Parameters and Trade-offs 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 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 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 Matrix voxel Small voxel few spins Large voxel more spins & high signal Slice width slice

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 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 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 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

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

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

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

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

Contrast to noise ratio (CNR)

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)

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

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

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

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