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RD1.1,RD2.1-2 | Imaging Modality Foundations — PBL Case

CLINICAL SETTING

It is 11 pm in the emergency department of a district hospital. A 34-year-old man is brought in by ambulance after a high-speed motorcycle collision. He is drowsy (GCS 12), has a heart rate of 118, blood pressure 98/64, bruising over the left chest and abdomen, an obviously deformed right thigh, and a scalp laceration. The on-call clinician must decide what imaging to obtain, in what order, and must clear the patient's suitability for each study before it is performed. The radiographer asks: 'What do you want, and is he safe for contrast?'

Trigger 1: First imaging decisions in the unstable trauma patient

The patient is haemodynamically borderline. The team must choose the initial imaging strategy for a multi-system trauma with a depressed conscious level. There is debate at the bedside: one colleague suggests an immediate MRI brain for the head injury, another suggests starting with ultrasound at the bedside and then proceeding to CT.

DISCUSSION POINTS

  • Why is CT, rather than MRI, the modality of choice for acute major trauma including the head injury here? Consider speed, availability, and what each modality demonstrates acutely.
  • What role does bedside ultrasound (e.g., focused assessment of the abdomen) play in the unstable patient, and what are its limitations — particularly across air-containing structures?
  • How do you reconcile the need for rapid CT with the principle of ALARA and the regulatory framework (AERB) governing radiation use?
Click to reveal Trigger 2: Contrast administration and renal/allergy screening under time pressure (discuss previous trigger first!)

Trigger 2: Contrast administration and renal/allergy screening under time pressure

A contrast-enhanced CT of the chest, abdomen and pelvis is planned to look for visceral and vascular injury. The accompanying relative mentions the patient 'is a diabetic on some sugar tablets' and 'had a bad reaction to a dye during a scan a few years ago'. No recent renal function is available.

DISCUSSION POINTS

  • Which screening domain is now critical, and what specific information do you need before iodinated contrast is given?
  • How should the reported prior 'reaction to dye' be interpreted versus a reported iodine or shellfish allergy — which actually predicts a contrast reaction?
  • How do diabetes, possible metformin use and unknown renal function change your plan for contrast, and what would you do if eGFR returned at 24 mL/min/1.73m²?
Click to reveal Trigger 3: Limbs, follow-up imaging and the magnetic-field hazard (discuss previous trigger first!)

Trigger 3: Limbs, follow-up imaging and the magnetic-field hazard

The patient stabilises after resuscitation. The deformed thigh needs definitive imaging, and the neurosurgical team later requests detailed imaging of a suspected spinal cord injury. During screening the patient — now more alert — reports an 'old metal rod in my leg' and that he 'worked as a welder'. The team considers MRI of the spine.

DISCUSSION POINTS

  • Which modality is first-line for the suspected long-bone fracture, and why are air/bone-related physical principles relevant to that choice?
  • Why is MRI the preferred modality for detailed spinal cord (soft-tissue/CNS) assessment once the patient is stable?
  • What magnetic-field safety issues do the orthopaedic implant and the welding history raise, and how would you decide whether MRI can proceed safely?

Group Task Assignments

  • Construct a sequenced imaging algorithm for this polytrauma patient from arrival to definitive imaging, naming the modality at each step and the clinical question it answers.
  • Build a one-page pre-imaging suitability checklist covering radiation, contrast (allergy + renal function + metformin) and magnetic-field domains, and apply it to this case.
  • Prepare a two-minute teaching point for junior staff on why 'CT for acute trauma, MRI for stable soft-tissue/CNS, USG and X-ray first where they answer the question' is the safe default.

Learning Issues

Research these questions and bring your findings to the discussion.

  1. [RD1.1] What are the general functioning principles, typical clinical use and relative radiation hazards of X-ray, fluoroscopy, CT, ultrasound, nuclear medicine and MRI, and how do they explain why air-containing structures are imaged better on X-ray than on ultrasound?
  2. [RD2.1] What clinical history must be obtained to determine a patient's suitability for radiation exposure, iodinated/gadolinium contrast administration and magnetic-field exposure, and why do iodine/shellfish allergies NOT predict contrast reactions?
  3. [RD2.2] How is the correct imaging modality selected by crossing pathology type (trauma, infection, tumour, congenital) and body system with the patient group (adult, child, pregnant), and why is CT first-line in acute trauma while MRI is preferred for stable CNS/soft-tissue assessment?