Mr Physics Week 4: Paramaters - Trade Offs

Front 1

•There are many ______ available to the operator when setting up a protocol.

Back 1

parameters

Front 2

• A ______ is defined as a “set of rules”

Back 2

protocol

Front 3

in MRI these _____ are a variety of parameters that are selected by the operator.

Back 3

PROTOCOLS

Front 4

•The four main considerations determining image quality are:

Back 4

- Signal to noise ratio (SNR)
- Contrast to noise ratio (CNR)
- Spatial resolution
- Scan time

Front 5

•The signal-to-noise ratio is defined as

Back 5

the ratio of the amplitude of the MR signal to the amplitude of the background noise.

Front 6

•The MR signal is

Back 6

the voltage induced in the receiver coil by the precession of the NMV in the transverse plane. It occurs at specific frequencies and time intervals (TE).

Front 7

•Noise is the undesired signal resulting from the MR system, the environment and the patient. T OR F?

Back 7

TRUE

Front 8

What occurs at all frequencies and randomly in time. ?

Back 8

Noise

Front 9

to increase the SNR usually requires ;

Back 9

increasing the signal relative to the noise.

Front 10

•The 8 Parameters that affect SNR are ;

Back 10

field strength,
proton density,
coil type and position,
TR,
TE,
flip angle,
number of signal averages
receive bandwidth.

Front 11

•The signal-to-noise ratio is affected by the strength of the magnet? Tor F

Back 11

true

Front 12

• The signal-to-noise ratio is affected by the strength of the magnet. At higher field strengths there are more spin up than spin down nuclei as fewer have enough energy to oppose the increased field. As a result;

Back 12

As a result the NMV increases thereby increasing the available signal.

Front 13

it is likely that measures to boost the SNR will be required WHEN?

Back 13

When scanning areas with a low proton density

Front 14

•The voxel is defined as 1.___ ___ __ _____ __ ___ ____ _____. Therefore the factors that affect the voxel volume are:2. a._____ _____
b.____ c.___ ______.

Back 14

1.the pixel area multiplied by the slice thickness
2..• slice thickness
• FOV
• the matrix.

Front 15

•Voxel volume is the product of ___ ___ __ ___ ____ ____ and the slice thickness.

Back 15

the two inplane pixel dimensions

Front 16

•The size of the voxel determines

Back 16

how much signal each voxel contains.

Front 17

Large voxels have a higher signal than small ones because

Back 17

there are more spins in a large voxel to contribute to the signal

Front 18

any setting of FOV, matrix size or slice thickness that results in large voxels leads to a

Back 18

leads to a higher SNR per voxel.

Front 19

• Small voxels improve resolution as they increase the likelihood of two points, close together in the patient, being in separate voxels and therefore distinguishable from each other. T or F?

Back 19

True

Front 20

Changing any dimension of the voxel changes the resolution and also affects ?

Back 20

SNR.

Front 21

•The size of the voxel determines

Back 21

how much signal each voxel contains.

Front 22

Large voxels have a higher signal than small ones because

Back 22

there are more spins in a large voxel to contribute to the signal

Front 23

any setting of FOV, matrix size or slice thickness that results in large voxels leads to a

Back 23

leads to a higher SNR per voxel.

Front 24

• Small voxels improve resolution as they increase the likelihood of two points, close together in the patient, being in separate voxels and therefore distinguishable from each other. T or F?

Back 24

True

Front 25

Changing any dimension of the voxel changes the resolution and also affects ?

Back 25

SNR.

Front 26

•Coarse matrix – is

Back 26

•Coarse matrix – is one with low number of frequency encoding and/or phase encoding and results in a low number of pixels in the FOV – low resolution

Front 27

•Fine matrix – is

Back 27

•Fine matrix – is one with high number of frequency encoding and/or phase encoding and results in high number of pixels in the FOV – high resolution

Front 28

•Halving the matrix doubles the dimension of each pixel along

Back 28

the phase encoding axis.

Front 29

•Halving the matrix doubles the dimension of each pixel along the phase encoding axis. There are less pixels to map over a given FOV dimension in the phase direction, therefore each pixel is larger along this axis. The SNR of each voxel ?

Back 29

doubles

Front 30

•When increasing the FOV, pixel dimensions along each axis increase because the same number of pixels needs to cover a larger FOV,( e.g. from a 12cm FOV to a 24cm FOV). The SNR of each voxel increases by a factor of ____ because the size of each pixel has doubled along each axis.

Back 30

four

Front 31

•Increasing the slice thickness increases the voxel volume along the dimension of the slice. Thick slices cover more of the patient’s body tissue and therefore have more spinning protons within them. SNR therefore

Back 31

increases in proportion to increase in slice thickness.

Front 32

•The TR determines how much _____ _______ is recovered between excitation pulses and how much is available to be flipped into the transverse plane.

Back 32

longitudinal magnetization

Front 33

•At very short TRs, very little longitudinal magnetization recovers so only a small amount of ______ _______ __ _____and therefore results in an image with ____ ___ .

Back 33

1. transverse magnetization is created
2. poor SNR.

Front 34

•Increasing the TR improves the SNR as more longitudinal magnetization, and therefore more transverse magnetization, is created. 1. T or F ?
Although short TRs are required for T1 weighting, reducing this parameter too much may severely compromise the SNR. 2. T or F ?

Back 34

1. TRUE
2. TRUE

Front 35

•The TE determines how much dephasing of transverse magnetization occurs between the ______ _____ ___ ___ _____.

Back 35

excitation pulse and the echo.

Front 36

At short TEs, as very little transverse magnetization has dephased, the 1. _____ _____ and therefore the SNR of the image is 2.____.

Back 36

1.signal amplitude
2. high.

Front 37

With a large flip angle all available longitudinal magnetization is converted into transverse magnetization and hence maximum signal amplitude True or False ?

Back 37

TRUE 90degree fkip angle = max trans. magnetization and hence max signal amplitude.

Front 38

the range of frequencies sampled during readout is ;

Back 38

the receive bandwidth

Front 39

Reducing the receive bandwidth reduces What?

Back 39

reduces the proportion of noise sampled relative to signal.( Reducing the receive bandwidth is a very effective way of boosting the SNR )

Front 40

•Halving the bandwidth increases the SNR by about 40%, but also increases -

Back 40

the sampling time.

Front 41

•The type of the coil used affects the amount of ____ _____ and therefore SNR

Back 41

signal received

Front 42

The contrast-to-noise ratio or CNRis defined as

Back 42

the difference in SNR between two adjacent areas – ability to distinguish areas of high signal from areas of low signal.

It is controlled by the same factors that affect SNR.

Front 43

most important image quality factor is ?

Back 43

The CNR is probably the most important image quality factor, as the objective of any examination is to produce an image where pathology is clearly seen relative to normal anatomy.

Front 44

The CNR is increased by the administration of these 4 things..
1.______
2.______
3.______
4._______

Back 44

a contrast agent,
magnetization transfer contrast,
chemical suppression techniques
T2 weighting

Front 45

•1/ Why does T2 have much lower SNR than T1 ?
2/ what else is T2 used for?

Back 45

1/ T2 has much lower SNR than T1 (due to longer TE) .
2/ • Better pathology visualization

Front 46

•The purpose of administering contrast agents is

Back 46

to increase the CNR between pathology and normal anatomy

Front 47

•By saturating out normal anatomy, pathology is often seen more clearly
•These can be used to suppress signal from either fat or water ; Name these techniques.

Back 47

chemical pre-saturation

Front 48

Chemical pre- saturation is done using ?

Back 48

• Partial saturation
• Full saturation

Front 49

.Small molecules =
•Medium molecule =
•Large molecule =

Back 49

water
fat
proteins

Front 50

•Magnetization transfer contrast (MTC) is applied :

Back 50

•MTC is applied before the excitation pulse

Front 51

Magnetization transfer contrast (MTC) is;

Back 51

•By selectively saturating the bound protons
•These pulses are either applied at a frequency away from the Larmor frequency where they are known as off resonant
•These protons, mainly bound to large proteins, membranes, and other macromolecules are called bound protons

Front 52

•Spatial resolution is defined as

Back 52

as the ability to distinguish between two points that are close together in the patient. It is entirely controlled by the size of the voxel.

Front 53

•The imaging volume is divided into 1. ______.
•Each slice displays an area of anatomy defined as the 2. ___ __ _____.

Back 53

1.slices
2. Field Of View (FOV).

Front 54

•The FOV is divided into 1.________ . There are usually either 2.___ or ___ pixels along the frequency axis of the FOV and either 3. ___ ___ ___ ___ or ___ pixels along the phase axis of the FOV.

Back 54

1. into pixels, the size of which is controlled by the matrix.
2. 256 or 512
3. 128, 192, 256, 384 or 512

Front 55

•The phase matrix determines -

Back 55

how many lines of K space are filled during data acquisition and therefore the scan time

Front 56

•The voxel is defined as 1. _____________ .
Therefore the factors that affect the voxel volume are 2.a _____
b _____
c _____

Back 56

1. the pixel area multiplied by the slice thickness.

2. • slice thickness
• FOV
• the matrix.

Front 57

•1. Square pixels always provide better spatial resolution than rectangular pixels T OR F ?
•2. If phase number is less than frequency number, the pixels are longer in frequency direction – spatial resolution is therefore reduced along the phase axis. T OR F ?
•3. Some systems automatically keep the pixels square T OR F ?
•4. FOV should always cover the required anatomy along the phase axis T OR F ?
•5. Some systems have an option called : rectangular FOV to maintain spatial resolution with an uneven matrix T OR F ?

Back 57

All True.

Front 58

Rectangular FOV does these two things.?

Back 58

•Te dimension of the FOV in the phase direction is reduced
•Reduces the scan time

Front 59

•The scan time is the time to complete data acquisition or the time to fill K space
•The factors that affect scan time are:

Back 59

- TR
- Phase matrix
- NEX

Front 60

• Ideally an image Has :

Back 60

Has high SNR, good spatial resolution and is acquired in a very short time
• Chart

Front 61

Volume imaging

Back 61

•The entire volume of tissue is excited and volume contains no gap
•SNR is superior and fewer NEX can be used

Front 62

•Ideally an image has:

Back 62

•Ideally an image has ;
High SNR, good spatial resolution and is acquired in a very short time

Front 63

Volume imaging is ;

Back 63

•The entire volume of tissue is excited and volume contains no gap

Front 64

•The entire volume of tissue is excited and volume contains no gap
•SNR is superior and fewer NEX can be used
This is what type of imaging ?

Back 64

Volume imaging

Front 65

•Volume imaging is

Back 65

the acquisition of magnetic resonance data from a volume rather than a single tomographic slice.

Front 66

when No slice gap is employed, what type of imaging protocol is being used?

Back 66

Volume imaging

Front 67

•In conventional imaging the slice thickness affects the SNR, in volume imaging the entire volume of tissue is excited and the SNR is 1. ______ and so 2. ___________________.

Back 67

1. superior
2. fewer NEX can be used.

Front 68

What are the DISADVANTAGES OF VOLUME IMAGING ?

Back 68

•The disadvantage of volume imaging are that in general, the scan times associated with them are very long

Front 69

•VOLUME IMAGING FORMULA FOR FINDING THE SCAN TIME IS;

Back 69

•Scan time= TR * NEX * PE * number of slices

Front 70

•To obtain equal resolution in every plane,

Back 70

each voxel should be symmetrical (isotropic) – have equal dimensions in every plane.

Front 71

•Non symmetrical voxels are named:

Back 71

anisotropic

Front 72

•To obtain equal resolution in every plane, each voxel should be symmetrical (isotropic) – have equal dimensions in every plane. - If this is not true, what is the result?

Back 72

poorer resolution.

Front 73

•Non symmetrical voxels are named ?

Back 73

: anisotropic

Front 74

volume imaging is used for scanning images of ?

Back 74

- Imaging of joints
- Brain
- ligaments