Myocardial bridging (MB) is a congenital coronary anomaly,1,2 which occurs when a segment of a coronary artery or its major branch travels through the myocardium instead of on the surface of the myocardium. The artery coursing within the myocardium is called a “tunneled artery”. In general, it is considered as a benign condition. However, a few MB patients with severe tunneled artery compression might suffer from myocardial ischemia and acute coronary syndromes,3–7 cardiac arrhythmias, and even sudden death.8–11 Correct diagnosis of MB in the coronary tree is helpful for guiding the consequence treatment for these patients. Most patients with MB need no special treatment and a few MB patients with severe tunneled artery compression will benefit from beta-blocker, and coronary vasodilators should be avoided.12–14 Intravascular ultrasound (IVUS) is now the golden standard for MB diagnosis. Currently, good anatomical view of the tunnel artery was also shown by noninvasive multislice CT (MSCT).15–17 Till now, there is no systemic study comparing invasive and non-invasive technique on MB diagnosis. This study was, therefore, designed to investigate the feasibility and accuracy of 16-slice MSCT in MB diagnosis in comparison with IVUS.
A total of 51 patients (38 men) with atypical or typical angina underwent invasive quantitative coronary angiography (QCA) and IVUS between August 2005 and November 2005 were chosen for this study. MSCT was performed 7 days before IVUS examination. Four main vessels (left main artery (LMA), left anterior descending (LAD), left circumflex (LCX), right coronary artery (RCA)) were examined. Exclusion criteria included irregular heart rate, allergy to iodinated contrast agents, serum creatinine >120 mmol/L, congestive heart failure (ejection fraction (EF)<40%), chronic bronchitis, chronic obstruction pulmonary disease and other contraindications to beta-blocker. Oral metaprolol (25 mg) was given to patients with heart rates over 65 beats per minute one hour before MSCT imaging to reduce motion artifacts caused by elevated heart rate and sublingual nitroglycerin (0.4 mg) was given one minute before image acquisition. The study protocol was approved by the local ethics committee and all participants gave informed consent.
Invasive techniques and analysis
QCA was obtained using 6F catheter and the transfemoral Judkins technique was used for all interventions. All angiograms were analyzed off-line with the Cardiovascular Medcon System (Medcon Telemedicine Technology, Inc., Whippany, NJ, USA). The determination of myocardial bridging was based on a typical “milking effect” in coronary artery. After passage of the guidewire across the target lesion, IVUS was performed (Atlantis 3.0F, 40 MHz coronary imaging catheter, SCIMED, Boston Scientific Corporation, USA), continuous ultrasound images were obtained from distal and proximal myocardial segments in real time and stop frames. The position of the IVUS probe was documented and numbered on X-ray film at the points where the IVUS images were taken. The determination of myocardial bridging was based on a typical “half moon” phenomenon, a specific echolucent phenomenon around the bridge segment which exists throughout the cardiac cycle.18 The following parameters were obtained: minimal/maximal luminal diameters of the myocardial segment were measured in systolic and diastolic phase, respectively. Minimal lumen diameter (MLD) of the normal proximal and distal segment to myocardial segment were measured in both systolic and diastolic phases.
MSCT technique and image reconstruction
MSCT was performed using a 16-detector row MSCT scanner (SOMATOM Sensation 16, Siemens, Forchheim, Germany). The scan parameters were chosen as follows: 16 mm × 0.75 mm collimation, tube voltage 120 kV, effective tube current-time product 750 mA, table feed 3.0 mm/rotation and tube rotation time 375 ms. The Adaptive Cardio Volume reconstruction approach applied with the scanner was used. A bolus of 100 ml contrast medium (ULTRAVIST 370) was injected through an antecubital vein at 4.5 ml/s during a 15–18 seconds period of breath holding; effective radiation dose was 58.5 mGy, and an effective slice thickness was 1.0 mm. Image reconstruction was gated by ECG using a half-scan reconstruction algorithm to perform the temporal resolution of 185 ms. Retrospective ECG-gated reconstruction was obtained at 30%-40% and 60%-70% of the R-R interval (representing relative systolic and diastolic phases). Data were transferred to a PC-based workstation (Wizard, Siemens Medical Solutions, Erlangen, Germany) after reconstruction. Two experienced radiologists blinded to the IVUS results evaluated the coronary segments according to a modified American Heart Association classification.18 By ECG-gated MSCT reconstruction, MB diagnosis was made when the tunnel artery (myocardial segment) covered with different thickness of myocardium and the lumen was narrowed during systolic phase and reconverted during diastolic phase. One tunnel artery axial image from systolic and diastolic phase was chosen to generate appropriate maximum intensity projection (MIP) reconstruction and curved multi-planar reconstructions (MPR).19,20
Data were presented as frequencies or mean ± standard deviation (SD). Statistical analysis was performed with SPSS 11.0 statistical package. The diagnostic performance of MSCT for the detection of myocardial bridging were compared with IVUS as standard reference method, and sensitivity, specificity, negative and positive predictive rates with the corresponding 95% confidence interval were determined. Differences in means were tested by the unpaired Student's t test and differences in proportions by the chi-square test. Quantitative lumen diameters were compared using bivariate correlation. A P value <0.05 was considered statistically significant.
General clinical characteristics were similar between patients with or without MB (Table). Forty-one out of 51 (80%) patients received metaprolol (25 mg) before the MSCT scan and 25 of them were current beta-blocker users. The mean heart rate was (64±3) beats/min during the scan procedure. The total scan time was (16.1±1.2) seconds. Scan image quality was sufficient in all the 51 patients and no patient or segment was excluded from the analysis.
Myocardial bridging detection by IVUS, QCA and MSCT
MB was present in 30 patients by IVUS and all MBs were located in LAD and no MB was detected in LMA, LCX, RCA. There were 28 single-MB segments located in the middle portion of the LAD and 2 double-MB segments located in both middle and distal portion of LAD in IVUS examination. Twenty-eight out of 30 MB were correctly diagnosed by MSCT and 2 patients with single MB were misdiagnosed as normal and these 2 myocardial segments were short (3.0 and 4.5 mm, respectively) and the degrees of systolic lumen compression were relatively small (2.8 to 2.7 mm and 2.0 to 1.9 mm, respectively). An MB detected in a sample case by IVUS and MSCT is shown in the figure. In comparison with IVUS, the sensitivity of detecting myocardial bridging by MSCT was 93% (28 of 30), specificity was 100%, positive predictive value and negative predictive value were 100% and 91%, respectively. MSCT identified the atherosclerosis proximal to MB segments and significant stenosis in the segments proximal to the MB in 5 out of 28 MB cases, which is comparable to the IVUS findings (8 out of 30). In comparison with IVUS, QCA correctly diagnosed 29 out of 30 MB cases and 1 patient with single MB at the middle portion of LAD was misdiagnosed by QCA as coronary stenosis. The sensitivity of detection by QCA for patients with or without myocardial bridging was 97% (29 of 30), specificity was 100%, positive predictive value and negative predictive value were 100% and 96%, respectively.
MB quantification by IVUS and MSCT
The lumen diameter of the tunnel artery derived from MSCT and IVUS significantly decreased from (2.9±0.3) mm to (2.4±0.4) mm (P<0.001) and from (3.3±0.3) mm to (2.6±0.5) mm (P<0.001) respectively during the cardiac cycle and a significant correlation between lumen diameters derived from MSCT and IVUS was observed (systolic phase: r=0.87, P<0.05; diastolic phase: r=0.92, P<0.05). Minimal and maximal diameters of MB during systolic and diastolic phase derived from MSCT were significantly smaller than those from IVUS ((2.4±0.4) mm vs (2.6±0.5) mm, P<0.05 and (2.9±0.3) mm vs (3.3±0.3) mm, P<0.05) while narrowing percent derived from the two methods was similar ((21.4±10.9)% vs (17.4±7.6)%, P>0.05).
The major finding of the present study is that the diagnostic accuracy on MB in LAD by non-invasive MSCT is comparable with that by invasive IVUS. Our study showed that twenty-eight out of 30 MB diagnosed by IVUS were correctly diagnosed by MSCT. Compared with IVUS, the sensitivity of detecting by MSCT for patients with myocardial bridging was 93%, specificity was 100%. Thus, most of the MB patients could be correctly diagnosed by non-invasive MSCT. This diagnostic tool is cheaper and safer, so MB patients will benefit from this technique and be treated properly.
Conventional invasive coronary angiography was currently standard for clinical evaluation of myocardial bridging, and the diagnosis was proved of the typical systolic vessel compression (milking effect), but it was difficult to demonstrate myocardial bridging when the systolic compression was relatively minimal. Some previous studies suggested that nitroglycerin may augment systolic myocardial compression and helped the diagnosis. However, the presence of atherosclerosis in the myocardial segment would be very difficult to detect using conventional coronary angiography. Therefore, intravascular ultrasound is accepted as a golden standard for the morphological assessment of myocardial bridging.
Coronary MSCT has recently been established as a reliable and non-invasive technique for the diagnosis of coronary lesions and congenital abnormalities.21 By MSCT, the accurate coronary evaluations were based on obtaining image of the coronary artery tree at the time of its maximal vasodilation and minimal motion artifacts. For this reason, the reconstruction window was usually positioned within the diastolic phase.22 But, for the accurate diagnosis of myocardial bridging, it must include the reconstruction window in systolic phase.23
Conventional invasive IVUS is currently the standard diagnosis method for clinical evaluation of myocardial bridging. Though the risk of adverse events is small, the potentially life-threatening consequences such as coronary artery dissection, arrhythmia, stroke (total complication rate: 1.8% and mortality rate: 0.1%) might occur during the invasive procedures.23,24 MSCT offers a safer, more comfortable and cost-effective (the prices of IVUS and MSCT examinations in China are 12 000 Yuan (1500 US$) and 750 Yuan (95 US$)) respectively. Since MSCT could follow the whole “journey” of tunneled artery, it could define the precise location and the length of the MB. It is worthy of noting that in 7 out of 28 MB cases, the MB diameters in systolic and diastolic phases could not be exactly determined by MSCT and further imaging quality improvement by MSCT is therefore warranted.
Study limitations are as follows: First, only symptomatic patients with an inconclusive or conclusive risk on exercise ECG stress testing were referred for catheterization, there was a high incidence of true myocardial bridging (59%) in our patients, tending to increase sensitivity. Second, the important limitation in MSCT was the need of lower heart rate, the dose of radiation, and the side effect of contract administration were also been considered.24
In conclusion, our study results may have important clinical implications, MSCT offers a reliable non-invasive method for myocardial bridging in LAD and atherosclerosis diagnosis with diagnosis accuracy comparable with invasive IVUS. Especially, in some patients with sinus rhythm but haven't the risk of coronary atherosclerosis disease, MSCT can be considered an alternative to invasive coronary angiography for patients with myocardial bridging.
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