Radiation dose reduction of chest CT with iterative reconstruction in image space - Part I: studies on image quality using dual source CT

Abstract

Objective: To determine whether the image quality (IQ) is improved with iterative reconstruction in image space (IRIS), and whether IRIS can be used for radiation reduction in chest CT.

Materials and methods: Standard dose chest CT (SDCT) in 50 patients and low dose chest CT (LDCT) in another 50 patients were performed, using a dual-source CT, with 120 kVp and same reference mAs (50 mAs for SDCT and 25 mAs for LDCT) employed to both tubes by modifying a dual-energy scan mode. Full-dose data were obtained by combining the data from both tubes and half-dose data were separated from a single tube. These were reconstructed by using a filtered back projection (FBP) and IRIS: full-dose FBP (F-FBP); full-dose IRIS (F-IRIS); half-dose FBP (H-FBP) and half-dose IRIS (H-IRIS). Objective noise was measured. The subjective IQ was evaluated by radiologists for the followings: noise, contrast and sharpness of mediastinum and lung.

Results: Objective noise was significantly lower in H-IRIS than in F-FBP (p < 0.01). In both SDCT and LDCT, the IQ scores were highest in F-IRIS, followed by F-FBP, H-IRIS and H-FBP, except those for sharpness of mediastinum, which tended to be higher in FBP. When comparing CT images between the same dose and different reconstruction (F-IRIS/F-FBP and H-IRIS/H-FBP) algorithms, scores tended to be higher in IRIS than in FBP, being more distinct in half-dose images. However, despite the use of IRIS, the scores were lower in H-IRIS than in F-FBP.

Conclusion: IRIS generally helps improve the IQ, being more distinct at the reduced radiation. However, reduced radiation by half results in IQ decrease even when using IRIS in chest CT.

Keywords: Chest CT; Image quality; Iterative reconstruction in image space; Radiation dose reduction.

Fig. 1

Standard dose contrast enhanced-chest CT in 48-year-old woman (BMI: 23 kg/m²) with lung cancer in left upper lobe. A-C. Transverse CT images with (A) full radiation dose FBP in B70f kernel and 5 mm slice thickness, (B) full radiation dose IRIS in I70f kernel and 5 mm slice thickness, and (C) half radiation dose IRIS in I70f kernel and 5 mm slice thickness. Objective noise measured in saline bag was 66.1 HU, 37.5 HU and 53.8 HU, respectively (not shown). BMI = body mass index, HU = Hounsfield unit, FBP = filtered back projection, IRIS = iterative reconstruction in image space

Fig. 2

Standard dose contrast enhanced-chest CT in 69-year-old man (BMI: 24 kg/m²) with mild emphysema. A-C. Transverse CT images with (A) full radiation dose FBP in B70f kernel and 1 mm slice thickness, (B) full radiation dose IRIS in I70f kernel and 1 mm slice thickness, and (C) half radiation dose with IRIS in I70f kernel and 1 mm slice thickness. Objective noise measured in saline bag was 59.9 HU, 35.7 HU and 47.6 HU, respectively (not shown). BMI = body mass index, HU = Hounsfield unit, FBP = filtered back projection, IRIS = iterative reconstruction in image space

Fig. 3

Low dose chest CT without enhancement in 37-year-old woman (BMI: 23 kg/m²) with small ground glass nodules. A-C. Transverse CT images with (A) full radiation dose FBP in B70f kernel and 5 mm slice thickness, (B) full radiation dose IRIS in I70f kernel and 5 mm slice thickness, and (C) half radiation dose IRIS in I70f kernel and 5 mm slice thickness. Objective noise measured in saline bag was 67.8 HU, 41.5 HU and 58.2 HU, respectively (not shown). BMI = body mass index, HU = Hounsfield unit, FBP = filtered back projection, IRIS = iterative reconstruction in image space

Fig. 4

Standard dose chest CT with enhancement in 75-year-old man (BMI: 21 kg/m²) with small lymph nodes in mediastinum. A-C. Transverse CT images with (A) full radiation dose FBP in B30f kernel and 5 mm slice thickness, (B) full radiation dose IRIS in I30f kernel and 5 mm slice thickness, and (C) half radiation dose IRIS in I30f kernel and 5 mm slice thickness. Objective noise measured in saline bag was 59.7 HU, 34.5 HU and 48.5 HU, respectively (not shown). Margins of mediastinal structures were slightly more blurred in (B) and (C) than in (A) due to excessive smoothing effect of IRIS image. BMI = body mass index, HU = Hounsfield unit, FBP = filtered back projection, IRIS = iterative reconstruction in image space