| BackgroundAortic elasticity is defined as the relative temporal change in vessel diameter or cross-sectional area through the cardiac cycle under per unit blood pressure. It depends on the geometrical and mechanical characteristic of the vessel, and is an important functional parameter of the aortic wall. The decrease of arterial elasticity or increase of stiffness is accepted as an independent influencing factor for a series of cardiovascular diseases. During the course of arterial diseases, the decrease of arterial elasticity usually occurred earlier than the morphological and structural changes which can be found by clinical methods. Therefore, aortic elasticity assessments early on may be benefit in preventing and curing arterial and cardiac diseases, and may also play an important role in lowing the incidence and death rate of the cardiovascular diseases.Now, it is impossible to observe the structural changes of the arterial wall directly and the quantitative assessment of the functional changes of the artery is difficult, thus, a variety of functional parameters are used to reflect the aortic elasticity indirectly. Including pulse pressure, augmentation Index (AI), C1/C2, and pulse wave velocity (PWV) and so on. These functional parameters can be obtained by a particular instrument or varied imaging techniques. Early, ultrasound was firstly used in the assessment of arterial elasticity, especially in the superficial artery. However, the repeatability and consistency is poor affected by the operators. With the continuous advances in imaging technology, MR was used in the evaluation of cardiac function, and then applied to the evaluation of aortic wall structure and function. However, it was not widely accepted because of the long scanning time but low spatial resolution, also the expensive cost.In the past, CT had been widely used in displaying morphology and anatomical structure of the vascular. Now, it is gradually applied in the evaluation of cardiovascular function. A previous study had reported that it was feasible to evaluate vessel elasticity by using electrocardiographically-gated MDCT through assessing changes of the cross-sectional area during the cardiac cycle. An additional tool is to use, DSCT thereby increasing available temporal resolution to83ms. This permits more accurate vessel measurements at higher heart rates, combined with ECG gating technology, all data during the cardiac cycle can be obtained, which provide a guarantee for the accurate assessment of arterial elasticity. However, there are still no adequate clinical data, the accuracy and clinical value of the technology needs further study.This was first study to evaluate aortic elasticity in people without relative cardiovascular diseases, all subjects were selected from people undergoing DSCT examination for suspected coronary artery or aortic disease. Based on this, elasticity was also assessed in patients with abdominal artery aneurysms.PurposeTo gain a new insight into the pulsating property and wall elasticity of the thoracic aorta in people without cardiovascular diseases and people with abdominal artery aneurysms using ECG-gated DSCT, the feasibility and potential clinical value of this technique in arterial function evaluation were discussed, aiming to find an objective and effective method for the comprehensive evaluation of the aortic diseases including morphology and function.Materials and Methods1. Experimental study:A silicone tube (inner lumen7.60mm) filled with Iohexol(350mg/ml) and saline(proportion1:32) was examined using DSCT after fixed in a certain radian. Ten different positions,10mm distance from each other, were measured with the vessel measurement prototype software, and the average measurement diameter was compared with the known diameter to verify the accuracy of the software.2. Study in people without cardiovascular disease:56subjects (male39, female17, average age46±10years) with no cardiovascular disease, selected from2,700people, from Dec.2008to Apr.2010undergoing ECG-gated DSCT examination, were divided into three groups according to their age (A<40years, B40-50years, C>50years). CT data were reconstructed in5%step throughout the RR interval. Diameter and area were measured at the curve of the ascending aorta (AA) and at the same level of the descending aorta (DA), about25±2.5mm above the sinotubular junction. The pulsation and elasticity of the aorta were evaluated, and elastic parameters are%Ao, AoD, AoC and AoSI.3. Study of abdominal artery aneurysm (AAA):11patients (male9, female2, average age65±9years) with AAA (R>5cm) were examined using ECG-gated DSCT. CT data were reconstructed in5%step throughout the RR interval. Areas were measured at the maximum lumen and AoD was used as the elastic parameter.Results 1. Accuracy of vessel measurement prototype software:Ten different positions along the silicone tube were measured by vessel measurement prototype software. The extracted average inner diameter was7.64mm. Compared with the known tube diameter of7.60mm the resulting relative error was0.53%. The discordance mainly occurred at the end or start plane of the trace.2. Results of people without cardiovascular disease The ECG-gated DSCT images showed good contrast between the enhanced vessels and the surrounding tissues for the aortic measurement2.1Evaluation of pulsation regulation The aortic diameter changes periodically according to the cardiac cycle. The AA achieved the maximum average diameter at an RR interval of24.02±4.99%,95%CI (22.68%,25.35%) and the minimum average diameter at an RR interval of93.5±4.04%,95%CI (92.4%,94.6%) respectively. For the DA, the maximum was at25.63±4.77%,95%CI (24.35%,26.9%), and the minimum was at96.6±4.58%,95%CI (95.38%,97.83%). The phases of the maximum and minimum average diameter were compared in relation to the heart rate and found that there were no significant correlations between the phases and the mean heart rate (for the AA, maximum average diameter phase r=0.251, p=0.189and minimum average diameter phase r=0.126,p=0.514; for the DA, they were r=0.141,p=0.3and r=0.087,p=0.552).2.2Aortic elasticity assessment There were significant differences between the AA and the DA for the diameter, area change over the cardiac cycle and AoC, whereas no significant difference for %Ao, AoD. The stiffness of the DA was different from that of the AA in all groups, but was only significant in group A.There was an age-dependent decrease of%Ao, AoD, AoC and increase of AoSI, the correlation coefficients between the age and the elastic parameters (%Ao, AoD, AoC and AoSI) were r=-0.747,-0.734,-0.545and0.650for the AA; the values were-0.690,-0.681,-0.568and0.627for the DA;(P<0.05or0.01). Whereas, the age-dependent trend was different between various age groups and between different elastic parameters, which was more significant in group A with the AoD (r=-0.711for the AA and-0.828for the DA).3. Results of patients with AAA3.1Evaluation of pulsation regulationBoth proximal of the aneurysm and the lumen of the aneurysm all changes according to the cardiac cycle. The area-versus-time curve at the proximal of the aneurysm demonstrates up-and-down trend similar to the thoracic descending aorta; while area-versus-time curve demonstrates multiple up-and-down trend without exactly regular pattern.3.2Aortic elasticity assessmentAoD at the proximal of the aneurysm and the aneurysm portion was (1.05±0.8) mmHg-1and (0.45±0.5) mmHg-1respectively, there were statistical differences between them (p<0.05). Comparing to the thoracic descending aorta, there was significant decrease at the proximal of the aneurysm which was (1.05±0.8) vs (2.17±0.58)(p<0.01). Conclusions1. As an new non-invasive method, the morphologic and functional evaluation can be obtained at the same time by using DSCT without additional radiation dose;2. The lumen of the aorta changes regularly according to the cardiac cycle, the maximum diameter occurred at the systolic time, and the minimum at late of the diastolic time, the time of descending aorta was later than that of the ascending aorta;3. There was an age-dependent decrease of the elasticity, whereas, the age-dependent trend was different between various age groups and between different elastic parameters;4. Elasticity at the aneurysm was significantly decreased comparing with the healthy arterial wall and the proximal part of it, the development of aneurysm related closely to the decrease of the elasticity.In conclusion, the properties of aortic pulsation and wall elasticity could be well shown by ECG-gated DSCT both in healthy people and patient with aneurysm, as a potentially useful noninvasive method, retrospective ECG-gated DSCT can provide valuable functional information of the aorta as well as showing the structural changes.
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