Knee Coil Selection, Position & Landmarking

Radiofrequency Coils & Hardware Setup

• Dedicated Rigid Knee Coil (Transmit/Receive or Receive-Only): Knee MRI is commonly performed using a dedicated multi-channel rigid knee coil (typically 8 to 18 channels) featuring a cylindrical or chimney-style architecture:
  • Transmit/Receive (T/R) Coils: Transmit RF excitation pulses locally within the coil volume rather than utilizing the scanner's body transmit coil. This dramatically reduces whole-body Specific Absorption Rate (SAR), optimizes $B_1$ field homogeneity, and delivers exceptional signal-to-noise ratio across small fields of view (FOV, 14 to 16 cm).
  • Receive-Only Coils: Rely on the body coil for RF excitation while multi-channel array elements receive signal. They provide excellent parallel imaging acceleration capabilities (SENSE / GRAPPA).
• Flexible Phased-Array Wrap Coils: For patients with severe joint contractures, morbid obesity, or bulky orthopedic splints who cannot fit into a rigid cylindrical coil, a flexible multi-channel wrap coil can be contoured around the joint.

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Patient Positioning Protocols

Precision patient positioning is necessary to resolve fine ligamentous structures without volume averaging.

1. Feet-First Supine (FFS) Orientation

• The patient is placed on the scanner couch in the feet-first supine (FFS) orientation.
• Clinical Benefit: Entering the bore feet-first significantly enhances patient comfort and substantially alleviates claustrophobia, as the patient's head remains outside or near the opening of the gantry during the examination.

2. Comfortable Joint Position and Imaging Planes

Place the knee in the coil's supported, tolerated position using the local protocol. Some protocols use slight external rotation; this is not a universal 5–10-degree requirement and neutral positioning does not by itself create a false ACL tear. The technologist plans slices from localizers to show the ACL and other structures. Never twist a painful, swollen or potentially unstable knee to achieve a memorized angle. Record and communicate limited movement.

3. Mild Flexion (5–10 Degrees) & Popliteal Support

• Place a contoured foam positioning sponge beneath the popliteal space to position the knee in 5 to 10 degrees of slight flexion.
• Rationale: Slight flexion relieves mechanical tension across the patellar tendon, quadriceps mechanism, and hamstring tendons, preventing muscle cramping and involuntary motion during long scans. Furthermore, slight flexion relaxes the anterior capsule and allows synovial fluid to distribute evenly around the cruciate ligaments.

4. Contralateral Limb Management & RF Safety

• The unaffected contralateral leg must be placed comfortably on the scan couch outside the coil, supported by foam bolsters and secured with a wide velcro strap to prevent motion.
• Thermal Safety: Ensure that the contralateral leg does not touch the active knee coil housing or cables. Maintain manufacturer-specified insulating separation between the limbs to prevent skin-to-skin contact loops that could cause RF thermal burns.

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Anatomic Landmarking

Precise laser alignment places the joint line at the magnetic isocenter (0, 0, 0) of the magnet and at the center of the dedicated coil:

Knee Landmarking Protocol:
1. Palpate Inferior Pole of the Patella (Joint Line)
2. Center Laser Crosshairs on the Landmark
3. Zero Table Counter ➔ Advance Table to Magnetic Isocenter

• Primary Landmark: The inferior pole of the patella.
  • External Correlation: The inferior pole of the patella can be a surface reference but is not an exact universal locator of the joint line; follow the coil mark and verify localizer coverage.
• Secondary Landmark: Center the laser crosshairs directly over the mid-patella or the palpable depression of the anterior joint line 1 cm below the patella.
• Set the console table counter to zero and advance the couch to the magnetic isocenter.

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Positioning Scenario

A swollen knee will not fit a rigid coil in the preferred position. Tell the technologist before attempting compression or rotation. A flexible approved coil and modified position may be needed. Maintain required skin and bore separation, confirm the actual joint target against the coil mark, and communicate pain or numbness promptly.

A Final Fit and Coverage Check

Before moving the table, confirm the ordered side with the patient and records, check that the joint is centered within the selected coil and ask about new pressure, pain or numbness. Inspect the cable path throughout table travel and maintain the manufacturer-required insulation between limbs, conductors and bore surfaces. Do not judge safe fit only from whether a coil latch closes. A contracture, bulky dressing or splint may require another approved configuration. Tell the technologist what limited the position so acquisition planning and the clinical record reflect the actual setup.