Benefits of ULF-MRI?
Ultra-low field (ulf-) MRI: Field strength ~Earth’s field
- Relatively cheaper
- No huge superconducting (Nb) coil
- Modern MRI systems’ installation cost typically more than hardware
- Relatively open imaging environment
- 10-15% of population cannot undergo stress of MRI
- Portable systems being developed
- Relaxed field homogeniety requirements
- High-field systems require 1 ppb (part per billion)
- Low-field can suffice with 1 ppt (part per thousand)
Ultimate goal for MR?
Ultimate(?) goal for MR: very narrow (1 Hz) linewidths
- Why 1 Hz? Inherent T1 for water ~ 1s “Natural linewidth limited”
- Improves spectroscopic resolution for NMR
- Improves imaging/spatial resolution for MRI
What limits linewidths? (MRI)
What limits linewidths? B-field (in)homogeniety!
- Inherent to coils – controllable(?)
- Environmental magnetic susceptibility variations (MR distortion)
- Sample/inherent magnetic susceptibility variations (MR contrast)
- Linewidth broadening due to inhomogeneity: ∆f = (γ/2π) ∆B = (γ/2π) (∆B/B) B ∝ B For given relative inhomogeneity ∆B/B (i.e. “shimming” fraction)
What’s a SQUID?
Superconducting QUantum Interference Device
a Superconducting ring, broken by Josephson junctions in which magnetic flux enclosed in the ring is QUantized giving rise to an Interference pattern with respect to flux that is linearized with a feedback circuit to make it a very sensitive Device!.
How sensitive is the SQUID?
Superconducting QUantum Interference Device
Ф0 = h / 2e = 2 x 10-15 T m2
Typical areas ~ cm2 - Typical amplifier noise ~ nV - Typical V-Phi transduction of mV per Ф0 - Typical SQUID system noise ~ µФ0
femtotesla sensitivities!
Strong pre-polarization field pulse for boosting the signal available for readout in ulf-MRI. Why isn’t the pre-polarized field left on throught the recording/reading?
The high tesla field can generate too much current and damage the squid.
MEG is used to image brain function via recordings of the magnetic field produced by?
Magnetoencephalography
MEG is used to image brain function via recordings of the magnetic field produced by post-synaptic currents in the dendrites.
Can MEG be used to identify the location of a brain tumor? Why or why not?
MEG measures electrical activity, a tumor doesn’t communicate with the rest of the brain. It is simply a growth and that is why SPECT/PET can image it through accumulation of radioactive sugar.
Which one has highest temporal resolution- MEG or fMRI?
MEG has much lower temporal resolution than fMRI. Therefore, a ten minute recording of both would gather more information with MEG than fMRI.
What does fMRI stand for?
functional Magnetic Resonance Imaging
What kind of signal(s) is/are used in fMRI?
Blood-Oxygen-Level-Dependent (BOLD) contrast imaging. BOLD effects of oxygenated or deoxygenated hemoglobin gives different signal due to its magnetic properties of Hb and HbO.
What can fMRI be used for?
fMRI can be used for:
- Behavioral experiments
- “Thought reading”
- Functionality mapping of the brain (oxygen level present)
