MRI Bone Marrow Signal
Understanding Normal and Pathological Marrow
Marrow Signal Patterns
Normal yellow marrow: T1 bright, T2 intermediate, STIR suppressed
Red marrow: T1 slightly less bright than fat, T2 intermediate
Oedema: T1 dark, STIR bright
Fatty replacement: T1 bright, STIR suppressed
Fibrosis/sclerosis: T1 dark, T2 dark, STIR dark
Tumour infiltration: T1 dark, T2 bright or intermediate, STIR bright
Key: The first step in marrow assessment: Is the T1 signal brighter or darker than muscle?
Critical Must-Knows
- Normal adult bone marrow is predominantly yellow (fatty) and appears BRIGHT on T1: loss of T1 brightness is the cardinal sign of marrow pathology.
- Marrow conversion: red (haematopoietic) marrow converts to yellow (fatty) marrow from distal to proximal and from diaphysis to metaphysis during childhood.
- STIR is the most sensitive sequence for detecting bone marrow oedema from any cause: fracture, infection, tumour, AVN.
- The 'double line sign' on T2 — a band of high signal surrounded by a low-signal rim — is pathognomonic for avascular necrosis.
- Modic changes classify vertebral endplate marrow signal: Type 1 (oedema), Type 2 (fatty), Type 3 (sclerotic).
Examiner's Pearls
- "On T1-weighted images, normal bone marrow is brighter than muscle. If marrow is DARKER than muscle on T1, it is abnormal.
- "Red marrow reconversion (physiological) can mimic pathological infiltration — look for symmetric distribution and residual fat signal.
- "Stress fractures show marrow oedema (bright STIR) BEFORE a fracture line becomes visible on plain radiographs or CT.
- "Transient osteoporosis of the hip: diffuse femoral head marrow oedema on STIR with no double line sign (distinguishes from AVN).
- "Tumour infiltration typically replaces the normal T1 bright marrow with dark signal and enhances with gadolinium.
Exam Warning
Bone marrow signal interpretation is a core MRI skill tested in fellowship examinations. You must be able to: explain normal marrow conversion, identify the T1 signal as the cornerstone of marrow assessment, differentiate oedema from infiltration from red marrow reconversion, identify the double line sign of AVN, and classify Modic changes. A common viva trap is failing to use the T1 signal as the primary reference when assessing marrow pathology — many candidates focus only on STIR, which is sensitive but not specific.
FBMNormal Marrow on T1
Memory Hook:FBM: Fat Bright Marrow on T1. If marrow is darker than muscle on T1, something is wrong.
FISTSCauses of Bone Marrow Oedema
Memory Hook:FISTS cause marrow oedema: Fracture, Infection, Surgery, Tumour, Stress/Transient.
1-2-3Modic Changes Classification
Memory Hook:Modic 1-2-3: Oedema (active) → Fat (stable) → Sclerosis (end-stage). Type 1 correlates with pain, Type 2 is most common, Type 3 is rare.
Overview
Bone marrow signal on MRI is one of the most clinically important imaging findings in orthopaedic practice. The ability to detect changes in marrow signal allows early diagnosis of occult fractures, stress injuries, avascular necrosis, infection, and tumour infiltration — often weeks or months before these conditions become visible on plain radiographs or CT.
The foundation of marrow assessment on MRI is the T1-weighted signal. Normal adult bone marrow is predominantly yellow (fatty) marrow, which appears bright on T1-weighted images due to the short T1 of fat. Any process that replaces, displaces, or alters this fatty marrow will reduce the T1 signal, making the abnormal area darker than the surrounding normal marrow. This simple observation — is the marrow bright or dark on T1? — is the single most important assessment in musculoskeletal MRI.
Red vs Yellow Marrow
At birth, the entire skeleton contains red (haematopoietic) marrow. During childhood and adolescence, red marrow progressively converts to yellow (fatty) marrow in a predictable pattern: from distal to proximal (feet and hands first, axial skeleton last) and within long bones from diaphysis to metaphysis. By approximately age 25, the distribution reaches the adult pattern with red marrow confined to the axial skeleton, proximal femora, proximal humeri, and some metaphyseal regions. Yellow marrow is T1 bright; red marrow is less bright but still usually brighter than muscle.
When Marrow Signal Is Abnormal
Abnormal bone marrow signal suggests a pathological process. Key patterns: (1) Oedema = T1 dark, STIR bright (fracture, infection, tumour, reactive). (2) Infiltration = T1 dark, may enhance with gadolinium (tumour, leukaemia, lymphoma). (3) Fatty replacement = T1 bright, STIR suppressed (chronic change, healed process). (4) Fibrosis/sclerosis = dark on T1 AND T2 (low signal on all sequences — Paget disease, chronic osteomyelitis, radiation therapy). (5) Red marrow reconversion = slightly dark on T1, often symmetric and in expected locations (physiological response to anaemia, chronic illness, smoking).
Clinical Imaging
Imaging Gallery
Systematic Approach
Systematic Bone Marrow Assessment on MRI
Systematic Marrow Signal Assessment
| Step | Assessment | Key Question |
|---|---|---|
| 1. T1 signal reference | Compare marrow signal to adjacent muscle on T1-weighted images | Is marrow brighter or darker than muscle? Normal marrow is ALWAYS brighter than muscle on T1 |
| 2. STIR signal | Check for bright signal on STIR indicating oedema or pathology | Is there abnormal bright STIR signal? Where is it located? Is it focal, multifocal, or diffuse? |
| 3. Distribution pattern | Assess whether signal changes are focal, multifocal, or diffuse | Focal = fracture, tumour, or infection. Multifocal = metastases, myeloma. Diffuse = systemic disease, reconversion |
| 4. Signal characteristics | Correlate T1, T2, and STIR signal to characterise the type of abnormality | T1 dark + STIR bright = oedema. T1 dark + T2 dark = sclerosis. T1 bright = fat. Enhancement = vascular |
| 5. Morphological features | Look for fracture lines, cortical disruption, periosteal reaction, soft tissue mass | These features narrow the differential: fracture line in stress injury, cortical disruption in tumour or infection |
| 6. Age and clinical context | Correlate the pattern with the patient's age, history, and presenting symptoms | Diffuse marrow oedema in a child may be normal red marrow; in an elderly patient may be metastatic disease |
The T1 Rule
The single most important rule in bone marrow MRI assessment is: normal bone marrow is ALWAYS brighter than muscle on T1-weighted images. If you see marrow that is darker than or equal in signal to muscle on T1, it is abnormal and requires further evaluation. This simple rule will catch the majority of significant marrow pathology.
Marrow Conversion and Reconversion
Normal Marrow Conversion
Understanding normal marrow conversion is essential for avoiding misdiagnosis, particularly in children, adolescents, and young adults where residual red marrow may mimic pathology.
Normal Marrow Conversion Timeline
| Age | Distribution | MRI Appearance |
|---|---|---|
| Birth | Entire skeleton contains red marrow | Diffusely intermediate-low T1 signal throughout skeleton — different from adult pattern |
| 5 years | Distal appendicular conversion well advanced (hands, feet converted to yellow) | Distal extremities now T1 bright; axial skeleton remains intermediate |
| 15 years | Epiphyses, diaphyses largely converted; metaphyses and axial skeleton still have red marrow residua | Most of the long bone shafts are now T1 bright; patchy metaphyseal areas may still be intermediate |
| 25 years | Adult pattern established: yellow marrow except for axial skeleton, proximal femora, proximal humeri | Normal adult T1 bright marrow; residual red marrow in expected locations |
| Adult | Stable yellow marrow distribution; slow conversion of residual red marrow continues with age | T1 bright throughout most of the appendicular skeleton; redder with reconversion stimuli |
Red Marrow Reconversion
Under certain physiological and pathological conditions, yellow marrow can reconvert back to red marrow in the reverse pattern (proximal to distal, metaphysis to diaphysis). This reconversion increases the proportion of red (haematopoietic) marrow at the expense of yellow (fatty) marrow.
Red Marrow Reconversion vs Pathological Infiltration
| Feature | Red Marrow Reconversion | Pathological Infiltration |
|---|---|---|
| Distribution | Symmetric; follows expected pattern (proximal to distal) | Often asymmetric, random, or focal |
| T1 signal | Slightly less bright than fat but STILL BRIGHTER than muscle | Darker than muscle on T1 |
| STIR signal | Mild increase; less intense than frank oedema | Markedly bright on STIR |
| Fat within the lesion | Scattered fat signal (small foci of T1 bright) within the marrow | Complete replacement of fat signal — no residual bright T1 foci |
| Enhancement | Mild, homogeneous if present | Avid, heterogeneous, may show mass effect |
| Causes | Chronic anaemia, heavy smoking, obesity, long-distance running, chronic illness | Metastases, lymphoma, leukaemia, myeloma, primary bone tumours |
Pathological Marrow Patterns
AVN (Osteonecrosis)
MRI is the most sensitive and specific imaging modality for detecting avascular necrosis, identifying signal changes months to years before plain radiographic changes appear.
T1 findings: The necrotic zone shows loss of normal bright T1 fatty marrow signal. A band of low T1 signal (the reactive interface between viable and necrotic bone) is characteristic.
T2 findings: The 'double line sign' is pathognomonic for AVN — a band of bright signal (fluid/granulation tissue) bordered by a low-signal rim (sclerotic reactive bone) at the necrotic interface. The double line sign has a specificity of nearly 100% for AVN.
STIR findings: The reactive zone and surrounding oedema are bright on STIR. The extent and pattern of STIR signal help with prognostication.
Femoral head AVN staging (MRI):
- Stage I: Normal radiograph, abnormal MRI (marrow oedema or subtle signal change)
- Stage II: Sclerosis on radiograph, characteristic MRI changes without collapse
- Stage III: Subchondral fracture (crescent sign) — MRI shows subchondral high signal on T2
- Stage IV: Femoral head collapse and secondary osteoarthritis
Key differential: transient osteoporosis of the hip shows diffuse marrow oedema of the entire femoral head WITHOUT the double line sign and WITHOUT a serpentine demarcation line. This distinction is critical because transient osteoporosis is self-limiting while AVN may require surgical intervention.
Modic Changes and Vertebral Marrow
Modic Endplate Classification
Modic changes are vertebral endplate and subchondral marrow signal changes associated with degenerative disc disease. They are classified into three types based on their MRI signal characteristics:
Modic Classification of Vertebral Endplate Changes
| Type | T1 Signal | T2/STIR Signal | Pathology | Clinical Correlation |
|---|---|---|---|---|
| Type 1 | Dark (hypointense) | Bright (hyperintense) | Active oedema and inflammation in the endplate | Most strongly associated with active low back pain; may convert to Type 2 or improve |
| Type 2 | Bright (hyperintense) | Bright or isointense | Fatty replacement of the endplate marrow | Most common type; more stable; may or may not be symptomatic |
| Type 3 | Dark (hypointense) | Dark (hypointense) | Sclerotic bone in the endplate | Least common; represents end-stage sclerosis; usually stable |
Clinical Significance of Modic Type 1
Modic Type 1 changes are the most clinically significant type because they represent active inflammation and are most strongly associated with axial back pain. They may also indicate low-grade infection (Modic Type 1 changes can be an early sign of discitis/osteomyelitis and must be interpreted in clinical context). Some authors have advocated antibiotic treatment for persistent Modic Type 1 changes that fail to respond to conservative management (the Modic Antibiotic Spine Therapy concept), though this remains controversial. Modic Type 1 changes may progress to Type 2 (fatty) over time or may persist and remain symptomatic.
Evidence Base
MRI for Occult Hip Fractures
- MRI detected 100% of occult hip fractures missed on initial plain radiographs.
- All patients with clinical suspicion and negative radiographs had fractures confirmed on MRI within 24 hours.
- Early MRI-guided diagnosis prevented delayed treatment and improved outcomes compared to historical observation protocols.
Sensitivity of MRI for Osteomyelitis
- MRI had pooled sensitivity of 90% and specificity of 82% for osteomyelitis across all anatomical sites.
- Specificity was lower in the diabetic foot (75%) due to overlap with neuropathic oedema and Charcot arthropathy.
- Gadolinium enhancement improved specificity for abscess detection but did not significantly improve overall sensitivity.
Diagnostic evidence strongly supports MRI for occult fractures and infection.
Australian Context
In Australia, MRI is the investigation of choice for suspected bone marrow pathology including occult hip fractures, stress injuries, and avascular necrosis. Medicare-funded MRI for bone marrow assessment requires specific clinical indications and referral from approved practitioners. Common funded indications include suspected hip fracture with negative radiographs, investigation of bone tumours, suspected osteomyelitis, and assessment of avascular necrosis.
The Australian and New Zealand Hip Fracture Registry data demonstrates the importance of rapid MRI access for occult hip fractures, supporting the recommendation for MRI within 24 hours when clinical suspicion is high and plain radiographs are negative. Several Australian orthopaedic units have implemented fast-track MRI protocols for suspected occult femoral neck fractures.
The AOANJRR provides data on revision arthroplasty outcomes that correlate with periprosthetic bone quality, which is assessed using marrow signal patterns on MRI. Australian sports medicine practice, particularly relating to elite athletes (AFL, cricket, rugby), relies heavily on MRI bone marrow signal for stress fracture grading and return-to-play decisions.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"A 65-year-old woman presents with hip pain after a fall. Plain radiographs of the pelvis are reported as normal. You are shown an MRI of the hip."
"You are shown an MRI of the hip in a 40-year-old male on long-term corticosteroids. The T2-weighted image shows a serpentine line in the femoral head with an inner bright band and outer dark band."
"An examiner shows you an MRI of the lumbar spine with bright T1 and bright T2 signal in the endplates of L4/5, and asks you to classify and discuss the changes."
MRI Bone Marrow Signal — Exam Day Reference
High-Yield Exam Summary
Normal Marrow Assessment
- •Normal marrow is T1 BRIGHT (brighter than muscle) — due to fat content
- •If marrow is darker than muscle on T1, it is abnormal
- •Conversion: distal to proximal, diaphysis to metaphysis (complete by age 25)
- •Red marrow reconversion is proximal to distal (reverse of conversion)
Pathological Patterns
- •Oedema: T1 dark, STIR bright (FISTS: Fracture, Infection, Surgery, Tumour, Stress)
- •Fatty replacement: T1 bright, STIR suppressed (chronic change)
- •Fibrosis/Sclerosis: T1 dark, T2 dark (Paget, chronic infection, radiation)
- •Infiltration: T1 dark, complete fat replacement, enhancement with gadolinium
AVN Diagnosis
- •Double line sign on T2 is pathognomonic (100% specific)
- •Outer dark line = reactive sclerosis; inner bright line = granulation tissue
- •Must scan bilaterally — 80% bilateral in steroid-related AVN
- •Distinguish from transient osteoporosis: TOH has diffuse oedema WITHOUT double line
Modic Changes
- •Type 1: T1 dark, T2 bright = oedema (PAIN associated; most clinically significant)
- •Type 2: T1 bright, T2 bright = fat (MOST COMMON; stable)
- •Type 3: T1 dark, T2 dark = sclerosis (LEAST COMMON; end-stage)
- •Must exclude discitis if clinical features suggest infection
Stress Fracture Grading (Fredericson)
- •Grade 1: Periosteal oedema only (STIR positive, T1 normal)
- •Grade 2: Marrow oedema on STIR, T1 normal
- •Grade 3: Marrow oedema on BOTH T1 (dark) and STIR (bright)
- •Grade 4: Visible fracture line — Grade 4b is a complete stress fracture