MRI
Which medical imaging does not involve electromagnetic spectrums?
Ultra sound
Define "electromagnetic spectrum."
The full range of electromagnetic radiation, organized by frequency or wavelength.
What is the most common nuclei examined in nuclear magnetic resonance imaging, and why?
Protons, because they are the most abundant in the body, therefore the easiest nuclei to work with because of the large concentration (but not the most sensitive nuclei).
What nuclei (other than protons) can be imaged using nuclear magnetic resonance?
Phosphorus (used for energy metabolism because it's in ATP), Carbon (can label carbons on metabolic compounds), Sodium (used for ion balance and movement of sodim in and out of the cell), Fluorine, and Potassium.
True or False? Each nuclei does not have it's own frequency?
False. Each nuclei has it's own frequency
Field strength is directly proportional to _____
Frequency (how fast the protons spin)
Bigger magnet in MRI = _____
Bigger Tesla
Explain "MRI is a bulk event."
MRIs average billions of protons in your body, not just a single proton.
What happens to a spinning proton when you put it in a magnetic field?
It will orient along that magnetic field.
Higher field strength = ____
More protons are spinning along the axis of that magnet. You get more signal the higher the magnetic field.
Explain and draw T1
T1 is the longitudinal relaxation time.
How long it takes the protons to reorient along Bo (magnetic field)
Explain and draw T2
Transverse relaxation time/ transverse magnetization being lost. Loss of signal in XY plain because of dephasing
Explain repetiton time (TR)
TR is the time between repetition of sequences.
Define TE (time to echo)
The moment the first echo is produced
How would you determine where a proton is in space? What is the equation?
Phase = (frequency)(time)
Compare how gradient echo and spin echo capture the echo.
Both are ways to change the distribution of protons along the XY plain, therefore realign protons and create an echo.
GE: uses gradients to create echo
When protons are in XY plain you use a gradient coil (which is along the main magnetic field) to switch its polarity so now everything is spinning backwards (spinning the opposite direction), the spin of protons align and it creates an echo.
SE: uses 180 degree RF to create echo.
Does T1 or T2 decay faster?
T2 decays faster. It decays as an exponential function
A long TR = ____ (more or less) signal. Why?
A long TR = more signal because the longer the TR, the more spins come back to the Z axis, so when you do the next pulse you have more signal.
A shorter TR = ___ (more or less) T1 sensitivity?
A shorter TR = more T1 sensitivity. Short TR = more signal, which maximizes contrast.
Define Larmor equation and write the equation
The frequency at which the protons spin (precess) is proportional to the strength of the magnetic field they experience.
w = y*B
w = frequency
y = gyromagnetic ratio (each proton has it's own)
B = strength of the magnetic field, in Tesla
What three factors (related to the coil) make the signal better?
1. More coils
2. Smaller coils
3. Close to the sample
What five factors are important to doing a good in vivo MRI study?
Filling factors: Coils, how close your coil is to the sample
Sequence parameters:
TR/TE: Setting up ranges of TR and TE in order to create either T1 or T2 sensitive sequences (T1 or T2 weighted sequences)
Motion: Need to eliminate motion of sample
Physiology: cerebral blood flow is sensitive to temperature and Co2
Explain "gradient coil."
Gradient coils create different magnetic frequencies in different locations so that spins in different locations precess at frequencies unique to their location, allowing for reconstruction of 2D or 3D images
Higher gradient strength = ____ (greater or lower) resolution
Higher gradient strength = greater resolution because you can measure smaller changes in space with higher gradient.
Higher gradient = increased delta frequency.
Tesla isn't as important.
What happens if you have a high gradient and a low Tesla, why?
This will just give you coise because there is not enough signal at each frequency to detect. You need higher magnetic fields to give you the signal to give you the resolution.
Explain K-space.
It is the data matrix for MRIs. Each line is a TR, it holds all of the TRs from the scan. At the end of the scan this data is turned into an image using Fourier Transformation.
Who discovered you can create an image using MR?
Paul Lauterbur
Who is responsible for the "birth or fast imaging, spin echo imaging"?
Peter Mansfield
Compare the images you get with spin echo vs gradient echo.
Spin echo: More sensitive to some tissue structures, such as grey and white matter.
Gradient echo: More sensitive to things that change the magnetic field. Better to use gradient echo for fMRIs
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