MRI Tutorial - how is contrast generated?

Question 1

How can T2WI be generated? Give approximate TR and TE values and why. What type of sequence is most appropriate? How does water appear on these images? (8 marks)

Answer 1

• Long TR to maximise signal from longitudinal component and minmise differences that T1 in different tissues will make to the signal ~ 3s for a long TR.
• Long TE to allow differences in T2 for different tissues to be seen and generate contrast ~ 100 ms.
• Use a spin-echo to gain more signal by re-focusing phases and compensate for spatial B-field inhomogenities.
• Water appears bright on T2WI as it has a long T2 and therefore decays slowly.

Question 2

How can T1WI be generated? Give approximate TR and TE values and why. What type of sequence is most appropriate? How does water appear on these images? (8 marks)

Answer 2

• Short TR to allow differences in T1 in tissues to generate contrast in the image, TR should be approx less than 750 ms.
• Short TE to maximise signal from M_x,y component and minimise differences from T2 in different tissues. TE should be approx 10 ms.
• Use a gradient echo as this allows faster imaging to be carried out and we do not care about obtaining T2 or T2*.
• Water appears dark on a T1WI as it recovers slowly due to a longer T1.

Question 3

What TR and TE should be selected on a PD scan and why? (3 marks)

Answer 3

• We want to maximise the signal generated and minimise the differences that T2 and T1 between different tissues will make.
• Long TR and a short TE will do this. TR ~ 3s, TE ~ 10 ms.

Question 4

What are the equations for transverse and longitudinal signal? And CNR (3 marks).

Answer 4

S_trans = S_trans(0)*exp(-TE/T2)

S_long = S_long(0)*[1-exp(-TR/T1)]

CNR = (S_a - S_b)/noise