Wrist & Hand MRI Positioning

Wrist & Hand MRI Positioning

Musculoskeletal Anatomy of the Wrist & Hand

The wrist (carpus) forms the biomechanical bridge connecting the forearm to the hand. Its osseous framework comprises eight carpal bones arranged in two distinct transverse rows:

Carpal Bone Architecture (Lateral to Medial):
- Proximal Row: Scaphoid ➔ Lunate ➔ Triquetrum ➔ Pisiform
- Distal Row:   Trapezium ➔ Trapezoid ➔ Capitate ➔ Hamate
Mnemonic: "Some Lovers Try Positions That They Cannot Handle"

1. The Proximal Carpal Row

• Scaphoid (Navicular): The largest bone in the proximal row, spanning across both rows to act as a mechanical strut. It articulates proximally with the scaphoid fossa of the distal radius. The scaphoid receives its arterial blood supply in a retrograde manner (from distal to proximal via branches of the radial artery entering at the waist). Consequently, fractures through the scaphoid waist carry a high risk of ischemic non-union and avascular necrosis (AVN) of the proximal pole .
• Lunate: The central, crescent-shaped bone articulating with the lunate fossa of the radius. Avascular necrosis of the lunate, known as Kienböck's disease, leads to progressive sclerosis, fragmentation, and carpal collapse, frequently associated with ulnar negative variance (short ulna).
• Triquetrum: A pyramidal bone articulating with the TFCC and lunate.
• Pisiform: A small pea-shaped sesamoid bone embedded entirely within the flexor carpi ulnaris (FCU) tendon, articulating anteriorly with the triquetrum outside the carpal tunnel.

2. The Distal Carpal Row

• Trapezium (Greater Multangular): Articulates with the first metacarpal to form the mobile saddle-shaped thumb carpometacarpal (CMC) joint.
• Trapezoid (Lesser Multangular): Articulates with the second metacarpal.
• Capitate: The largest carpal bone, occupying the physical center of the wrist. Its rounded head articulates proximally with the concavity of the scaphoid and lunate.
• Hamate: Positioned on the medial side, featuring a prominent anterior hook-like projection—the hook of the hamate (hamulus)—which forms the medial boundary of the carpal tunnel and the lateral wall of Guyon's canal.

The Triangular Fibrocartilage Complex (TFCC)

The TFCC is the primary soft-tissue stabilizer of the distal radioulnar joint (DRUJ) and the ulnocarpal articulation, serving as a fibrocartilaginous shock absorber transmitting approximately 20% of axial compressive load across the wrist. It comprises:

1. The TFC Articular Disc: A biconcave fibrocartilage disc originating from the hyaline cartilage of the sigmoid notch of the radius and inserting into the fovea and styloid process of the ulna. The central disc is avascular and aneural, whereas the peripheral 15–20% is vascularized and capable of surgical repair.
2. The Meniscus Homologue & Radioulnar Ligaments: Dorsal and volar radioulnar ligaments that anchor the disc to the radius and ulna.
3. Ulnocarpal Ligaments: The ulnolunate and ulnotriquetral ligaments.
4. Extensor Carpi Ulnaris (ECU) Tendon Sheath: Forms the dorsal fibrous reinforcement of the complex.

Intrinsic Interosseous Ligaments

• Scapholunate Interosseous Ligament (SLIL): A C-shaped ligament binding the scaphoid and lunate. It possesses three distinct subregions: dorsal, membranous, and palmar. The dorsal band is the thickest and biomechanically most critical. Complete rupture of the dorsal SLIL leads to scapholunate dissociation and Dorsal Intercalated Segment Instability (DISI), where the lunate tilts abnormally into dorsal extension.
• Lunotriquetral Interosseous Ligament (LTIL): Binds the lunate and triquetrum. Its palmar band is mechanically strongest. Disruption precipitates Volar Intercalated Segment Instability (VISI), where the lunate tilts abnormally into volar flexion.

The Carpal Tunnel & Dorsal Extensor Compartments

• The Carpal Tunnel: An inelastic fibro-osseous space bounded deeply by the carpal bones and roofed anteriorly by the thick transverse carpal ligament (flexor retinaculum). It transmits exactly 10 anatomical structures:
  • One Nerve: The median nerve, positioned superficially beneath the flexor retinaculum.
  • Nine Tendons: Four tendons of the flexor digitorum superficialis (FDS), four tendons of the flexor digitorum profundus (FDP), and one tendon of the flexor pollicis longus (FPL).
  • CRITICAL EXAM PRINCIPLE: The flexor carpi radialis (FCR) tendon does NOT traverse the carpal tunnel; it travels in its own separate, dedicated fibrous compartment within the lateral flexor retinaculum. The ulnar nerve and artery pass medial to the tunnel through Guyon's canal.
• Dorsal Extensor Compartments: Six specialized synovial compartments situated beneath the extensor retinaculum on the dorsal wrist:
  • Compartment 1: Abductor pollicis longus (APL) and extensor pollicis brevis (EPB) — the site of de Quervain's tenosynovitis.
  • Compartment 2: Extensor carpi radialis longus (ECRL) and brevis (ECRB).
  • Compartment 3: Extensor pollicis longus (EPL), which hooks sharply around Lister's tubercle of the distal radius.
  • Compartment 4: Extensor digitorum and extensor indicis.
  • Compartment 5: Extensor digiti minimi.
  • Compartment 6: Extensor carpi ulnaris (ECU).

---

Radiofrequency Coils & Hardware Setup

• Dedicated Phased-Array Wrist Coil: Clinical wrist MRI is commonly acquired using a dedicated multi-channel wrist coil (typically 8 to 16 channels, rigid cylindrical or solenoid design). The tight cylindrical design provides an exceptional filling factor, maximizing SNR across miniature FOVs (typically 8 to 10 cm).
• Dedicated Hand Array: When imaging the metacarpals, phalanges, or MCP joints, a flat multi-channel flexible matrix coil or specialized hand array is deployed to cover the expanded anatomical volume from the mid-carpals to the fingertips.
• Cable Safety: Inspect coil cables for insulation cracks. Route cables straight out of the gantry without loops, maintaining separation from the patient's skin using non-conductive foam padding.

---

Patient Positioning Protocols

Positioning the wrist requires selecting a posture that reconciles field homogeneity with motion prevention.

1. The Prone "Superman" Position

• The patient lies prone with the affected arm extended straight overhead, with the hand and wrist centered inside the dedicated coil at the magnet's physical isocenter.
• Advantages: Unmatched static magnetic field ($B_0$) homogeneity, optimal gradient linearity, and pristine spectral fat suppression.
• Disadvantages: Highly fatiguing for the shoulder and neck. Patients frequently develop involuntary fine-motor tremors of the fingers and wrist, introducing severe phase-encoding ghosting artifacts on high-resolution acquisitions.

2. The Supine with Arm at Side Position

• The patient lies supine with the arm extended down along the hip, hand in neutral or slight pronation.
• Advantages: Excellent patient tolerance and physical stability. Supported posture may reduce motion, but tremor and voluntary movement remain possible.
• Disadvantages: The wrist is located 20 to 25 cm off-isocenter. Requires careful localized volume shimming or the use of STIR / Dixon sequences to overcome off-axis $B_0$ field inhomogeneity.
• Safety Padding: Manufacturer-specified insulating padding must be placed between the wrist coil and the patient's lateral hip to maintain the required skin and equipment separation; padding alone does not prevent aliasing.

3. The Supine with Arm Across Abdomen Position (STRICTLY DISCOURAGED)

Immobilization Protocol

• Finger Extension & Support: Place a contoured foam sponge beneath the palm and fingers to maintain gentle, passive extension. The patient must never clench their fist inside the coil, as fist clenching dramatically increases intracompartmental carpal tunnel pressures and artificially compresses the median nerve and flexor tendons.
• Strapping: Secure the fingers and distal forearm snugly with soft velcro straps to eliminate micro-motion.
• Joint Alignment: Ensure the wrist is maintained in a strictly neutral position (0 degrees of flexion/extension, 0 degrees of radial/ulnar deviation). Excessive wrist flexion, extension, or deviation distorts carpal ligament tension and alters TFCC morphology.

---

Anatomic Landmarking

Accurate laser landmarking places the tiny carpal anatomy directly at the center of the gradient field:

• Wrist MRI Landmark: Center the horizontal and vertical positioning laser crosshairs directly on the mid-carpal joint line.
  • External Anatomical References: The mid-carpal joint line corresponds externally to the distal wrist flexion crease, the depression of the anatomical snuffbox (between the EPL and EPB/APL tendons), or the palpable dorsal prominence of the head of the capitate.
• Hand / Digit Landmark: For examinations focused on the metacarpals, phalanges, or digits, center the laser crosshairs directly over the third metacarpophalangeal (MCP) joint (the knuckle of the middle finger).

---

Hand, Thumb and Nonthumb Digits

The wrist, whole hand and individual digit require different coil coverage and centering. Confirm the order, side and exact digit. Metacarpophalangeal (MCP) joints connect metacarpals to proximal phalanges; nonthumb fingers have proximal and distal interphalangeal joints (PIP and DIP), while the thumb has two phalanges and one interphalangeal joint. The mobile thumb CMC joint is more proximal than the thumb MCP. A wrist-centered coil does not automatically cover the fingertip or show the thumb in the intended plane.

For a finger study, support the specific digit, use the prescribed approved coil and center the intended joint or lesion rather than always the third MCP. The thumb lies obliquely to the other fingers, so the technologist plans planes relative to the thumb itself. Do not force a swollen digit straight or remove a prescribed splint without authorization. Report a tight ring or restricted circulation before positioning. Check localizer coverage with the technologist and document any limitation.