State-of-the-art Magnetic Resonance Imaging in Vascular Thoracic Outlet Syndrome




Vascular thoracic outlet syndrome is caused by compression of subclavian/axillary vessels during their passage from the thoracic cavity to the axilla. Early diagnosis and treatment is important to prevent debilitating outcomes of vascular thoracic outlet syndrome. Contrast-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) with equilibrium phase using provocative arm positioning is the optimal examination to determine presence, degree of vascular compression, and complications of vascular thoracic outlet syndrome. This article reviews thoracic outlet anatomy, disorders of the vascular component, and typical imaging findings by contrast-enhanced 3D MRA.


Key points








  • The use of contrast-enhanced 3D MRA using provocative arm positioning provides comprehensive information on arterial and venous compression by surrounding organs.



  • Equilibrium phase images increase detection venous abnormalities when enhancement is not adequate on the venous phase of contrast-enhanced 3D MRA.



  • Contrast-enhanced 3D MRA with equilibrium phase images is a valuable tool, however should be used as complementary tests in diagnosis of vascular thoracic outlet syndrome.






Introduction


Thoracic outlet syndrome (TOS) is a constellation of symptoms caused by impingement of the subclavian vessels and brachial plexus during their passage from the thoracic cavity to the axilla. TOS has been classified into several types, including neurogenic TOS (nTOS), arterial TOS (aTOS), and venous TOS (vTOS). The true incidence of TOS is controversial, and has been reported to range from 0.3% to 8%. nTOS accounts for almost 90% of cases of TOS, whereas less than 10% of patients have only vascular or combined symptoms. The female/male ratio for nTOS is 3.5:1 and there is no sex predilection for the arterial type. vTOS is traditionally considered to be male predominant; however, the largest study reported a similar proportion of men and women.


Underlying causes for compression are congenital or acquired factors, such as cervical rib, long C7 transverse process, exostosis, hypertrophic callus, congenital fibromuscular anomalies, posture, repetitive movements, and posttraumatic fibrosis of the scalene muscle. aTOS is almost always associated with bone abnormalities, such as cervical rib, callus, or exostosis, whereas venous disorder is caused by repetitive injury to the subclavian vein by the first rib, clavicle, subclavius muscle, anterior scalene muscle, and costoclavicular ligament. Both vTOS and aTOS usually develop in young, healthy patients with few if any comorbid conditions.


The symptoms caused by arterial compression are pain, claudication, pallor, and coldness; however, pain and edema can be seen with vTOS. Edema of the upper extremity is the hallmark for venous compression, especially when it is associated with thrombosis (effort thrombosis, which is also known as Paget-Schroetter syndrome). Pain is the most common feature in aTOS, although some patients with venous compression experience paresthesia caused by swelling rather than nerve compression. Although rare, potential severe complications have been reported, including venous gangrene of the hand, pulmonary embolism as a result of venous compression or digital ischemia, and stroke associated with aTOS ( Table 1 ).



Table 1

Vascular TOS: contrast-enhanced 3D MRA protocols for 1.5-T and 3-T MR imaging scanners


























aTOS vTOS
Sex Female/male equally
Age (y) 20–30 20–35
Risk factors Bone abnormalities (cervical rib and articulating with the first rib as a pseudoarthrosis; anomalous first rib; long C7 transverse process)
Congenital fibrocartilaginous bands associated with the anterior scalene muscle
Hypertrophic callus from healed clavicle or first rib fracture
Vigorous exercise or activity involving affecting extremity
Repetitive injury contributed by first rib, clavicle, subclavius muscle, and fibrous costoclavicular ligament leads to perivenous fibrosis and endothelial injury
Posture
Sign and symptoms Pain
Claudication
Pallor
Coldness
Paresthesia
Hand swelling caused by edema
Feeling of tightness worsens with exertion
Venous engorgement with collateralization
Potential severe complications Digital ischemia
Stroke
Venous gangrene of the hand
Pulmonary embolism


TOS can be diagnosed with history and a physical examination that includes provocative tests. However, imaging is required to identify vascular abnormalities. Conventional digital subtraction angiography (DSA) has been considered the reference standard for diagnosis of vascular TOS. However, DSA has potential risks, including nephrotoxicity from iodinated contrast agent, arterial puncture site complications, ionizing radiation, and rarely stroke. Also, it requires separate procedures for arteries and veins; therefore, DSA is reserved for minimally invasive interventions and preoperative evaluation of the vessel anatomy to determine surgical approach or bypass graft planning. Computed tomography angiography (CTA) can be used as an alternative to DSA to reveal vascular anatomy and disorders using multiplanar reformatted techniques and maximal intensity projections (MIPs). However, similar to DSA, it requires iodinated contrast agent and considerable irradiation, particularly with multiphase acquisitions during the arm abduction and rest positions. Also, bolus injection at a high rate results in a streak artifact from contrast material in the superior vena cava, which may obscure disorders in these regions.


Contrast-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) with provocative arm positioning has emerged as the primary imaging tool to evaluate patients with TOS. Contrast-enhanced 3D MRA is a noninvasive, user-independent imaging modality that does not affect renal function when used in the recommended amount in patients who have normal renal function (glomerular filtration rate >30 mL/min). For vascular TOS assessment, flow-based bright-blood MR venography (MRV) including time of flight (TOF) has been used. However, it is limited to a two-dimensional implementation because of signal saturation of slow flow. TOF is also more time consuming and images may be impaired by breathing artifacts.


This article reviews the anatomy of the thoracic outlet, MR imaging techniques for evaluation of vascular TOS, and imaging features of vascular TOS.




Introduction


Thoracic outlet syndrome (TOS) is a constellation of symptoms caused by impingement of the subclavian vessels and brachial plexus during their passage from the thoracic cavity to the axilla. TOS has been classified into several types, including neurogenic TOS (nTOS), arterial TOS (aTOS), and venous TOS (vTOS). The true incidence of TOS is controversial, and has been reported to range from 0.3% to 8%. nTOS accounts for almost 90% of cases of TOS, whereas less than 10% of patients have only vascular or combined symptoms. The female/male ratio for nTOS is 3.5:1 and there is no sex predilection for the arterial type. vTOS is traditionally considered to be male predominant; however, the largest study reported a similar proportion of men and women.


Underlying causes for compression are congenital or acquired factors, such as cervical rib, long C7 transverse process, exostosis, hypertrophic callus, congenital fibromuscular anomalies, posture, repetitive movements, and posttraumatic fibrosis of the scalene muscle. aTOS is almost always associated with bone abnormalities, such as cervical rib, callus, or exostosis, whereas venous disorder is caused by repetitive injury to the subclavian vein by the first rib, clavicle, subclavius muscle, anterior scalene muscle, and costoclavicular ligament. Both vTOS and aTOS usually develop in young, healthy patients with few if any comorbid conditions.


The symptoms caused by arterial compression are pain, claudication, pallor, and coldness; however, pain and edema can be seen with vTOS. Edema of the upper extremity is the hallmark for venous compression, especially when it is associated with thrombosis (effort thrombosis, which is also known as Paget-Schroetter syndrome). Pain is the most common feature in aTOS, although some patients with venous compression experience paresthesia caused by swelling rather than nerve compression. Although rare, potential severe complications have been reported, including venous gangrene of the hand, pulmonary embolism as a result of venous compression or digital ischemia, and stroke associated with aTOS ( Table 1 ).



Table 1

Vascular TOS: contrast-enhanced 3D MRA protocols for 1.5-T and 3-T MR imaging scanners


























aTOS vTOS
Sex Female/male equally
Age (y) 20–30 20–35
Risk factors Bone abnormalities (cervical rib and articulating with the first rib as a pseudoarthrosis; anomalous first rib; long C7 transverse process)
Congenital fibrocartilaginous bands associated with the anterior scalene muscle
Hypertrophic callus from healed clavicle or first rib fracture
Vigorous exercise or activity involving affecting extremity
Repetitive injury contributed by first rib, clavicle, subclavius muscle, and fibrous costoclavicular ligament leads to perivenous fibrosis and endothelial injury
Posture
Sign and symptoms Pain
Claudication
Pallor
Coldness
Paresthesia
Hand swelling caused by edema
Feeling of tightness worsens with exertion
Venous engorgement with collateralization
Potential severe complications Digital ischemia
Stroke
Venous gangrene of the hand
Pulmonary embolism


TOS can be diagnosed with history and a physical examination that includes provocative tests. However, imaging is required to identify vascular abnormalities. Conventional digital subtraction angiography (DSA) has been considered the reference standard for diagnosis of vascular TOS. However, DSA has potential risks, including nephrotoxicity from iodinated contrast agent, arterial puncture site complications, ionizing radiation, and rarely stroke. Also, it requires separate procedures for arteries and veins; therefore, DSA is reserved for minimally invasive interventions and preoperative evaluation of the vessel anatomy to determine surgical approach or bypass graft planning. Computed tomography angiography (CTA) can be used as an alternative to DSA to reveal vascular anatomy and disorders using multiplanar reformatted techniques and maximal intensity projections (MIPs). However, similar to DSA, it requires iodinated contrast agent and considerable irradiation, particularly with multiphase acquisitions during the arm abduction and rest positions. Also, bolus injection at a high rate results in a streak artifact from contrast material in the superior vena cava, which may obscure disorders in these regions.


Contrast-enhanced three-dimensional (3D) magnetic resonance angiography (MRA) with provocative arm positioning has emerged as the primary imaging tool to evaluate patients with TOS. Contrast-enhanced 3D MRA is a noninvasive, user-independent imaging modality that does not affect renal function when used in the recommended amount in patients who have normal renal function (glomerular filtration rate >30 mL/min). For vascular TOS assessment, flow-based bright-blood MR venography (MRV) including time of flight (TOF) has been used. However, it is limited to a two-dimensional implementation because of signal saturation of slow flow. TOF is also more time consuming and images may be impaired by breathing artifacts.


This article reviews the anatomy of the thoracic outlet, MR imaging techniques for evaluation of vascular TOS, and imaging features of vascular TOS.




Anatomy


The thoracic outlet is an area located between thorax, shoulder, and under the clavicle. Subclavian vessels course through several narrow passageways in the thoracic outlet ( Fig. 1 ). First, the scalene triangle (interscalene triangle) is delineated by the anterior scalene muscle anteriorly, middle scalene muscle posteriorly, and the medial surface of the first rib inferiorly. The interscalene triangle contains the subclavian artery and brachial plexus. Second, the costoclavicular space is bordered by the clavicle superiorly, the subclavius muscle and costoclavicular ligament anteriorly, and by the first rib and anterior scalene muscle posteriorly. This space contains the subclavian vein. Third, the retropectoralis minor space (subcoracoid space) is limited by the pectoralis minor anteriorly, subscapularis muscle superiorly, and chest wall inferiorly and posteriorly. This space contains the neurovascular bundle.




Fig. 1


Three potential spaces of the thoracic outlet that can cause vascular compression: scalene triangle, costoclavicular space, and pectoralis minor space.

( From Klaassen Z, Sorenson E, Tubbs RS, et al. Thoracic outlet syndrome: a neurological and vascular disorder. Clin Anat 2014;27(5):724–32; with permission.)




Magnetic resonance imaging and protocols


Contrast-enhanced 3D MRA, equilibrium phase venography, and T2-weighted images with combination of provocative maneuvers including both arm abduction and adduction are used to assess vascular abnormalities in patients with TOS. Multiphase acquisition of contrast-enhanced 3D images yields arterial and venous phase images; in addition, equilibrium phase acquisition improves detection of venous abnormalities such as stenosis and the presence of thrombus. Equilibrium phase images are also helpful for identifying extrinsic masses compressing the vascular structures. For better characterization of extra-anatomic structures such as soft tissue masses, T2-weighted sequences should be acquired.


Patient Preparation and Positioning


A 20-gauge peripheral intravenous line should be obtained in the forearm or antecubital fossa, before placing the patient on the table in a supine position. Multichannel phased-array coils provide better signal and excellent coverage. Patients should be instructed to hold their breath during the image acquisitions. All sequences should be acquired during 150° to 160° of bilateral arm abduction with head and neck in the neutral position and repeated during arm adduction position with additional contrast administration.


Magnetic Resonance Protocols and Image Acquisition


MRA examination can be performed on a 1.5-T or 3-T scanner; therefore imaging parameters should be adjusted accordingly. Before injecting contrast agent, T2-weighted sequences (HASTE [Half-Fourier acquisition single-shot turbo spin-echo]) and precontrast mask images should be obtained. For contrast-enhanced MRA, 20 mL of intravenous extracellular contrast agent (ECA) such as gadobenate dimeglumine (0.5 mol/L; MultiHance, Bracco Diagnostics) can be used with a 20-mL saline flush at a rate of 2 mL/s during abduction positioning. In addition, 15 mL of ECA should be administered for the same contrast-enhanced sequences in the rest position; that is, with the arms next to the torso. The timing of the MRA acquisition can be determined by fluoroscopic real-time monitoring or by using a test bolus, or via automatic triggering at contrast agent arrival. For each arm positioning, 2 sets of coronal oblique 3D slabs should be acquired. Recently a blood-pool agent (BPA), gadofosveset trisodium (gadofosveset) was approved by the US Food and Drug Administration for MRA in the United States and single-injection BPA MRA has been shown to have diagnostic image quality and high vessel contrast, similar to double-injection imaging. The gadolinium-based contrast agents shorten T1 relaxation time of the blood and signal of the vascular contrast is independent of flow dynamics. 3D gradient echo (GRE) sequences with ultrashort recovery time (TR) and echo time (TE) are performed to obtain fast imaging speed and a large field of view with high spatial resolution in a single held breath. For optimal contrast enhancement, the center of k-space should be obtained when the contrast agent concentration is at its peak in the vessel of interest. Equilibrium phase imaging should be initiated around 5 minutes after injection with both axial and coronal orientations during breath holding. This sequence also produces T1-weighted 3D images using interpolation and/or partial Fourier technique. Typically, the less symptomatic arm is used for injection and contrast-enhanced imaging (3D MRA and volume interpolated gradient echo) is performed in abduction first, with identical sequences repeated in adduction. The detailed pulse sequences and their imaging parameters are described in Table 2 .


Sep 18, 2017 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on State-of-the-art Magnetic Resonance Imaging in Vascular Thoracic Outlet Syndrome
Premium Wordpress Themes by UFO Themes