Newer Sequences for Spinal MR Imaging: Smorgasbord or Succotash of Acronyms?
Jeffrey S. Ross
,a
a From the Department of Radiology, Cleveland Clinic Foundation, Cleveland, OH.
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FIG 1. EPI diffusion-sequence structure. The 90° and 180° RF pulses are followed by a bipolar, trapezoidal frequency encode gradient (Gx) for rapid collection of multiple echos. DWI is applied by symmetrical gradients along a frequency-encoded direction (black rectangles). Subsequent sequence acquisitions would apply diffusion weighting along phase (Gy) and slice-select (Gz) directions
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FIG 2. Half Fourier schematic. Slightly more than half of the data can be collected and used to "fill in" the remainder of k-space because the data is assumed to be symmetrical. This partial data acquisition shortens imaging time.
FIG 3. Interpolation schematic. Matrix is expanded by addition of "place holder" data, allowing reconstruction, for example, of a 512 matrix from a 256 data set. Appearance of image will be filtered due to more heavy weighting from central k-space data.
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FIG 4. 3D GE with slice interpolation (36/15/1). Three contiguous slices reconstructed at 1.5 mm and acquired at 3 mm with slice interpolation. The advantage is that neural foramina are encompassed by multiple images with very-thin-slice reconstruction.
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FIG 5. Fast FLAIR sequence structure. Typical FSE sequence structure of multiple 180° pulses is modified by addition of a 180° inversion pulse, followed by a delay time until the alpha pulse (inversion time or TI). CSF is suppressed by appropriate selection of inversion time, which for FLAIR is approximately 2000 ms. Effective TE is determined by low-amplitude phase-encoding steps (central k-space). S = slice-select direction, R = "read" or frequency-encode direction, P = phase-encode direction
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FIG 6. False-negative fast FLAIR for demyelinating disease.
A, Sagittal T1-weighted image (500/12/2) demonstrates a markedly enlarged cord with slighted decreased signal centrally.
B, Sagittal T2-weighted FSE (4620/112/ 3) shows diffuse increased signal throughout cervical cord.
C, Sagittal FSE FLAIR (6000/105/ 2) shows low signal at C7-T1 level, but no abnormal increased signal as on the FSE.
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FIG 7. Fast STIR sequence structure. This is analogous to FLAIR sequence, except that TI time is shorter to null fat signal, and low-amplitude phase-encode steps are acquired earlier. S = slice-select direction, R = "read" or frequency- encode direction, P = phase-encode direction
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FIG 8. T2W vs. FLAIR vs. STIR in demyelinating disease.
A, Fusiform enlargement of cord without enhancement is shown on sagittal T1-weighted sequence (500/12/2).
B, Abnormal high signal within cord is shown on sagittal FSE T2-weighted sequence (4620/112/ 3).
C and D, FSE spin-density weighted (2000/10/2), and FSE STIR (1200/14/4), respectively.
E, Abnormal cord signal is not revealed by fast FLAIR sequence (6000/105/ 2). Lesion is most conspicuous on FSE T2-weighted and fast STIR sequences.
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FIG 9. Chronic demyelinating disease.
A, Sagittal T2-weighted FSE (4620/112/ 3) shows faint focal increased signal in cervical cord at C1 and C3 levels.
B, Sagittal FSE FLAIR (6000/105/2) also shows very indistinct abnormal signal at those two levels.
C, High lesion-to-cord contrast is achieved with fast STIR sequence (1200/14/4).
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FIG 10. Echo-planar diffusion imaging of the normal cervical cord. Three orthogonal directions of diffusion gradients are applied: anteroposterior (A); transverse (B); and through-plane (C). Notice the least signal from the cord with through-plane diffusion encoding (parallel to white matter tracts) reflecting direction of relatively fastest water diffusion
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FIG 11. MT contrast. Axial gradient echo slice without (A) and with (B) application of off-resonance MT pulse. Application of MT dramatically improves cord/CSF contrast.
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FIG 12. True FISP sequence structure with balanced gradients. Net effect of gradients allows spins that are stationary as well as those moving with constant velocity to reach a steady state. Gz = slice select gradient, Gy = phase encode gradient, and Gx = frequency encode gradient.
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FIG 13. True FISP (17/8/2, 70° flip angle).
A, Sagittal 2-mm slice from one of the two sequences acquired with different RF phase (combined to produce final image) demonstrates areas of banding or signal loss related to nonuniform resonant offset.
B, Combined final sequence shows more uniform high-signal CSF with relative suppression of soft-tissue signal.
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FIG 14. 3D CISS (12/6/3, 70° flip angle). Axial 2-mm section through cervical spine shows sharp interface between cord/intradural dorsal and ventral roots (arrows) and the CSF. There is slight truncation artifact surrounding the cord, manifest as curvilinear low signal.
FIG 15. PSIF diffusion-sequence structure (aka, FISP backwards).Diffusion weighting is applied as a single gradient along slice-select direction. Acquired signal is an RF echo. Echo occurs prior to alpha pulse because it is generated by the refoccussing of magnetization that has resided in transverse plane over at least one previous complete TR cycle. Gz = slice-select gradient, Gy = phase- encode gradient, Gx = frequency-encode gradient.
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FIG 16. Diffusion true positive in patient with myeloma (PSIF 22/2/10, 75° flip angle).
A, Sagittal T1-weighted image shows diffuse abnormal marrow signal with mild compression fracture.
B, Sagittal PSIF sequence with diffusion gradient shows high signal from compression fracture comfirming malignant origin.
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FIG 17. Diffusion false positive in trauma (PSIF 22/2/10, 75° flip angle) found in a 17-year-old who sustained a flexion injury at C3–4 after going over handlebars of waterski.
A and B, Sagittal T1-weighted (A) and T2-weighted (B) images show anterior wedge deformities of C3 and C4 bodies.
C, Diffusion sequence shows slight increased signal from bodies, falsely suggesting a cellular infiltrate.
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AJNR - American Journal of Neuroradiology