Lecture Two: Anatomical MRI Flashcards
What wavelength does MRI use?
Radiowaves
What normally determines resolution?
The wavelength used in imaging, MRI is the exception as they use fancy tools to improve resolution
What can MRI be used for?
- Can modulate contrast to highlight pathology
- Can indicate Anatomy and Function (fMRI)
Whats a property of all molecules?
Spin (quantum property)
Dissarrangement (not aligned)
Describe in brief terms MRI
Based on NRI, radiowaves excite particles under the presence of a magenetic field where they align in a plane. The decay of excite releases energy that is detected and generates an image
Lots of other factors naturally
Describe the use of radiowaves in MRI
Radiowaves at a specific frequency excite particles (ground -> excited) to an excited state.
Under the influence of magentic fields the particles align.
Excited particles ‘decay’ over time and release radiowaves that are detected by coils.
What specific particle is typically excited?
H+ as it is abundant in water and used mainly in clinical imaging.
What is B0?
Head to foot direction of magnetic field in MRI (Z plane)
Describe what happens when H are excited by radiowaves?
Radiowaves excite H particles from their aligned B0, into the X,Y plane. (flip), (flip is typically 90 degrees)
Here they start to rotate around the central axis. Procession = particular frequency they rotate at. (also known as wobbles)
What is measured in MRI?
The procession, as these wobbles release radiowaves that are detected
Whats the relevant equation for procession?
Resonate Hz of precision =
w = y.B0
What is the magentic field measured in?
Teslas (is the unit for B0)
How can the relaxation / decay from an excited state be divided into?
T1 and T2, describes how the H+ relaxes
What is T1?
The time for the axis plane to return to Z
What is T2?
Decay of X,Y measurement
Whats faster T2 or T1?
T2 is faster than T1
Process is known as rephasing
How is dephasing significant in MRI?
When depahsing occurs the total signal output of molecules decreases and is not detected. (summation is reduced)
Occurs naturally
What is a spin echo?
Radio Hz burst over time, this uses gradients to maintain the molecules in an excited state so their alignment produces radio signals that summate and produce an image.
Describe what a spin echo does the the molecules;
- Radio Hz alters spin direction / wobble
- Increase in radio Hz is used to rephase
Describe the MRI process using echo spin;
T1 = 90 degree phase Hz pulse
- Decay of wobble
T2 = 180 degree phase Hz pulse
- Rephase
signal aquired
Echo time 2x (T2-T1)
This T1,T2 (ACQ) is repeated many times to develop an image
How is an image built up?
Repeated rephasing
What is Tr?
Time between 90 degree repeats
What is Te?
Time between 90 degree Hz and Acq
How is contrast altered?
By altering Tr and TE
How is MRI generated from NMR?
MRI is generated by gradients from NMR
- Gradients of magnetic fields generated by coils
- 3 axis of gradients
What is the gradients function?
Gradients act to change Hz of nuclie rotation.
- These Hz components can be used to spatially locate tissues
- Back projections create images
Why is MRI a slow process?
Because of the time it takes for relaxation from the excited state to occur
Describe the physical basis of NMR used in MRI;
- NMR ; nuclie with ODD atomic weight have SPIN and magnetic moment
- When placed in a static magnetic field B0 they develop a net alignment with the field
- NMR sensitivity is related to the size of the magnetic field used
What are MRI measurements sensitive to?
NMR sensitivity and relative abundance of nuclie being excited
Whats the equation for radio Hz used in MRI?
Lamor Frequency
w = y B0
When nuclie relax what does the frequency of radio waves released depend on?
Strength of B0
Atomic species
Chemical of physical environment
Describe how T1 and T2 relate to tissues
Solids ; Very short T1 and T2 (ms) Water; Very long T1,T2 (S) Tissues; T2 usually shorter than T1 (30 vs 800ms) WM has shorter T1 than GM Tumors have longer T1 Fat has shorter T1 than water
What is usually the strength of the magnetic field?
1.5T
How is an image constructed?
Using gradients to spatially locate the signal
What are the uses of MRI?
- Proton density imaging
- T1 and T2 weighting
- T1 weighting shows good WM/GM contrast
- T2 weighting shows bright CSF
- Angiograms
- DWI for early stroke
- BOLD
- In vivo spectroscopy
What are the advantages of MRI?
- Signal can be dependant on tissue type due to the individual environment of nuclie
- Ability to change contrast with a wide variety of imaging protocols
- Sensitivity to motion (Angiograms DWI)
- Good resolution (1mm)
- Non-invasive (no ionising radiation)
What are the disadvantages of MRI?
- Motion artefacts can destroy BOLD signal
- Contraindications (pacemakers, metal clips, pins etc)
- Claustrophobia due to small bore
