Increasing iterative reconstruction strength at low tube voltage in coronary CT angiography protocols using 3D‐printed and Catphan® 500 phantoms

Abstract Purpose The purpose of this study was to investigate the effect of increasing iterative reconstruction (IR) algorithm strength at different tube voltages in coronary computed tomography angiography (CCTA) protocols using a three‐dimensional (3D)‐printed and Catphan® 500 phantoms. Methods A 3D‐printed cardiac insert and Catphan 500 phantoms were scanned using CCTA protocols at 120 and 100 kVp tube voltages. All CT acquisitions were reconstructed using filtered back projection (FBP) and Adaptive Statistical Iterative Reconstruction (ASIR) algorithm at 40% and 60% strengths. Image quality characteristics such as image noise, signal–noise ratio (SNR), contrast–noise ratio (CNR), high spatial resolution, and low contrast resolution were analyzed. Results There was no significant difference (P > 0.05) between 120 and 100 kVp measures for image noise for FBP vs ASIR 60% (16.6 ± 3.8 vs 16.7 ± 4.8), SNR of ASIR 40% vs ASIR 60% (27.3 ± 5.4 vs 26.4 ± 4.8), and CNR of FBP vs ASIR 40% (31.3 ± 3.9 vs 30.1 ± 4.3), respectively. Based on the Modulation Transfer Function (MTF) analysis, there was a minimal change of image quality for each tube voltage but increases when higher strengths of ASIR were used. The best measure of low contrast detectability was observed at ASIR 60% at 120 kVp. Conclusions Changing the IR strength has yielded different image quality noise characteristics. In this study, the use of 100 kVp and ASIR 60% yielded comparable image quality noise characteristics to the standard CCTA protocols using 120 kVp of ASIR 40%. A combination of 3D‐printed and Catphan® 500 phantoms could be used to perform CT dose optimization protocols.

which carries risks of cancer, especially to the radiosensitive organs. 5 In a previous multicenter study, 6 the authors reported the mean effective dose of CCTA could achieve up to 12 mSv among the participating centers. Therefore, an effective strategy to further reduce the dose while maintaining the image quality is highly desirable.
Coronary computed tomography angiography is usually performed with a tube voltage of 120 kVp. 7,8 Scanning acquisitions at a low tube voltage of 100 kVp has been suggested as an effective way to reduce radiation dose in non-obese patients while maintaining image quality. 9,10 The low tube voltage helps to enhance coronary vessels as a result of the increased attenuation of iodinated contrast material. 11,12 However, this low tube voltage can also deteriorate the image quality by increasing the image noise. 13 Iterative reconstruction (IR) algorithm can compensate the increment of noise, especially when using the low exposure factors. 14,15 As a result, dose reduction will be achieved while maintaining image quality. Iterative reconstruction also offers a selection of strength levels resulting in different amounts of noise reduction. 16,17 However, high IR strengths may result in "blooming" artifacts that typically affect the visualization of small structures. 18 As a result, the level of the IR algorithm strengths must be carefully considered so as to balance its impact on image quality.
Previous literature had studied and established the use of threedimensional (3D)-printed cardiac insert phantom for evaluating the effect of IR strengths on image quality and dose reduction potential. [19][20][21] The cardiac insert phantom was constructed using a 3D printing technology. In contrast to other phantoms, such as Catphan ® (Phantom Laboratory, Salem, NY) and ACR (American College of Radiology), this phantom can be placed in an anthropomorphic chest phantom. Therefore, this combination provides an appropriate simulation of a patient's attenuation properties during CCTA along with anatomically relevant cardiac structures for the assessment of image quality.
The purpose of this study was to investigate the effect of changing iterative reconstruction (IR) algorithm strength at different tube voltages in CCTA protocols using a 3D-printed and Catphan ® 500 phantoms.

2.A. | Phantoms
Two phantoms were used in this present study, (a) 3D-printed cardiac insert phantom, and a (b) Catphan ® 500 (The Phantom Laboratory, Greenwich, NY, USA) phantom (see Fig. 1). The 3D-printed cardiac insert phantom, simulating the contrast-enhanced region of the ascending aorta and coronary vessels in CCTA, was constructed using 3D printing technology and placed within an anthropomorphic chest phantom (Lungman N-01, Kyoto Kagaku Co., Ltd., Kyoto, Japan). The development of the 3D-printed cardiac insert phantom was described in our previous work. 22 The contrast medium used was Ultravist 370 (Schering Health Care Ltd, Burgess Hill, UK). For the Catphan ® 500 phantom, two modules of CTP528 and CTP515 were included. The modules were used to assess the axial spatial resolution and low contrast detectability.

2.B. | Acquisitions and reconstruction
The 3D-printed cardiac insert phantom was positioned within the Filtered back projection (FBP) and two different IR strengths of ASIR 40% and 60% were investigated. The 120 kVp and ASIR 40% are the current CCTA protocols used in the institution where the phantom was scanned. Table 1 shows the summary of CT parameters and reconstruction settings used in this present study.
The attenuation values (Hounsfield Units, HU) and the image noise were measured by placing a region of interest (ROI) within the contrast-enhanced region simulating the ascending aorta in the 3Dprinted cardiac insert phantom [ Fig. 2(a)]. The signal-noise ratio (SNR) and the contrast-noise ratio (CNR) were calculated according to Eqs. (1) and (2), respectively. The CNR was obtained by using the HU values and image noise of the contrast material (CM) and the oil to simulate the myocardial fat in Eq. (2) [ Fig. 2 The axial spatial resolution was measured using the images   (Fig. 3). The image datasets of ASIR 60% with 120 kVp resulted in the lowest image noise and the highest SNR and CNR (P < 0.05), whereas the FBP with 100 kVp protocol showed the highest image noise and the lowest SNR and CNR (Fig. 3). There was no significant difference in HU values between the image datasets of the 120 and 100 kVp (Fig. 3). There was no significant difference (P > 0.05) between 120  (Fig. 3). Table 3 shows the MTF results obtained using the Catphan ® 500 phantom when the strengths of IR algorithm were increased. For both tube voltages, the spatial frequency of MTF was mildly affected by the different IR strength levels (variation < 10%). However, between 120 and 100 kVp protocols, the spatial frequency of MTF was strongly affected (variation > 10%), indicating changes in the spatial resolution and thus, image quality. Figure 4 illustrates the changes in low contrast resolution for FBP and different strengths of IR at 120 and 100 kVp. The low contrast object diameter ranged from 3 to 15 mm. From the graph, the image datasets reconstructed with FBP at 100 kVp required the highest contrast to detect the smallest size of object diameter of 3 mm. The ASIR 60% at 100 kVp has a higher contrast resolution compared to the FBP at 120 kVp for the smallest object diameter.
The highest contrast resolution was the ASIR 60% at 120 kVp. produced comparable SNR to the current local CCTA protocols of 120 kVp and ASIR 40% resulting in a > 50% reduction in radiation dose as measured by the CTDI vol . In our previous systematic review, 22

| CONCLUSION
Changing the IR strength has yielded different image quality noise characteristics. In this study, the use of 100 kVp and ASIR 60% yielded comparable image quality noise characteristics to the standard CCTA protocols using 120 kVp of ASIR 40%. In addition, a combination of 3D-printed and Catphan ® 500 phantoms could be used to perform CT dose optimization protocols.

CONF LICT OF I NTEREST
No conflict of interest.