High Yield Overview
Paediatric Imaging: Special Considerations
—Growth Plate Closure
14-16Girls y, Boys 16-18y
—Bone Age Assessment
3Greulich-Pyle or TW
—Paediatric CT Dose
50-75%Reduce vs adult
—MRI Sedation Age
6Typically < years
Salter-Harris Classification
Type I: Through physis (SALTER)
Type II: Above physis (metaphysis) - most common
Type III: Lower (epiphysis)
Type IV: Through all (metaphysis + physis + epiphysis)
Type V: Crush/compression of physis
Key: Higher types = worse prognosis for growth disturbance
Critical Must-Knows
- Ossification centres appear and fuse in predictable sequences
- CRITOE mnemonic for elbow ossification (ages 1,3,5,7,9,11)
- Salter-Harris classification for physeal injuries (Types I-V)
- ALARA principle - minimise radiation especially in children
- Normal variants mimic pathology: accessory ossicles, growth plates
Examiner's Pearls
- "Always compare with contralateral side when uncertain
- "Growth plate injuries may be radiographically occult - suspect clinically
- "Toddler's fracture: spiral tibial fracture, often subtle
- "Plastic deformity unique to paediatric bone
- "MRI preferred for suspected physeal bar or AVN
Exam Warning
Paediatric imaging questions commonly test: ossification centre sequences (especially elbow), Salter-Harris classification interpretation, and distinguishing normal variants from pathology. Always mention radiation safety and ALARA when discussing paediatric CT.
Ossification Centre Development
Understanding the sequence and timing of ossification centre appearance is fundamental to interpreting paediatric radiographs. Secondary ossification centres appear at predictable ages and their absence or premature appearance may indicate pathology.
Mnemonic
C-R-I-T-O-ECRITOE - Elbow Ossification Centres
C
C = Capitellum (1 year)
R
R = Radial head (3 years)
I
I = Internal (medial) epicondyle (5 years)
T
T = Trochlea (7 years)
O
O = Olecranon (9 years)
E
E = External (lateral) epicondyle (11 years)
Memory Hook:Medial epicondyle ALWAYS ossifies before trochlea - if medial epicondyle appears 'absent' with visible trochlea, suspect avulsed fragment trapped in joint
Salter-Harris Classification
Salter-Harris Classification of Physeal Injuries
| Type | Description | Frequency | Growth Disturbance Risk | Mnemonic |
|---|---|---|---|---|
| Type I | Separation through physis only | 6% | Rare (unless blood supply) | SALTER = Slip Along |
| Type II | Through physis + metaphyseal fragment | 75% | Rare | Above (metaphysis) |
| Type III | Through physis + epiphyseal fragment | 8% | Moderate - involves joint | Lower (epiphysis) |
| Type IV | Through metaphysis, physis, and epiphysis | 10% | High - disrupts germinal layer | Through Everything |
| Type V | Crush injury to physis | 1% | Very High - often missed initially | ERasure (compression) |
Type V Characteristics
Often normal initial X-ray. Diagnosed retrospectively when growth arrest occurs. Mechanism: axial loading. Common sites: ankle, knee. MRI may show physeal widening/signal change acutely.
Rang Type VI
Added classification: peripheral physeal injury (e.g., lawnmower injury). Causes localised growth disturbance and angular deformity due to peripheral bridge formation.
Normal Variants vs Pathology
Common Normal Variants Mimicking Pathology
| Variant | Location | Key Features | Differentiation from Pathology |
|---|---|---|---|
| Bipartite patella | Superolateral patella | Smooth, well-corticated margins | Bilateral in 50%, no oedema on MRI |
| Os trigonum | Posterior talus | Round, smooth accessory ossicle | Fused by 15y; if symptomatic = os trigonum syndrome |
| Accessory navicular | Medial foot | Type II most symptomatic | Synchondrosis may be painful with overuse |
| Irregular ossification | Distal femoral epiphysis | Irregular, sclerotic appearance | Normal finding age 2-8, no treatment needed |
| Sinding-Larsen-Johansson | Inferior patella pole | Fragmentation of pole | Traction apophysitis, not avulsion |
Rule of Twos
If in doubt, get comparison views of the contralateral side. Bilateral symmetry suggests normal variant; unilateral findings warrant further investigation.
Radiation Safety in Children
Children have 10-15x higher lifetime radiation-induced cancer risk compared to adults due to longer life expectancy and actively dividing cells. ALARA principle is paramount.
Paediatric Radiation Dose Reduction Strategies
| Strategy | Implementation | Dose Reduction |
|---|---|---|
| Size-based protocols | Reduce mAs/kVp based on weight | 50-75% |
| Reduce number of phases | Single phase CT rather than multiphasic | 60-70% |
| Limit scan coverage | Scan only area of interest | Variable |
| Shield radiosensitive organs | Gonadal/thyroid shields when possible | 90% to shielded area |
| Consider alternatives | MRI or ultrasound instead of CT | 100% (no radiation) |
Mnemonic
A-L-A-R-AALARA Principle
A
A = As
L
L = Low
A
A = As
R
R = Reasonably
A
A = Achievable
Memory Hook:Document justification for any CT in children. Consider if ultrasound or MRI could answer the clinical question.
MRI Considerations in Children
Preferred Over CT
Physeal/epiphyseal injuries, Osteomyelitis assessment, Occult hip pathology (Perthes, SCFE), Soft tissue masses, Spinal dysraphism, AVN staging
Unique Paediatric MRI Features
Red marrow signal normal until 25y (low T1, high T2 STIR), Cartilaginous epiphyses show intermediate signal, Unfused physes appear as high T2 signal band
Common Paediatric Fracture Patterns
Unique Paediatric Fracture Types
| Fracture Type | Mechanism | Imaging Features | Management Implications |
|---|---|---|---|
| Torus (buckle) | Axial compression | Buckling of cortex, intact periosteum | Stable, splint 3-4 weeks |
| Greenstick | Bending force | Incomplete fracture, one cortex intact | May need completion to reduce |
| Plastic deformity | Bending force | Bowing without visible fracture line | Unique to children, may need correction |
| Toddler's fracture | Rotational force | Spiral tibial fracture, often subtle | High clinical suspicion, often occult |
| Supracondylar humerus | FOOSH | Displaced posterior fat pad, sail sign | Check for neurovascular injury |
Toddler's Fracture Key Points
Age 9 months to 3 years. Non-displaced spiral fracture of tibial diaphysis. May be radiographically occult initially. Repeat X-ray at 7-10 days shows periosteal reaction. MRI confirms diagnosis if needed.
Bone Age Assessment
Bone Age Assessment Methods
| Method | Technique | Advantages | Limitations |
|---|---|---|---|
| Greulich-Pyle | Compare left hand/wrist to atlas | Quick, widely used | Inter-observer variability |
| Tanner-Whitehouse (TW3) | Score individual bones mathematically | More reproducible | Time-consuming |
| Automated (AI-based) | Computer analysis of hand X-ray | Consistent, rapid | Requires software, validation ongoing |
Advanced Bone Age Causes
Hyperthyroidism, Growth hormone excess, Precocious puberty, Obesity, CAH (congenital adrenal hyperplasia)
Delayed Bone Age Causes
Hypothyroidism, Growth hormone deficiency, Constitutional delay, Chronic illness, Malnutrition, Turner syndrome
Hip Imaging in Children
DDH Imaging by Age
| Age | Modality of Choice | Key Measurements |
|---|---|---|
| Less than 4 months | Ultrasound (Graf method) | Alpha angle greater than 60° normal, Beta angle less than 55° normal |
| 4-6 months | Ultrasound or X-ray | Transition period, depends on ossific nucleus |
| Greater than 6 months | X-ray (AP pelvis) | Hilgenreiner, Perkin, Shenton lines; acetabular index |
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A 4-year-old presents with elbow pain after a fall. X-ray shows a visible trochlea but you cannot identify the medial epicondyle ossification centre."
EXCEPTIONAL ANSWER
This is a displaced medial epicondyle avulsion fracture with the fragment trapped in the joint. Using CRITOE, the medial epicondyle (I) should ossify before the trochlea (T). If the trochlea is visible but the medial epicondyle is 'absent', the epicondyle must be displaced. The fragment is likely incarcerated in the joint space, appearing as an intra-articular ossific density.
KEY POINTS TO SCORE
CRITOE sequence: medial epicondyle (5y) before trochlea (7y)
Absent expected ossification centre = displaced fragment
Medial epicondyle avulsion associated with elbow dislocations
Treatment: Open reduction if fragment in joint
COMMON TRAPS
✗Assuming normal anatomy without applying CRITOE
✗Missing incarcerated fragment on poorly positioned films
✗Not checking for associated ulnar nerve injury
VIVA SCENARIOStandard
EXAMINER
"A 2-year-old presents with refusal to weight-bear for 2 days. Initial X-rays appear normal. What is your differential diagnosis and management plan?"
EXCEPTIONAL ANSWER
The most likely diagnosis is a toddler's fracture - a non-displaced spiral fracture of the tibial diaphysis that may be radiographically occult initially. This is common in children aged 9 months to 3 years. Management includes immobilisation in a below-knee cast and repeat X-ray in 7-10 days looking for periosteal reaction. If diagnosis is urgent or uncertain, MRI will show the fracture and marrow oedema.
KEY POINTS TO SCORE
Toddler's fracture: common in 9 months to 3 years
Non-displaced spiral tibial diaphysis fracture
Often occult on initial X-ray
Periosteal reaction visible at 7-10 days
MRI confirms if needed urgently
COMMON TRAPS
✗Dismissing symptoms due to normal X-ray
✗Not considering NAI (non-accidental injury) in differential
✗Ordering CT (radiation, may still miss non-displaced fracture)
VIVA SCENARIOStandard
EXAMINER
"You are asked to review a CT scan of a 6-year-old with abdominal trauma. The radiologist mentions incidental bone lesions. The parent asks about radiation exposure."
EXCEPTIONAL ANSWER
I would acknowledge the legitimate concern about radiation in children. Children have 10-15 times higher lifetime cancer risk from radiation compared to adults due to longer life expectancy and actively dividing cells. However, if the CT was clinically indicated for trauma, the immediate benefit outweighs theoretical long-term risk. I would explain that paediatric protocols should reduce dose by 50-75% compared to adults using size-based protocols. The ALARA principle guides all imaging decisions in children.
KEY POINTS TO SCORE
Children have 10-15x higher radiation cancer risk vs adults
ALARA principle: As Low As Reasonably Achievable
Paediatric protocols reduce dose 50-75%
Size-based mA adjustment, limit coverage, single phase
Document justification for CT in children
COMMON TRAPS
✗Dismissing parental concerns
✗Not knowing paediatric dose reduction strategies
✗Unable to justify CT indication
Paediatric Imaging Essentials
High-Yield Exam Summary
CRITOE Elbow Ossification
- •Capitellum: 1 year
- •Radial head: 3 years
- •Internal (medial) epicondyle: 5 years
- •Trochlea: 7 years
- •Olecranon: 9 years
- •External (lateral) epicondyle: 11 years
Salter-Harris (SALTER)
- •Type I: Slip Along (physis only) - 6%
- •Type II: Above (metaphysis) - 75%, most common
- •Type III: Lower (epiphysis) - 8%, involves joint
- •Type IV: Through Everything - 10%, highest risk
- •Type V: Erasure (crush) - 1%, often missed
Radiation Safety
- •Children: 10-15x lifetime cancer risk vs adults
- •ALARA: As Low As Reasonably Achievable
- •Reduce paediatric CT dose 50-75%
- •Consider US/MRI alternatives first
- •Shield gonads/thyroid when possible
Common Traps
- •Missing entrapped medial epicondyle (CRITOE)
- •Normal X-ray doesn't exclude toddler's fracture
- •Bipartite patella is bilateral in 50%
- •Always image BOTH hips in SCFE
- •Normal variants: compare contralateral side