Updated: 6/9/2021

Hip Magnetic Resonance Imaging

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  • Introduction
    • MRI basics
      • patient is positioned in a large magnetic field
        • protons in body align with the magnetic field
        • external radiofrequency current pulses through patient and stimulates protons, causing to spin out of equilibrium
        • once external radiofrequency current is discontinued energy is released by protons
        • sensors detect the energy released by the protons and the time it takes the protons to return to equilibrium
        • Fourier transformation is used to convert sensor data at each location into image data
      • relaxation time
        • time required for protons to align with external magnetic field corresponds to T1 and T2
          • T1 - displays tissues high fat content as bright
          • T2 - displays tissues with high water content as bright
      • gadolinium contrast
        • paramagnetic agent (contains heavy metal)
        • shortens T1 interval
          • increases signal intensity of T1 images
        • direct hip arthrography
          • contrast injected directly in femoroacetabular joint under fluoroscopic guidance
        • indirect hip arthrography
          • intravenous contrast injected
          • patient then exercises the femoracetabular joint
          • contast diffuses into the femoracetabular joint
      • magnetic strength
        • 1.5 T
          • standard MRI magnetic strength
          • faster scan times than 1.2 T magnets
        • 3.0 T
          • extemely high resolution
            • good for visualing small bones, small ligaments, neurolgic tissues, and vascular tissues
          • higher machine cost
          • studies can be perfromed faster than 1.5T MRI
          • increased artifact than 1.5T
            • flow artifact from movement of blood and body fluids
      • Tissue Appearance on MRI
      • Tissue
      • T1
      • T2
      • Synovial fluid
      • Dark
      • Bright
      • Blood
      • Dark
      • Bright
      • Fat
      • Bright
      • Dark
      • Inflammation
      • Dark
      • Bright
      • Muscles
      • Gray
      • Dark Gray
      • Paramagnetic substances (Iron, hemosiderin, gadolinium, etc.)
      • Bright
      • Dark
  • Normal Anatomy
    • Osteology
      • Pelvis
        • innominate bone
          • ilium
          • ishcium
          • pubis
        • fuse together at the triradiate cartilage
        • acetabulum
          • socket
            • contains articular surface, cotyloid fossa, and acetabular notch
          • highly constrained joint
          • congruity increased with labrum
      • Femur
        • strongest bone in the body
        • femoral head
          • ball that articulates with acetabulum
            • spherical in shape
        • greater trochanter
          • insertion of abductors
            • gluteus minimus
            • gluteus medius
          • vastus ridge
            • origin of vastus lateralus
    • Labrum
      • rim of fibrocartilage that surrounds the acetabulm
        • triangular in shape on cross section
        • horseshoe shaped surrounding acetabulum
      • increases congruity and depth of the femoroacetabular joint
      • adds to the suction cup stability of the acetabulofemoral joint
      • originates from the transverse acetabular ligament
      • small percentage of patients do not have a labrum
        • approximately 3%
    • Ligaments
      • ligamentum teres
        • ligamentous connection between femoral head and acetabulum
          • fovea of femoral head
          • cotyloid fossa of acetabulum
        • in patients with developmental hip dysplasia the ligamentum teres can be hypertrophied and elongated
      • iliofemoral ligament
        • also referred to as ligament of Bigelow or Y-ligament
        • anterior to femoroacetabular joint and is major ligamentous stabilizer of the femoroacetabular joint
    • Muscles
      • Muscle on MRI
      • Muscle
      • MRI 
      • Iliopsoas
      • Pectineus
      • Sartorius
      • Rectus femoris
      • Vastus lateralis
      • Gluetus maximus
      • Gluteus medius
      • Gluteus minimus
      • Tensor fascia lata
      • Semitendinosus
      • Semimembranosus
      • Biceps femoris
      • Quadratus femoris
      • Piriformis
    • Nerves
      • Nerves on MRI
      • Nerve
      • MRI
      • Femoral nerve
      • Sciatic nerve
      • Obturator nerve
    • Blood vessels
      • Blood vessels on MRI
      • Blood vessel
      • MRI
      • Femoral artery
      • Femoral vein
  • Anatomic Variants
    • Labral variants
      • sublabral sulci
        • present in up to 25% of patients
          • most common positions
            • posterosuperior (48%)
            • anterosuperior (44%)
        • can be mistaken for a labral tear
          • can be differentiated from labral tears by:
            • sulci have smooth labral edges
              • tears tend to have rough edges
            • sulci tend to span all anatomic positions
              • tears tend to be more focal
            • sulci are not associated with perilabral pathology
              • labral tears are typically associated with chondral lesions and perilabral cysts
      • transverse acetabular ligament labral junction sulcus
        • can appear in the anterior an posterior intersection of the transverse acetabular ligament and the labrum
          • can be present in up to 33% of hips
          • usually present on the inferior acetabulum
            • uncommon place for a labral tear to occur
        • can occur in conjunction with a periligamentous recess anteromedial to the ligamentum teres
      • perilabral sulcus
        • anatomic space between the joint capsule and labrum
          • more apparent adjacent to the superior labrum than the anterior and posterior labrum
        • can be mistaken for a perilabral cyst
        • MRI arthrogram with joint distension can differentiate between perilabral cyst and perilabral sulcus
      • anterosuperior cleft
        • partial extension of fluid into the anterosuperior labrum on sagittal and coronal images
          • usually due to labral hypertrophy from developmental ip dysplasia
    • Nonlabral variants
      • synovial herniation pits
        • also known as Pitt pits
        • predominantly found on the anterosuperior femoral neck
          • usually subchondral or subcortical
        • found in up to 33% of patients with femoroacetabular impingement
          • thought to be a reactive lesion to impingement against anterior acetabulum or pressure caused by iliopsoas
      • os acetabuli
        • ossicle adjacent to the superior acetabular rim
        • due to perisistent secondary ossification center that remains unfused to the acetabulum
          • also thought be be from ossification of the superior labrum
      • accessory iliacus tendon
        • seperated iliacus tendon from the iliopsoas tendon
        • present in up to 66% of patients undergoing hip arthrograms
        • can be mistaken for a longitudinal tear of the iliopsoas tendon
          • can be differentiated by a tear with fat suppression MRI
            • fat containing fascia interposed between the two tendons
      • plicae
        • synovial folds that are embyronic remnants and are located at the interface of the articular surface
        • mostly are asymptomatic
          • can sometimes slip into the articular surface leading to mechanical symptoms and mass effect
      • pectinofoveal fold
        • also known as the middle retinaculum of Weitbrecht
        • extends from lesser trochanter to fovea capitis
        • contains the posteroinferior retinacular arteries and branches of the medial femoral circumflex artery
        • found in 95% of hip MRI arthrograms
        • almost never becomes symptomatic
      • supraacetabular fossa
        • typically located at the 12 O'clock position of the acetabulum
        • can be filled with synovial fluid or cartilage
          • cartilage most common
        • can be mistaken for an acetabular cartilage defect
          • differentiated by lack of underlying bone marrow changes and preserved catilage during arthroscopy
        • does not communicate with the acetabular fossa
      • stellate crease
        • area of the acetabulum that is bare of hyaline cartilage
        • usually medial to where supraacetabular fossa would be located
        • in continuity with the acetabular fossa
  • Pathology
    • Femoroacetabular impingement
      • impingement between the acetabulum and femoral neck
        • most often the anterolateral femoral neck and anterosuperior acetabulum
      • two types of impingement:
        • cam impingement
          • nonspherical femoral head articulates with a hemispheric acetabulum
        • pincer impingement
          • overcoverage of the femoral head by the acetabular labrum or bone
      • high association with labral tears
      • radiographic assessment
        • head-neck offset
          • assessed with frog-leg lateral radiographs
            • >0.17 is considered normal
        • alpha angle
          • can be assessed on frog-leg lateral radiographs or axial T1 MRI
            • oblique axials are preferred for measuring alpha angle
              • slices are made parallel to the femoral neck axis
          • measured as the angle between the region of the aspheric femoral head and axis of the femoral neck
          • <50° considered normal
          • occaisonally decreased marrow signal will be present at the site of the cam lesion on T1 weighted imaging
            • corresponds to reactive bone marrow edema
        • acetabular version
          • best assessed on straight axials
            • retroversion determined by the anterior wall projecting more lateral than posterior wall
              • corresponds to cross-over sign on pelvis radiographs
    • Labral tears
      • intrasubstance tear or detachment of the labrum from the acetabulum
        • occurs most commonly in the anterosuperior quadrant
          • interval between anterior labrum and iliopsoas tendon can be mistaken for a labral tear
        • freuqently occurs in conjunction with femoroacetabular impingement and developmental hip dysplasia
      • radiographic assessment:
        • MRI arthrogram is the most sensitive study for diagnosing labral tears
          • axial T1 and sagittal T1 are best for charactizing anterosuperior labrum
    • Hip avascular necrosis
      • necrosis of the bone involving the femoral head
        • occurs in the epiphysis
        • idiopathic etiology is most common
      • radiographic assessment
        • usually presents with hypointense signal on T1 weighted imaging localized to the epiphysis
        • modified Kerboul combined necrotic angle
          • summation of the angle of necrotic zones in the coronal and mid-sagittal sections of T1 MRI
          • prognosticates the risk of femoral head collapse
        • Modified Kerboul Combined Necrotic Angle
        • Risk group
        • Combined Angle
        • Low-risk
        • <190°
        • Medium-risk
        • between 190° and 240°
        • High-risk
        • >240°
    • Transient osteoporosis of the hip
      • localized hyperemia of the proximal femur with decreased venous return and increase marrow pressure
      • radiographic assessment:
        • hypointense signal on T1 and hyperintense singal on T2 that spans the metaphysis and epiphsysis
          • best visualized on coronal imaging
    • Trochanteric bursitis
      • inflammation involving the greater trochanteric bursa
      • can be classified based on location of inflammation on MRI imaging
        • sub-gluteus maximus (most common)
        • sub-gluteus medius
        • sub-gluteus minimis
      • radiographic assessment
        • high signal present in the trochanteric bursa on T2 sequences
          • best visualized on the coronal and axial images
    • Osteomyelitis
      • bacterial of fungal infection of bone
        • most often due to hematogenous spread
        • frequently involves the metaphysis in pediatric patients
          • sluggish blood flow in metaphysis allows for baceterial adhesion to marrow trabeculae
        • radiographic assessment:
          • MRI is most sensitive and specific for diagnosing early osteomyelitis
            • decreased signal on T1 sequence
              • "penumbra sign" - intraosseus abscess with dark central portion and bright internal wall
            • increased signal on T2 sequence
            • abscesses will have rim enhancement with intravenous gadolinium
    • Ischiofemoral impingement
      • impingement of soft tissue beween the lesser trochanter and the ischial tuberosity
      • radiographic assessment:
        • axial T1 weighted images can be used to calculate distance between lesser trochanter and ischial tuberosity
          • distance <15 mm is considered abnormal
          • diagnosis is presence of symptoms with distance <15 mm
          • fatty atrophy of the quadratus femoris may be present
        • axial T2 weighted images
          • hyperintense signal in the quadratus femoris muscle belly
            • can extend to the hamstrings and iliopsoas muscle
      • treatment:
        • nonoperative
          • correction of LLD, physical therapy, NSAIDs, injections
        • operative
          • address concomitant pathology
            • DDH, gluteal tendon tear, hamstring pathology
          • lesser trochanter resection
    • Osteoarthritis
      • degenerative changes of the femoroacetabular joint that leads to cartilage loss
      • radiographic assessment:
        • diagnosis predominantly made with plain radiographs
        • MRI has low utility in diagnosis and treatment modification
          • can detect early focal cartilage defects
            • best seen with T1 weighted images with intraarticular contrast
            • chondrosis grading:
              • grade 1: signal changes with preserved thickness
              • grade 2: signal changes with <50% loss of thickness
              • grade 3: signal changes with >50% loss of thickness
              • grade 4: full thickness cartilage defect with underlying bone marrow edema and subchondral cysts
            • cartilage degeneration typically occurs on the acetabulum first
    • Metastic disease
      • most common cause for detructive bone lesions in adults
      • proximal femur is the most common site for metastasis next to the spine
        • 50% femoral neck
        • 20% pertrochanteric region
        • 30% subtrochanteric
      • radiographic assessment
        • most surgical decision making is done with standard AP and lateral hip and full-length femur radiographs
        • magnetic resonance imaging
          • bright on T2 sequence
          • dark on T1 sequence
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