British Journal of Radiology 74 (2001),862-873 © 2001 The British Institute of Radiology
High resolution CISS imaging of the spine
N Ramli, FRCR
A Cooper, HDCR, MBA
and
T Jaspan, FRCP, FRCR
Imaging Centre, Radiology Department, University Hospital, Nottingham NG7 2UH, UK
Correspondence: T Jaspan
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Abstract
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Spatial resolution remains one of the major problems and goals in spinal imaging. The high spatial resolution afforded by a novel sequence, constructive interference in steady state (CISS), provides a further refinement to MRI, the modality of choice in the investigation of suspected intraspinal pathology. Both complex and subtle abnormalities are more fully elucidated using CISS. It is now used in our institution as an adjunct to conventional imaging sequences in the diagnostic evaluation of complex intraspinal pathology. The anatomical information provided by CISS is of particular value in planning surgical interventions, most notably in the management of intraaxial and extraaxial cystic abnormalities, dysraphic malformations and disturbances of cerebrospinal fluid circulation, including post-traumatic and post-surgical scarring.
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Introduction
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As early as 1986, Hennig et al [1] found that MRI myelography afforded similar information to that provided by X-ray myelography [1]. Recent developments in MR hardware technology and pulse sequences have refined the technique further. Among the newer pulse sequences is constructive interference in steady state (CISS), a true fast imaging sequence with steady-state free precession. Pulsatile cerebrospinal fluid (CSF) flow is minimized by acquiring the sequence with flow compensation applied over each TR cycle, rather than over each echo as in the case of conventional compensation techniques. Turbulent flow, however, is not suppressed, with phase dispersion resulting in signal loss. Two data sets are acquired successively with alternating and non-alternating radio frequency pulses, which are subsequently combined to produce a myelographic image with excellent CSF-to-cord contrast [2]. The morphological characteristics of intrinsic cord abnormalities with ill defined margins and variable signal characteristics on T1 and T2 weighted images are, however, poorly evaluated owing to the intrinsic lack of contrast between such structures on CISS imaging.
All our patients undergo conventional spin echo (SE) T1 weighted and T2 weighted turbo spin echo (TSE) MRI. A three-dimensional (3D) CISS examination is undertaken in selected cases based upon the clinical context and review of the images at the time of the examination. Lesions that are relatively isointense with CSF on T1 and T2 weighted images may be obscured, missed or underestimated. In addition, the comparatively thick slices employed for 2D Fourier transform SE and TSE images (3–4 mm) result in partial volume averaging and decreased spatial resolution in comparison with the submillimetre capability of the 3D Fourier CISS sequence. The short TE employed in the CISS sequence results in limited signal loss from magnetic susceptibility effects, and the low flip angle reduces T1 weighting. The increased sensitivity of the CISS sequence is a consequence of the accentuation of the T2 values between CSF and pathological structures, and the higher intrinsic resolution between neural structures, CSF and lesions surrounded by CSF. Thus, the extremely thin wall of cystic structures such as arachnoid cysts, which may easily evade detection with conventional sequences can be resolved.
The loss of signal from pulsatile CSF motion, an inherent problem with 2D T2 weighted fast SE techniques, is minimized by acquiring a free induction with steady-state precession (FISP) sequence with flow compensation applied over each TR cycle, rather than over each echo as in conventional compensation techniques. The well recognized flow voids, particularly in the dorsal thecal sac in the cervicothoracic region or in individuals with capacious thecal sacs, are therefore diminished. Turbulent flow, such as that seen in a complex hydrosyrinx, is not, however, suppressed using this technique, resulting in signal loss due to phase dispersion.
The purpose of this pictorial review is to illustrate the usefulness of CISS in various spinal pathologies and to discuss its potential role in a spinal imaging protocol. This technique is used at our institution, to investigate a wide range of spinal pathologies when routine conventional MR sequences provide suboptimal anatomical information, or to elucidate complex spinal pathology. All our patients were examined on a Siemens 1.5 T Vision scanner (Siemens, Erlangen, Germany) using a spine array surface coil. In the cervical spine the T1 weighted imaging parameters are 450/12/2 (TR/TE/excitations), matrix of 256 x 512, field of view (FOV) of 300, acquisition time 3 min 53 s and in the thoracolumbar region 500/12/2 (TR/TE/excitations), matrix of 240 x 512, FOV of 340 and acquisition time 4 min 34 s. For T2 weighted imaging, the FSE scan parameters are 4000/112eff/3 (cervical) or 4500/112eff/2 (thoracolumbar), echo train length 15, matrix 300–512, FOV 300 and acquisition time 4 min 4 s (cervical) or 4 min 34 s (thoracolumbar). For the CISS sequence TR=12.25 ms, TE=5.90 ms, flip angle is 70°, FOV is 260 mm, with a 192 x 256 matrix. The acquisition time is approximately 9 min 4 s. A 32 mm volume slab is acquired in the sagittal plane, which is divided into 42 partitions giving an effective slice thickness of 1.03 mm. The raw data are reviewed on an independent console with multiplanar reconstruction (MPR) into axial, coronal and sagittal planes.
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Normal anatomy
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Owing to high intrinsic contrast, CISS images provide exquisite information about intrathecal contents, dura and root sheaths. Normal variants such as perineural cyst and conjoint nerve roots are clearly visible on CISS images (Figure 1
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Figure 1. Reconstructed coronal CISS image showing a thoracic perineural cyst within a neuroforaminal canal (arrow).
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Discovertebral disease
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The CISS sequence has been reported to be equivalent to contrast myelography for demonstrating the thecal sac and dural root sleeve at each intervertebral level [3, 4]. The sequence could not differentiate disc from osteophytes, but clearly shows the site and effects on the thecal sac and spinal cord (Figure 2). The millimetre thick slices obtainable with CISS imaging improve the spatial resolution and enable good definition of the spinal and neuroforaminal canal dimensions in patients with spondylotic disease. The susceptibility effects of adjacent bony structures remain a problem in accurately defining the degree of stenosis in severe neuroforaminal canal narrowing in the cervical region. However, this difficulty is inherent to gradient echo sequences, which are routinely used in the axial plane to evaluate the neuroforaminal canals.
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Figure 2. (a) Sagittal CISS image demonstrating discovertebral impingement upon the thecal sac and cord. (b)Reconstructed axial CISS image clearly delineates a paracentral disc protrusion flattening and deforming the left side of the cord, in conjunction with an osteophyte arising posteriorly projecting into the lateral recess (arrow).
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Vascular abnormalities
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With the aid of MPR, CISS can clearly demonstrate the engorged pial venous plexus associated with spinal dural fistulae or arteriovenous malformations, seen as complex serpinginous structures in the CSF (Figure 3). Oblique coronal reconstructions may localize the site of the fistula. Mild to moderate cord expansion secondary to congestive venous myelopathy is a frequent finding. Whilst CISS readily identifies cord swelling, the accompanying alteration in the cord signal is best appreciated on T2 weighted images.
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Figure 3. (a) Sagittal CISS image of the thoracic region. The high contrast and spatial resolution highlights an enlarged perimedullary venous plexus surrounding the cord, associated with a dural arteriovenous fistula. (b)Coronal reconstructed CISS image demonstrating serpiginous veins on the dorsal surface of the cord. (c) Spinal angiography demonstrates the fistula and the perimedullary venous drainage corresponding to the coronal CISS image.
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The nidus of an intraspinal arteriovenous malformation may be identified as a complex but more focal filling defect in the CSF, either within the cord or on the cord surface. Flow-related or intranidal aneurysms and venous aneurysmal pouches can also be accurately identified (Figure 4). The findings on CISS imaging complement conventional spinal angiographic images.
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Figure 4. Perimedullary arteriovenous malformation. An enlarged dorsal perimedullary venous complex with an associated venous aneurysm (arrow) on the dorsal surface of the cord is demonstrated on sagittal (a) and reconstructed coronal (b) CISS images. (c) Vertebral artery angiogram showing supply from the anterior spinal artery axis and the corresponding angiographic features. The aneurysm is faintly filled in on this early arterial phase image.
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Several studies have used MR angiography to image a dilated venous plexus in the spine by phase contrast [5–7] or post-gadolinium 3D time-of-flight techniques [8, 9]. The CISS sequence can demonstrate the features of a spinal vascular abnormality without the use of contrast agents, being a useful adjunct for localizing the nidus and possibly the site of fistulation. Nevertheless, spinal angiography remains the gold standard technique for assessing vascular malformations of the spine.
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Trauma
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The high CSF-to-cord contrast on CISS clearly demonstrates the late complications of spinal cord injury. Early myelomalacia is better appreciated on T2 weighted imaging, where focal ill defined hyperintensity can be seen. Local pial adhesions, cord tethering, atrophy and/or expansion with intramedullary CSF-filled cavities indicative of post-traumatic syringohydromyelia, are better visualized than with conventional SE imaging (Figure 5). The full extent of these cavities and the presence of septations are also better appreciated with CISS, which is important if surgical decompression is to be undertaken.
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Figure 5. Sagittal (a) and axial (b) CISS images showing the late complications of a stab wound into the thoracocervical cord, with focal cystic myelomalacia and arachnoidal adhesions tethering the cord. (c)Sagittal T1 weighted section demonstrates the cystic myelomalacia but cannot identify the adhesions and tethering.
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Archnoiditis
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Arachnoiditis associated with tumour, trauma, past surgery or infection can be classified into three types: Type 1, conglomeration of adherent roots in the centre of the thecal sac; Type 2, roots adherent to the thecal sac peripherally (empty sac syndrome); and Type 3, soft tissue mass obliterating the subarachnoid space [10]. CISS clearly shows the level, extent and type of arachnoiditis (Figure 6). Adhesive changes and focal entrapment of CSF pouches are better appreciated owing to the high intrinsic contrast resolution of the CISS sequence (Figure 7).
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Figure 6. Sagittal (a) and axial (b) CISS images demonstrating arachnoiditis associated with past surgery and Myodil myelography. Residual Myodil is filling the thecal cul-de-sac (arrowhead). Note the "empty" thecal sac with the spinal roots adherent to the posterior thecal wall (arrow).
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Figure 7. (a) Sagittal CISS image clearly demonstrating multiple arachnoidal adhesions and focal compressive entrapped cerebrospinal fluid pouches within the subarachnoid space. (b) The corresponding sagittal T2 weighted image dose not have the spatial resolution to define the extent of the adhesions and the smaller arachnoidal cysts.
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Congenital abnormalities
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Neurofibromatosis Type 1 (NF-1) is the most common neurocutaneous syndrome. Spinal abnormalities occur in 60% of NF-1 cases. Scoliosis is the most common skeletal abnormality in NF-1. The relationship between the spinal cord and the adjacent dysplastic vertebral body is best displayed on the reconstructed sagittal and coronal planes in line with the spinal cord (Figure 8). Dural ectasia with scalloping of the posterior vertebral body and widened neural foramina secondary to nerve root tumours or lateral meningocoeles may also be clearly depicted, the high contrast between CSF and solid structures clearly differentiating between the latter.
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Figure 8. Sagittal reconstructed CISS image showing the expanded subarachnoid space associated with dural ectasia in a child with neurofibromatosis. There are no associated neurofibromata.
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The metameric haustrations seen within syringomyelic cavities associated with the Chiari I malformation are defined in greater detail than on conventional SE imaging (Figure 9). The complex craniocervical abnormalities associated with the Chiari II malformation, an almost inevitable accompaniment of myelomeningocoeles, are optimally assessed employing the CISS sequence (Figure 10) [11]. Post-operative complications of repaired myelomeningocoele, such as dural tears, thecal sac and cord herniation, late tethering andcord cavitation, are also well demonstrated (Figure 11).
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Figure 9. (a) T1 weighted and (b) T2 weighted sagittal images of the cervical region in a patient with Chiari I malformation associated with syringomyelia. (c) Sagittal CISS image of the same patient demonstrates the metameric haustration in greater clarity and the cephalad extent of the syringomyelia.
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Figure 10. Sagittal CISS image in Chiari II malformation demonstrating a characteristic cervicomedullary kink and cystic dilatation of the caudal fourth ventricle. Forking of the caudal end of the cerebral aqueduct is also clearly demonstrated along with tectal beaking and clival concavity.
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Figure 11. Repaired cervical myelomeningocoele with late retethering. (a) T1 weighted sagittal image shows a slender cord cavity (arrow). (b) Sagittal CISS image clearly delineates neural tissue extending dorsally to the surgical repair site and more clearly defines the short segment hydrosyringomyelia (arrow).
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Tumours
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Whilst conventional MR sequences are usually sufficiently sensitive in demonstrating intraspinal and extraspinal pathology, CISS images may better define the precise local anatomy and provide useful additional information for the surgeon (Figure 12). In one case, cystic degeneration of a tumour with scalloping of the adjacent vertebral body supported a pre-operative diagnosis of benign schwannoma. Additionally, the surgeon was provided with unexpected morphological information about the complex cystic nature of the tumour and local adhesions (Figure 13).
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Figure 12. (a) Posterior extradural duplication cyst demonstrated on sagittal CISS image, with a lentiform capsule outlining the cyst. (b) Reconstructed axial CISS image showing cord atrophy and compression.
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Figure 13. (a) T2 weighted sagittal image of the thoracolumbar spine showing an intraspinal schwannoma. (b) Post-gadolinium T1 weighted section showing marked enhancement of the tumour. (c) Sagittal CISS image clearly defines the margins and morphology of the tumour and its relationship to the cauda equina.
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Advantages and Limitations
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CISS imaging provides superior topographical information regarding extradural and intradural pathology. CISS has the advantage of combining the best features of X-ray myelography, such as fine detail of nerves and neural tethering, and MRI in showing structural abnormalities of nerve roots, compression and displacement of the thecal sac, syringomyelia and diastomyelia. There are, however, limitations, which include a long acquisition time (9 min), inherent lack of tissue characterization, and CSF flow artefacts, which become a problem at the cervicothoracic junction and in narrowed CSF spaces.
Received for publication February 18, 2000.
Revision received November 1, 2000.
Accepted for publication November 27, 2000.
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Abstract
Introduction
