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Ann Thorac Surg 2001;71:1229-1232
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Contrast-enhanced magnetic resonance angiography of mammary artery grafts after minimally invasive coronary bypass surgery

^a Department of Cardiothoracic Surgery, Heart Center/Klinikum Wuppertal, University of Witten/Herdecke, Wuppertal, Germany
^b Department of Radiology, Heart Center/Klinikum Wuppertal, University of Witten/Herdecke, Wuppertal, Germany

Accepted for publication October 18, 2000.

Address for reprint requests to Dr Vetter, Department of Cardiothoracic Surgery, Heart Center, Arrenberger St 20, 42217 Wuppertal, Germany

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. The aim of the study was to investigate the application of the contrast-enhanced magnetic resonance angiography (CE-MRA) for the visualization of left internal mammary artery (LIMA) bypass.

Methods. A total of 30 patients with LIMA bypass (22 men, 8 women, 35 to 77 years) received a CE-MRA 4 to 20 days after surgery. The non–ECG-triggered CE-MRA was performed during expiration using a body array coil at a 1.5 Tesla scanner (Magnetom-Vision). A three-dimensional gradient-echo sequence with slice interpolation technique was applied. For the three-dimensional visualization, single coronal slices were postprocessed with maximal intensity projection. Of 30 patients 22 agreed to a comparative coronary angiography.

Results. Five bypasses were identified up to the end-to-side anastomosis. A total of 80% of the bypass course was detectable in 13 patients and 60% in 11 patients. In two LIMA bypasses only 30% of the proximal part could be viewed; one was found by conventional coronary angiography to be occluded. The other conventional coronary angiography showed the LIMA bypass to be patent.

Conclusions. The complete course of the LIMA bypass to the left anterior descending coronary artery can be visualized by the MRA technique. The most reliable imaging of the distal anastomosis can be realized by reducing the negative influence of the beating heart.

	Introduction

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Minimally invasive coronary bypass surgery is established for patients with one-vessel disease. For this, the left internal mammary artery (LIMA) is connected to the Ramus interventricularis anterior. Coronary angiography is considered to be the standard procedure for the evaluation of graft patency. Magnetic resonance (MR) imaging appears to be a sensitive and specific noninvasive technique for the assessment of venous and internal mammary coronary artery bypass grafts [1–3].

The aim of the present study was to determine the value of contrast-enhanced three-dimensional (3D) MR angiography in assessing the patency of arterial coronary bypass grafts.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
Patients
We studied 30 patients (22 men, eight women) who had undergone minimally invasive coronary artery bypass surgery (MIDCAB) with LIMA graft to the left anterior descending (LAD) artery. Their mean age (±SD) at MR imaging was 58.5 years ± 8.7 (range 35–77 years). The mean interval between surgery and MR imaging was 7 days ± 5.8 (range 4 to 20 days). All patients were regularly scheduled for follow-up conventional angiography as part of another study protocol (for evaluating the quality of the distal anastomosis after MIDCAB) and gave written informed consent for that study and for MR angiography, both of which were approved by the review board of our hospital. Of 30 patients 22 agreed to a comparative coronary angiography.

Magnetic resonance
Magnetic resonance angiography was performed with a 1.5 T whole-body system (Magnetom Vision, Siemens Medical Systems, Erlangen, Germany) equipped with 25 mT/m high-performance gradients that enabled a 600-microsecond rise time. A four-element phased-array body coil was used to optimize signal detection. A 3D radio-frequency, spoiled, fast, low-angle shot sequence without ECG triggering was used (repetition time ms/echo time ms = 5/2). Sagittal data acquisitions were performed in 22 patients and coronal data acquisitions in 8 patients.

The base matrix size in the readout direction was 512 (reduced to 70% in the phase-encoding direction) with a 500-mm field of view. The rectangular field of view was adapted to the patient’s constitution, and the imaging orientation was 250 to 313 mm in the sagittal orientation and 250 to 375 mm in the coronal orientation. The 3D volume had a thickness of 96 mm and was subdivided into 26 to 40 partitions, which yielded a maximum acquisition time of approximately 30 seconds.

The mean section thickness was 2.8 for sagittal acquisitions and 3.3 mm for coronal acquisitions, with an in-plane resolution of 1.37 mm² (0.98 x 1.40 mm). The mean acquisition time was 29.6 ± 1.9 seconds (range, 26.9 to 32.2 s) in the sagittal slab orientation and 30.9 ± 1.7 seconds (range, 26.9 to 35.5) in the coronal slab orientation.

Each data acquisition was enhanced with a 0.1-mmol Gd-DTPA/kg body weight (Magnevist; Schering, Berlin, Germany). Contrast material was automatically administered with a 20-gauge needle and an MR-compatible power injector into an antecubital vein. To optimize contrast enhancement, the transit time of the contrast agent was evaluated with a sequential TurboFLASH (Siemens Medical Systems) (fast low-angle shot) sequence combined with a 2-ml bolus of contrast material and the same injection parameters. After a time–intensity analysis, the individual start delay to cover the central k-space acquisition with contrast material was calculated as follows: Td = Tt - (Ta/2) + 4, where Td is the delay in seconds between the start of contrast material injection and data acquisition. It is the transit time (in seconds) of the contrast agent between the injection site and the volume of interest, and Ta is the 3D data acquisition time (in seconds). Each graft was divided into four sections (origin of the LIMA at the subclavian artery, 60% of the bypass course, 80% of the bypass course, and distal end-to-side-anastomosis), and each section was judged to be visible or nonvisible. LIMA bypasses whose course could be viewed 60% or more by CE-MRA were interpreted as patent. Graft patency was assessed by two radiologists experienced in MR (U.K. and B.M.C.) who were oblivious to the results of conventional angiography. The titanium clips used did not cause signal void.

Comparison of coronal and transversal orientation
For the evaluation of the patency of the LIMA bypasses it is necessary to represent the proximal course of the vessel over a significant distance and in continuity. This can be performed by CE-MRA in coronal and transversal orientation, respectively. To compare the diagnostic value of both the coronal and transversal orientation, in 8 patients a transversal orientation was chosen. In a transversal orientation the bypass graft is illustrated mainly perpendicular to its course and thereby it is possible to record the proximal 80% of the LIMA bypass.

Conventional angiography
Conventional angiography was performed using the Sones technique. The angiograms of selective catheterization of the LIMA were analyzed by experienced cardiologists.

Statistical analysis
The sensitivity, specificity, and positive predictive value of CE-MRA in the assessment of graft patency was calculated. Sensitivity was calculated as truly patent grafts/(truly patent grafts + false occluded grafts). Specificity was calculated as truly occluded grafts/(truly occluded + false patent grafts). Positive predicted value was calculated as true patent grafts/(true patent grafts + false patent grafts) and negative predicted value as true occluded/(true occluded + false occluded) and the test efficiency (sum of positive and negative predicted value). The results of the conventional angiography served as the standard of reference.

The Fisher exact test was used to compare proportions (sensitivity, specificity, and positive predictive value). A p value less than 0.05 was considered to indicate a statistically significant difference.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
The origin of the LIMA at the subclavian artery could be demonstrated in 29 of 30 patients.

Data acquisition was performed in coronal (n = 22) and sagittal (n = 8) orientation. With sagittal orientation, the bypass is predominantly depicted as being vertical to the course, and the examination process enabled the proximal 80% of the LIMA course to be depicted.

With the coronal orientation, 21 of the 22 LIMA bypasses originating at the subclavian artery were visible. A total of 60% of the LIMA bypass course was visible with through-flow in 8 out of 22 patients, in 80% of the bypass course in 10 out of 22 patients, and the entire bypass course including distal end-to-side anastomosis in 5 out of 22 patients. With the sagittal orientation, seven of the eight LIMA bypass origins were visible, 60% of the LIMA bypass course was visible with through-flow in 3 patients, and 80% of the bypass course in 3 patients. The entire bypass course including anastomosis was not visible in any of the patients for technical reasons related to the examination. The results of the comparison between the coronal and sagittal orientations are shown in Table 1. The diagnostic value is not improved by the higher disintegration of layers during the sagittal technique (p > 0.05, one-sided Gauss test to compare the relative frequency). The coronal layer guide permits display of the entire LIMA bypass with no loss of diagnostic safety.

	Comment

Top Abstract Introduction Material and methods Results Comment References

The benchmark for bypass examination is conventional angiography. Conventional angiography is an invasive method that can be associated with complications. Magnetic resonance angiography examinations are noninvasive and can be performed, on average, in a 20-minute examination period without distressing the patient.

The aim of our investigation was to examine the potential for using CE-MRA for the bypass check. In many earlier, well publicized studies, the assessment of LIMA bypasses proved to be especially problematic. This is attributed in part to the small lumen of IMA bypasses (1 to 2 mm) by comparison with venous bypasses (3 to 6 mm), and in part to the titanium clips during the course of this bypass. Gomes and colleagues [6], for example, could identify only 45% of the IMA bypasses. In other studies IMA bypasses have not been examined or have been only in a small number of cases [7]. With the 3-D gradient-echo sequence with coronal data acquisitions used in our study, with layer interpolation in the breath-hold technique, it is basically possible to view the entire course of the LIMA bypass (Fig 1). The titanium clips used caused no signal voids. With this technique we achieved a sensitivity of 95.5% and a specificity of 66.7%. In this connection, the low specificity of 66.7% can be partly attributed to the low number of occluded bypasses in our study (Table 3).

View larger version (134K): [in this window] [in a new window]   Fig 1. Three-dimensional visualization of a left internal thoracic artery graft to the ramus interventricularis anterior by contrast-enhanced magnetic resonance angiography.

A combination with assessment of flow velocities and flow profiles by means of phase-contrast techniques [11, 12], or a combination with systolic and diastolic flow measurement in LIMA bypasses by means of duplex sonography, could facilitate noninvasive examination of the functioning LIMA bypass. The first results from a study of ours that is currently in progress are very promising.

The results of this investigation demonstrate the possibility of using CE-MRA for a reliable evaluation of graft patency. Because this procedure can be performed noninvasively with high evidence it can be an optimal screening method.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Herbert O. Vetter Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Vetter, H. O. Articles by Cramer, B. M. Search for Related Content PubMed PubMed Citation Articles by Vetter, H. O. Articles by Cramer, B. M. Related Collections Coronary disease Minimally invasive surgery