Objective To investigate the effect of Karl iterative reconstruction on CT image quality enhancement.

Methods ① Phantom study: A 120 kVp/140 mAs tube current was set as the standard radiation dose. Low-dose setting is implemented with a 50% current reduction, i.e., 70 mAs, whereas the voltage is set to 120 kVp. Two experimental groups with algorithms of filtered back projection(FBP) and Karl iterative reconstruction(with a noise reduction of 1–9 levels) were compared. The noise power spectrum(NPS) and standard deviation metrics were adopted to assess the noise degree. ② Clinical study: In the phantom study, 120 patients, including 61 males and 59 females with ages ranging from 35 to 75 and a BMI of(23.95±0.27) kg/m², were recruited and randomly divided into two groups: standard dose and low dose. These groups were scanned via FBP and Karl iterative reconstruction at level 5(Karl 5), respectively. The image qualities of the two groups were assessed with various objective metrics, such as CT dose index of volume(CTDIvol), dose length product(DLP), effective dose(ED), and signal-to-noise ratio(SNR), as well as by human subjective evaluation. Student t-test was adopted to assess the significance of difference for the values of objective metrics from the two groups, whereas subjective evaluation was quantified with an χ² test.

Results ① Phantom study: With the standard dose, the average noise degree for Karl iterative reconstruction was lower than that for FBP. With an increased level of Karl iterative reconstruction, the noise degree will be lowered(t=5.14–47.50, all P<0.01). Referring to the NPS curves, the Karl 1−9 algorithm can attain the goal of noise reduction and remain keep the image texture unchanged(t=2.49, P=0.42). With the low dose, i.e., 50% current reduction, the image noise(6.40±0.16) Hu of Karl 5 was close to that(6.30±0.38) Hu of FBP with insignificant difference(t=28.34, P=0.423), compared with other levels of Karl iterative reconstruction. Meanwhile, the magnitudes of NPS curves for Karl 5 and FBP insignificantly differed. ② Clinical study: The CTDIvol(5.56±0.01) mGy and DLP(170.74±18.40) mGy·cmvalue of the low-dose group were significantly lower than those(11.06±0.01) mGy, (348.93±26.16) mGy·cm of the standard-dose group(t=4757.7, P=0.003; t=39.23, P=0.005). The ED values(2.58±0.16) mSv of the low-dose group were significantly less by 51.5% than those(5.01±0.17) mSv of the standard-dose group(t=37.94, P=0.004). The noise degree and SNR values for images from Karl 5 with low dose and FBP with standard dose were insignificantly different(noise degree, t=0.24, P=0.38; t=1.51, P=0.70; SNR, t=0.45, P=0.45; t=0.08, P=0.72). The results of subjective quality assessments for the images from Karl 5 and FBP were insignificantly different in terms of the usage of mediastinum(χ²=2.32, P=0.317; χ²=1.38, P=0.268) and lung(χ²=0.97, P=0.614; χ²=0.59, P=0.760 ) window settings for image reading.

Conclusions Karl iterative reconstruction at different levels can effectively reduce noise with different degrees. For the 50% current reduction to 70 mAs, the image quality of Karl 5 is comparable with that of standard-dose FBP.