Hanson MRI Tutorial Pdf
<p>Apr 21, 2012 02:51 PM
 
see page 5  put a pocket compass near apowerful magnet...
needle aligns with the field;  it is usu embedded inliquid to damp its oscillations.
the needle oscillates as it aligns...  the stronger the bar magnet... the faster it oscillates!!
 
As the need oscillates it emets weak magnetic fields (radio waves) at the same freq as the needle.
 
page 10
 
B0 is the mag field imposed by the big 3T  field that is perpendicular to the patient.
 
The net magnetization is equivalent to only around <b>3 per million nuclear spins oriented</b>
<b>along the direction of the field (3 ppm at 1 tesla)</b>. 
This means that the magnetization of a million partially aligned hydrogen nuclei in the scanner 
equals a total magnetization of only<b> 3 completely aligned nuclei</b>.
With the gigantic number of hydrogen nuclei (about 1027) found in the body, the net magnetization
still becomes measurable. It is proportional to the field: A large field produces a high
<b>degree of alignment and thus a large magnetization and better signal-to-noise ratio</b>.
 
Precession at 42 Mhz.  Eventually it returns to equilibrium (relaxation) but it takes about 100 ms.
Meanwhile radio waves are emitted.
 
Precession freq is called the <b>Larmor freq</b>. ....
 
most important equation in MR  is   f= gamma * B0.  Larmor equation;
 
gamma =  42 MHz per Tesla for protons.  yromagnetic ratio
 
so, for protons in a 1.5 tesla scanner = 63 Mhz.
 
The powerful STATIONARY FIELD pushes the magnetization so that it precesses.
 
Likewise the (perpendicular)  radio waves (which change mag field direction over time)
push the magnetization around the radio wave field.
 
But the RF field is many thousand times weaker than the static field.
 
page 11:   figure:  look at NET MAGNETIZATION  (the THICK ARROW)
 
RESONANCE:  pushing a child on a swing in synchrony...
 
page 12:  analogy:  horses on a merry-go-round in action...
 
observed from the ground horses have complex motion...  but  if u stand next to them on
merry-go-round then they just move up and down...
 
change to rotating frame of ref and get rid of  Larmor-precession.
 
this make the effect of B0 appear to disappear...  (see diagram at top of page 13; also watch the
Hanson  Bloch simulation videos)
 
<b>5)  RELAXATION:  always reread this..</b>. its important...
 
page 14  TR = repetition time is the time btwn repetitions (repeating measurements of a line)
 
typical cycle is  excitation - wait - measure -wait - repeat  sequence.
 
5.2  causes of relaxation:
 
page 20  IMAGING:
 
radio waves used in MRI  are several meters long....  cannot tell whether pt is in or out of the scanner...
 
This principle was introduced by<b> Paul Lauterbur in 1973, and it resulted in the Nobel
Prize in Medicine in 2003. Basically, Lauterbur made the protons give their own locations away by
making the frequency of the emitted radio waves reflect the position. Lauterbur shared the prize
with Sir Peter Mansfield</b>, who also contributed greatly to the development of techniques used in
magnetic resonance imaging (MRI).
 
<b>9.2  Gradient Coils.</b>.
 
make the field inhomogeneous  say from L ear to R ear...  so resonance freq varies in the direction of the field gradient.
 
see fig 12... sagittal slice.
 
page 22
 
9.4  
 
 
 
 
 </p>