MRI of the Hip: Detailed Anatomy

Magnetic resonance imaging (MRI) utilizes magnet and radio waves to produce diagnostic images that allow a doctor to visualize the hips. This medical imaging method can detect stress fractures or bone bruises that a regular X-ray usually misses.

According to a study, MRI is the modality of choice when determining X-ray results’ abnormalities and the diagnosis of various hip conditions. This modality is considered safe, non-invasive, and depicts accurate anatomical details⁽¹⁾. 

How Does Hip MRI Work in Medical Diagnosis?

MRI is a medical imaging tool that evaluates various causes of pain surrounding the hip joint. This tool can show changes in the cartilage and the underlying bone, helping doctors detect arthritis’ early signs⁽²⁾. 

Several tendons insert around the hip and may become inflamed or degenerated. MRI determines the causes of hip pain that may originate from nearby structures, like the pubic bones, sacroiliac joints, or the lower lumbar spine⁽³⁾. 

Other sources of pain that can be detected by MRI include tumors, infection, or necrosis of the bone (avascular necrosis or AVN)⁽⁴⁾. 

Anatomy of the Hip

Understanding the anatomy of the hip is essential for diagnosing its pathology. The alignment and the marrow are the critical elements of the osseous (bony) structures on MR imaging⁽⁵⁾. 

Normal variations that involve the cartilage include the supra-acetabular fossa (acetabular cartilage) and the stellate lesion. Meanwhile, labral sulcus and absent labrum are normal variations in the labrum (ring of cartilage). 

The hip’s essential muscles are the sartorius, rectus femoris, gluteus minimus and medius, iliopsoas, adductors, and hamstrings. 

The iliofemoral, pubofemoral, and ischiofemoral ligaments represent the thickenings of the joint capsule. These ligaments reinforce and stabilize the hip joint⁽⁶⁾. 

The sciatic nerve, the femoral nerve, and the obturator nerve are the largest nerves in the hip and thigh. 

Hip Conditions or Pathologies

Hip Fractures

Hip fractures encompass a wide range of types and various imaging techniques. The most common hip fracture types are stress fractures and occult fractures (not detectable using radiograph).

Despite their ease of use and immediate results, radiographs have low reliability in picking up fractures in their early stages⁽⁷⁾. 

MRI is considered the gold standard in detecting hip fractures due to its high sensitivity. A study demonstrated that radiographs have only a 15% to 35% sensitivity on initial stress fracture exam, increasing to 30% to 70% on follow-up visits⁽⁸⁾. 

Meanwhile, MRI had 100% sensitivity, 95% accuracy, 93% positive predictive value, 86% specificity, and 100% negative predictive value⁽⁹⁾. 

T-1 weighted and T2-weighted scans are the most common MRI sequences. T1-weighted images are generated using short TE (time to echo) and TR (repetition time), while T-2 weighted images are generated using longer TE and TR times⁽¹⁰⁾. 

Time to echo refers to the time between the application of the radiofrequency pulse and the peak of the echo signal induced in the coil. Meanwhile, repetition time is the time between the application of successive pulse sequences to the same slice⁽¹¹⁾. 

It was suggested that a combination of a T1- and T2-weighted MRI is the best method of detecting occult fractures⁽¹²⁾. 

This combination allows the differentiation of fractures from soft tissue injuries. 

When reading MRI for hip OA, one should look for the following:

• Low-intensity signal located within or on the edge of the bone
• T2-weighted MRI – edema surrounding bone
• Displacement of bone (in the case of large fracture)

Hip Osteoarthritis (OA) 

Osteoarthritis is a multi-factorial condition that involves the breakdown of hyaline cartilage along articulating joints. A radiograph or MRI is the most commonly used source of imaging for hip OA. 

The gold standard for imaging hip OA is MRI because the articular cartilage is visible. The MRI also has a higher resolution of surrounding tissues⁽¹³⁾. 

When reading MRI for hip OA, one should look for the following:

• Joint narrowing
• Reduced signal frequency of hyaline cartilage
• Increased edema 
• Osteophyte formation
• Fast spin-echo images, such as fast spin-echo T2-weighted fat-suppressed images
• Gradient-echo images, such as T1-weighted 3-dimensional fat-suppressed images

Trochanteric Bursitis or Tendonitis

Trochanteric bursitis or tendonitis is the inflammation of the tendons or greater trochanter around the same region⁽¹⁴⁾. 

This condition is likely positive with maximum hip flexion, adduction, internal or external rotation, and positive palpation to the affected bursa or tendon. 

Additionally, tendonitis has a positive isometric contraction with activation to the affected muscle group.  

A radiograph is not as helpful in diagnosing trochanteric bursitis as soft tissues and muscles are not visible to any degree⁽¹⁵⁾. 

When suspicious of hip bursitis or tendonitis, a T2-weighted MR image is considered the most beneficial. An MR image can detect asymmetric sites of fluid accumulation and edema⁽¹⁶⁾. 

A fluid sensitive T2 helps detect soft tissue lesions, muscular strain, or tears that may accompany trochanteric bursitis or tendonitis. This MRI allows for high sensitivity of detecting the hip condition⁽¹⁷⁾.

When reading MRI for hip OA, one should look for the following:

• Asymmetrical collection of fluid in the high signal intensity peripheral of bone
• The presence of bursal distension

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