Magnetic Resonance Imaging (MRI) Practice Test

In a dual contrast spin echo sequence with echo times of 25ms and 90ms, the second echo image has what compared to the first?

1. Less T2 contrast and higher SNR

2. More T2 contrast and lower SNR

3. Equal T2 contrast

4. Higher T1 contrast

The correct answer is: More T2 contrast and lower SNR

In a dual contrast spin echo sequence, the timing of the echoes allows for the manipulation of T2 weighting in the images obtained. The first echo is captured at a shorter echo time (25ms), which provides a balance of T1 and T2 contrast. In contrast, the second echo is acquired at a longer echo time (90ms), leading to more pronounced T2 contrast because T2 relaxation effects become more significant over a longer time period. As the echo time increases, the signal intensity of tissues with shorter T2 relaxation times decreases more rapidly than that of tissues with longer T2 relaxation times. Therefore, the longer echo time enhances the differences between tissues with varying T2 values, resulting in more T2 contrast in the second echo. However, as echo time increases, the signal-to-noise ratio (SNR) typically decreases due to the inherent decay of the signal over time as well as potential contributions of noise that may become more pronounced. Thus, while the second echo yields more T2 contrast, it generally does so at the expense of SNR. This underscores the nature of imaging where adjustments in timing can favor specific contrasts but may also impact overall image quality as indicated by SNR.