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J Am Coll Cardiol, 2001; 37:2031-2035 © 2001 by the American College of Cardiology Foundation |

* Division of Cardiology of the Department of Medicine, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA
Department of Radiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA
Manuscript received December 31, 2000; revised manuscript received February 27, 2001, accepted March 14, 2001.
Reprint requests and correspondence: Dr. João A. C. Lima, Division of Cardiology, Blalock 569, The Johns Hopkins Hospital, 600 North Wolfe Street, Baltimore, Maryland 21287-6568
jlima{at}mri.jhu.edu
| Abstract |
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We sought to determine the feasibility and potential of transesophageal magnetic resonance imaging (TEMRI) for quantifying atherosclerotic plaque burden in the aortic arch and descending thoracic aorta in comparison with transesophageal echocardiography (TEE).
BACKGROUND
Improved morphologic assessment of atherosclerotic plaque features in vivo is of interest because of the potential for improved understanding of the pathophysiology of plaque vulnerability to rupture and progression to clinical events. Magnetic resonance imaging (MRI) is well suited for atherosclerotic plaque imaging. Performing MRI using a radio frequency (RF) receiver probe placed near the region of interest improves the signal-to-noise ratio (SNR).
METHODS
High-resolution images of the thoracic aortic wall were obtained by TEMRI in 22 subjects (8 normals, 14 with aortic atherosclerosis). In nine subjects, we compared aortic wall thickness and circumferential extent of atherosclerotic plaque measured by TEMRI versus TEE using a Bland-Altman analysis. Additional studies were performed in a human cadaver with pathology as an independent gold standard for assessment of atherosclerosis.
RESULTS
In clinical and experimental studies, we found similar measurements for aortic plaque thickness but a relative underestimation of circumferential extent of atherosclerosis by TEE (p = 0.001), due in large part to the lower SNR in the near field.
CONCLUSIONS
Using TEMRI allows for quantitative assessment of thoracic aortic atherosclerotic plaque burden. This technique provides good SNR in the near field, which makes it a promising approach for detailed characterization of aortic plaque burden.
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| Methods |
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coaxial cable with an extended inner conductor, housed inside an 8- or 12F Levin gastric tube and connected via tuning, matching and decoupling circuitry to the scanner (Fig. 1). The circuitry provides high-speed diode switching to decouple the antenna during external RF pulses and allows signal reception between pulses. A BALUN circuit is interposed to block the transmission of unbalanced currents. In theory, without the BALUN, if the MRI connector cable is inadvertently left in a loop configuration during scanning, induced currents might be transmitted to the patient as heat.
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2 mm thickness) in the distal aortic arch or descending thoracic aorta. The MRI was performed within one day after TEE. Eight normal controls (3 women, mean age 30 ± 8 years) were also studied by TEMRI. None of the volunteers had a history of smoking, hypertension (HTN), hypercholesterolemia, diabetes or a familial history of premature coronary artery disease (CAD). The study was approved by the Johns Hopkins Committee on Clinical Investigation. Written informed consent was obtained from each subject. We performed additional studies in a formalin-fixed human male cadaver with diffuse aortic atherosclerosis. A vitamin Efilled balloon catheter was placed below the origin of the left subclavian artery under TEE guidance and sutured into position for cross-registration among TEE, TEMRI and histopathology.
Placement of the TEMRI probe.
The probe was advanced through the nose into the stomach, using topical benzocaine spray when needed. Proper positioning was confirmed by aspiration and auscultation, and adjusted once after scout images when necessary. The distal extremity of the receiver was placed at the gastroesophageal junction. As shown previously (9,12), because the sensitivity of the TEMRI antenna is maintained over much (
20 cm) of its length, it was not necessary to reposition it to image the upper portion of the descending aorta or aortic arch (Fig. 2E).
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=90°, echo train length (ETL) 24, 1 NEX (number of excitations), no phase wrap, in-plane resolution 0.47 x 0.47 mm). The breath-hold duration varied from 18 to 28 heart beats/slice. The TEE studies. The TEE studies were performed with conventional equipment (Hewlett Packard 5500) by an expert echocardiographer using an omniplane probe. The probe was advanced toward the level of the diaphragm and a gradual pullback was performed. Care was taken to obtain high-quality images by optimizing focal length, gain settings and the contact between the probe and the esophagus. Images were recorded on S-VHS videotapes. Horizontal-plane TEE images were compared with TEMRI.
The TEE/TEMRI comparison.
In nine subjects with good-quality TEE images and for whom TEE and TEMRI images contained specific anatomic landmarks (origin of the left subclavian, pulmonary artery, carina, left main stem bronchus, crura of the diaphragm, left atrium), plaque thickness, and extent were analyzed independently by two observers (K.A.S., J.G.) (NIH Image 1.62, Frederick, Maryland). Maximum and minimum aortic wall thickness (WT), excluding adventitia, was measured in the visible circumference of the descending aorta. To establish a basis for an arbitrary definition of abnormal aortic WT, the TEMRI slices from normal subjects were obtained in similar anatomic locations. Allowing multiple measurements per individual, we generated 82 WT measurements from various independent sites in the normal aortic wall, with a mean value (±SD) of 1.03 ± 0.32 mm (Fig. 3). A WT
2.0 mm (mean + 3 SD) was defined as abnormal. Subsequently, we measured the circumferential extent of the plaque in patients as the number of radial degrees
2.0 mm WT, with good intraobserver (r = 0.970, p < 0.0001) and interobserver correlations (r = 0.77, p = 0.0094). Anatomic landmarks were used for cross-registration between TEE and TEMRI. Planimetry of aortic WT was performed on paired TEE images to determine maximum and minimum values and the extent of abnormal thickening. Comparisons were analyzed by the Bland-Altman method and significance determined by two-tailed paired t tests. The reproducibility of measurements was assessed by linear regression analysis.
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| Results |
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20 cm without the need for repositioning the device. Maximum and minimum WTs were 3.5 ± 1.2 mm and 1.2 ± 0.8 mm by TEE, and 3.3 ± 1.5 mm and 1.0 ± 0.7 mm by TEMRI, respectively (NS), for the entire patient pool. Although the correlation was good between the two techniques for measurements of circumferential plaque extent (r = 0.77, p = 0.009), relative underestimation of the extent of disease was found by TEE, particularly at the higher range of values as reflected by the Bland-Altman analysis (Fig. 2F). Each point in Figure 2F represents an individual study patient.
The most important limitation of TEE in the quantification of aortic atherosclerosis is further demonstrated in Figure 4 , which represents comparisons among TEE, TEMRI and pathology from a cadaver, using an intra-aortic vitamin Efilled balloon catheter as an artificial landmark for image registration. Figure 4 shows that TEMRI allows for evaluation of the aorta over its entire circumference, whereas TEE aortic imaging is hampered by near-field limitations inherent to the ultrasound method. These results further support the observation that TEMRI provides a more accurate measure of aortic circumferential plaque extent than TEE.
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| Discussion |
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Limitations of TEE for aortic plaque imaging. Transesophaseal echocardiography is not ideal for aortic plaque imaging because the aortic wall is not visualized in its 360° entirely due to near-field signal losses. The probe is not a fixed reference point, making TEE an imperfect tool for plaque burden quantification, particularly in areas with many plaques and few anatomic landmarks. Finally, tissue characterization is poor. Despite these limitations, TEE has been used as a reference for aortic atherosclerosis quantification (6), and it has provided useful information about aortic atherosclerosis (13) and its relationship to cardiovascular clinical events (14). In addition, TEE provides readily available data about plaque mobility. In contrast, TEMRI may have several potential advantages over TEE for monitoring plaque behavior. First, it allows imaging in any plane with precise registration to a fixed reference frame. Second, TEMRI has potential for assessment of plaque composition (68).
Limitations of the TEMRI technique. The TEMRI technique does have limitations. As opposed to TEE, it is not portable to the patients bedside and does not currently allow true real-time imaging. Also, if the center does not have an available MRI scanner, the acquisition cost of an MRI machine compared to a TEE probe and ultrasound machine is considerable. The nonuniformity of SNR is another limitation of TEMRI, but is a property shared by TEE. Compared to surface-coil MRI, TEMRI is slightly more invasive and, therefore, carries more potential risk. In principle, TEMRI can be combined with phased-array torso coil MR imaging to further enhance the SNR at the aortic wall. Studies are ongoing to rigorously compare the SNR obtainable at the aortic wall using TEMRI versus the best available phased-array torso coil to determine the incremental benefit of TEMRI over the more conventional MRI technique.
Conclusions. The TEMRI technique is a promising tool for quantification of atherosclerotic plaque extent in the aortic arch and the descending thoracic aorta. Because the entire circumference of the aorta can be visualized at any level and orientation, the relationship of individual plaques to structural landmarks is straightforward, making the technique ideal for serial studies. Its minimally invasive nature and lack of a requirement for sedation, which itself carries additional risks and costs, are additional advantages. Finally, the potential for detailed assessment of plaque composition makes TEMRI an important addition to cardiovascular medicine and clinical investigation.
| Acknowledgments |
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| Footnotes |
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| References |
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