Parallel Imaging of the Cervical Spine at 3T: Optimized Trade-Off between Speed and Image Quality

J. Fruehwald-Pallamar; P. Szomolanyi; N. Fakhrai; A. Lunzer; M. Weber; M.M. Thurnher; M. Pallamar; S. Trattnig; D. Prayer; I.M. Noebauer-Huhmann

doi:10.3174/ajnr.A3101

Abstract

BACKGROUND AND PURPOSE: Patients with cervical spine syndrome often experience pain during the MR examination. Our aim was to compare the quality of cervical spine MR images obtained by parallel imaging with those of nonaccelerated images, with the goal of shortening the examination time while preserving adequate image quality.

MATERIALS AND METHODS: A phantom study and examinations of 10 volunteers and 26 patients were conducted on a clinical 3T scanner. Acquisitions included axial T2WI, sagittal T2WI, T1WI, and T2TIRM sequences. Nonaccelerated sequences and accelerated sequences with different numbers of averages and different accelerations, with a scanning time reduction of 67%, were performed. For quantitative analysis, the SNR was obtained from the phantom measurements, and the NU was calculated from the volunteer measurements. For qualitative analysis, 3 independent readers assessed the delineation of anatomic structures in volunteers and the visibility of degenerative disease in patients.

RESULTS: In the phantom study, as expected, the SNR of the nonaccelerated images was higher than the SNR of the same sequence with parallel imaging. In vivo, the NU was higher when applying fewer averages or parallel imaging, compared with the nonaccelerated images. The analysis of the subjective parameters in the volunteers and patients showed that a scanning time of 48% of the original protocol could be obtained by combining the following sequences: sagittal T1WI with 1 average; sagittal T2WI with acceleration factor 3; sagittal T2TIRM with acceleration factor 2; and axial T2* GRE with acceleration factor 2.

CONCLUSIONS: Parallel imaging of the cervical spine at 3T allows shortening of the examination time by 52%, preserving adequate image quality.

ABBREVIATIONS:

CNR: contrast-to-noise ratio
GRAPPA: generalized autocalibrating partially parallel acquisition
GRE: gradient recalled-echo
NU: nonuniformity
pMRI: parallel MR imaging
T2TIRM: T2 turbo inversion-recovery magnitude

Up to 50% of individuals older than 50 years of age have some degree of disk degeneration.^1,2 Patients are symptomatic due to radiculopathy, cervical myelopathy, or muscular and ligamentous imbalance, as well as such sequelae as osteochondrosis and facet joint osteoarthritis. The annual incidence of cervical radiculopathy is reportedly 83 per 100,000 population, whereas the prevalence is 3.5 per 1000 population, with a peak incidence in the sixth decade of life.^3⇓–5

In patients with symptoms resistant to therapy or inconclusive findings on projection radiographs, MR imaging is the technique of choice for the evaluation of the cervical spine. The cervical spine bodies, joints, disks, and ligaments, as well as the spinal cord, the nerve roots, and the surrounding soft-tissue, can be assessed with MR imaging. For sufficient image quality, patients must avoid any movement that might cause motion artifacts. For patients with painful spine conditions, in particular, it is important to reduce the examination time while still obtaining diagnostic images. One possibility for decreasing scanning time is to reduce the number of acquisitions. Another technique, pMRI, became possible with the introduction of multicoil arrays.⁶ In the past few years, pMRI has been increasingly used to accelerate MR images and reduce scanning time.^{7⇓⇓⇓⇓⇓–13} In pMRI, compared with conventional MR imaging, the number of phase-encoding steps in the same FOV can be reduced, with an array of multiple independent receiver coils that acquire signals simultaneously.

With the GRAPPA technique, it is possible to omit phase-encoding steps from the abundant information received from the various coil elements during acquisition.¹⁴

Several studies, mostly on 1.5T scanners, have proved that with the use of parallel imaging, the examination time can be reduced substantially while preserving good image quality.^15⇓–17 The benefits of a shorter breath-hold time for pMRI has been proved in cardiac, thoracic, and liver imaging^16,18 and even with free breathing in cardiac MR imaging.¹⁹ In contrast-enhanced MR angiography, pMRI is used to achieve a higher temporal resolution. In general, the higher SNR of 3T systems can be used for better image quality or reduced scanning time. It is possible to test higher acceleration factors on higher than 1.5T units. Imaging of the cervical spine should benefit from the use of higher field strengths in terms of scanning time reduction, especially when dealing with patients who cannot lie still for a long time.

However, to apply this technique routinely in cervical spine imaging, we had to evaluate the image quality and clinical utility at 3T systematically. The aim of our study was, therefore, to apply parallel imaging at 3T in cervical spine imaging. For this purpose, we tested accelerated and nonaccelerated sequences objectively and subjectively in a phantom, in volunteers, and in patients.

Materials and Methods

The MR imaging examination was performed on a 3T clinical scanner (Tim Trio; Siemens, Erlangen, Germany) with a maximum gradient strength of 38 mT/m and a gradient slew rate of 170 mT/m/ms. We used an array of dedicated head, spine, and neck coils. For sagittal measurements (phantom, volunteers, and patients), the standard neck coil (3T Neck Matrix, a total imaging matrix coil; Siemens) and the standard head coil (3T Head Matrix, a total imaging matrix coil; Siemens) were used. All axial measurements were performed with the standard spine coil (3T Spine Matrix, a total imaging matrix coil; Siemens).

First, a phantom study was conducted to evaluate the protocol and to assess the SNR of the accelerated and nonaccelerated sequences. Acquisitions included axial T2*-weighted sequences (2D spoiled gradient-echo multiecho sequences²⁰), sagittal T2-TSE, sagittal T1-TSE, and sagittal T2TIRM sequences. Detailed sequence parameters are listed in the Table.

View this table:

Imaging parameters and examination times

Those 4 sequences were measured by using GRAPPA, with an acceleration factor of 1–4, as well a reduced number of averages (sagittal T2WI, sagittal T1WI).

We examined 1 cylindric phantom (a plastic bottle filled with 3.7-g nickel sulphate × 6H₂O + 5-g sodium chloride per 1000-g H₂O, distilled; the signal intensity of the content mimics CSF), 10 healthy volunteers (mean age, 35 years; range, 19–53 years), and 26 patients with pain and/or neurologic deficits due to suspected degeneration (mean age, 46 years; range, 21–75 years). In the phantom study, the image quality of the sequence with an acceleration factor of 4 was insufficient (low SNR). Consequently, all volunteers and patients were only examined with acceleration factors of 2 and 3.

The institutional ethics committee approved this study, and written, informed consent was obtained from the 10 volunteers and 26 patients.

Quantitative: Phantom

For quantitative analysis, the SNR was calculated for each sequence and acceleration factor from the phantom measurements. A large region of interest covering major parts of the phantom (corresponding to the location of relevant structures in vivo) and a second region of interest placed on the background next to the phantom were drawn on an image in the middle of the stack (Fig 1). The ROIs were drawn large to average the B₁ inhomogeneity in the center of the phantom.

Fig 1.

Region-of-interest measurements in the phantom.

SNR was calculated by a modified dual acquisition method⁸ by using the following formula:

where SI is the signal intensity of a region of interest placed on an image of the phantom in the middle of the stack, and SD_{SI 1 − SI2} is the SD of the signal intensity of the same region of interest on the “subtracted image,” representing the noise.

Quantitative: Volunteers

In the volunteer examinations, 3 ROIs (levels C2, C4, and C6) were placed in the spinal cord (sc), in the paravertebral muscle (m), and in the background air in a central section of the stack (Fig 2). From these measurements, the NU was calculated by using the following formula:

Fig 2.

The region-of-interest measurements in a volunteer.

with SI_sc taken as the mean signal intensity of the ROIs in the spinal cord, and SD_sc taken as the SD of these ROIs.

In the 10 volunteers, the CNR was also calculated by

where SI_sc is the mean signal intensity measured by the ROIs in the spinal cord, SI_m is the mean signal intensity of the ROIs measured in the muscles, and SD_air is the SD of the ROIs in the air.

Qualitative: Volunteers

Qualitative analysis was performed by 3 independent readers (J.F.-P., I.M.N.-H., N.F., who had MR imaging experience of 5, 14, and 4 years, respectively), on a commercial PACS workstation. The readers were blinded to the technique and volunteer/patient name. They interpreted the images in a random order.

In the volunteer examinations, the delineation of 6 anatomic structures (facet joints, central spinal canal, neural foramina, nerve roots, disks, spinal cord) was rated as 5 (excellent visibility), 4 (good), 3 (acceptable), 2 (poor), or 1 (not visible). The overall subjective impression of the image was assessed in the same manner. Artifacts (motion, aliasing, susceptibility, section overlap) were graded as absent ⁵, minimal ⁴, mild ³, moderate ², or severe ¹. We determined that a sequence with >10% of “problematic” ratings (1 or 2) should be excluded from the protocol. For this calculation, we used the total number of ratings (all readers and patients/volunteers).

Qualitative: Patients

In the patients, we evaluated the following degenerative changes: abnormal posture, disk degeneration, disk herniations, Modic changes, spondylophytes, facet arthropathy, spinal stenosis, neural foramina stenosis, and compression myelopathy. “Disk degeneration” was defined as height and signal loss on the sagittal T2 and turbo-inversion recovery magnitude sequences. “Disk herniation” was defined as focal extension of the disk past the vertebral body. Endplate changes were defined according to the classification of Modic and Ross.²¹ Facet joint degeneration and neural foramina stenosis were evaluated according to the criteria described by Morvan.²² There is no consensus definition of cervical spinal canal stenosis. Cervical spinal canal stenosis was defined as the absence of CSF signal around the spinal cord. Myelopathy was defined as high signal changes in the spinal cord on the T2-weighted sequences.

The 3 readers rated the visibility of pathology as 5 (excellent visibility), 4 (good), 3 (acceptable), 2 (poor), or 1 (not visible). A cutoff at 10% of missed pathologies was defined for exclusion of an accelerated sequence.

Data were analyzed by using the Statistical Package for the Social Sciences for Windows (Version 17.0; SPSS, Chicago, Illinois). Nominal data are presented by using absolute frequencies and percentages. Metric data are presented by using mean ± SD when normal and distributed and median (minimum, maximum) when skewed. To compare the original with the accelerated sequences with respect to SNR, CNR, and NU, we used 1-way ANOVAs for repeated measures, followed by simple contrasts in case of significant results. A P value ≤ .05 indicated a significant result. As corrections for multiple comparisons decrease the power to detect a difference, no such corrections were applied.