Concerning image contrast in spin–echo imaging: a. The best contras

1.	Concerning image contrast in spin–echo imaging: a. The best contrast between tissues with different T1s is obtained with short repetition times (TRs) and short echo times (TEs) b. T2 weighting is obtained with short TEs and long TRs c. An image obtained with a TE of 15 ms and a TR of 2000 ms would be proton density (PD) weighted d. Fat appears with a high signal on T1-, T2- and PD-weighted sequences e. The fat signal is partially suppressed on fast (turbo) spin–echo imaging
Answer» a. True. T₁ weighting is obtained with a short TR, usually between 300 and 800 ms (close to the T₁s of the tissues being imaged). With increasing TR, the longitudinal magnetization in all tissues would recover more, diminishing the T₁ contrast. Short TEs are used to reduce the effect of T₂ relaxation on contrast (see below). b. False. Relatively long TEs (90–140 ms, close to the T₂s of the tissues being imaged) are used so measurable differences in the degree of transverse magnetization can develop. With short TEs, these differences are small; therefore, short TEs reduce the effect of T₂ weighing. Long TRs are used to allow near-full recovery of longitudinal magnetization and eliminate differences in contrast due to T₁. c. True. As described above, a short TE and long TR reduce the effect of T₂ and T₁, respectively. The remaining contrast is only dependent on the density of hydrogen nuclei (PD weighted). d. True. Due to a short T₁ and a higher PD in comparison with most tissues, fat is bright on these sequences. In T₂-weighted images, fat is less bright than water, but still brighter than most other tissues. e. False. On fast spin–echo images, fat appears hyperintense. This results from the multiple refocusing pulses, which suppress a phenomenon known as J-coupling (an interaction between different nuclei within the fat molecules), resulting in fat having a longer T₂ and therefore appearing brighter.

Concerning image contrast in spin–echo imaging: a. The best contrast between tissues with different T1s is obtained with short repetition times (TRs) and short echo times (TEs) b. T2 weighting is obtained with short TEs and long TRs c. An image obtained with a TE of 15 ms and a TR of 2000 ms would be proton density (PD) weighted d. Fat appears with a high signal on T1-, T2- and PD-weighted sequences e. The fat signal is partially suppressed on fast (turbo) spin–echo imaging

Answer»

a. True. T₁ weighting is obtained with a short TR, usually between 300 and 800 ms (close to the T₁s of the tissues being imaged). With increasing TR, the longitudinal magnetization in all tissues would recover more, diminishing the T₁ contrast. Short TEs are used to reduce the effect of T₂ relaxation on contrast (see below).

b. False. Relatively long TEs (90–140 ms, close to the T₂s of the tissues being imaged) are used so measurable differences in the degree of transverse magnetization can develop. With short TEs, these differences are small; therefore, short TEs reduce the effect of T₂ weighing. Long TRs are used to allow near-full recovery of longitudinal magnetization and eliminate differences in contrast due to T₁.

c. True. As described above, a short TE and long TR reduce the effect of T₂ and T₁, respectively. The remaining contrast is only dependent on the density of hydrogen nuclei (PD weighted).

d. True. Due to a short T₁ and a higher PD in comparison with most tissues, fat is bright on these sequences. In T₂-weighted images, fat is less bright than water, but still brighter than most other tissues.

e. False. On fast spin–echo images, fat appears hyperintense. This results from the multiple refocusing pulses, which suppress a phenomenon known as J-coupling (an interaction between different nuclei within the fat molecules), resulting in fat having a longer T₂ and therefore appearing brighter.

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