The research advancements on the advantages and limitations of coronary computed tomography angiography at low tube voltage: a literature review

Introduction

Coronary computed tomography angiography (CCTA) has emerged as a compelling alternative to invasive angiography for the screening and follow-up of coronary artery disease, and it has been widely applied in clinical practice, with a growing focus on low radiation and low contrast agent dose (1-3). Lowering tube voltage stands out as a highly effective strategy for radiation dose reduction, particularly when tube current remains constant, resulting in a radiation dose reduction proportional to the square of the tube voltage (4,5). Furthermore, the lower tube voltage has the added benefit of augmenting the iodine photon absorption (6,7), which creates a high-attenuation effect within the vessel lumen. thereby reducing contrast agent dosage and subsequently lowering the risk of kidney injury.

However, low tube voltage X-ray spectra demonstrate reduced penetration capabilities, leading to an increase in image noise and a substantial influence on the evaluation of coronary artery plaques and pericardial fat based on computed tomography (CT) values. Numerous research studies have highlighted that, compared to standard tube voltage, low tube voltage CCTA leads to notable disparities in plaque attenuation, quantitative composition, and qualitative characterization (8-11). These discrepancies may introduce potential challenges in the clinical diagnosis of coronary artery disease and prognostic assessment. This article aims to provide a comprehensive review of the advantages, limitations, and potential strategies for low tube voltage CCTA imaging in the context of coronary artery disease. We present this article in accordance with the Narrative Review reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-57/rc).

Methods

A systematic literature search was conducted using the PubMed database. The search strategy combined the following keywords with Boolean operators: “coronary computed tomography angiography”, “low tube voltage”, “lumen attenuation”, “plaque composition”. Synonyms and abbreviations (e.g., “low kV”, “atherosclerotic plaque”, “pericoronary adipose tissue”) were included to ensure comprehensive coverage. The search was restricted to articles published between 2010 and 2024, with language filters applied to retrieve English-only publications. Initial results were screened based on titles and abstracts to exclude irrelevant studies or non-English records. Studies focusing on the interplay between low tube voltage techniques, coronary plaque characterization, luminal contrast dynamics, and peri-coronary adipose tissue (PCAT) quantification were prioritized for inclusion (Table 1).

Advantages of low tube voltage CCTA

Reducing radiation dose

In current CCTA practice, commonly used low tube voltage settings include 100, 90, 80, and 70 kV. Clinically, the selection of tube voltage is no longer solely based on empirical methods but is optimized through automatic kV selection algorithms (such as Auto kV or Care kV). These algorithms adjust the tube voltage in real-time based on the patient’s body size and scanning region to improve image quality and reduce radiation dose. In a study conducted by Masuda et al. (12), 116 patients with a body mass index (BMI) <23.9 kg/m² underwent CCTA examinations, the results demonstrated that the 100 kV group exhibited a reduction of approximately 45% in radiation dose compared to the 120 kV group (563.7±82.8 vs. 819.1±115.1 mGy·cm, P<0.01), moreover, the 100 kV group achieved comparable diagnostic performance in terms of plaque characterization and assessment of stenosis severity. Wu et al. (13) conducted further research involving 154 patients with an average BMI of 23.5±2.8 kg/m² who underwent CCTA examinations at an 80 kV tube voltage, they discovered that the effective radiation dose (ED) in the 80 kV group was reduced by approximately 63% compared to the 120 kV group (2.39±0.45 vs. 0.88±0.16). The study by Jia et al. demonstrated that the use of 70 kVp combined with a high pitch (3.2) technique can reduce radiation dose by 94.8%. However, its application strictly depends on stable electrocardiogram (ECG) gating signals. If premature atrial contractions (PAC) or premature ventricular contractions (PVC) occur during the scan, the high pitch may result in incomplete Z-axis coverage, thereby affecting image continuity. Therefore, this technique is more suitable for patients with a regular rhythm and a heart rate ≤65 bpm (14). Researchers employed an adaptive approach in a study involving 796 patients who underwent CCTA examinations using a 70 kV tube voltage with a high pitch. In comparison to the conventional group, the low tube voltage group achieved a further reduction in ED by approximately 94.8%, while both groups demonstrated similar diagnostic performance in identifying coronary artery stenosis of ≥50%. Studies have also explored the application of low tube voltage CCTA in obese patients, Li et al. (15) conducted CCTA examinations on 100 patients with a BMI of approximately 28 kg/m². The high-strength deep learning image reconstruction (DLIR-H) algorithm reduces common “spotty” artifacts in low tube voltage images by suppressing high-frequency noise components, while preserving the sharpness of vascular edges. This approach also resulted in higher subjective and objective image quality ratings. Wang et al. (16) enrolled 30 patients with suspected coronary artery disease and BMI ≥28 kg/m², who underwent CCTA scanning at a tube voltage of 100 kV. The images were reconstructed using the DLIR algorithm [medium-strength deep learning image reconstruction (DLIS-M) and DLIS-H groups] and compared to those obtained with adaptive statistical iterative reconstruction [(ASIR-V) group]. The results showed that both DLIS-M and DLIS-H groups had lower image noise, higher contrast-to-noise ratio (CNR), and better subjective image quality than the ASIR-V group. Moreover, the subjective image score was highest in the DLIS-H group. Zhu et al. (17) also obtained analogous results. Therefore, the use of the DLIR algorithm can significantly reduce CCTA image noise and improve image quality in obese individuals. With continuous advancements in imaging technology, low tube voltage is increasingly being used in clinical practice.

Enhanced lumen attenuation

Reducing the tube voltage not only lowers radiation exposure but also brings X-ray photon energy closer to the binding energy of iodinated contrast agents. This, in turn, enhances the interaction of X-rays with the human body, particularly through the photoelectric effect, thus increasing the CT attenuation within iodinated vascular lumens. Consequently, it allows for a reduction in the required contrast agent dose while maintaining consistent CT attenuation. In a study conducted by Zhang et al. (18), which involved 90 patients with a BMI <25 kg/m², low tube voltage in combination with low-concentration contrast agents was used for CCTA examinations. The results revealed that the CT attenuation within the vascular lumens of the three major coronary arteries did not significantly differ among the three groups: 100 kV combined with 270 mg/mL contrast agent (Group A), 120 kV combined with 350 mg/mL contrast agent (Group B), and 120 kV combined with 370 mg/mL contrast agent (Group C). Nevertheless, the contrast agent dose in Group A was notably reduced by 21.27% and 24.83% compared to Groups B and C, respectively (P<0.05). Pan et al. (19) further conducted CCTA examinations on a population with a BMI <26 kg/m². They found that the lumen contrast of the aorta and the three major coronary arteries did not significantly differ between the group using 80 kV combined with 270 mgI/mL contrast (Group A1) and the group using 100 kV combined with 400 mgI/mL contrast (Group B1). Nonetheless, the iodine intake was reduced by approximately 17.7% in Group A1 (14.9±1.5 vs. 18.1±2.5 g). Albrecht et al. (20) investigated contrast agent injection protocols for CCTA examinations at different tube voltages, wherein, for every 10 kV reduction in tube voltage (from 120 to 70 kV), the contrast agent volume was correspondingly decreased by 10 ml (from 90 to 40 mL). The results revealed that, in comparison to the high tube voltage group (>100 kV), the low tube voltage group (≤100 kV) exhibited an almost 38.1% reduction in contrast agent dosage, while concurrently achieving an approximately 43.4% increase in lumen attenuation. Furthermore, both groups demonstrated comparable diagnostic accuracy in identifying coronary artery stenosis, as well as receiving analogous subjective and objective image quality ratings. Zhang et al. (21) evaluated the application value of reducing tube voltage and iodine delivery rate according to body weight in CCTA. The subjects in the control and experimental groups were divided into four subgroups based on their body weight. The control group utilized a tube voltage of 100 kVp and an iodine flow rate of 1.1, 1.3, 1.4, and 1.5 gI/s. In contrast, the experimental group used the following settings:70 kVp and 0.8 gI/s in 50–59 kg group, 80 kVp and 1.0 gI/s in 60–69 kg group, 80 kVp and 1.1 gI/s in 70–79 kg group, and 100 kVp and 1.5 gI/s in 80–89 kg group, respectively. The results demonstrated that the experimental group exhibited higher CT values in both the aortic root and left anterior descending artery (LAD) near-segment compared to those in the control group; additionally, ED was reduced by approximately 37.3%, contrast agent dosage decreased by 15.5%, and overall subjective image quality score improved when compared to that observed in the control group.

Improving lipid recognition

Accurate assessment of lipid plaques is crucial for clinical treatment and prognosis. Multiple studies have indicated that as the tube voltage decreases, the attenuation values of lipid plaques also decrease, resulting in reduced overlap with the attenuation values of fibrous plaques. Consequently, low tube voltage CCTA plays a significant role in precise lipid plaque identification, carrying substantial clinical relevance. In a study by Tanami et al. (22) with histopathology as the gold standard, the impact of tube voltage on the overlap of CT attenuation between lipid and fibrous plaques was investigated, the findings demonstrated a gradual reduction in overlap as the tube voltage decreased. Specifically, the attenuation values for lipid plaques were 23.9, 23.1, 21.8, and 20.6 HU for the 140, 120, 100, and 80 kV groups, respectively. The corresponding overlap percentages were 81%, 76.2%, 70%, and 55%. Furthermore, the diagnostic performance for identifying lipid plaques exhibited improvement, with the respective areas under the curve (AUC) [standard error (SE)] being 0.651 (0.066), 0.682 (0.064), 0.772 (0.056), and 0.813 (0.047). Similarly, Masuda et al. (12) also reported that using 100 kV CCTA examination results in slightly higher accuracy in differentiating between fibrous and lipid/fibrous-lipid components as compared to 120 kV. Despite a notable increase in coronary artery lumen attenuation values in the 100kV group compared to the 120 kV group, by approximately 15.7% (323.1±81.2 and 279.3±61.8 HU), the CT threshold for diagnosing lipid plaques remained significantly lower (30.5 and 37.4 HU).

Limitations of low tube voltage CCTA

Impact on image quality

While low tube voltage scanning offers several advantages, one of its primary challenges lies in the increased image noise it tends to produce. Previous research has shown that as tube voltage decreases, both image quality and diagnostic accuracy tend to decrease as well. For instance, Mousavi et al. (23) conducted CCTA examinations on a cohort of 54 patients with a BMI ≤29 kg/m², their findings revealed a substantial increase in noise in the 100 kV group when compared to the 120 kV group (61.00±24.04 vs. 29.00±4.24). Similarly, studies by Wang et al. (24) reported significantly higher image noise in the 80 kV group (40.3±5.6 HU) as opposed to the 100 kV group (24.6±4.0 HU) and the 120 kV group (18.8±2.2 HU). Interestingly, there was no significant difference in image noise between the 70 and 80 kV groups (25). In another investigation, Holmquist et al. (26) employed a body phantom to simulate patients with normal body weight, their experiments demonstrated that, in comparison to the 120 kV group, the lumen attenuation in the 80 and 70 kV groups increased by approximately 1.6 and 2.0 times, respectively. However, image noise exhibited a more substantial increase, approximately 1.9 and 2.5 times, respectively. This phenomenon may be attributed to the limitations of second-generation dual-source CT in providing adequate tube current at lower tube voltages, resulting in reduced X-ray energy and an attendant increase in image noise.

Beam hardening artifacts and challenges in large patients

Although the application of low tube voltage techniques has made progress in large patients, the accompanying beam hardening artifacts should not be overlooked. The reduction in X-ray penetration at low kVp leads to photon starvation, which is especially pronounced in patients with a BMI ≥30 kg/m². Such artifacts may manifest as blurred vessel edges or an overestimation of calcified plaques, interfering with the accurate assessment of plaque composition. For example, Booij et al. (27) found in phantom experiments that the measurement error for calcified volume at 70 kVp was approximately 22% higher compared to 120 kVp, while using a calcium-aware reconstruction algorithm reduced the error to 8%. Additionally, the high attenuation of iodine contrast agents can exacerbate the beam hardening effect, especially at low kVp, where the attenuation difference between high iodine concentration in the vessel and surrounding tissues increases, further leading to artifact formation. Zhang et al. (28) noted that at low tube voltage (80 kVp), even when the tube current was increased to 1,300 mA, the coefficient of variation for calcium scoring was still 15% higher than that of the standard voltage group, suggesting that merely increasing mA is not sufficient to fully resolve this issue.

Impact on plaque assessment

Low tube voltage influence on plaque assessment through two primary pathways, the first is a direct pathway driven by X-ray energy, where changes in attenuation values directly affect the assessment of plaque composition. Studies have shown that reducing tube voltage generally leads to an increase in the volume of necrotic cores, fibrous-lipid, and calcified components, while reducing the volume of fibrous components (12). For instance, in a study conducted by Masuda et al. (12) involving 116 patients undergoing CCTA, the 100 kV group exhibited a 22.1 HU increase in attenuation values for fibrous plaques, accompanied by an 8.19 HU decrease in lipid plaques or fibrous-lipid plaques when compared to the 120 kV group. Similarly, a study by Takagi et al. (8) involving 1,236 patients undergoing CCTA, found that the 80 kV group showed a significant increase in fibrous-lipid (β=5.4) and necrotic core (β=1.8) components when compared to the 120 kV group. In addition, Vattay et al. (29) utilized state-of-the-art photon-counting CT to investigate the impact of different virtual mono-energetic levels on the volume of coronary artery plaque components. By reconstructing virtual mono-energetic images at 10 keV intervals (ranging from 40 to 70 keV), they observed that as the keV level increased, there was a decrease in the average attenuation of plaques, resulting in an increased volume of low-attenuation plaques. This observation contrasts with earlier studies, and the discrepancy may be due to the failure to account for the indirect impact of lumen attenuation on low-attenuation plaques. As keV increased, there was a notable decrease in lumen attenuation (30). Furthermore, a study by Marwan et al. (31) involving 150 patients who underwent coronary calcium scoring revealed that the 100 kV group exhibited significantly higher calcium volume and Agatston scores compared to the 120 kV group, even though the calcium assessment threshold had been raised to 147 HU in the 100 kV group. However, Allio et al. (32) reported contrasting results as they examined 1,511 patients for coronary artery calcium scoring. They found that the 80 kV group tended to underestimate the Coronary Artery Calcification Score (CACS) compared to the 120 kV group, the proportions of patients with a CACS of 0 are 17.5% (264 cases) and 29% (437 cases), respectively. This discrepancy may be attributed to the increased image noise during low tube voltage scanning, which diminishes spatial resolution and can result in the oversight of smaller calcifications. The second pathway involves an indirect impact through alterations in lumen attenuation. Takagi et al. (8) categorized patients into low attenuation (<350 HU), normal attenuation (350–500 HU), and high attenuation (>500 HU) groups based on lumen attenuation values. The results indicated that as lumen attenuation increased, the proportion of fibro-fatty plaques (26%, 13%, and 4%) and necrotic cores (1.6%, 0.3%, and 0.0%) decreased, while the proportion of calcified plaques significantly increased (16%, 27%, and 40%). However, due to enhanced lumen attenuation resulting from low tube voltage, the difference between calcification and lumen attenuation values decreased, potentially leading to the underdiagnosis of calcified plaques. Baliyan et al. (9) found that the low tube voltage group (70–90 kV) had a higher rate of missed calcified plaques (approximately 41%) compared to the standard tube voltage group (100–120 kV). Tischendorf et al. (10) further confirmed that when using low tube voltage in combination with conventional contrast agent concentrations, the rate of missed calcified plaques approached nearly half, with the rate reaching 57.1% at 70 kV. Consequently, they concluded that the use of low tube voltage in combination with lower contrast agent concentrations was beneficial for the assessment of calcified plaques.

Impact on coronary perivascular fat evaluation

PCAT based on CCTA can serve as an indicator of coronary artery wall inflammation, and it contributes to early risk stratification and prognostic prediction for patients with coronary artery disease. However, recent research indicates that the low tube voltage has a significant impact on the attenuation values of pericoronary adipose tissue, leading to significant decreases in PCAT with a reduction in voltage. In a study conducted by Ma et al. (33) in which PCAT was the dependent variable and various factors, including tube voltage and BMI served as independent variables, a comprehensive multivariate linear regression analysis was performed. The findings revealed a significant positive correlation between PCAT and tube voltage. This suggests that as the tube voltage increases, PCAT values exhibit a more pronounced increase, while there is no significant association found with BMI. The mean PCAT values for the 70, 80, 90, 100, and 120 kV groups were −95.6±9.6, −90.2±11.5, −87.3±9.9, −82.7±6.2, and −79.3±6.8 HU, respectively. Etter et al. (34) also reported that, regardless of the specific imaging equipment and reconstruction techniques used, PCAT exhibited a direct proportionality to changes in tube voltage. Similarly, Pan et al. (11) investigated the effect of different tube voltages on pericoronary fat and found that the pericoronary fat attenuation index (FAI) was directly proportional to the tube voltage, and the magnitude of the change increased with decreasing tube voltage. Consequently, when comparing PCAT values obtained from CCTA examinations conducted at different tube voltages, it is imperative to account for the influence of tube voltage variations.

Methods for improvement

Firstly, in response to the impact of low tube voltage on image quality, recent research demonstrates that the combination of iterative reconstruction (IR) algorithms can effectively mitigate the noise associated with low tube voltage. Park et al. (35) conducted low tube voltage (100 kV) CCTA examinations on 40 patients in each group, with BMI <25 and 25–30 kg/m², respectively. The results revealed that, when compared to filtered back projection (FBP), the IR group exhibited noise reduction of 65.0% and 68.1%, respectively. Remarkably, the compensation effect was found to be more pronounced in overweight patients. it is worth mentioning that the use of IR can increase infrared intensity, potentially leading to changes in noise texture and causing plastic-looking, speckled, or excessively smoothed appearances in the images, and DLIR-H has shown promise in ameliorating these artifacts. Wang et al. (36) found that, compared to FBP, the combination of 70 kV with DLIR-H reduced image noise by 57.5%, while increasing signal-to-noise ratio (SNR) and CNR by a factor of 2 and 2.4, respectively, thereby enhancing both subjective and objective image quality. Secondly, concerning the impact of low tube voltage on plaque assessment, the majority of studies emphasize a reduction in contrast agent dosage to achieve comparable luminal enhancement and subsequently mitigate indirect effects. Yuan et al. (37) evaluated image quality on low contrast media with DLIR algorithm, CCTA reconstructed with DLIR could be realized with adequate enhancement in coronary arteries, excellent image quality and diagnostic confidence at low contrast dose. Additionally, Subtraction CCTA (S-CCTA) technology has been proven to be an effective approach for reducing these indirect effects. Takamura et al. (38) found that S-CCTA with low tube voltage significantly reduced the rate of missed detection of calcified plaques in comparison to conventional CCTA scans. This could be attributed to the elimination of luminal enhancement, masking the presence of calcifications. However, research on methods specifically addressing direct voltage-related impacts is somewhat limited, some researchers have proposed the use of adaptive thresholding to enhance this aspect. For example, Masuda et al. (12) recommended the determination of optimal diagnostic threshold values for assessing plaque characteristics under different voltages, the study indicated that using optimal diagnostic thresholds of 30.5 HU for 100 kV and 37.4 HU for 120 kV resulted in similar diagnostic performance for coronary artery plaque nature in both groups. Gräni et al. (25) applied low tube voltage in conjunction with KV-adaptive threshold calculations for computing CACS, they discovered that as tube voltage decreased, the threshold range for the same risk level gradually expanded, for instance, for a risk level of 1, the threshold ranges were 130–199 HU for 120 kV, 145–222 HU for 100 kV, 177–271 HU for 80 kV, and 207–318 HU for 70 kV, post-application of KV-adaptive thresholds, the CACS values in the 80 and 70 kV groups exhibited a strong correlation with standard tube voltage (r>0.97, P<0.001). Finally, with regard to the influence of low tube voltage on pericardial fat attenuation values, some studies recommend the use of a correction factor (K) to account for the effect of low tube voltage. Etter et al. (34) calculated K values for different tube voltages, such as 80, 100, 120, and 140 kV, using the formula Ki = PCATMAi/PCATMA120 (where MA represents the mean value, and i represents voltages other than 120 kV), the resulting K values were 1.267, 1.08, 1, and 0.947, respectively. In a study conducted by Kuneman et al. (39), which involved 198 patients undergoing CCTA scans at both 100 and 120 kV, a conversion factor (K=1.11485) was used to correct PCAT values acquired at 100 kV. The study discovered that, in patients with acute coronary syndromes, PCAT attenuation at culprit lesion sites was significantly higher compared to that at non-culprit lesion sites. In recent years, the introduction of automatic kV selection algorithms, such as Care kV, has significantly optimized the applicability of low tube voltage techniques. These algorithms adjust the combination of tube voltage and tube current in real-time based on the patient’s body size and the scanning region, rather than relying on traditional empirical selection. For example, Rajendran et al. (40) confirmed in a multicenter study that Care kV technology can reduce image noise by 18% in patients with BMI ≥28 kg/m², while also decreasing radiation dose by 35%. Additionally, the combination of iodine flow rate adaptive techniques, such as dual-flow injection protocols, can balance the conflict between vascular enhancement and beam hardening artifacts. Im et al. (3) suggested that in patients with BMI >25 kg/m², using low-concentration contrast agents (270 mgI/mL) combined with IR can reduce the artifact index to a level comparable to the standard voltage group (P=0.12) while maintaining vascular CT values >350 HU.

Conclusions

In conclusion, the gradual application of low-tube-voltage CCTA in clinical practice has demonstrated significant advantages in reducing radiation dose, minimizing contrast agent usage, and improving lipid characterization. However, there are also drawbacks, including reduced image quality and an impact on plaque and fat assessment. While techniques like DLIR, optimal thresholding, and conversion factors show promise in addressing some of these issues, further research is imperative to promote the extensive adoption of low tube voltage technology.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-24-57/rc

Funding: None.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-57/coif). The authors have no conflicts of interest to declare.

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