Impact of Increasing Levels of Advanced Iterative Reconstruction on Image Quality in Low-Dose Cardiac CT Angiography

Abstract

Purpose: To investigate the effects of an advanced iterative reconstruction (IR) technique on subjective and objective image quality (IQ) in low-dose cardiac CT angiography (CCTA).

Materials and Methods: 30 datasets of prospectively triggered “step-and-shoot” CCTA scans acquired on a 256-slice CT scanner with optimized exposure settings were processed on a prototype IR system using filtered back-projection (FBP) and 4 levels of advanced IR (iDose4, Philips) providing incremental rates of IR (level 2, 4, 6, 7). In addition, the effects of different reconstruction kernels (semi-smooth [CB], standard with edge-enhancement [XCB]) and a “multi-resolution” feature [MR] to preserve the noise power spectrum were evaluated resulting in a total of n = 480 image sets. Contrast-to-noise ratios (CNR) were computed from regions of interest at 9 coronary locations. The subjective IQ was rated on a 4-point-scale with “classic” image appearance and noise-related artifacts as main criteria.

Results: At an effective dose of 1.7 ± 0.7 mSv, the CNR significantly improved with every increasing level of IR (range: 14.2 – 27.8; p < 0.001) with the best objective IQ at the highest level of IR (level 7). The subjective IQ, however, was rated best at the medium level of IR (level 4) with minimal artifacts and a more “classic” image appearance when compared to higher IR levels. The XCB kernel provided better subjective ratings than CB (p < 0.05) and the MR feature further increased the IQ at a high level of IR.

Conclusion: The objective IQ of low-dose CCTA progressively improves with an increasing level of IR. The best subjective IQ, however, is reached at medium levels of IR combined with an edge-enhancing kernel allowing for preservation of a “classic” image appearance suggesting application in the clinical routine.

Key Points:

• iterative reconstruction (IR) improves image quality in low-dose coronary CTA

• objective image quality (CNR) enhances with increasing level of IR

• best subjective image quality is reached at medium level of IR

• „multi-resolution“ algorithm further improves image quality at a higher level of IR

Citation Format:

• Kröpil P, Bigdeli AH, Nagel HD et al. Impact of Increasing Levels of Advanced Iterative Reconstruction on Image Quality in Low-Dose Cardiac CT Angiography. Fortschr Röntgenstr 2014; 186: 567 – 575

Zusammenfassung

Ziel: Untersuchung von Auswirkungen einer fortgeschrittenen iterativen Rekonstruktionstechnik (IR) auf die subjektive und objektive Bildqualität (IQ) in der Niedrigdosis CT-Coronarangiografie (CCTA).

Material und Methoden: 30 Datensätze prospektiv getriggerter „step-and-shoot“ CCTA wurden an einem 256-Schicht CT mit optimierten Dosisparametern akquiriert. Jeder Rohdatensatz wurde 16 mal an einem Prototyp für fortgeschrittene IR (iDose4, Philips) prozessiert, resultierend in n = 480 Bilddatensätzen. Neben gefilterter Rückprojektion (FBP) wurden 4 Stufen zunehmender IR sowie zusätzlich zwei Faltungskerne (CB und XCB) und ein „multi-resolution“ Algorithmus zur Bewahrung des Rauschleistungsspektrums verglichen. Kontrast-zu-Rausch Verhältnisse (CNR) wurden an 9 koronaren Lokalisationen bestimmt, die subjektive IQ auf einer 4-Punkt-Skala bewertet mit „klassischem“ Bildeindruck und rauschbedingten Artefakten als Hauptkriterien.

Ergebnisse: Bei einer effektiven Dosis von 1.7 ± 0.7 mSv wurde die mittlere CNR durch jede zunehmende IR-Stufe signifikant erhöht (Spanne 14.2 – 27.8; p < 0.001) mit bester objektiver IQ bei höchster IR-Stufe. Die subjektive IQ wurde bei mittlerer IR-Stufe (Level 4) am besten bewertet mit reduzierten rauschbedingten Artefakten und bewahrtem „klassischen“ Bildeindruck. Der kantenbetonende XCB-Faltungskern ergab bessere subjektive Bewertungen als der glättende CB-Kern (p < 0.05). „Multi-resolution“ führte zu einer weiteren Verbesserung der IQ bei hohen IR Stufen.

Schlussfolgerung: Die objektive IQ der Niedrigdosis CCTA verbessert sich mit steigendem Ausmaß iterativer Rekonstruktion. Die subjektive IQ ist hingegen bei mittlerer IR kombiniert mit kantenbetonendem XCB Kernel optimal durch ein Gleichgewicht von Rauschreduktion und Erhalt des „klassischen“ Bildeindrucks.

Kernaussagen:

• iterative Rekonstruktionstechnik verbessert die Bildqualität der Niedrigdosis CT-Coronarangiografie

• die objektive Bildqualität (CNR) steigt mit dem Ausmaß iterativer Rekonstruktion

• optimale subjektive Bildqualität wird bei mittlerer Stufe iterativer Rekonstruktion erreicht

• der „multi-resolution“ Algorithmus verbessert zusätzlich die Bildqualität bei hoher Stufe iterativer Rekonstruktion

Key words

cardiac CT - iterative reconstruction - image quality - CT angiography - dose - iDose

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