We found that the total mean attenuation values of PAs were significantly higher in group A than in group B (532.7 ± 243 HU vs. 380.6 ± 232 HU, respectively) (Table 2). Additionally, total DLP and effective dose values were significantly lower in group A than in group B. Twenty-one patients (21%) in both groups were detected for PTE.
Pulmonary angiography is the gold standard in the diagnosis of PTE and the sensitivity and specificity of this technique has been reported to be 90% and 94%, respectively (13). Pulmonary angiography is an invasive procedure and is not widely available. On the other hand, CTPA is a non-invasive radiological imaging method using an multi-detector CT (MDCT) device with high sensitivity and specificity, low false negativity or false positivity ratios. CTPA is used widely in daily clinical applications for PTE diagnosis in many clinics (14).
One disadvantage of CTPA is the health risk associated with the radiation dose and amount of CM used. The radiation dose constitutes an important problem especially in less risky pulmonary embolism, in young female patients and patients who clinically have low probability for pulmonary embolism (12). Alterations in technical parameters without significantly affecting the diagnostic quality can decrease the radiation dose. These technical parameters include decreasing the tube current (mA) and gantry rotation time (s), decreasing the kV, using automatic tube current modulation, decreasing the scanning area and increasing the table speed (7).
Different voltage amounts such as 140 KV - 80 kV were used in CTPA examinations in different clinics (13-15). Keeping other factors constant, decreasing the tube voltage from 120 kV to 80 kV decreases the radiation dose by at least 60%. However, decreasing the tube voltage increases noise while increasing the image contrast. As a result, signal noise ratio (SNR) is decreased since the relative increase in noise is more than the increase in image contrast (6, 12). On the other hand, it was demonstrated that diagnostic imaging quality in CTPA is preserved even when the tube voltage applied is decreased to 80 kV due to the increase in photo-electric event (14). Decreasing the tube voltage from 140 kV to 120 kV and even to 80 kV increases X-ray attenuation in the order of 2 and 1.6, respectively (15). In our study, the HU numbers are higher in group A than group B thanks to lower kV values in group A.
Image noise can increase appreciably when low kV is used. This is particularly important in wide body areas such as the abdomen. However, low kV applications in the lungs do not constitute a problem since there is a very significant density difference between the interstitium surrounding the air-filled alveoli and vascular structures. X-ray attenuation and absorption in the alveoli are significantly low. This increases the image clarity of the neighboring parenchymal and vascular structures (15). The aim of the current study was to take advantage of this low X-ray attenuation and absorption in the thoracic cavity and investigate the feasibility of using low tube voltage in CTPA captures.
Although the side effects of CM are well known, contrasted CT examinations using iodine-based CMs are routinely used in radiological examinations. On the other hand, renal toxicity is the most important complication of iodine-based CMs. Amongst the patients administered with CM, 15% reported nephropathy in the absence of any other significant risk factor. This incidence may rise up to 80% in the presence of additional risk factors such as diabetes, cardiac failure, hyperglycemia, old age, and over use of CM (10, 16, 17). Many recent studies suggest a reduction in the amount of CM during CT examinations, especially in angiography to minimize renal toxicity (10, 18).
On the other hand, considering that low tube voltage increases CM opacification, many authors have suggested that both volume of CM used and tube voltage should be lowered together (11, 19). In this study, we aimed to acquire better attenuation values with better image quality by using low dose CM and low kV value in patients who underwent CTPA scan for suspected PTE.
Schuller-Weidekamm et al. (12) detected significantly higher attenuation values at 100 kV compared to 140 kV in central and peripheral PAs with an average density of 268 ± 63 HU at 140 kV and 379 ± 95HU at 100 kV in the main PA. Wintersperger et al. (15) reported an average arterial attenuation value of 432 ± 80 HU at 100 kV and 333 ± 90 HU at 120 kV where the mAs value was held constant in aortoiliac CT angiography. These authors reported a significant decrease in the radiation dose taken by the patient.
In a previous study, we separated the cases into two groups as standard dose (group A; 120 kV and 1 mL/kg, CM) and low dose (group B; 100 kV and 0.5 mL/kg CM) in order to investigate the feasibility of using low CM volume and kV values in carotid computed tomography angiography (CTA) using 128 slice MDCT (6). We observed that in low dose applications (group B), the arterial attenuation values were higher and the exposed radiation dose was lower (6).
Viteri-Ramirez et al. (20) used a dual-source CT system (similar to the current study) on 70 patients with a pre-diagnosis of PTE. The patients were divided into two groups: n = 35, 80 kv/60 mL for group A and n = 35, 100 kV/80 mL for group B. The average attenuation values were 362.4 ± 100.2 HU for group A and 262.4 ± 134.3 HU for group B and the difference was statistically significant. Total DLP values were measured as 64.5 ± 43.5 mGy-cm in group A and 161 ± 69.1 mGy-cm in group B and effective dose values were measured as 1.1 ± 0.7 mSv in group A and 2.7 ± 1.2 mSv in group B. The difference in total DLP and effective dose between the two groups was statistically significant (P < 0.001). In the current study, amongst the 100 patients with a pre-diagnosis of PTE the total mean attenuation values for PAs were measured 532.7 ± 243 HU in group A (n = 50, 0.5 mg/kg CM and 80 kV) and 380.6 ± 232 HU in group B (n = 50, 1 mg/kg CM and 100 kV). Total DLP (76.5 ± 17.3 mGy.cm in group A, 162.1 ± 31.3 mGy.cm in group B) and effective dose values (1.2 ± 0.2 in group B and 2.4 ± 0.2 in group B) were found to be significantly lower in group A (P < 0.001).
Viteri-Ramirez et al. (20) used a protocol whereby the tube voltage was decreased from 100 kV to 80 kV similar to the current study. These authors used a standard dose of 80 mL CM in group B while a lower CM volume of 60 mL was used in group A. In the current study, a standard CM dose of 1 mL/kg was used in group B while the dose was halved to 0.5 mL/kg in group A. In spite of the lower CM volume, we observed higher average attenuation values of PAs in group A. Additionally, we think that there may be a significant decrease in the dose of radiation with the lower tube voltage in group A. Moreover, allergic reaction and possibility of CM-induced nephropathy can be significantly decreased.
The time of initiation of CT angiography after the application of CM is important for which bolus injection and bolus tracking methods are routinely used. In this case, image capturing starts automatically when CM attenuation value reaches a pre-determined threshold (21, 22). In our study, 30 mL serum physiological was given at 5 mL/s after application of CM at 5 mL/s to transfer any remnant CM in the injector line and other spaces using "bolus tracking" technique in 128 slice MDCT.
Increasing the scanning rate and shortening the examination time provides an opportunity to decrease the CM dose in CT angiography applications. Previously, doses between 140 and 160 mL were used in PA CT angiography; this was reduced to less than 100 mL with the new MDCT devices (14). We aimed to decrease CM amount further by using a dose of 0.5 mL/kg. The heaviest patient in our cohort was 120 kg for whom 60 mL CM was used while the leanest patient was 55 kg and the CM amount used was 22.5 mL. An average of 40 - 45 mL CM was used, which is well below the amount that is used routinely.
The limitations of our study include a relatively small cohort of 100 patients and a lack of data on body mass index since most of the CTPA were carried out as an emergency procedure. Additionally, comparative CTPA examinations using two separate kV values and CMs could not be carried out in the same patient to minimize exposure to radiation and CM.
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