Spin-Warp Imaging
What is spin-warp imaging? Isn't this just "regular" MRI?
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Since its introduction in the early 1980s, the Fourier transform-based spin-warp imaging technique of Edelstein and colleagues has been the dominant method for spatial encoding the MR signal. If you think about "regular MRI" as acquiring data using successive application of phase- and frequency-encoding gradients, you are thinking about some variant of the spin-warp method.
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A prototype conventional 2D spin-echo pulse sequence using spin-warp imaging is illustrated above. Here an MR signal is generated after each pair of 90°-180° degree RF-pulses. Slice-select gradients are turned on simultaneously with each RF-pulse so that only a single slice is stimulated.
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Frequency-encoding is performed using a dephase lobe between the 90°- and 180°-pulses and a readout lobe after the 180°-pulse. The dephase lobe imparts a frequency-dependent phase shift to protons along this axis as a function of their spatial position within the gradient. The phases of these spins are inverted by the 180°-pulse then rephased into an echo by the readout lobe.
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The digitized values of the MR signal at each time point are then inserted into a single horizontal row of the k-space matrix, filling it from left-to-right.
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The unique feature of the spin-warp sequence is that a variable-amplitude phase-encoding gradient is applied during signal evolution, typically between the 90°- and 180°-pulses. Additional MR signals are collected for this slice during the next TR interval with the same frequency-encoding gradient but with a different phase-encoding gradient. The phase-encode gradient provides a method for differentiating signals according to spatial location along this direction.
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With each successive application of the phase-encode gradient, the digitized MR signal is used to fill another row of k-space. For routine MR imaging, this process is typically repeated on the order 256 times. Once all the rows have been filled with data, Fourier transform methods can be used to reconstruct the image.
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If you've made it this far through the k-space Q&A's I suggest you carve out a quarter hour of time to watch the two videos below by the late Sir Paul Callaghan. Dr. Callaghan carefully lays out the principles of 2DFT spin-warp imaging and demonstrates how k-space and the MR signal are related.
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References
Edelstein WA, Hutchison JMS, Johnson G, Redpath T. Spin warp NMR imaging and applications to human whole-body imaging. Phys Med Biol 1980; 25:751-6.
Wald L. MR image encoding. (From MIT OpenCourseWare http://ocw.mit.edu)
Edelstein WA, Hutchison JMS, Johnson G, Redpath T. Spin warp NMR imaging and applications to human whole-body imaging. Phys Med Biol 1980; 25:751-6.
Wald L. MR image encoding. (From MIT OpenCourseWare http://ocw.mit.edu)
Related Questions
Where do you get the data to fill k-space? How does this relate to MR signals and echoes?
Where do you get the data to fill k-space? How does this relate to MR signals and echoes?