Hip and Pelvis: Coil Coverage & Supported Setup

Bilateral Pelvis vs. Dedicated Unilateral Hip Protocols

Clinical MR imaging of the hip falls into two distinct technical categories, dictated by the suspected pathology:

1. Bilateral Pelvis Protocol (Screening & Marrow Disorders)

• Field of View (FOV): Large FOV, typically 36 to 42 cm, encompassing both iliac crests, both hip joints, the sacrum, and the pubic symphysis.
• Primary Purpose: Symmetrical, comparative evaluation of both proximal femora and the pelvic skeleton.
• Key Indications:
  • Avascular Necrosis (AVN / Osteonecrosis): Crucial because AVN can be bilateral in patients with relevant risk factors (such as chronic systemic corticosteroid therapy, sickle cell disease, or alcohol abuse). Symmetrical coronal T1 and STIR images allow early detection of asymptomatic contralateral disease.
  • Transient Osteoporosis of the Hip (TOH): Marked by extensive, reversible bone marrow edema throughout the femoral head and neck, occasionally bilateral or migratory.
  • Sacroiliitis & Spondyloarthropathies: Bilateral assessment of the sacroiliac (SI) joints for erosions, subchondral sclerosis, and bone marrow edema in ankylosing spondylitis.
  • Occult Insufficiency Fractures: Detection of non-displaced trabecular microfractures of the sacral ala, pubic rami, or femoral necks following low-energy falls in osteoporotic patients with normal plain radiographs.

2. Dedicated Unilateral Hip Protocol (Labrum & Micro-Pathology)

• Field of View (FOV): Small FOV, typically 16 to 20 cm, centered strictly over the symptomatic joint.
• High-Resolution Matrix: Utilizes high-resolution matrix acquisitions (e.g., 384 × 384 or 512 × 512) yielding sub-millimeter in-plane spatial resolution (0.4 to 0.5 mm) with thin slice partitions (2.5 to 3.0 mm).
• Specialized Oblique Prescription: Images are acquired in specialized planes, particularly coronal-oblique (parallel to the long axis of the femoral neck) and axial-oblique (planned to the femoral-neck axis under the protocol), to profile the femoral head-neck junction and acetabular labrum without partial-volume averaging.
• Key Indications:
  • Acetabular Labral Tears: High-resolution depiction of tears, detachment, or paralabral cysts at the chondrolabral junction.
  • Femoroacetabular Impingement (FAI): Structural osseous deformities causing abnormal mechanical conflict during hip motion:
    • Cam Morphology: Loss of normal spherical concavity at the anterosuperior femoral head-neck junction, producing an osseous "bump" or "pistol-grip" deformity (quantified on axial-oblique images as an alpha angle > 55°). During hip flexion and internal rotation, this bump shears into the acetabular rim, causing chondral delamination and labral tears.
    • Pincer Morphology: Acetabular over-coverage of the femoral head caused by coxa profunda (deep socket), acetabular protrusion, or cranial retroversion. During motion, the femoral neck impacts against the excessively deep acetabular rim, crushing the underlying labrum.
    • Mixed Morphology: Features combined cam and pincer elements (seen in >70% of FAI patients).
  • Gluteal Tendon Tears: Full-thickness or partial-thickness tears of the gluteus medius/minimus tendons at the greater trochanter.
  • Athletic Pubalgia ("Sports Hernia"): High-resolution evaluation of the common aponeurosis of the rectus abdominis muscle and adductor longus tendon at the pubic crest.

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Radiofrequency Coils & Hardware Configuration

• Phased-Array Torso / Cardiac Array: Routine bilateral pelvic imaging and most unilateral hip studies utilize a large multi-channel torso or cardiac phased-array coil (16 to 32 receive channels) wrapped snugly across the anterior pelvis, paired with the embedded multi-element table spine array underneath the patient.
• Dedicated Rigid / Semi-Flexible Hip Coils: When available, dedicated multi-channel hip coils (conformal rigid or shaped wrap arrays) provide an elevated filling factor and superior signal-to-noise ratio (SNR) over the targeted coxofemoral joint, crucial for direct MR arthrography.
• Coil Management & Cable Safety: Coil cables must be routed in a straight longitudinal path down the table toward the gantry exit without forming conductive loops. Non-conductive foam pads manufacturer-specified insulating padding must be placed between coil cables and the patient's skin, as well as between the patient's thighs, to prevent capacitive coupling and RF thermal burns.

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Patient Positioning Protocols

Patient positioning directly influences the diagnostic accuracy of hip MRI. The Imaging Assistant must execute specific technical steps to align internal bony structures.

1. Supine Position and Tolerated Rotation

Supine positioning and mild internal rotation are common for routine hip studies, but the degree depends on the examination and patient. Do not force rotation in trauma, suspected fracture, recent arthroplasty, restrictions or severe pain. Support the legs and heels, preserve the approved position and let the technologist determine the protocol and oblique imaging planes. MRI does not use the x-ray rule of projecting the greater trochanter away from the femoral neck. Do not tape toes together to overcome resistance.

2. Arm Positioning & Aliasing Prevention

• Arms Across Chest: The patient's arms should be crossed comfortably over their upper chest or placed along the upper abdomen away from the pelvic coil.
• Rationale: If the patient's arms are allowed to rest along their sides at hip level, the broad transverse dimension of the body will exceed the selected field of view (FOV). In unilateral or bilateral pelvis imaging with a tight FOV, anatomical tissue from the hands and wrists outside the FOV wraps into the image along the phase-encoding axis, creating severe aliasing (wrap-around) artifacts directly across the femoral heads and acetabula.

3. Patient Comfort & Immobilization

• Support: Use padding or a small support as allowed by the patient condition and protocol; a knee support is not universally contraindicated. Do not sacrifice comfort or precautions for a textbook pose.
• Hearing Protection & Call Bulb: High-gradient pelvic sequences generate substantial acoustic noise. Insert earplugs (NRR 25–30 dB) and place pneumatic headphones before securing the coil. Provide the squeeze call bulb in the patient's hand.

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Anatomic Landmarking

Accurate laser centering ensures the target anatomy translates to the magnetic isocenter (0, 0, 0), where static field homogeneity ($B_0$) is highest and gradient linearity is optimal:

Pelvic & Hip Landmarking Workflow:
1. Palpate Bony Landmarks (ASIS / Pubic Symphysis / Greater Trochanter)
2. Center Laser Crosshairs (Midline for Pelvis; 1–2" Inferior/Medial to ASIS for Unilateral)
3. Zero Table Coordinate Counter ➔ Advance Table to Magnetic Isocenter

• Bilateral Pelvis Landmark:
  • Palpate the anterior superior iliac spines (ASIS) bilaterally.
  • Center the vertical and horizontal laser crosshairs on the anterior midline, midway between the level of the ASIS and the superior border of the pubic symphysis (approximately 2 inches [5 cm] superior to the pubic symphysis).
• Unilateral Hip Landmark:
  • Palpate the ASIS on the symptomatic side.
  • Center the laser crosshairs the protocol-specific hip center using the coil reference and localizers.
  • External Correlation: This point corresponds directly to the level of the palpable femoral arterial pulse in the inguinal fold and the lateral prominence of the greater trochanter.
  • Zero the table display and advance the couch to isocenter.

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Positioning Scenario

A patient with a possible femoral-neck fracture cannot internally rotate the leg. Maintain the supported position and request the technologist's assessment rather than forcing or taping the feet. A focused unilateral study and a large-field pelvic study have different coverage needs; confirm the order and coil before moving the patient.