The Application Of Low Dose Retrospective ECG-gated CTA In Evaluation Of Aortic Dissection

Part I:Low dose retrospective ECG-gated thoracoabdominal aorta CTA in patients with acute aortic dissection:feasibility and additional diagnostic valueObjectives:To assess feasibility and additional diagnostic value of low dose ECG-gated thoracoabdominal aorta CT angiography (CTA) in patients with acute aortic dissection (AD).Materials and methods:From January2012to November2012, a total of49consecutive patients were prospective enrolled in the study. The ages were ranged from30to73years old (mean age,49.25±10.4years), and there were33men and16women. Low dose ECG-gated thoracoabdominal aorta CTA was performed by dual-source CT (DSCT). Functional imaging was reconstructed in5%steps between0and95%of the R-R interval from the ECG with1-mm slice thickness,0.8-mm increments, I26f reconstruction kernel and24cm FOV in patients with acute AD. Aortic intraluminal attenuation, image noise and signal noise ratio(SNR)were assessed as objective image quality parameters at datasets acquired with full tube current (70%R-R interval). The ROIs were placed in the ascending aorta, descending aorta at the level of the right pulmonary trunk plane and at the level of2cm above the celiac trunk ostium. Blinded image interpretation of70%R-R interval image was performed on a3D workstation by two experienced vascular radiologists. Subjective image quality was categorized into three groups:grade3(no artifacts, all anatomical details assessable); grade2(mild artifacts, majority of the anatomical structures assessable); grade1(severe artifacts, no anatomical structures assessable). Diagnostic confidence of intimal flap rapture and origin of aorta brunches were determined for both the mono-phases (70%R-R interval) and multi-phases (0%-95%R-R interval). Diagnostic confidence was rated on a3-point scale (1=uncertain,2=moderately confident,3=fully confident). Motion artifact of intimal flap of each phase was assessed at ascending aorta, thoracic aorta (at the level of the right pulmonary trunk plane) and abdominal aorta (at the level of2cm above the celiac trunk ostium) by a consensus reading of two radiologists. The frequency distributions of intimal flap motion artifacts during a cardiac cycle were analyzed. True lumen collapse was sorted as complete collapse (true lumen collapsed at every R-R intervals at the same level) and partial collapse (true lumen collapsed at one or more R-R intervals at the same level).The relationship of true lumen collapse and dynamic obstruction were analyzed. The volume CT dose index (CTDIvol) and dose length product (DLP) were provided by the scanner. The effective dose (ED) was determined by using DLP and appropriate normalized coefficients k (k=0.0142mSv.mGy-1.cm-1). All statistical analyses were performed with statistical software SPSS17.0The intra-observer agreement in image quality grading was evaluated by kappa test, and K-values of0.41-0.6corresponded to moderate agreement, K-values of0.61-0.80corresponded to good agreement, and0.81-1.00corresponded to perfect agreement. The intraluminal attenuation difference of ascending aorta and abdominal aorta were compared with the paired two-tailed Student’s t-test. The grades of two datasets diagnostic confidence were compared using Wilcoxon rank sum test. A value of p<0.05was considered statistically significant.Results:Aortic intraluminal attenuation of ascending aorta, descending aorta and abdomen aorta were386.9±63.6HU,381.4±55.3HU and377.6±60.0HU respectively. Image noise of ascending aorta, descending aorta and abdomen aorta were26.8±6.4,23.2±5.5and24.4±5.1respectively. SNR of ascending aorta, descending aorta and abdomen aorta were14.7±2.9,16.8±2.3and15.3±2.7respectively. There were no statistically significant differences in intraluminal contrast attenuation between ascending aorta and abdomen aorta (p>0.05).The inter-observer agreement for subjective image quality was excellent (k=0.83).Assessment of ascending aorta wall, descending aorta wall,aortic valve and the origin of the coronary arteries subjective image quality were2.83±0.42、3±0、2.94±0.24、2.88±0.33和2.92±0.27.89.8%structures of aorta were depicted with no motion artifacts. Only1aortic valve (2.0%) was depicted severe artifacts. The diagnostic confidence in multi-phases images was statistically higher compared to that in mono-phase images at ascending aorta intimal flap rapture(P<0.05), but not at the descending aorta and abdominal aorta flap rapture (P>0.05).The diagnostic confidence in multi-phases images was no statistical differences compared to that in mono-phase images at the origin of aorta brunches(P>0.05).128of460(25.7%) intimal flaps had motion artifacts at ascending aorta.122of680(17.9%) intimal flaps had motion artifacts at thoracic aorta.83of620(13.4%) intimal flaps had motion artifacts at abdomen aorta.72%intimal flaps with motion artifacts were at0%～40%of R-R interval. True lumen collapse was found in13of31cases.4cases were complete collapse and9cases were partial collapse.7visceral vessels (3superior mesenteric arteries,2celiac trunks and2renal arteries) in4patients were diagnosed with typical dynamic obstruction. True lumens were completely collapsed at the level of the involved branch origins in these4cases. The CTDIvol, DLP, and ED were16.1±6.8(7.6-33.6) mGy,896.4±373.4(450.1～1850.1) mGy.cm, and12.7±5.3(6.4～26.3) mSv respectively.Conclusions:We have demonstrated the feasibility of low dose retrospective ECG-gated CTA covering the entire aorta while keeping the radiation dose in standard limits. In patients with acute AD, low dose retrospective ECG-gated CTA multi-phases images can provide more substantial diagnostic information with potential therapeutic consequences than mono-phases images. Retrospective ECG-gated thoracoabdominal aorta CTA multi-phases images can reflect the actual status of the true lumen and provide more information about true lumen collapse. This information may be helpful to diagnosis and differential diagnosis of dynamic abstraction. Part II Low dose retrospective ECG-gated thoracoabdominal aorta CTA evaluating abdomen aortic intimal flap motion in patients with acute aortic dissection:feasibility and intimal flap motion characterizationObjectives:To evaluate the feasibility of low dose retrospective ECG-gated thoracoabdominal aorta CT angiography (CTA) assessing abdominal aortic intimal flap motion and investigate the motion characteristics of intimal flap in acute aortic dissection (AD).Materials and methods:From January2012to November2012, a total of31consecutive patients (22men,9women; age range,30-72years, mean age±standard deviation,50.35±9.6years) were retrospective enrolled in the study. Low dose ECG-gated thoracoabdominal aorta CTA was performed by dual-source CT (DSCT). Imaging was reconstructed in5%steps between0and95%of the R-R interval from the ECG with1-mm slice thickness,0.8-mm increments, I26f reconstruction kernel and24cm FOV in patients with acute AD. The true lumen short axis diameter (TLD) and the false lumen short axis diameter (FLD) were measured at the level of2cm above the celiac trunk ostium at different phases of a cardiac cycle by two independent observers. The configuration of the intimal flap was classified as curved toward the false lumen, curved toward the true lumen and flat. It was assessed independently by the two observers. In case of disagreement in classifying between the observers, a final decision was obtained by consensus.All time instances were expressed as percentages of the R-R interval to enable comparisons of intimal flap motion amplitude for all patients. The motion amplitude of intimal flap was quantified by the change of the TLD at every5%R-R interval. It was expressed as the absolute value of relative change between the TLD (RCTLD) of adjacent phases of R-R interval. On the basis of measurements, the following variables were obtained:(1) Maximum true lumen diameter (TLDmax) and minimum true lumen diameter (TLDmin) during a cardiac cycling.(2)The maximum change of TLD (MCTLD) and relative decrease of TLD (RDTLD) during a cardiac cycle:MCTLD was defined as the difference between TLDmax and TLDmin; RDTLD was defined as the ratios of MCTLD to TLDmax. MCTLD reflect the range of intimal flap motion and RDTLD reflect the extent of TLDmax decreased during a cardiac cycle.(3) RCTLD and maximum motion phase (MMP) of TLD over a cardiac cycle: RCTLD defined as the absolute difference between the TLD of adjacent time intervals, which reflects the change of TLD at a given percentage of time intervals. MMP was defined as the phase of R-R interval where the maximum RCTLD was at. The design formulas were as follow:MCTLD=TLDmax-TLDmin;RDTLD=(MCTLD/TLDmax)×100%RCTLDn=|TLD (n+1)-TLDn|Where "n" represents a given phase of the R-R interval and "(n+1)" represents the next adjacent phase.Interobserver variation of diameter measurement was evaluated by using the Bland-Altman analysis. Interobserver agreement on classifying the intimal flap configuration was calculated before consensus reading by using Kappa statistics. A kappa value of more than0.81corresponded to excellent interobserver agreement, with values of0.61-0.80corresponding to good agreement. Bonferroni post-hoc tests were used for multiple pair wise comparisons of FLD, TLD and RCTLD within time intervals. A value of p<0.05was considered statistically significant. All statistical analyses were performed with statistical software SPSS17.0.Results:Onset time of acute AD was from2hours to12days.20of the31(64.5%) patients were DeBakey I, and11of the31(35.5%) were DeBakey Ⅲ. The Bland-Altman analysis of interobserver variation of diameter measurements demonstrated good results. The measurement error of TLD and FLD were0.085and0.078respectively, whereas the95%limits of agreement werel.630to-1.460and1.410to-1.254mm respectively. Overall interobserver agreement on intimal flap configuration classifying was excellent (k=0.93). Peak of group-averaged FLD (19.44±4.07mm) and trough of group-averaged TLD (8.67±5.16mm) were found at15%of the R-R interval. There were no significant statistical differences comparing to other R-R intervals (p>0.05). The TLDmax, TLDmin, MCTLD and RDTLD in all patients were12.2±4.1mm (2.6～17.4)、6.7±4.1mm (0～15.3)、5.5±2.6mm (1.8～10.2)和49.5%±23.5%(12%～100%) respectively. All of the MMP were at middle and later systolic phase and early diastolic phase (5%～40%of R-R interval). Group-averaged intimal flap motion was synchronized to the heart pulsation. Peak intimal flap motion amplitude (1.82±1.69mm) was found at15%of the R-R interval; group-averaged intimal flap motion amplitude in this interval was higher than those at any other time intervals (0%-10%and55%-95%:p<0.05;20%-50%:p>0.05). The minimum intimal flap motion amplitude was found at75%(0.440±0.31mm) of the R-R interval, and second minimum (0.464±0.31mm) was found at70%of the R-R interval. Intimal flap motion amplitude in these two R-R intervals was less than that in other R-R intervals (15%,20%:p<0.05;0%-10%and25%-95%:p>0.05).3types of intimal flap configurations were found in every R-R intervals.312of620(50.3%) intimal flaps curved toward the false lumen,226of620(36.5%) intimal flaps curved toward the true lumen, and82of620(13.2%) intimal flaps were flat. The CTDIvol, DLP, and ED were16.9±7.4(9.6～33.6) mGy,936.4±353.1(540.3～1850.1) mGy.cm and13.4±6.1(7.4～26.3) mSv respectively.Conclusions:Low dose retrospective ECG-gated thoracoabdominal aorta CTA can assess the abdominal intimal flap motion in acute AD. Abdominal intimal flap is a dynamic structure, and the highest motion of intimal flap was at systolic phase. The position and configuration of intimal flap varied greatly during a cardiac cycle. None ECG-gated CTA or any one phase images from retrospective ECG-gated CTA cannot reflect the actual status of the true lumen. Retrospective ECG-gated thoracoabdominal aorta CTA can reflect the actual status of the true lumen and provide more information about true lumen collapse.