4.5 Musculoskeletal Joints

4.5 Musculoskeletal Joints

Introduction to Musculoskeletal Joint MRI

Musculoskeletal (MSK) joint imaging represents a significant portion of clinical MRI. Achieving diagnostic-quality images requires an understanding of joint anatomy, dedicated coil selection, and precise slice planning. Because joints consist of complex, multi-planar structures (cartilage, ligaments, tendons, and labral structures), scan planes must be tailored to specific anatomical landmarks to prevent volume averaging and diagnostic errors.

Coil Selection Principles

In MSK imaging, coil selection directly impacts the spatial resolution and Signal-to-Noise Ratio (SNR).

• Phased-Array Coils: Multi-channel, form-fitting coils are the gold standard. They combine the high SNR of small surface coils with the large field of view (FOV) of volume coils. Phased-array coils support parallel imaging (e.g., SENSE or GRAPPA), reducing scan times for long fast spin echo (FSE) trains.
• Volume Coils: Transmit/receive volume coils (e.g., rigid knee coils) provide uniform RF excitation (B1 homogeneity) and signal reception across the entire joint.
• Local Surface Coils: Used for small, superficial structures (e.g., temporomandibular joints or single fingers). They provide high SNR close to the coil surface but suffer from rapid signal drop-off as depth increases.

Upper Extremity Joint Protocol and Slice Planning

1. Shoulder

• Positioning: The patient is supine, head-first. The arm is placed in a neutral to slightly externally rotated position. Internal rotation must be avoided, as it causes the infraspinatus tendon to wrap tightly, mimicking a rotator cuff tear on coronal views.
• Coil: Phased-array shoulder coil, positioned close to the shoulder joint with the humeral head at isocenter.
• Slice Prescriptions:
  • Axial: Prescribed from a coronal localizer. Slices are perpendicular to the glenohumeral joint line. They cover from the acromioclavicular (AC) joint superiorly to the inferior margin of the glenoid labrum.
  • Coronal Oblique: Prescribed from the axial localizer. Slices are oriented parallel to the long axis of the supraspinatus tendon or the scapular blade. This plane profiles the supraspinatus tendon, subacromial space, and superior/inferior labrum.
  • Sagittal Oblique: Prescribed from the axial localizer. Slices are oriented parallel to the glenoid fossa (perpendicular to the supraspinatus tendon). This plane is vital for evaluating rotator cuff muscle bulk, labral tears, and subacromial arch morphology.

2. Elbow

• Positioning: Ideally, the patient is placed prone or supine in the "Superman" position (arm extended overhead, palm down). This places the elbow at the magnet's physical isocenter, maximizing B0 homogeneity and fat suppression. If the patient cannot tolerate this, the arm is placed at the side, but off-center positioning decreases fat suppression quality.
• Coil: Dedicated elbow coil (phased-array) or small extremity coil.
• Slice Prescriptions:
  • Axial: Perpendicular to the humeral shaft, covering from 5 cm proximal to the joint line down to the radial tuberosity.
  • Coronal: Parallel to a line connecting the medial and lateral humeral epicondyles. This displays the collateral ligaments (ulnar and radial) in profile.
  • Sagittal: Perpendicular to the coronal plane (parallel to the humeral shaft), covering from the medial to lateral cutaneous margins.

3. Wrist

• Positioning: Like the elbow, the "Superman" position is preferred to keep the wrist at isocenter. The wrist is secured to prevent micro-motion.
• Coil: Dedicated wrist coil (multi-channel phased-array) or high-density surface coil.
• Slice Prescriptions:
  • Axial: Perpendicular to the long axis of the radius and ulna, covering from the distal radioulnar joint (DRUJ) through the proximal metacarpals.
  • Coronal: Aligned parallel to the pronator quadratus muscle or a line connecting the radial and ulnar styloid processes. This profiles the triangular fibrocartilage complex (TFCC) and scapholunate ligament.
  • Sagittal: Perpendicular to the coronal plane, covering from the radial to ulnar borders.

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Lower Extremity Joint Protocol and Slice Planning

1. Hip

• Positioning: Supine, feet first. Crucially, the feet are internally rotated 15 degrees and taped together. This rotation extends the femoral necks, placing them parallel to the coronal plane to prevent foreshortening of the femoral head-neck junction.
• Coil: Large body phased-array coil, centered over the pelvic girdle.
• Slice Prescriptions:
  • Axial: Perpendicular to the pelvic midline (axial localizer). Slices cover from the anterior superior iliac spine (ASIS) down to 2 cm below the lesser trochanter.
  • Coronal: Parallel to a line running through the centers of both femoral necks (bilateral hip protocol) or parallel to the femoral neck axis (unilateral).
  • Sagittal: Perpendicular to the coronal plane, aligned to the femoral shaft.

2. Knee

• Positioning: Supine, feet first. The knee is placed in the coil and rotated 5 degrees externally. This rotation aligns the anterior cruciate ligament (ACL) parallel to the sagittal plane, allowing it to be visualized on a single sagittal slice.
• Coil: Transmit/receive volume knee coil or dedicated phased-array.
• Slice Prescriptions:
  • Axial: Prescribed from the sagittal localizer, parallel to the femoral condyle articular surfaces (or patellar joint line), covering the patella through the tibial tuberosity.
  • Coronal: Prescribed from the axial localizer, parallel to the posterior margins of the femoral condyles.
  • Sagittal: Prescribed from the coronal/axial localizers, angled 10 to 15 degrees externally (parallel to the lateral femoral condyle or the ACL).

3. Ankle

• Positioning: Supine, feet first. The ankle is positioned at a 90-degree angle (dorsiflexed) to put tension on the ligaments and Achilles tendon, preventing folding artifacts.
• Coil: Dedicated foot/ankle coil.
• Slice Prescriptions:
  • Axial/Axial-Oblique: Perpendicular to the tibia/fibula shaft, covering from the distal tibia to the plantar aspect of the calcaneus.
  • Coronal: Parallel to the bimalleolar line (connecting the medial and lateral malleoli).
  • Sagittal: Parallel to the long axis of the Achilles tendon or perpendicular to the coronal plane.

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Contrast Mechanisms and Pulse Sequence Selection in Joint Imaging

In joint imaging, pulse sequences must balance tissue contrast and spatial resolution:

• Proton Density (PD) and PD Fat-Suppressed (PD FS): These are the workhorse sequences of MSK joint imaging. PD provides excellent detail of fibrocartilage (menisci and labrum), articular cartilage, and ligaments. Adding fat suppression (PD FS) improves contrast between fluid/edema (high signal) and surrounding tissues.
• T1-Weighted (T1W) SE: Excellent for defining anatomy, bone marrow infiltration, and trabecular bone patterns. T1 is the primary sequence for identifying fractures, osteomyelitis, and avascular necrosis (AVN).
• T2-Weighted (T2W) FSE with Fat Saturation: Highly sensitive to fluid, joint effusions, and bone marrow edema.
• Gradient Recalled Echo (GRE): Utilized to evaluate thin articular cartilage layers and detection of hemosiderin (e.g., pigmented villonodular synovitis - PVNS) due to susceptibility effects.
• MR Arthrography (Direct vs. Indirect):
  • Direct Arthrography: Involves intra-articular injection of a dilute gadolinium mixture (typically 1:200 dilution with saline or iodinated contrast) directly into the joint under fluoroscopic guidance, followed by T1-weighted fat-saturated sequences. It is the gold standard for labral tears in the shoulder and hip, and TFCC tears in the wrist.
  • Indirect Arthrography: Contrast is injected intravenously, and the patient is instructed to exercise the joint for 10-15 minutes prior to scanning to allow contrast to diffuse into the joint space.

MSK Joint Protocol Optimization (Summary Table)