Radiation physics — X-rays, ionising radiation and radiation protection

ACEM Primary, ANAT-12

Overview

Medical imaging is the cornerstone of emergency diagnosis. Every modality exploits a distinct physical principle to generate tissue contrast, and understanding those principles allows the clinician to choose the right test, interpret its output intelligently, and recognise its limitations. For the ACEM Primary, imaging knowledge sits at the intersection of physics, anatomy, and clinical reasoning, examiners expect candidates to explain why a chest X-ray is acquired postero-anteriorly rather than antero-posteriorly, why MRI is superior to CT for cord compression, and why ultrasound is first-line in undifferentiated hypotension.

The six major modalities encountered in emergency medicine are plain radiography, fluoroscopy, computed tomography (CT), nuclear medicine (including PET), magnetic resonance imaging (MRI), and ultrasound (US). Each carries trade-offs between spatial resolution, contrast resolution, ionising radiation exposure, acquisition speed, availability, and cost. A working mental model of these trade-offs transforms imaging from a "black box" into a rational diagnostic tool.

Radiation safety is woven through every decision involving X-rays, CT, and nuclear medicine. The principle of ALARA (As Low As Reasonably Achievable) underpins all ionising modalities and is especially relevant in paediatric patients and pregnant women, where the ED clinician must weigh diagnostic yield against biologically significant doses.

Plain Radiography

Physical Principle

Plain radiography uses X-rays, electromagnetic radiation produced when high-velocity electrons are decelerated at a tungsten anode (bremsstrahlung radiation) or cause inner-shell electron transitions (characteristic radiation). The X-ray beam passes through the patient; differential attenuation by tissues creates contrast on the detector.

Five radiographic densities, from most to least attenuating:

Key principle: Contrast is seen only at the interface between densities. A pleural effusion silhouettes the hemidiaphragm because fluid and diaphragm share the same density, this is the silhouette sign, fundamental to chest X-ray interpretation.

Projection Matters

• PA (postero-anterior) chest X-ray: X-rays travel from back to front; the heart lies anteriorly and magnifies less, giving a truer cardiac silhouette (cardiothoracic ratio < 0.5 is normal)
• AP (antero-posterior) projection used for portable/supine films: heart is magnified ~15-20%, mediastinum appears wider, always note the projection before interpreting
• Lateral decubitus: detects small free pleural fluid (as little as 50 mL) or demonstrates free intraperitoneal gas when erect film is impractical

Radiation Dose

Effective dose from a chest X-ray is approximately 0.02-0.1 mSv, equivalent to a few days of background radiation. A two-view lumbar spine series delivers ~1.5 mSv. These values contextualise CT doses discussed below.

Fluoroscopy

Fluoroscopy is real-time, continuous X-ray imaging displayed as a live video stream. In emergency practice it is used intraoperatively (orthopaedic reduction, foreign-body retrieval) and for contrast studies (swallow studies, cystourethrography).

The trade-off is dose: fluoroscopy delivers dose continuously, typical screening rates range from 1-10 mGy/min depending on technique and equipment. Long procedures accumulate skin doses that can exceed deterministic thresholds (> 2 Gy) causing radiation dermatitis, making time minimisation essential.

Clinically: Inadvertent prolonged fluoroscopy in the ED (e.g. during a difficult central line insertion) can deliver a surprisingly high skin dose; the operator should use pulsed mode, last-image-hold, and collimation.

Computed Tomography

Physical Principle

CT acquires multiple X-ray projections around the patient using a rotating X-ray tube and detector array. Raw projection data undergoes filtered back-projection (or iterative reconstruction in modern scanners) to produce cross-sectional images with Hounsfield Units (HU) quantifying attenuation.

Hounsfield scale (water = 0 HU by definition):

Key principle: CT's great strength over plain radiography is its ability to distinguish tissues with similar densities, acute subdural haematoma (dense) versus chronic (hypodense) versus acute-on-chronic, something impossible on plain film.

Windowing

Raw CT data contains far more HU range than the human eye can perceive simultaneously. Windowing selects a centre level (WL) and width (WW) to optimise contrast for a specific tissue:

Contrast Agents

Iodinated contrast (non-ionic, iso- or low-osmolar) increases attenuation of vascular structures and enhances perfused tissue. Timing of image acquisition relative to bolus injection defines the phase:

• Arterial phase (~25-30 s): aortic dissection, active arterial haemorrhage, CT pulmonary angiography
• Portal venous phase (~60-70 s): solid organ injury, bowel pathology, most abdominal trauma
• Delayed phase (~3-5 min): urinary tract (excretion), some malignancy characterisation

Risks of iodinated contrast include contrast-induced nephropathy (risk elevated with eGFR < 30-45 mL/min/1.73 m²), anaphylactoid reactions (severe reactions ~0.01-0.04%), and, relevant in emergency, contrast extravasation at the IV site. Metformin should be withheld 48 hours post-contrast if significant renal impairment is present or develops.

Radiation and CT

CT accounts for a minority of imaging studies but the majority of medical radiation exposure. Typical effective doses:

Radiation-induced cancer risk is modelled linearly with dose (the linear no-threshold model), though individual risk from a single scan is small. In children, radiosensitivity is higher and latency longer, reinforcing the principle of dose reduction through paediatric protocols (lower kVp, mA, iterative reconstruction).

Nuclear Medicine

Principle

Nuclear medicine introduces a radiopharmaceutical, a radiolabelled tracer with biological activity, into the patient. Gamma rays emitted by the radionuclide are detected externally by a gamma camera (SPECT) or by coincidence detection of annihilation photons (PET).

The image reflects physiological function rather than anatomy, making it fundamentally different from CT or plain film.

Common Agents in Emergency Contexts

Clinically: The V/Q scan remains an important alternative to CTPA for PE diagnosis in patients with contrast allergy, renal impairment, or who are pregnant (lower breast dose than CTPA). A normal perfusion scan effectively excludes PE; a high-probability scan (multiple segmental mismatched defects) is diagnostic.

Limitations

Spatial resolution is poor (7-15 mm) compared with CT. Acquisition time is long (30-90 min). Availability outside business hours is limited in most Australian EDs. The dose from a V/Q scan is approximately 1-2 mSv.

Magnetic Resonance Imaging

Physical Principle

MRI exploits nuclear magnetic resonance of hydrogen nuclei (protons). A strong external magnetic field (typically 1.5 or 3 Tesla) aligns proton magnetic moments. Radiofrequency (RF) pulses tilt the magnetisation; as protons relax back to equilibrium, they emit RF signals detected by receiver coils.

Two independent relaxation time constants determine image contrast:

• T1 relaxation (longitudinal recovery): fat is bright; water/CSF is dark, T1-weighted images are excellent for anatomy and post-contrast enhancement
• T2 relaxation (transverse decay): water/CSF is bright; fat is moderately bright, T2-weighted images highlight oedema, fluid, and most pathology

Clinically: Cord oedema from central cord syndrome appears as T2 hyperintensity within the cord, visible on MRI within hours of injury, invisible on CT. This is why MRI is mandatory for suspected spinal cord injury with neurological deficit.

Gadolinium Contrast

Gadolinium-based contrast agents shorten T1 relaxation, causing enhancing tissues to appear bright on T1-weighted sequences. Risks include nephrogenic systemic fibrosis in patients with severe renal impairment (eGFR < 30 mL/min/1.73 m²), now rare with modern macrocyclic agents, and gadolinium deposition in the brain with repeated doses, clinical significance of which remains under investigation.

MRI Safety

The strong static magnetic field is a constant hazard:

• Ferromagnetic projectile risk, any unscreened ferrous object becomes a missile in the bore
• Implanted devices: pacemakers (most modern devices are MRI-conditional), cochlear implants, aneurysm clips, orthopaedic metalwork, all require checking against manufacturer data
• Acoustic noise from gradient coils necessitates hearing protection
• No ionising radiation, major advantage over CT, especially in pregnancy and children

Sequences Relevant to Emergency Medicine

Key principle: Diffusion-weighted imaging (DWI) detects cytotoxic oedema within minutes of ischaemic stroke onset, far earlier than CT, which may appear normal for 6-24 hours. In the undifferentiated comatose patient, DWI-MRI is invaluable.

Ultrasound

Physical Principle

Ultrasound uses high-frequency sound waves (2-20 MHz) generated by a piezoelectric crystal in the transducer. Sound travels through tissue; at interfaces between structures of differing acoustic impedance, waves are reflected back to the transducer. Time-of-flight and amplitude of returning echoes reconstruct a 2D image.

Echogenicity describes how bright a structure appears:

• Hyperechoic (bright): bone cortex, calculi, fat, air interfaces
• Hypoechoic (dark): muscle, lymph nodes, many solid tumours
• Anechoic (black): fluid, blood, urine, bile, ascites
• Posterior acoustic shadowing: seen deep to calculi (complete shadow) or air
• Posterior acoustic enhancement: seen deep to fluid-filled structures (gallbladder, bladder)

Frequency-Resolution Trade-off

$$\text{Resolution} \propto \text{frequency}, \quad \text{Penetration} \propto \frac{1}{\text{frequency}}$$

High-frequency probes (7-15 MHz, linear) resolve superficial structures (vessels, tendons, soft tissue) exquisitely but cannot penetrate deeply. Low-frequency probes (1-5 MHz, curvilinear, phased array) sacrifice resolution for depth, needed for abdominal organs and cardiac imaging.

Emergency Ultrasound Applications

Clinically: In the haemodynamically unstable trauma patient, a positive FAST (free fluid in Morrison's pouch, splenorenal space, or pelvis) justifies immediate operative intervention without CT, avoiding the "doughnut of death" (sending an unstable patient to the CT scanner).

Doppler Ultrasound

Colour Doppler superimposes flow direction (red/blue by convention) on B-mode images. Spectral Doppler generates a velocity-time waveform. Together they interrogate vascular patency, flow direction, and vessel resistance, essential for diagnosing testicular torsion (absent intratesticular flow), mesenteric ischaemia, and portal hypertension.

Limitations

Ultrasound is highly operator-dependent, limited by obesity, bowel gas (acoustic shadowing from air prevents deep penetration), subcutaneous emphysema, and patient cooperation. Bone cortex reflects almost all sound, making intra-osseous and intracranial structures inaccessible (except through fontanelles in neonates).

Comparative Summary

Key principle: No single modality is universally superior. The art is matching the clinical question to the modality that answers it fastest, safest, and most accurately, the definition of rational, patient-centred imaging.

Sources

• ACEM policy documents
• Life in the Fast Lane CCC (open educational resource)
• Deranged Physiology (open educational resource)