There are increasing concerns about the potential future malignancy risk associated with exposure to ionizing radiation generated in computed tomography (CT) scans.1,2 In addition, there is concern about the risk of contrast agent-induced nephropathy in patients with decreased renal function because contrast agent-induced nephropathy is now a major cause of hospital-acquired acute renal failure.3 Therefore, the radiation dose and contrast agent dose must be kept to a minimum, while obtaining a diagnostically adequate image quality.
Scanning with low tube voltage has advantages of reducing the radiation and contrast agent doses. Low tube voltage technique can yield higher attenuation levels from iodinated contrast agents because of an increased photoelectric effect and lesser Compton scattering.4–8 However, this technique has the disadvantage of increasing image noise.9 The introduction of iterative reconstruction (IR) could reduce the quantum noise associated with filtered back projection (FBP) techniques.10–12 Previous report suggested that coronary CT with low tube voltage and low contrast agent dose with hybrid IR (HIR) can dramatically decrease radiation and contrast agent doses, while maintaining adequate image quality.13 Recently, knowledge-based iterative model reconstruction (IMR) (Philips Medical Systems, Cleveland, OH) has become available.14 Iterative model reconstruction models the process of physical data acquisition as accurately as possible through the iterative minimization of the difference between measured raw data and estimated image via a penalty-based cost function. Iterative model reconstruction is reportedly effective in coronary CT, decreasing image noise and increasing the contrast-to-noise ratio (CNR).15–19 In addition, previous report suggested that IMR technique can yield better image quality in cardiac CT, compared with the FBP and HIR technique.16 However, to our knowledge, there have been no reports about the usefulness, advantages, and disadvantages of the IMR technique in coronary CT with low tube voltage and low contrast agent dose compared with the HIR technique. We hypothesized that an 80-peak kilovoltage (kVp) coronary CT using low contrast agent dose and IMR can yield better image quality than an 80-kVp CT with low contrast agent dose and HIR.
The purpose of this study was to evaluate the feasibility of a low-contrast and low-radiation dose protocol for cardiac CT using 80 kVp and IMR.
MATERIALS AND METHODS
This prospective study received institutional review board approval; previous informed consent was obtained from all prospectively enrolled patients, and written informed consent was waived for retrospectively enrolled patients.
Patients
Between October 2013 and January 2014, we enrolled 35 patients (prospective group) for the electrocardiography (ECG)–triggered axial coronary CT angiography with 80 kVp and low contrast agent dose. Of the 35 patients of the prospective group, 5 fulfilled our exclusion criteria of a history of allergic reaction to iodinated contrast agents, severe renal failure, or normal renal function (estimated glomerular filtration rate of <30 mL/min/1.73m2 and ≥60 mL/min/1.73 m2, respectively); and heart rate of greater than 75 beats/min or atrial fibrillation; pregnant or lactating women were also considered ineligible. Consequently, we assigned 30 patients to undergo 80-kVp prospective ECG-gated coronary CT angiography in our study. In addition, we retrospectively selected 30 patients who have undergone prospective ECG-gated coronary CT angiography with 120 kVp with conventional contrast agent dose between June and August 2011 as control. In this study, we enrolled the historical consecutive patients as the control group with the high radiation dose setting and FBP reconstruction, because IRs were routinely used in our hospital and it might be against the patient's benefit to enroll the prospective control group with the high radiation dose setting and FBP reconstruction. We evaluated the male-female ratio, age, and body weight between the prospective and control groups. The excluded normal renal patients were assigned to our conventional coronary CT angiography with 120 kVp with standard contrast agent dose. In addition, the patients with tachycardia or atrial fibrillation were assigned to the ECG-triggered helical coronary CT angiography scan.
CT Scanning and Contrast Infusion Protocols
Patients with a heart rate greater than 55 beats/min received 20-mg metoprolol tartrate orally (Selocken; AstraZeneca, London, England) 60 minutes before scanning. Five minutes before scanning, all patients received 0.3-mg nitroglycerin (Nitropen; Nippon Kayaku, Tokyo, Japan) sublingually to dilate the coronary arteries. All patients were scanned using a 256-row multidetector-row CT (Brilliance iCT, Philips Healthcare, Cleveland, OH).
The 30 control patients were examined with the conventional 120-kVp protocol, after the standard dose of contrast agent (370-mg iodine per kilogram of body weight) was delivered over 15 seconds.20 The scan parameters for the control group were as follows: rotation time, 0.27 seconds; beam collimation, 128 × 0.625 mm; section thickness, 0.9 mm; interval, 0.45 mm; and tube current, 992 mA. Thirty patients were assigned to receive the 80-kVp protocol (prospective group), after a 40% lower dose of contrast agent (222-mg iodine per kilogram of body weight) was delivered over 12 seconds.
The scan parameters for the prospective group were the same between the control group and 80-kVp group, except for tube current, injection duration, and scan delay. Detailed scanning parameters for each protocol are shown in Table 1. In all examinations, Iopamidol 370 mg/mL (Iopamiron-370; Nihon Schering, Osaka, Japan) was delivered via a 20-gauge catheter inserted into an antecubital vein using a power injector (DUAL SHOT GX; Nemoto-Kyorindo, Tokyo, Japan).
The scan start time was determined with a computer-assisted bolus tracking program (Bolus Pro Ultra; Philips Medical Systems)21 with trigger threshold of 110 Hounsfield units (HU) in the aortic arch. Real-time (120 kVp, 15 mAs; 80 kVp, 30 mAs) serial monitoring studies began 10 seconds after the start of contrast injection. The scans were started 8 and 9 seconds (80- and 120-kVp protocol, respectively) after triggering. The patients were instructed to hold their breath with tidal inspiration during scanning.
Computed Tomographic Image Reconstruction
Image reconstruction was performed in a 21.5-cm display field of view. The 120- and 80-kVp images were reconstructed using a standard FBP algorithm with a standard cardiac kernel and XCB filter (control group and protocol A). A third image set (protocol B) was generated by processing the 80-kVp image set using HIR (iDose, Philips Healthcare) and XCB. We selected a hybrid-iterative level of 60%, as recommended by the vendor for cardiac imaging. The fourth image set (protocol C) was generated by processing the 80-kVp image set using IMR and XCB. For IMR, the prototype provided 3 levels (L1-L3). Of the 3 level noise reduction settings for IMR, L3 provides the maximum noise reduction. We selected a L1 for cardiac imaging, as recommended by the vendor. Because these were cardiac scans, a “cardiac routine” setting specific for cardiac anatomy was used for the IMR.
Contrast and Radiation Doses for Each Protocol
To estimate the contrast agent dose, we compared the total amount of contrast agent and the injection rate for each protocol. To estimate the CT radiation dose, the CT volume dose index (CTDIvol) and dose-length product (DLP) were recorded. The effective dose (ED) for cardiac CT was derived from the product of the DLP and a conversion coefficient for the chest according to European Commission guidelines on quality criteria for CT (k = 0.014 mSv×Gy−1×cm−1).22
Quantitative Image Analysis
A board-certified radiologist with 8 years of cardiac CT experience performed quantitative image analysis on reconstructed 0.9-mm-thick transverse images. The mean attenuation of the ascending aorta was measured in a circular region of interest (ROI) placed in the origin of the left main trunk (ROIAo). Attempts were made to select an ROI of 400 mm2 in the ascending aorta, a size large enough to be unaffected by pixel variability and small enough to exclude the vessel wall or perivascular fat. We measured 3 ROIs in 3 different sequential slices. To evaluate the image noise, we measured the SD of the attenuation at ROIAo. To minimize bias from single measurements, we calculated the mean of measurements from all ROIs. The CNR was calculated as CNR = ROIAo / noise. The noise reduction rate for HIR and IMR was calculated as [1 − image noise (protocol B) / image noise (protocol A)] and [1 − image noise (protocol C) / image noise (protocol A)].
Qualitative Image Analysis
To evaluate the image quality from the different protocols, we performed qualitative image analysis of axial images at a window level and width of 90 and 750 HU, respectively (standard cardiac window setting) on a PACS viewer (Synapse; Fuji Film Medicals, Tokyo, Japan).
Two board-certified radiologists with 14 and 5 years of experience with cardiac CT independently graded image contrast, image noise, image sharpness, unfamiliar image texture, and overall image quality. The CT data sets were randomized, and the radiologists were blinded to the acquisition parameters; they were allowed to adjust the window level and width. Using a 4-point subjective scale, they independently graded image contrast and overall image quality (1, unacceptable; 2, acceptable; 3, good; 4, excellent). Image noise was similarly graded as grade 1 being poor/not evaluable (severe artifacts impairing accurate evaluation); grade 2, adequate (moderate artifacts, acceptable for routine clinical diagnosis); grade 3, good (minor artifacts, good diagnostic quality); and grade 4, excellent (no artifacts, unrestricted evaluation). Image sharpness was graded by evaluating aortic wall sharpness as blurry being grade 1; poorer than average, grade 2; better than average, grade 3; sharpest, grade 4. We also evaluated the “unfamiliar texture” using a four-point subjective scale (1, unacceptable; 2, acceptable; 3, good; 4, excellent) because previous report suggested that the HIR sometimes causes the noise texture and the blocky appearance, which were unfamiliar to radiologists and may influence the reader's judgment.23,24 Interobserver disagreement was resolved by consensus.
Statistical Analysis
All numeric values are reported as mean (SD). To compare age, body weight, CTDIvol, DLP, and ED between the 120-kVp and 80-kVp groups, we used the 2-tailed Student t test. For multiple comparisons of attenuation values, image noise, and CNR between the 120-kVp group, protocols A, B, and C, we used Dunnett method and treated 120-kVp values as control. The visual scores assigned to the 120-kVp, protocols A, B, and C were compared using the Kruskal-Wallis test, and 120-kVp values were treated as control. If there was a statistically significant difference among the different groups, we performed pairwise comparisons with the Steel-Dwass test. Differences of P < 0.05 were considered statistically significant. The scale for the κ coefficients for interobserver agreement was as follows: less than 0.20, poor; 0.21 to 0.40, fair; 0.41 to 0.60, moderate; 0.61 to 0.80, substantial; and 0.81 to 1.00, near perfect. Statistical analyses were performed with the free statistical software R (R, version 2.6.1; The R Project for Statistical Computing; http://www.r-project.org/).
RESULTS
Patient Characteristics
Table 2 shows the characteristics of the 2 patient groups. There were no significant differences between the 80- and 120-kVp groups with respect to sex, age, and body weight (P > 0.05).
Contrast and Radiation Dose of Each Protocol
The amount of contrast medium, injection rate, CTDIvol, DLP, and ED values for each protocol are shown in Table 2. The amount of contrast agent used for 80-kVp scanning was more than 40% lower than that for the 120-kVp protocol (35.7 [8.4] vs 59.7 [9.0] mL, P < 0.01). The ED was approximately 74% lower with the low-dose protocol (1.4 [0.1] vs 5.4 [0.5] mSv, P < 0.01).
Quantitative Image Analysis
Table 3 and Figure 1 summarize the results of the quantitative image analysis. The mean attenuation of the ascending aorta of the 120-kVp protocol is significantly lower than that of protocols A, B, and C (120-kVp vs protocol A; 431.4 [51.6] vs 515.8 [64.4] HU, P < 0.01; 120-kVp vs protocol B, 431.4 [51.6] vs 515.7 [64.5] HU, P < 0.01; 120-kVp vs protocol C; 431.4 [51.6] vs 517.0 [64.8] HU, P < 0.01). Mean image noise of protocol A was 261% higher than that of the 120-kVp protocol (119.0 [39.9] vs 33.3 [7.3] HU, P < 0.01). Mean image noise of protocol B was 81% higher than that of the 120-kVp protocol (60.4 [18.8] vs 33.3 [7.3] HU, P < 0.01). There was no significant difference in image noise between protocol C and 120-kVp protocol (27.4 [7.3] vs 33.3 [7.3] HU, P = 0.61). The CNR of the 120-kVp protocol was higher than those of protocols A and B (120-kVp vs protocol A; 13.3 [2.5] vs 4.7 [1.5], P < 0.01; 120-kVp vs protocol B; 13.3 [2.5] vs 9.2 [2.5], P < 0.01). The CNR of protocol C was significantly higher than that of the 120-kVp protocol (19.8 [3.9] vs 13.3 [2.5], P < 0.01). The noise reduction rate for IMR was significantly higher than that for HIR (76.2% [4.6%] vs 50.0% [1.6%], P < 0.01).
Qualitative Image Analysis
Table 4 and Figure 2 show the result of qualitative image analysis. On visual evaluation, there was no statistically significant overall difference in image contrast between the 120-kVp protocol and protocol A (P = 0.60) and between the 120-kVp protocol and protocol B (P = 0.20). However, the mean score of image contrast of protocol C was higher than that of the 120-kVp protocol (P < 0.05). The mean scores of image noise, image sharpness, and overall image quality of the 120-kVp protocol were higher than those of protocols A and B (P < 0.05). There were no statistically significant differences in image noise, image sharpness, and overall image quality between the 120-kVp protocol and protocol C (P = 0.73, P = 0.97, and P = 0.49, respectively). The mean scores of unfamiliar image texture of protocol A was lower than that of the 120-kVp protocol (P < 0.05). There was no significant difference in unfamiliar image texture between the 120-kVp protocol and protocol B (P = 0.35) and between the 120-kV protocol and protocol C (P = 0.57).
There were moderate to substantial interobserver agreements regarding image contrast, image noise, image sharpness, unfamiliar image texture, and for overall image quality (κ = 0.41, κ = 0.76, κ = 0.54, κ = 069, and κ = 0.57, respectively).
Representative cases are shown in Figures 3 and 4.
DISCUSSION
Our study suggests that low tube voltage coronary CT can reduce the radiation dose and contrast agent dose. The 80-kVp coronary CT with low contrast agent dose and IMR can yield significantly lower image noise and 74% radiation dose reduction compared with the 120-kVp protocol with standard contrast agent dose. The 80-kVp coronary CT with low contrast agent dose with IMR yields as much inconspicuous unnatural texture as the 120-kVp protocol with standard contrast agent dose.
Previous reports suggested that low tube voltage CT is well suited to reduce radiation dose and contrast agent dose; however, this technique increased image noise and prevented clinical use.7,25 Previous report suggests that 80-kVp abdominal CT with high tube current can decrease the contrast agent dose by 40% without deterioration of image quality.25 However, the high tube current technique is not well suited for a low-radiation protocol because the radiation dose is proportional to the tube current.26 It has been reported that the HIR technique is well suited for a low-contrast and low-radiation dose protocol for cardiac CT.13 The use of HIR has been shown to reduce the image noise elevated by low tube voltage in coronary CT.13,27,28
Our study suggested that the IMR technique is more suited for the low tube voltage scan and low contrast agent dose protocol than the HIR technique because the noise reduction rate for IMR is significantly higher than that for the HIR (50.0% [1.6%] vs 76.2% [4.6%], P < 0.01) and unfamiliar texture of the 80-kVp protocol with IMR with low contrast agent dose is not a serious problem. In addition, the 80-kVp protocol with IMR with low contrast agent dose could yield almost same image quality in qualitative image analysis compared with the 120-kVp protocol with standard contrast agent dose.
Iterative model reconstruction technique can reduce more image noise and artifact than HIR technique in this study, as previous report suggested.29 Hybrid IR involves 2 denoising components, that is, iterative maximum likelihood-type sonogram restoration and local structure model fitting on image data, which iteratively decrease uncorrelated noise.27,30 However, a certain amount of image noise and some artifacts continue to be present. On the other hand, IMR techniques use statistical models, system models, and motion models to produce image data that truly correspond to measured projection data. The goal of reconstruction is to iteratively minimize the penalty-based cost function that addresses data mismatch. Previous report suggested that accurate simulation of the cone beam x-ray and the absorbing voxels by a “system physical model” could suppress various artifacts.31
In our study, the presence of unfamiliar texture with the IMR technique is not a serious problem; however, previous reports suggested that other full IR techniques in abdominal CT have the disadvantages of yielding pixelated, blotchy, and unfamiliar images for radiologists.32–34 We speculated that there are 2 reasons that IMR technique yields inconspicuous unfamiliar image texture images unlike the other full IR technique. First, the reason is that the window width setting of cardiac CT is generally wider than that of abdominal CT. We inferred that this wide window setting suppress the unfamiliar image texture of IR. Second, the reason is that the tuning of the reconstruction parameters of IMR for the cardiac study might have the possibility to suppress the unfamiliar image texture. Previous IR techniques maintained fixed reconstruction parameters for all examinations, but IMR can change the tuning for IR based on the body area and the experimental FBP reconstructed images through penalty factors and the cost function. Therefore, it might be possible to tune the reconstruction parameters of IMR to mimic the conventional appearance of cardiac images using the FBP technique.
Our study had some limitations. First, the sample size is relatively small. Therefore, more patients are required to reach a solid conclusion. Second, our study focused on the subjective and objective assessments of image quality; however, we did not evaluate the diagnostic performance of among the 4 protocols. Third, we used a single parameter setting (L1 and “cardiac routine”) in reconstruction using IMR, although the IMR technique offers many reconstruction parameter settings. However, the comparison among all parameters of IMR and other reconstruction techniques was difficult and statistically complicated. Finally, patients of the 120-kVp protocol were enrolled retrospectively. We could not enroll the control group and prospective group in that same time because many reports suggested that the HIR and IMR techniques are well suited for radiation dose reduction in coronary CT.13,15–17,27 Therefore, we did not enroll additional subjects with the high radiation dose protocol for the benefit of the patients.
In conclusion, the 80-kVp protocol with IMR with low contrast agent dose technique yields higher image quality, 74% radiation dose reduction, and 40% contrast agent dose reduction compared with the 120-kVp protocol and reduces more image noise than the HIR technique.
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Keywords: imaging; computed tomography; coronary angiography; coronary artery disease; diagnosis
Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.
Source
Journal of Computer Assisted Tomography. 40(6):941-947, November/December 2016.
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