Parameters and Trade-offs

Report
Parameters and Tradeoffs
Signal to noise ratio
Contrast to noise ratio
Spatial resolution
Scan time
Volume imaging
Introduction
The choice of pulse sequence determines the
weighting and the quality of the images as well
as their sensitivity to pathology
 The timing parameters selected determines the
weighting of the images
 The quality of the image is controlled by
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Signal to noise ratio (SNR)
Contrast to noise ratio (CNR)
Spatial resolution
Scan time
Signal to Noise Ratio
(SNR)
Signal to noise ratio (SNR)
SNR = amplitude of the signal / average
amplitude of the noise
 The signal is the voltage induced in the
receiver coil by the precession of the NMV
 The noise is generated by the presence of
the patient in the magnet, and the
background electrical noise of the system

Noise
Is constant for the patient
 Depends on the
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– build of the patient
– Area under examination
– Inherent noise of the system

Occurs at every frequency and is random
in time
Signal
Is cumulative and depends on many
factors and can be altered
 Increasing the signal decreases the SNR
 Any factor that affects the signal
amplitude in turn affects the SNR

Factors that affect SNR
Proton density of the area under
examination
 Voxel volume
 TR, TE and flip angle
 NEX (number of excitations)
 Receive bandwidth
 Coil type

Proton density

Areas with low proton density (PD)
produce low signal and therefore low SNR
– E.g. Lungs, cortical bone

Areas with high PD have high signal &
high SNR
– E.g. Bladder, renal pelvis
Voxel volume
Large voxels have more nuclei therefore
high signal and high SNR
 Small voxels have less nuclei therefore low
signal and low SNR
 Any parameter that change the voxel
volume changes the SNR

– Voxel volume = pixel area x slice thickness
– Pixel area = FOV dimensions/matrix size
Voxel Volume
voxel
Small
voxel
few
spins
Matrix
slice
Slice
width
Large voxel
more spins &
high signal
TR, TE and flip angle

Spin echo pulse sequences have more signal
than gradient echo sequences, because;
– All the longitudinal magnetization is converted into
transverse magnetization by 900 flip angle
– Gradient echo pulse sequences only convert a portion
of the longitudinal magnetization into transverse
magnetization as the flip angle is not 900
– The 1800 rephasing pulse is more efficient at
rephasing than the rephasing gradient of gradient
echo sequences
The lower the flip angle, the lower the
SNR
 Long TR increases SNR and a short TR
reduces SNR (TR controls the amount of
longitudinal magnetization that is allowed
to recover)
 Long TE reduces SNR and short TE
increases SNR (TE controls the amount of
transverse magnetization that is allowed
to decay before an echo is collected)

Number of signal averages (NSA)
or NEX
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This is the number of times data is collected
with the same amplitude of phase encoding
slope.
Increase in NEX increase the amount of data
stored in each line of k-space.
The data consists of both signal and noise.
The noise increases randomly and the increase
of signal is not random.
Therefore the increase of NEX increases the SNR
by √2(=1.4)
NEX Vs SNR
SNR
3
2
1
1
2
3
4
5
6
7
8
NEX
Receive bandwidth
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This is the range of frequencies that are
sampled during the application of readout
gradient.
Reducing the receive bandwidth results in less
noise being sampled relative to signal.
Therefore SNR increases when receive
bandwidth is decreased.
But Sampling tine increases and increases the
minimum TE available
Increases the chemical shift artefact
Bandwidth versus SNR
Bandwidth +/- 16 kHz
signal
noise
noise
noise
Bandwidth
+/- 4 kHz
signal
noise
noise
noise
SNR increases with decrease of bandwidth
Type of coil
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The type of coil used affects the amount of signal
received and therefore the SNR
Volume coils with quadrature excitation increase SNR as
two coils are used to receive signal
Surface coils placed close to the area under examination
also increase the SNR
The use of the appropriate receiver coil plays extremely
important role in optimizing SNR
The volume of tissue imaged should optimally fill the
sensitive volume of the coil
Large coils increase the likelihood of aliasing as tissue
outside the FOV more likely to produce signal
RF coil types

Commonly used
– Volume coil or Bird-cage coil
– Surface coil
– Phased array coil

Specific
– Solenoidal coil
– Helmholtz pair
How to increase SNR
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Use spin echo sequences where possible
Try not to use a very short TR and a very long
TE
Use the correct coil and ensure that it is well
tuned
Use a coarse matrix
Use a large FOV
Select thick slices
Use as many NEX as possible
FOV, Resolution & SNR
FOV = 24cm
FOV = 12 cm
High SNR
High resolution, low SNR
Slice thickness, Resolution & SNR
10 MM SLICE
3 MM SLICE
High resolution, low SNR
NEX & SNR
NEX = 4, TIME= 6 MIN
High SNR
PARTIAL AVERAGING, TIME 56 S
Contrast to noise ratio
(CNR)
CNR
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Defined as the difference in the SNR between
two adjacent areas
Controlled by the same factors that affect SNR
Most critical factor affecting image quality
Determines the eyes ability to distinguish areas
of high signal from areas of low signal
(Although the SNR of T2 weighted image is
lower than T1 weighted image the ability to
distinguish tumour from normal tissue is greater
because of the high signal compared to the low
signal of surrounding anatomy, i.e. CNR is
higher)

Image contrast depends on:
–
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–
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–
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–
–
–
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TR
TE
TI
Flip angle
Flow
Turbo factor (in fast spin echo)
T1
T2
Proton density
Magnetization transfer coherence (MTC)*
* (see note)
Spatial Resolution
This is the ability to distinguish between two
points as separate and distinct.
 Controlled by the voxel size
 Small voxels results in good spatial resolution
 The voxel size is affected by

– Slice thickness
– FOV
– Number of pixels or matrix
The spatial resolution is increased by using
rectangular FOV for rectangular anatomy
 Rectangular FOV reduces the scan time

Matrix & resolution
Matrix 256 x 256
Matrix 512 x 256
Scan time
Short scan times are important to
minimize the possibility of patient
movement
 Factors that affect the scan time are

– TR – time of each repetition
– Number of phase encodings – number of lines
of K space
– NEX – number of times data is collected with
the same phase encoding gradient
How to reduce the scan time
Use the shortest TR possible
 Select the coarsest matrix possible
 Reduce the NEX to a minimum

Trade-offs
There are many trade-offs when selecting
parameters within a pulse sequence
 Ideally an image should have

– high SNR,
– Good spatial resolution
– a very short scan time.

But when improving one factor inevitably
reduces one or both of other two
Decision making

Selections depend on
– The area to be examined
– Condition and co-operation of the patient
– Clinical throughput required
Tips to improve image quality
Choose the correct coil
 Make sure that the patient is comfortable
ad immobilized
 Ascertain from the radiologist what
sequences are required
 SNR is the most important quality factor
 Keep the scan time as short as possible

Volume Imaging
Advantages to demonstrate very small
lesions
 Slice thickness can be reduced
 Entire volume of tissue is excited & No
slice gap
 SNR is superior
 Fewer NEX can be used
 Can look at anatomy in any plane
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The scan times are relatively long therefore used
in conjunction with faster pulse equences
Slices are sectioned out by a series of phase
encoding steps along the slice select axis
Therefore the scan time increases
But as SNR is increased with increased number
of slices the NEX can be reduced
Isotropic (equal dimensions) pixel give equal
resolution in any plane
Scan time = TR x NEX x number of phase
encodings x number of slice encodings
Conclusion
Manipulating SNR, image contrast, spatial
resolution and scan time is a real art and
takes some time and experience
 Even after many years you may get
things wrong occasionally
 Perseverance is important to get good
image quality


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