Intra-individual comparison between abdominal virtual mono-energetic spectral and conventional images using a novel spectral detector CT

Abstract

Objectives: To quantitatively and qualitatively assess abdominal arterial and venous phase contrast-enhanced spectral detector computed tomography (SDCT) virtual mono-energetic (MonoE) datasets in comparison to conventional CT reconstructions provided by the same system.

Materials and methods: Conventional and MonoE images at 40-120 kilo-electron volt (keV) levels with a 10 keV increment as well as 160 and 200 keV were reconstructed in abdominal SDCT datasets of 55 patients. Attenuation, image noise, and contrast- / signal-to-noise ratios (CNR, SNR) of vessels and solid organs were compared between MonoE and conventional reconstructions. Two readers assessed contrast conditions, detail visualization, overall image quality and subjective image noise with both, fixed and adjustable window settings.

Results: Attenuation, CNR and SNR of vessels and solid organs showed a stepwise increase from high to low keV reconstructions in both contrast phases while image noise stayed stable at low keV MonoE reconstruction levels. Highest levels were found at 40 keV MonoE reconstruction (p<0.001), respectively. Solid abdominal organs showed a stepwise decrease from low to high energy levels in regard to attenuation, CNR and SNR with significantly higher values at 40 and 50 keV, compared to conventional images. The 70 keV MonoE was comparable to conventional poly-energetic reconstruction (p≥0.99). Subjective analysis displayed best image quality for the 70 keV MonoE reconstruction level in both phases at fixed standard window presets and at 40 keV if window settings could be adjusted.

Conclusion: SDCT derived low keV MonoE showed markedly increased CNR and SNR values due to constantly low image noise values over the whole energy spectrum from 40 to 200 keV.

Fig 1. Attenuation values assessed in the suprarenal aorta in arterial and venous contrast phase imaging.

The highest values are found for MonoE at 40 keV reconstruction level, respectively. (values: mean ± SD)

Fig 2. Image noise for the whole energy spectrum from 40 to 200 keV in both contrast phases (values: Mean ± SD).

Note that image noise values are not significantly increased in the low keV MonoE reconstruction levels compared to the high keV reconstructions.

Fig 3. Absolute attenuation, standard deviation (SD), SNR and CNR in the infrarenal aorta in arterial contrast phase imaging for conventional and MonoE reconstructions.

Due to the substantial increase of absolute attenuation and constantly low SDs, SNR and CNR values are markedly elevated at low keV reconstruction levels (Values: Mean ± SD).

Fig 4. Waterfall plot showing overall image quality rating in arterial and venous contrast phase with standard abdominal window settings where 0 corresponds to the reference standard (conventional imaging).

Best image quality was rated for MonoE at 70 keV in arterial and 80 keV in venous phase.

Fig 5. SDCT images of an 83-year old patient in arterial and venous contrast phase.

For a more realistic comparability window settings were kept constant (center 60 HU; width 360 HU), except for the 40 keV MonoE reconstructions with adjusted window settings. Compared to conventional images MonoE at low keV reconstruction levels showed increased attenuation and contrast without excessive noise in both contrast phases. If window levels were adjustable MonoE at 40 keV was rated best for overall image quality, whereas reconstruction levels of 70 keV were rated best and superior to conventional images if readers were not allowed to adjust window settings. Note extensive increase in parenchymal attenuation in venous contrast phase in MonoE at 40 keV reconstruction level with standard windowing presets.