Parameters and Parameters and Trade-offs Trade-offs Signal to noise ratioSignal to noise ratioContrast to noise ratioContrast to noise ratioSpatial resolutionSpatial resolutionScan timeScan timeVolume imagingVolume imaging

IntroductionIntroduction

The choice of pulse sequence determines The choice of pulse sequence determines the weighting and the quality of the the weighting and the quality of the images as well as their sensitivity to images as well as their sensitivity to pathologypathology

The timing parameters selected The timing parameters selected determines the weighting of the imagesdetermines the weighting of the images

The quality of the image is controlled by The quality of the image is controlled by – Signal to noise ratio (SNR)Signal to noise ratio (SNR)– Contrast to noise ratio (CNR)Contrast to noise ratio (CNR)– Spatial resolutionSpatial resolution– Scan timeScan time

Signal to Noise Ratio Signal to Noise Ratio (SNR)(SNR)

Signal to noise ratio (SNR)Signal to noise ratio (SNR)

SNR = amplitude of the signal / SNR = amplitude of the signal / average average amplitude of the noiseamplitude of the noise

The signal is the voltage induced in The signal is the voltage induced in the receiver coil by the precession of the receiver coil by the precession of the NMVthe NMV

The noise is generated by the The noise is generated by the presence of the patient in the presence of the patient in the magnet, and the background magnet, and the background electrical noise of the systemelectrical noise of the system

NoiseNoise

Is constant for the patientIs constant for the patient Depends on the Depends on the

– build of the patientbuild of the patient– Area under examinationArea under examination– Inherent noise of the systemInherent noise of the system

Occurs at every frequency and is Occurs at every frequency and is random in timerandom in time

Signal Signal

Is cumulative and depends on many Is cumulative and depends on many factors and can be alteredfactors and can be altered

Increasing the signal decreases the Increasing the signal decreases the SNRSNR

Any factor that affects the signal Any factor that affects the signal amplitude in turn affects the SNRamplitude in turn affects the SNR

Factors that affect SNRFactors that affect SNR

Proton density of the area under Proton density of the area under examinationexamination

Voxel volumeVoxel volume TR, TE and flip angleTR, TE and flip angle NEX (number of excitations)NEX (number of excitations) Receive bandwidthReceive bandwidth Coil typeCoil type

Proton densityProton density

Areas with low proton density (PD) Areas with low proton density (PD) produce low signal and therefore low produce low signal and therefore low SNRSNR– E.g. Lungs, cortical boneE.g. Lungs, cortical bone

Areas with high PD have high signal Areas with high PD have high signal & high SNR& high SNR– E.g. Bladder, renal pelvisE.g. Bladder, renal pelvis

Voxel volumeVoxel volume

Large voxels have more nuclei Large voxels have more nuclei therefore high signal and high SNRtherefore high signal and high SNR

Small voxels have less nuclei Small voxels have less nuclei therefore low signal and low SNRtherefore low signal and low SNR

Any parameter that change the voxel Any parameter that change the voxel volume changes the SNRvolume changes the SNR– Voxel volume = pixel area x slice Voxel volume = pixel area x slice

thicknessthickness– Pixel area = FOV dimensions/matrix sizePixel area = FOV dimensions/matrix size

Voxel VolumeVoxel Volume

Matrix

sliceSlice width

voxel

Small voxel few spins

Large voxel more spins & high signal

TR, TE and flip angleTR, TE and flip angle

Spin echo pulse sequences have more Spin echo pulse sequences have more signal than gradient echo sequences, signal than gradient echo sequences, because;because;– All the longitudinal magnetization is converted All the longitudinal magnetization is converted

into transverse magnetization by 90into transverse magnetization by 9000 flip angle flip angle– Gradient echo pulse sequences only convert a Gradient echo pulse sequences only convert a

portion of the longitudinal magnetization into portion of the longitudinal magnetization into transverse magnetization as the flip angle is transverse magnetization as the flip angle is not 90not 9000

– The 180The 18000 rephasing pulse is more efficient at rephasing pulse is more efficient at rephasing than the rephasing gradient of rephasing than the rephasing gradient of gradient echo sequencesgradient echo sequences

The lower the The lower the flip angleflip angle, the lower , the lower the SNRthe SNR

LongLong TRTR increases SNR and a short increases SNR and a short TR reduces SNR (TR controls the TR reduces SNR (TR controls the amount of longitudinal magnetization amount of longitudinal magnetization that is allowed to recover)that is allowed to recover)

Long TELong TE reduces SNR and short TE reduces SNR and short TE increases SNR (TE controls the increases SNR (TE controls the amount of transverse magnetization amount of transverse magnetization that is allowed to decay before an that is allowed to decay before an echo is collected)echo is collected)

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

This is the number of times data is This is the number of times data is collected with the same amplitude of collected with the same amplitude of phase encoding slope.phase encoding slope.

Increase in NEX increase the amount of Increase in NEX increase the amount of data stored in each line of k-space.data stored in each line of k-space.

The data consists of both signal and noise.The data consists of both signal and noise. The noise increases randomly and the The noise increases randomly and the

increase of signal is not random.increase of signal is not random. Therefore the increase of NEX increases Therefore the increase of NEX increases

the SNR by the SNR by √√2(=1.4) 2(=1.4)

NEX Vs SNRNEX Vs SNR

1 2 3 4 5 6 7 8 NEX

SNR

3

2

1

Receive bandwidthReceive bandwidth

This is the range of frequencies that are This is the range of frequencies that are sampled during the application of readout sampled during the application of readout gradient.gradient.

Reducing the receive bandwidthReducing the receive bandwidth results in results in less noise being sampled relative to signal.less noise being sampled relative to signal.

Therefore Therefore SNR increasesSNR increases when receive when receive bandwidth is decreased.bandwidth is decreased.

But But Sampling tine increasesSampling tine increases and increases and increases the minimum TE availablethe minimum TE available

Increases the chemical shift artefactIncreases the chemical shift artefact

Bandwidth versus SNRBandwidth versus SNR

Bandwidth +/- 16 kHz

Bandwidth +/- 4 kHz

noise

noise noise

noise

noise

noise

signal

signal

SNR increases with decrease of bandwidth

Type of coilType of coil The type of coil used affects the amount of signal The type of coil used affects the amount of signal

received and therefore the SNRreceived and therefore the SNR Volume coils with quadrature excitationVolume coils with quadrature excitation increase increase

SNR as two coils are used to receive signalSNR as two coils are used to receive signal Surface coilsSurface coils placed close to the area under placed close to the area under

examination also increase the SNRexamination also increase the SNR The use of the appropriate receiver coil plays The use of the appropriate receiver coil plays

extremely important role in optimizing SNRextremely important role in optimizing SNR The The volume of tissuevolume of tissue imaged should optimally imaged should optimally fillfill

the the sensitive volumesensitive volume of the coil of the coil Large coils increaseLarge coils increase the likelihood of the likelihood of aliasingaliasing as as

tissue outside the FOV more likely to produce tissue outside the FOV more likely to produce signalsignal

RF coil typesRF coil types

Commonly usedCommonly used– Volume coil or Bird-cage coilVolume coil or Bird-cage coil– Surface coilSurface coil– Phased array coilPhased array coil

SpecificSpecific– Solenoidal coilSolenoidal coil– Helmholtz pairHelmholtz pair

How to increase SNRHow to increase SNR

Use spin echo sequences where possibleUse spin echo sequences where possible Try not to use a very short TR and a very Try not to use a very short TR and a very

long TElong TE Use the correct coil and ensure that it is Use the correct coil and ensure that it is

well tunedwell tuned Use a coarse matrixUse a coarse matrix Use a large FOVUse a large FOV Select thick slicesSelect thick slices Use as many NEX as possibleUse as many NEX as possible

FOV, Resolution & SNRFOV, Resolution & SNR

FOV = 24cm

High SNR

FOV = 12 cm

High resolution, low SNR

Slice thickness, Resolution & Slice thickness, Resolution & SNRSNR

10 MM SLICE 3 MM SLICE

High resolution, low SNR

NEX & SNRNEX & SNR

NEX = 4, TIME= 6 MIN

High SNR

PARTIAL AVERAGING, TIME 56 S

Contrast to noise Contrast to noise ratio (CNR)ratio (CNR)

CNRCNR Defined as the difference in the SNR Defined as the difference in the SNR

between two adjacent areasbetween two adjacent areas Controlled by the same factors that affect Controlled by the same factors that affect

SNRSNR Most critical factor affecting image qualityMost critical factor affecting image quality Determines the eyes ability to distinguish Determines the eyes ability to distinguish

areas of high signal from areas of low signalareas of high signal from areas of low signal (Although the SNR of T2 weighted image is (Although the SNR of T2 weighted image is

lower than T1 weighted image the ability to lower than T1 weighted image the ability to distinguish tumour from normal tissue is distinguish tumour from normal tissue is greater because of the high signal compared greater because of the high signal compared to the low signal of surrounding anatomy, to the low signal of surrounding anatomy, i.e. CNR is higher)i.e. CNR is higher)

Image contrast depends on:Image contrast depends on:– TRTR– TETE– TITI– Flip angleFlip angle– FlowFlow– Turbo factor (in fast spin echo)Turbo factor (in fast spin echo)– T1T1– T2T2– Proton densityProton density– Magnetization transfer coherence (MTC)*Magnetization transfer coherence (MTC)*

* (see note)* (see note)

Spatial ResolutionSpatial Resolution This is the ability to distinguish between This is the ability to distinguish between

two points as separate and distinct.two points as separate and distinct. Controlled by the voxel sizeControlled by the voxel size Small voxels results in good spatial Small voxels results in good spatial

resolutionresolution The voxel size is affected byThe voxel size is affected by

– Slice thicknessSlice thickness– FOVFOV– Number of pixels or matrixNumber of pixels or matrix

The spatial resolution is increased by using The spatial resolution is increased by using rectangular FOV for rectangular anatomyrectangular FOV for rectangular anatomy

Rectangular FOV reduces the scan timeRectangular FOV reduces the scan time

Matrix & resolutionMatrix & resolution

Matrix 256 x 256 Matrix 512 x 256

Scan timeScan time

Short scan times are important to Short scan times are important to minimize the possibility of patient minimize the possibility of patient movementmovement

Factors that affect the scan time areFactors that affect the scan time are– TR – time of each repetitionTR – time of each repetition– Number of phase encodings – number of Number of phase encodings – number of

lines of K spacelines of K space– NEX – number of times data is collected NEX – number of times data is collected

with the same phase encoding gradient with the same phase encoding gradient

How to reduce the scan How to reduce the scan timetime

Use the shortest TR possibleUse the shortest TR possible Select the coarsest matrix possibleSelect the coarsest matrix possible Reduce the NEX to a minimumReduce the NEX to a minimum

Trade-offsTrade-offs

There are many trade-offs when There are many trade-offs when selecting parameters within a pulse selecting parameters within a pulse sequencesequence

Ideally an image should have Ideally an image should have – high SNR, high SNR, – Good spatial resolution Good spatial resolution – a very short scan time.a very short scan time.

But when improving one factor inevitably But when improving one factor inevitably reduces one or both of other tworeduces one or both of other two

Decision makingDecision making

Selections depend onSelections depend on– The area to be examinedThe area to be examined– Condition and co-operation of the Condition and co-operation of the

patientpatient– Clinical throughput requiredClinical throughput required

Tips to improve image Tips to improve image qualityquality

Choose the correct coilChoose the correct coil Make sure that the patient is Make sure that the patient is

comfortable ad immobilizedcomfortable ad immobilized Ascertain from the radiologist what Ascertain from the radiologist what

sequences are requiredsequences are required SNR is the most important quality factorSNR is the most important quality factor Keep the scan time as short as possibleKeep the scan time as short as possible

Volume ImagingVolume Imaging

Advantages to demonstrate very Advantages to demonstrate very small lesionssmall lesions

Slice thickness can be reducedSlice thickness can be reduced Entire volume of tissue is excited & Entire volume of tissue is excited &

No slice gapNo slice gap SNR is superiorSNR is superior Fewer NEX can be usedFewer NEX can be used Can look at anatomy in any planeCan look at anatomy in any plane

The scan times are relatively long therefore The scan times are relatively long therefore used in conjunction with faster pulse used in conjunction with faster pulse equencesequences

Slices are sectioned out by a series of phase Slices are sectioned out by a series of phase encoding steps along the slice select axisencoding steps along the slice select axis

Therefore the scan time increasesTherefore the scan time increases But as SNR is increased with increased But as SNR is increased with increased

number of slices the NEX can be reducednumber of slices the NEX can be reduced Isotropic (equal dimensions) pixel give equal Isotropic (equal dimensions) pixel give equal

resolution in any planeresolution in any plane

Scan time = TR Scan time = TR x x NEX NEX xx number of phase number of phase encodings encodings x x number of slice encodingsnumber of slice encodings

Manipulating SNR, image contrast, Manipulating SNR, image contrast, spatial resolution and scan time is a spatial resolution and scan time is a real art and takes some time and real art and takes some time and experienceexperience

Even after many years you may get Even after many years you may get things wrong occasionallythings wrong occasionally

Perseverance is important to get Perseverance is important to get good image qualitygood image quality

ConclusionConclusion