MRI physics midterm

rf excitation pulse

a short burst of Radio frequency energy applied to the area of the body being imaged

whats needed for a strong fid

magnetic field must be as homogeneous as possible

FID function

generates raw data thats manipulated by subsequent pulses

FID only occurs when??

after uniti

what process releases energy and moves back into the longitudinal phase is

relaxation

releasing enrrgy plus dephasing happens

simultaneous but independent

precessional path

The route the magnetic moment takes around the axis

phase is the

POSITION! Of mag moment on their precessional path

frequency vs phase in mri

frequency describes the rate Or speed of procession of the magnetic moment. While phase describes the directional relationship between two protons magnetic moments

transverse magnetization

wat we can measure in mri scanner

excitation

The amount of the MMV is flipped or tilted away from the longitudinal plane is called the flip angle. It's a term it is determined by the length and duration of the RF exccitation pulse.

Flip angle is determined by what?

length and duration of a RF excitation pulse

short vs long rf excitation

short is flipping it with less than 90°. Long RX excitation pulse has a flip angle equal to 90 degrees

Flip angle has another name

The amount that hydrogen nuclear are flipped into the transverse plane

The faster of a magnetic field changes,,

then the greater the EMF (electromotive force) will be

what effect does a 180 refocusing pulse have in a spin echo sequence?

it compensates for magnetic field inhomongeneties

what is the significance of the term echo train length in a fast spin echo sequences

It refers to the number of echoes generated per TR

balanced spin echo uses

phase cycling

similar t1 relaxation and t2 decay times =

stronger signal intesnsity

bigger difference between t1 relaxation and t2 decay times =

less signal !

brocas area

speech

fmri good for

pre surg planning and mapping brain safely

BOLD

contrast properties of oxygenated and deoxygenated hemoglobin in the blood. We need oxygenated blood to scan because it has a better mag field

oxygenated hemoglobin

diamagnetic

deoxygenated equals

disturbs Magnetic field. not as stable as oxygenated blood

spin echo disadvantage

Sensitive to motion and longer scan time

conventional CSE spin echo

Pulse sequence use an MRI to generate high-quality images by rephasing spin dephasing (occurs by inhomonogeniteis)

voxel

3-D

spatial encoding

Howie map location of signals in the body to create a detailed image. Pinpoints or origin of MRI signal in 3-D space

gradient coil task

Sliced selection, phase encoding and frequency encoding

slice selection

Isolate single slice of the body

phase encoding

alters phase of spinning protons in axis direction

slice

Thin, cross-sectional layer of body

pixel

2-D location performed by gradient coils

gradient coils

Create a magnetic field superimpose over static magnetic field in XYZ directions

frequency encoding

also known as read out gradient, varies Precess of protons

thin slice

Steep SSG/narrow transmit bandwidth

thick slice

Shallow SSG, broad transit bandwidth

gradient SSG

Only during RF excitation

Carrier frequency

Center of transmit bandwidth

isoCenter

Best area of the body to take pictures for MRI

slice of interest

Excited by RF pulse at same or similar frequencies as processional frequency of that area of the body

sliced thickness relies on what two factors

Sliced selection gradient amplitude or slope. or transmit bandwidth

sampling time

FEG applied and the signal is red critical time when info or signal frequency is encoded

when is the FEG turned on

When the signal is received. This depends on image plane for specific gradient

FOV

Field of view area of anatomy covered in image can be adjusted by adjusting FEG's amplitude or slope

steady state

Keeps the strong picture. Longitude no transverse plane run into each other. 30° to 45° flip angle. (all that aoply)

proton density

Small flip angle, long TR short TE

what does gradient echo look for in anatomy imaging?

Look through blood and brain/angios

balanced spin echoes have

Faster Scan times. reduce artifacts. Angeo have a good detail, but very loud

FID

Shows half signal, but we need to de-phase with gradient echo then re-phase with gradient echo

coherent

In phase or rephasing

incoherent

Out of phase or dephasing

FID also

is after RF pulse and results From magnetic field in homogeneity and T2 decay

In gradient echo the variable rf pulse must be less than

90degrees

Gradient exho faster but

inhomogenietes not accounted for! and these mag susceptibilities messes with gradient echo

FEG function

Dephase and rephase. bipolar part. permits short TE shorter TR shorter than spin exho

what is the result of gradient spoiling?

Magnetic moments are not in sync

gradients are used to rephrase what type of magnetization

transverse

spoiling

no signal

gradient dephasing

Spoilers

rephasing aka

rewinders

artifact

magnetic susceptibility

what is the number one artifact in MRI?

Movement

echo planar imaging

ultra fast mri imaging technique

middle k space

Equals good signal

steep gradient equals

Low signal

median gradient equals

Medium signal

shallow gradient equals

High signal

IR

spin echo starts at 180°. rf pulse flips longitudinal magnetization

slice select happens when

During RF pulse with the body slice being imaged

phase encoding gradient

After 90 but before 180° various phase of signal across body turned on after the SSG is turned off

Single spin echo

T1 weighting

dual echo

for pd/ t2 weighting

atomic nunber

sum of protons in nucleus

mass number

sum ofprotons and nuetrons in nucleus

MR active nuclei are

only nuclei with an odd mass number or atomic weight. Characterized by their tendency to align their access of rotation to an applied magnetic field

protium

isotope of hydrogen

isotope

atoms of element with same number of protons but diff number of neutrons!

saturation happens when

Spins are unable to absorb more energy or to be stimulated and release more energy

pulse sequences

Forms Magnitude and timing of RF pulses, which are the basis of contrast generation in MRI

Ferradays law

Changing the magnetic field generates electric current

what does widening the transmit bandwidth do?

Decreases chemical shift of artifact

what does widening the transmit bandwidth do?

Decreases chemical shift of artifact

X

sagittal left to right

Y

coronal - front to back

PEG on graph

Y AXIS

PEG on graph

Y AXIS

slice thickness determines…

Bandwidth and gradient strength. width of slice in bandwidth- gradient strength

Gradient flip angles have … what types of fov

steep= small fov

shallow= large fov

Z

axial. head to foot

IR

Starts at 180

narrow bandwidth equals what slice

thin

large bandwidth equals

thick slice

What application from gradient is turned on during echo or during signal turning on?

FEG!

gradient is what than spin echo

faster because of rapid imaging

gradient high flip angle maximizes-

T1 weighting and T1 recovery becomes prominant

gradient low flip angle

Minimizes T1 weighting and preserves longitudinal magnetization