Comparison of wideband steady-state free precession and T₂-weighted fast spin echo in spine disorder assessment at 1.5 and 3 T

Abstract

Wideband steady-state free precession (WB-SSFP) is a modification of balanced steady-state free precession utilizing alternating repetition times to reduce susceptibility-induced balanced steady-state free precession limitations, allowing its use for high-resolution myelographic-contrast spinal imaging. Intertissue contrast and spatial resolution of complete-spine-coverage 3D WB-SSFP were compared with those of 2D T₂-weighted fast spin echo, currently the standard for spine T₂-imaging. Six normal subjects were imaged at 1.5 and 3 T. The signal-to-noise ratio efficiency (SNR per unit-time and unit-volume) of several tissues was measured, along with four intertissue contrast-to-noise ratios; nerve-ganglia:fat, intradural-nerves:cerebrospinal fluid, nerve-ganglia:muscle, and muscle:fat. Patients with degenerative and traumatic spine disorders were imaged at both MRI fields to demonstrate WB-SSFP clinical advantages and disadvantages. At 3 T, WB-SSFP provided spinal contrast-to-noise ratios 3.7-5.2 times that of fast spin echo. At 1.5 T, WB-SSFP contrast-to-noise ratio was 3-3.5 times that of fast spin echo, excluding a 1.7 ratio for intradural-nerves:cerebrospinal fluid. WB-SSFP signal-to-noise ratio efficiency was also higher. Three-dimensional WB-SSFP disadvantages relative to 2D fast spin echo are reduced edema hyperintensity, reduced muscle signal, and higher motion sensitivity. WB-SSFP's high resolution and contrast-to-noise ratio improved visualization of intradural nerve bundles, foraminal nerve roots, and extradural nerve bundles, improving detection of nerve compression in radiculopathy and spinal-stenosis. WB-SSFP's high resolution permitted reformatting into orthogonal planes, providing distinct advantages in gauging fine spine pathology.

FIG. 1

Simulated¹⁶ CSF balanced-SSFP (a = 1, solid red) and several (a = 0.75 [dotted blue], 0.58 [solid blue], 0.41 [dotted green]) WB-SSFP signals as a function of the off-resonance frequencies at 1.5 T, T₁/T₂ = 3300 ms/160 ms, T₁/T₂ = 15, TR_L = 6.0 ms, and a flip angle of 30°. Passband widths are balanced- SSFP: 167 Hz and WB-SSFP with a = 0.75, 0.58, 0.41: 190, 211, and 236 Hz.

FIG. 2

CNR comparison between WB-SSFP and FSE at 1.5 T (a) and 3 T (b) for various spinal tissues. Error bars reflect the standard deviation of tissue SNRs as measured over multiple regions-of-interest (ROIs).

FIG. 3

Axial FSE (a) and WB-SSFP (b) of a healthy volunteer at 1.5 T obtained at the same resolution. Fat (plus), disc (elipse), CSF (star), intradural nerves (circle), and nerve ganglia (minus) used for SNR and CNR calculations are shown. The dorsal root ganglia and cauda equina (arrow heads) are better seen with WB-SSFP, whereas WB-SSFP’s CSF signal is less hyperintense and more spatially uniform than that of FSE.

FIG. 4

A 75-year-old female patient with left-sided back pain imaged at 3 T. a: conventional axial T₂-weighted FSE and (b) axial WB-SSFP. WB-SSFP demonstrates better visualization and delineation of the intradural nerve roots and foraminal segments of the L3 nerves bilaterally (arrows).

FIG. 5

a–f: Three-Tesla coronal WBSSFP images of a patient with left-sided radicular pain. Bone impingement upon left S1 nerve (d–f, orange arrows) and the ensuing deformation of the nerve is clear. The sciatic nerves can be easily followed on both sides (yellow arrowheads).

FIG. 6

Three-Tesla C-spine 3D WB-SSFP. A coronal acquisition (a and b) was reformatted into axial (c). d: Curved-plane reformatting was then performed along white line, tracing the C2 nerve (orange arrows). Dark blood vessels (yellow arrowhead) demonstrate the “black blood” contrast. Images (e) and (f) demonstrate larger coverage, showing spinal vein (e) and brachial plexus (f), although off-resonance artifacts (large white arrows) create darker bands, limiting C-spine superior–inferior coverage to 70–90 mm.

FIG. 7

Cervical spine trauma patient scanned at 1.5 T. Sagittal FSE image (a) shows an acute dens fracture (yellow arrowhead). Impingement upon the left C2 nerve roots (yellow arrow) is better seen in the 2.6-mm axial 3D WB-SSFP image (c) than in the 4-mm axial FSE image (b). A fracture of the right posterolateral ring of the C2 vertebra is well demonstrated by all three images, while edema (orange arrow) is better seen with FSE.

FIG. 8

Three-Tesla high-resolution 3D WB-SSFP of a patient with multilevel spinal stenosis acquired in the coronal plane, and reformatted into sagittal and axial views. (a–c): Lower lumbar spine with L3–L4 pathology (orange arrow-heads), (d–f):Upper lumbar spine with L1–L2 pathology (yellow arrows).