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