Magnetic Resonance Imaging Flashcards
What EMR is used in MRI?
Radiowaves (and a Magnetic field is used)
T1 Hypo intense is what colour
dark
T2 Hyper bright is what colour
White
Ferro-magnetic
Strongest force of magnet
Stays magnet
SAR
Specific adsorption rate
Heats up patient
Homogeneity
All the same
All the magnets are in rows, that’s why it’s 50cm
Radio frequency
We want coils close cause inverse square law
Gradient coil
Pairs of the coils
On gradient axes to the patient
Paramagnetic
Slightly magnetic
Gadolinium
Didgeridoo
Vibration MRI
Same frequency of sound
Super-conducting magnet
No resistance
Using liquid helium low temp
So it’s superconducting as there is no resistance
Pixel
A picture element
Pixels are bigger in MRI compared to CT
Poor signal to noise frequency
Matrix
Made up of pixels
You want a large matrix small pixel
But MRI has large pixel and small matrix
Larmour Frequency
Larmour frequency equation
Frequency is equal to magnetic field strength
Voxel
Volume element to give pixel
M0
Bodies magnetic field
Influence by the MRI to line up
1 Tesla is what gauss
10000
Missile effect
If you bring metal into MRI room it will launch towards MRI machine
Deflection
Twist to follow the MRI scanner
MR safe
Safe for MRI
MR conditional
You might be able to scan
MR unsafe
Not safe for MRI
In medical emergency
Take the patient out the room as the cardiac team may have metal equipment
Usually MRI have their own trolley for this reason
5 guass
5 guass line is how far the metal field strength is
Always in the MRI room
Implant effects the scan how
They are safe
But it makes area dark so you can’t see there
But if you don’t know it’s there smear the artefact across the image
Uncontrolled quench
Controlled quench
Is to let the helium gas out safely, warm it up slowly
If it was to leak into the room it would cause dissipation of oxygen so you couldn’t breathe
Could explode
How to deal with claustrophobia
Patients with bad claustrophobia will not be able to have MRI
Cognitive methods
reassurance
eye masks
mirrors on the head coil so the patient can see you at the scanner
mild oral medication such as low dose diazeplam
IV sedation is not safe for MRI because it can be magnetic the equipment
Radio frequency burns
radio frequency waves raise the patient’s temp
different sequences in MRI have different SAR levels
changes can be made to the MRI protocol to limit the heating but it takes longer
radio frequency burns can be caused by skin-to-skin contact that can form a loop which leads to heating
ECG and overweight patients are also a risk
Sponge pads may be used to prevent skin to skin contact
What is the most common element and why is it used for MRI
Hydrogen is chosen because:-
- It is very abundant in the body (approx
10% of body mass is Hydrogen) - It gives the strongest MRI signal
Hydrogen has a single spinning proton (+ve) • A moving (spinning) charge creates a magnetic field • This results in the nucleus acting like a small bar magnet
MRI involves 3 physical principles:-
- Nuclear - the nuclei of certain atoms
behave like small bar magnets - Magnetic – when placed in a strong
magnetic field, these small bar
magnets align with the field and
precess - Resonance - if a radio frequency pulse
is applied to these nuclei, some
resonate and absorb energy and re-
emit the energy as an MRI signal
Protons from M0 change to what
B0
some hydrogens may spin in the opposite direction
M0 perpendicular to B0 to measure it
Advantages of MRI
No rad
Soft tissue contrast resolution
safer contrast, fewer contraindications
Multimodal- functional, anatomical angiographic in one exam
Disadvantages of MRI
longer time Claustrophobic metal devices, magnets Noisy Expensive to buy, and less staff, equipment for the MRI
Pregnancy is a contraindication in what trimester
the first trimester
Proton density
T1
T2
How long are the tr and te?
long tr short te its proton density
short tr and short te its T1
Long tr and long te its T2
T1, what colour is water
Water is dark
T2, what colour is water
Light
TR means what
Time to repeat
TE means what?
Time to echo
Precessional frequency means what
How fast the hydrogen spins around B0
how fast it wobbles
RF pulse has to be the same frequency in order to flip the hydrogen from longitude to transverse plane (to take MRI picture)
What is measured?
During excitation using the RF all the atoms are put on the same spin path at the same precession
We measure the point of the rotation as it causes a net magnetic field in the direction they are all pointing
Same transverse magnetism
How do the spins depend on the tissues?
After the RF is stopped the atoms stay to go back to normal and stop spinning again
Different tissues return back at different rates
Spins recover at different rates depending on the type of tissue
T1 recovery curve
Recovery of longitudinal magnetization
Rate of different tissues to go back to normal spin
T2 relaxation curve
Transverse magnetization
Decay process
moving out of phase with each other once the RF is switched off
Receiver coils do what?
Have to be close to the part measuring
collects the signal whilst recovery/relaxation takes place
FLAIR meaning
Fluid Attenuated Inversion Recovery - Protons in tissue water
What different coils are used in MRI?
Y coil
Z coil
X coil
Transceiver coil
What does the information that is given on the screen with an MRI image represent?
What T1 or T2 it is
Patient prep
Remove all the metal objects from the patient
Keys, coins, mobile phone, hair-grips, watch
Credit cards and travel cards will be wiped if taken into the room
Clothing with metal attached? Artificial limbs or callipers
What contra-indications there are to MRI?
Pregnant Metal fragments in body e.g gunshot Any metal implants Cardiac pacemaker Claustrophobia (any contradictions to contrast)
Explain how an MRI image is created.
Magnetic resonance imaging (MRI) uses the movement of protons within a magnetic field to generate an image.
The coils send different RFs to the tissues and depending on the coil will affect which way the atoms will face and spin.
Then you wait some time as the tissues start to go back to their original spin.
Then the Transceiver coil will measure the magnetic field given off the tissues (the tissues will give off different MF depending on which ones it is, some tissues go back to normal faster than others)
What must you do if someone has a neurotransmitter device?
Check that its MRI compatible/safe
Will have to use a special low frequency to ensure that the MRI doesn’t affect the neurotransmitter device.
Recognize the different sequences (T1 and T2) that may be used.
You can tell by what the colour of the water is
or the graphs in the PowerPoint.
