History
A 52-year-old female patient, suffering from spinal canal stenosis, myelopathy and radiculopathy, underwent spinal osteosynthesis a year ago. She returned to the hospital complaining of chronic and refractory lower back pain, which had worsened over the past 5 months, with radiation into and paresis of the right lower limb. A Dual Energy (DE) CT was performed to assess the integrity and position of the osteosynthesis device and to rule out osteolytic destruction as well as compression of the spinal canal and neuroforamina.
Diagnosis
DECT images displayed at 70 keV (equivalent to 120 kV of conventional CT) showed severe metal artifacts, caused by the osteosynthesis device, which was intact. The artifacts were significantly reduced in images displayed at 140 keV. A lucency was seen around the screw margins in the right ilium, indicating osteolysis. A right foraminal stenosis, compressing the nerve root at L5-S1, was also visualized. There were no signs of a spinal canal stenosis.
Courtesy of Radiology Department, Hospital das Clínicas – InRad, Sao Paulo, Brazil
Fig. 1: Cinematic volume rendered images, using monoenergetic images displayed at 140 keV (Figs. 1a & 1b), clearly show an intact osteosynthesis device implanted in the lower back. The same structures are blurred and hard to interpret when using monoenergetic images displayed at 70 keV (Fig. 1c).
Courtesy of Radiology Department, Hospital das Clínicas – InRad, Sao Paulo, Brazil
Fig. 2: An axial image, displayed at 140 keV (Fig. 2a), clearly shows osteolytic changes (arrow) around the right iliac screw margins. The 70 keV image is more challenging to interpret (Fig. 2b) due to severe metal artifacts. Both images are presented at the same window settings.
Courtesy of Radiology Department, Hospital das Clínicas – InRad, Sao Paulo, Brazil
Fig. 3: Metal artifacts are significantly reduced in an axial image displayed at 140 keV (Fig. 3a), however a right foraminal stenosis, compressing the nerve root at L5-S1 (arrow), is more clearly demonstrated in the image displayed at 70 keV (Fig. 3b). Both images are presented at the same window settings.
Comments
CT is commonly performed for postoperative evaluation in orthopedic patients with spinal osteosynthesis. However, metal artifacts caused by the osteosynthesis devices impose a significant diagnostic challenge on accurate CT interpretation. To overcome this challenge, a TwinSpiral scan technique is applied, to acquire two successive scans in opposite directions and at different kV and mA settings. The second scan uses a tin filter to optimize the spectral separation of materials, improving image results. This approach has a lower temporal coherence, and is therefore focused on imaging static structures, such as a spine with an osteosynthesis device. The acquired images are processed using syngo.CT DE Monoenergetic Plus, which allows users to display monoenergetic images within a range of 40–190 keV. Metal artifacts can be effectively reduced by generating images extrapolated to higher photon energies, such as 140 keV in this case, facilitating the identification of osteolytic changes around a loosening screw. On the other hand, it is also observed that contrast resolution for soft tissue assessment is well preserved at lower photon energies, such as 70 keV in this case, leading to a better appreciation of the compressed nerve root. TwinSpiral DE allows monoenergetic images to be displayed at different photon energies levels assisting accurate diagnosis and optimizing workflow.