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Jay's COMPLETE MRI Mess

MRI physics and safety made easy.
by

Jay Alexander

on 28 October 2013

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Transcript of Jay's COMPLETE MRI Mess

Jay's
MRI
Mess
WELCOME...
to...
Steve (70% water)
This guy is cool...
Cause he invented this...
This "thing" helped to prove his law,
FARADAY'S LAW
. This states that a MOVING MAGNETIC FIELD will produce a POSITIVE ELECTRIC CHARGE.
As well, a MOVING ELECTRIC CHARGE will produce a MAGNETIC FIELD.
Check it out !!!
Current
Magnetric
Field
Current
(magnet)
(Wire)
(Happy guy)
is kind of like...
which means...
which means
Steve is a MAGNET.
So this proton
(with its positive charge)
A spinning top, that "wobbles" or
PRECESSES
.
The moving electric charge produces a MAGNETIC FIELD!!!
BUT... He is a "RANDOM" Magnet.
All of his protons, or
"Spins"
as we call them, are producing
magnetic fields randomly in all directions.
So SUPPOSE we throw Steve into
A BIG POWERFUL MAGNET...
The spins now line up with the strong "MAIN"
Magnetic field !!!
AND...they start to spin at the same speed.
The speed depends on the strength of the field.
The stronger the field, the faster they spin !!!
Remember this?
At our magnet's field strength (1.5 TESLA)
The protons spin at approximately 63 mhz.
Side note
HERTZ
A is one CYCLE per second.
MEGAHERTZ
A is one MILLION cycles per second.
(Those protons are spinning seriously fast !!!)
So now we know...
When all of these spins are aligned to the main magnetic
field, their "little fields" combine to form...
A NET MAGNETIC VECTOR.
(a "combined force" so to speak)
Steve is not a RANDOM MAGNET anymore!!!
When we place someone into a powerful magnetic field,
their hydrogen protons align to that field and spin
according to it's strength. This creates a
NET VECTOR.
MAGNETIC
We now have the in
"M"
MRI...
Brainiac
Really old MRI
So then, some brainiacs figured out how to
manipulate those MAGNETIC VECTORS in order
to do
GREAT THINGS
!!!
And to do this... They used
RADIO FREQUENCY.
So now we need to "TALK RADIO"...
Radios work with
RADIO WAVES
. These waves all travel at the same
speed (the speed of light), but have different
WAVELENGTHS
.
13 Hz
6 Hz
1 second
You can measure how many waves pass through or
"FREQUENT"
a point in a given time. This is the
FREQUENCY
of the wave. (Remember the HERTZ?)
105300000 Hz (105.3 Mhz)
YAY !!!
Radio stations "push out" a certain frequency group of radio waves (known as bandwidth). You then tell your radio to "listen" or TUNE to that frequency group, and ignore all of the others. The radio then takes the information carried within that bandwidth and changes it to audible output. This is FREQUENCY MODULATION... or FM.
In NORTH AMERICA, radio stations use Frequency between
87 and 109 Mhz.
Don't worry... I AM going somewhere with this...
RESONANCE TIME
side track #1
side track #2
RESONANCE
is the transfer of energy through LIKE FREQUENCIES.
This means if two or more things share a common "natural frequency", energy
in different forms can pass from one to the other.
One example of this is the singer breaking the wine glass. If she hits
the "perfect frequency" that matches the "natural frequency" of the
glass, it breaks.
Musical tuning forks are another example. If you strike a fork
tuned in the key of "C" and place another "C" tuned fork next
to it, the second fork will begin to hum along with the first.
RESONANCE is the "R" in MRI !!!
OK... BACK TO STEVE...
So we have created a
NET MAGNETIC VECTOR
inside of Steve's
body, We can now use this new vector to do great things. BUT... How will we measure this new magnetism when its drowning within the
HUGE MAIN MAGNETIC FIELD
??? We need to make Steve's magnetic field different...
Let's use THOSE RADIO WAVES !!!
Remember when I told you that protons spin at approximately 63 Mhz within our magnetic field?
63 Mhz
What would happen if we hit
Steve with some radio waves
in a frequency of 63 Mhz???
Through the
MAGIC OF RESONANCE
Steve's magnetic field will become
ENERGIZED
and "TIP" away from the main field !!!
Main field
Steve's field
90°
When the radio frequency is turned off, it returns back to where it started. We turn the radio frequency on just long enough to
tip Steve's field 90 degrees. We call this a
90° PULSE.
Now we can place a loop or near this newly tipped magnetization. Just like FARADAY told us, when the magnetic field passes the loop, it will generate an
ELECTRIC SIGNAL
!!!
COIL
Now as we all know...
Steve isn't made up of JUST WATER. He has protons in him that
make up a lot of things. The most predominant however are WATER
and FAT.
The
MAGNETIC VECTORS
generated from water
and fat behave differently in the magnet.
AND...
So now we know...
We can make a
MAGNETIC VECTOR
by placing protons in a strong magnetic field.
With the right
RADIO FREQUENCY
and the
MAGIC OF RESONANCE
, we can "tip" that
vector away from the main field.
Just as
FARADAY'S LAW
states, this new
magnetic vector will induce a current in a COIL that can be measured.
When we apply our 90° PULSE, both vectors will tip 90 degrees
just the same...
main field
The difference is the
RATE OF RELAXATION
, or how fast each
vector will RETURN TO THE MAIN FIELD after the pulse
is turned off.
main field
FAT relaxes quickly (about 500 milliseconds)
WATER relaxes slower (at least 2000 milliseconds)
1/2 a second
2-15 seconds
If we apply another 90° pulse, FAT will have more protons ready to be tipped again!!!
main field
90°
So if we wait 500 milliseconds after the first 90° pulse, and apply another
pulse, the VECTOR FROM FAT will be larger.
90°
If we now turn on our
COIL
, the current or SIGNAL induced in the coil will be
stronger from FAT than WATER.
The resulting image will show fluid as
dark compared to fat. This is called a weighted image.
T1
T1 weighted images are great
for showing a patient's
anatomy
in general.
If we wait longer than 2000 milliseconds, the signal from fat will be fully
"RELAXED"
. Now instead of tipping the vectors again, we can just go ahead and use the signal still present from water.
Now we will have a weighted image. Fluid will present
as bright in these images.
T2
T2 weighted images are great for showing
any
pathology
that may be present.
We can manipulate the time we wait as well as how far we tip our vectors. This gives us a multitude of useful images.
THAT'S ALL FINE AND DANDY BUT...
With all of those protons producing signals
from
EVERYWHERE
in the body, how does the MRI
sort them out into an image????
We use GRADIENTS !!!!
If we add more power to one side of the magnet, the field at that side will increase. We call this a
gradient magnetic field
. The protons within the stronger portion of the field will spin faster, or with a higher FREQUENCY. The MRI can now sort the protons by their frequency along the gradient field.
Now we can apply ANOTHER GRADIENT in a quick burst. This will speed some protons up, knocking them out of sync or
PHASE
with each other. The protons spin at the same speed, but some are ahead, and some are behind the others. The MRI can now sort the protons by their phase.
When this is finished, we get a nice grid of protons, sorted or
ENCODED
by frequency and phase. This grid is what eventually becomes a
SLICE
or image.
There are 3 sets of coils within the magnet that produce gradient fields. One for phase encoding, one for frequency encoding, and another for separating the grid into individual slices. This is the
slice selection gradient
. These gradients can be fully manipulated. This allows us to scan in
multiple planes
and with various slice thickness.
In order to perform all of these
functions in a short time, these
gradients get switched on and off
very rapidly during scanning...
This causes the magnet to rattle and
knock
VERY LOUDLY !!!
I can now CONGRATULATE you on becoming a complete
like these guys.
SUPER GENIUS
THANK YOU FOR HELPING SORT OUT...
GENIUS
NOT Madonna
(cause I KNEW you were going to ask me about that...)
And... because you managed to sit through all of this physics stuff...
(unless...you have any questions...)
I can upgrade you all to
Status....

BUT... we still have this


MAGNET to deal with....

ALMOST
SUPER GENIUS
(Like me...)
CRAZY,
TERRIFYING,
WHEELCHAIR
TWISTING
ELECTRO MAGNETS MADE
SUPER EASY...
Take some wire, wrap it around an iron
core, add current...
and
FARADAY'S LAW
STRIKES AGAIN!!!
Most MRI's contain
SUPERCONDUCTING

MAGNETS
SUPERCONDUCTIVITY
OCCURS WHEN CERTAIN
METALS ARE COOLED TO NEAR ABSOLUTE ZERO.
(SERIOUSLY COLD)
AT THESE TEMPERATURES THEY HAVE NO ELECTRICAL
RESISTANCE. ELECTRONS CAN FLOW FREELY THROUGH
WITHOUT EVER BEING STOPPED.
THESE COLD METALS CAN CARRY HUGE CURRENTS
FOR LONG PERIODS OF TIME WITHOUT LOSING ENERGY!!
Our magnet uses the same idea as the simple electromagnet, but uses 1000's of loops in the wire. It is then supercooled with liquid helium. A massive amount of current is then applied, which travels through the loops virtually FOREVER.
It's like golfing in space !!!
With no resistance, the golf ball will travel forever !!!
Many loops of wire coupled with
massive current in a helium bath
gives us
A VERY STRONG MAGNET !!!
Our magnet is rated at 1.5 TESLA.

1 TESLA is 10,000 GAUS.

A fridge magnet is around 5 GAUS.
So remembering that current is running through
this magnet indefinitely, we now know that the
magnet is
ALWAYS ON...
Or in other words... THE DANGER NEVER GOES AWAY !!!
SUPERCONDUCTION
Made super easy...
Chairs...
Walkers (flyers)...
Wheelchairs...
IVACs...
Supply Carts...
Floor Polishers...
Beds...
Techs...
Oxygen Bottles...
Some metals are "magnet friendly".
They are known as NON FERROUS
metals.
Most devices that are implanted in
the body are made from high grade
metals which are non ferrous.
When these metals are placed
in the magnet, they cause a
disturbance in the magnetic field.
This in turn causes artifacts in
the resulting images.
The pix look bad!!!
So how do we keep people
safe in this thing ???
WE USE A SCREENING
FORM !!!
Everyone must fill out a MRI SCREENING
FORM prior to entering the scan room.
This form ensures the person has no
contraindications to being in a high magnetic
field. These include:
Pacemakers
Aneurysm clips
Metal in the eyes
Pregnancy
Recent surgeries
The form also helps us in finding out if a patient is claustrophobic so we can
aide them in getting their exam done.
Alright.... We have gone over how this thing works
And... how not to have stuff like this happen...
What is MRI Anyhow???
MRI
is a "SOFT TISSUE" imaging modality...
X-RAY utilizes radiation to
acquire images of bone and
certain soft tissues.
MRI utilizes tiny magnetic
fields produced by hydrogen
atoms within water molecules.
NO RADIATION !!!
The human body contains countless hydrogen protons
This allows MRI to image
a wide variety of body tissues...
MRI is extremely useful
in imaging structures
such as...
The Brain
The Spine
Various Joints
But how does this
thing manage to get these images without RADIATION ???
Let's just start by saying...
So... What happens when these things make it into the magnet room ???
Full transcript