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 SE T2 FSE “CYST”

T1 SE T2 FSE CYST?

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

Image Contrast 100% noise

Image Contrast 80% noise

Image Contrast 60% noise

Image Contrast 40% noise

Image Contrast 20% noise

Image Contrast 0% noise

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

SNR

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 Magnet S/N effectDownside linear increaseless T1 weighting at high fields increased chemical shift effects in RO direction

Coil Selection S/N effectDownside 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 Size S/N effectDownside linear increase in either RO or PE direction linear increase with increased slice thickness decreased resolution

Decreased Phase Encodings S/N effectDownside 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 Averages S/N effectDownside square root increase in signal to noise linear increase in scan time

Decreased Receiver BW S/N effectDownside 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

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 water plus fat water only