PEDIATRICS
SPINE
CME
ABBREVIATION KEY: SSFP ⫽ steady-state free precession BFFE ⫽ balanced fast field echo FIESTA ⫽ fast imaging employing steady-state acquisition FISP ⫽ fast imaging with steadystate precession
Pediatric Spine Disorders: Appearance on Steady-State Free Precession MR Images
Received November 6, 2012; accepted after revision August 17, 2013.
CME Credit
From the Department of Radiology and Imaging Sciences (B.P.S.), Emory University Hospital, Atlanta, Georgia; Department of Radiology and Biomedical Imaging (M.M., J.D.M.), University of California, San Francisco, San Francisco California; and Departments of Neurosurgery (P.P.S.) and Diagnostic Imaging (K.W.M.), UCSF Benioff Children’s Hospital, Oakland, California.
The American Society of Neuroradiology (ASNR) is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to provide continuing medical education for physicians. The ASNR designates this enduring material for a maximum of one AMA PRA Category one creditTM. Physicians should claim only the credit commensurate with the extent of their participation in the activity. To obtain credit for this activity, an online quiz must be successfully completed and submitted. ASNR members may access this quiz at no charge by logging on to eCME at http://members.asnr.org. Nonmembers may pay a small fee to access the quiz and obtain credit via http://members.asnr.org/ecme.
Please address correspondence to Kenneth W Martin, MD, Department of Diagnostic Imaging, UCSF Benioff Children’s Hospital, 747 52nd Street, Oakland, CA 94609; email:
[email protected]
B.P. Soares, M. Mabray, J.D. MacKenzie, P.P. Sun, and K.W. Martin
http://dx.doi.org/10.3174/ng.3140087
ABSTRACT Pediatric spinal disorders are routinely imaged using conventional T1- and T2-weighted sequences. Steady-state free precession sequences are a cornerstone in the evaluation of cranial nerves and inner ear structures and may be used as a problem-solving tool in pediatric spinal pathology due to their high spatial resolution and contrast between CSF and soft tissues. The exquisite anatomic detail provided by steady-state free precession facilitates the depiction and characterization of subtle and complex findings in pediatric spinal disorders. Learning Objective: To understand the value of steady-state free precession MR images in the depiction and characterization of subtle and complex findings in pediatric spinal disorders.
INTRODUCTION
Despite the advances in imaging technology during the past decade, spatial resolution remains one of the limitations of spinal MR imaging. Steady-state free precession (SSFP) is a sequence that provides excellent contrast resolution between CSF and soft tissues, given its characteristic strong signal intensity in tissues that have a high T2/T1 ratio.1 SSFP is a fundamental sequence in the imaging evaluation of the cranial nerves, cerebellopontine angle, and inner ear structures.1 It is also a valuable tool in the diagnostic and postoperative evaluation of hydrocephalus due to its capability of delineating the ventricular walls, detecting abnormal cystic structures, and assessing the patency of ventriculostomies.2,3 In children, SSFP is commonly used for the evaluation of the cochlear
nerve and inner ear in the setting of congenital hearing loss and for the diagnostic and postoperative evaluation of hydrocephalus.2 Although not routinely used in spinal imaging, SSFP has been used in a variety of adult spinal disorders4 and in the preoperative evaluation of neonates with lumbosacral lipoma and myeloschisis.5 In this article, we demonstrate that the high spatial resolution and the water/softtissue contrast of SSFP imaging provide exquisite anatomic detail for the depiction of subtle and complex findings in the pediatric spine. Technique
The SSFP technique relies on a gradient echo pulse sequence with small flip angles and short relaxation times to create a nonzero steady-state between pulse repetitions Neurographics 4:133–138 September 2014 www.neurographics.com
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Fig 1. Tethered cord. Sagittal T1, T2, and SSFP images demonstrate a thickened filum terminale (arrow). The T2 axial image confirms that the filum is thicker than adjacent nerve roots (small arrow). The conus medullaris lies abnormally low at L3 (arrowhead). Note also the dysmorphic appearance of the sacrum.
for both the longitudinal and transverse relaxation values of the interrogated tissues.6 The volumetric acquisition and submillimeter spatial resolution allow reconstructions in multiple planes for high detail imaging of the pediatric neuroaxis. This sequence is also known as balanced fast field echo (BFFE) (Philips Healthcare, Best, the Netherlands), fast imaging employing steady-state acquisition (FIESTA) (GE Healthcare, Milwaukee, Wisconsin), or True fast imaging with steady-state precession (FISP) (Siemens, Erlanger, Germany). The images displayed in this pictorial essay were acquired at 1.5T (Intera/Achieva; Philips Healthcare). We acquired the sequence in the sagittal plane using a 1.0-mm section thickness with 0.5-mm intervals, resulting in 0.33 ⫻ 0.33 mm pixels. Acquisition time was approximately 4 minutes for every 3 cm of coverage. Pediatric Spinal Disorders
Spinal Dysraphism. Spinal dysraphism can be categorized as closed or open, on the basis of the presence or absence of intact skin covering. In closed spinal dysraphisms, there may be a subcutaneous mass, containing fat, CSF, or neural tissue. Simple spinal dysraphisms without a subcutaneous mass include abnormalities of the filum terminale and dermal sinuses. Complex spinal dysraphisms include split cord malformations and caudal regression syndrome.7 Tethered Cord (Simple Spinal Dysraphism). MR imaging demonstrates the level of the conus medullaris, the thickness of the filum terminale, and associated anomalies. The conus medullaris is normally at or above the L2 inferior endplate.8 The normal filum is thin and should have a diameter equal to or less than that of adjacent nerve roots; thickening should raise the possibility of a tethered cord (Fig 1).9 The thecal sac may be widened, and the dorsal dura 134
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Fig 2. Meningocele manque ´. Sagittal SSFP image demonstrates a lowsignal-intensity band tethering the dorsal surface of the cord to the posterior thecal sac (arrow).
Fig 3. Dorsal dermal sinus. Sagittal T1, T2, and SSFP images demonstrate a fibrous band (arrow), which attaches to the dermatome on the spinal cord matching the entry point of a dorsal dermal sinus passing through the spinal canal (arrowhead).
Fig 4. Epidermoid tumor. Sagittal T1, T2, and postcontrast T1 images demonstrate a mildly heterogeneous cystic mass displacing the conus (red arrow). On SSFP and DWI images (lower row), the heterogeneous content and reduced diffusion (bright signal intensity on DWI and low signal intensity on the apparent diffusion coefficient map [not shown]) are characteristic of epidermoid tumors. Neurographics 4:133–138 September 2014 www.neurographics.com
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Fig 6. Arachnoid cyst. Sagittal SSFP image depicts the wall and homogeneous content of a circumscribed intradural extra-medullary cyst (arrow) with mild mass effect on the spinal cord.
Fig 5. Split cord malformation. Coronal SSFP image shows a dominant left hemicord and a smaller right hemicord within a single thecal sac.
may be tented posteriorly. The meningocele manque´, a fibrous or meningeal band tethering the cord to the thecal sac,10 is well-demonstrated on SSFP images (Fig 2). Associated fibrolipomas of the filum can be noted as high signal intensity on T1-weighted imaging and low signal intensity on T2. Areas of increased T2 signal intensity and low T1 signal intensity can be seen in the central spinal cord representing a syrinx or myelomalacia. Dorsal Dermal Sinus, Dermoid, and Epidermoid Tumors. The subcutaneous portion of a dorsal dermal sinus tract can be appreciated as a low signal intensity tract on T1-weighted imaging. The T2 appearance can be of high signal intensity if the tract is filled with fluid. The intrathecal components can be difficult to see on conventional MR imaging but can be seen on SSFP images as a hypointense linear or curvilinear structure (Fig 3). Contrast-enhanced images with fat suppression may help to identify the sinus if there is granulation tissue present from prior infection or inflammation. 136
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Fig 7. Syrinx. Sagittal SSFP images acquired before (left) and after (right) posterior fossa decompression for Chiari malformation show a decrease in the size of a multiseptate syrinx throughout the cervicothoracic cord. Note the detailed depiction of the multiple septations.
The dermal sinuses may be associated with intraspinal extramedullary dermoids/epidermoids that have characteristic heterogeneous signal intensity on SSFP images (Fig 4). DWI confirms the diagnosis by showing reduced diffusion in the lesion.
Fig 8. Leptomeningeal metastasis. Sagittal SSFP image shows a nodule attached to the cauda equina in a patient with a suprasellar germ cell tumor (arrow), also shown on a coronal reformation (arrowhead). The nodule is not seen on conventional postcontrast T1- or on T2-weighted images.
Split Cord Malformation/Diastematomyelia (Complex Spinal Dysraphism). Split cord malformations will demonstrate a division of the spinal cord into 2 symmetric or asymmetric hemicords each containing a central canal (Fig 5).11 In diastematomyelia, a fibrous or osseous spur may be identified. If there is an associated syrinx, it will follow CSF signal intensity. Arachnoid Cyst. An arachnoid cyst should follow CSF signal intensity on all imaging sequences. An arachnoid cyst will be suppressed and will be low intensity on FLAIR imaging like CSF, which distinguishes it from an epidermoid cyst. However, FLAIR is not routinely used in spinal imaging. Differentiation from an epidermoid cyst may also be possible on DWI, because the epidermoid cyst has reduced diffusion. Whereas conventional MR images indicate the presence of the lesion by displacement of the spinal cord, nerves, or vessels, SSFP images are uniquely able to depict the walls of the cyst (Fig 6). Syrinx/Hydromyelia
A syrinx/hydromyelia appears on MR imaging as a CSF intensity collection within the spinal cord, which may enlarge the involved cord section. The collection may be within the substance of the cord (syrinx) or may dilate the central canal (hydromyelia). Often incomplete septations can be seen within the syrinx cavity, and these are nicely demonstrated by SSFP images (Fig 7). Increased signal intensity can be seen on T2-weighted images within the parenchyma at the ends of the syrinx. This is thought to be from microcystic or astrogliotic changes resulting from pulsations of the cyst on the adjacent cord. It is possible to have flow artifacts within the syrinx cavity from CSF pulsations. Leptomeningeal Metastasis. CSF-disseminated metastases are seen as nodular extramedullary lesions or a diffuse coat-
Fig 9. Postoperative pseudomeningocele. Sagittal T2 image shows a postoperative fluid collection in the posterior epidural soft tissues. SSFP image demonstrates the precise location and size of the dural defect (arrow).
ing of the cord that enhances after the administration of contrast. MR imaging is generally insensitive without the use of contrast, and sensitivity may be increased by the use of higher doses of contrast. Typical locations of leptomeningeal metastasis include the lower thoracic and lumbar cord surface, thecal sac, and roots of the cauda equina. SSFP images may have the potential to improve the sensitivity for small leptomeningeal metastases and differentiation from Neurographics 4:133–138 September 2014 www.neurographics.com
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tion of subtle and complex findings in the pediatric spine. In addition to the current routine use for the evaluation of cranial nerves, inner ear structures, and hydrocephalus, SSFP may be a helpful adjunct for the evaluation of pediatric spine pathology.
Fig 10. Nerve root avulsion. Transverse SSFP image shows a right cervical pseudomeningocele (arrow) and truncation of the corresponding nerve roots (arrowhead) in a child with traumatic brachial plexopathy. The left nerve roots are normal.
venous structures on the surface of the cord and normal nerve roots (Fig 8). Nerve Root Avulsion and Pseudomeningocele
Abnormalities seen in traumatic nerve root injury include nerve root avulsion, pseudomeningocele, thickened or enhancing nerve roots suggesting neuroma or scar, and lowlying nerve roots without pseudomeningocele formation. Injuries are more commonly preganglionic (ie, proximal to the dorsal root ganglion). Preganglionic avulsions are not amenable to surgical repair. The pseudomeningocele presents as a dilated nerve root sheath and follows CSF signalintensity characteristics (Fig 9). The nerve roots may abruptly terminate, and the actual disruption can be clearly depicted on SSFP images (Fig 10). CONCLUSIONS
The high spatial resolution and water/soft-tissue contrast of SSFP imaging provides exquisite anatomic detail for depic-
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