MRI Sequences in Orthopaedics
Choosing the Right Sequence for Each Clinical Question
Pulse Sequence Selection Guide
T1-weighted: Short TR (400-600ms), Short TE (10-20ms) → Anatomy, fat, gadolinium
T2-weighted: Long TR (3000-6000ms), Long TE (80-120ms) → Pathology, fluid, oedema
PD-weighted: Long TR (2000-4000ms), Short TE (10-30ms) → Meniscus, cartilage, ligament detail
STIR: TI = 160ms (1.5T), Long TR, Intermediate TE → Bone marrow oedema, most sensitive fluid detection
GRE (T2*): Short TR, Variable TE, Flip angle → Cartilage surface, haemosiderin, loose bodies
Key: TR controls T1 contrast, TE controls T2 contrast — this relationship governs all sequence selection
Critical Must-Knows
- Pulse sequence selection determines what pathology you can see — the wrong sequence can hide the diagnosis.
- T1-weighted (short TR, short TE): anatomy, fat, subacute blood. Post-gadolinium enhancement seen on T1.
- T2-weighted (long TR, long TE): fluid and oedema detection. Pathology is bright — the 'pathology sequence'.
- Proton Density (long TR, short TE): optimal for meniscal and cartilage assessment. High SNR.
- STIR: fat suppression via inversion — most sensitive for bone marrow oedema, robust near metal.
Examiner's Pearls
- "TR controls T1 weighting (short TR = T1); TE controls T2 weighting (long TE = T2).
- "PD-weighted images have the highest signal-to-noise ratio — making them optimal for detecting subtle meniscal tears.
- "STIR nulls fat at a specific inversion time (TI approximately 160ms at 1.5T) — provides uniform fat suppression.
- "Gradient echo sequences are susceptible to metal and field inhomogeneity — avoid near implants.
- "Magic angle artefact: tendons at 55 degrees to B0 appear bright on short TE sequences (T1, PD) but NOT on T2.
Exam Warning
Understanding MRI sequence selection is essential for viva examinations. You will be asked to specify which sequences you want for specific clinical scenarios and explain WHY. Common exam traps include: confusing PD and T2 weighting, not knowing the difference between STIR and chemical fat suppression, recommending gradient echo near metal implants, and failing to recognise magic angle artefact on PD or T1 images.
TR-TEMRI Sequence Parameters
Memory Hook:TR governs T1, TE governs T2 — this is the single most important rule for understanding MRI sequence selection.
TOPSSSequence Selection by Clinical Question
Memory Hook:TOPSS covers the five most common sequence selection decisions in orthopaedic MRI.
SCATFat Suppression Techniques
Memory Hook:SCAT: STIR vs Chemical fat saturation — know when to use each for perfect fat suppression.
Overview
MRI pulse sequence selection is the critical decision point that determines whether pathology will be visible on the resulting images. Each pulse sequence produces a different image contrast by manipulating the timing parameters (TR, TE, TI, flip angle) that govern how tissue relaxation properties contribute to the final signal. Choosing the wrong sequence for a clinical question can render the diagnosis invisible.
In orthopaedic practice, the core sequences used routinely are: T1-weighted, T2-weighted, proton density (PD)-weighted, STIR, and gradient echo (GRE). Advanced sequences including diffusion-weighted imaging (DWI), dynamic contrast-enhanced (DCE) MRI, and quantitative mapping are increasingly used for specific indications.
The key principle is: TR controls T1 contrast, TE controls T2 contrast. A T1-weighted image uses short TR and short TE. A T2-weighted image uses long TR and long TE. A PD-weighted image uses long TR and short TE (maximising signal-to-noise ratio while minimising both T1 and T2 contrast).
Sequence Determines Diagnosis
A meniscal tear is best seen on PD-weighted images (high SNR for fibrocartilage). Bone marrow oedema is best detected by STIR (most sensitive for fluid signals). A rotator cuff tear is best characterised on T2-weighted images (fluid in the torn tendon gap). Gadolinium enhancement for tumour and infection requires T1 post-contrast. Each clinical question has an optimal sequence.
Protocol Selection
Standard orthopaedic MRI protocols include at least three sequences in two planes: typically T1, T2/PD, and STIR in sagittal and coronal planes for most joints. Axial images are added for specific indications. The radiologist selects the protocol, but the referring orthopaedic surgeon should specify the clinical question to ensure the correct sequences are included.
Clinical Imaging
Imaging Gallery
Systematic Approach
Systematic Sequence Selection
When ordering or interpreting an orthopaedic MRI, follow this systematic approach:
Systematic MRI Sequence Assessment
| Step | Action | Rationale |
|---|---|---|
| 1. Define the clinical question | What specific pathology are you looking for? | Different pathologies require different sequences — a meniscal tear protocol differs from a tumour staging protocol |
| 2. Select anatomical plane | Which planes best demonstrate the suspected pathology? | Sagittal for ACL and meniscus; coronal for MCL, LCL, and rotator cuff; axial for labrum and peroneal tendons |
| 3. Choose core sequences | T1 (anatomy) + T2 or STIR (pathology) + PD (if meniscus/cartilage) | These three categories cover the vast majority of orthopaedic diagnostic needs |
| 4. Add contrast if needed | Gadolinium for tumour, infection, or synovitis | T1 fat-suppressed post-contrast; consider dynamic contrast-enhanced for tumour vascularity |
| 5. Consider metal-specific protocols | MAVRIC-SL, SEMAC, spin echo over gradient echo, STIR over chemical fat sat | Standard protocols fail near metal implants — specific modifications are required |
| 6. Review all sequences systematically | Do not interpret a single sequence in isolation | T1 alone misses oedema; T2 alone misses fatty infiltration. Cross-reference findings across sequences |
Core Sequences
T1-Weighted Imaging
Parameters: Short TR (400-600ms), Short TE (10-20ms)
T1-weighted images are the anatomical backbone of any MRI examination. They provide excellent contrast between fat (bright), muscle (grey), and fluid (dark), and their consistent appearance makes them the reference sequence for anatomical orientation.
What T1 shows best:
- Anatomy: Fat planes between structures provide natural contrast boundaries
- Fatty infiltration: Chronic rotator cuff tears with fatty replacement of muscle appear bright
- Fatty bone marrow: Normal adult bone marrow is bright on T1 due to fat content. Loss of this normal brightness (dark marrow) indicates pathology: infiltration, infection, tumour, or oedema
- Subacute haemorrhage: Methaemoglobin in subacute blood appears bright on T1
- Gadolinium enhancement: T1 post-contrast is the sequence used to detect enhancement; gadolinium shortens T1, making enhancing tissue brighter
What T1 misses:
- Oedema (dark on T1, may be subtle against dark background structures)
- Early soft tissue inflammation without fat replacement
- Fluid-filled structures blend into dark background
T1 is essential but never sufficient alone — always correlate with fluid-sensitive sequences.
Advanced Sequences
Advanced MRI Sequences in Orthopaedics
| Sequence | Mechanism | Primary Orthopaedic Application |
|---|---|---|
| Gradient Echo (GRE/T2*) | Uses gradient refocusing instead of 180-degree RF pulse; sensitive to magnetic susceptibility | Haemosiderin detection (PVNS shows blooming), cartilage surface assessment (3D GRE), loose bodies, calcification |
| Diffusion-Weighted Imaging (DWI) | Measures random (Brownian) motion of water molecules; restricted diffusion appears bright | Infection vs tumour differentiation, abscess detection, vertebral body fracture (acute vs chronic), cellularity of tumours |
| MR Arthrography (direct) | Intra-articular injection of dilute gadolinium then T1-weighted imaging | Gold standard for labral tears (hip, shoulder), SLAP lesions, loose bodies, cartilage defects, capsular pathology |
| Dynamic Contrast-Enhanced (DCE) | Serial T1 images during gadolinium bolus injection tracking enhancement curves | Tumour vascularity mapping, distinguishing viable tumour from necrosis, monitoring treatment response |
| T2 Mapping | Quantitative measurement of T2 values in articular cartilage pixel by pixel | Early cartilage degeneration detection before morphological changes visible on standard sequences |
| MAVRIC-SL / SEMAC | Multi-spectral imaging around metallic implants with slice and frequency encoding corrections | Assessment of periprosthetic soft tissues around hip and knee arthroplasty components |
DWI for Acute vs Chronic Vertebral Fractures
Diffusion-weighted imaging can distinguish acute benign osteoporotic vertebral fractures from pathological fractures (malignant infiltration). Acute benign fractures show high DWI signal in the early phase but increasing ADC values over time (reflecting oedema). Malignant fractures show persistently restricted diffusion (high DWI, low ADC) due to hypercellularity. This distinction is clinically important when a new vertebral compression fracture is found in an elderly patient with a history of malignancy.
Region-Specific Protocols
Standard Knee MRI Protocol
The standard knee MRI includes:
- Sagittal PD fat-suppressed: Gold standard for meniscal tears, ACL assessment, patellar tendon
- Coronal T1: Anatomy, medial/lateral compartment, bone stock
- Coronal PD fat-suppressed or T2 fat-suppressed: Collateral ligaments, bone marrow oedema
- Axial PD fat-suppressed: Patellofemoral joint, patellar tracking, transverse meniscal ligament
- Sagittal T1 (optional): Anatomical reference, fatty infiltration
Key structures to assess systematically: ACL, PCL, MCL, LCL, posterolateral corner, medial and lateral menisci, articular cartilage (all compartments), patellar tracking, bone marrow signal, popliteal fossa.
This protocol covers 95% of knee pathology.
Evidence Base
Sensitivity of MRI Sequences for Meniscal Tears
- Proton density-weighted images had the highest sensitivity for meniscal tears (93%) compared to T2 (85%) and GRE (79%).
- Fat-suppressed PD sequences provided additional improvement in sensitivity by suppressing competing fat signal.
- Meniscal tear sensitivity was higher on 3T compared to 1.5T scanners due to improved signal-to-noise ratio.
STIR versus Fat-Suppressed T2 for Bone Marrow Oedema
- STIR was more sensitive than fat-suppressed T2 for detecting bone marrow oedema in the foot and ankle (sensitivity 96% vs 84%).
- Chemical fat suppression was less uniform at the periphery and near tendons, leading to missed lesions.
- STIR provided more reliable fat suppression across the imaging volume, particularly near metal from previous surgery.
These studies establish PD and STIR as the optimal sequences for their respective indications.
Australian Context
In Australia, MRI protocols for orthopaedic indications are standardised within each institution but may vary between centres. The RANZCR publishes guidelines for minimum acceptable sequences for common orthopaedic indications, ensuring consistent image quality across Australian imaging centres. Medicare-eligible MRI examinations require scanning on equipment meeting specific standards, and clinical indications must meet Medicare criteria for funded imaging.
Australian orthopaedic trainees should be familiar with requesting MRI appropriately: specifying the clinical question, the suspected pathology, and whether contrast or arthrography is indicated. In the Australian public hospital system, communication with the radiologist about the clinical question often determines which sequences are included in the protocol, making effective multidisciplinary communication essential.
MR arthrography is widely available at Australian tertiary centres for both hip and shoulder labral assessment. The trend towards 3T scanning in Australia has improved SNR and spatial resolution but requires protocol optimisation to manage increased susceptibility artefact and specific absorption rate (SAR) considerations.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"You are asked by an examiner: 'You have requested a knee MRI for a 25-year-old footballer with suspected ACL injury. What sequences would you request and why?'"
"An examiner asks you: 'What is the difference between STIR and fat-suppressed T2? When would you use one over the other?'"
"You are asked to explain magic angle artefact and describe a clinical scenario where it could lead to misdiagnosis."
MRI Sequences in Orthopaedics — Exam Day Reference
High-Yield Exam Summary
TR and TE Control
- •TR controls T1 contrast: Short TR = T1-weighted (fat bright, water dark)
- •TE controls T2 contrast: Long TE = T2-weighted (water bright)
- •Long TR + Short TE = PD (highest SNR, gold standard for meniscus)
- •STIR: Inversion at TI 160ms (1.5T) nulls fat signal
Best Sequence for
- •Meniscal tears: PD fat-suppressed (sagittal) — highest sensitivity
- •Bone marrow oedema: STIR — most sensitive, uniform fat suppression
- •Rotator cuff: Coronal oblique T2 fat-suppressed
- •Tumour/Infection: T1 + gadolinium with fat suppression
- •Haemosiderin (PVNS): GRE/T2* — susceptibility blooming artefact
Fat Suppression
- •STIR: Uniform suppression, works near metal, CANNOT use with gadolinium
- •Chemical fat sat: Higher SNR, faster, but fails near metal
- •Post-gadolinium: MUST use chemical fat sat (NOT STIR)
- •Near metal: Always choose STIR over chemical fat sat
Magic Angle Artefact
- •Occurs at 55 degrees to B0 — tendons appear falsely bright
- •Visible on short TE (T1, PD) — DISAPPEARS on T2 (long TE)
- •Classic locations: supraspinatus, Achilles, peroneal tendons
- •Always confirm tendon pathology on T2 before diagnosing a tear
MR Arthrography
- •Gold standard for labral tears (hip and shoulder)
- •Sensitivity 87% vs 66% for non-arthrographic MRI
- •Dilute gadolinium injected intra-articularly
- •T1 fat-suppressed is the key post-arthrography sequence