What does a STIR (Short Tau Inversion Recovery) finding on MRI (Magnetic Resonance Imaging) indicate?

What is a STIR Finding on MRI?

STIR (Short Tau Inversion Recovery) is a specialized MRI sequence that detects increased water content in tissues, appearing as bright (hyperintense) signal on imaging, and is highly sensitive for identifying edema, inflammation, or fluid accumulation in bone marrow, soft tissues, muscles, and fascia. 1

Technical Basis

STIR sequences are fluid-sensitive MRI sequences that suppress fat signal while highlighting areas of increased water content 1. These sequences are available on most MR scanners and provide excellent contrast for detecting pathologic processes involving tissue edema 2.

Clinical Significance by Anatomic Location

Musculoskeletal Applications

STIR hyperintensities in bone and soft tissue typically represent:

• Bone marrow edema - indicating inflammation, stress reactions, or traumatic injury 3
• Fascial fluid or edema - particularly important in detecting necrotizing fasciitis, where STIR has 93% sensitivity 3
• Muscle inflammation - seen in muscular dystrophies, myositis, and other inflammatory muscle conditions 2, 4
• Soft tissue edema - including subcutaneous and fascial inflammation 3, 4

Specific Pathologic Conditions

In sacroiliac joints and spine (axial spondyloarthropathy):

• Bone marrow edema on STIR indicates active inflammation with sensitivity of 79-82% and specificity of 89-97% 3, 5
• However, bone marrow edema can be nonspecific and seen in up to 30% of healthy controls, requiring careful interpretation 3, 5, 1
• Deep bone marrow edema lesions extending at least 1 cm deep to the articular surface are more specific for pathologic inflammation 1

In diabetic foot (Charcot neuroarthropathy):

• STIR shows denervation edema-like signal in foot muscles, fascial edema, joint effusions, and bone marrow edema 3
• Helps differentiate active from remission stages of disease 3

In soft tissue infections:

• STIR is superior to CT for detecting fascial fluid and edema, with the absence of fluid signal essentially excluding necrotizing fasciitis 3
• Fascial thickening ≥3 mm on STIR sequences is an excellent predictor of necrotizing fasciitis (86% sensitivity) 3

In osteomyelitis:

• High bone marrow signal on STIR sequences combined with low signal on T1-weighted images suggests osteomyelitis with sensitivity ~90% and specificity ~85% 3

Cardiac Applications

STIR hyperintensities in myocardium represent myocardial edema from acute inflammation, particularly in myocarditis and acute myocardial infarction 1, 6. STIR accurately distinguishes acute from chronic myocardial infarction with 95% sensitivity and 99% specificity 6.

Trauma and Stress Injuries

STIR detects stress fractures and bone bruises by showing diffuse bright signal representing edema 7. In stress fractures of the femoral neck, abnormal STIR signal typically resolves within 3-6 months in 90% of patients 7.

Critical Interpretation Principles

STIR should NEVER be used in isolation - always combine with conventional T1-weighted sequences to differentiate acute inflammatory changes from chronic structural changes 3, 1. This combined approach is essential because:

• T1-weighted images show chronic structural changes (sclerosis, erosions, fat infiltration) 3
• STIR shows acute inflammatory/edematous changes 3, 1
• Together they provide complete assessment of both active and chronic disease 3

Common Pitfalls and Caveats

Non-specificity: STIR hyperintensities represent "edema-like increased signal intensity" rather than a specific diagnosis 8. The same STIR appearance can occur in:

• Traumatic processes (bone bruise) 8
• Inflammatory conditions 8
• Osteoarthritic changes 8
• Neoplastic processes 8
• Normal variants (up to 30% of healthy controls in sacroiliac joints) 3, 5

Contrast enhancement is generally not needed: STIR sequences alone are sufficient for detecting inflammation in most clinical scenarios, and contrast medium does not significantly increase diagnostic accuracy 3, 1. The exception is when assessing tissue viability or abscess formation, where contrast-enhanced imaging may provide additional information 3.

Clinical correlation is mandatory: Given the non-specific nature of STIR findings, interpretation must always be integrated with clinical presentation, other imaging sequences, and laboratory findings to arrive at the correct diagnosis 3, 5.