Abstract

Article Series: Dual Energy CT – Scientific Evidence and Clinical Application (5/7) – Extremities

posted by Thorsten R. C. Johnson, M.D. | Dec 20, 2010

This article is part of the seven-article series on “Dual Energy CT – Scientific Evidence and Clinical Application” and covers Dual Energy applications of the extremities comprising peripheral arteries, tendons and ligaments, gout, bone marrow edema, and metal artifact reduction.

Extremities

Peripheral Arteries

Figure 9 A maximum intensity projection shows multiple occlusions and collaterals in the entire vasculature of the legs.

Similar to the arteries of head and neck, it is feasible to differentiate bone and arteries in runoff angiography datasets and display the vasculature without superimposition of bones. Studies have confirmed that Dual Energy based bone removal is more reliable and faster than conventional, software based bone removal [118-122]. The resulting maximum intensity projections provide an excellent overview of the entire vasculature of abdomen, pelvis, and the entire legs in one single image. Additionally, the software implemented in Dual Source systems provides the option to add calcified plaques back into the MIP image. Thus, the plaques can be switched on and off in order to visualize both the residual lumen and the plaque burden in the area of a stenosis. These images are very helpful for communicating the angiographic findings, e.g. to vascular surgeons.

The algorithm further contains a beam hardening correction that aims to identify the interface between plaque and vessel lumen more exactly, and studies have shown that the residual lumen is quantified more precisely with this technique [123]. Further studies have confirmed the efficacy of plaque removal but also showed limitations in small diameters and low enhancement of calcified crural arteries [124]. Therefore, in our experience the combination of runoff angiography in Dual Energy technique and an additional dynamic acquisition of the lower legs with a second small bolus of contrast material provide the best option to clarify the situation in critical limb ischemia.

Tendons and Ligaments

Figure 10 Tendons of the ankle are visualized well. However, the ligaments which are of diagnostic importance are too thin to visualize with this technique.

Initial experiments had shown that tendons and ligaments can be identified in a Dual Energy dataset due to their weak spectral properties in comparison to other tissue with similar CT density [125-126]. This would be an attractive option to evaluate tendons and ligaments along with the bones, e.g. in joint evaluation after trauma. There have been some reports on successful clinical application of this technique [127-128]. However, the contrast to noise ratio is very weak, so that there is considerable noise in the surrounding tissue and the signal in the tendon itself is not always continuous, depending on the adjacent bony structures and the resulting beam hardening. Therefore, the diagnostic value in clinical practice is quite limited and certainly no competition to MRI [129].

Gout

Figure 11 Volume rendered image showing deposits of uric acid in the right index finger in blue. Contrast enhancement is color coded in red.

Initial reports had shown that it is not only possible to identify uric acid in kidney stones, but also in the soft tissue around joints as morphological substrate of gout [130]. The method proved to be very helpful in clinical practice as it confirms the diagnosis without joint fluid aspiration and additionally shows the extent of gout tophi and any affection of the adjacent joint [131-136].

Bone Marrow Edema

Bone marrow edema is a very sensitive sign for any pathological process in bones in MRI. Similarly, Dual energy CT can identify water in the bone marrow and thus identify bone bruises which are normally not visible in spongy bone in CT. Initial reports confirmed the feasibility and very good agreement with MRI [137].

Metal Artifact Reduction

Figure 12 Normal (left) and high energy monoenergetic reconstruction (right) showing that metal artifacts can be reduced significantly.

Metal artifacts pose a significant problem in clinical CT. Especially after implantation of any metallic prostheses or other hardware, the implant itself, the interface between implant and bone, and the surrounding tissue are most important to exclude a fracture of the hardware, loosening, infection, or hematoma. However, metal artifacts severely impair image quality, often rendering it impossible to answer the relevant diagnostic questions.

Metal artifacts consist of photon starvation and beam hardening. The former results in lack of transmission and can only be reduced by increasing the energy and amount of photons. The latter can be corrected based on Dual Energy information, because there is a difference in beam hardening between the low and high energy acquisition; thus it is feasible to extrapolate an image with the beam hardening properties at very high photon energies. Initial own studies confirmed that a significant reduction of metal artifacts is feasible with this technique. In contrast to other Dual Energy protocols, we applied a higher current on the filtered high energy tube in order to increase the share of high energy photons [138]. Further research will be required to determine the optimal tube current relation.