Tissue Contrast• intrinsic factors

– relative quantity of protons• tissue proton density

– relaxation properties of tissues• T1 & T2 relaxation

• secondary factors– flow– contrast agents

Contrast

• the ability to discriminate different tissues based on their relative brightness

Basic Principle

• relatively intermediate intensity structures look bright on a dark background– important to remember with fatsat

• relatively intermediate intensity structures look dark on a light background

Caveat

• windowing affects the relative contrast of tissues– intensity values of pixels are relative

to one another, unlike CT• windowing can make a solid tumor

look like a “cyst”

T1 SET2 FSE

“CYST”

T1 SET2 FSE

CYST?

T2 FSE T2 FSE

CYST?

Summary

• need visible differences in intensity to discriminate tissues

• surrounding tissues can make an intermediate signal tissue appear dark or bright

• windowing affects image and tissue contrast

Noise

• constant at a given machine setup• reduces the ability to visualize low

contrast structures• adds to or subtracts from the

average signal intensity of a given pixel

Noise

• increasing the available signal will reduce the relative effects of noise

• machine parameters must be chosen to maximize signal without significantly extending exam times

• S/N is a relative measure allowing for comparison in a variety of circumstances

frequency

SI

frequency

SI

Signal versus Noise

• high signal• high SNR

• low signal• low SNR

Noiseless Conditions

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Image Contrast

100% noise

Image Contrast

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Factors Affecting SNR

• strength of main magnet• coil selection• voxel size• phase encoding• number of averages• receiver bandwidth• pulse sequence parameters

SNR

SNR K voxel size measurementsbandwidth= • •( )

• stronger main magnet• proper imaging coil• larger voxel size• decreased phase encoding• increased number of averages• decreased receiver bandwidth• (pulse sequence parameters)

Factors INCREASING SNR

Stronger Main MagnetS/N effect Downside

• linear increase • less T1 weighting at high fields

• increased chemical shift effects in RO direction

Coil SelectionS/N effect Downside

• increase in signal with surface coils

• quadrature provides 40% increase S/N over linear

• phased array increased over quadrature

• limited coverage with surface coils

• more complex coils are more expensive

Larger Voxel SizeS/N effect Downside

• linear increase in either RO or PE direction

• linear increase with increased slice thickness

• decreased resolution

Decreased Phase EncodingsS/N effect Downside

• square root increase in signal to noise

• linear decrease in scan time

• decreased resolution in PE direction

• Gibb’s phenomenon in PE direction

Increased Signal AveragesS/N effect Downside

• square root increase in signal to noise

• linear increase in scan time

Decreased Receiver BWS/N effect Downside

• square root increase in signal to noise

• increase in chemical shift artifact in RO direction

Pulse Sequence Parameters

• SE imaging– increased TR provides nonlinear

increase in SNR with linear increase in scan time

– decreased TE provides nonlinear increase in SNR with no effect on scan time and less T2 weighting

Pulse Sequence Parameters

• GE imaging– complex effects– maximum SNR typically between 30

and 60 degrees– long TR sequences (2D)

• increase SNR with increased flip angle– short TR sequences (TOF & 3D)

• decreased SNR with increased flip angle

SNR Application

• pituitary imaging– baseline:

• 16 cm FOV, 3 mm slice thickness, 192 phase encodes, 4 NEX

– new goal:• reduced scan time, same SNR

FOV RO PE

Slice Thickness

(mm) NEX

Imaging Time (TR=500 msec)

Relative SNR

160 256 192 3 4 6.40 43.30160 256 170 4 2 2.83 43.39190 256 192 3 2 3.20 43.18160 256 144 3 3 3.60 43.30

SNR Example

Fat Suppression and SNR

• non fat-suppressed image– each image pixel comprised of signal

from water and fat in the imaging voxel• fat-suppression

– reduces total signal by suppression of fat from the voxel

– reduces SNR

frequency

SI

frequency

SI

Fat Suppression• without fat

suppresion• high SNR

• with fat suppression

• lower SNR

waterplusfat water only