MRI Image Quality

GGamal amal FFathalla athalla MM.. MMahdalyahdaly

gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com

Experta Medica

Factors affecting digital image Factors affecting digital image QualityQuality

Signal to Noise Ratio:Signal to Noise Ratio:The amount of true data building the image The amount of true data building the image

relative to the amount of false data relative to the amount of false data interfering with the image data during image interfering with the image data during image reconstruction.reconstruction.

In MRI, being unavoidable, noise should not In MRI, being unavoidable, noise should not exceed signal & the ratio SNR should be exceed signal & the ratio SNR should be around 1.around 1.

In CT, the noise sources are much less, so a In CT, the noise sources are much less, so a different noise index was proposed.different noise index was proposed.

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Factors affecting Signal to Factors affecting Signal to Noise Ratio (SNR)Noise Ratio (SNR)

Receive Bandwidth (RBW) describes Receive Bandwidth (RBW) describes the window of frequencies, the the window of frequencies, the receive coil tuned to receive. receive coil tuned to receive.

The received frequencies are of both The received frequencies are of both Signal & Noise.Signal & Noise.

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Sources of the Noise in MRISources of the Noise in MRI• Random fluctuations in electrical current (electronic noise): exists in all electrical conductors including the MR coils with which we measure the signal, but it also includes the electrically conducting tissues of the patient. •Human tissue contains many ions such as sodium, potassium and chloride which are electrically charged atomic particles carrying electrical currents within the body, e.g. in nerve conduction.•These currents generate fluctuating magnetic fields which induce a noise voltage in the coil.•Pulsatile flow is also implicated in noise generation as the pulsed spins generate random electric fluctuations as well.• Remedy Tips: use a small or dedicated anatomy coil. Where large fields of view are essential, phased array coils are usually best.

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Receive Bandwidth (RBW)Receive Bandwidth (RBW) The frequency range of the receiver can The frequency range of the receiver can

be related to the frequency used to be related to the frequency used to encode each pixel and therefore encode each pixel and therefore determine the extent of the chemical-shift determine the extent of the chemical-shift artifact.artifact.

For example, for a 256 frequency matrix For example, for a 256 frequency matrix and bandwidth of 32 kHz, there are 125 Hz and bandwidth of 32 kHz, there are 125 Hz per pixel so the fat–water shift is per pixel so the fat–water shift is approximately 2 pixels. Typical approximately 2 pixels. Typical bandwidths vary from 6.5 kHz to 1 MHz.bandwidths vary from 6.5 kHz to 1 MHz.

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Bandwidth ( back to Larmor Equation)

/2

Where is the larmor frequency Gyromagnetic ratio Gyromagnetic ratio /2= 42.57 MHz / Tesla for = 42.57 MHz / Tesla for protonproton

RBW RBW being the whole repertoire of received frequencies, we can say:being the whole repertoire of received frequencies, we can say:

RBW = 42.57 RBW = 42.57

For 1T systems, RBW should be in the range of 42.57 KHzFor 1T systems, RBW should be in the range of 42.57 KHzFor 1.5T systems, RBW is in the range of 64 KHzFor 1.5T systems, RBW is in the range of 64 KHzFor 3T systems, RBW is in the range of 128 KHzFor 3T systems, RBW is in the range of 128 KHz

The question is how can we select the suitable RBW from these The question is how can we select the suitable RBW from these ranges & is it dependable on the pulse sequence?ranges & is it dependable on the pulse sequence?

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Effect of RBW on SNRWe should consider three factors: •The Fat/water phase shift•The SNR •The Spatial Resolution

Good SNR with Narrow RBW Poor SNR with Wide RBWNoise remains always Constant & Fluctuating

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Effect of RBW on SNR

Sampling rate controls the receive bandwidth•To narrow the receive bandwidth the system samples slower & consequently the read out time increases & vice versa

•Slower sampling requires a longer readout time

•Faster sampling allows a shorter readout time

Select BW to balance minimum TE, minimum ES versus SNR Consider the impact on chemical shift

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Factors affecting Signal to Noise Ratio (SNR)

RBW : As the RBW widens, SNR decreases.

Number of Excitations ( NEX): As NEX increases, SNR also increases because each excitation generates part of the signal with constant fluctuating noise.

Field of View (FOV): As FOV increases, SNR also increases due to increase of signal producing spins.

Slice Thickness: As it increases, SNR also increases.

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Spatial Resolution It is the ability to discriminate between 2 neighboring points in any image i.e. to see image details.

To be read on a 2D screen, the 3D voxel content is interpreted into 2D pixel

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Voxel and pixelVoxel and pixel

Voxel =Voxel =(Volume element)(Volume element)

Pixel =Pixel =(Picture (Picture

element)element) Experta Medica

To be read on a 2D screen, the 3D voxel content is interpreted into 2D pixel

The system averages

the intensities In the pixel, therebydecreasing spatial resolution

Partial volume averaging (PVA)

Most approaches to enhance spatial resolution are in deed to overcome PVA

Field of view (FOV):Field of view (FOV):The diameter of reconstructed imageThe diameter of reconstructed image

MatrixMatrix: : Orderly array of cells fashioned in rows and Orderly array of cells fashioned in rows and

columns. columns.

In MRI, we have a frequency matrix &In MRI, we have a frequency matrix & a phase matrixa phase matrix

Spatial Resolution

Factors affecting spatial resolution:

• RBW: as RBW widens, spatial resolution increases as the data building the image ( signal frequencies) increases.

• Matrix (Phase & Frequency dimensions): as matrix increases, PVA decreases increasing spatial resolution.

• Thickness (Slice dimension): as thickness decreases, PVA decreases increasing the spatial resolution.

• FOV: as FOV decreases, PVA decreases, increasing the spatial resolution

128 256 512

2020

512

512

256

256

Matrix ZIP is a zero-fill interpolation to reconstruct images in a 512x512or 1024x1024 matrix even though acquired in a different gradient encoding configuration ( i.e. to spread image data on a larger matrix).

Spatial Resolution Matrix ZIP

2222

Matrix ZIP increases the visibility of resolution inherent in theImage: Note the increased sharpness of the vessel edges

256 encoded/512 recon 256 encoded/256 recon

Spatial Resolution Matrix ZIP

Contrast Resolution Contrast or Grey Scale Resolution is the ability to resolve the grey color into more color grades.

Contrast Resolution demonstrates the ability to differentially diagnose different lesions according to their color index on the grey scale.

In MRI, we have several contrasts in each pulse sequence.

Each pulse sequence has its weighting (contrast – creating) factor(s).

Grey ScalingThe MRI reconstruction process results in a 2D matrix of floating point numbers in the computer which range from near 0.0 up to value equal to 1.0

These numbers correspond to the average signal intensities of the tissue contained in each voxel

The MR images are normalized and truncated to integer values that encompass 4096 values, between -1000 and 3095 (typically)

Rescaling these values to definite shades of grey produces the Grey Scale.

Window width / window Window width / window levellevel

The human tissue MR signal intenseties extend over a narrow range (window) of the total signal spectrum.

Window width represents the MR signal intensities of all the tissues of interest which are displayed as various shades of grey.Tissues with intensities outside this range are displayed as either black or white.

Window Level represents the central number of all the intensities covered by the window width. This should be set as close as possible to the mean MR intensity of the tissue of interest.

Both WW & WL can be set independently & their respective settings affect the contrast & brightness of the displayed image.Unlike CT, in MRI, we have multiple contrasts in each pulse sequence with different windowing.

The Concept of Contrast (or The Concept of Contrast (or Weighting)Weighting)

ContrastContrast = difference in RF signals — emitted = difference in RF signals — emitted by water protons — between different tissuesby water protons — between different tissues

T1 weighted example: gray-white contrast is T1 weighted example: gray-white contrast is possible because possible because T1T1 is different between is different between these two types of tissuethese two types of tissue

T2 Decay

MRSignal

T1 Recovery

MRSignal

50 ms50 ms 1 s1 s

Static Contrast Imaging Methods

Optimal TR and TE for Proton Density Contrast

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T2 Decay

MR

Sig

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t (ms)t (s)

MR

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TRTR TETE

T1 Recovery

Optimum TR corresponds to the end of T1 recovery

Optimum TE corresponds to the beginning of T2 decay

Proton Density ContrastProton Density Contrast

Technique: use very long time between RF shots (large TR) and very short delay between excitation and readout window (short TE)

Useful for anatomical reference scans

Several minutes to acquire 256256128 volume

~1 mm resolution

Proton Density Weighted ImageProton Density Weighted Image

Optimal TR and TE for T2* and T2 Contrast

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T2 Decay

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T1 Recovery

TRTR TETE

T1 ContrastT1 Contrast T2 ContrastT2 Contrast

T2* and T2 ContrastT2* and T2 Contrast Technique: use long TR and intermediate to long TE in SE & FSE Use an itermediate TE & TR for GRE with a narrow flip angle ( 15-30) In F-GRE, use a very short TE & TR & very narrow flip angle. Useful for functional (T2* contrast) and anatomical (T2 contrast to enhance fluid contrast) studies 1mm resolution for anatomical scans or 4 mm resolution [better is possible with better gradient system, and a little longer time per volume]

T2 Weighted T2 Weighted ImageImage

Optimal TR and TE for T1 Contrast

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T1 contrast T2 contrast

T2 Decay

MR

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MR

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Recovery

TRTR TETE

T1 Contrast

Technique: use intermediate timing between RF shots (intermediate TR) and very short TE, also use large flip angles for GRE & MEMP

Useful for creating gray/white matter contrast for anatomical reference

~1 mm resolution

T1 Weighted T1 Weighted ImageImage

-S-So

SSoS = SS = Soo * (1 – 2 e * (1 – 2 e –t/T1–t/T1))

S = SS = Soo * (1 – 2 e * (1 – 2 e –t/T1’–t/T1’))

Inversion Recovery for Extra T1 Inversion Recovery for Extra T1 ContrastContrast

                                                        

        

T2T2 Inversion RecoveryInversion Recovery(CSF Attenuated)(CSF Attenuated)

In summary, TR controls T1 weighting and In summary, TR controls T1 weighting and TE controls T2 weighting. Short T2 tissues TE controls T2 weighting. Short T2 tissues are dark on T2 images, but short T1 tissues are dark on T2 images, but short T1 tissues are bright on T1 images.are bright on T1 images.

Motion Contrast Imaging MethodsMotion Contrast Imaging Methods Can “prepare” magnetization to make Can “prepare” magnetization to make

readout signal sensitive to different motion readout signal sensitive to different motion propertiesproperties– Flow weighting (bulk movement of blood) Flow weighting (bulk movement of blood) – Diffusion weighting (scalar or tensor)Diffusion weighting (scalar or tensor)– Perfusion weighting (blood flow into capillaries) Perfusion weighting (blood flow into capillaries)

MR AngiogramMR Angiogram

•Time-of-Flight Time-of-Flight ContrastContrast

•Phase ContrastPhase Contrast

Time-of-Flight ContrastTime-of-Flight Contrast

No Flow

Medium Flow

High Flow

No Signal

Medium Signal

High Signal

Vessel

Acquisition

Saturation Excitation

Vessel Vessel

90o

Excitation

ImageAcquisition

RF

Gx

Gy

Gz

90o

Saturation

Time to allow fresh flow enter the slice

Pulse Sequence: Time-of-Flight Contrast

Phase Contrast (Velocity Encoding)

Externally AppliedSpatial Gradient G

Externally AppliedSpatial Gradient -G

Blood Flow v2

0

2)()(

GvT

dtvtxGdtvtxGT T

T

Time

T2T0

90o

Excitation

Phase

Image

Acquisition

RF

Gx

Gy

Gz

G

-G

Pulse Sequence: Phase Contrast

MR AngiogramMR Angiogram

Diffusion Weighted Imaging Diffusion Weighted Imaging SequencesSequences

Dtl 2

Externally AppliedExternally AppliedSpatial Gradient Spatial Gradient GG

Externally AppliedExternally AppliedSpatial Gradient -Spatial Gradient -GG

TimeTime

TT 2T2T00

322

32 TGD

oeSS

Pulse Sequence: Gradient-Echo Pulse Sequence: Gradient-Echo Diffusion WeightingDiffusion Weighting

90o

Excitation

Image

Acquisition

RF

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Gy

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-G

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Excitation

Image

Acquisition

RF

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Pulse Sequence: Spin-Echo Diffusion Weighting

Advantages of Advantages of DWIDWI

1.1. The absolute magnitude of the diffusion The absolute magnitude of the diffusion coefficient can help determine proton pools coefficient can help determine proton pools with different mobilitywith different mobility

2.2. The diffusion direction can indicate fiber tracksThe diffusion direction can indicate fiber tracks

3.3. DWI defines the diffusion characteristics of tumorsDWI defines the diffusion characteristics of tumors

Diffusion Anisotropy

Determination of fMRI Using the Directionality of

Diffusion Tensor

Display of Diffusion Tensor Using Ellipsoids

Diffusion ContrastDiffusion Contrast

PerfusionPerfusionDiffusionDiffusion

Diffusion and Perfusion ContrastDiffusion and Perfusion Contrast

Part III.3Part III.3Some fundamental acquisition methods Some fundamental acquisition methods

And their k-space viewAnd their k-space view

k-Space Recap

Kx = Kx = /2/200ttGx(t) dtGx(t) dt

Ky = Ky = /2/200ttGx(t) dtGx(t) dt

Equations that govern k-space trajectory:Equations that govern k-space trajectory:

These equations mean that the k-space coordinatesThese equations mean that the k-space coordinatesare determined by the area under the gradient waveformare determined by the area under the gradient waveform

dxdyeyxIkkS ykxkiyx

yx )(2),(),(

Gradient Echo ImagingGradient Echo Imaging Signal is generated by magnetic field Signal is generated by magnetic field

refocusing mechanism only (the use of refocusing mechanism only (the use of negative and positive gradient)negative and positive gradient)

It reflects the uniformity of the magnetic fieldIt reflects the uniformity of the magnetic field Signal intensity is governed by Signal intensity is governed by S = So eS = So e-TE/T2*-TE/T2*

where where TETE is the echo time (time from is the echo time (time from excitation to excitation to

the center of k-space)the center of k-space) Can be used to measure T2* value of the Can be used to measure T2* value of the

tissuetissue

MRI Pulse Sequence for Gradient MRI Pulse Sequence for Gradient Echo ImagingEcho Imaging

digitizer ondigitizer on

ExcitationExcitation

SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding

PhasePhase EncodingEncoding

ReadoutReadout

K-space view of the gradient echo K-space view of the gradient echo imagingimaging

Kx

Ky

123.......n

Multi-slice acquisitionMulti-slice acquisition

Total acquisition time =Total acquisition time = Number of views * Number of excitations * TRNumber of views * Number of excitations * TR

Is this the best we can do?Is this the best we can do?

Interleaved excitation methodInterleaved excitation method

readoutreadout

ExcitationExcitation

SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding

PhasePhase EncodingEncoding

ReadoutReadout

readoutreadout readoutreadout

…………

…………

…………

TRTR

Spin Echo ImagingSpin Echo ImagingSignal is generated by radiofrequency

pulse refocusing mechanism (the use of 180o pulse )

It doesn’t reflect the uniformity of the magnetic field

Signal intensity is governed by S = So e-TE/T2

where TE is the echo time (time from excitation to

the center of k-space)Can be used to measure T2 value of

the tissue

MRI Pulse Sequence for Spin Echo Imaging

digitizer ondigitizer on

ExcitationExcitation

SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding

PhasePhase EncodingEncoding

ReadoutReadout

9090 180180

K-space view of the spin echo imaging

Kx

Ky

123.......n

Fast ImagingFast ImagingHow fast is “fast imaging”?How fast is “fast imaging”?

In principle, any technique that can generate In principle, any technique that can generate an entire image an entire image with sub-second temporal resolution can be with sub-second temporal resolution can be called fast imaging.called fast imaging.

For fMRI, we need to have temporal resolution For fMRI, we need to have temporal resolution on the order of on the order of a few tens of a few tens of msms to be considered “fast”. Echo- to be considered “fast”. Echo-planar imaging, planar imaging, spiral imaging can be both achieve such spiral imaging can be both achieve such speed.speed.

Echo Planar Imaging (EPI)Echo Planar Imaging (EPI)

Methods shown earlier take multiple RF shots to Methods shown earlier take multiple RF shots to readout enough data to reconstruct a single imagereadout enough data to reconstruct a single image– Each RF shot gets data with one value of phase Each RF shot gets data with one value of phase

encodingencoding If gradient system (power supplies and gradient coil) If gradient system (power supplies and gradient coil)

are good enough, can read out all data required for are good enough, can read out all data required for one image after one RF shotone image after one RF shot– Total time signal is available is about Total time signal is available is about 22T2* T2* [80 ms][80 ms]

Must make gradients sweep back and forth, doing all Must make gradients sweep back and forth, doing all frequency and phase encoding steps in quick frequency and phase encoding steps in quick successionsuccession

Can acquire 10-20 low resolution 2D images per Can acquire 10-20 low resolution 2D images per secondsecond

......

...

Pulse Sequence K-space View

Why EPI?Why EPI?

Allows highest speed for dynamic contrastAllows highest speed for dynamic contrast Highly sensitive to the susceptibility-induced Highly sensitive to the susceptibility-induced

fieldfield changes --- important for fMRIchanges --- important for fMRI Efficient and regular k-space coverage and Efficient and regular k-space coverage and

good good signal-to-noise ratiosignal-to-noise ratio Applicable to most gradient hardwareApplicable to most gradient hardware

Spiral ImagingSpiral Imaging

t = TE

RFRF

GxGx

GyGy

GzGz

t = 0

KK-Space -Space Representation of Representation of

Spiral Image Spiral Image AcquisitionAcquisition

Why Spiral?Why Spiral?

• More efficient More efficient kk-space trajectory to improve-space trajectory to improve throughput.throughput.• Better immunity to flow artifacts (no gradient atBetter immunity to flow artifacts (no gradient at the center of k-space)the center of k-space)• Allows more room for magnetization preparation,Allows more room for magnetization preparation, such as diffusion weighting.such as diffusion weighting.

Under very homogeneous magnetic field, images look good …

Gradient-Recalled EPI Images Under Homogeneous Field

Gradient Recalled Spiral Images Under Homogeneous Field

However, if we don’t have a homogeneous field …(That is why shimming is VERY important in fast imaging)

Distorted EPI Images with Imperfect x-Shim

A

B

C

D

Distorted Spiral Images with Imperfect x-Shim

A

B

C

D

Any Question???. Again Any Question???.

Thank you!

Have a nice dayHave a nice day