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Volume fast spin-echo imaging of the cervical spine
Volume Fast Spin-Echo Imaging of the Cervical Spine Catherine Maldjian, MD, Richard J. Adam, MD, Naveed Akhtar, MD Akbar Bonakdarpour, MD, Orest B. Boyko, MD, PhD
Routine magnetic resonance (MR) imaging of the cervical spine at our institution currently consists of multiple pulse sequences performed in sagittal and axial planes. An axial three-dimensional (3D) gradient-echo (GRE) pulse sequence is also incorporated into the routine protocol. Standard imaging time is approximately 30 minutes. Use of volume fast spin-echo (SE) imaging could potentially shorten imaging time by over 50%. This study was conducted to evaluate prospectively the usefulness of a T2-weighted 3D volume fast SE pulse sequence in assessment of the cervical spine and to compare it with current standard imaging protocol. The main advantages of 3D imaging over two-dimensional (2D) protocols are multiplanar reconstruction capability, thinner sections, and shorter imaging times.
Eighteen patients (10 women, eight men) were referred for MR imaging of the cervical spine. The patients ranged in age from 26 to 74 years (mean, 46.1 years). Indications were neck pain and suspected spondylosis. All patients were imaged on a 1.5-T Signa MR unit with Horizon gradients (GE Medical Systems, Milwaukee, Wis) and a phased-array spinal coil or volume neck coil. Routine imaging consisted of (a) sagittal Tl-weighted SE imaging with a repetition time (TR) of 500-600 msec, an echo time (TE) of 10-20 msec (500-600/10-20), two signal averages, 256 x 256 matrix, 24-26-cm field of
Acad Radio11999:6:84-88
From the Department of Diagnostic Imaging, Temple University Hospital, Broad and Ontario Sts, Philadelphia, PA 19146. Received March 2, 1998; revision requested April 15; revision received September 3; accepted September 4. Address reprint requests to C.M. ©AUR 1999
84
a.
b.
a,
b.
Figure 1. (a) Routine sagittal T2-weighted fast SE image of the
Figure 2. (a) Sagittal T2-weighted fast SE image shows multilevel
spinal canal demonstrates a disk bulge at the C5-C6 level with no impingement on the spinal cord. (b) Partition image obtained from a T2-weighted volume acquisition at the same location demonstrates the disk bulge at C5-C6 equally well. Contrast between cord and cerebrospinal fluid is more pronounced.
disk herniations with a focal area of cord compression and myelomalacia at the C4-C5 level (arrow). (b) Sagittal partition image obtained from a T2-weighted volume acquisition in the same patient shows the area of cord edema (arrow) with greater conspicuity.
view (FOV), 3-mm section thickness, and 1-mm gap; (b) sagittal proton-density-weighted 2D fast SE imaging with 1,500/39, echo train length of 4, 3-mm section thickness, and 1-mm gap; (c) sagittal T2-weighted 2D fast SE imaging with 2,500/91, echo train length of 8, 3-mm section thickness, and 1-mm gap; (d) selected axial T1weighted imaging (two or three images per disk space), four signal averages, 256 x 128 matrix, 16-cm FOV, 3mm section thickness, 1-mm gap; and (e) axial 3D GRE volume imaging with 35/15, 5 ° flip angle, two signal averages, 256 x 128 matrix, 22-cm FOV, and 2-mm section thickness with no skip. The 3D volume fast SE examination was performed as a sagittal T2-weighted acquisition with the following parameters: 3,000/106, two signal averages, 256 x 256 matrix, 24 x 24-cm FOV, 1.3-mm section thickness, and zero gap. Imaging was performed in 10 slabs, each of which contained six sections. Images from the sections between adjacent slabs that overlapped were discarded; therefore, approximately 40 sagittal direct images were available for interpretation. Imaging time was 9 minutes 50 seconds (less than 10 minutes). Axial reconstructions were obtained by a radiologist from the sagittal partitions by sectioning parallel to the intervertebral disk; the reconstruc-
tion was accomplished within 10 minutes in each case. Two neuroradiologists independently evaluated images of the spinal cord, spinal canal, intervertebral disks, and neural foramina on the volume acquisitions for pathologic conditions. They were blinded to the results of the conventional studies. Spinal stenosis or neural foraminal narrowing, when detected, was graded for severity as mild, moderate, or severe. These results were then compared with reports from the routine examinations to assess the degree of concordance and discordance in interpretation.
With both imaging protocols, 10 patients had no evidence of herniated disks, spinal stenosis, or marked neural foraminal narrowing. The anatomic detail of the spinal canal was seen equally well on the volume fast SE partition images and on the 2D sagittal images (Fig 1). In six patients, nine cases of herniated nucleus pulposus were identified with both protocols. In three cases, acquired spinal stenosis was seen with both protocols. In one patient, cervical cord signal representing myelomalacia was identified with both protocols (Fig 2). Mild to moderate levels of spinal stenosis were seen in three cases with
85
a. b. c. Figure 3. (a) Parasagittal T2-weighted fast SE image through the neural foramina does not adequately demonstrate their anatomy and contents. (b) Axial reconstruction from the sagittal volume acquisition demonstrates selection of an oblique sagittal plane through the neural foramina (white line) to optimize the image of the neural foraminal anatomy and its contents. (c) Oblique sagittal reconstruction demonstrates the neural foramina to better advantage compared with conventional sagittal images. Multiplanar reconstruction capability is one of the main advantages of volume imaging.
both protocols. In one instance, on the volume acquired images, the stenosis was overrated as severe by one of the two radiologists. A computed tomography myelogram obtained in this case confirmed the condition as mild to moderate spinal stenosis. Five cases of marked neural foraminal narrowing (moderate or severe) were identified on the 3D images. Neural foraminal narrowing was also identified on 2D images in these cases but was characterized as mild in two cases. In one of these two cases, an electromyographic examination showed radiculopathy at the same level. The high resolution and reconstruction capability of the volume acquisitions frequently allowed nerve roots to be traced effectively from the spinal canal through the neural foramina. DISCUSSION
The diagnostic accuracy of volume fast SE imaging in the detection of degenerative disk disease was comparable to that of standard fast SE imaging. The instances of dis-
86
crepancy in neural foraminal narrowing may in part be due to subjective differences in interpretation. Moreover, this narrowing was detected more often with the volume fast SE imaging sequence. Prior studies have demonstrated the strength of 3D imaging over 2D fast SE imaging in delineating the nerve root anatomy and foraminal encroachment (1). Thus, volume fast SE images may be more accurate in portraying foraminal narrowing, which may be responsible for the mild discrepancies we encountered in this area. Volume fast SE imaging can also be used to perform oblique sagittal reconstructions in the plane of the cervical neural foramina, which provide a more optimal view of the neural foramina in this region than the view provided by straight sagittal images (Fig 3). In the case in which one radiologist overestimated mild to moderate spinal stenosis as severe, the error may have resulted from cerebrospinal fluid pulsation artifacts. With GRE pulse sequences, flow compensation can be applied simultaneously along section-selection and frequency-selection directions, but with fast SE pulse se-
a,
b,
Figure 4. (a) Axial image from a 3D GRE acquisition is compared with (b) an axial reconstruction from the volume fast SE acquisition. The right neural foramen (arrow) appears less distinct on the GRE images due to the pronounced T2* effects. Because T2* effects are less prominent on the volume fast SE acquisition in b, the borders of the disk at the neural foramen are more clearly depicted, and the dimensions of the neural foramen can be better assessed.
quences it can be applied in a single direction only. This fact may account for the lower signal intensity of cerebrospinal fluid on reconstructed 3D fast SE images compared with that of axial GRE images. Application of flow compensation in both directions on a fast SE acquisition usually results in severe image artifact. A case of cord edema-gliosis was seen equally well on the volume fast SE images and on the 2D fast SE images. Spinal canal assessment was also concordant. The quality of the partition images was good to excellent. Subjectively, contrast between cord and cerebrospinal fluid on the partitions is not markedly different from the contrast normally observed on 2D fast SE images. Reconstructed axial images were consistently rated of lower quality. This likely resulted from our use of anisotropic voxels, as the section thickness used in the 3D acquisition exceeded the in-plane resolution of the partitions, which results in a loss of image quality and a reduction in resolution on the axial reconstructed images. The use of thinner partitions could eliminate this problem but would also increase imaging time and might not markedly change the diagnostic value of the study. In spite of the lower quality of the reconstructed images, the results of volume fast SE imaging were still comparable to those of direct axial 3D GRE images, most likely due to its better signal-to-noise ratio. Moreover, because the partition images were of such high quality, this technique could replace standard sagittal 2D fast SE imaging protocols. The volume fast SE pulse sequence may therefore provide the same information as
combined 2D fast SE sagittal and 3D GRE sequences, the standard protocol for cervical spinal imaging at many institutions. Standard MR imaging of the cervical spine at our institution currently consists of sagittal 2D fast SE and axial 3D GRE acquisitions. Use of a sagittal 3D fast SE pulse sequence may decrease overall imaging time. Multiplanar reconstruction capability can also be used to depict axial and coronal planes, in addition to specific oblique planes, as necessary. Further advantages of volume fast SE imaging include its better signal-to-noise ratio, its use of thinner sections (resulting in greater resolution), and its decrease in susceptibility artifacts (T2* effects). This latter characteristic may enable better assessment of neural foraminal dimensions (Fig 4). Potential disadvantages of volume fast SE imaging include motion and truncation artifacts in two planes, and loss of resolution on reconstructed images. Prior investigators evaluating 3D imaging of the spine encountered difficulties that were probably in part due to the pulse sequences selected. In one instance, this sequence was a Tl-weighted pulse sequence that involved administration of an intravenous gadolinium-based contrast agent; in another instance, the sequence was a GREbased pulse sequence (1,2). The merits of 3D imaging in delineating neural foraminal encroachment and disk disease have been demonstrated with 3D GRE acquisition (3). Its success in this area has been attributed mainly to decreased partial volume averaging effects due to thinner
87
sections (1.5-2.0 mm). The main problems encountered were in the area of cord assessment, which resulted from marked truncation artifacts. This problem, which limited its usefulness and prohibited its use as a solitary diagnostic pulse sequence, can be addressed by use of a 3D T2weighted fast SE pulse sequence. A prior investigation that compared T2-weighted 3D fast SE imaging with 2D imaging on a 0.5-T system in patients with spondylosis demonstrated the two protocols to be comparable in the detection of abnormalities (4). Those authors also demonstrated facilitation in visualization of nerve root origins; however, contrast was found to be inadequate on the 3D images. Although contrast may be reduced with a 3D pulse sequence, this poses less of a problem on a high field magnet because of the greater attainable inherent signal-to-noise ratio. Although volume fast SE imaging has some weaknesses, it generates a greater signal-to-noise ratio than the 3D GRE acquisitions currently in use and may be promising as a single comprehensive, rather than adjunctive, pulse sequence. The improved contrast provided by volume fast SE may enable better evaluation of the spinal cord. Reconstruction time is not prohibitive, because in
88
most instances the sagittal images are of high quality and are sufficient for evaluation of most of the anatomy. Multiplanar reconstruction may be reserved for the evaluation of specific areas of suspected disk disease or neural foraminal encroachment. The partition images were comparable to those of 2D sagittal imaging and may adequately display contrast-sensitive areas, such as the spinal cord. The main advantages of volume fast SE imaging over current cervical spine MR imaging protocols involve its decreased imaging time and its capability for multiplanar reconstruction into conventional, as well as unconventional, planes. 1EFERENCE~
1. Yousem DM, Atlas DW, Goldberg H, Grossman R. Degenerative narrowing of the cervical spine neural foramina: evaluation with high resolution 3DFT gradient-echo MR imaging. Am J Neuroradio11991 ; 12:229-236. 2. Ross JS, Ruggieri PM, Tkach JA, et al. Gd-DTPA-enhanced 3D MR imaging of cervical degenerative disc disease: initial experience. Am J Neuroradio11992; 13:127-136. 3. Tsuruda JS, Norman D, Dillon W, et al. Three-dimensional gradient-recalled MR imaging as a screening tool for the diagnosis ef cervical radiculopathy. Am J Neuroradio11989: 10:1263-1271. 4. Swainson C J, Hutchinson CE, Watson Y. A comparison of 2-D and 3-D FSE Imaging in MR of the cervical spine. Clin Radiol 1997; 52:194-197.
Routine magnetic resonance (MR) imaging of the cervical spine at our institution currently consists of multiple pulse sequences performed in sagittal and axial planes. An axial three-dimensional (3D) gradient-echo (GRE) pulse sequence is also incorporated into the routine protocol. Standard imaging time is approximately 30 minutes. Use of volume fast spin-echo (SE) imaging could potentially shorten imaging time by over 50%. This study was conducted to evaluate prospectively the usefulness of a T2-weighted 3D volume fast SE pulse sequence in assessment of the cervical spine and to compare it with current standard imaging protocol. The main advantages of 3D imaging over two-dimensional (2D) protocols are multiplanar reconstruction capability, thinner sections, and shorter imaging times.
Eighteen patients (10 women, eight men) were referred for MR imaging of the cervical spine. The patients ranged in age from 26 to 74 years (mean, 46.1 years). Indications were neck pain and suspected spondylosis. All patients were imaged on a 1.5-T Signa MR unit with Horizon gradients (GE Medical Systems, Milwaukee, Wis) and a phased-array spinal coil or volume neck coil. Routine imaging consisted of (a) sagittal Tl-weighted SE imaging with a repetition time (TR) of 500-600 msec, an echo time (TE) of 10-20 msec (500-600/10-20), two signal averages, 256 x 256 matrix, 24-26-cm field of
Acad Radio11999:6:84-88
From the Department of Diagnostic Imaging, Temple University Hospital, Broad and Ontario Sts, Philadelphia, PA 19146. Received March 2, 1998; revision requested April 15; revision received September 3; accepted September 4. Address reprint requests to C.M. ©AUR 1999
84
a.
b.
a,
b.
Figure 1. (a) Routine sagittal T2-weighted fast SE image of the
Figure 2. (a) Sagittal T2-weighted fast SE image shows multilevel
spinal canal demonstrates a disk bulge at the C5-C6 level with no impingement on the spinal cord. (b) Partition image obtained from a T2-weighted volume acquisition at the same location demonstrates the disk bulge at C5-C6 equally well. Contrast between cord and cerebrospinal fluid is more pronounced.
disk herniations with a focal area of cord compression and myelomalacia at the C4-C5 level (arrow). (b) Sagittal partition image obtained from a T2-weighted volume acquisition in the same patient shows the area of cord edema (arrow) with greater conspicuity.
view (FOV), 3-mm section thickness, and 1-mm gap; (b) sagittal proton-density-weighted 2D fast SE imaging with 1,500/39, echo train length of 4, 3-mm section thickness, and 1-mm gap; (c) sagittal T2-weighted 2D fast SE imaging with 2,500/91, echo train length of 8, 3-mm section thickness, and 1-mm gap; (d) selected axial T1weighted imaging (two or three images per disk space), four signal averages, 256 x 128 matrix, 16-cm FOV, 3mm section thickness, 1-mm gap; and (e) axial 3D GRE volume imaging with 35/15, 5 ° flip angle, two signal averages, 256 x 128 matrix, 22-cm FOV, and 2-mm section thickness with no skip. The 3D volume fast SE examination was performed as a sagittal T2-weighted acquisition with the following parameters: 3,000/106, two signal averages, 256 x 256 matrix, 24 x 24-cm FOV, 1.3-mm section thickness, and zero gap. Imaging was performed in 10 slabs, each of which contained six sections. Images from the sections between adjacent slabs that overlapped were discarded; therefore, approximately 40 sagittal direct images were available for interpretation. Imaging time was 9 minutes 50 seconds (less than 10 minutes). Axial reconstructions were obtained by a radiologist from the sagittal partitions by sectioning parallel to the intervertebral disk; the reconstruc-
tion was accomplished within 10 minutes in each case. Two neuroradiologists independently evaluated images of the spinal cord, spinal canal, intervertebral disks, and neural foramina on the volume acquisitions for pathologic conditions. They were blinded to the results of the conventional studies. Spinal stenosis or neural foraminal narrowing, when detected, was graded for severity as mild, moderate, or severe. These results were then compared with reports from the routine examinations to assess the degree of concordance and discordance in interpretation.
With both imaging protocols, 10 patients had no evidence of herniated disks, spinal stenosis, or marked neural foraminal narrowing. The anatomic detail of the spinal canal was seen equally well on the volume fast SE partition images and on the 2D sagittal images (Fig 1). In six patients, nine cases of herniated nucleus pulposus were identified with both protocols. In three cases, acquired spinal stenosis was seen with both protocols. In one patient, cervical cord signal representing myelomalacia was identified with both protocols (Fig 2). Mild to moderate levels of spinal stenosis were seen in three cases with
85
a. b. c. Figure 3. (a) Parasagittal T2-weighted fast SE image through the neural foramina does not adequately demonstrate their anatomy and contents. (b) Axial reconstruction from the sagittal volume acquisition demonstrates selection of an oblique sagittal plane through the neural foramina (white line) to optimize the image of the neural foraminal anatomy and its contents. (c) Oblique sagittal reconstruction demonstrates the neural foramina to better advantage compared with conventional sagittal images. Multiplanar reconstruction capability is one of the main advantages of volume imaging.
both protocols. In one instance, on the volume acquired images, the stenosis was overrated as severe by one of the two radiologists. A computed tomography myelogram obtained in this case confirmed the condition as mild to moderate spinal stenosis. Five cases of marked neural foraminal narrowing (moderate or severe) were identified on the 3D images. Neural foraminal narrowing was also identified on 2D images in these cases but was characterized as mild in two cases. In one of these two cases, an electromyographic examination showed radiculopathy at the same level. The high resolution and reconstruction capability of the volume acquisitions frequently allowed nerve roots to be traced effectively from the spinal canal through the neural foramina. DISCUSSION
The diagnostic accuracy of volume fast SE imaging in the detection of degenerative disk disease was comparable to that of standard fast SE imaging. The instances of dis-
86
crepancy in neural foraminal narrowing may in part be due to subjective differences in interpretation. Moreover, this narrowing was detected more often with the volume fast SE imaging sequence. Prior studies have demonstrated the strength of 3D imaging over 2D fast SE imaging in delineating the nerve root anatomy and foraminal encroachment (1). Thus, volume fast SE images may be more accurate in portraying foraminal narrowing, which may be responsible for the mild discrepancies we encountered in this area. Volume fast SE imaging can also be used to perform oblique sagittal reconstructions in the plane of the cervical neural foramina, which provide a more optimal view of the neural foramina in this region than the view provided by straight sagittal images (Fig 3). In the case in which one radiologist overestimated mild to moderate spinal stenosis as severe, the error may have resulted from cerebrospinal fluid pulsation artifacts. With GRE pulse sequences, flow compensation can be applied simultaneously along section-selection and frequency-selection directions, but with fast SE pulse se-
a,
b,
Figure 4. (a) Axial image from a 3D GRE acquisition is compared with (b) an axial reconstruction from the volume fast SE acquisition. The right neural foramen (arrow) appears less distinct on the GRE images due to the pronounced T2* effects. Because T2* effects are less prominent on the volume fast SE acquisition in b, the borders of the disk at the neural foramen are more clearly depicted, and the dimensions of the neural foramen can be better assessed.
quences it can be applied in a single direction only. This fact may account for the lower signal intensity of cerebrospinal fluid on reconstructed 3D fast SE images compared with that of axial GRE images. Application of flow compensation in both directions on a fast SE acquisition usually results in severe image artifact. A case of cord edema-gliosis was seen equally well on the volume fast SE images and on the 2D fast SE images. Spinal canal assessment was also concordant. The quality of the partition images was good to excellent. Subjectively, contrast between cord and cerebrospinal fluid on the partitions is not markedly different from the contrast normally observed on 2D fast SE images. Reconstructed axial images were consistently rated of lower quality. This likely resulted from our use of anisotropic voxels, as the section thickness used in the 3D acquisition exceeded the in-plane resolution of the partitions, which results in a loss of image quality and a reduction in resolution on the axial reconstructed images. The use of thinner partitions could eliminate this problem but would also increase imaging time and might not markedly change the diagnostic value of the study. In spite of the lower quality of the reconstructed images, the results of volume fast SE imaging were still comparable to those of direct axial 3D GRE images, most likely due to its better signal-to-noise ratio. Moreover, because the partition images were of such high quality, this technique could replace standard sagittal 2D fast SE imaging protocols. The volume fast SE pulse sequence may therefore provide the same information as
combined 2D fast SE sagittal and 3D GRE sequences, the standard protocol for cervical spinal imaging at many institutions. Standard MR imaging of the cervical spine at our institution currently consists of sagittal 2D fast SE and axial 3D GRE acquisitions. Use of a sagittal 3D fast SE pulse sequence may decrease overall imaging time. Multiplanar reconstruction capability can also be used to depict axial and coronal planes, in addition to specific oblique planes, as necessary. Further advantages of volume fast SE imaging include its better signal-to-noise ratio, its use of thinner sections (resulting in greater resolution), and its decrease in susceptibility artifacts (T2* effects). This latter characteristic may enable better assessment of neural foraminal dimensions (Fig 4). Potential disadvantages of volume fast SE imaging include motion and truncation artifacts in two planes, and loss of resolution on reconstructed images. Prior investigators evaluating 3D imaging of the spine encountered difficulties that were probably in part due to the pulse sequences selected. In one instance, this sequence was a Tl-weighted pulse sequence that involved administration of an intravenous gadolinium-based contrast agent; in another instance, the sequence was a GREbased pulse sequence (1,2). The merits of 3D imaging in delineating neural foraminal encroachment and disk disease have been demonstrated with 3D GRE acquisition (3). Its success in this area has been attributed mainly to decreased partial volume averaging effects due to thinner
87
sections (1.5-2.0 mm). The main problems encountered were in the area of cord assessment, which resulted from marked truncation artifacts. This problem, which limited its usefulness and prohibited its use as a solitary diagnostic pulse sequence, can be addressed by use of a 3D T2weighted fast SE pulse sequence. A prior investigation that compared T2-weighted 3D fast SE imaging with 2D imaging on a 0.5-T system in patients with spondylosis demonstrated the two protocols to be comparable in the detection of abnormalities (4). Those authors also demonstrated facilitation in visualization of nerve root origins; however, contrast was found to be inadequate on the 3D images. Although contrast may be reduced with a 3D pulse sequence, this poses less of a problem on a high field magnet because of the greater attainable inherent signal-to-noise ratio. Although volume fast SE imaging has some weaknesses, it generates a greater signal-to-noise ratio than the 3D GRE acquisitions currently in use and may be promising as a single comprehensive, rather than adjunctive, pulse sequence. The improved contrast provided by volume fast SE may enable better evaluation of the spinal cord. Reconstruction time is not prohibitive, because in
88
most instances the sagittal images are of high quality and are sufficient for evaluation of most of the anatomy. Multiplanar reconstruction may be reserved for the evaluation of specific areas of suspected disk disease or neural foraminal encroachment. The partition images were comparable to those of 2D sagittal imaging and may adequately display contrast-sensitive areas, such as the spinal cord. The main advantages of volume fast SE imaging over current cervical spine MR imaging protocols involve its decreased imaging time and its capability for multiplanar reconstruction into conventional, as well as unconventional, planes. 1EFERENCE~
1. Yousem DM, Atlas DW, Goldberg H, Grossman R. Degenerative narrowing of the cervical spine neural foramina: evaluation with high resolution 3DFT gradient-echo MR imaging. Am J Neuroradio11991 ; 12:229-236. 2. Ross JS, Ruggieri PM, Tkach JA, et al. Gd-DTPA-enhanced 3D MR imaging of cervical degenerative disc disease: initial experience. Am J Neuroradio11992; 13:127-136. 3. Tsuruda JS, Norman D, Dillon W, et al. Three-dimensional gradient-recalled MR imaging as a screening tool for the diagnosis ef cervical radiculopathy. Am J Neuroradio11989: 10:1263-1271. 4. Swainson C J, Hutchinson CE, Watson Y. A comparison of 2-D and 3-D FSE Imaging in MR of the cervical spine. Clin Radiol 1997; 52:194-197.