High Yield Overview

Paediatric Imaging Principles

Radiation Safety, Modality Selection & the Growing Skeleton

ALARAAs Low As Reasonably Achievable — critical in children

3-5xChildren are 3-5x more radiosensitive than adults

USSFirst-line for DDH, effusion, soft tissue pathology

CRITOEElbow ossification centre appearance order

MRINo radiation — preferred for complex paediatric assessment

RemodellingGreater in younger children, near the physis, in the plane of motion

CTMinimised in children due to radiation risk — use only when essential

Image GentlyCampaign: adjust technique for child size

Paediatric Imaging Modality Selection Hierarchy

Ultrasound: DDH (less than 6mo), joint effusion, soft tissue, guided aspiration — NO radiation

Radiograph: Fracture screening, alignment, ossification centre assessment — LOW radiation

MRI: Complex fractures (physeal), infection, tumour, cartilage — NO radiation

CT: Minimise in children — use ONLY for complex fractures, spinal trauma, or tumour characterisation

Fluoroscopy: Intraoperative only — mandatory ALARA (pulse mode, collimation, shielding)

Key: Non-ionising modalities (USS, MRI) first. CT only when absolutely essential and with paediatric protocols.

Critical Must-Knows

Children are 3-5 times more radiosensitive than adults due to rapidly dividing cells and longer remaining lifespan for stochastic effects to manifest.
ALARA principle is PARAMOUNT in paediatric imaging: use non-ionising modalities (USS, MRI) first whenever possible.
Ossification centres appear sequentially: CRITOE for the elbow, specific timetables for the hip, knee, and wrist.
The growing skeleton creates unique imaging challenges: unfused growth plates mimic fractures, ossification centres mimic avulsion fractures, and cartilaginous structures are radiolucent.
Ultrasound is the preferred first-line investigation for many paediatric conditions: DDH (before 6 months), joint effusion, soft tissue masses, and guided procedures.

Examiner's Pearls

"
CRITOE: Capitellum (1yr), Radial head (3yr), Internal (medial) epicondyle (5yr), Trochlea (7yr), Olecranon (9yr), External (lateral) epicondyle (11yr) — the order of elbow ossification centre appearance.
"
The trapped medial epicondyle: following an elbow dislocation, the medial epicondyle avulsion may be trapped within the joint mimicking the trochlea. Compare with the contralateral elbow and check CRITOE sequence — if the trochlea appears BEFORE the medial epicondyle, it is a displaced medial epicondyle.
"
Image Gently campaign principles: reduce dose (lower kVp, mAs), use appropriate collimation (include only what is necessary), and scan once (avoid repeat/unnecessary imaging).
"
Remodelling potential: greatest in younger children, closer to the physis, and in the plane of motion of the adjacent joint. Angular deformity opposite the direction of joint motion has the LEAST remodelling potential.
"
Greenstick and torus (buckle) fractures are unique to children due to the more porous, elastic nature of the paediatric cortex.

Exam Warning

Paediatric imaging principles are frequently tested in fellowship examinations. You must know: why children are more radiosensitive (dividing cells, longer lifespan, smaller body = higher organ doses), ALARA implementation, CRITOE ossification centre sequence, the trapped medial epicondyle pitfall, age-appropriate imaging selection (USS for DDH, MRI for complex injuries), and the unique fracture patterns of the paediatric skeleton (greenstick, torus, physeal). Classic traps: ordering CT instead of MRI for paediatric assessment, not knowing CRITOE, and confusing ossification centres with fractures.

Mnemonic

CRITOEElbow Ossification Centre Sequence

Capitellum (1 year)

First to appear. Present by age 1. The primary landmark for other centres

Radial head (3 years)

Appears by age 3. Small, round ossification that should not be confused with a fracture fragment

Internal (medial) epicondyle (5 years)

Appears by age 5. THE most important centre — it is the one avulsed in paediatric elbow dislocations

Trochlea (7 years)

Appears by age 7. ALWAYS appears AFTER the medial epicondyle. If it is 'present' without a medial epicondyle — the epicondyle is TRAPPED in the joint

Olecranon (9 years)

Appears by age 9. Apophyseal centre — contributes to the proximal ulna. Can be confused with a fracture

External (lateral) epicondyle (11 years)

Last to appear, by age 11. All centres have appeared by early adolescence

Memory Hook:CRITOE: 1-3-5-7-9-11 years. If the trochlea appears 'before' the medial epicondyle — the medial epicondyle is TRAPPED in the joint.

Mnemonic

CHILDWhy Children Are More Radiosensitive

Cells dividing rapidly

Children's cells are dividing more rapidly (growth). Dividing cells are MORE sensitive to radiation damage (Law of Bergonié and Tribondeau)

Higher organ doses per mAs

Smaller body = less tissue attenuation = higher organ doses for the same machine settings. Paediatric protocols MUST reduce exposure parameters

Immature DNA repair

Children have less robust DNA repair mechanisms than adults, leading to more unrepaired radiation-induced DNA damage

Longer lifespan remaining

Stochastic effects (cancer induction) may take decades to manifest. A child has 60-80 years for a radiation-induced cancer to develop (vs 20-30 years in a 50-year-old)

Dose accumulation over time

Cumulative lifetime radiation dose is higher when imaging begins in childhood. Each additional scan adds to the total

Memory Hook:CHILD: why we must be extra careful — Children have dividing cells, Higher doses per mAs, Immature DNA repair, Longer lifespan, and Dose accumulation.

Mnemonic

PGTBSPaediatric Fracture Patterns

Plastic deformation (bowing)

The bone bends without fracturing — unique to children. No visible fracture line. May require comparison views. Most common in ulna/fibula

Greenstick fracture

Incomplete fracture — one cortex fractures while the opposite cortex bends (like breaking a green stick). The intact periosteum on the compression side acts as a tether

Torus (buckle) fracture

Cortical compression failure — the cortex buckles outward without disrupting. Most common at distal radius. Subtle — look for cortical irregularity

Bow fracture (complete)

A complete fracture but with thick periosteum preventing significant displacement. More common in younger children

Salter-Harris (physeal)

Growth plate fractures — classified I-V. The growth plate is the weakest link in the paediatric musculoskeletal chain (weaker than ligaments)

Memory Hook:PGTBS: Plastic deformation, Greenstick, Torus, Bow, Salter-Harris — the spectrum of paediatric fracture patterns.

Overview

Paediatric imaging requires a fundamentally different approach from adult imaging. Three principles underpin all paediatric imaging decisions: (1) radiation safety — children are 3-5 times more radiosensitive than adults and have a longer remaining lifespan for stochastic effects to manifest; (2) the growing skeleton — cartilaginous structures are radiolucent, ossification centres appear sequentially and can mimic pathology, and growth plates create unique injury patterns; (3) clinical context — children cannot reliably describe symptoms, examination findings may be non-specific, and the differential diagnosis for musculoskeletal complaints is different from adults.

ALARA in Paediatric Imaging

The ALARA principle (As Low As Reasonably Achievable) is paramount in paediatric imaging. Practical implementation: (1) Always consider whether imaging is necessary — clinical assessment alone may suffice (Ottawa ankle rules apply from age 6). (2) Use non-ionising modalities FIRST (USS for soft tissue, MRI for complex assessment). (3) If ionising imaging is needed, use PAEDIATRIC PROTOCOLS with reduced kVp and mAs. (4) COLLIMATE tightly — include ONLY the anatomy needed (avoid whole-body scatter). (5) Shield radiosensitive organs (gonads, thyroid) when they are in or near the beam. (6) Scan ONCE — avoid unnecessary repeat imaging. The Image Gently campaign provides protocols for paediatric dose reduction.

The Growing Skeleton Challenge

Key imaging challenges: (1) Ossification centres: appear at predictable ages but can be confused with fracture fragments. The most critical example is the TRAPPED MEDIAL EPICONDYLE in elbow dislocation — compare with CRITOE sequence and obtain comparison views of the uninjured side. (2) Growth plates: the physis is the weakest link (weaker than ligaments) — what would be a ligament injury in an adult is a physeal fracture in a child. Salter-Harris Type I may have a NORMAL radiograph. (3) Remodelling: children can correct angular deformity through growth. Greatest potential in younger children, near the physis, in the plane of motion. (4) Non-ossified cartilage: articular cartilage and epiphyseal cartilage are radiolucent — MRI or USS may be needed to evaluate these structures.

Clinical Imaging

Imaging Gallery

Paediatric elbow comparison views showing ossification centre assessment — Paediatric elbow imaging demonstrating the importance of understanding ossification centre development (CRITOE sequence) for accurate radiographic interpretation. Comparison views of the contralateral uninjured elbow are essential for differentiating normal ossification centres from fracture fragments.Credit: Open-i (NIH) (Open Access (CC BY))

Paediatric fracture patterns unique to the growing skeleton — Paediatric fracture patterns demonstrating the unique injury patterns of the growing skeleton: torus (buckle) fractures from cortical compression, greenstick fractures with intact opposite cortex, and physeal (growth plate) injuries that require specific classification (Salter-Harris) and management.Credit: Open-i (NIH) (Open Access (CC BY))

Systematic Approach

Paediatric Imaging Modality Selection

Age-Appropriate Imaging Selection for Paediatric Conditions

Clinical Scenario	Preferred Imaging	Rationale
DDH screening (less than 6 months)	Ultrasound (hip USS, Graf classification)	Femoral head not yet ossified. USS shows cartilaginous anatomy. No radiation
DDH assessment (more than 6 months)	AP pelvis radiograph (Perkins, Hilgenreiner lines)	Femoral head ossification centre now visible. Radiograph is standard
Limping child (2-5 years)	AP pelvis + frog lateral radiograph. Blood tests (CRP, FBC)	Differential: irritable hip vs Perthes vs septic arthritis. USS if effusion suspected
Elbow injury (child)	AP + lateral elbow radiograph. Comparison views if needed	Assess fat pads, anterior humeral line, CRITOE centres. Compare with contralateral side
Suspected physeal injury (normal X-ray)	MRI (if management would change)	Salter-Harris I may have normal radiographs. MRI shows physeal oedema and confirms the diagnosis
Joint effusion/septic arthritis	Ultrasound (detects effusion + guides aspiration)	NO radiation. Real-time guided aspiration for diagnostic synovial fluid analysis
Suspected NAI	Skeletal survey (full-body radiograph series)	Standardised protocol: AP/lateral skull, AP chest, AP abdomen, AP all limbs. Follow-up repeat survey at 2 weeks
Complex fracture/tumour assessment	MRI (NO radiation, excellent soft tissue contrast)	Avoids CT radiation. Shows cartilaginous structures, physeal involvement, marrow pathology

Clinical Applications

Ossification Centre Assessment

Understanding the sequence and timing of ossification centre appearance is fundamental to paediatric imaging interpretation. The key clinical relevance is distinguishing NORMAL ossification centres from fracture fragments.

Elbow (CRITOE): The most commonly tested ossification centre sequence. Capitellum (1yr), Radial head (3yr), Internal (medial) epicondyle (5yr), Trochlea (7yr), Olecranon (9yr), External (lateral) epicondyle (11yr). The critical clinical application is the TRAPPED MEDIAL EPICONDYLE: following a paediatric elbow dislocation, the medial epicondyle (which has been avulsed by the ulnar collateral ligament) can become trapped within the joint. On the post-reduction radiograph, it may be misinterpreted as the trochlea. KEY RULE: if the trochlea is 'present' but the medial epicondyle is NOT visible in its normal position, the medial epicondyle is trapped in the joint and requires open surgical removal.

Hip: The femoral head ossification centre appears at 3-6 months. Its absence before this age means that DDH assessment requires ultrasound (radiographs cannot visualise the cartilaginous femoral head). After 6 months, the AP pelvis radiograph using Perkins and Hilgenreiner lines becomes the standard assessment tool.

Wrist: The carpal ossification centres appear in a roughly circular sequence — the capitate (1-3 months) and hamate (2-4 months) appear first. The pisiform is the last carpal bone to ossify (approximately 10-12 years). The distal radial epiphysis ossification centre appears at approximately 1 year.

Comparison views: When uncertain whether an ossification centre is normal or a fracture fragment, obtain comparison views of the contralateral (uninjured) side. Both limbs should show symmetric ossification patterns.

Evidence Base

Radiation Risk in Paediatric Imaging

Cohort Study

Pearce MS, Salotti JA, Little MP, McHugh K, Lee C, Kim KP, Howe NL, Ronckers CM, Rajaraman P, Sir Craft AW, Parker L, Berrington de González A • The Lancet (2012)

Key Findings:

CT scans in childhood were associated with a dose-dependent increase in leukaemia and brain tumour risk.
Cumulative doses of 50 mGy to the head approximately tripled the risk of brain tumours.
The highest risk was in children scanned before age 5 years.

Clinical Implication: This landmark study provides direct evidence linking childhood CT exposure to increased cancer risk — it underpins the principle that CT should be MINIMISED in children.

Limitation: Absolute risk remains low. The benefit of necessary CT imaging usually outweighs the risk. The key is avoiding UNNECESSARY CT imaging.

Source: Pearce MS et al. Lancet 2012;380(9840):499-505

Image Gently Campaign Impact

Quality Improvement Study

Goske MJ, Applegate KE, Boylan J, Butler PF, Callahan MJ, Coley BD, Farley S, Frush DP, Hernanz-Schulman M, Jaramillo D, Johnson ND, Kaste SC, Morrison G, Strauss KJ, Tuggle N • American Journal of Roentgenology (2008)

Key Findings:

The Image Gently campaign successfully reduced paediatric CT radiation doses by 20-50% across participating institutions.
Child-size-specific CT protocols were developed for different body regions.
The campaign demonstrated that diagnostic image quality could be maintained with reduced radiation parameters.

Clinical Implication: The Image Gently campaign provides practical, evidence-based protocols for reducing radiation dose in children without compromising diagnostic quality.

Limitation: Implementation varies between institutions. Ongoing education and protocol auditing are needed to maintain standards.

Source: Goske MJ et al. AJR 2008;190(2):273-4

Evidence strongly supports radiation dose reduction in paediatric imaging.

Australian Context

In Australia, paediatric imaging follows guidelines established by RANZCR, the Australasian College of Emergency Medicine (ACEM), and paediatric radiology subspecialty standards. Australian children's hospitals (Royal Children's Hospital Melbourne, Westmead Children's Hospital, Queensland Children's Hospital) have dedicated paediatric imaging protocols with radiation dose optimisation programs aligned with the Image Gently campaign.

DDH screening in Australia follows the NHMRC selective screening policy: clinical examination at birth and 6-8 weeks, with selective ultrasound for at-risk infants (breech presentation, first-degree family history, clinical instability). Universal USS screening is NOT current Australian practice.

Australian radiation safety legislation, overseen by ARPANSA (Australian Radiation Protection and Nuclear Safety Agency), mandates specific dose reference levels for paediatric imaging. Australian diagnostic reference levels (DRLs) are published for common paediatric examinations and are typically 50-70% lower than adult DRLs.

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

EXAMINER

"A 7-year-old boy falls off monkey bars and presents with elbow pain and swelling. The lateral elbow radiograph shows a posterior fat pad sign but no visible fracture."

EXCEPTIONAL ANSWER

The positive posterior fat pad sign on the lateral elbow radiograph is ALWAYS abnormal and, in the context of trauma, indicates an intra-articular fracture until proven otherwise. The posterior fat pad is normally hidden within the olecranon fossa and is only pushed out by a significant effusion (haemarthrosis from fracture). The anterior fat pad may also be elevated (sail sign), which is less specific on its own but supports the diagnosis when combined with a posterior fat pad. In a 7-year-old with this mechanism (fall from height onto outstretched hand), the most likely occult fractures are: (1) Radial head/neck fracture — common but may be subtle on radiographs. (2) Supracondylar fracture — the most common elbow fracture in children. Look carefully at the anterior humeral line (should bisect the middle third of the capitellum). If it passes through the anterior third, there may be an undisplaced extension-type supracondylar fracture. (3) Lateral condyle fracture — requires careful assessment as it may need surgical fixation. (4) Coronoid fracture — rare but can occur. I would systematically assess: (1) The anterior humeral line. (2) The radiocapitellar line (should pass through the capitellum on all views). (3) All ossification centres using CRITOE — at age 7: Capitellum (1), Radial head (3), Internal/medial epicondyle (5), and Trochlea (7) should all be PRESENT. If any is absent, it may be avulsed. (4) COMPARISON VIEWS of the contralateral uninjured elbow if abnormality is suspected. Management: Given the positive posterior fat pad, I would TREAT AS A FRACTURE even without a visible fracture line. This means: above-elbow backslab with elbow at 90 degrees, arm elevated in a collar and cuff sling, urgent fracture clinic review in 5-7 days with repeat radiographs. At 1-2 weeks, the fracture callus may become visible on radiographs, confirming the fracture site. If there is any concern about a displaced fracture, associated dislocation, or clinical neurovascular compromise, I would seek URGENT orthopaedic review and consider MRI for further assessment.

KEY POINTS TO SCORE

Posterior fat pad sign in trauma = occult fracture until proven otherwise

Most common occult fractures: radial head/neck, supracondylar, lateral condyle

Check anterior humeral line, radiocapitellar line, and CRITOE centres

Comparison views of contralateral elbow help differentiate ossification centres from fragments

Treat as fracture: above-elbow backslab, fracture clinic review, repeat radiograph at 7-14 days

COMMON TRAPS

✗Dismissing the posterior fat pad sign as normal (it is ALWAYS abnormal)

✗Not checking the anterior humeral line for a subtle supracondylar fracture

✗Not using CRITOE to assess whether all expected ossification centres are present

✗Not treating as a fracture when no fracture line is visible

VIVA SCENARIOStandard

EXAMINER

"An examiner asks you why radiation dose management is particularly important in children and what principles you would apply."

EXCEPTIONAL ANSWER

Radiation dose management is critically important in children for several reasons, summarised by the CHILD mnemonic. First, WHY children are more radiosensitive: (1) Cells dividing rapidly: the Law of Bergonié and Tribondeau states that cells are most sensitive to radiation during mitosis. Children's tissues are rapidly growing and dividing, making them inherently MORE radiosensitive than adult tissues. (2) Higher organ doses: children's smaller bodies provide less tissue for radiation attenuation. Using adult exposure parameters on a child results in HIGHER organ doses. This is why paediatric-specific protocols are essential. (3) Immature DNA repair: children have less robust DNA repair mechanisms, resulting in more persistent radiation-induced DNA damage. (4) Longer remaining lifespan: stochastic effects (radiation-induced cancer) may take 10-40 years to manifest. A child exposed at age 5 has 75+ years for a cancer to develop, compared to 30 years for a 50-year-old. (5) Dose accumulation: starting imaging in childhood means a longer period of cumulative exposure. The evidence supporting this concern is strong: the landmark Pearce et al. Lancet 2012 study demonstrated a dose-dependent increase in leukaemia and brain tumours in children who received CT scans, with the highest risk in children under 5. The principles I would apply — ALARA: (1) Justify every examination: ask whether the imaging result will change management. Apply clinical decision rules where validated. (2) Use non-ionising modalities FIRST: USS for soft tissue, joint effusion, DDH. MRI for complex fracture/physeal assessment, infection, tumour. Reserve CT for specific indications where it is clearly superior. (3) USE PAEDIATRIC PROTOCOLS: reduce kVp and mAs for the child's body size. The Image Gently campaign provides age/weight-specific protocols. (4) COLLIMATE: restrict the radiation field to the anatomy of interest only. Avoid including the gonads, thyroid, or eyes in the beam unnecessarily. (5) SHIELD: use gonadal and thyroid shielding when these organs are in or adjacent to the primary beam. (6) SCAN ONCE: avoid unnecessary repeat imaging. If previous imaging exists from another institution, obtain it rather than repeating.

KEY POINTS TO SCORE

CHILD: Cells dividing rapidly, Higher organ doses, Immature DNA repair, Longer lifespan, Dose accumulation

Pearce et al. Lancet 2012: direct evidence linking childhood CT to increased cancer risk

Non-ionising modalities first: USS and MRI preferred over CT

Image Gently: paediatric-specific protocols reducing dose by 20-50%

ALARA: justify, use appropriate modality, paediatric protocols, collimate, shield, scan once

COMMON TRAPS

✗Not knowing the specific evidence (Pearce et al. 2012)

✗Not mentioning the Law of Bergonié and Tribondeau

✗Not listing practical dose reduction strategies

✗Not discussing the ALARA principle

VIVA SCENARIOChallenging

EXAMINER

"A 4-year-old has sustained an elbow dislocation. The post-reduction radiograph appears satisfactory, but you notice that the trochlea ossification centre appears to be present while the medial epicondyle is not visible."

EXCEPTIONAL ANSWER

This is a critically important finding that represents a TRAPPED MEDIAL EPICONDYLE — one of the most important and commonly tested pitfalls in paediatric elbow imaging. Understanding why: according to the CRITOE sequence, the ossification centres appear in a FIXED order. The Internal (medial) epicondyle appears at approximately age 5 years, and the Trochlea appears at approximately age 7 years. The trochlea ALWAYS appears AFTER the medial epicondyle. Therefore, in a child of any age, you CANNOT have a trochlea without a medial epicondyle. If the trochlea appears to be present but the medial epicondyle is NOT visible in its normal anatomical position, the structure you are seeing is actually the DISPLACED MEDIAL EPICONDYLE trapped within the joint — NOT the trochlea. What has happened: during the elbow dislocation, the ulnar collateral ligament (UCL) exerts traction on its attachment at the medial epicondyle, causing an avulsion fracture. When the elbow is reduced, the avulsed medial epicondyle can be dragged INTO the joint and become trapped between the articular surfaces (usually between the trochlea and the olecranon). On the post-reduction radiograph, this trapped fragment within the joint mimics a trochlea ossification centre. Clinical significance: a trapped medial epicondyle within the joint: (1) Acts as a mechanical block to full elbow motion. (2) Will cause ongoing joint incongruity and damage to the articular surfaces. (3) The ulnar nerve is at risk (it runs immediately posterior to the medial epicondyle). (4) Requires OPEN SURGICAL REMOVAL and fixation of the medial epicondyle back to its anatomical position. My actions: (1) Obtain comparison views of the contralateral uninjured elbow to confirm that the trochlea is NOT present at this age. (2) If the clinical and radiographic findings confirm a trapped medial epicondyle, arrange URGENT return to theatre for open reduction and internal fixation. (3) Inform the surgical team about the trapped fragment and the need for re-operation.

KEY POINTS TO SCORE

CRITOE: trochlea ALWAYS appears AFTER the medial epicondyle — you cannot have one without the other

If trochlea 'present' without medial epicondyle = the medial epicondyle is TRAPPED in the joint

Mechanism: UCL avulses medial epicondyle during dislocation, fragment enters joint during reduction

Requires open surgical removal and fixation — a missed diagnosis causes ongoing joint damage

Always obtain comparison views of the contralateral elbow when in doubt

COMMON TRAPS

✗Accepting the post-reduction radiograph as satisfactory without checking CRITOE sequence

✗Not recognising that the trochlea cannot be present without the medial epicondyle

✗Not obtaining comparison views of the contralateral elbow

✗Not recognising this as a surgical emergency requiring re-operation