Chest, Thoracic & Brachial Plexus MRI

Chest, Thoracic & Brachial Plexus MRI

Mediastinal & Thoracic Visceral Anatomy

The thoracic cavity is anatomically partitioned into the pulmonary cavities laterally and the mediastinum centrally. The mediastinum is subdivided into distinct anatomical compartments, each harboring specific visceral organs and pathological entities:

• Superior Mediastinum: Extends from the superior thoracic aperture (thoracic inlet) down to the horizontal plane connecting the sternal angle (angle of Louis) to the T4–T5 intervertebral disc space. It contains the thymus gland (or thymic remnant in adults), brachiocephalic (innominate) veins, superior vena cava (SVC), aortic arch and its three branches, trachea, esophagus, thoracic duct, and the vagus and phrenic nerves.
• Anterior Mediastinum: Lies anterior to the pericardium and posterior to the sternal body. It houses the inferior thymus, internal thoracic lymph nodes, and loose connective tissue. Common masses include the "4 Ts": Thymoma, Teratoma, Thyroid (substernal goiter), and "Terrible" Lymphoma.
• Middle Mediastinum: Bounded by the anterior and posterior pericardial surfaces. It contains the heart, pericardial sac, ascending aorta, superior and inferior vena cava terminations, main pulmonary artery and its branches, pulmonary veins, carina, right and left mainstem bronchi, and hilar lymph nodes.
• Posterior Mediastinum: Positioned posterior to the pericardium and diaphragm, anterior to the T5–T12 vertebral bodies. It contains the descending thoracic aorta, azygos and hemiazygos veins, esophagus, thoracic duct, sympathetic trunk, and posterior intercostal neurovascular bundles. Neurogenic tumors (schwannomas, neurofibromas) predominate here.

The Great Thoracic Vessels

The ascending thoracic aorta emerges from the left ventricle, arching superiorly and posteriorly toward the left to form the aortic arch. The aortic arch typically gives rise to three primary branches:

1. Brachiocephalic (Innominate) Artery: Bifurcates into the right subclavian and right common carotid arteries.
2. Left Common Carotid Artery: Supplies the left head and neck.
3. Left Subclavian Artery: Supplies the left upper extremity and vertebrobasilar circulation.

The aorta then transitions into the descending thoracic aorta, coursing along the left side of the spine before traversing the aortic hiatus of the diaphragm at the T12 level.

Brachial Plexus Neuroanatomy & Spatial Tunnels

The brachial plexus is an intricate network of nerve fibers providing motor and sensory innervation to the shoulder, chest wall, and entire upper extremity. It arises from the ventral rami of spinal nerve roots C5, C6, C7, C8, and T1, with variable contributions from C4 or T2.

Brachial Plexus Architecture & Spatial Tunnels:
Roots (C5–T1) ➔ Interscalene Triangle (Trunks: Superior, Middle, Inferior)
➔ Costoclavicular Space (Divisions: Anterior & Posterior) ➔ Retropectoral Space (Cords: Lateral, Posterior, Medial)
➔ Axilla & Upper Limb (Terminal Branches: Musculocutaneous, Axillary, Radial, Median, Ulnar)

The plexus is organized sequentially into Roots, Trunks, Divisions, Cords, and Terminal Branches (mnemonic: "Reach To Drink Cold Beer"):

1. Roots: The ventral rami of C5–T1 emerge from the neural foramina between the transverse processes of the cervical and upper thoracic spine.
2. Trunks (Interscalene Triangle): Within the interscalene triangle—bounded by the anterior scalene muscle anteriorly, the middle scalene muscle posteriorly, and the first rib inferiorly—the five roots unite into three trunks:
   • Superior Trunk: Formed by the union of C5 and C6.
   • Middle Trunk: Formed by the continuation of C7.
   • Inferior Trunk: Formed by the union of C8 and T1. The subclavian artery accompanies the trunks through this space, while the subclavian vein runs anterior to the anterior scalene muscle.
3. Divisions (Costoclavicular Space): Behind the clavicle, within the costoclavicular space (between the clavicle and the first rib), each trunk divides into an anterior division (supplying flexor compartments) and a posterior division (supplying extensor compartments).
4. Cords (Retropectoral Space): Deep to the pectoralis minor tendon, within the retropectoral/subcoracoid space, the six divisions recombine into three cords named according to their spatial orientation relative to the axillary artery:
   • Lateral Cord: Formed by the anterior divisions of the superior and middle trunks.
   • Posterior Cord: Formed by the posterior divisions of all three trunks (superior, middle, inferior).
   • Medial Cord: Formed by the anterior division of the inferior trunk.
5. Terminal Branches: At the axilla, the cords divide into five major peripheral nerves: the Musculocutaneous, Axillary, Radial, Median, and Ulnar nerves.

Patient Positioning, Coil Selection & Landmarking

Patient Positioning & Coil Arrangement

• Patient Orientation: The patient is positioned head-first supine on the scanner table.
• Arm Positioning:
  • For Brachial Plexus Examinations: Both arms must be placed straight along the patient's sides with palms facing the thighs. Support the shoulders in the prescribed neutral position. Do not apply forceful shoulder depression, which can narrow rather than open the costoclavicular space; provocative positions are used only when specifically prescribed.
  • For Thorax / Chest Examinations: Arms are rested alongside the torso or lightly crossed over the upper pelvis to keep them outside the primary chest field of view (FOV).
• Coil Selection: A high-density multi-channel phased-array torso/body coil is positioned securely across the anterior chest wall. For dedicated brachial plexus imaging, a specialized neurovascular array or a head-and-neck coil combined with upper torso phased-array elements provides superior coverage from the skull base through the T2 vertebral level and across both axillae.

Landmarking Coordinates

• Thorax / Chest Centering: The transverse laser crosshair is landmarked at the sternal notch or the angle of Louis (manubriosternal joint), corresponding to the T4–T5 vertebral disc space and placing the aortic arch, heart, and hila at magnet isocenter.
• Brachial Plexus Centering: The transverse laser crosshair is landmarked at the C7–T1 vertebral interspace, approximately at the level of the sternoclavicular joint or supraclavicular fossa. The sagittal laser crosshair is oriented along the mid-sagittal plane for bilateral studies, or shifted slightly toward the symptomatic side for dedicated unilateral brachial plexus imaging.

Physiological Motion Suppression & Gating Mechanics

Thoracic MRI is uniquely susceptible to physiological motion artifacts: cardiac pulsatile blood flow and cardiac chamber motion, respiratory diaphragmatic motion, and vascular pulsatility from the aorta and pulmonary arteries.

1. Cardiac Gating (ECG & VCG)

To eliminate motion blur and ghosting from the beating heart and pulsating great vessels, acquisitions are synchronized to the cardiac cycle using Vectorcardiography (VCG) or electrocardiography (ECG):

• Electrode Placement: Four MR-compatible non-ferrous electrodes are applied to the anterior chest over the precordium, arranged to maximize the amplitude of the electrical R-wave while minimizing magnetohydrodynamic (MHD) voltage distortion.
• MHD Effect: Flowing conductive blood ions (e.g., sodium, chloride) in the aortic arch moving perpendicularly through the strong static magnetic field ($B_0$) generate an induced electrical voltage ($V = v \times B_0 \times d$). This voltage artificially elevates the T-wave on the monitor trace. Advanced VCG algorithms utilize spatial vector analysis to accurately distinguish the true QRS complex from the flow-induced elevated T-wave, preventing false triggering.
• Acquisition Window: Sequences are triggered to acquire data during ventricular diastole, when cardiac motion and blood acceleration are minimized.

2. Respiratory Motion Suppression

• Respiratory Bellows: A pneumatic bellow strap is placed snugly around the upper abdomen/lower rib cage, tracking chest expansion and generating a waveform used for prospective triggering.
• Prospective Navigator Gating: A 2D pencil-beam navigator echo is placed across the right diaphragmatic dome. The scanner dynamically samples diaphragmatic excursion and accepts imaging data only when the liver-lung boundary resides within an end-expiratory "acceptance window" (typically 3–5 mm), permitting free-breathing acquisitions with high spatial resolution.
• Breath-Holding: T1-weighted gradient-echo or dynamic post-contrast sequences require coaching the patient to maintain suspended end-expiration breath-holds (15–20 seconds).

Clinical Indications & Pathological Conditions

Pancoast Tumor (Superior Sulcus Carcinoma)

A Pancoast tumor is a malignant neoplasm (most commonly non-small cell lung carcinoma) arising in the superior pulmonary sulcus at the extreme thoracic apex. Due to its location, it directly invades adjacent structures:

• Neural Invasion: Infiltration into the C8, T1 nerve roots and the inferior trunk of the brachial plexus causes intractable neuropathic shoulder pain radiating down the medial arm, forearm, and fourth/fifth digits (ulnar nerve distribution).
• Cervical Sympathetic Chain & Stellate Ganglion: Direct invasion produces classic Horner syndrome, characterized by the clinical triad of ipsilateral ptosis (drooping eyelid), miosis (pupillary constriction), and anhidrosis (loss of facial sweating).
• Vascular Encasement: Extends to encase or occlude the subclavian artery and vein.

MRI is superior to CT in delineating whether the tumor invades the neural foramina, spinal canal, or brachial plexus trunks, determining surgical resectability.

Thoracic Outlet Syndrome (TOS)

Thoracic Outlet Syndrome encompasses symptomatic compression of the neurovascular bundle passing through the three thoracic outlet spaces:

• Neurogenic TOS (95% of cases): Compression of the brachial plexus trunks (predominantly the inferior trunk) within the interscalene triangle or costoclavicular space, often exacerbated by a cervical rib, hypertrophied anterior scalene muscle, or anomalous fibrous band.
• Venous TOS (Paget-Schroetter Syndrome): Compression and thrombosis of the subclavian vein in the costoclavicular space, frequently seen in competitive throwing athletes.
• Arterial TOS: Compression and aneurysm formation of the subclavian artery.

MRI protocols for TOS frequently incorporate dynamic postural maneuvers: scanning the patient in the neutral position (arms at sides) followed by hyperabduction (arms elevated overhead) to reveal positional compression.

Acute Aortic Syndromes & Mediastinal Masses

• Aortic Dissection: Tear in the aortic intima permitting pressurized blood to separate the aortic media into a true lumen and a false lumen:
  • Stanford Type A: Involves the ascending aorta; represents an immediate surgical emergency requiring open repair.
  • Stanford Type B: Confined to the descending aorta distal to the left subclavian artery; managed medically with tight blood pressure control unless end-organ ischemia develops.
• Mediastinal Tumors: Evaluating tissue invasion, vascular encasement, and neural extension in thymomas, lymphomas, and posterior mediastinal neurogenic schwannomas.

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