MERGE/MEDIC/M-FFE
We have been using a sequence called MERGE for spinal imaging that shows excellent contrast between cord, CSF and disk. How does this work?
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MERGE ("Multiple Echo Recombined Gradient Echo") is a spoiled T2*-weighted sequence for spinal and musculoskeletal imaging developed by GE Healthcare. The corresponding Siemens sequence is called MEDIC ("Multi-Echo Data Image Combination"), while the Philips sequence is called M-FFE ("Merged Fast Field Echo"). Hitachi offers ADAGE ("Additive Arrangement Gradient Echo"). Acquisition may be in either 2D or 3D mode.
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The basic sequence is pictured right. By reversing the frequency-encoding gradient rapidly, several individual gradient echoes can be generated at different TEs. The number of echoes is limited by T2*-decay, but typically between 3-5 echoes are recorded.
The individual echoes are magnitude reconstructed and then combined into a single image using a sum of squares algorithm. |
MERGE/MEDIC pulse sequence.
De = dephase, Re = rephase gradient lobes. Typical parameters TR=500-1000, TE=5-20, α = 20-30° |
MEDIC C-spine image
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MERGE Hindfoot image
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References
Martin N, Malfair D, Zhao Y, et al. Comparison of MERGE and axial T2-weighted fast spin-echo sequences for detection of multiple sclerosis lesions in the cervical spinal cord. AJR 2012; 199:157–162.
Merkel R, Laine AF, Wu EX. Combined MR data acquisition of multicontrast images using variable acquisition parameters and k-space data sharing. IEEE Trans Med Imaging 2003;22:806-823.
Nitz WR. Fast and ultrafast non-echo-planar MR imaging techniques. Eur Radiol 2002; 12:2866-2882.
Martin N, Malfair D, Zhao Y, et al. Comparison of MERGE and axial T2-weighted fast spin-echo sequences for detection of multiple sclerosis lesions in the cervical spinal cord. AJR 2012; 199:157–162.
Merkel R, Laine AF, Wu EX. Combined MR data acquisition of multicontrast images using variable acquisition parameters and k-space data sharing. IEEE Trans Med Imaging 2003;22:806-823.
Nitz WR. Fast and ultrafast non-echo-planar MR imaging techniques. Eur Radiol 2002; 12:2866-2882.
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