Dual-layer spectral computed tomography aortography using a seventy-five-percent-reduced iodine dose protocol and multiparameter spectral imaging: comparison with conventional computed tomography imaging

Abstract

Background: Computed tomography angiography (CTA) is the recommended diagnostic and follow-up imaging modality for acute aortic dissection (AD). However, the high-contrast medium burden associated with repeated CT aortography follow-ups remains a significant concern. This prospective study aimed to assess whether an ultra-low contrast dose (75% cutoff) aortic CTA protocol on dual-layer spectral CT could achieve comparable image quality with the full dose protocol. We also investigated the image quality of the virtual noncontrast (VNC) images derived from the ultra-low dose protocol.

Methods: This study included 37 consecutive patients who were referred to aortic CTA from May 2022 to August 2022. The enrolled patients underwent full-dose contrast CTA and ultra-low dose (reduced to 25% of conventional) contrast CTA on dual-layer spectral CT in 1 day. Virtual monochromatic images (VMIs) were reconstructed with 40 and 70 keV. The VNC images were reconstructed for both protocols. Objective image quality evaluation, recorded as signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs), was compared between the groups using 1-way analysis of variance and post hoc analysis with Bonferroni correction. Subjective image quality was also compared between the groups. Finally, VNC images derived from the low-dose (VNC_low) and full-dose (VNC_full) protocols were compared to the true noncontrast (TNC) images.

Results: Neither CNR nor SNR was lower for the 40-keV images reconstructed from the ultra-low dose group compared to the conventional images. Both were significantly higher than those of the 70-keV images. Regarding subjective image quality, vessel enhancement was not significantly different between the 40-keV VMI and full-dose images [ascending aorta (AAO): 4.37±0.46 vs. 4.57±0.48, P=0.096; brachiocephalic arteries: 4.34±0.45 vs. 4.51±0.49, P=0.152; abdominal aortic side branch: 4.42±0.48 vs. 4.51±0.49, P=0.480]. The VNC_low images were similar to the TNC images but significantly different from the VNC_full images (P<0.001).

Conclusions: Ultra-low contrast aortic CTA with a 75%-reduced iodine dose using dual-layer spectral CT and the derived VNC achieved image quality comparable to that of conventional CTA and TNC images.

Keywords: Aortic computed tomography aortography (aortic CTA); aortic dissection (AD); dual-layer spectral CT (DLCT); virtual noncontrast images (VNC images).

Figure 1

Flowchart of the study. CTA, computed tomography angiography; TNC, true noncontrast; VNC, virtual noncontrast; VMI, virtual monochromatic image.

Figure 2

Case-based comparison of the CT angiography images among the full-dose image, 70-keV VMI, and 40-keV VMI groups at the AAO, DAO, and AA levels. Note that the vessel enhancement of the 40-keV images was similar to that of the conventional images, and the enhancement of the above 2 was significantly better than that of the 70-keV images. White dotted circles indicate an implanted stent. VMI, virtual monochromatic image; AAO, ascending aorta; DAO, descending aorta; AA, abdominal aorta; CT, computed tomography.

Figure 3

Case-based comparison of the VNC images derived from the 75% cutoff ultra-low (VNC_low) and standard full dose (VNC_full) contrast protocol with reference to the TNC images at the AAO, DAO, and AA levels. Note that significant remnants of iodine (black asterisk) could be found in the VNC_full images, but only slight amounts were found in the VNC_low images. In addition, the intramural hematoma (arrows) of clinical significance could be found in the TNC and VNC_low images but would be mistaken as a thrombus in the VNC_full images. White dotted circles indicate an implanted stent. VNC, virtual noncontrast; TNC, true noncontrast; AAO, ascending aorta, DAO, descending aorta, AA, abdominal aorta.