Brachial Plexus MRI for Neurogenic Thoracic Outlet Syndrome

Version 1.0 — August 2026

1. Executive Summary

1.1 Added Value over the Generic Protocol

The generic Brachial Plexus MRI protocol is designed primarily for traumatic plexopathy, tumour/neurofibromatosis surveillance, and inflammatory plexitis (Parsonage-Turner syndrome) — all conditions in which the plexus is imaged in a single, neutral arm position. Neurogenic thoracic outlet syndrome (nTOS) is fundamentally different: it is a dynamic, positional compressive neuropathy, and the master page itself explicitly flags “thoracic outlet syndrome (requires dynamic sequences)” as a scenario the generic protocol does not fully address. This child page documents exactly what changes: dual-position (neutral and provocative-abduction) acquisition, the specific anatomic compartments of the thoracic outlet that must be interrogated, perineural fat-signal assessment as the key direct sign, and the differential diagnosis against the far more common cervical radiculopathy and ulnar/median entrapment neuropathies that mimic nTOS clinically.

1.2 Limits of the Dedicated Protocol

Even with dedicated dynamic imaging, MRI’s diagnostic performance in nTOS remains an area of genuine, acknowledged uncertainty in the literature: nTOS is fundamentally a clinical diagnosis, supported — not replaced — by imaging, electrodiagnostic testing, and diagnostic scalene block. A structurally unremarkable dynamic MRI does not exclude nTOS, and conversely, some degree of positional plexus compression can be demonstrated in asymptomatic volunteers, particularly at the costoclavicular space. This child page documents MRI’s specific, genuine contribution — direct visualisation of compression and its anatomic cause — without overstating its stand-alone diagnostic authority.

2. Clinical Context

2.1 Clinical Presentation

Neurogenic TOS accounts for the large majority of all TOS presentations in adults (arterial and venous TOS are comparatively rare) and typically presents with positional upper-limb paraesthesia, pain, and — in more advanced or “true” nTOS — hand weakness and intrinsic muscle atrophy, classically in an ulnar (lower trunk/medial cord) distribution, exacerbated by arm elevation or overhead activity. Symptom onset is frequently insidious and associated with repetitive overhead activity, prior clavicular or first-rib trauma, or an anatomic predisposing variant (cervical rib, elongated C7 transverse process, anomalous fibrous band).

2.2 Anatomic Basis — the Three Compression Sites

The thoracic outlet comprises three sequential anatomic compartments through which the brachial plexus (and subclavian vessels) must pass, each a potential compression site: the interscalene (scalene) triangle, bounded by the anterior and middle scalene muscles and the first rib; the costoclavicular space, between the clavicle and first rib; and the retropectoralis minor (subcoracoid) space, beneath the pectoralis minor tendon. Unlike arterial and venous TOS, which localise predominantly to the costoclavicular space, neurogenic compression can occur at any of the three sites, and dynamic MRI must interrogate all three rather than assuming a single dominant location.

2.3 Differential Diagnosis (Clinical)

nTOS is notoriously difficult to distinguish clinically from cervical radiculopathy (particularly C8-T1), ulnar neuropathy at the elbow, carpal tunnel syndrome, and non-specific brachialgia — a diagnostic overlap explicitly identified as central to the difficulty of nTOS diagnosis in both the ACR Appropriateness Criteria and the Society for Vascular Surgery reporting standards. Dedicated brachial plexus MRI is specifically requested when this clinical overlap cannot be resolved by history, examination, and electrodiagnostic testing alone.

3. Indications, Timing, and Patient Selection

3.1 When the Dedicated Protocol Is Indicated

Per the ACR Appropriateness Criteria for thoracic outlet syndrome, dedicated MRI of the brachial plexus with provocative positioning is appropriately requested when neurogenic TOS remains a genuine diagnostic consideration after clinical assessment, particularly to identify a surgically correctable anatomic cause (cervical rib, anomalous fibrous band, hypertrophic or accessory scalene musculature) or to support surgical planning (e.g. selection for first-rib resection).

3.2 When a Standard Neck MRI Is Not Sufficient

A routine cervical spine or general neck MRI is explicitly insufficient for nTOS assessment: it neither extends far enough laterally to cover the full costoclavicular and retropectoralis minor spaces, nor includes the dynamic/provocative-position acquisition on which nTOS diagnosis specifically depends. This is one of the most frequently identified ordering errors in the imaging literature — a dedicated brachial plexus protocol, explicitly requested as such, is required.

3.3 When Vascular TOS Assessment Should Be Added

Because arterial and venous compression can coexist with neurogenic compression, and because clinical differentiation between subtypes is imperfect, a contrast-enhanced MR angiography component in the provocative position should be added whenever clinical or examination findings suggest a vascular contribution (differential arm pulses/pallor, arm swelling or venous distension) — this vascular-specific protocol design is addressed by MR angiography technique documented elsewhere on MRIninja and is not duplicated in this neurogenic-focused child page.

3.4 Red Flags Modifying Urgency

Acute-onset arm swelling, cyanosis, or absent/diminished pulses raise concern for acute venous (Paget-Schroetter, effort thrombosis) or arterial TOS respectively — these are not the focus of this neurogenic-specific child page and warrant urgent vascular imaging pathways rather than the elective, dynamic neurogenic protocol described here.

4. Dedicated Protocol Design

4.1 Mandatory Core Sequences

The table below lists the complete mandatory protocol for neurogenic thoracic outlet syndrome — the five generic-protocol core sequences (1-5) plus the three nTOS-specific additions (6-8) detailed in Section 4.3. See Section 4.2 for the full comparison against the generic protocol.

4.2 Protocol Delta vs the Generic Brachial Plexus Protocol

4.3 Mandatory Dedicated Sequences

• Coronal and sagittal oblique T1-weighted sequences through the thoracic outlet in the neutral position, for baseline anatomy and osseous assessment (cervical rib, first rib morphology).
• Coronal and sagittal oblique fluid-sensitive fat-suppressed sequences (STIR, as per the generic protocol’s standard fat-suppression convention) in the neutral position, assessing baseline perineural fat signal and any intrinsic plexus signal abnormality.
• The same fluid-sensitive fat-suppressed sequence repeated in the provocative (abducted/externally rotated) position — this dual-position acquisition is the single most important departure from the generic protocol and the acquisition most consistently identified across the reviewed literature as necessary for meaningful nTOS assessment.

4.4 Conditional and Advanced Sequences

• 3D heavily T2-weighted sequences (per the generic protocol’s use of CISS/FIESTA-C/DRIVE-family sequences) may be added in the neutral position where fine discrimination of individual plexus elements from adjacent vasculature is required, though these are typically less practical to repeat in the provocative position given positioning and coil constraints.
• Axial sequences through the costoclavicular space in both positions provide a complementary cross-sectional view of space narrowing, particularly useful for direct measurement.

4.5 Rationale per Disease-Specific Sequence

Provocative-position fluid-sensitive fat-suppressed sequence (the core nTOS-specific acquisition) The rationale for this sequence is direct: neurovascular compression in TOS is frequently absent or minimal in the neutral position and becomes apparent specifically with the shoulder abducted, with the most consistent finding being a more marked reduction in the costoclavicular space on abduction. Loss of the normal perineural fat plane around the plexus elements in the provocative position — sometimes describable as the plexus appearing to lose its normal fat-signal “halo” — has been specifically identified as a useful sign for selecting patients for first-rib resection in nTOS. Failure to include this provocative acquisition is explicitly identified in the literature as one of the most common, avoidable protocol errors.

Bilateral, comparative acquisition The rationale for deliberately imaging both sides, even when symptoms are unilateral, is that some degree of positional narrowing of the costoclavicular space and even mild plexus displacement can be demonstrated in asymptomatic volunteers on abduction — meaning an isolated finding of positional narrowing, without a comparison to the contralateral (or population-normal) side, has limited specificity on its own.

Neutral-position osseous-sensitive sequences (T1, and correlation with prior/concurrent radiography or CT when available) The rationale for deliberate osseous assessment is that a substantial proportion of anatomically-explicable nTOS relates to identifiable congenital or acquired bony/fibrous variants — cervical rib, an elongated C7 transverse process, or anomalous fibromuscular bands (including rare variants such as the subclavius posticus muscle) — findings that carry direct surgical planning relevance distinct from the dynamic soft-tissue compression assessment itself.

4.6 Dedicated Planes and Field of View

Coronal oblique planes, angled along the long axis of the brachial plexus as per the generic protocol, remain the primary imaging plane but the field of view must be deliberately extended inferiorly to include the full costoclavicular space and the proximal retropectoralis minor region — a wider coverage than is typically required for the generic protocol’s trauma/tumour indications, where the field of view is more often centred on the supraclavicular plexus alone.

4.7 Contrast Strategy

Neurogenic TOS assessment does not routinely require intravenous gadolinium contrast; the diagnostic information sought (positional compression, perineural fat signal, osseous/fibrous anatomic cause) is derived from the T1/fluid-sensitive dual-position acquisition described above. Contrast is reserved specifically for cases where a mass lesion, active inflammatory plexitis, or a vascular component requiring MR angiography is separately suspected — in which case standard macrocyclic GBCA dosing and post-contrast fat-suppressed T1 as per site-wide contrast policy apply, with STIR (not spectral fat suppression) used for any non-contrast fluid-sensitive sequence per generic-protocol convention, and Dixon or spectral fat saturation reserved specifically for the post-contrast series where a vascular/mass work-up is added.

4.8 Sequence Matching to Clinical Question

5. MRI Semiotics of Neurogenic Thoracic Outlet Syndrome

5.1 Direct Signs

The principal direct sign is demonstrable narrowing of one or more thoracic outlet compartments — most consistently the costoclavicular space — with associated plexus displacement, flattening, or signal change, specifically or predominantly in the provocative position, with relative preservation of normal anatomy and fat planes in the neutral position. Loss of the normal perineural fat signal (“fat halo”) around the compressed plexus segment in the provocative position is a specifically described, clinically useful finding.

5.2 Indirect and Secondary Signs

Secondary signs include an identifiable anatomic substrate for compression — cervical rib, elongated C7 transverse process, hypertrophic scalene musculature (including the scalenus minimus variant), an anomalous fibrous band, or an aberrant muscle such as the subclavius posticus — and, in more chronic or severe cases, denervation-pattern muscle oedema/atrophy in the plexus-dependent forearm and hand musculature, analogous in mechanism to the denervation changes described for traumatic plexopathy in the generic protocol but distributed according to the specific trunk/cord affected (most often the lower trunk/medial cord in classic nTOS).

5.3 Severity, Extent, and Site Localisation

Reporting should specify which of the three compartments (interscalene, costoclavicular, retropectoralis minor) shows the compression, whether it is present only in the provocative position or also, to a lesser degree, in neutral, and whether the abnormality is unilateral or bilateral (asymmetric bilateral compression is a recognised pattern given the frequency of anatomic predisposition being bilateral even when symptoms are unilateral).

5.4 Relevant Classification and Reporting Frameworks

The Society for Vascular Surgery reporting standards for TOS (Illig et al.) provide the current consensus framework for case definition, severity grading, and standardised terminology across neurogenic, venous, and arterial TOS, and are the appropriate reference framework for structuring a report intended to support multidisciplinary (vascular surgery, thoracic surgery, physiotherapy) decision-making rather than radiology-only interpretation.

5.5 Differential Diagnosis on Imaging

The principal imaging differential is a structurally normal or near-normal study in a patient with a genuine clinical nTOS presentation — reflecting the condition’s substantial clinical-imaging discordance rather than a true alternative diagnosis. True structural alternative diagnoses to consider and specifically exclude on the same study include cervical disc herniation/foraminal stenosis with radiculopathy (assessed on the cervical spine component of the generic protocol when included), a discrete brachial plexus mass or nerve sheath tumour, and inflammatory plexitis (Parsonage-Turner), both of which are addressed by the generic protocol and should prompt reporting outside the neurogenic-TOS-specific framework if identified.

5.6 Mimickers and Pitfalls

Positional costoclavicular space narrowing and even mild plexus displacement on abduction can be demonstrated in a substantial proportion of asymptomatic individuals — the single most important interpretive pitfall in this protocol, and the specific reason bilateral, comparative, symptom-correlated interpretation (Section 5.3) rather than an isolated positive finding on the symptomatic side alone is required for a clinically meaningful report.

6. Reporting Framework

6.1 Structured Reporting Template

Compartments assessed: interscalene triangle, costoclavicular space, retropectoralis minor space (bilateral, neutral and provocative position). Positional compression identified: site(s), side(s), position(s) in which present. Perineural fat signal: preserved / lost, by compartment and position. Anatomic substrate identified: cervical rib / elongated C7 transverse process / anomalous fibrous band or muscle / hypertrophic scalene musculature / none identified. Denervation change: present/absent, muscle groups involved. Comparison with asymptomatic/contralateral side: concordant / discordant with symptomatic side.

6.2 Mandatory Reporting Elements

Every report should explicitly state whether the provocative-position acquisition was successfully obtained and diagnostic (positioning-related technical failure is a recognised limitation of this protocol), and should explicitly avoid over-calling isolated costoclavicular narrowing as diagnostic of nTOS without correlating it against the comparison side and the specific clinical distribution of symptoms.

6.3 Critical/Actionable Findings

An identifiable, surgically addressable anatomic cause (cervical rib, discrete fibrous band, aberrant muscle) is the most directly actionable finding, since it provides a specific surgical target beyond generic scalenectomy/first-rib resection, and should be flagged explicitly and separately from the general compression assessment.

6.4 Common Reporting Errors

Reporting costoclavicular narrowing on the provocative position alone, without stating whether the same finding is present on the asymptomatic/contralateral side or in normal volunteers generally, overstates the finding’s specificity; conversely, reporting a “normal” study without explicitly confirming that a genuine provocative-position acquisition was obtained risks a false-reassurance report when the key diagnostic manoeuvre was technically inadequate.

7. Technical Pitfalls

7.1 Positioning Failure

The provocative position must reproduce genuine shoulder abduction/external rotation (the classic “hands-up” or Roos-type position) sufficient to provoke costoclavicular narrowing; an inadequately abducted or inconsistently positioned “provocative” acquisition — a common technical shortfall given coil and bore-space constraints — undermines the entire diagnostic rationale of the dedicated protocol.

7.2 Sequence-Specific Technical Considerations

Motion and positioning-related artefact is more pronounced in the provocative position than in a standard neutral-arm brachial plexus acquisition, given the more extreme and less comfortable patient positioning required and typically longer effective scan time in that position; technologists should anticipate a higher rate of motion-degraded provocative-position series and plan sequence ordering accordingly (acquiring the provocative series earlier, before patient fatigue, is a reasonable practical strategy).

7.3 When the Generic Protocol Alone Is Insufficient

Any brachial plexus MRI performed for suspected nTOS using only the generic, neutral-position protocol should be recognised as inherently limited for this specific indication — per Section 3.2, a request explicitly for the dedicated dynamic protocol, not the generic one, is required whenever nTOS is the leading clinical concern.

8. MRI Technologist Pearls

8.1 Positioning Strategy

Achieving genuine, reproducible provocative-position abduction within the practical constraints of a closed-bore scanner and receive coil placement requires deliberate attention — coil selection and patient coaching for the “hands-up” position should be planned before the patient enters the bore, not improvised once positioning proves difficult.

8.2 Sequence Ordering Logic

Given the higher risk of motion/positioning degradation in the provocative position (Section 7.2), acquiring the provocative-position series before the neutral-position series (reversing the generally intuitive “baseline first” ordering) can reduce the risk of a non-diagnostic key acquisition due to patient fatigue or positioning drift over a long examination.

8.3 Fast Salvage Protocol

If time or patient tolerance is genuinely limited, prioritise the bilateral, provocative-position fluid-sensitive fat-suppressed sequence (Section 4.3) above all other sequences in this protocol — it is the single acquisition most directly tied to this child page’s diagnostic rationale and the one most frequently reported as omitted in incomplete studies.

8.4 Disease-Specific Common Avoidable Errors

Requesting or performing only a standard neck/cervical spine MRI when nTOS is the clinical question (Section 3.2); omitting the provocative-position acquisition entirely; failing to image bilaterally for comparison; and failing to extend the field of view far enough inferiorly/laterally to include the full costoclavicular and retropectoralis minor compartments.

9. Quality Control Checklist

• Both neutral and genuine provocative (abducted/externally rotated) positions successfully acquired and diagnostic quality confirmed before the patient leaves the department.
• Bilateral coverage confirmed, with both sides assessed on the same sequences for direct comparison.
• Field of view confirmed to extend through all three thoracic outlet compartments (interscalene, costoclavicular, retropectoralis minor) bilaterally.
• Osseous/fibrous anatomic survey (cervical rib, C7 transverse process, scalene anatomy) explicitly reviewed, not only the soft-tissue plexus itself.
• Clinical indication (specifically “neurogenic TOS” / “dynamic brachial plexus assessment”) confirmed as correctly communicated to protocol against, distinguishing this request from a routine plexus or neck MRI order.

11. Evidence Gaps and Ongoing Debate

• MRI’s stand-alone diagnostic accuracy for nTOS remains genuinely debated. As acknowledged in the systematic scoping review of published protocols, MRI protocol reporting across the existing literature is frequently incomplete, and no fully standardised, universally validated dynamic MRI protocol for nTOS yet exists — the dual-position approach documented in this child page reflects the most consistent current practice, not a single, universally mandated technique.
• The specificity of positional costoclavicular narrowing is incompletely characterised. Because some degree of positional narrowing is demonstrable in asymptomatic individuals, the precise diagnostic threshold distinguishing clinically significant compression from a normal anatomic variant remains an active area of investigation rather than a settled quantitative cut-off.
• Optimal correlation between imaging findings and surgical outcome is still being established. While loss of perineural fat signal in the provocative position has been proposed as a useful predictor of benefit from first-rib resection, this and related imaging-based patient-selection criteria are based on comparatively small case series to date, and larger prospective validation remains an open area of research, as explicitly acknowledged in the Society for Vascular Surgery reporting standards’ call for more consistent outcome reporting in NTOS specifically.

12. Evidence-Based References

A. Guidelines / Consensus / Society Recommendations

B. Systematic Reviews / Meta-analyses

C. Important Prospective / Original Studies

D. Technical MRI Papers

The technical MRI literature specific to nTOS positional acquisition is directly represented by the Szaro et al. systematic scoping review already listed in Category B above; a separate, non-duplicative Category D entry is not populated for this child page to avoid citing the same source twice under different categories.

E. Landmark Historical References

No landmark historical (pre-modern-MRI-era) reference specific to neurogenic thoracic outlet syndrome MRI was identified as distinct from the original studies already listed in Category C; Category E is therefore not populated for this child page.

End of document — Brachial Plexus MRI for Neurogenic Thoracic Outlet Syndrome — Child Protocol under the MRIninja Neck / Brachial Plexus master page — v1.0 — August 2026 Parent page: Brachial Plexus MRI — Generic Standard Protocol