Lecture 7 - MRI Basics Flashcards
What quality of hydrogen atoms is used during MRI imaging?
Hydrogen atoms precess or spin around the axis that exists between their positive and negative poles.
What happens to hydrogen atom “spins” when placed in a strong, static magnetic field?
Hydrogen atoms normally spin or precess at random, in random directions. When placed in a strong, static magnetic field hydrogen atoms precess in parallel or anti-parallel to the direction of the magnetic field (the magnetic field generated in an MRI machine is known as Bo).
Although MRI machines are non-invasive imaging machines that have view health risks, what are some safety concerns to keep in mind?
The strong electromagnetic field of MRI machines may cause nausea. Furthermore, the magnetic field is very strong and therefore no magnetic metal can in the room when the MRI is on.
What does MRI and fMRI stand for?
Magnetic Resonance Imaging and Functional Magnetic Resonance Imaging
Define Larmor Frequency
The Larmor frequency is the frequency at which protons/hydrogen ions precess and is dependent on the strength of the magnetic field they are in.
During MRI imaging how are protons manipulated to precess in the same phase?
A radio frequency pulse is applied perpendicular to the Bo/static magnetic field at the Larmor Frequency.
What two effects does the Radio Frequency pulse have on the precessing of ions?
Applying a Radio Frequency pulse at the Larmor Frequency perpendicular to the magnetic field generated by the MRI machine makes the hydrogen ions/protons precess at the same frequency and also tilts the magnetisation vector away from the magnetisation vector generated by the MRI magnet.
What are the two effects off turning of Radio Frequency pulse and how do they help construct an image?
When Radio Frequency pulse is switched off two signals can be obtained from the effect this has. First, the hydrogen ions re-align with Bo/the magnetic vector created by the MRI magnet. The time it takes for this is called T1 recovery or Longitudinal Relaxation. Secondly, the magnetic vector generated the RF pulse, which is tilted away from Bo, decays. The time it takes for this to happen is called transversal decay or T2 decay.
What are the main types of tissue being looked at in MRIs?
White matter (high in fat), grey matter, and Cerebral Spinal Fluid (high in liquid/water).
In regards to MRI imaging what do T1 recovery and T2 decay refer to?
Why does Magnetic Resonance Imaging have low temporal resolution?
MRI images are constructed by sequence of changes to magnetic field strengths and applied Radio Frequency pulses at specific times for single slices of the brain. To generate a whole brain image many slices need to be measured, which in turn takes a lot of time. So, whilst MRI has good spatial resolution it has low temporal resolution.
With respect to superconductors used for MRI scanning, what does a stronger static magnetic field allow for?
Higher spatial resolution.
What is the Lamor frequency and how is this affected by the magnetic field?
The lamor frequency is the frquency at which hydrogen ions precess. The frequency at which hydrogen ions precess is altered by the strength of the magnetic field.
When in the magnetic field generated by the MRI machine what happens to the hydrogen ions?
The hydrogen ions align with Bo (parallel or anti-parallel) and they also precess at the same frequency.
However, their precessing is out of phase.
How are hydrogen ions manipulated to precess in phase?
A radio frequency of the same frequency as the lamor frequency is applied perpendicular to Bo.
This causes the hydrogen ions not only to precess in phase, but to also align their magnetic fields with the transversal field.
