High Yield Overview
MRI Imaging Principles
Magnetic Resonance for Orthopaedics
1.5-3TClinical Field Strength
T1Fat = Bright
T2Fluid = Bright
0Ionizing Radiation
MRI Signal Characteristics
Fat
PatternT1: Bright | T2: Intermediate | STIR: Dark
TreatmentShort T1
Water/Fluid
PatternT1: Dark | T2: Bright | STIR: Bright
TreatmentLong T1, Long T2
Muscle
PatternT1: Intermediate | T2: Intermediate | STIR: Intermediate
TreatmentReference standard
Cortical Bone
PatternT1: Dark | T2: Dark | STIR: Dark
TreatmentNo mobile protons
Fibrocartilage
PatternT1: Dark | T2: Dark | PD: Dark
TreatmentOrganized collagen
Critical Must-Knows
- T1-weighted: Fat is BRIGHT, fluid is DARK. Best for anatomy. Short TR, short TE.
- T2-weighted: Fluid is BRIGHT, fat is intermediate. Best for pathology/edema. Long TR, long TE.
- STIR: Fat-suppressed T2. Fluid bright, fat dark. Best for bone marrow edema.
- Proton Density (PD): Best for menisci and ligaments. Long TR, short TE.
- Gadolinium enhances on T1: Post-contrast T1 shows vascularized tissue/inflammation bright.
Examiner's Pearls
- "T1 = Anatomy, T2 = Pathology (water/edema).
- "STIR nulls fat signal - excellent for bone bruise/edema.
- "Menisci are dark on all sequences - bright signal = tear.
- "ACL should be parallel to Blumensaat line on sagittal - disruption = tear.
- "Gadolinium is contraindicated in severe renal impairment (NSF risk).
Clinical Imaging
Imaging Gallery
Click to expand
Click to expand
Click to expand
MRI Safety - Absolute Contraindications
Absolute Contraindications
- Cardiac pacemaker (non-MRI conditional)
- Cochlear implants (most)
- Ferromagnetic aneurysm clips
- Metallic ocular foreign body
- Some older cardiac valves
Relative Contraindications
- Claustrophobia (sedation may help)
- First trimester pregnancy (caution, not absolute)
- Renal impairment + Gd (NSF risk if eGFR less than 30)
- Implants - check MRI compatibility
MRI Physics Basics
How MRI Works
- Strong Magnetic Field (B0): Aligns hydrogen protons along field direction
- Radiofrequency Pulse (RF): Tips protons out of alignment
- Relaxation: Protons return to equilibrium, emitting RF signal
- Signal Detection: Coils detect emitted RF and create image
T1 and T2 Relaxation
T1 vs T2 Relaxation
| Parameter | T1 (Longitudinal) | T2 (Transverse) |
|---|---|---|
| Definition | Recovery of longitudinal magnetization | Decay of transverse magnetization |
| What it measures | Spin-lattice interaction | Spin-spin interaction |
| Fat signal | BRIGHT (short T1) | Intermediate |
| Water signal | DARK (long T1) | BRIGHT (long T2) |
| Clinical use | Anatomy, fat, hemorrhage | Pathology, edema, fluid |
Pulse Sequence Parameters
TR (Repetition Time)
Time between RF pulses.
- Short TR (less than 600ms): T1-weighting
- Long TR (greater than 2000ms): T2 or PD weighting
TE (Echo Time)
Time between RF pulse and signal measurement.
- Short TE (less than 30ms): T1 or PD weighting
- Long TE (greater than 80ms): T2-weighting
Key Pulse Sequences
T1-Weighted Imaging
T1-Weighted
Parameters: Short TR, Short TE
Appearance:
- Fat: BRIGHT (high signal)
- Fluid/Water: DARK (low signal)
- Muscle: Intermediate
Uses:
- Anatomical detail
- Post-gadolinium imaging
- Fat-containing lesions (lipoma)
- Subacute hemorrhage (methemoglobin)
T2-Weighted Imaging
T2-Weighted
Parameters: Long TR, Long TE
Appearance:
- Fluid/Water: BRIGHT (high signal)
- Fat: Intermediate-bright
- Muscle: Intermediate-dark
Uses:
- Pathology detection (edema, effusion, cysts)
- Soft tissue abnormalities
- Joint effusions
- Tumors (often bright)
STIR (Short Tau Inversion Recovery)
STIR
Key Feature: Fat suppression via inversion pulse
Appearance:
- Fat: DARK (suppressed)
- Fluid/Edema: BRIGHT
- Muscle: Intermediate
Uses:
- Bone marrow edema (bone bruise)
- Stress fractures
- Infection
- Tumor extent
- Best sequence for pathology near fat
Proton Density (PD)
Proton Density
Parameters: Long TR, Short TE
Appearance:
- Maximizes signal from tissues with high proton density
- Intermediate contrast between T1 and T2
Uses:
- Meniscal tears (best sequence)
- Ligament assessment (ACL, MCL)
- Articular cartilage
- Labral tears
Sequence Selection Summary
| Sequence | TR | TE | Fat | Fluid | Best For |
|---|---|---|---|---|---|
| T1 | Short | Short | Bright | Dark | Anatomy |
| T2 | Long | Long | Intermediate | Bright | Pathology |
| PD | Long | Short | Intermediate | Intermediate | Menisci/ligaments |
| STIR | Long | Long + IR | Dark | Bright | Bone edema |
| T1 + Gd | Short | Short | Bright | Variable | Enhancement |
Orthopaedic Applications
Meniscus Evaluation
Mnemonic
SIGMeniscal Tear Signs
S
Signal
Increased intrameniscal signal reaching articular surface
I
Irregular
Irregular contour, truncation, or displacement
G
Grade 3
Signal contacts articular surface = tear
Memory Hook:Normal meniscus is uniformly DARK on all sequences
Ligament Assessment
ACL Tear Signs:
- Discontinuity of fibers
- Abnormal orientation (not parallel to Blumensaat line)
- Increased signal on T2/PD
- Secondary signs: bone bruise pattern, anterior tibial translation
Rotator Cuff:
- Full-thickness tear: Fluid signal gap from bursal to articular surface
- Partial tear: High signal not extending full thickness
- Tendinopathy: Increased signal without discontinuity
Bone Marrow Edema
Best detected on STIR or fat-suppressed T2:
- Bone bruise (contusion)
- Stress fracture
- Avascular necrosis
- Infection (osteomyelitis)
- Tumor infiltration
Gadolinium Contrast
Indications
- Tumor characterization: Enhancement suggests vascularity
- Infection: Rim enhancement in abscess, diffuse in cellulitis
- Post-operative assessment: Scar vs recurrent pathology
- MR arthrography: Intra-articular injection for labral/capsular detail
Complications
Nephrogenic Systemic Fibrosis (NSF)
- Rare but serious complication of gadolinium in renal impairment
- Risk highest with linear agents, minimal with macrocyclic
- Contraindicated if eGFR less than 30 mL/min
- Check renal function before gadolinium administration
- Symptoms: skin thickening, fibrosis of internal organs
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
MRI Physics Viva
EXAMINER
"Explain the difference between T1 and T2 weighted imaging and when you would use each."
EXCEPTIONAL ANSWER
T1 and T2 refer to different relaxation processes. T1 (longitudinal relaxation) measures how quickly protons realign with the magnetic field - tissues with short T1 (fat) appear bright. T2 (transverse relaxation) measures how quickly protons dephase - tissues with long T2 (fluid) appear bright. T1 is best for anatomy as fat provides natural contrast. T2 is best for pathology as edema, effusions, and most pathological processes contain water and appear bright.
KEY POINTS TO SCORE
T1: Fat bright, fluid dark - best for anatomy
T2: Fluid bright, fat intermediate - best for pathology
T1 uses short TR/short TE, T2 uses long TR/long TE
Both are affected by tissue composition
COMMON TRAPS
✗Saying T2 shows 'water' rather than 'fluid/edema'
✗Confusing TR and TE effects
✗Not mentioning clinical applications
LIKELY FOLLOW-UPS
"What is STIR and when would you use it?"
"What sequence is best for meniscal tears?"
"How does gadolinium affect T1 signal?"
VIVA SCENARIOStandard
MRI Contraindications
EXAMINER
"A patient with a pacemaker needs imaging of their knee. Can they have an MRI?"
EXCEPTIONAL ANSWER
Traditional pacemakers are an absolute contraindication to MRI due to risk of device malfunction, lead heating, and induced currents. However, newer 'MRI-conditional' pacemakers exist that can safely undergo MRI under specific conditions (field strength, SAR limits, monitoring). I would check the device manufacturer and model, review the MRI compatibility documentation, consult cardiology, and if MRI-conditional, ensure appropriate protocols are followed. If non-MRI-compatible, alternative imaging such as CT or ultrasound would be considered.
KEY POINTS TO SCORE
Traditional pacemakers = absolute contraindication
MRI-conditional devices exist - check documentation
Requires cardiology consultation and specific protocols
Alternative imaging if MRI not possible
COMMON TRAPS
✗Blanket statement that all pacemakers are contraindicated
✗Not knowing about MRI-conditional devices
✗Forgetting to mention alternatives
LIKELY FOLLOW-UPS
"What other implants are contraindicated?"
"What are the risks of MRI with metallic implants?"
"Is pregnancy a contraindication to MRI?"
VIVA SCENARIOStandard
Bone Marrow Edema
EXAMINER
"What MRI findings would you expect in a tibial stress fracture?"
EXCEPTIONAL ANSWER
On MRI, a stress fracture shows: (1) Bone marrow edema - best seen on STIR or fat-suppressed T2 as high signal within the medullary cavity; (2) Low signal line on T1 representing the fracture line, often perpendicular to cortex; (3) Periosteal edema - high signal surrounding the cortex on fluid-sensitive sequences; (4) In later stages, callus formation and cortical thickening. The edema pattern is typically linear or geographic, unlike the more diffuse pattern of infection or the focal pattern of tumor.
KEY POINTS TO SCORE
Bone marrow edema on STIR (bright signal)
Low signal fracture line on T1
Periosteal edema on fluid-sensitive sequences
Pattern helps differentiate from infection/tumor
COMMON TRAPS
✗Only mentioning one sequence
✗Not describing the fracture line appearance
✗Forgetting differential considerations
LIKELY FOLLOW-UPS
"How would you differentiate from osteomyelitis?"
"What is the role of CT in stress fractures?"
"When would you use a bone scan instead?"
MRI Principles Exam Day Cheat Sheet
High-Yield Exam Summary
Signal Characteristics
- •T1: Fat BRIGHT, Fluid DARK
- •T2: Fluid BRIGHT, Fat intermediate
- •STIR: Fat DARK (suppressed), Edema BRIGHT
- •PD: Best for menisci and ligaments
Sequence Parameters
- •T1: Short TR, Short TE
- •T2: Long TR, Long TE
- •PD: Long TR, Short TE
- •TR controls T1, TE controls T2 weighting
Clinical Applications
- •T1 = Anatomy, hemorrhage (subacute), post-Gd
- •T2 = Pathology, edema, effusion, tumors
- •STIR = Bone marrow edema, stress fractures
- •PD-FS = Menisci, ligaments, cartilage
Contraindications
- •Pacemaker (non-MRI conditional) = Absolute
- •Cochlear implant = Usually absolute
- •Ferromagnetic aneurysm clips = Absolute
- •Gadolinium: eGFR less than 30 = Contraindicated