The brachial plexus is a complicated network of nerves stemming from the ventral branches of spinal nerves C5 to T1.1 The brachial plexus plays an important role in the innervation to the upper extremity. The pathologic conditions related to the brachial plexus include primary and secondary tumors, trauma, entrapment, and irradiation. Nevertheless, the examination for brachial plexus is challenging because of its dimensional pathway and orientation.2
Magnetic resonance imaging is renowned for its excellent soft tissue contrast and spatial resolution. Recently, there is an increased interest in diagnosing brachial plexopathies via magnetic resonance imaging (MRI) techniques.3–5 One of the current standard MRI protocols used in assessing the brachial plexus is short inversion time inversion recovery (STIR), known for suppressing the signal from tissues with a short T1 relaxation time similar to fat.6 As a consequence, brachial plexus lesions could be nicely delineated by the maximum-intensity projection (MIP) and multiplanar reformatting (MPR).1,2,6
In the aforementioned literature, the 3-dimensional T2-weighted STIR (3D-T2-STIR) sequence was implemented without contrast agent administration.1,2,6 In such cases, the outlines of brachial plexus tend to be not clear because of overlapping with vessels. On the other hand, it is evidenced that T1 relaxation time of blood could be reduced as short as that of fat tissues after contrast agent (gadolinium diethylenetriamine-pentaacetic acid [Gd-DTPA]) administration.7 Based on this framework, the 3D-T2-STIR images without and with contrast agent administration were compared to see if the image quality had been improved. The quantitative analyses would be achieved by rating the subjective diagnostic ability and evaluating the objective contrast ratio (CR) between the brachial plexus and the surrounding tissues.
MATERIALS AND METHODS
The study protocol was approved by the local human experiments and ethics committee. From June 2011 to December 2012, a total of 30 patients with brachial plexus diseases were recruited in this study and their age ranged from 16 to 85 years (mean, 53.9 ± 16.55 years; 13 female and 17 male, including 6 patients with brachial plexus injuries, 4 with neurogenic tumors, 1 with an intraspinal meningioma, 6 with tumors near the brachial plexus, and 13 with unremarkable findings). Informed written consent was acquired from every subject after explaining the details of this study.
All images were performed on a 3-Tesla MRI system (Magnetom Trio with TIM system; Siemens, Erlangen, Germany). Before the scan, patients were educated to avoid motion from deep breathes or swallows. The combination of the body coil and the neck coil was used for imaging acquisition. The 3D-T2-STIR imaging used an inversion recovery (IR) 3D turbo spin echo (TSE) with variable flip angle RF excitations (SPACE, Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions).8 The scan parameters were as follows: TR/TE = 4500/108 milliseconds, flip angle = 180 degrees, echo train length = 103, TI = 220 milliseconds, slice thickness = 1.1 mm, slices oversampling = 30%, slice per slab = 40, field of view (FOV) = 280 × 280 mm, matrix size = 256 × 256, number of acquisitions = 1.4, bandwidth = 673 Hz/pixel. The acquisition time for every scan was 8 minutes 44 seconds. The images were obtained both before and after administration of the contrast agent. The contrast agent (Gd, Magnevist; Schering, Berlin, Germany) of 0.2 mL/kg was administered at a rate of 3 mL/s followed by a 20-mL saline flush at the same rate through a 20- to 22-gauged intravenous needle in the antecubital vein by the power bolus injector. For the images with contrast agent, the 3D-T2-STIR protocol began immediately after the bolus injection finished.
For each 3D data set, an MIP image of the 3D-T2-STIR sequence was constructed via the Siemens workstation (Numaris/4 syngo MR B17). Multiplanar reformatting technique was also performed through Siemens workstation. The selected regions of interest (ROIs) are elucidated in Figure 1. The ROIs for nerves were plotted on both sides of the brachial plexus (C5–C7) about 2 cm away from the thecal sac edge in the MIP image. For surrounding tissues, ROIs were plotted just adjacent to those of nerves. There were a total of 12 circles in every MIP image, and areas of circles were uniformly 5 mm2. Signals of the 6 ROIs in brachial plexus were averaged as the Signalnerve, and those in surrounding tissues were averaged as the Signaltissue. Then CR was calculated before and after contrast agent by the following equation:
The paired Student t test was used to assess the significance of differences in CR between images without and with contrast agents. To evaluate the image qualities between MIP images of 3D-T2-STIR sequence without and with contrast agent, the diagnostic abilities for both runs were scored by the experienced radiologist as follows: 5 = excellent, 4 = good, 3 = fair, 2 = poor, and 1 = not seen. The paired nonparametric Wilcoxon signed rank test was used to test whether the diagnostic ability was improved after injecting the contrast agent. Statistical analyses were performed with SPSS (Statistical Product and Service Solutions, version 18.0, Chicago, Ill). A value of P < 0.05 was considered significant.
The 3D-T2-STIR sequences without and with contrast agent were successfully acquired in all subjects. Figure 2 shows examples of 3D-T2-STIR images. Raw images (Fig. 2A, B) without contrast agent show that the vessels (black arrows) will overlap with the brachial plexus in the MIP image without contrast agent (Fig. 2C). After contrast medium administration, the MIP image with contrast agent (Fig. 2D) shows the vessels in Figure 2C are suppressed. Then the outlines of the brachial plexus get sharp and clear.
Other examples of 3D-T2-STIR images are given in Figure 3. There is a high signal mass over the paraspinal region. The relationship between the mass and brachial plexus is hard to determine without contrast agent (Fig. 3A). After contrast agent administration, the outlines of brachial plexus roots are identified better because of improved CR (Fig. 3B). Postcontrast MPR images with multiangled reconstructions reveal that the right C7 and C8 nerves are involved with disruptions and a focal swelling (Fig. 3C, D).
The CRs for brachial plexus and surrounding tissues without and with contrast agent are shown in Figure 4. For brachial plexus, the signal intensity without and with contrast agent were 400.67 ± 60.68 and 394.37 ± 55.33, respectively. With P = 0.68, the signal intensities for brachial plexus without and with contrast agent were not significantly different. For surrounding tissues, however, the signal intensities were significantly suppressed with the contrast agent (P < 0.05); those without and with contrast agent were 188.69 ± 31.48 and 121.41 ± 16.81, respectively. In addition, images with contrast agent exhibited a higher CR (0.53 ± 0.04) than that without contrast agent (0.35 ± 0.05) (P < 0.05; Fig. 5). The higher CR also explained the improved diagnostic ability in images with contrast agent administration.
The mean scores of the diagnostic ability were 1.7 and 4.5 for 3D-T2-STIR images without and with contrast agent, respectively (P < 0.05; Table 1). The significantly higher score of enhanced images implied that the diagnostic ability would be improved with the help of the contrast agent.
This study provided evidence that, with the help of contrast agent, the diagnostic ability of 3D-T2-STIR images can be improved via suppressing signals of surrounding tissues and paraspinal vessels. This has not been fully discussed before because the administration of gadolinium chelates was not encouraged in the STIR technique.9 However, we found that CR would be increased and the diagnostic ability was improved under the presence of contrast agent. This may be because the signals of surrounding tissues and vessels were further suppressed.
In the vicinity of the brachial plexus, there exist some vessels,6,10 including paraspinal vessels, subclavain veins, internal jugular veins, and others. At 3T MR scanner, the T1 relaxation times for nerves and blood in vessels are 141011 and 1664 milliseconds, respectively.12 After a short period of TI, both nerves and blood in vessels are still hyperintense with respect to the fat and surrounding tissues (Fig. 2A-C) because of their intrinsic MR properties. After contrast agent administration, the T1 relaxation time of blood in vessels will be reduced7 whereas that of nerves remains the same.13 We have successfully improved the CR and the diagnostic ability by decreasing T1 relaxation time of vessels. As shown in Figures 2 and 3, signals of blood vessels were appropriately suppressed after contrast agent administration whereas those of the nerves remained hyperintense. Consequently, clear outlines of the brachial plexus were delineated in MIP images without effort.
In the literature, there are other T2-weighted protocols for evaluating the brachial plexus. For example, CISS (constructive interference in the steady state), 3D True-FISP (fast imaging with steady state precession), and 3D FSE-cube (3-dimensional isotropic resolution fast spin echo sequence) are good ways to image the brachial plexus.1,14,15 The advantages of these techniques are that they provide details of pathological lesions and anatomical structures. However, in these techniques, there is no report to achieve the task of showing the clear outlines of the complicated brachial plexus in 1 MIP image. In addition, the enhancement technique applied in these T2-weighted protocols for brachial plexus has never been reported. Therefore, our suggested technique is unique and helpful for diagnostic ability in the future.
There are some limitations in this study. First, our included cases have various etiologies of brachial plexus diseases. Further prospective studies focusing on the certain types of brachial plexus lesions are suggested in future work. Second, the protocol with contrast agent administration is relatively invasive when compared with that without contrast agent and it is not suitable for renal insufficiency patients. The application of this technique might be limited in clinical use. Third, it takes 8 minutes 44 seconds to complete the examination. A long scan time might increase the chance of motion artifact, especially for patients with brachial plexus lesions. It is worthy to have further research on reducing the examination time and simultaneously maintaining a good image quality. Last, division and cord segments of the brachial plexus gather into a bundle. Therefore, it is difficult to distinguish each nerve unless the image resolution improves.
In conclusion, the MIP image of 3D-T2-STIR technique with contrast agent is superior to that without. It might be a better way to evaluate anatomies and pathologies of the brachial plexus. These advantages would improve the understanding and neurosurgical planning for brachial plexopathies in the future.
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Keywords:Copyright © 2014 Wolters Kluwer Health, Inc. All rights reserved.
brachial plexus; 3-dimensional; STIR; contrast agent; contrast ratio; magnetic resonance imaging