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Aim: Since dose reduction techniques such as tube current modulation may impact noise and consequently the performance that can be achieved with CT images, it is important to establish quality monitoring. We have studied whether it would be possible and relevant to implement an (automatical) procedure to retrieve and possibly alert for patients with relatively high noise levels in CT in comparison to similar cases. Proper alerting would make clinical quality supervision more efficient. Material and methods: Two homogeneous phantoms consisting of different diameters were scanned following a routine CT thorax protocol on a Siemens SOMATOM Force scanner and noise power spectra were calculated for the different phantom sizes. Next, forty-four patients, scanned with the same CT thorax protocol and reconstructed with a hard kernel (lung) and soft kernel (liver), were retrieved from PACS. Noise power spectra (NPS) were calculated for regions in the lung and liver, and evaluated over different frequency ranges. We hypothesized that the high frequency part correlates better with dose than the low frequency part that is determined by anatomical noise. Therefore we focused on the correlation of high frequency noise and dose versus patient size. Water equivalent diameters (WED) were calculated as a metric of patient size. Additionally, all patients were rated subjectively by an experienced thorax radiologist for their overall image quality and presence of diagnostically acceptable noise. Statistical correlations and outliers were investigated. Results: While the correlations between NPS and dose and patient size were not significant for the lung, a positive correlation of NPS measured in the liver with CTDIvol and WED was found (e.g. R2 = 0.31 for NPS(high frequencies) versus WED). The combined visualization of NPS at high frequencies, WED and CTDIvol showed some interesting outliers, however they did not receive lower image quality ratings. Conclusions: This work described how the Siemens SOMATOM Force scanner balances patient size, dose and image noise for a routine CT thorax protocol. However, since the outliers in both dose and (high frequency) noise levels still result in adequate to very good image quality scores, it is suggested that (straightforward) dose outlier based alerting should be the first task in dose-quality surveys on the particular scanner.
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