What factors are known to increase contrast to noise ratio (CNR)?

• A. T1 weighting and short TEs
• B. T2 weighting and long TEs
• C. TE shortening and TE lengthening
• D. High field strength and short relaxation times

Answer: (B)

Increasing the contrast to noise ratio (CNR) is essential for improving image quality in magnetic resonance imaging (MRI). T2 weighting is particularly effective in enhancing contrast because it emphasizes differences between tissues that have varying T2 relaxation times, which is crucial for distinguishing between pathologic and normal tissues. Longer echo times (TEs) allow for greater relaxation effects to occur, enhancing the contrast between tissues that have different T2 times.

Shortening the TE will decrease the T2 contrast since there is less time for spins to relax before the signal is measured. On the other hand, increasing the TE allows for the full manifestation of T2 differences between various tissues, thereby increasing the CNR. Therefore, T2 weighting combined with longer TEs is known to improve CNR by highlighting differences in tissue characteristics effectively.

High field strength and short relaxation times could enhance overall signal strength but does not directly relate to how T2 weighting and long TEs can influence contrast effectively.