Congress:
EuroSafe Imaging 2018
Keywords:
Aneurysms, Radiation safety, Diagnostic procedure, Audit and standards, CT-Angiography, Vascular, Radioprotection / Radiation dose, Interventional vascular, Action 2 - Clinical diagnostic reference levels (DRLs), Action 3 - Optimisation, diagnostic reference levels, image quality, Dosimetric comparison
Authors:
M. A. Fink, A. Steuwe, M. Cattelaens, L. Born, W. Stiller, H. U. Kauczor, F. Rengier
DOI:
10.1594/esi2018/ESI-0037
Description of activity and work performed
To date,
the CTA based follow-up after EVAR has been regularly preformed as a triphasic examination using single-energy technique.
This triphasic protocol usually includes a non-contrast scan limited to the region of the stent graft,
an arterial scan covering the abdominal aorta down to the groin and as indicated also the thoracic aorta,
and a delayed phase again limited to the region of the stent graft typically acquired 60-70 s after contrast media injection.
Depending on the postinterventional result and the occurrence of complications,
such acquisitions can be necessary multiple times in the patient’s lifetime resulting in a substantial cumulative radiation exposure.
The here described approach for saving radiation dose is to replace the noncontrast and the delayed phase by a single delayed phase using the dual-source dual-energy technique.
Thus,
the postinterventional CT protocol is reduced to a biphasic examination with the arterial phase as before in single-energy technique and the delayed phase in dual-source dual-energy technique limited to the region of the stent graft typically acquired 60-70 s after contrast media injection.
Virtual noncontrast images are reconstructed from the dual-energy delayed phase (Fig.
1).
Standard acquisition parameters used for the arterial and delayed phase are summarised in Table 1.
For the acquired dual-energy dataset several postprocessing options exist to generate clinically useful information (Fig.
2).
First of all,
the two separate data sets of the delayed phase (80 kVp data set and Sn140 kVp data set) can be combined into a single weighted average image dataset.
The weighting factor can be chosen according to individual preferences and typically lies between 0.3-0.7.
We suggest a weighting factor of 0.5.
For higher contrast to noise ratio,
the weighting factor can be changed to 0.7 for the 80 kVp data.
The weighting towards lower photon energies thereby increases the weighting of the higher photoelectric absorption of iodine leading to a better visualization of contrast agent in the aortic/prosthetic lumen.
For reduced noise,
the weighting factor can be changed to 0.3 for the 80 kVp data.
For further analyses,
the dual-energy software package of syngo.via (Siemens Healthineers,
Forchheim,
Germany) provides application profiles to create VNC and color-coded (application: LiverVNC) as well as virtual monoenergetic images (application: Monoenergetic+) (Fig.
2).
The color coding of iodine and calcium (iodine map,
hard plaque imaging) may help in the detection of small endoleaks and their differentiation from aortic plaques (Fig.
3).
Virtual monoenergetic images are computed data sets for a specific energy level ranging between 40-190 keV.
The chosen energy level determines the degree of contrast and noise with lower energy levels resulting in higher contrast but higher noise and higher energy levels resulting in lower contrast with the benefit of lower noise.
Virtual monoenergetic images are not part of our clinical routine but can be helpful in difficult cases e.g.
in cases with low contrast in the weighted average images.
One potential pitfall of the biphasic dual-energy CT protocol are patients after implantation of the Nellix® EndoVascular Aneurysm Sealing System (Endologix,
Irvine,
USA) due to the contrast material injected into the endobags of the sealing system during the implantation.
This contrast material is then slowly resorbed over a period of several months to years.
The virtual noncontrast algorithm cannot differentiate between contrast material injected as part of the CT scan and contrast material within the endobags.
As a consequence,
the contrast material in the endobags will disappear in the VNC images and will be colored in iodine maps (Fig.
4).