MRI Flashcards
properties of hydrogen nuclei, longitudinal magnetization, precssional frequency, resonant frequency, Larmor frequency, radiofrequency pulses
What properties do hydrogen nuclei have that make them ideal for MRI imaging?
Atomic nuclei with odd mass numbers have useful magnetic properties. Hydrogen nuclei
What happens to hydrogen nuclei when they are placed in a magnetic field ?
The magnetic moments of hydrogen nuclei are normally randomly aligned but when put in a strong magnet they align by spinning up or down. High energy nuclei will spin down against the magnetic field direction and the low energy nuclei will down with the field.
During an MRI scan what causes the net magnetization of the patient to be parallel to the main magnetic field direction?
The majority of hydrogen nuclei are in a low energy state and during a MRI scan they spin up making the net magnetization of the patient parallel to the main magnetic field direction
What is longitudinal magnetization?
when the overall magnetisation of the patient is in the longitudinal direction along the Z axis or the main magnetic field direction.
What happens to the precessional frequency of our hydrogen nuclei as we increase the magnetic field strength ?
As the magnetic field frequency increases the precessional frequency of the hydrogen nuclei also increases.
What is Larmor frequency?
how the magnetic moments of hydrogen nuclei are precessing at a rate directly proportional to the magnetic field
What is precessional frequency
when the magnetic field exerts a torque on MR active nuclei causing a secondary spin called precession which makes a circular path around the magnetic field strength
What is the Larmor equation and what does it describe?
Precssional frequency = Constant x magnetic field strength.
The precssional frequency of the magnetic moments is directly proportional to the external magnetic field strength.
What is resonant frequency?
resonance is an energy transition that occurs when an object is subjected to a frequency the same as its own. It is induced by applying a radiofrequency pulse at the same frequency as the precessing magnetic moments. The resonant frequency of hydrogen nuclei depends on the field of strength of the magnet
What happens when we apply a radiofrequency at the resonant frequency of our precessing hydrogen nuclei?
Excitation which is when the hydrogen nuclei absorb this energy flipping it into the higher energy state causing it to flip down against the magnetic field direction .
What happens when the radiofrequency pulse flips the overall magnetisation.
The overall magnetization of the patient is flipped into the transverse plane towards the receiver coil because there are equal moments of nuclei spinning in low and high energy states.
