Neuroimaging selection and radiation-aware practice — formative SAQs

SAQ 1 (10 marks)

A 3-year-old boy is brought to the emergency department after falling off a playground slide. He cried immediately, did not lose consciousness, is alert and interactive with a Glasgow Coma Scale of 15, has not vomited, has no palpable skull fracture or signs of basilar skull fracture, no severe headache, and his parents feel he is acting normally. The fall was approximately one metre onto a rubber surface. The parent asks whether he needs a brain scan. [1] [5]

1. Apply the PECARN prediction rule and state the imaging disposition for this child, defending your reasoning. (4) [1]
2. The parent has read online that a CT scan will give her child cancer. Explain the attributable lifetime cancer risk of a single paediatric head CT in plain language, citing the evidence that underpins it. (3) [2] [3] [4]
3. Outline the four levers of the ALARA principle and give one practical example of each as it applies to this child and to children generally. (3) [5]

Model answer — SAQ 1

(1) PECARN application and disposition (4). This child is in the two-years-and-over PECARN arm. I work through the six predictors: altered mental status (absent — GCS 15, alert and interactive); signs of basilar skull fracture (absent — no haemotympanum, racoon eyes, Battle sign, or cerebrospinal-fluid leak); loss of consciousness (absent); severe mechanism of injury (absent — a one-metre fall onto a rubber surface is below the threshold of a fall of more than 1.5 metres or a motor-vehicle crash with ejection); severe headache (absent); and vomiting (absent). Meeting none of the predictors places him at very low risk of a clinically-important traumatic brain injury, with a negative predictive value well over 99 percent in the derivation cohort. CT is therefore not indicated. I observe him briefly in the department and discharge him with written head-injury advice, giving the explicit return precautions of persistent or repeated vomiting, worsening headache, increasing drowsiness, a seizure, or any focal neurology. I explain to the parent that the scan is more likely to cause harm than to find anything, and that the validated rule supports observation over imaging. [1]

(2) Attributable cancer risk in plain language (3). A single paediatric head CT delivers an effective dose of the order of 1 to 2 mSv, comparable to several months of natural background radiation (which is around 2 to 3 mSv per year in most regions). The attributable lifetime cancer risk is small and stochastic — meaning probabilistic, with no safe threshold assumed — and is of the order of one excess cancer per 10,000 scans in the cohort estimates. I frame this for the parent as a real but very small increase over the background lifetime cancer risk of roughly one in three, and I compare it with the everyday risks the family accepts. The evidence base is the lifetime-risk model of Brenner (2001), the UK CT-in-childhood retrospective cohort of Pearce (2012) which demonstrated a dose-response with cumulative brain and marrow dose, and the Dutch cohort of Meulepas (2019) with longer follow-up and refined dose estimates; these studies do not forbid CT, they justify minimising and justifying every scan. [2] [3] [4]

(3) The four levers of ALARA (3). First, justification: is any imaging needed, is it needed now, and what question does it answer — for this child, no imaging is needed because the question (is there intracranial injury) is answered by the validated PECARN rule and a well-looking child. Second, optimisation of the dose: when CT is the modality, use a paediatric protocol that scales the milliampere-seconds and kilovoltage peak to the child, limits the field, uses a single phase, and applies iterative reconstruction. Third, substitution of a non-ionising modality: where ultrasound or MRI answers the question, choose it — for example, a rapid MRI protocol for shunt surveillance, or cranial ultrasound in the neonate. Fourth, limitation of repeat scanning: cumulative dose accumulates across a lifetime, so each repeat request is justified on its own merits, and the shunted or oncology child is the population at greatest cumulative risk. [5]

SAQ 2 (10 marks)

A 7-year-old girl is referred by her general practitioner after a first afebrile generalised tonic-clonic seizure witnessed at home overnight. The seizure lasted two minutes, has fully resolved, and on assessment she is alert and back at her baseline, with a normal general and neurological examination. She has been developing normally and there is no family history of epilepsy. [6]

1. State the most appropriate imaging strategy and the modality of choice, and defend it against the alternatives of urgent CT and no imaging. (4) [6]
2. Outline the practical workflow for obtaining the imaging in this child, including the techniques used to avoid sedation in a young or uncooperative child, and the trade-offs of each. (3) [5]
3. The MRI incidentally reveals a small pineal cyst. Describe the principles of communicating this incidental finding to the family without causing harm, and the appropriate follow-up. (3) [5]

Model answer — SAQ 2

(1) Imaging strategy and defence (4). The imaging strategy is an elective MRI brain with a dedicated epilepsy protocol, performed as an outpatient. The rationale is that the American Academy of Neurology practice parameter supports MRI as the modality of choice for a child with an apparent first unprovoked seizure who has returned to baseline and has a normal examination, and the MRI-yield data of Shinnar show that a meaningful proportion of such children have structural abnormalities (focal cortical dysplasia, a low-grade tumour such as a ganglioglioma or dysembryoplastic neuroepithelial tumour, a vascular malformation, or mesial temporal sclerosis) that MRI is uniquely positioned to detect. I would defend against urgent CT on the grounds that CT is reserved for the child with a focal deficit, a prolonged postictal state, or an altered conscious state in whom an acute haemorrhage or mass must be excluded; this well child does not meet those criteria, and CT delivers ionising radiation without the soft-tissue sensitivity of MRI. I would defend against no imaging on the grounds that an unrecognised structural lesion would be missed, with implications for prognosis and for the consideration of antiseizure medication. The epilepsy protocol includes high-resolution coronal T2 and fluid-attenuated inversion recovery sequences through the temporal lobes, with three-dimensional T1 and susceptibility-weighted imaging. [6]

(2) Workflow and sedation-avoidance techniques (3). This 7-year-old is likely to cooperate with explanation, play therapy, and — where available — a mock-scanner desensitisation programme that familiarises her with the noise and the bore of the magnet; with these, many school-age children complete a brain MRI without sedation. In younger children or those who cannot cooperate, the age-stratified approach applies: a feed-and-wrap technique in the neonate and young infant (feeding, swaddling, foam immobilisation) succeeds without sedation in a high proportion; a mock scanner and play therapy in the cooperative preschool child; and procedural sedation or general anaesthesia in the child who cannot otherwise keep still, with the explicit trade-off that the small but real risk of sedation is weighed against the diagnostic benefit of a justified MRI. I would communicate clearly with the radiology team about the clinical question and the protocol, and I would document the discussion and the plan. [5]

(3) Communicating the incidental pineal cyst (3). A small asymptomatic pineal cyst is a common and almost always benign incidental finding that does not, in itself, warrant intervention or surveillance in a child without symptoms of raised intracranial pressure, Parinaud syndrome, or hydrocephalus. I would communicate the finding honestly but without causing alarm: I would name it (a pineal cyst), explain what it is and what it is not (a common normal-variant structure, not a tumour), state that it does not require treatment or routine follow-up in an asymptomatic child, and give the family a clear safety-net of the symptoms that would prompt review (a new or worsening headache, visual change, or any sign of raised pressure). I would document the discussion and the safety-net in the correspondence to the general practitioner, and I would avoid cascading into further unnecessary imaging. The general principle is to communicate the finding, reassure with the evidence, provide a safety-net, and avoid harm from over-investigation. [5]