MRI Flashcards
What are the goals of imaging?
- assessment of human anatomy, physiology, and function in-vivo, NON-INVASIVELY
- diagnostic/prognostic information (e.g. location/type of injury, stroke, etc.)
- improve our understanding of: normal physiology, mechanisms of disease, treatment strategies
List 3 brain imaging techniques.
- computed tomography (x-ray CAT scan)
- positron emission tomography (PET)
- magnetic resonance imaging (MRI)
What kind of electromagnetic waves does MRI use? Is the wavelength larger or smaller than x ray?
radio; larger than x-ray
What are the pros of MRI?
- contrast mechanisms
- no ionizing radiation
- soft tissue contrast (unlike CT)
- any slice orientation
What are the cons of MRI?
- scan time
- contra-indications (i.e. not everyone can have MRI scan because it is a large magnet)
- patient monitoring
- availability
What basic concept in chemistry did MRI evolve from?
NMR
What does NMR measure?
intrinsic molecules in the body
What does NMR stand for?
- Nuclear: looking at nuclei of atoms within molecules
- Magnetic: sample placed in strong magnetic field
- Resonance: deals with spinning frequency of nuclear spins
TRUE or FALSE: atomic number is the number of protons+ neutrons; mass number is the number of protons
FALSE:
- atomic number = # protons
- mass number = # protons + neutrons
A nucleus yields a magnetic moment unless the number of each nucleon type is _____________.
even
What exactly is imaged in MRI? Based on this, what does MRI detect?
protons of the hydrogen atom in water; detect change in magnetic properties of the proton
TRUE or FALSE: water has constant properties in various tissues, which makes it useful in MRI.
FALSE: water has DIFFERENT properties in various tissues, which makes it useful in MRI
Describe how MRI gives us tissue contrast.
- water has different properties in various tissues
- MRI takes advantage of differences in: concentration, magnetic properties, diffusion, flow, blood oxygen, etc.
When is blood flow considered paramagnetic? diamagnetic?
- paramagnetic = deoxygenated blood
- diamagnetic = oxygenated blood
What does the signal intensity of each voxel represent?
macroscopic average of the water properties in each compartment within the tissue (i.e. activity of H2O molecules)
TRUE or FALSE: one water molecule fits into one voxel
FALSE: many water molecules can fit into 1 voxel
How big is a single voxel?
1mm x 1mm
How thick are slices in MRI scans?
~ 1-5 mm thick
What does the radiofrequency (RF) coil do in MRI?
excites and detects the H signal from the tissue
What are the main hardware components of an MRI scanner?
- magnet
- RF coil
- gradient coil
- computer console
How is a static magnetic field (Bo) created in MRI? What does it do?
- large magnet polarizes the sample by creating a net nuclear magnetization along the axis of the applied static magnetic field
- nuclear magnetic moments align themselves along Bo (like how a compass needle lines up towards north pole)
What are MRI scanners characterized by?
strength of their magnetic field
How strong is the magnetic field in a:
- normal clinical scanner
- our strongest scanner
- Earth’s magnetic field
- normal clinical scanner = 1.5 Tesla
- our strongest scanner = 4.7 Tesla
- Earth’s magnetic field = 0.00005 Tesla
What is the function of longitudinal magnetization (Mo) in MRI?
prepare the signal for transmission, by aligning the protons
TRUE or FALSE: signal-to-noise in image is inversely proportional to magnetic field strength
FALSE: signal-to-noise in image GOES UP LINEARLY with field strength (i.e. increase Bo (Tesla) = increase resolution and speed of MRI)
What are some disadvantages of using a strong magnetic field in MRI?
- distortions
- cost
- power
- acoustic noise
What do RF pulses do in MRI? Why do we need radiofrequency pulses (B1) in MRI? (i.e. how does it affect magnetization?)
- need RF pulses to excite the magnetic field (Bo) that we created because we cannot detect magnetization along the z-axis (longitudinal magnetization)
- i.e. change LONGITUDINAL magnetization (which cannot be detected) to TRANSVERSE magnetization
What are the letters that symbolize static magnetic field, RF pulses, longitudinal magnetic field, transverse magnetization, gradient coil?
- static magnetic field = Bo
- RF pulses = B1
- longitudinal magnetic field = Mo or Mz
- transverse magnetization = Mxy
- gradient coil = G
Why do we need to convert Mz to Mxy in MRI with B1 rf pulses?
- Mz cannot be detected
- Mxy is measurable because it induces a current in an appropriately tuned RF coil
Why is the Larmor frequency important in MRI? What is another name for the Larmor frequency?
- it is the rate of spin of the proton magnetic moment around the magnetic field
- also called the precessional frquency
What are 2 types of brain RF coils?
- standard “birdcages”
- phased array
How is the MR signal that is detected by the RF coil converted into a reconstructed image of the brain?
the FOURIER TRANSFORM (FT) is able to pull apart individual frequencies from a complex signal to form a reconstructed image of the brain
What do gradient coils (G) create in MRI? What are they used for?
- creates inhomogeneous linearly-varying magnetic field that adds or subtracts a small amount from the main static magnetic field
- used for SPATIAL LOCALIZATION of the signal from the nuclear spins
TRUE or FALSE: magnetic field and precessional frequency vary as a LINEAR function of position
TRUE
What are the benefits of stronger gradients in MRI?
- faster acquisition
- can yield more signal and contrast potential
- more signal means higher spatial resolution
Describe the gradient-echo MRI pulse sequence.
- magnetize the sample (Bo)
- excitation of magnetization (B1)
- gradients - spatial localization of the signal (G)
- acquire the data - precession of magnetization (computer console)
Order, strength, and timing of RF and G pulses - all of these parts affect __________________________.
final image signal and contrast
TRUE or FALSE: different MRI pulse sequences are used for different body parts
TRUE
What are the main classes of MRI sequences? What is the difference?
GRADIENT-ECHO:
- faster, sensitive to magnetic field inhomogeneities
SPIN-ECHO:
- refocuses field inhomogeneities
- bread and butter sequence used for proton-density, T1-, and T2-weighting
Describe the spin-echo pulse sequence.
arrange different gradient pulses to create different images
What are the key acquisition parameters of MRI?
- slice thickness
- plane of acquisition
- number of slices
- field of view (FOV)
- matrix (in-plane resolution)
- repetition time (RT)
- echo time (TE)
- flip angle
- number of averages (signal-to-noise)
TRUE or FALSE: more time is needed to make brain slice images thicker in MRI
FALSE: more time needed to make then THINNER
Can stroke often be detected using the typical spatial resolution in MRI?
no, should use high spatial resolution
What does 3D MPRAGE stand for? What does it do?
- 3D Magnetization Prepared Rapid Acquisition with Gradient Echo
- excite entire 3D brain volume with first RF pulse + alter delay before acquisition to maximize gray/white contrast
What can contrast in MRI be particularly useful for?
detecting acute stroke
what are some water properties that can be imaged using MRI?
- proton density
- T1
- T2
- FLAIR
- diffusion
- perfusion
- flow
- blood oxygenation
What is one of the most important sources of tissue contrast in MRI?
differences in nuclear relaxation
Explain nuclear relaxation in terms of Mxy and Mz.
transverse magnetization (Mxy) DECAYS away and the longitudinal magnetization (Mz) RECOVERS over time
What IV contrast agent is used for MRI? How does it affect T1? Provide an example of what we would be able to see with Gd.
- Gadolinium (Gd) - DTPA
- shortens T1 (i.e. gets through leaky parts of BBB)
- e.g. Gd allow us to visualize metastases
What does FLAIR stand for? What does it do? What kind of image can you get with FLAIR?
- Fluid Attenuated Inversion Recovery
- uses difference in T1 between CSF and brain tissue to NULL SIGNAL FROM CSF
- get T 2-weighted image without confounding bright signal from CSF
What can DTI be used to image?
properties in brain connectivity (e.g. white matter tracts)
What does cerebral perfusion in MRI allow us to measure/image?
blood flow in the brain
TRUE or FALSE: in MRI, you can see increased blood flow in individuals with long covid
FALSE: DECREASED blood flow
What can MRA be used to image?
vasculature in the brain
What can fMRI be used to image? How does it work?
- image task-specific brain activation
- signal intensities change because of differing levels of oxygen
What does MR spectroscopy measure? Provide some examples.
- other molecules in tissue (i.e. not water)
- e.g. phosphorus spectroscopy to detect ATP
- e.g. carbon spectroscopy to measure glucose
TRUE or FALSE: MR spectroscopy can be used to measure temperature
TRUE
TRUE or FALSE: MRI can only be used to image the brain
FALSE: can also image spinal cord, and other body parts
What are some MRI safety issues?
- projectiles
- metal implants
- tissue heating (SAR)
- acoustic noise
- peripheral nerve stimulation
- nausea/dizzy (high magnetic fields)
How are MRI safety issues addressed?
pre-screening and secure scanner environment
