensitive to endogenous water (freely diffusible tracer) in the blood vessels perfusing into tissue
75
How does ASL work?
- RF pulse is used to label the blood water in large arteries
- Time is allowed for the blood water to reach the perfusing region
- An image is captured in the region of interest
76
What are two images taken in ASL and why?
- Control and labelled image
- The subtraction of labelled image from the control image will give a perfusion image proportional to cerebral blood flow (CBF)
77
What data can be aquired using ASL?
- By using several delay times in the data acquisition, data can be generated for certain parameters...
- Arterial transit time (when apply the RF pulse, the time taken to reach this area of the brain)
- Tissue transit time (from the extracellular structures to the particular tissues to be images)
- Cerebral blood flow
78
Oultine how Diffusion tractography of dMRI is used in biomedical research
* Diffusion tractography visualises white matter connectivity in the brain and other tissues like cardiac fibers and tumors.
* A b-value of 800 s/mm² helps characterize carcinoma masses through ADC and dynamic post-contrast imaging with techniques like MIP (Baliyan et al., 2016).
79
Outline how ASL and FLAIR are used in biomedical research
* ASL (Arterial Spin Labeling) and FLAIR (Fluid Attenuated Inversion Recovery) are complementary MRI techniques.
* ASL measures cerebral blood flow, revealing areas of increased perfusion, such as in hypervascular tumors.
* FLAIR suppresses cerebrospinal fluid (CSF) signal, highlighting areas of edema or lesions.
* Together, these techniques provide valuable insights into both the functional (ASL) and structural (FLAIR) aspects of brain pathology.
80
How are advanced diffusion techniques able to measure BBB function?
- Apply using blood water to measure the BBB function in two factors
- The extracellular (De) and intracellular (Di) are the diffusivities measure in the blood vessels and the brain tissues
- k is an important factor in measuring the exchange rate across the barrier and its sensitivity to perfusion
- Powell et al 2023
Imaging in Biomedical Research
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