Benefits of Photon-Counting CT Myelography for Localization of Dural Tears in Spontaneous Intracranial Hypotension

Previous Myelographic Techniques

In recent years, the 2 main techniques used for localization of dural tears have included DSM and dynamic EID CTM.⁷ These procedures are performed with the patient positioned prone when a ventral dural tear is suspected and decubitus when a lateral dural tear is anticipated.⁸ DSM provides higher spatial and temporal resolution and typically has a lower radiation dose compared with dynamic CTM on EID scanners.⁹ By contrast, the main advantage of dynamic EID CTM is that it provides cross-sectional detail, which often allows one to determine not only the level but also the precise laterality of a dural tear (for example, left/right ventrolateral versus midline ventral). Furthermore, dynamic CTM can reveal osseous spicules or osteophytes that frequently cause ventral dural leaks, while such lesions are usually occult on DSM.¹⁰ Dynamic CTM was initially described in 2003 by Luetmer and Mokri.¹¹ Since that time, the technique has been improved with newer-generation CT technology.^12⇓-14 To date, all described techniques for dynamic CTM have used EID CT scanners, most recently including modern dual-energy CT scanners with fast scan speeds.^12,13 PCCT represents a fundamental shift in CT technology, using a novel detector mechanism that permits a wide variety of the aforementioned improvements.

Dynamic PC-CTM Technique

In our practice, all patients with suspected SIH undergo noncontrast MR imaging of the entire spine. If this demonstrates extradural CSF, a dural tear is assumed to be present.¹⁵ If the extradural CSF collection is largely ventral, a ventral dural tear is suspected. If the extradural collection is predominantly dorsolateral or if there is a large or irregular meningeal diverticulum, greater consideration is given to the possibility of a lateral leak site. On the basis of these factors, prone-versus-decubitus positioning for subsequent myelography is chosen. The decision among DSM, dynamic EID CTM, and dynamic PC-CTM is currently based on the radiologist’s preference and/or equipment availability.

For dynamic PC-CTM, patients are placed prone or decubitus on the PCCT scanner table (NAEOTOM Alpha; Siemens), using a custom cushion to elevate the pelvis. Lumbar puncture is performed under CT guidance at L2–L3 or below using a 22-ga Quincke spinal needle. Intrathecal needle positioning is confirmed with a tiny (∼0.2 mL) injection of Omnipaque 300 (GE Healthcare). A low-dose monitoring scan is set up approximately 2 spinal levels below the lowest level at which extradural CSF is present. Next, 2 mL of contrast is injected while the monitoring scan is continuously repeated every 5 seconds. As soon as intrathecal contrast is visible on the monitoring scan, 2–4 scans of the spine are initiated, with the exact number of scans depending on patient-specific factors (for example, more scans may be obtained if the patient has undergone previously unrevealing myelograms). The scan range may be limited to the spinal levels at which extradural CSF is present or may include the entire spine, largely depending on proceduralist’s preference. An additional 1–2 mL of contrast is injected immediately before every scan. The scans are obtained as quickly as allowed by the scanner, with usually only 2–3 seconds of delay between scans.

Each scan is obtained in standard resolution mode (SR, 144 × 0.4 mm detector collimation), except for the final scan, which is obtained in the ultra-high-resolution mode (UHR, 120 × 0.2 mm detector collimation). The SR mode provides greater temporal resolution due to a shorter scan time, though the resulting section thickness is limited to 0.4 mm. The UHR mode usually has approximately double the scan time but can provide a section thickness as low as 0.2 mm. The rationale for performing a UHR scan at only the final acquisition is to provide at least 1 high-resolution scan without impairing the temporal resolution during the most time-sensitive portion of the examination. Both scan modes confer spectral sensitivity even when a single x-ray source is used, meaning that images can be reconstructed at any desired virtual monoenergetic level. All scans use a rotation time of 0.5 seconds and a CARE keV IQ level of 200. The pitch is 1.2 (SR) or 1.0 (UHR). Automatic exposure control is used, with a manual tube potential of 140 kV.

In our protocol, all scans are reconstructed with the lowest section thickness allowed by the scan mode (0.4 mm for SR and 0.2 mm for UHR). A low-energy threshold (referred to as T3D by the manufacturer and including all photon energies from 25 to 140 keV) is applied to all series. Separately, we reconstruct 40-keV virtual monoenergetic images (VMIs) to maximize iodine conspicuity (thus, 2 series are created for each scan performed). We use a Br48 kernel (quantum iterative reconstruction setting of 3) for the 40-keV VMIs and a Br56 kernel (quantum iterative reconstruction setting of 4) for T3D reconstructions. These are both relatively sharp kernels that provide high spatial resolution while maintaining an acceptable noise level. As with previously described techniques, the site of the dural tear is determined by direct visualization of a contrast column leaking into the epidural space (Fig 1). The approximate leak site can also be inferred by the lowest spinal level at which epidural contrast is seen, though this is a less definitive finding, and direct visualization of the exact level is strongly preferred.

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FIG 1.

Ventral dural tear localized on PC-CTM after negative prone DSM and negative prone dynamic EID CTM. Sagittal low-energy threshold (T3D) reconstructions from the first scan during dynamic PC-CTM demonstrate a ventral leak at T2–T3 (A, arrow). The leak remained visible on the second scan obtained 4 seconds later (not shown) but was barely visible on the third scan obtained after another 4 seconds (B, arrow), largely because of dense contrast that had already leaked into the epidural space. An axial 0.4-mm image from the first scan (C) clearly shows the precise site of the dural tear, which is slightly to the left of midline (C, arrow). This leak was occult on prone DSM (unsubtracted image D, arrow), as well as on prone dynamic EID CTM (not shown), both of which were performed before the PC-CTM.