Parameters Flashcards
What is T1?
Spin-lattice decay constant
Rate of recovery of the longitudinal component of the magnetic moment. The spins of the protons interact with the lattice structure of the tissue, occurring when the Larmor frequency of the spins overlaps with the vibrational frequency of the lattice of the tissue. This is referred to as ‘spin-lattice’ interaction. These interactions release the energy absorbed from the RF pulse.
What is T2?
Spin-spin decay constant
Spin-spin interactions, the length of which dependent on the molecular structure. Mobile structures, with fast, rapid motion, have a long T2 period, such as CSF. Immobile, larger, and more highly bound structures have shorter T2 periods.
What is T2*?
Decay time resulting from intrinsic magnetic properties of the material. T2* is the decay time resulting from both intrinsic and extrinsic magnetic field variations. T2 is always longer than T2*
What is gyromagnetic ratio?
Ratio of magnetic moment to angular moment, represented by γ. This is unique for each nucleus
What is precession?
Precession is the ‘wobbling’ motion that a nucleus with a magnetic moment makes when placed in an external magnetic field. Like a spinning top, the nucleus wobbles around the direction of B_0. The frequency at which protons precess around B_0 is the Larmor frequency.
What is the relationship between T1, T2, and molecular size / motion / interaction?
Larger molecular size often results in longer T1 but will result in shorter T2 due to quicker dephasing. Similarly, free molecules have less efficient dephasing, causing longer T2. Free molecules will also result in a longer T1 due to slowed energy transfer.
These curves are different because of the dependencies of the different decays. T1 refers to the release of energy back to the lattice and depends on the rate of energy dissipation into the surrounding molecular structures. Both T1 and T2 relaxation depend on the main magnetic field strength as well as biological variation within patients.
(recall curve diagram)
What is susceptibility and what are the three types?
The extent to which material becomes magnetized when placed in a magnetic field.
- Diamagnetic: materials that have slightly negative susceptibility and oppose applied magnetic fields due to their paired electrons. Examples of this are calcium, water, and most other organic materials.
- Paramagnetic: materials that have slightly positive susceptibility and enhance local magnetic fields due to their unpaired electrons. Examples of this are O2 and deoxyhemoglobin.
- Ferromagnetic: materials that are superparamagnetic. Ferromagnetic materials have a uniform alignment and strong magnetization.
What is the purpose of a 90 degree RF pulse?
A 90 degree (RF) pulse is a burst of electromagnetic energy at the Larmor frequency that is applied perpendicular to the magnetic field (B_0). This serves to flip the net magnetization vector from alignment in the longitudinal (z) direction into the transverse (xy) plane. Once the magnetization is within the transverse plane, it can be measured as an MRI signal
What is an isospin?
Spin quantum number (I)
No magnetic moment if I=0 (even-even)
I = integer if proton and neutron both odd
I = non-integer if one odd and one even
What is FID?
Free Induction Decay (T2 relaxation)
