Assessment of the Hepatic Veins in Poor Contrast Conditions using Dual Energy CT: Evaluation of a Novel Monoenergetic Extrapolation Software Algorithm

Abstract

Purpose: To evaluate a novel monoenergetic post-processing algorithm (MEI+) in patients with poor intrahepatic contrast enhancement.

Materials and Methods: 25 patients were retrospectively included in this study. Late-phase imaging of the upper abdomen, which was acquired in dual-energy mode (100/140 kV), was used as a model for poor intrahepatic contrast enhancement. Traditional monoenergetic images (MEI), linearly weighted mixed images with different mixing ratios (MI), sole 100 and 140 kV and MEI+ images were calculated. MEI+ is a novel technique which applies frequency-based mixing of the low keV images and an image of optimal keV from a noise perspective to combine the benefits of both image stacks. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the intrahepatic vasculature (IHV) and liver parenchyma (LP) were objectively measured and depiction of IHV was subjectively rated and correlated with portal venous imaging by two readers in consensus.

Results: MEI+ was able to increase the SNR of the IHV (5.7 ± 0.4 at 40keV) and LP (4.9 ± 1.0 at 90keV) and CNR (2.1 ± 0.6 at 40keV) greatly compared to MEI (5.1 ± 1.1 at 80keV, 4.7 ± 1.0 at 80keV, 1.0 ± 0.4 at 70keV), MI (5.2 ± 1.1 M5:5, 4.8 ± 1.0 M5:5, 1.0 ± 3.5 M9:1), sole 100 kV images (4.4 ± 1.0, 3.7 ± 0.8, 1.0 ± 0.3) and 140 kV images (2.8 ± 0.5, 3.1 ± 0.6, 0.1 ± 0.2). Subjective assessment rated MEI+ of virtual 40 keV superior to all other images.

Conclusion: MEI+ is a very promising algorithm for monoenergetic extrapolation which is able to overcome noise limitations associated with traditional monoenergetic techniques at low virtual keV levels and consequently does not suffer from a decline of SNR and CNR at low keV values. This algorithm allows an improvement of IHV depiction in the presence of poor contrast.

Key points:

• The evaluated new image-based algorithm for virtual monoenergetic imaging allows calculating low virtual keV images from dual energy datasets with significantly improved contrast-to-noise ratios.

• The image based novel monoenergetic extrapolation algorithm applies frequency-based mixing of the low keV images and an image of optimal keV from a noise perspective to combine the benefits of both image stacks.

• When compared to traditional monoenergetic images, the novel monoenergetic algorithm has improved contrast-to-noise ratios for both low and high virtual keV images.

• Contrast-enhanced dual energy images with poor contrast conditions can be significantly improved, e.g. late phase imaging of the liver.

Citation Format:

• Schabel C, Bongers M, Sedlmair M et al. Assessment of the Hepatic Veins in Poor Contrast Conditions using Dual Energy CT: Evaluation of a Novel Monoenergetic Extrapolation Software Algorithm. Fortschr Röntgenstr 2014; 186: 591 – 597

Zusammenfassung

Zielsetzung: Die Evaluierung eines neuartigen monoenergetischen Nachverarbeitungsalgorithmus (MEI+) bei Patienten mit eingeschränkter intrahepatischer Kontrastierung.

Material und Methoden: 25 Patienten wurden retrospektiv in der Studie eingeschlossen. Computertomografien des oberen Abdomen in später Kontrastmittelphase, welche im Dual-Energy-Modus (100/140 kV) akquiriert wurden, wurden als Modell für eine schwache intrahepatische Kontrastierung verwendet. Es wurden linear gewichtete Mischbilder (MI), traditionelle monoenergetische (MEI), reine 100 und 140 kV sowie MEI+ Bilder berechnet. MEI+ ist eine neuartige Technik, welche frequenzbasiert rauschbehaftete virtuelle niedrig-keV Bilder mit Bildern optimaler keV (aus Perspektive des Bildrauschens) kombiniert und so die Vorteile beider Bilddatensätze vereint. Signal-zu-Rauschen (SNR) und Kontrast-zu-Rauschen (CNR) intrahepatischer Gefäße (IHV) und des Leberparenchyms (LP) wurden objektiv gemessen. Subjektiv wurde die Abgrenzbarkeit der IHV von zwei Untersuchern im Konsensus beurteilt und mit portal-venösen Bildern verglichen.

Ergebnisse: MEI+ verbesserte SNR von IHV (5,7 ± 0,4 bei 40keV) und LP (4,9 ± 1,0 bei 90keV) sowie CNR (2,1 ± 0,6 bei 40keV) erheblich, verglichen mit MEI (5,1 ± 1,1 bei 80keV; 4,7 ± 1,0 bei 80keV; 1,0 ± 0,4 bei 70keV), MI (5,2 ± 1,1 M5:5; 4,8 ± 1,0 M5:5; 1,0 ± 3,5 M9:1), reinen 100 kV Bildern (4,4 ± 1,0; 3,7 ± 0,8; 1,0 ± 0,3) und 140 kV Bildern (2,8 ± 0,5; 3,1 ± 0,6; 0,1 ± 0,2). Die subjektive Beurteilung schätzte MEI+ bei 40 keV gegenüber allen anderen Bildern als überlegen ein.

Schlussfolgerung: MEI+ ist ein vielversprechender Algorithmus für die monoenergetische Extrapolation, welcher die Limitation eines hohen Bildrauschens bei niedriger virtueller keV überwindet und keine Abnahme von SNR und CNR bei niedrigen keV verursacht. MEI+ ermöglicht eine signifikante Verbesserung der IHV-Darstellung bei eingeschränkter Kontrastbedingung.

Key words

CT - monoenergetic - liver - contrast - dual energy - post-processing

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