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Understanding Tissue Contrast and Signal-to-Noise Ratio in Imaging

This overview discusses the intrinsic and secondary factors affecting tissue contrast in imaging, focusing on the role of tissue proton density and relaxation properties (T1 and T2). Emphasis is placed on how relative brightness helps discriminate between tissues, the significance of windowing in image contrast, and the impact of noise on visualization of low-contrast structures. Key factors that influence the signal-to-noise ratio (SNR), such as magnet strength, coil selection, and voxel size, are also explored to optimize imaging conditions for accurate diagnosis.

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Understanding Tissue Contrast and Signal-to-Noise Ratio in Imaging

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Presentation Transcript

  1. Tissue Contrast • intrinsic factors • relative quantity of protons • tissue proton density • relaxation properties of tissues • T1 & T2 relaxation • secondary factors • flow • contrast agents

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

  3. 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

  4. 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”

  5. “CYST” T2 FSE T1 SE

  6. CYST? T2 FSE T1 SE

  7. CYST? T2 FSE T2 FSE

  8. 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

  9. 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

  10. 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

  11. high signal high SNR low signal low SNR SI SI frequency frequency Signal versus Noise

  12. Image Contrast 100% noise

  13. Image Contrast 80% noise

  14. Image Contrast 60% noise

  15. Image Contrast 40% noise

  16. Image Contrast 20% noise

  17. Image Contrast 0% noise

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

  19. SNR

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

  21. linear increase less T1 weighting at high fields increased chemical shift effects in RO direction Stronger Main MagnetS/N effect Downside

  22. 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

  23. Larger Voxel SizeS/N effect Downside • linear increase in either RO or PE direction • linear increase with increased slice thickness • decreased resolution

  24. 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

  25. Increased Signal AveragesS/N effect Downside • square root increase in signal to noise • linear increase in scan time

  26. Decreased Receiver BWS/N effect Downside • square root increase in signal to noise • increase in chemical shift artifact in RO direction

  27. 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

  28. 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

  29. SNR Application • pituitary imaging • baseline: • 16 cm FOV, 3 mm slice thickness, 192 phase encodes, 4 NEX • new goal: • reduced scan time, same SNR

  30. SNR Example

  31. 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

  32. without fat suppresion high SNR with fat suppression lower SNR SI SI frequency frequency Fat Suppression water plus fat water only

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