Image Production Flashcards
What is Heisenberg’s Uncertainty principle?
we cannot know the exact direction of a particle’s spin at any point in time
What two factors determine the Larmor frequency?
Gyromagnetic ratio and magnetic field strength (B0)
Why is protium used in MRI?
protium is an isotope of hydrogen. Hydrogen is very abundant in the human body, and because its solitary proton gives it a relatively large magnetic moment.
What is the gyromagnetic ratio of hydrogen?
42.58 Mhz/T
What are the results of resonance?
all magnetic moments move to the same position, with the same precessional path around B0.
What is gradient rise time, and what is it measured in?
The time it takes for the gradient to reach its full amplitude. Measure in microseconds or milliseconds
What is slew rate and how is it measured?
Acceleration of the gradient magnetic field to its maximum amplitude. Measure in in Tesla/meter/second or millitesla/meter/millisecond
What is duty cycle?
The time the gradients are on during one TR period
How is SAR measured?
Watts per kilogram
What is the primary biological effects of radiofrequency fields?
Tissue heating and thermal heating
FDA limits for main magnetic field
4 Tesla for entire population
8 Tesla for those over 1 month old
What temperature are the cryogens cooled to?
4 kelvin
define ferromagnetic
iron and iron-like substances that can generate a relatively strong magnetic field
define paramagnetic
Have a very weak magnetic field.
define diamagnetic
Have no magnetic field.
T1 relaxation time
the recovery of 63% longitudinal magnetization of a specific tissue’s original magnetization.
T2 relaxation time
the time it takes for 63% of the transverse magnetization to decay, or the time it takes the spins to de-phase to 37% of their original value
Spin-spin relaxation
The return of the transverse magnetization to its
equilibrium value, zero (T2 decay)
spin-lattice relaxation
The return of the longitudinal magnetization to its
equilibrium value along the Z axis (T1 recovery)
Fat has a ____ t1 relaxation time and a ____ t2 relaxation time
Short & short
Water has a ____ t1 relaxation time and a _____ t2 relaxation time
Long & long
Hydrogen in fat recovers _____ along the longitudinal axis than water and loses transvers magnetization _____ than water
Faster & faster
T1 weighted imaging uses a ____ TR and a _____ TE
Short & short
T2 weighted imaging uses a _____ TR and a _____ TE
Long & long
Proton Density imaging uses a ____ TR and a _____ TE
Long TR, Short TE
Increasing the TR will \_\_\_\_\_ SNR \_\_\_\_\_ scan time \_\_\_\_\_ t1 weighting \_\_\_\_\_ available number of slices
Increase SNR
Increase scan time
Reduce t1 weighting
Increase slice #
Decreasing the TR will: \_\_\_\_\_ SNR \_\_\_\_\_ scan time \_\_\_\_\_ t1 weighting \_\_\_\_\_ the available # of slices
Lower SNR
Reduce scan time
Increase t1 weighting
Reduce the # of slices
TR range for a t1 weighted image
350-700 ms
TR range for a PD weighted image
1500-3000 ms
Tr range for a t2 weighted image
2000-6000 ms
TE range for a t1 weighted image
10-30 ms
TE range for a PD weighted image
10-30 ms
Define superparamagnetic
Demonstrate positive magnetic susceptibilities stronger than paramagnetic substances and weaker than ferromagnetic substances. Often send as t2 contrast agents
TE range for a t2 weighted image
70-120 ms
How can you reduce scan time?
Reduce TR Reduce NSA/NE Increase parallel imaging factor Utilize RecFOV (rectangular FOV) Utilize half-Fourier/halfscan Use coarsest matrix possible
When increasing NEX/NSA, SNR increases by what rate?
The square root of the %increase
(2 NEX to 6, 300% increase, 3 times, square root of 3, 1.73 times more SNR
How can you increase SNR?
Increase NEX/NSA Increase FOV Decrease phase matrix Increase TR Increase slice thickness Reduce bandwidth Reduce ETL (echo train length)
Spin Echo scan time formula
TR X phase matrix x NEX
Fast spin echo scan time formula
(TR x phase matrix x NEX)/ETL
2D gradient echo scan time formula
TR x phase matrix x NEX
3D gradient echo sequence time formula
TR x phase matrix x NEX x #slices
How is pixel size determined?
Field of view / matrix
What is gradient moment nulling?
Flow compensation. Compensated for first order motion (protons moving with constant velocity) and slow moving vessels
Ernst angle?
Optimal flip angle that yields the maximum signal for a particular spin in e least amount of time
Diffusion Tensor
Displays the mobility of water molecules in all three coordinates
At 1 Tesla the difference in precessional frequencies between fat and water is ____ hz
147
For Inversion Recovery, the null point is _____ the T1
0.69 times
Does gadolinium pass thru the intact blood-brain barrier?
No, only if the barrier is disrupted
On gradient echo sequences, what controls T1 weighting?
Flip angle. Larger flip angle produces more T1 weighting
The strength of the magnetic field is determined by the _________ of the current
Amount
The strength of the earth’s magnetic field is…
0.5 gauss
Permanent magnets generally have a field strength of
0.06 to 0.35 Tesla
Resistive magnets generally have a field strength up to
0.3 Tesla
When substances have an ______ they are said to be MR active
Uneven mass number
Not atomic number
A radio wave is a/an _________ electromagnetic field
Oscillating
If the flip able is doubled the RF power is… it
Increased 4 times
The FID signal can be changed from a plot of signal over time into a split of signal over frequency known as…
NMR spectrum
Fourier transform occurs in the …
Array processor
The t2 time is about ______ in fat
50 msec
Water has a t2 time of about…
200 msec
Fat has a T1 recovery of about
150 msec (short)
Water has a T1 recovery of about
2000 msec (long)
The purpose of the gradient coils is to produce a magnetic field that
varies in intensity in a given direction
In a spin echo sequence the slice selection gradient is applied during…
The 90 degree RF pulse and the 180 degree RF pulse
With each repitition, the phase encoding gradient will be applied with a different…
Amplitude and/or polarity
Which operator selectable acquisition parameters affect SNR?
All of them
RF shielding can be accomplished by lining the scan room with
Copper
A short TI suppresses …
Suppresses fat
A long TI suppresses…
Water
At an intermediate TE, the differences between _____ will be demonstrated
Gray and white matter
Echoes encoded with high amplitude phase encoding steps are weighted more for ______ and reside in the ______ of k-space.
Spatial resolution, or edge detail
Outer edges
Does number of frequency encodings affect scan time?
No
Scan time formula for 2d acquisition SE or GRE
TR x Ny (# of phase encodings) x NSA
Scan time formula for FSE
TR x Ny (# of phase encodings) x NSA / ETL
3D scan time formula
TR x Ny (# of phase encodings) x NSA x NSl (# of slices)
Time of flight MRA sequences use ______ gradient echo acquisitions
T1 (spoiled or incoherent)
Phase contrast MRA sequences use _______ gradient echo acquisitions
T2 (steady state or coherent)
2D TOF is optimal for ______ flow in _____ vessels
Slow
Large
3D TOF is optimal for _____ flow in ______ vessels
Fast
Small
_________ flip angles are used with 3d TOF than 2D TOF
Lower
The lower the field strength the ________ the T1 time
Lower
Zero fill reduces scan time at the expense of ______
Spatial resolution
Cost of gradient moment nulling
Increase minimum TE, signal dephasing is increased, vascular signal can be decreased
In GRE reducing the flip angle has the same effect as…
Increasing the TR
Vascular signal is increased when slices are acquired _______ the flow
against
Routine weight-based contrast administration is typically __________
0.2ml/kg
