MRI FINAL

For resonance to occur

a RF pulse must be applied at 90 degree from the longitudinal plane at the precessional frequency of hydrogen, gaining transversal magnetization

precession hydrogen at 3T

127.74 MHz

larmour eq

ω = γB

what happens when a RF pulse is applied at 90 degree from the longitudinal plane at the precessional frequency of hydrogen

This places hydrogen into phase. This resultant coherent magnetization precess at the larmour frequency in the transverse plane. 

the signal has

a frequency equal to the larmour frequency of hydrogen, regardless of the origin of signal in the patient

The system must be able to locate

signal spatially in three dimensions, so that it can position each signal at the correct point on the image.

How does a system locate signal

first locates a slice. Once a slice is selected, the signal is located or encoded along both axis of the image

three primary functions that gradients perform during MR scanning

slice selection, phase encoding, and frequency encoding

magnetic isocentre

the center point of the axis of all three gradients

what happens in the magnetic isocentre

The magnetic field strength and therefore the precessional frequency remain unaltered here even when the gradients are applied.

first way of selecting slice thickness

We send in an RF pulse that has a range of frequencies.

The wider the range, the thicker slice of protons will be excited

In sequence a, we used a range of RF frequency from 64 mHz to 65 mHz, to produce slice 1

If however in sequence b we use a frequency range from 64 mHz to 64.5 mHz , we will produce slice 2 which is thinner than slice 1

second way of selecting slice thickness

• In figure c, we use the same range of frequencies or bandwidth, the slice thickness can be modified by the slope of the gradient

• By using a steeper gradient , the precessional frequencies will vary to a larger degree

• In figure a and c , the same band width containing frequencies between 64 and 65 mHz was used both times

• Because of the steeper gradient in c was used, we produce a thinner slice

To obtain equal resolution in each plane at every angle of obliquity

each voxel must be symmetrical, or isotropic. .

isotropic voxel

voxel has equal dimensions in every plane

anisotropic

the volume has poorer resolution in the planes other than the one that was acquired.

course matrix

one with a low number of frequency and/or phase encoding and results in a low number of pixels in the FOV

for a given FOV a course matrix results in

large pixels and voxels.

fine matrix

one with a high number of frequency and/or phase encoding and results in a large  number of pixels in the FOV

for a given FOV a fine matrix results in

small pixels and voxels

pixel

building unit of a digital image

The brightness of the pixel represents

the MRI signal strength generated by a unit volume of patient tissue (voxel).

The voxel represents

a volume of tissue within the patient,

the voxel is determined by

the pixel area and the slice thickness

The pixel area is determined by

the size of the FOV and the number of pixels in the FOV or matrix.

To optimize image quality, the SNR must be

the highest possible

how to get high SNR

Use spin echo pulse sequences where possible

Try not to use a very short TR and a very long TE

Utilize the correct coil

Use of coarse matrix

Use of large FOV

Thicker slices

Use more NEX or signal averages

To improve image quality the spatial resolution must be

high

The spatial resolution can be increased/maintained by

selecting as thin a slice as possible

Selecting a fine matrix

Selecting a small FOV

scan times should be, why

To reduce the likelihood of patient movement, scan times should be as short as possible.

To achieve the shortest scan time

Use the shortest TR possible

Select the coarsest matrix possible

Reduce the NEX to a minimum

SNR is proportional to

Pixel area/FOV squared

Slice thickness

Proton density

NEX

Number of phase encodings (base resolution)

Number of frequency encodings (base resolution)

TR, TE, and flip angle.

spatial resolution determined by

FOV

Matrix size

Slice thickness

scan time is proportional to

TR

Number of phase encodings

NEX

if you increase NEX to maximize SNR what happens

scan time increases

if you decrease matrix to maximize SNR what happens

scan time decreases (p/matrix) and resolution decreases

if you increase slice thickness to maximize SNR what happens

resolution decreases

if you decrease receive bandwidth to maximize SNR what happens

increase the minimum TE and increase chemical shift

if you increase FOV to maximize SNR what happens

decrease resolution

if you increase TR to maximize SNR what happens

decrease T1 weighting and increase number of slices

if you decrease TE to maximize SNR what happens

decrease T2 weigthing

if you decrease slice thickness to maximize resolution what happens

decrease SNR

if you increase matrix to maximize resolution what happens

decrease SNR and increase scan time (p/matrix)

if you decrease FOV to maximize resolution what happens

decrease SNR

if you decrease TR to minimize scan time what happens

increase T1 weighting, decrease SNR, decrease # of slices

if you decrease phase matrix to minimize scan time what happens

decrease resolution, increase SNR,

if you decrease NEX to minimize scan time what happens

decrease SNR, increase movement artifact

if you decrease slice number in volume imaging to minimize scan time what happens

decrease SNR

TR being increased benefits and limitations

increase SNR but Increase Scan time
increase number of slices but decrease T1 weighting

TR being decreased benefits and limitations

decrease scan time but decrease SNR
increase T1 weighting but decrease number of slices

TE being increased benefits and limitations

increased T2 weighting but decrease SNR

TE being decreased benefits and limitations

increased SNR but decreased T2 weighting

increase NEX benefits and limitations

increased SNR but increased scan time
increased signal averaging

decrease NEX benefits and limitations

decrease scan time but decrease SNR and signal averaging

increase slice thickness benefits and limitations

increased SNR but decreased resolution
increased coverage but increased partial voluming

decrease slice thickness benefits and limitations

increased resolution but decreased SNR
decreased partial voluming but decreased coverage

increased FOV benefits and limitations

increased SNR but decreased resolution
increased coverage
decreased Aliasing (pFOV)

decreased FOV benefits and limitations

increased resolution but decreased SNR, decreased coverage, increased aliasing (pFOV)

increase (p)matrix benefits and limitations

increased resolution but increased scan time and decrease SNR if pixel small

decrease (p)matrix benefits and limitations

decreased scan time but decrease resolution
increased SNR if pixel large but decreased resolution

increase receive bandwidth benefits and limitations

decreased chemical shift but decreased SNR
decreased minimum TE but decreased SNR

decrease recieve bandwidth benefits and limitations

increased SNR but increased chemical shift and increased minimum TE

large coil benefits and limitations

increased area of received signal but decreased SNR, more sensitive to artifacts, aliasing with small FOV

small coil benefits and limitations

increased SNR, less sensitive to artefacts, less prone to aliasing with small FOV but decreased area of recieved signal

TR and what it is measured in

TR is the time from the application of one RF pulse to the application of the next RF pulse for each slice and is measured in ms. 

what does TR determine

amount of longitudinal relaxation that is allowed to occur between the end of RF pulse and the application of the next
amount of T1 relaxation that has occurred when the signal is read

Echo time

TE: Time in milliseconds from the application of the RF pulse to the peak of the signal induced in the coil

TE determines

 much decay of transverse magnetization is allowed to occur

TE controls

the amount of T2 relaxation that has occurred when the signal is read

most common applications for inversion recovery technique

STIR and FLAIR imaging

STIR used where and for what

just about every body part and is known for its uniform Fat Suppression

FLAIR starts with

starts with a 180 degree pulse used to null specific signal

turbo spin echo

spin echo that is much shorter than spin echo.

how does turbo spin echo work

Uses multiple 180 degree rephrasing pulses per TR.

turbo factor or echo train length

The number of 180 rephrasing pulses performed

turbo spin echo

how is a gradient echo created

A magnetic field gradient is applied to refocus the dephasing spins instead of 180 pulse

This means an uneven magnetic field is added onto existing magnetic field

The gradient is switched on for a short time

This creates an even larger increase in field inhomogeneities

Which in turn causes a larger increase in internal magnetic field inhomogeneities inside the tissue slice

This causes the protons in the transverse plane to dephase faster

The magnetic field gradient is switched off

After a short time it is switched on again same strength, but in the opposite direction

Large flip angles produce more

T1 weighting

Longer TE’s produce

more T2* weighting

With fast scans there are often

intense signals from vessels

advantages of gre

• Decreased scan time

• Increased sensitivity to flow

• Volume acquisitions possible

disadvantages of gre

• Decreased signal to noise ratio

• Increased sensitivity to magnetic susceptibility artifacts

• Increased gradient noise

T1 spin echo weighted images are produced using

short TR and short TE
One 180 RF pulse is applied after 90 degree excitation pulse

A short TE in a t1 spin echo ensures

minimal T2 decay

in a T1 spin echo, The differences in T2 time

do not dominate the echo and its contrast

Short TR in a T1 spin echo insures that

T1 time dominates the echo

T2 spin echo created using

a long TR and long TE
One 180 RF rephasing pulse is applied after 90 degree excitation pulse

in t2 spin echo, A long TR

minimizes the T1 contrast

in t2 spin echo a long TE

ensures T2 contrast dominates the echo

phase mismapping

ghosting/ produces replications of moving anatomy across the image in the phase encoding direction

ghosting comes from

anatomy that moves periodically during the scan, such as the chest wall during respiration, pulsatile movement of vessels and CSF, swallowing and eye movement

phase encoding direction can be determined by

the direction of the phase mismapping or ghosting artefact

magnetic susceptibility artifact

produces distortion of the image together with large signal voids

magnetic susceptibility

ability of a substance to become magnetized

different tissues magnetize to ______ which results in ____.
what does that cause

Different tissues magnetize to different degrees, which  result in different precessional frequency and phase.
This causes dephasing at the interface of these tissues and a signal loss.

the main causes of magnetic susceptibility artefacts are

metal within imaging volume

natural occurring magnetic susceptibility artefact

the iron content of hemorrhage, as these magnetize to a much greater degree than the surrounding tissue.

Ferromagnetic objects have a very high

magnetic susceptibility and cause image distortion.

Magnetic susceptibility artefact  is more prominent in ______ because

gradient echo sequences bc gradient reversal cannot compensate for phase difference at the interface.

ways to fix magnetic susceptibility artefact

Remove all metal objects. Always ensure the patient has removed all metal objects when possible before the scan.

Use turbo or fast spin echo sequences instead of gradient echo. The multiple 180 degree rephasing pulses is very effective at compensating for phase differences between fat and water. Gradient echo is very poor at this.

Decrease the TE. Longer echo times allow for more dephasing between tissues with susceptibility differences, therefore using a short TE reduces this artefact. Broad receive bandwidths also reduce the TE.

aliasing/wrap artefact

Wrap or aliasing produces an image where the anatomy that exists outside the FOV is folded onto the top of the anatomy inside the FOV.