High Yield Overview
MRI Sequences: T1, T2, STIR, PD Selection
1T TR
400-800Short (ms)
2T TR
2000Long (greater than ms)
—STIR TI
150-175ms (nulls fat)
—PD Optimal For
—Menisci, ligaments
Signal Intensity Reference
T1 Bright: Fat, subacute blood, gadolinium, protein
T1 Dark: Water, calcium, air, fibrous tissue
T2 Bright: Water, fluid, oedema
T2 Dark: Calcium, cortical bone, fibrous tissue, air
Key: Know what's bright and dark on each sequence to interpret pathology
Critical Must-Knows
- T1: Anatomy, fat bright, water dark, marrow assessment
- T2: Pathology detection, water bright, excellent for oedema
- STIR: Fat-suppressed T2, marrow oedema, fractures
- PD: Meniscal and ligament detail, intermediate weighting
- Fat saturation removes fat signal for better pathology detection
Examiner's Pearls
- "T1: Best for anatomy, marrow infiltration (goes dark)
- "T2: Fluid is bright - effusions, cysts, oedema
- "STIR: Uniform fat suppression, detects bone oedema
- "PD fat-sat: Workhorse for knee, shoulder assessment
- "Gadolinium enhances on T1 sequences
Exam Warning
MRI sequence selection is a favourite viva topic. You must understand the physics basis for T1 vs T2 contrast, know which sequences show specific pathologies best, and be able to justify sequence selection for common clinical scenarios.
Fundamental Sequence Characteristics
Core MRI Sequence Parameters
| Sequence | TR | TE | Primary Contrast | Key Use |
|---|---|---|---|---|
| T1-weighted | Short (400-800ms) | Short (10-20ms) | T1 relaxation | Anatomy, marrow, gadolinium |
| T2-weighted | Long (greater than 2000ms) | Long (80-120ms) | T2 relaxation | Fluid, pathology detection |
| PD-weighted | Long (greater than 2000ms) | Short (15-30ms) | Proton density | Menisci, ligaments, cartilage |
| STIR | Long | Long | T2 with fat nulling | Marrow oedema, occult fractures |
| Gradient Echo | Variable | Variable | T2* decay | Cartilage, haemosiderin |
Mnemonic
WW2 Bombed Water Towers (T2)T1 vs T2 Memory Aid
W
W = Water bright on T2
T
T2 = Type 2 (pathology detection)
T
T1 = Anatomy (fat is bright)
D
Dark on T1 + Bright on T2 = fluid/oedema
Memory Hook:If in doubt about sequence: T1 = anatomy (fat bright, water dark), T2 = pathology (water bright)
T1-Weighted Imaging
T1 Signal Intensity
| Tissue/Substance | T1 Signal | Reason |
|---|---|---|
| Fat | Bright (hyperintense) | Short T1 relaxation time |
| Yellow marrow | Bright | Fat content |
| Subacute blood (methaemoglobin) | Bright | Paramagnetic effect |
| Gadolinium (post-contrast) | Bright | T1 shortening |
| Proteinaceous fluid | Bright | Protein-water binding |
| Water/fluid | Dark (hypointense) | Long T1 relaxation |
| Cortical bone | Dark (signal void) | No mobile protons |
| Air | Dark (signal void) | No mobile protons |
| Calcium | Dark | No mobile protons |
T2-Weighted Imaging
T2 Signal Intensity
| Tissue/Substance | T2 Signal | Clinical Example |
|---|---|---|
| Water/fluid | Bright (hyperintense) | Joint effusion, cysts, oedema |
| Oedema | Bright | Bone marrow oedema, soft tissue inflammation |
| Fat | Intermediate to bright | Less bright than T1 |
| Muscle | Intermediate | Normal muscle signal |
| Fibrous tissue | Dark (hypointense) | Ligaments, tendons, scars |
| Cortical bone | Dark (signal void) | No mobile protons |
| Calcium/calcification | Dark | Chondrocalcinosis, dystrophic calc |
| Haemosiderin | Dark | Chronic haemorrhage, PVNS |
Fat Suppression Techniques
Fat Suppression Methods
| Technique | Mechanism | Advantages | Limitations |
|---|---|---|---|
| Chemical fat sat | RF pulse at fat frequency | Fast, widely available | Fails at air-tissue interfaces, field inhomogeneity |
| STIR | Inversion recovery nulls fat | Uniform suppression, robust | Longer scan time, cannot use with gadolinium |
| Dixon | Separates fat/water chemically | Uniform, works near metal | Post-processing required |
| Spectral presaturation | Saturates fat spins | Good with gradient echo | Sensitive to field inhomogeneity |
STIR vs Fat-Sat T2
STIR provides uniform fat suppression regardless of field inhomogeneity. Better for large FOV, near metal. Fat-sat T2 is faster but may fail near air/metal interfaces. Use STIR for spine, whole body, near hardware.
STIR + Gadolinium Warning
NEVER use STIR after gadolinium contrast. Gadolinium-enhanced tissues have shortened T1 and may be nulled along with fat, losing enhancement information. Use fat-sat T1 post-contrast instead.
Proton Density Imaging
PD-Weighted Imaging Characteristics
| Feature | Description | Clinical Use |
|---|---|---|
| Contrast mechanism | Based on proton number in tissue | Intermediate T1/T2 weighting |
| Fat signal | Bright to intermediate | Good tissue contrast |
| Fluid signal | Intermediate (less bright than T2) | Less conspicuous than T2 |
| Meniscal detail | Excellent | Best sequence for meniscal tears |
| Ligament detail | Excellent | ACL, collaterals, rotator cuff |
| Cartilage assessment | Good | Surface and internal detail |
PD Fat-Saturated: The Workhorse
PD fat-sat combines excellent soft tissue resolution with pathology detection. Standard for knee (menisci, ligaments) and shoulder (rotator cuff, labrum). Provides good cartilage assessment with reasonable fluid sensitivity.
Sequence Selection by Pathology
Optimal Sequences for Common Pathologies
| Pathology | Best Sequence(s) | Rationale |
|---|---|---|
| Meniscal tear | PD or PD fat-sat | Excellent meniscal detail, intrameniscal signal |
| Ligament tear (ACL, RTC) | PD fat-sat, T2 fat-sat | Shows internal signal, oedema |
| Bone marrow oedema | STIR or T2 fat-sat | Uniform fat suppression highlights oedema |
| Occult fracture | STIR | Most sensitive for marrow oedema |
| Infection/osteomyelitis | T1 (marrow) + STIR (oedema) + Gd T1 fat-sat | Combination approach |
| Tumour staging | T1 (extent) + T2 (characterisation) + Gd T1 fat-sat | Complete assessment |
| Cartilage | PD fat-sat, 3D gradient echo | Surface detail, defects |
| Labrum | PD fat-sat ± MRA | Contrast improves tear detection |
| AVN | T1 (marrow) + STIR (oedema) | Band pattern, extent |
| Haemosiderin/PVNS | Gradient echo (T2*) | Blooming artefact from iron |
Regional Protocol Selection
Standard Knee MRI Protocol
| Sequence | Plane | Primary Target |
|---|---|---|
| PD fat-sat | Sagittal | Menisci, ACL, cartilage |
| PD fat-sat | Coronal | Collaterals, meniscal body |
| PD fat-sat | Axial | Patellofemoral, retinaculum |
| T1 or T2 | Sagittal | Anatomy, bone detail |
| STIR (optional) | Coronal | Bone marrow oedema if query fracture |
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"You are asked to design an MRI protocol for a 25-year-old with suspected ACL tear after a twisting injury. The referrer also mentions the patient heard a 'pop' and has significant effusion."
EXCEPTIONAL ANSWER
For suspected ACL tear, I would request: (1) Sagittal PD fat-sat - this is the workhorse sequence for ACL assessment, showing the ligament fibres and any intrinsic signal abnormality. (2) Coronal PD fat-sat - for assessing the collateral ligaments (MCL often injured with ACL) and meniscal body. (3) Axial PD fat-sat - for patellofemoral assessment and meniscal horn detail. (4) Sagittal STIR or T2 fat-sat - to assess bone marrow oedema pattern (pivot-shift bruising: lateral femoral condyle and posterior lateral tibial plateau is classic for ACL injury). The effusion will be visible on all sequences but most conspicuous on T2/STIR.
KEY POINTS TO SCORE
PD fat-sat is workhorse for knee soft tissue
STIR/T2 fat-sat for bone marrow oedema (pivot-shift pattern)
Coronal for MCL (commonly associated injury)
Bone bruise pattern helps confirm mechanism
COMMON TRAPS
✗Not including fat-suppressed sequences
✗Missing associated injuries (MCL, meniscus)
✗Not recognising bone bruise significance
VIVA SCENARIOStandard
EXAMINER
"A 45-year-old presents with worsening back pain, night sweats, and elevated inflammatory markers. The referring clinician suspects spinal infection."
EXCEPTIONAL ANSWER
For suspected spinal infection, I would request: (1) Sagittal T1 - to assess marrow infiltration (infection replaces normal bright marrow fat with dark signal), vertebral body anatomy, and extent. (2) Sagittal STIR - highly sensitive for marrow oedema, will show involved vertebral bodies brightly. (3) Sagittal and Axial T1 fat-sat post-gadolinium - enhancement patterns help differentiate infection from tumour, shows epidural/paraspinal abscess. (4) Axial T2 - for cord compression assessment, epidural collection. Key findings: T1 hypointensity in adjacent vertebral bodies, disc involvement (infection crosses disc, tumour spares disc), paraspinal phlegmon/abscess, epidural extension.
KEY POINTS TO SCORE
T1 shows marrow replacement (goes dark)
STIR sensitive for marrow oedema
Post-Gd T1 fat-sat for abscess, enhancement pattern
Disc involvement key differentiator from tumour
COMMON TRAPS
✗Using STIR after gadolinium (incorrect)
✗Missing epidural extension (cord compression)
✗Not differentiating from tumour (disc involvement)
VIVA SCENARIOStandard
EXAMINER
"A 60-year-old with hip pain is referred for MRI. There is concern for avascular necrosis."
EXCEPTIONAL ANSWER
For AVN assessment, key sequences are: (1) Coronal T1 - essential for detecting the geographic marrow abnormality, shows the characteristic band pattern (dark line separating normal from abnormal marrow). (2) Coronal STIR - shows reactive oedema in surrounding marrow, helps stage disease activity. (3) Sagittal T1 and STIR for extent and contralateral hip comparison. Classic MRI features: 'Double-line sign' on T2 - inner bright line (granulation tissue) with outer dark line (sclerotic bone). T1 shows geographic demarcation with dark band. In late stages, femoral head collapse and subchondral fracture visible. Staging uses extent of involvement and presence of collapse.
KEY POINTS TO SCORE
T1 best for detecting marrow abnormality
Double-line sign on T2 pathognomonic
Dark band on T1 represents interface
STIR shows reactive oedema, activity
Always image both hips (bilateral in 50%)
COMMON TRAPS
✗Missing early AVN (may be subtle)
✗Not imaging contralateral hip
✗Confusing transient osteoporosis (diffuse oedema) with AVN
MRI Sequences Quick Reference
High-Yield Exam Summary
T1 vs T2 Signal
- •T1: Fat BRIGHT, Water DARK
- •T2: Water BRIGHT, Fat intermediate
- •Both: Cortical bone, air, calcium = DARK
- •Marrow pathology: T1 dark (replaces fat)
Sequence Selection
- •Anatomy/marrow: T1
- •Fluid/oedema: T2 or STIR
- •Menisci/ligaments: PD fat-sat
- •Bone marrow oedema: STIR (most sensitive)
- •Post-contrast: T1 fat-sat (NEVER STIR)
Fat Suppression
- •STIR: Uniform, robust, near metal OK
- •Chemical fat-sat: Fast, fails at interfaces
- •Use STIR for spine, whole body, hardware
- •NEVER STIR + gadolinium (nulls enhancement)
Regional Workhorse Sequences
- •Knee: PD fat-sat (menisci, ligaments)
- •Shoulder: PD fat-sat (rotator cuff, labrum)
- •Spine: T1 + T2 + STIR (complete assessment)
- •Infection/tumour: Add Gd T1 fat-sat