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Detection of myocardial bridge and evaluation of its anatomical properties by coronary multislice spiral computed tomography
European Journal of Radiology 61 (2007) 130–138
Detection of myocardial bridge and evaluation of its anatomical properties by coronary multislice spiral computed tomography Yoko Kawawa a,∗ , Yukio Ishikawa b , Tatsuya Gomi a , Masashi Nagamoto a , Hitoshi Terada a , Toshiharu Ishii b , Ehiichi Kohda a a
Department of Radiology, Toho University Medical Center Ohashi Hospital, 2-17-6 Ohashi, Meguro-ku, Tokyo 153-8515, Japan b Department of Pathology, Toho University School of Medicine, 5-21-16 Ohmori-nishi, Ohta-ku, Tokyo 143-8540, Japan Received 4 May 2006; received in revised form 26 August 2006; accepted 29 August 2006
Abstract Background: Myocardial bridge (MB) is a common anatomical condition, under which a part of the coronary artery running in the epicardial adipose tissue, is covered with myocardial tissue. It regulates atherosclerosis development and sometimes evokes coronary heart disease through haemodynamic alterations. We attempted to efficiently detect MB and evaluate the anatomical properties of MB by coronary multislice spiral computed tomography (MSCT). Methods: Sixteen-row MSCT was conducted on 148 patients with coronary heart disease. MSCT images were reconstructed and reformed with transverse scans, curved planar reformat and three-dimensional volume-rendered images. The MB, over 1.0 mm in thickness, was identified by the presence of the “step-down and step-up” appearance. After “trial and error” essays, we could consistently examine the frequency of MB and evaluate the anatomical properties of MB, especially its thickness, together with coronary wall lesions. Results: Twenty-three patients (15.8%) had MB over 1.0 mm in thickness: 21 MBs (87.5%) were located in the left anterior descending artery with a mean thickness and length of 1.8 ± 0.7 and 20.0 ± 8.6 mm. Moreover, although the tunneled segment beneath MB was always free of coronary wall lesions, 79.2% (19/24) of the segments proximal to MB demonstrated coronary wall lesions. Of special significance were three symptomatic MB patients without any atherosclerotic lesion throughout all the coronary arteries. Conclusion: Coronary MSCT is a new imaging technique that provides a non-invasive diagnostic tool for MB and yields much better results of MB detection than previous imaging methods. © 2006 Elsevier Ireland Ltd. All rights reserved. Keywords: Myocardial bridge; Coronary multislice spiral computed tomography (coronary MSCT); Coronary heart disease
1. Introduction The myocardial bridge (MB) is a common anatomical condition, under which a part of the coronary artery running in the epicardial adipose tissue is covered with myocardial tissue (Fig. 1) [1]. The coronary artery covered by MB has been also called tunneled artery. MB appears almost exclusively in the left anterior descending coronary artery (LAD) [2], and its frequency at autopsy is sometimes over 50% [3]. It is known that MB ∗
Corresponding author. Tel.: +81 3 3468 1251; fax: +81 3 3481 7333. E-mail addresses: [email protected] (Y. Kawawa), [email protected] (Y. Ishikawa), [email protected] (T. Gomi), [email protected] (M. Nagamoto), [email protected] (H. Terada), [email protected] (T. Ishii), [email protected] (E. Kohda). 0720-048X/$ – see front matter © 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2006.08.029
in LAD consistently influences the distribution of atherosclerotic lesions along the entire course of this artery, where the segment beneath MB is free of atherosclerotic development despite frequent plaque formation in the segment proximal to MB [2,4]. MB is generally benign, but symptomatic cases have been described: ischaemic heart disease, ventricular fibrillation, atrioventricular block and sudden death [1,5]. These symptoms are considered to be caused by coronary ischaemia attributed to a reduction in blood flow subsequent to coronary compression by MB at systole or delayed arterial relaxation at diastole, or both [6,7], which may be naturally influenced by blood flow changes characteristic of the inherent anatomical properties of MB. In contrast, the frequency of MB detected by coronary angiography is less than 5%, which is significantly lower than that by autopsy studies [1]. The detection by coronary angiography depends on transient systolic constriction of the bridged
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in the three cases. The MSCT findings of soft tissue density covering the coronary artery, which disclosed the same contrast enhancement as myocardial tissue as well as the “step-down and step-up” appearance of the tunneled artery demonstrated in the MB, accorded with the coronary angiographic findings of MB that had already been evaluated from the typical transient systolic constriction (Fig. 2). 2.2. Study population The 148 subjects (111 men and 37 women; between 19 and 87 years old; mean 65.7 ± 11.9 years) had consecutively undergone diagnostic coronary MSCT at Toho University Medical Center Ohashi Hospital from January 2004 to October 2005 for coronary heart diseases from various reasons (angina pectoris, myocardial infarction, Kawasaki’s disease and annuloaortic ectasia) or were follow-up patients after having undergone percutaneous coronary intervention (PCI) and coronary arterial bypass graft placement (CABG). Apart from the above, this study was approved by the ethical committee in our institute, and written informed consent for the present study was also provided by all patients. 2.3. Coronary MSCT studies (Table 1)
Fig. 1. Macroscopic view of myocardial bridge (MB) in the left anterior descending artery (LAD) of an autopsied heart (the two arrows indicate the entrance and exit of the MB).
segment and the “milking effect” [5], which are based on indirect images only suggesting the presence of MB. The imaging techniques of intravascular ultrasound (IVUS) and intracoronary doppler (ICD) are also known [6,7] to directly detect the bridged muscle itself. In addition, recent developments of multislice spiral computed tomography (MSCT) in radiography have made possible the clear detection of the entire running course of coronary arteries and MB itself, as well as of coronary wall lesions [8,9]. MSCT provides an accurate demonstration of the anatomical characteristics of MB in LAD, such as location, thickness and length, which modulate coronary atherosclerotic evolution [10]. In this study, we used 16-row MSCT to detect MB among patients with coronary heart diseases and evaluated the anatomical characteristics of MB with a view of providing raw data for the consideration of haemodynamic changes attributed to the presence of MB. 2. Patients and methods 2.1. Preliminary comparison between MSCT and angiography Before examining the coronary artery by our MSCT described in Sections 2.3–2.6 below, we first confirmed that the results of MB detection by MSCT accorded with that by conventional coronary angiography for an each identical patient
Coronary MSCT images were examined with a 16-row MSCT scanner (Aquillion-16, Toshiba, Tokyo, Japan). Patients were pre-medicated with beta-blocker to maintain heart rates at less than 70 beats/min, for the improvement of the desirable high resolution of the Z-direction imaging. The coronary MSCT imaging sequence was obtained craniocaudally at tube voltage 120 kV, tube current 300 mA s, gantry rotation time 400 ms, detector collimation 16 mm × 0.5 mm and helical pitch 3.2, during a single breath hold (Table 1). At first, the pre-contrast MSCT images were obtained to evaluate coronary calcification. A bolus injection of 100 ml contrast medium (Ioversol, Optiray 320 mg/ml, Tyco Healthcare Japan, Tokyo, Japan) was then administered intravenously at the rate of 4.0 ml/s before the infusion of 50 ml saline also at 4 ml/s. An automated bolus-tracking system was used to synchronize the arrival of the contrast agent with the initiation of the scan. The scan was started after the Table 1 Scan parameters and technique of contrast agent administration Scan parameters Gantry rotation time Number of slices per rotation Individual detector width Helical pitch Tube voltage Tube current Contrast agent: Ioversol, Optiray 320 Volume Concentration Injection rate Injection site Synchronization technique
400 ms 16 0.5 mm 3.2 120 kV 300 mA s 100 ml 400 mgI/ml 4.0 ml/s Antecubital vein Bolus tracking
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attenuation in the ascending aorta exceeded 200 Hounsfield units (HU). The total scan time was approximately 25 s. A 512 × 512 matrix with an 18-cm field-of view was used. A volume data set of the heart was acquired in all patients.
additional reconstruction data were obtained in increments and decrements of 10%.
2.4. Image reconstruction
MSCT images were evaluated by transverse scans and secondary reformation, namely, the curved planar reformat (CPR) and the three-dimensional volume-rendered (3D-VR) image. The images were obtained through the workstation (Virtual Place Advance, AZE, Tokyo, Japan) by using a volumerendering technique.
The initial retrospective ECG-gated reconstruction was generated with the reconstruction window starting at the enddiastolic phase, that is 75% of the R-peak to R-peak interval. When the data were insufficient due to motion artifacts,
2.5. Image reformation and evaluation
Fig. 2. A 49 year-old man with angina pectoris (patient No. 23). Conventional coronary angiography (a: systolic phase, b: diastolic phase) demonstrates typical transient systolic constriction (arrow) in the LAD for MB. On axial images (c), the curved planar reformat (CPR) image (d) and the three-dimensional volume-rendered (3D-VR) image (e) of the left coronary multislice spiral computed tomography (MSCT), the MB covered with myocardial tissue is situated in the middle of the LAD (arrow, both arrows in (d and e) indicate the entrance and exit of MB, respectively). There is no significant coronary wall lesion in the segment proximal to MB as well as no stenotic lesion in the other coronary branches. The thickness and length of MB are 2.2 and 14.5 mm.
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Fig. 2. (Continued ).
2.6. Determination of MB Identification of MB was determined by two conditions: (1) the presence of the “step-down and step-up” appearance, namely, a significant tortuosity of the segment beneath MB at the entrance (step-down) and the exit (step-up) sites, the same as the findings by angiography; (2) soft tissue density covering the coronary artery, which had the same contrast enhancement as myocardial tissue. In this study, the covering tissue, with the same threshold with myocardial tissue and thickness over 1.0 mm, was regarded as MB. A limitation to distinguishing the myocardium from the coronary arterial wall or atherosclerotic plaque was set at a thickness under 1.0 mm.
evaluated. Coronary wall lesions were divided into two categories of calcified lesion and non-calcified plaque by coronary MSCT imaging. Coronary wall lesions were regarded as calcified when the CT threshold in the plaque exceeded 100 HU in non-enhanced MSCT images and as non-calcified plaque when the lesion exhibited intraluminal low density plaque without calcification. ‘Stenosis’ meant a significant reduction (over 50%) in the diameter of the arterial lumen at the site of the lesion compared with that in the segment distal to the lesion.
2.7. Evaluation and measurements of MB All MSCT data were evaluated in a blinded manner by two observers (Y.K. and E.K.) by mutual agreement. The thickness and the length of MB were reviewed on the basis of the acquired CPR images with an adequate window level and window width for soft tissue evaluation through the workstation. The thickness of the MB was regarded as the widest part between the surface of the covering myocardium and the tunneled artery (Fig. 3); the length was regarded as the distance of the covering myocardial tissue between the entrance and the exit of the tunneled artery (Fig. 3). 2.8. Evaluation of the types of coronary wall lesions in the segment proximal to MB The presence or absence of coronary wall lesions in the tunneled artery and the arterial segment proximal to MB were
Fig. 3. Measurements of MB on coronary MSCT. The thickness of the MB is evaluated as the widest part from the myocardial surface surrounded by epicardial adipose tissue to the contrast enhancement of the tunneled artery (arrowhead to arrowhead) on the CPR image. The length of the MB is also evaluated as the distance from the entrance to the exit of MB (arrow to arrow).
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3. Results 3.1. Visualization of MB by coronary MSCT Normal coronary arteries were demonstrated good contrast enhancement for the lumen in all but two patients showing motion artifacts of arrhythmia or breathing. On transverse scan and CPR images, MB was evaluated by coronary MSCT, and the tunneled artery was found with the covering myocardial tissue (Figs. 2, 4 and 5).
grounds of 23 (15.8%) with MB in the coronary arterial branch are indicated in Table 2. They included 21 men and 2 women aged between 38 and 78 years old (mean 65.0 ± 10.2 years) with usually one MB confined in LAD; however, one patient (No. 19) had two MBs: one in the mid-segment of LAD and the other in the diagonal segment (D1). At the time of this study, 16 (70%) of the 23 patients were diagnosed with angina, and three patients (Nos. 13, 15 and 16) had a stent placed in the segment proximal to MB because of significant stenosis (Fig. 5).
3.2. Frequency of MB and clinical backgrounds
3.3. Types of coronary wall lesions in the segment beneath and proximal to MB
Two patients were excluded from this study because of motion artifacts. Among the 146 patients, the clinical back-
The segments beneath the MBs were always free of coronary wall lesions in the 24 coronary branches of the 23 patients. In
Fig. 4. A 62 year-old woman with angina pectoris (patient No. 4). Axial images (a and b), CPR image (c) and 3D-VR image (d) of left coronary MSCT. The MB is situated in the middle of the LAD (arrow, the two arrows in (d and e) indicate the entrance and exit of the MB) and low density plaque is shown just proximal (arrowhead) to the entrance of the MB. The thickness and length of the MB are 2.6 and 29.8 mm.
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Fig. 5. A 47 year-old man underwent percutaneous coronary intervention with stent at the proximal LAD (arrowhead) because of angina pectoris (patient No. 13). Axial (a and b), CPR (c) and 3D-VR (d) images of left coronary MSCT. The MB covered with myocardial tissue is situated in the middle of the LAD (arrow, the two arrows in (d and e) indicate the entrance and exit of the MB). The MB shows typical “step-down and step-up” appearance on the CPR image. The thickness and length of the MB are 2.2 and 25.7 mm.
contrast, the arterial segment proximal to the MB had significant coronary wall lesions (Fig. 4) in 19 coronary branches (79.2%) including those in the three patients fitted with stents in the segments proximal to MBs. Three cases with angina (No. 21–23) had MBs in the mid-segment of LAD; however, no significant coronary wall lesions were observed in the segments proximal to MB and no stenotic lesions in the other coronary branches (Fig. 2). 3.4. MB location, thickness and length In 19 patients (82.6%), the MB was located in the midportion of LAD; in two patients (patient No. 6 and 12), it
was located in the distal and the proximal portion of LAD, respectively. In another three patients, the MB was located in the second diagonal branch (patient No. 2), one in the middle LAD and another in the first diagonal branch (patient No. 19), and one in the obtuse marginal branch (patient No. 20). In the 23 patients, the thickness of the MB ranged from 1.1 to 3.7 mm (mean ± S.D.; 1.8 ± 0.7 mm), and the length ranged from 10.5 to 50.2 mm (mean ± S.D.; 20.0 ± 8.6 mm). The longest MB (50.2 mm) was located in the mid-portion of LAD (patient No. 18) completely occluding the LAD lumen in the segment proximal to MB and necessitating CABG.
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Table 2 Summary of 23 cases with myocardial bridge No.
Age/sex
Clin. diag.
Stenosis
Lesion
Location
Thickness (mm)
Length (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
78/M 62/F 58/M 62/F 71/M 74/M 68/M 64/M 71/M 67/M 56/M 38/M 47/M 74/M 76/M 61/M 67/M 66/M 73/M
Angina Angina Angina Angina Angina Angina Angina Angina Angina Angina Angina MI/PCI MI/PCI Angina/PCI Angina/PCI MI/PCI OMI Angina/CABG Angina
20 21 22 23
64/M 78/M 71/M 49/M
Angina Angina Angina Angina
Y Y Y Y N N N N Y N N N N Y N Y Y Y N Y N N N N
Calc. Plaque Plaque Plaque Calc. Calc. + plaque Calc. + plaque Calc. + plaque Calc. Calc. Calc. + plaque Plaque Stent Calc. + plaque Calc. + stent Plaque (in-stent) Calc. + plaque Occlusion None Plaque None None None None
Mid. LAD 2nd diagon. Mid. LAD Mid. LAD Mid. LAD Dist. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Prox. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD 1st diagon. Obtuse Mid. LAD Mid. LAD Mid. LAD
2.6 1.1 1.2 2.6 3.7 1.1 2.2 2.1 1.4 2.4 1.5 2.1 2.2 1.4 1.4 2.7 1.7 2.0 1.3 1.4 1.1 1.3 1.1 2.2
24.9 20.9 21.9 29.8 20.5 10.5 14.3 17.0 24.6 27.4 10.8 27.5 25.7 16.7 11.7 15.1 12.3 50.2 17.4 19.1 24.1 11.0 12.8 14.5
Abbreviations: Clin. diag., clinical diagnosis; MI, myocardial infarction; PCI, post-coronary intervention; OMI, old myocardial infarction; CABG, coronary-aortic graft bypass; Y, yes; N, no; calc., calcified lesion; plaque, elevated lesion without calcification; Mid. LAD, middle portion of the left anterior descending coronary artery (LAD); RCA, the right coronary artery; 2nd diagon., the second diagonal branch; Dist. LAD, distal portion of LAD; Prox. LAD, proximal portion of LAD; 1st diagon., the first diagonal branch; Obtuse, obtuse marginal branch.
4. Discussion
angiography to the detection of MB; it also discloses a higher frequency of MB in coronary heart diseases.
4.1. Evaluation of MB with coronary MSCT 4.2. Recognition of MB by coronary MSCT The efficiency of MB detection was greatly improved by our newly devised modification of MSCT after simple “trial and error” essays. Consequently, the study clearly proved that coronary MSCT made possible the detection of MB much more objectively and non-invasively than the previous examination methods, such as conventional angiography, IVUS or ICD. MSCT revealed MB in 15.8% of the present 146 Japanese patients with various coronary heart diseases, which is evidently higher than the results of previous methods. The rate of MB detected by coronary angiography among coronary heart diseases has ranged between 0.8 and 7.5% in several studies [4], in each of which the occurrence of MB has been estimated mainly through focal reduction in coronary diameter or by the milking effect during systole; however, the designation of focal diameter reduction in coronary arteries has varied among angiographers [11]. In addition, a retrospective review of angiograms with special attention to the assessment of the arterial diameter has disclosed a higher prevalence of MB [11]. These results suggest that the diagnosis of MB by coronary angiography is somewhat dependent on the perception level of the individual angiographer. In contrast, MB can be detected consistently by coronary MSCT regardless of the perception level of the observer. We thus consider this method effectively more conducive than coronary
Autopsy studies on Japanese subjects have demonstrated, regardless of the anatomical properties of MB, a frequency of MB in LAD of 45% in 642 consecutive autopsied hearts [12], and of 29.5% in 200 cases with MBs over 1mm thick [10]. Consequently, the frequency of MBs over 1 mm thick is estimated to be approximately 13.3% (45% × 0.295) in autopsied Japanese subjects with non-coronary heart disease, which is similar to the present result (15.5%) obtained from patients with coronary heart disease examined by MSCT. However, MBs in 21 (87.5%) of 24 patients with MB in LAD were located in its middle segment, which is in good agreement with previous results of angiographic and autopsy studies [12]. The significance of MB in coronary heart diseases has been controversial. Long-term prognosis of isolated MBs is considered to be benign [13], and pathological studies have also shown a tendency for fewer anterior-wall infarctions in patients with MB in LAD [14]. On the other hand, a considerable number of symptomatic cases are attributed directly to the presence of MB, in which the release of the bridged muscle by surgical myotomy, stent placement in the MB segment and a regimen of -blocker has been effective for the improvement of symptoms [1,2], suggesting that some, but not all, cases of MB are potentially
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pathological. The anatomical properties of MB, such as thickness and length, may play a significant role in the haemodynamic alterations of blood flow in LAD with MB because of LAD compression exerted by the MB muscle; they are important regulators for atherosclerotic evolution in the entire course of LAD [1,10]. Furthermore, the thickness and length of MB are sometimes directly associated with pathological conditions, such as ischaemic heart disease or sudden death. Indeed, previous surgical and autopsy findings have indicated that the development of ischaemia is linked directly to a reduction in blood flow, depending on the length and thickness of the bridged muscle [15], and the deeply situated coronary artery, called as thick MB, is associated with sudden death [16]. Considering these associations of the anatomical characteristics of MB with pathological conditions, although overall MBs are thought to be benign [13], those with certain anatomical features are probably pathological. Based on such backgrounds, we evaluated the anatomical properties of MB and found that the mean thickness and length of MBs examined were 1.8 ± 0.7 mm (between 1.1 and 3.7 mm) and 20.0 ± 8.6 mm (between 10.5 and 50.2 mm), being considerably thicker and longer than those of previous autopsy studies (0.86 and 14.4 mm), respectively [10]. The difference is simple due to differences of methodology between the current coronary MSCT and histomorphometric measurements on the autopsied materials, as MBs less than 1 mm thick are so far as undetectable by the former one. Furthermore, the length of MBs is significantly correlated with their thickness [10]. Thus, the thickness and length of MBs examined by MSCT were found to be larger in this study, reflecting evident differences between the examination methods of autopsy and MSCT. Coronary MSCT is, however, amply helpful in the evaluation of the anatomical features of MB, compared with coronary angiography by which MB itself is not directly detectable. Further examination of the anatomical properties of MB in clinically pathologic cases using coronary MSCT concomitantly with autopsy analyses may elucidate the anatomical features of the pathologic MB. 4.3. Association of MB with atherosclerotic lesions In 23 patients with MB and ischaemic heart disease, 19 coronary branches had a significant number of lesions, such as calcified lesions and/or non-calcified plaque, in the coronary segment proximal to MB; however, the 23 patients had no coronary wall lesions in the segment beneath MB. This unique finding has frequently been described in the previous reports [1,2,6]. This difference of lesion distribution is caused exclusively by haemodynamic alteration of blood flow in the coronary artery [17], as well as by endothelial dysfunction in the segment proximal to MB [1,18]. In 16 (70%) of the 23 patients, the coronary wall lesion in the segment proximal to MB was considered responsible for ischaemic heart disease. PCI including stent placement was effectively applied to the stenotic segment proximal to MB in 5 of 16 patients. Considering these findings, the presence of MB may more or less be related to the occurrence of ischaemic heart disease through coronary compression leading to atherosclerotic evolution in the segment proximal to MB. In contrast, the three patients with angina showed not only
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absence of significant coronary wall lesions in the segment proximal to MB, but also absence of stenosis in the other coronary arteries. In these three patients with MB in LAD, the associated symptom might have been caused by the presence of MB itself, because MB compressed LAD at systole and yielded delayed diastolic relaxation of LAD resulting in blood flow reduction in the peripheral myocardium [6]. 4.4. Study limitations In a recent study, coronary MSCT has been described as still having certain limitations in fully evaluating coronary artery disease because of insufficient specificity when compared with coronary angiography [19]. Nevertheless, MSCT is an appropriate tool to at least acquire local anatomical images of MB as was done in the present study. Moreover, current coronary MSCT is capable of demonstrating only myocardial tissue more than 1 mm thick, and the distinction of the myocardium from either the arterial wall or coronary plaque is unclear. Since the anatomical properties of the tunneled artery are, however, believed to closely relate to coronary ischaemia and sudden death, coronary MSCT examinations are preponderantly useful in evaluating properties such as the location, length and depth of the tunneled artery. Further improvement of spatial, time and contrast resolution by MSCT is needed. In this study, we focused on efficiently detecting MB itself and evaluating its anatomical aspects. We, therefore, used only diastolic data images. However, we must mention that it would be possible to evaluate not only superficial and thinner MB but also haemodynamic effects of MB, when we also deal with systolic data images. Further investigations are needed from such view-points. Finally, the high exposure to radiation up to 13.0 mSv under coronary MSCT, remains a matter of concern [20]. Further finetuning the current modulation of the prospective X-ray tube and development of new devices for reducing radiation exposure are highly desirable. 5. Conclusion Coronary MSCT has recently been established as a reliable and non-invasive technique for the diagnosis of intracoronary lesions and periarterial abnormalities. Furthermore, it provides several potential advantages over other invasive techniques and is a new evolving imaging technique allowing a non-invasive diagnostic option for MB. Because of its capability of accurately locating the site and measuring the thickness as well as the length of MB, coronary MSCT is a useful tool for defining the anatomical properties of MB. References [1] Moehlenkamp S, Hort W, Ge J, Erbel R. Update on myocardial bridging. Circulation 2002;106:2616–22. [2] Ishii T, Asuwa N, Masuda S, Ishikawa Y, Kiguchi H, Shimada K. The effects of a myocardial bridge on coronary atherosclerosis and ischaemia. J Pathol 1998;185:4–9.
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Detection of myocardial bridge and evaluation of its anatomical properties by coronary multislice spiral computed tomography Yoko Kawawa a,∗ , Yukio Ishikawa b , Tatsuya Gomi a , Masashi Nagamoto a , Hitoshi Terada a , Toshiharu Ishii b , Ehiichi Kohda a a
Department of Radiology, Toho University Medical Center Ohashi Hospital, 2-17-6 Ohashi, Meguro-ku, Tokyo 153-8515, Japan b Department of Pathology, Toho University School of Medicine, 5-21-16 Ohmori-nishi, Ohta-ku, Tokyo 143-8540, Japan Received 4 May 2006; received in revised form 26 August 2006; accepted 29 August 2006
Abstract Background: Myocardial bridge (MB) is a common anatomical condition, under which a part of the coronary artery running in the epicardial adipose tissue, is covered with myocardial tissue. It regulates atherosclerosis development and sometimes evokes coronary heart disease through haemodynamic alterations. We attempted to efficiently detect MB and evaluate the anatomical properties of MB by coronary multislice spiral computed tomography (MSCT). Methods: Sixteen-row MSCT was conducted on 148 patients with coronary heart disease. MSCT images were reconstructed and reformed with transverse scans, curved planar reformat and three-dimensional volume-rendered images. The MB, over 1.0 mm in thickness, was identified by the presence of the “step-down and step-up” appearance. After “trial and error” essays, we could consistently examine the frequency of MB and evaluate the anatomical properties of MB, especially its thickness, together with coronary wall lesions. Results: Twenty-three patients (15.8%) had MB over 1.0 mm in thickness: 21 MBs (87.5%) were located in the left anterior descending artery with a mean thickness and length of 1.8 ± 0.7 and 20.0 ± 8.6 mm. Moreover, although the tunneled segment beneath MB was always free of coronary wall lesions, 79.2% (19/24) of the segments proximal to MB demonstrated coronary wall lesions. Of special significance were three symptomatic MB patients without any atherosclerotic lesion throughout all the coronary arteries. Conclusion: Coronary MSCT is a new imaging technique that provides a non-invasive diagnostic tool for MB and yields much better results of MB detection than previous imaging methods. © 2006 Elsevier Ireland Ltd. All rights reserved. Keywords: Myocardial bridge; Coronary multislice spiral computed tomography (coronary MSCT); Coronary heart disease
1. Introduction The myocardial bridge (MB) is a common anatomical condition, under which a part of the coronary artery running in the epicardial adipose tissue is covered with myocardial tissue (Fig. 1) [1]. The coronary artery covered by MB has been also called tunneled artery. MB appears almost exclusively in the left anterior descending coronary artery (LAD) [2], and its frequency at autopsy is sometimes over 50% [3]. It is known that MB ∗
Corresponding author. Tel.: +81 3 3468 1251; fax: +81 3 3481 7333. E-mail addresses: [email protected] (Y. Kawawa), [email protected] (Y. Ishikawa), [email protected] (T. Gomi), [email protected] (M. Nagamoto), [email protected] (H. Terada), [email protected] (T. Ishii), [email protected] (E. Kohda). 0720-048X/$ – see front matter © 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2006.08.029
in LAD consistently influences the distribution of atherosclerotic lesions along the entire course of this artery, where the segment beneath MB is free of atherosclerotic development despite frequent plaque formation in the segment proximal to MB [2,4]. MB is generally benign, but symptomatic cases have been described: ischaemic heart disease, ventricular fibrillation, atrioventricular block and sudden death [1,5]. These symptoms are considered to be caused by coronary ischaemia attributed to a reduction in blood flow subsequent to coronary compression by MB at systole or delayed arterial relaxation at diastole, or both [6,7], which may be naturally influenced by blood flow changes characteristic of the inherent anatomical properties of MB. In contrast, the frequency of MB detected by coronary angiography is less than 5%, which is significantly lower than that by autopsy studies [1]. The detection by coronary angiography depends on transient systolic constriction of the bridged
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in the three cases. The MSCT findings of soft tissue density covering the coronary artery, which disclosed the same contrast enhancement as myocardial tissue as well as the “step-down and step-up” appearance of the tunneled artery demonstrated in the MB, accorded with the coronary angiographic findings of MB that had already been evaluated from the typical transient systolic constriction (Fig. 2). 2.2. Study population The 148 subjects (111 men and 37 women; between 19 and 87 years old; mean 65.7 ± 11.9 years) had consecutively undergone diagnostic coronary MSCT at Toho University Medical Center Ohashi Hospital from January 2004 to October 2005 for coronary heart diseases from various reasons (angina pectoris, myocardial infarction, Kawasaki’s disease and annuloaortic ectasia) or were follow-up patients after having undergone percutaneous coronary intervention (PCI) and coronary arterial bypass graft placement (CABG). Apart from the above, this study was approved by the ethical committee in our institute, and written informed consent for the present study was also provided by all patients. 2.3. Coronary MSCT studies (Table 1)
Fig. 1. Macroscopic view of myocardial bridge (MB) in the left anterior descending artery (LAD) of an autopsied heart (the two arrows indicate the entrance and exit of the MB).
segment and the “milking effect” [5], which are based on indirect images only suggesting the presence of MB. The imaging techniques of intravascular ultrasound (IVUS) and intracoronary doppler (ICD) are also known [6,7] to directly detect the bridged muscle itself. In addition, recent developments of multislice spiral computed tomography (MSCT) in radiography have made possible the clear detection of the entire running course of coronary arteries and MB itself, as well as of coronary wall lesions [8,9]. MSCT provides an accurate demonstration of the anatomical characteristics of MB in LAD, such as location, thickness and length, which modulate coronary atherosclerotic evolution [10]. In this study, we used 16-row MSCT to detect MB among patients with coronary heart diseases and evaluated the anatomical characteristics of MB with a view of providing raw data for the consideration of haemodynamic changes attributed to the presence of MB. 2. Patients and methods 2.1. Preliminary comparison between MSCT and angiography Before examining the coronary artery by our MSCT described in Sections 2.3–2.6 below, we first confirmed that the results of MB detection by MSCT accorded with that by conventional coronary angiography for an each identical patient
Coronary MSCT images were examined with a 16-row MSCT scanner (Aquillion-16, Toshiba, Tokyo, Japan). Patients were pre-medicated with beta-blocker to maintain heart rates at less than 70 beats/min, for the improvement of the desirable high resolution of the Z-direction imaging. The coronary MSCT imaging sequence was obtained craniocaudally at tube voltage 120 kV, tube current 300 mA s, gantry rotation time 400 ms, detector collimation 16 mm × 0.5 mm and helical pitch 3.2, during a single breath hold (Table 1). At first, the pre-contrast MSCT images were obtained to evaluate coronary calcification. A bolus injection of 100 ml contrast medium (Ioversol, Optiray 320 mg/ml, Tyco Healthcare Japan, Tokyo, Japan) was then administered intravenously at the rate of 4.0 ml/s before the infusion of 50 ml saline also at 4 ml/s. An automated bolus-tracking system was used to synchronize the arrival of the contrast agent with the initiation of the scan. The scan was started after the Table 1 Scan parameters and technique of contrast agent administration Scan parameters Gantry rotation time Number of slices per rotation Individual detector width Helical pitch Tube voltage Tube current Contrast agent: Ioversol, Optiray 320 Volume Concentration Injection rate Injection site Synchronization technique
400 ms 16 0.5 mm 3.2 120 kV 300 mA s 100 ml 400 mgI/ml 4.0 ml/s Antecubital vein Bolus tracking
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attenuation in the ascending aorta exceeded 200 Hounsfield units (HU). The total scan time was approximately 25 s. A 512 × 512 matrix with an 18-cm field-of view was used. A volume data set of the heart was acquired in all patients.
additional reconstruction data were obtained in increments and decrements of 10%.
2.4. Image reconstruction
MSCT images were evaluated by transverse scans and secondary reformation, namely, the curved planar reformat (CPR) and the three-dimensional volume-rendered (3D-VR) image. The images were obtained through the workstation (Virtual Place Advance, AZE, Tokyo, Japan) by using a volumerendering technique.
The initial retrospective ECG-gated reconstruction was generated with the reconstruction window starting at the enddiastolic phase, that is 75% of the R-peak to R-peak interval. When the data were insufficient due to motion artifacts,
2.5. Image reformation and evaluation
Fig. 2. A 49 year-old man with angina pectoris (patient No. 23). Conventional coronary angiography (a: systolic phase, b: diastolic phase) demonstrates typical transient systolic constriction (arrow) in the LAD for MB. On axial images (c), the curved planar reformat (CPR) image (d) and the three-dimensional volume-rendered (3D-VR) image (e) of the left coronary multislice spiral computed tomography (MSCT), the MB covered with myocardial tissue is situated in the middle of the LAD (arrow, both arrows in (d and e) indicate the entrance and exit of MB, respectively). There is no significant coronary wall lesion in the segment proximal to MB as well as no stenotic lesion in the other coronary branches. The thickness and length of MB are 2.2 and 14.5 mm.
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Fig. 2. (Continued ).
2.6. Determination of MB Identification of MB was determined by two conditions: (1) the presence of the “step-down and step-up” appearance, namely, a significant tortuosity of the segment beneath MB at the entrance (step-down) and the exit (step-up) sites, the same as the findings by angiography; (2) soft tissue density covering the coronary artery, which had the same contrast enhancement as myocardial tissue. In this study, the covering tissue, with the same threshold with myocardial tissue and thickness over 1.0 mm, was regarded as MB. A limitation to distinguishing the myocardium from the coronary arterial wall or atherosclerotic plaque was set at a thickness under 1.0 mm.
evaluated. Coronary wall lesions were divided into two categories of calcified lesion and non-calcified plaque by coronary MSCT imaging. Coronary wall lesions were regarded as calcified when the CT threshold in the plaque exceeded 100 HU in non-enhanced MSCT images and as non-calcified plaque when the lesion exhibited intraluminal low density plaque without calcification. ‘Stenosis’ meant a significant reduction (over 50%) in the diameter of the arterial lumen at the site of the lesion compared with that in the segment distal to the lesion.
2.7. Evaluation and measurements of MB All MSCT data were evaluated in a blinded manner by two observers (Y.K. and E.K.) by mutual agreement. The thickness and the length of MB were reviewed on the basis of the acquired CPR images with an adequate window level and window width for soft tissue evaluation through the workstation. The thickness of the MB was regarded as the widest part between the surface of the covering myocardium and the tunneled artery (Fig. 3); the length was regarded as the distance of the covering myocardial tissue between the entrance and the exit of the tunneled artery (Fig. 3). 2.8. Evaluation of the types of coronary wall lesions in the segment proximal to MB The presence or absence of coronary wall lesions in the tunneled artery and the arterial segment proximal to MB were
Fig. 3. Measurements of MB on coronary MSCT. The thickness of the MB is evaluated as the widest part from the myocardial surface surrounded by epicardial adipose tissue to the contrast enhancement of the tunneled artery (arrowhead to arrowhead) on the CPR image. The length of the MB is also evaluated as the distance from the entrance to the exit of MB (arrow to arrow).
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3. Results 3.1. Visualization of MB by coronary MSCT Normal coronary arteries were demonstrated good contrast enhancement for the lumen in all but two patients showing motion artifacts of arrhythmia or breathing. On transverse scan and CPR images, MB was evaluated by coronary MSCT, and the tunneled artery was found with the covering myocardial tissue (Figs. 2, 4 and 5).
grounds of 23 (15.8%) with MB in the coronary arterial branch are indicated in Table 2. They included 21 men and 2 women aged between 38 and 78 years old (mean 65.0 ± 10.2 years) with usually one MB confined in LAD; however, one patient (No. 19) had two MBs: one in the mid-segment of LAD and the other in the diagonal segment (D1). At the time of this study, 16 (70%) of the 23 patients were diagnosed with angina, and three patients (Nos. 13, 15 and 16) had a stent placed in the segment proximal to MB because of significant stenosis (Fig. 5).
3.2. Frequency of MB and clinical backgrounds
3.3. Types of coronary wall lesions in the segment beneath and proximal to MB
Two patients were excluded from this study because of motion artifacts. Among the 146 patients, the clinical back-
The segments beneath the MBs were always free of coronary wall lesions in the 24 coronary branches of the 23 patients. In
Fig. 4. A 62 year-old woman with angina pectoris (patient No. 4). Axial images (a and b), CPR image (c) and 3D-VR image (d) of left coronary MSCT. The MB is situated in the middle of the LAD (arrow, the two arrows in (d and e) indicate the entrance and exit of the MB) and low density plaque is shown just proximal (arrowhead) to the entrance of the MB. The thickness and length of the MB are 2.6 and 29.8 mm.
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Fig. 5. A 47 year-old man underwent percutaneous coronary intervention with stent at the proximal LAD (arrowhead) because of angina pectoris (patient No. 13). Axial (a and b), CPR (c) and 3D-VR (d) images of left coronary MSCT. The MB covered with myocardial tissue is situated in the middle of the LAD (arrow, the two arrows in (d and e) indicate the entrance and exit of the MB). The MB shows typical “step-down and step-up” appearance on the CPR image. The thickness and length of the MB are 2.2 and 25.7 mm.
contrast, the arterial segment proximal to the MB had significant coronary wall lesions (Fig. 4) in 19 coronary branches (79.2%) including those in the three patients fitted with stents in the segments proximal to MBs. Three cases with angina (No. 21–23) had MBs in the mid-segment of LAD; however, no significant coronary wall lesions were observed in the segments proximal to MB and no stenotic lesions in the other coronary branches (Fig. 2). 3.4. MB location, thickness and length In 19 patients (82.6%), the MB was located in the midportion of LAD; in two patients (patient No. 6 and 12), it
was located in the distal and the proximal portion of LAD, respectively. In another three patients, the MB was located in the second diagonal branch (patient No. 2), one in the middle LAD and another in the first diagonal branch (patient No. 19), and one in the obtuse marginal branch (patient No. 20). In the 23 patients, the thickness of the MB ranged from 1.1 to 3.7 mm (mean ± S.D.; 1.8 ± 0.7 mm), and the length ranged from 10.5 to 50.2 mm (mean ± S.D.; 20.0 ± 8.6 mm). The longest MB (50.2 mm) was located in the mid-portion of LAD (patient No. 18) completely occluding the LAD lumen in the segment proximal to MB and necessitating CABG.
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Table 2 Summary of 23 cases with myocardial bridge No.
Age/sex
Clin. diag.
Stenosis
Lesion
Location
Thickness (mm)
Length (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
78/M 62/F 58/M 62/F 71/M 74/M 68/M 64/M 71/M 67/M 56/M 38/M 47/M 74/M 76/M 61/M 67/M 66/M 73/M
Angina Angina Angina Angina Angina Angina Angina Angina Angina Angina Angina MI/PCI MI/PCI Angina/PCI Angina/PCI MI/PCI OMI Angina/CABG Angina
20 21 22 23
64/M 78/M 71/M 49/M
Angina Angina Angina Angina
Y Y Y Y N N N N Y N N N N Y N Y Y Y N Y N N N N
Calc. Plaque Plaque Plaque Calc. Calc. + plaque Calc. + plaque Calc. + plaque Calc. Calc. Calc. + plaque Plaque Stent Calc. + plaque Calc. + stent Plaque (in-stent) Calc. + plaque Occlusion None Plaque None None None None
Mid. LAD 2nd diagon. Mid. LAD Mid. LAD Mid. LAD Dist. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Prox. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD Mid. LAD 1st diagon. Obtuse Mid. LAD Mid. LAD Mid. LAD
2.6 1.1 1.2 2.6 3.7 1.1 2.2 2.1 1.4 2.4 1.5 2.1 2.2 1.4 1.4 2.7 1.7 2.0 1.3 1.4 1.1 1.3 1.1 2.2
24.9 20.9 21.9 29.8 20.5 10.5 14.3 17.0 24.6 27.4 10.8 27.5 25.7 16.7 11.7 15.1 12.3 50.2 17.4 19.1 24.1 11.0 12.8 14.5
Abbreviations: Clin. diag., clinical diagnosis; MI, myocardial infarction; PCI, post-coronary intervention; OMI, old myocardial infarction; CABG, coronary-aortic graft bypass; Y, yes; N, no; calc., calcified lesion; plaque, elevated lesion without calcification; Mid. LAD, middle portion of the left anterior descending coronary artery (LAD); RCA, the right coronary artery; 2nd diagon., the second diagonal branch; Dist. LAD, distal portion of LAD; Prox. LAD, proximal portion of LAD; 1st diagon., the first diagonal branch; Obtuse, obtuse marginal branch.
4. Discussion
angiography to the detection of MB; it also discloses a higher frequency of MB in coronary heart diseases.
4.1. Evaluation of MB with coronary MSCT 4.2. Recognition of MB by coronary MSCT The efficiency of MB detection was greatly improved by our newly devised modification of MSCT after simple “trial and error” essays. Consequently, the study clearly proved that coronary MSCT made possible the detection of MB much more objectively and non-invasively than the previous examination methods, such as conventional angiography, IVUS or ICD. MSCT revealed MB in 15.8% of the present 146 Japanese patients with various coronary heart diseases, which is evidently higher than the results of previous methods. The rate of MB detected by coronary angiography among coronary heart diseases has ranged between 0.8 and 7.5% in several studies [4], in each of which the occurrence of MB has been estimated mainly through focal reduction in coronary diameter or by the milking effect during systole; however, the designation of focal diameter reduction in coronary arteries has varied among angiographers [11]. In addition, a retrospective review of angiograms with special attention to the assessment of the arterial diameter has disclosed a higher prevalence of MB [11]. These results suggest that the diagnosis of MB by coronary angiography is somewhat dependent on the perception level of the individual angiographer. In contrast, MB can be detected consistently by coronary MSCT regardless of the perception level of the observer. We thus consider this method effectively more conducive than coronary
Autopsy studies on Japanese subjects have demonstrated, regardless of the anatomical properties of MB, a frequency of MB in LAD of 45% in 642 consecutive autopsied hearts [12], and of 29.5% in 200 cases with MBs over 1mm thick [10]. Consequently, the frequency of MBs over 1 mm thick is estimated to be approximately 13.3% (45% × 0.295) in autopsied Japanese subjects with non-coronary heart disease, which is similar to the present result (15.5%) obtained from patients with coronary heart disease examined by MSCT. However, MBs in 21 (87.5%) of 24 patients with MB in LAD were located in its middle segment, which is in good agreement with previous results of angiographic and autopsy studies [12]. The significance of MB in coronary heart diseases has been controversial. Long-term prognosis of isolated MBs is considered to be benign [13], and pathological studies have also shown a tendency for fewer anterior-wall infarctions in patients with MB in LAD [14]. On the other hand, a considerable number of symptomatic cases are attributed directly to the presence of MB, in which the release of the bridged muscle by surgical myotomy, stent placement in the MB segment and a regimen of -blocker has been effective for the improvement of symptoms [1,2], suggesting that some, but not all, cases of MB are potentially
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pathological. The anatomical properties of MB, such as thickness and length, may play a significant role in the haemodynamic alterations of blood flow in LAD with MB because of LAD compression exerted by the MB muscle; they are important regulators for atherosclerotic evolution in the entire course of LAD [1,10]. Furthermore, the thickness and length of MB are sometimes directly associated with pathological conditions, such as ischaemic heart disease or sudden death. Indeed, previous surgical and autopsy findings have indicated that the development of ischaemia is linked directly to a reduction in blood flow, depending on the length and thickness of the bridged muscle [15], and the deeply situated coronary artery, called as thick MB, is associated with sudden death [16]. Considering these associations of the anatomical characteristics of MB with pathological conditions, although overall MBs are thought to be benign [13], those with certain anatomical features are probably pathological. Based on such backgrounds, we evaluated the anatomical properties of MB and found that the mean thickness and length of MBs examined were 1.8 ± 0.7 mm (between 1.1 and 3.7 mm) and 20.0 ± 8.6 mm (between 10.5 and 50.2 mm), being considerably thicker and longer than those of previous autopsy studies (0.86 and 14.4 mm), respectively [10]. The difference is simple due to differences of methodology between the current coronary MSCT and histomorphometric measurements on the autopsied materials, as MBs less than 1 mm thick are so far as undetectable by the former one. Furthermore, the length of MBs is significantly correlated with their thickness [10]. Thus, the thickness and length of MBs examined by MSCT were found to be larger in this study, reflecting evident differences between the examination methods of autopsy and MSCT. Coronary MSCT is, however, amply helpful in the evaluation of the anatomical features of MB, compared with coronary angiography by which MB itself is not directly detectable. Further examination of the anatomical properties of MB in clinically pathologic cases using coronary MSCT concomitantly with autopsy analyses may elucidate the anatomical features of the pathologic MB. 4.3. Association of MB with atherosclerotic lesions In 23 patients with MB and ischaemic heart disease, 19 coronary branches had a significant number of lesions, such as calcified lesions and/or non-calcified plaque, in the coronary segment proximal to MB; however, the 23 patients had no coronary wall lesions in the segment beneath MB. This unique finding has frequently been described in the previous reports [1,2,6]. This difference of lesion distribution is caused exclusively by haemodynamic alteration of blood flow in the coronary artery [17], as well as by endothelial dysfunction in the segment proximal to MB [1,18]. In 16 (70%) of the 23 patients, the coronary wall lesion in the segment proximal to MB was considered responsible for ischaemic heart disease. PCI including stent placement was effectively applied to the stenotic segment proximal to MB in 5 of 16 patients. Considering these findings, the presence of MB may more or less be related to the occurrence of ischaemic heart disease through coronary compression leading to atherosclerotic evolution in the segment proximal to MB. In contrast, the three patients with angina showed not only
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absence of significant coronary wall lesions in the segment proximal to MB, but also absence of stenosis in the other coronary arteries. In these three patients with MB in LAD, the associated symptom might have been caused by the presence of MB itself, because MB compressed LAD at systole and yielded delayed diastolic relaxation of LAD resulting in blood flow reduction in the peripheral myocardium [6]. 4.4. Study limitations In a recent study, coronary MSCT has been described as still having certain limitations in fully evaluating coronary artery disease because of insufficient specificity when compared with coronary angiography [19]. Nevertheless, MSCT is an appropriate tool to at least acquire local anatomical images of MB as was done in the present study. Moreover, current coronary MSCT is capable of demonstrating only myocardial tissue more than 1 mm thick, and the distinction of the myocardium from either the arterial wall or coronary plaque is unclear. Since the anatomical properties of the tunneled artery are, however, believed to closely relate to coronary ischaemia and sudden death, coronary MSCT examinations are preponderantly useful in evaluating properties such as the location, length and depth of the tunneled artery. Further improvement of spatial, time and contrast resolution by MSCT is needed. In this study, we focused on efficiently detecting MB itself and evaluating its anatomical aspects. We, therefore, used only diastolic data images. However, we must mention that it would be possible to evaluate not only superficial and thinner MB but also haemodynamic effects of MB, when we also deal with systolic data images. Further investigations are needed from such view-points. Finally, the high exposure to radiation up to 13.0 mSv under coronary MSCT, remains a matter of concern [20]. Further finetuning the current modulation of the prospective X-ray tube and development of new devices for reducing radiation exposure are highly desirable. 5. Conclusion Coronary MSCT has recently been established as a reliable and non-invasive technique for the diagnosis of intracoronary lesions and periarterial abnormalities. Furthermore, it provides several potential advantages over other invasive techniques and is a new evolving imaging technique allowing a non-invasive diagnostic option for MB. Because of its capability of accurately locating the site and measuring the thickness as well as the length of MB, coronary MSCT is a useful tool for defining the anatomical properties of MB. References [1] Moehlenkamp S, Hort W, Ge J, Erbel R. Update on myocardial bridging. Circulation 2002;106:2616–22. [2] Ishii T, Asuwa N, Masuda S, Ishikawa Y, Kiguchi H, Shimada K. The effects of a myocardial bridge on coronary atherosclerosis and ischaemia. J Pathol 1998;185:4–9.
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