Lateral Decubitus Digital Subtraction Myelography: Tips, Tricks, and Pitfalls

Abstract

Digital subtraction myelography is a valuable diagnostic technique to detect the exact location of CSF leaks in the spine to facilitate appropriate diagnosis and treatment of spontaneous spinal CSF leaks. Digital subtraction myelography is an excellent diagnostic tool for assessment of various types of CSF leaks, and lateral decubitus digital subtraction myelography is increasingly being used to diagnose CSF-venous fistulas. Lateral decubitus digital subtraction myelography differs from typical CT and fluoroscopy-guided myelograms in many ways, including equipment, supplies, and injection and image-acquisition techniques. Operators should be familiar with techniques, common pitfalls, and artifacts to improve diagnostic yield and prevent nondiagnostic examinations.

Fig 1.

SSCSFL diagnostic imaging algorithm flow diagram.

Fig 2.

Supplies for DSM.

Fig 3.

Patient positioning. A custom-made wedge is used to position the hips higher than the shoulders to create cranial flow of the contrast from the lumbar region by gravity. Reprinted with permission from the Mayo Foundation for Medical Education and Research. All rights reserved.

Fig 4.

Defining the true horizontal axis and spine tilt on DSM. By placing the C-arm in a vertical position with respect to the floor (A, dotted arrows) while maintaining the long axis of the image intensifier orthogonal to the C-arm (B, solid arrow), the long axis of the image intensifier and the long axis of the image are aligned horizontally, parallel to the floor (B and C, solid arrow). The angle between this line and the line connecting the spinous processes (C, dotted line) defines the spine tilt with respect to the floor.

Fig 5.

Examples of diagnostic DSM. Full-FOV DSM images from a lower cervical-to-mid-/lower thoracic run clearly delineate a CSF-venous fistula in the unsubtracted image (A) and subtracted image (B). A digitally magnified unsubtracted image from another patient shows a very subtle CSF-venous fistula (C), which is better seen in a subsequent CT myelogram (D).

Fig 6.

Effect of patient rotation on the image. A, An unsubtracted image from a DSM study obtained with the patient in the true right lateral decubitus position and the image intensifier horizontal shows the contrast outlining the exiting nerve roots and filling small nerve sheath diverticula. B, An unsubtracted image from a DSM study from the same patient in the oblique left lateral decubitus position and the image intensifier horizontal shows partial obscuration of the contrast in the nerve sheaths due to an overlying contrast column within the thecal sac. Note the differences in the position of the spinous processes indicating the patient obliquity.

Fig 7.

Uneven contrast flow. Consecutive DSM images at 1 frame per 2 seconds, with the leading edge of the contrast column denoted by the solid arrows. A and B, Initial optimal flow with 1-level progression per second, but subsequent images in C and D show contrast progressing more rapidly, 2–4 levels per second. Note that only a thin layer of contrast is visible at the convex aspect of the spine curvature and less contrast is available to opacify a potential CSF-venous fistula at this level (D, dotted arrow).

Fig 8.

Tenting of the thecal sac and confirming intrathecal contrast flow. A, Lateral view spot image with a test injection shows contrast outlining the nerve roots, confirming intrathecal needle positioning. B, Lateral view spot image with a test injection in a different patient shows tenting of the thecal sac and subdural contrast despite the needle tip appearing to be well within the bony spinal canal.

Fig 9.

Mixed subdural and intrathecal injection. Unsubtracted (A) and subtracted (B) images from the DSM study demonstrate a subdural contrast injection superimposed on some intrathecal contrast. Note the masslike contrast extending cranially (arrows) rather than normal layering meniscus of intrathecal contrast. Unsubtracted image from the DSM study from another patient shows a small amount of masslike subdural contrast column extending cranially (C, arrow), which is confirmed on the subsequent decubitus CT myelogram (D, arrow).