MRI physics Flashcards
What are advantages and disadvantages of Superconductive magnets?
Advantages:
- High field strength and homogeneity
- Low power consumption
- High SNR
- Fast scanning
Disadvantages:
- High capital cost and cryogen cost
- Acoustic noise
- Motion artifacts
- Techical complexity
What is diamagnetic susceptibility and what are examples?
- Slightly negative susceptibility and opposes the applied magnetic feild
- Materials:
- calcium
- water
- most organic materials (C,O)
What is paramagnetic suscptibility and what are examples?
- Slightly positive susceptibility and enhances local magnetic field, but no measurable self-magnetism
- Examples:
- Molecular O2
- some blood degredation products
- Gadolinium
What is ferromagnetic susceptibility and what are examples?
- Supramagnetic augmenting the external magnetc field and can exhibit self-magnetism
- Examples:
- iron
- cobalt
- nickel
What is the gyromagnetic ratio for H?
H = 42.58 MHz/T
(1.5T = 63.87 MHz)
What is the Larmor equation?
At equilibrium, what do Mz and Mxy equal?
Mz = M0
Mxy = 0
What is the difference between T2* and T2 decay/relaxation?
- T2* depends on both intrinsic (spin-spin) and extrinsic (field inhomogeneities) relaxation factors
- T2 depends only on intrinsic spin-spin interactions
What is the T2 decay constant?
Time after the 90-degree RF pulse (time 0) over which the signal decays to 37% of the maximal transverse magnetization
What causes T2 shortening?
- Slow molecular tumbling rates
- Rigidly bound molecules (tumbling slower than the Larmor frequency)
- Large macromolecules
Arrange from long to short the T2 relaxation times of:
Grey matter, CSF, white matter
CSF (long) > grey matter > white matter (hypo)
How does magnetic field strength affect T2 relaxation?
Magnetic field strength has no effect on T2 relaxation (unlike T1)
What causes T1 shortening (hyperintensity)?
- intermediat molecular tumbling (close to Larmor), size and protein binding
- fat stores in adipose and marrow tightly bound and close to Larmor
- protein binding in fluids (mucin) close to Larmor)
Arrange tthe following from short to long T1 relaxation times:
CSF, fat, grey matter, white matter
fat (hyper) < white matter < grey matter < CSF (hypointense) Mmm
How does B0 impact T1 relaxation?
Larger B0 can cause increased T1 relaxation times - due to less overlap of lattice with precessional frequencies
What is the T1 relaxation constant?
Time needed to recover 63% of the longitudinal magnetization (Mz) (after the 90 degree RF pulse)
What are the typical TR and TE for T1?
TR = 400-600, TE = 2-20
What molecules can cause T1 shortening?
Gd-DTPA and methemoglobin
T2 shortening?
Gd-DTPA, deoxyhemoglobin and intracellular methemoglobin
What is the difference in resonance frequencies of the protons in water and fat?
224 Hz (3.5 ppm)
WHat are the TR and TE in PD weighting?
TR = 2000-4000, TE = < 40
What is the typical TI for STIR?
TI = 140-180 ms, TR = 2500
WHat is the typical TI and TR of FLAIR?
TI = 2400, TR = 7000
How can you make a GRE sequence more T1W? more T2W or PD?
- T1W GRE: Short TE, large FA (70-90)
- PD GRE: Short TE, medium FA (40-50)
- T2W GRE: Longer TE, small FA (5-30)
What determines slice thickness?
the RF bandwidth (range of frequencies used, Hz) and the SEG field strength (slope, Hz/cm)
What determines the thickness of a voxel?
slice encoding gradient strength and RF frequency bandwidth
What information is provided by the center and periphery of k-space?
- Center: signal and contrast - contains low frequencies
- Periphery: spatial resolution - high frequencies
How do you calculate FOV?
FOV = sample bandwidth ÷ gradient strength
What is the trade off of having a smaller/narrower bandwidth?
Increased SNR at the expense of increased TE with more T2 decay and increased chemical shift artifact
What is the trade off with increasing NEX?
- Increased SNR
- reduces artifacts due to signal averaging
- increases scan time (in order to double SNR, NEX = 4, quadruples scan time)
In regard to the ACQUISITION matrrix, what occurs when you increase it?
decreases voxel size, thus:
- reduces SNR
- increases spatial resolution
- increases scan time in MPEG is increased (more k-space to fill)
What three parameters define the spatial resolution of an MR image?
- Dimension of the FOV
- Slice thickness
- Size on the image matrix
What is one way to increase resolution?
- Increase the matrix size, this will decrease the pixel size
What is the relation ship between k-space line spacing and FOV?
- ∆k inversely related to FOV
- E.g. wider ∆k, smaller FOV
In basic terms, how does phase-wrap artifact (or aliasing) occur?
- An object containing protons outside the prescribed FOV but within the bore will e subject to the same PE gradient
- This Fourier transformation erroneously calculates the signal as arising from within the prescribed FOV, wrapped around to the opposite side of the image
What is the relationship between FOVFE and the GFE and receiver bandwidth
- If GFE increases, then FOVFE decreases
- Narrowing rBw decreases FOVFE
In basic terms, how does frequency wrap -around, or aliasing occur?
- Sampling frequency determines the maximum frequency (fFEmax) in the echo that can be sampled accurately (similar to Doppler US and PRF)
- Max freq we wish to record is determined by total rBW, so equal to rBW/2
- If higher frequencies in the signal from outside the rBW/FOVFE range, will be sample insufficiently and wrap around.
How does TR relate to T1 weighting?
- TR determines how much longitudinal magnetization difference there is between tissues and emphasizes these differences when TR is short
- Short TR = good T1 contrast
What is the relationship of TE and T2?
- TE determines how much transverse magnetization was allowed to decay prior to the signal read out
- Shorter the TE, the less difference in residual transverse magnetization between tissues
- Long TE maximizes differences in tissues
- Long TE = good T2 contrast
What combination on TR and TE are used for proton density weighted images and why?
- Long TR and short TE
- Minimizes both T1 and T2
What is the relationship between SNR and spatial resolution?
inversely related
What controllable factors influence SNR?
- VOXEL size
- Larger voxels generate more signal
- Receiver bandwidth
- SNR inversely related to the square root of rBW
- Number of excitations (NEX)
- When NEX doubled, SNR increased by a factor of root 2
- Number of PE steps (# of pixels in the PE direction [NPE])
- SNR is a function of the square root of NPE
- increase in NPE causes some increase in SNR
- However, if NPE is doubled there is a net decrease in SNR (0.7 factor)
- SNR divided by two since pixels two times smaller in PE direction
- SNR is a function of the square root of NPE
- Longer TR = more signal
- Longer TE = less signal
- Flip angle <90, lower SNR (lower amplitude of transverse magnetization)
- Type of coil
- Decreasing acquisition time decreases SNR
In regard to T1 weighted imaging, how does gadolinium affect the nearby protons?
- The strong paramagnetic effect of gadolinium shortens the T1 relaxation time of these protons, thereby causing a dramatic increase in signal on T1W images
What is the major benefit of FSE/TSE and roughly how is this accomplished?
- Faster scan times
- More 180 RF pulses within a TR, fills k-space faster
Why does fat tend to be brighter on FSE/TSE in comparison to standard SE?
- The multiple 180 RF pulses used reduce the spin-spin interactions in fat (j-coupling), thereby strengthening its T2
How is the SS-FSE accomplished in regard to k-space?
- Only a little over half of the lines of k-space are filled within 1 TR after a single 90 RF
Why is STIR more efficient than fat sat techniques for nulling fat?
- STIR is not sensitive to magnetic field inhomogeneities
Why can’t STIR sequences be used with gadolinium?
- The T1 of enhancing tissue is shortened and closer to that of fat, leading to suppression of signal from enhancing tissues on STIR images
Motion artifacts occur in what direction?
phase encoding
Why does entry slice phenomenon occur?
blood in the center of the stack has experienced more excitations than blood at the entry slice
What causes the flow artifact?
Fast flowing blood will not have experienced the 180 degree rephasing pulse, thus will not have signal in spine echo sequences
How can you correct for TOF and entry slice phenomenon?
saturation band adjacent to the FOV or in the slice direction
How can you correct flow related intravoxel dephasing?
flow compensation, aka gradient moment rephasing
In what direction does the zipper artifact occur?
Frequency encoding
What causes the spike/herring bone artifact?
- loose electrical connection or build up os static electricity
- occurs due to production of spike noise, resulting in a bad data point in the raw data (k-space)
How can cross-excitation be prevented?
using an interslice gap of at least one-third of the slice thickness
In what direction does wrap around artifact occur?
phase encoding, hence the term “phase wrap”
How can you correct truncation/Gibb’s artifact?
due to undersampling →increase number of phase encoding steps (increase matrix and voxel size)
In what direction does chemical shift artifact occur?
frequency encoding direction
How can you minimize chemical shoft artifacts?
increase reciever bandwidth or use fat suppression techniques
On what type of sequence do phase cancellation/black boundry artifacts occur?
gradient echo sequences - when fat and water are exactly 180 degress out of phase
