Individualized Scan Protocol Based on Body Mass Index in Dual-Energy Computed Tomography Pulmonary Angiography to Reduce Radiation and Contrast Doses with Improved Image Quality

Background

Pulmonary embolism (PE) is considered the third major cardiovascular emergency after coronary heart disease and stroke, with a high mortality and morbidity rate [1–3]. Therefore, an accurate and rapid diagnosis of pulmonary embolism is imperative for the clinical treatment of patients. Computed tomography pulmonary angiography (CTPA) is widely used in clinical work because of its high sensitivity and specificity [4–6]. It is considered the criterion standard for the detection of PE [7] and provides excellent details for visualizing pulmonary arteries and is a simple and noninvasive method for diagnosing thrombosis [8,9]. The image quality of CTPA depends mainly on the contrast between blood vessels and surrounding tissues. The degree of vascular enhancement is closely related to the patient’s body mass index (BMI) and the volume and flow rate of iodine contrast media [10]. At present, due to the limitation of the pulmonary circulation and CT imaging parameters, the radiation dose and iodine intake in CTPA is relatively high. Although it maximizes the image contrast and diagnostic ability, the beam-hardening artifact in the superior vena cava is high, which interferes with the display of adjacent structures and easily leads to misdiagnosis and missed diagnosis of small emboli around it, resulting in false-negative results [10]. At the same time, higher contrast dosage can potentially cause a series of clinical problems, such as the incidence of contrast-induced nephropathy (CIN) and contrast extravasation [11]. The incidence of CIN is increasing yearly, and it is the third cause of iatrogenic acute renal failure, next to abnormal renal blood perfusion and nephrotoxic drugs [11,12]. Although recent reports suggest that the adverse effects of contrast agents may be exaggerated, the European Society of Urogenital Radiology currently recommends the use of the lowest possible contrast dose to reduce the incidence of CIN and other adverse reactions [13].

The radiation dose is closely related to the scanning parameters, such as tube voltage, tube current, scanning time and pitch, and is greatly affected by the patient’s body size [14,15]. How to achieve acceptable image quality at a reduced contrast media and radiation dose on an individual patient level is a hot research topic.

The Revolution CT scanner (GE Healthcare) in our routine clinical application is capable of dual-energy CT (DECT) imaging, with rapid tube voltage switching between 80 kVp and 140 kVp (<0.5 ms) to acquire 101 monochromatic images in the range of 40–140 keV [16]. It is also equipped with the adaptive statistical iterative reconstruction-V algorithm (ASIR-V, GE Healthcare) to reduce image noise. The combination of DECT with ASIR-V has been extensively used for CTA imaging of various human body parts. However, to the best of our knowledge, there are few studies investigating the selection of optimal keV and ASIR-V levels in DECT for CTPA with reduced contrast and radiation dose based on patient BMI. This study aimed to demonstrate the value of using an individualized DECT scanning protocol in CTPA to reduce radiation dose and iodine dose while maintaining or improving image quality and reducing artifacts in the superior vena cava by combining the optimal monochromatic images with an appropriate ASIR-V reconstruction strength.

Material and Methods

PATIENT POPULATION:

This was a prospective study and was approved by the Ethics Committee of our institution, the First Affiliated Hospital of Dalian Medical University, and all patients provided signed informed consent. A total of 142 consecutive patients with clinically suspected PE who underwent CTPA examination in our hospital from August 2020 to December 2021 were considered, of whom we excluded patients with acute agitation (n=2), after lung resection (n=5), massive pleural effusion (n=3), contrast media allergy and liver and kidney dysfunction (n=3), and pacemaker or metal implant (n=2). Finally, 127 patients were enrolled in the study and randomly divided into the standard group (63 cases) and the individualized group (64 cases). The patient enrollment flowchart is shown in Figure 1.

CTPA SCANNING PROTOCOL:

All CT scans were performed on a 256-row single-source DECT scanner (Revolution CT, GE Healthcare, Milwaukee, WI, USA). The main scanning parameters of the standard group were tube voltage of 120 kVp and tube current of 150 mA. The individualized group was scanned using a DECT imaging mode with rapid kVp switching between 80 and 140 kVp in less than 0.5 ms. The tube current in the individualized group was selected according to patient’s BMI (BMI ≤20 kg/m², 200 mA; 20< BMI ≤23 kg/m², 240 mA; 23< BMI ≤25 kg/m², 280 mA; BMI >25 kg/m², 320 mA). The other scanning parameters were consistent with those of the standard group. The gantry rotation time was 0.5 s, and 80 mm of z-axis coverage was chosen. The pitch was 1.531: 1, and slice thickness and interval were both 5 mm.

The nonionic contrast media loversol (350 mgI/mL) was used, with an injection rate of 5 mL/s in the amount of 60 mL in the standard group, which is a standard injection protocol in our institution. The individualized group was injected with a low-iodine contrast media intake at 130 mgI/kg. The contrast media that first passed through the pulmonary artery was strongly associated with BMI. Larger patients have a greater volume of blood distribution than smaller patients, so patients with a high BMI require a higher flow rate and/or increased contrast dose [17]. We controlled the injection time to 7 s. After contrast injection, 30 mL of saline flush was injected at a flow rate of 5 mL/s. To obtain an optimal intraluminal contrast enhancement, the Smart Prep technique was used, with a region of interest placed on the pulmonary trunk, and the trigger threshold was 70 HU. The arterial-phase scan started 2 s after triggering. The scan range was from the upper edge of the lung apex to the lower edge of the bilateral costophrenic angle.

IMAGE RECONSTRUCTION:

The images of the standard group were reconstructed with a 1.25-mm slice thickness using ASIR-V at the standard 40% strength (40%ASIR-V). The virtual monochromatic images of the individualized group were reconstructed with a 1.25-mm slice thickness at the energy levels of 55–70 keV (interval 5 keV) with 40% to 80% ASIR-V (interval 10%). All images were transferred to the GE ADW 4.7 workstation for analysis with the GSI software. The images in the 2 groups were also reconstructed with maximum intensity projection and curved planar reconstruction for review.

OBJECTIVE IMAGE EVALUATION:

The regions of interest were placed in the pulmonary trunk, left pulmonary artery (the left upper and lower lobe arteries, and left lower segmental arteries), right pulmonary artery (the right upper and lower lobe arteries), and superior vena cava, with two-thirds of the target vessel diameter to measure the CT value and standard deviation (SD). The background was selected on the trapezius, with an average area of 50 mm². The copy-and-paste function for regions of interest was used among the multiple monochromatic images in the individualized group for consistency. Each target was measured 3 times to get the mean value. The signal-to-noise ratio (SNR) was evaluated as the mean CT value of the target vessel divided by the standard deviation. The contrast-to-noise ratio (CNR) was calculated as CNR=(CT _target – CT _background)/SD _background. The beam hardening artifacts of superior vena cava were measured, and an artifacts index (AI) was calculated as AI=SD2artifact-SD2background, with a higher AI value indicating higher beam-hardening artifact.

SUBJECTIVE IMAGE EVALUATION:

In a randomized blinded design, 2 radiologists with more than 5 years of CTPA diagnostic experience independently evaluated the subjective image quality using a 5-point scale: 5, excellent contrast and sharp edge of vessels; 4, good contrast and sharp edge of vessels; 3, moderate vessel contrast; 2, fair image quality and unclear vessel edge; and 1, bad image quality and blurred vessel edge. The beam-hardening artifacts of the superior vena cava were recorded (1, no obvious artifacts; 2, obvious artifacts that affect the diagnosis). Because the degree of enhancement was not consistent in each patient, 2 senior radiologists could automatically adjust the window set to the best viewing condition based on their own diagnostic experience.

RADIATION DOSE AND CONTRAST MEDIA INTAKE:

Radiation dose was recorded on the device as CT dose-index volume (CTDI_vol) in the unit of mGy and dose-length product (DLP) in the unit of mGy·cm, and effective dose (ED) was calculated using the formula ED=DLP×k, in which k represents the radiation dose conversion factor with a value of 0.014 mSv/(mGy·cm) for the chest. The patient’s BMI and the amount of contrast media dosage were recorded.

STATISTICAL ANALYSIS:

All statistical analyses were performed using SPSS version 24.0 (IBM Corp, Armonk, NY, USA) and GraphPad Prism version 8.0.1 (GraphPad Software, San Diego, CA, USA), with normally distributed data presented as mean±standard deviation. The chi-square test was used to analyze sex differences. The differences in age, height, weight, BMI, radiation dose, and iodine intake were analyzed by unpaired t test between the 2 patient groups. Analysis of variance (ANOVA) was used to compare the CT value, SNR value, CNR value, and AI value among the multiple image groups. The Kappa test was used for the consistency of image subjective quality scores between the 2 radiologists, and the Kruskal-Wallis H test was used for the comparison of image quality subjective scores among the multiple image groups. A P value <0.05 was regarded as statistically significant.

Results

PATIENT POPULATION:

There were 65 men and 62 women in the study, with an age range of 30 to 87 (mean, 63.22±11.51) years and a BMI range of 17.3 to 33.22 (mean, 24.07±3.16) kg/m2. There were no significant differences in sex, age, height, weight, and BMI between the standard and individualized groups (P>0.05). The difference in personal factors is shown in Table 1.

OBJECTIVE IMAGE EVALUATION:

The objective measurements of images in the individualized group were significantly different than those of the standard group (P<0.05). In general, for images with a fixed energy level, CNR and SNR values increased, and beam-hardening artifacts (AI values) decreased as the ASIR-V strength increased. For the same ASIR-V strength, CNR and SNR values decreased, and AI values decreased as energy levels increased. Compared with images in the standard group (AI=63.38±34.39), all images in the individualized group had significantly lower beam-hardening artifacts in the superior vena cava (Table 2).

SUBJECTIVE IMAGE EVALUATION:

The subjective scores of image quality indicated an excellent consistency between the 2 observers (Kappa=0.948, P<0.05). The subjective scores were significantly different (P<0.05). The 60 keV with 80%ASIR-V image group had the highest image quality score. In the standard group, 69.84% of patients had beam-hardening artifacts that significantly affected the diagnosis, but only 7.81% were in the individualized group (Tables 3, 4 and Figure 2).

RADIATION DOSE AND CONTRAST MEDIA INTAKE:

Compared with the values in the standard group, CTDIvol, DLP, and ED in the individualized group were significantly decreased by 31.87%, 34.01%, and 33.93%, respectively (all P<0.05). The contrast media intake of the individualized group was 9.04±1.40 gI, 56.95% lower than that (21 gI) in the standard group (Figure 3).

Discussion

We showed that an individualized scan protocol based on BMI can reduce the radiation and contrast dose while maintaining adequate image quality and decreasing the superior vena cava artifacts in dual-energy computed tomography pulmonary angiography using 9.04±1.40 g iodine. Wu et al [18] also demonstrated that satisfactory image quality can be obtained with a lower contrast agent dose for CTPA. A prerequisite for accurate diagnosis of pulmonary embolism is the adequate enhancement of the pulmonary artery. Factors such as contrast dosage, concentration, injection flow rate, and patient BMI need to be considered [19]. Patients with a high BMI have a larger blood flow distribution [20], and the dosage and injection flow rate of contrast media need to be increased, so it is necessary to realize an individualized contrast agent injection protocol. Reducing the amount of contrast media can minimize the incidence of adverse reactions, but it may impair the image quality. To counter the decrease of attenuation caused by the contrast dose reduction in the low-dose research in CTPA, low tube voltages are often used. Alobeidi et al [21] used an 80 kVp in conjunction with an 80-mgI/kg body weight iodine dose to achieve a low iodine dose scan because the low kVp enhanced iodine attenuation while having a greater superior vena cava CT value. Boos et al [22] demonstrated the feasibility of applying a 70-kVp, high pitch, and 40-mL iodine contrast in patients with a BMI less than 35 kg/m2. Faggioni et al [23] showed the feasibility of 80-kV CTPA in lean patients using 40 mL of contrast medium with either a moderate (320 mg I/mL) or high (400 mg I/mL) iodine concentration. However, the use of low-kVp is often ruled out for large patients due to the severe loss of detection efficiency using low-kVp for imaging large patients [24,25]. However, DECT uses signals from both the low (80 kVp) and high (140 kVp) X-ray spectrums to generate a total of 101 sets of virtual monochromatic images in the range of 40 to 140 keV to better balance contrast and image noise. Using the lower energy images generated in DECT, the system can significantly improve the contrast between blood vessels and surrounding tissues [26,27] and optimize microvascular imaging while controlling the image noise to acceptable levels, and it has great potential in reducing the amount of contrast media. Image noise can further be reduced by using advanced reconstruction algorithms, such as the iterative reconstruction algorithm. The ASIR-V algorithm is a multi-model iterative reconstruction algorithm that integrates the system noise model, object model, and physical model. It is possible to accomplish varying degrees of noise reduction, considerably increase image contrast, and improve image quality by adjusting the algorithm’s strengths, all of which have been employed extensively in clinical situations. In the present study, we explored the best combination of monochromatic images and ASIR-V algorithm in CTPA and adopted an individualized contrast agent injection according to patients’ BMI to achieve individualized double-low dose (low radiation dose and contrast dose) CTPA. At the same time, we adopted 1.532 high-pitch technology to improve the scanning speed. A low contrast media injection protocol of 130 mgI/kg was used based on the patient’s weight, and the iodine contrast dose was 9.04±1.40 g of iodine, which was 56.95% less than that in the conventional CTPA scanning protocol.

The study by Borges et al [28] showed that the average transmission time from the elbow vein injection of contrast media to the pulmonary artery trunk was 7 s; therefore, in the individualized group, we fixed the contrast media injection time to 7 s. At the same time, to enhance the vascular enhancement effect and reduce the artifacts of the superior vena cava, 30 mL of saline flush was injected at 5 mL/s. We found that in the standard group, 69.84% (44 patients) had obvious beam-hardening artifacts in the superior vena cava that affected the diagnosis, while only 7.81% (5 patients) of the individualized group had the same. By measuring the SD value of the area with superior vena cava artifacts and calculating the artifact index, it was found that with the increase of ASIR-V weight, the artifact index showed a downward trend. It showed that ASIR-V with a high weight could also reduce the beam-hardening artifacts and improve image quality. The injection rate in the individualized group ranged from 2.5 mL/s to 5.3 mL/s (mean, 3.67±0.59 mL/s), which was about 26% lower than that in the standard group (5 mL/s). Patients’ physical comfort was better, and adverse reactions, such as fever and mild vomiting, were less for the patients in the individualized group.

We used the dual-energy CT scanning mode and selected the mA value according to the patient’s BMI to achieve individualized low radiation dose scanning. Compared with the conventional 120 kVp scanning group, the effective radiation dose of the individualized group was reduced by about 34%. Lenga et al [29] compared the radiation dose and image quality of single-energy CT and dual-source dual-energy CT in contrast-enhanced CT of the chest by analyzing 200 patients. The study demonstrated that dual-source dual-energy CT can be routinely performed for chest contrast-enhanced CT without increased radiation dose or decreased image quality, which is consistent with our study conclusions. In the present study, images with photon energies from 55 keV to 70 keV (5 keV interval) were generated for the individualized group. With the decrease in photon energy, vascular contrast increased significantly [15], while image noise also increased. To reduce the image noise, we reconstructed the virtual monochromatic images with 40% to 80% ASIR-V (10% interval) at each energy level. ASIR-V is a multi-model iterative reconstruction algorithm that integrates the system noise model, object model, and physical model and achieves different degrees of noise reduction by changing the weight of the algorithm [30,31]. Combined with subjective and objective image analysis, we showed that 60 keV plus 80%ASIR-V generated the best image quality in the individualized double-low CTPA group. Meanwhile, compared with the standard scanning mode, the SNR and CNR of each vessel were significantly improved. Our research showed that the individualized group could realize the purpose of low radiation dose and contrast media dose scanning, which is more beneficial for patients with obesity or overweight, and can obtain satisfactory diagnostic image quality.

This study also had some limitations. We used only a single parameter, BMI, for the study and grouping, which may not be the most relevant index. We did not calculate the difference in the detection rate of emboli in each group because it was not the same patient.

Conclusions

Compared with the conventional CTPA scanning protocol, the use of a BMI-dependent DECT protocol with the combination of appropriate ASIR-V weight in CTPA further reduced the contrast agent dose and radiation dose, and improved image quality and reduced superior vena cava artifacts, with the 60 keV images reconstructed using 80%ASiR-V having the best image quality.