Applied in a direction that is perpendicular to alignment of protons.
causes extra protons to align in a different direction and tip out of alignment.
Spin Echo Pulse Sequence
Description
Magnetic Induction
Recorded b the MRI
Relaxation
Relax back to equilibrium (when gummy bear pressed is released, it takes longer for it to return back to original position but it takes a little longer) -relaxation rate is used to generate signal to look at contrast in different tissues
How you do an MRI
The tissue in the body feels a magnetic field
a short radio wave is transmitted
a signal is generated in the body
And an image is reconstructed
When a Magnet is Turned on
Protons in the body align, some with the magnetic field, and some against
The anti-parallel and parallel protons cancel out and only the few that align with the field are detectable.
Only a few align with the field, but when you have so many protons in your body the small number becomes significant
Protons Precess (Spin), and the rate is calculated by Larmor frequency and the strength of the magnetic field determines rate of spin
Magnetic Precession
Some protons align inside the field and the spins align themselves parallel or anti-parallel with the field.
Larmor Equation
The rate at which the protons are spinning (w) at depends on the gyromagnetic ration (g=42 megahertz/tesla) and external magnetic field strength (B).
w=g*B
Protons are spinning at the rate of radio waves! (RF - radiofrequency)
Note
Description
Laboratory vs Rotating Frame
LF - not spinning at the same frequency as the protons RF - spinning at the same frequency as the protons
RF gets turned off
Hydrogen protons return to original alignment - "go home" or relax - Longitudinal relaxation (Mz) in the direction of the magentic field -Transverse relaxation (Mxy) the xy plane where you get the signal from, so the signal is going to change over time
Magnetic Relaxation
d
Signal Envelope
Detect NMR signal from receiver coil using a radio receiver
How does a radio work?
Taking RF and converting it into an image that eyes can see
Making an Image
T1 varies according to tissue type and T2 is also tissue specific Relaxation curves determine speed of the protons going back
MRI Image Contrast
A and B are two different tissues, A has a shorter relaxatioon time than B, and then you get a difference