Imaging studies play a significant role in assessment of thoracic outlet syndrome. In this article, we discuss the etiology and definition of thoracic outlet syndrome and review the spectrum of imaging findings seen in patients with thoracic outlet syndrome. We then discuss an optimized technique for computed tomography and MRI of patients with thoracic outlet syndrome, based on the experience at our institution and present some representative examples. Based on our experience, a combination of computed tomography angiography and MRI (with postural maneuvers) effectively demonstrate thoracic outlet syndrome abnormalities.
Key points
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This article describes imaging techniques for assessing patients suspected of thoracic outlet syndrome.
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Our institutional protocol for imaging and management of TOS is described.
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It describes the imaging manifestations of thoracic outlet syndrome and illustrates them with the help of examples.
Introduction
Thoracic outlet syndrome (TOS) is a constellation of symptoms caused by the compression of neurovascular structures as they traverse the superior thoracic outlet. , The thoracic outlet has 3 anatomic compartments through which neurovascular structures must pass to reach the upper extremity: the interscalene triangle, the costoclavicular space, and the retropectoralis minor space. TOS can have a neurogenic component, a vascular component, or both. Compression of the vascular structures leads to swelling, edema, cyanosis, and/or decreased blood flow to the upper extremity, whereas neurogenic TOS leads to pain, numbness, dysesthesia, and weakness of the upper extremity. A classification, proposed by Wilbourn, includes 5 diverse syndromes: (a) arterial vascular TOS, (b) venous vascular TOS, (c) traumatic neurovascular (traumatic) TOS, (d) neurogenic TOS, and (e) nonspecific or disputed TOS.
Recognition of the first 4 types of TOS is relatively straightforward because they present a constellation of clinical features in relation to a specific anatomic derangement. In contrast, nonspecific (disputed) TOS presents with symptoms of unclear etiology and there are no consistent electrodiagnostic or vascular imaging abnormalities. , Nonspecific TOS may have as much as an 8% prevalence rate in select cohorts. However, owing to the controversial nature of nonspecific TOS, and lack of predefined diagnostic criteria, we primarily focus on the other 4 types of TOS in this article.
TOS that is not attributable to the first 4 types is uncommon, , with the neurogenic TOS accounting for 80% to 90% of patients with TOS. , Neurogenic TOS has an incidence of approximately 1 per million people. Vascular forms TOS are relatively uncommon; the venous forms represent 3% to 4% of TOS and arterial forms represent about 1% to 2%. ,
A diagnosis of TOS is usually made through a combination of physical examination (history and provocative tests) and diagnostic modalities (including electrodiagnostic tests and imaging studies). Imaging is necessary to demonstrate neurovascular compression and to determine the nature and location of the compressed structure and the structure producing the compression. Radiologic assessment is the principal method for diagnosis confirmation in vascular and neurovascular TOS.
In this article, we first describe computed tomography (CT) and MRI protocols used at our institution for assessing suspected TOS. Response to botulinum toxin (Botox; Allergan, Irvine, CA) injection into different muscle groups within the thoracic outlet—technical details for which are also presented in this article—has been reported to be useful in the triage process. We then illustrate various imaging abnormalities that we have seen in our cohort.
Thoracic outlet syndrome imaging protocols
Computed Tomography Imaging: Neurogenic and Arterial Thoracic Outlet Syndrome
Assessment of TOS requires a contrast-enhanced CT angiogram conducted on a multidetector CT scanner. The CT scan must be performed from the aortic arch to the skull base. A specialized protocol is required for conducting this study because the usual imaging protocol used for CT angiogram of the head and neck is sometimes compromised by the inflow artifact from the intravenously injected, dense, undiluted contrast flowing into the subclavian and brachiocephalic veins in the thoracic outlet region. Patients are placed in a supine position, with their arms alongside their body in the neutral position to decrease beam-hardening in the vicinity of the studied neurovascular structures. To minimize radiation dose, we do not perform a noncontrast scan before a contrast-enhanced scan.
To further decrease the radiation dose, and to opacify both the subclavian and axillary artery and vein, we perform a multiphasic injection of contrast material, composed of 105 mL of Omnipaque-350 (Iohexol-350, GE Healthcare, Waukesha, WI) and 165 mL of normal saline, administered using a dual-barrel injector programmed as demonstrated on graph shown in Fig. 1 . We do not supplement the CT examination with dynamic scans, or perform any scans with stress maneuvers, allowing for further radiation dose reduction.
The CT images are reconstructed at both 1-mm and 2-mm slice thicknesses, with dedicated 3-dimensional views of the thoracic outlet. Thick maximum intensity projection images of the bilateral subclavian veins and arteries as well as double oblique multiplanar reformatted image series, oriented axially with respect the subclavian artery lumens, are also produced to view the vessels in cross-section. Additional 3-dimensional volume rendered images, MIPs, and oblique multiplanar reformatted series may be created at the time of interpretation as deemed necessary by the radiologist.
Computed Tomography Imaging: Venous Thoracic Outlet Syndrome
In clinically suspected cases of venous TOS, a modification of the standard CT protocol described elsewhere in this article is implemented. A second intravenous cannula is placed in the antecubital vein of the unaffected or less affected side. A physician or technologist wearing a lead apron and lead-lined goggles performs a hand injection of 60 mL of a 16.6% contrast mixture to coincide with the start and finish of the autoinjector (at a rate of approximately 1 mL/s) that is simultaneously injecting contrast on the contralateral side using the protocol described elsewhere in this article. The purpose of this injection timing is to make sure that dilute contrast is flowing into the veins on the affected side to depict any extrinsic compression, luminal thrombus, venous varix, or any other abnormality.
MRI protocol
For MRI of TOS, we use a 3.0-T MRI scanner using phased-array body and neck coils. The imaging is performed first on the affected side, followed by the contralateral side. Our protocol includes a localizing sequence, followed by coronal, sagittal, and axial T1-weighted images, and sagittal/coronal T2-weighted images. We do not normally acquire any postcontrast, gadolinium-enhanced images. Contrast is only used if tumor or infection workup needs to be performed as a source of symptoms.
Imaging is divided into 2 sets of image sequences. The first set of images are acquired with patients in the supine position with arms at their sides and palms facing up. The second set of images is acquired with one arm in an abduction external rotation (ABER) position, which is achieved by flexing the elbow and placing the patient’s hand posterior to the contralateral aspect of the neck, with the head turned toward the side being examined.
The technical details of the image sequences are as follows:
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Triplane gradient echo coronal localizer: TR/TE 20/5 ms; number of excitations (NEX) 1; matrix 128 x 256; slice thickness 10 mm; field of view (FOV) 40 cm.
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Coronal T1-weighted fast spin echo (FSE): TR/TE 500/11 ms; NEX 4; matrix 269 × 384; slice thickness 5 mm; FOV 30 cm.
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Axial FSE T1-weighted: TR/TE 733/11; NEX 3; matrix 269 × 384; slice thickness 3 mm; FOV 20 cm.
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Sagittal FSE T1-weighted: TR/TE 590/11; NEX 4; matrix 269 × 384; slice thickness 3 mm; FOV 20 cm).
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Sagittal FSE T2-weighted: TR/TE 4150/49; NEX 3; matrix 269 × 384; slice thickness 3 mm; FOV 20 cm. Coronal T2-weighted images with similar technical parameters.
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The following sagittal FSE T1-weighted images are acquired with the affected arm in abduction and external rotation position: TR/TE 551/11 ms; NEX 4; matrix 269 × 384; slice thickness 3 mm; FOV 20 cm.
Ultrasound-guided botulinum toxin injection
Ultrasound guided injection of botulinum toxin into the muscles of the thoracic outlet, most notably the anterior scalene muscle, is an important diagnostic and potentially therapeutic adjunct in the evaluation of neurogenic TOS. In our experience, patients who report decrease or complete remission from their symptoms after botulinum toxin injection are more likely to benefit from surgical decompression of the thoracic outlet from first rib resection and scalene release.
Injection of botulinum toxin is accomplished as follows. With the patient lying supine, the head is rotated approximately 20° opposite to the injection site, to expose the region of sternocleidomastoid muscle. Using an ultrasound unit with a multifrequency (7.5–12.0 MHz) linear transducer, a preliminary assessment is made to identify the anterior scalene muscle, trunks of brachial plexus and location of vascular structures. Scanning is performed primarily in the transverse plane. A frequency of 12 MHz is used, with between 3 and 6 focal zones. The anterior scalene muscle is scanned along its craniocaudal extension, aiming for optimal visualization, which is usually through its lower half. Once an adequate approach is identified, the skin site is marked adjacent to the lateral short axis of the transducer.
One hundred units of botulinum toxin type A are reconstituted in 2 mL of 0.9% sterile nonpreserved saline. Eighteen units (0.36 mL) of reconstituted botulinum toxin are drawn into a separate tuberculin syringe. The syringe is connected to a 25G, 1.5-inch needle for the anterior scalene muscle injection. For pectoralis minor muscle, 23 units (0.46 mL) of reconstituted botulinum toxin are drawn into a separate tuberculin syringe. Both injections are done with a lateral-to-medial needle approach. Color Doppler ultrasound guidance is typically not required, because the vascular structures are readily identified on gray-scale images. The skin surrounding the entry point marks are cleaned with betadine solution and sterile drapes are placed. The ultrasound transducer is protected with a sterile cover containing a small amount of gel within it. A free-hand technique is used, orienting the needle–syringe set parallel to the transducer’s imaging plane and angled approximately 45° to the transducer’s footprint. When needed, the angle of needle entry is adjusted to avoid superficial vessels. The needle is slowly advanced with intermittent back-and-forth movements to visualize its tip, allowing for adjustment of the entry angle as needed. Once the needle tip is visualized within the belly of the muscle, reconstituted botulinum toxin is slowly administered under real-time monitoring. Adjustments in needle position during injection are made to maximize the distribution of medication throughout the muscle cross-section.
Classification of thoracic outlet syndrome and imaging manifestations
TOS could be classified based on the classification by Wilbourn , , : (1) arterial vascular, (2) venous vascular, (3) neurogenic, and (4) neurovascular TOS (coexistence of neurogenic and vascular components owing to posttraumatic or postoperative fibrous scarring or granulation tissue). Neurogenic TOS could be further classified into the following categories based on radiologic findings. ,
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Bony abnormalities
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C7 transverse process abnormalities such as elongation or obvious abnormality in the shape of transverse process. The transverse process of C7 is deemed elongated if it extends beyond the margins of the transverse process of T1 vertebral body.
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Cervical rib arising from C7 or, less commonly, another cervical vertebral body. TOS symptoms can be attributed to a cervical rib, accessory scalene fibers, or other soft tissue bands originating from such as rib. Sometimes there is distinct cervical rib with, at other times, the osseous projection is fused with the transverse process of the C7 vertebrae.
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First rib or clavicle abnormalities such as exostosis, callus, tumor, or congenital malformation (eg, a malformed first rib originating from the T1 vertebral body that does not articulate with the manubrium).
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Combined anomalies implicating first and second rib such as incomplete formation and/or pseudoarthrosis.
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Soft tissue abnormalities including muscular abnormalities, accessory fibrous bands, and congenital abnormalities.
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Narrowing of anatomic compartments of thoracic outlet
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Interscalene triangle.
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Costoclavicular space.
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Retropectoralis minor spaces.
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Vascular abnormalities such as aberrant vasculature, venous thrombosis, and vasculitis. For patients with venous TOS, diagnosis was confirmed by venography and ultrasound examinations. In this report, we do not provide details on assessment of venous TOS.
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Postoperative complications such as hematoma, chyloma, infections, and abscess formation, reossification along the resected rib, and granulation tissue formation.
We illustrate imaging manifestations of some of these etiologies of TOS in Figs. 2–7 .
