1. Parameters and Trade-offs
Signal to noise ratio
Contrast to noise ratio
Spatial resolution
Scan time
Volume imaging

2. 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
Signal to noise ratio (SNR)
Contrast to noise ratio (CNR)
Spatial resolution
Scan time

3. Signal to Noise Ratio (SNR)

4. 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

5. Noise Is constant for the patient Depends on the
build of the patient
Area under examination
Inherent noise of the system
Occurs at every frequency and is random in time

6. 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

7. 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

8. 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

9. 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

10. Voxel Volume Matrix voxel Small voxel few spins
Large voxel more spins & high signal
Slice width
slice

11. 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

12. 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)

13. Number of signal averages (NSA) or NEX
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)

14. NEX Vs SNR
SNR 3 2 1
NEX

15. Receive bandwidth
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

16. Bandwidth versus SNR SNR increases with decrease of bandwidth
Bandwidth +/- 16 kHz
signal
noise
noise
noise
Bandwidth +/- 4 kHz
signal
noise
noise
noise
SNR increases with decrease of bandwidth

17. Type of coil
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

18. RF coil types Commonly used Specific Volume coil or Bird-cage coil
Surface coil
Phased array coil
Specific
Solenoidal coil
Helmholtz pair

19. How to increase SNR 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

20. FOV, Resolution & SNR FOV = 24cm FOV = 12 cm High SNR
High resolution, low SNR

21. Slice thickness, Resolution & SNR
10 MM SLICE
3 MM SLICE
High resolution, low SNR

22. NEX & SNR
PARTIAL AVERAGING, TIME 56 S
NEX = 4, TIME= 6 MIN
High SNR

23. Contrast to noise ratio (CNR)

24. CNR 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)

25. Image contrast depends on:TE TI
Flip angle
Flow
Turbo factor (in fast spin echo) T1 T2
Proton density
Magnetization transfer coherence (MTC)*
* (see note)
MTC is the pre saturation of bound protons, which would otherwise change the T1 values of free protons by trransfer of magnetization

26. 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

27. Matrix & resolution
Matrix 256 x 256
Matrix 512 x 256

28. 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

29. How to reduce the scan time
Use the shortest TR possible
Select the coarsest matrix possible
Reduce the NEX to a minimum

30. 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

31. Decision making Selections depend on The area to be examined
Condition and co-operation of the patient
Clinical throughput required

32. 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

33. 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

34. Therefore the scan time increases
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

35. 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