Which of the following are true regarding opposed-phase imaging? a. I

1.	Which of the following are true regarding opposed-phase imaging? a. It is most commonly used with gradient-echo sequences b. In a 1.5 T magnet, the magnetization of fat and water becomes out of phase every 220 ms c. A gradient echo obtained with a TE of 6.9 ms at 1.5 T would represent a summation of signals from fat and water d. The most common application of out-of-phase imaging is in the diagnosis of hepatic and adrenal lesions e. Out-of-phase images show a characteristic ‘India ink’ appearance
Answer» a. True. It relies on the phase differences arising due to different resonance frequencies of water and fat. The echo time (TE) is chosen such that the echo forms when the water and fat signals have a 180° phase difference between them. In standard spin-echo sequences, these phase differences would be eliminated by the 180° refocusing pulse. b. False. The difference in Larmor frequency between fat and water at 1.5 T is approximately 220 Hz. Fat and water will therefore be in phase at every multiple of 1000/220 = 4.6 ms (i.e. at 4.6, 9.2, 13.8 ms, etc.). Hence, they will be out of phase mid-way between these time points (i.e. at 2.3, 6.9, 11.5 ms etc.). c. False. At this time point, fat and water are out of phase, so the resulting signal would represent the difference in the fat and water magnetizations in that voxel. d. True. Fatty lesions of these organs that contain both fat and water will show signal hypointensity on the out-of-phase images in comparison with the corresponding in-phase images. e. True. At interfaces where voxels contain both water and fat, in the out-of-phase images the magnetization cancels, resulting in a one-voxel thin black line as if drawn with ink.

Which of the following are true regarding opposed-phase imaging? a. It is most commonly used with gradient-echo sequences b. In a 1.5 T magnet, the magnetization of fat and water becomes out of phase every 220 ms c. A gradient echo obtained with a TE of 6.9 ms at 1.5 T would represent a summation of signals from fat and water d. The most common application of out-of-phase imaging is in the diagnosis of hepatic and adrenal lesions e. Out-of-phase images show a characteristic ‘India ink’ appearance

Answer»

a. True. It relies on the phase differences arising due to different resonance frequencies of water and fat. The echo time (TE) is chosen such that the echo forms when the water and fat signals have a 180° phase difference between them. In standard spin-echo sequences, these phase differences would be eliminated by the 180° refocusing pulse.

b. False. The difference in Larmor frequency between fat and water at 1.5 T is approximately 220 Hz. Fat and water will therefore be in phase at every multiple of 1000/220 = 4.6 ms (i.e. at 4.6, 9.2, 13.8 ms, etc.). Hence, they will be out of phase mid-way between these time points (i.e. at 2.3, 6.9, 11.5 ms etc.).

c. False. At this time point, fat and water are out of phase, so the resulting signal would represent the difference in the fat and water magnetizations in that voxel.

d. True. Fatty lesions of these organs that contain both fat and water will show signal hypointensity on the out-of-phase images in comparison with the corresponding in-phase images.

e. True. At interfaces where voxels contain both water and fat, in the out-of-phase images the magnetization cancels, resulting in a one-voxel thin black line as if drawn with ink.

Discussion

No Comment Found

Related InterviewSolutions

Reply to Comment