Emergency and Trauma · Imaging physics
Emergency CT Protocol and Dose One-Liners
High-yield one-liners on emergency CT acquisition protocols, contrast timing, radiation dose metrics and dose-reduction strategies in trauma and emergency imaging.
Rapid-revision one-liners covering emergency CT protocol design, contrast timing principles, radiation dose metrics and dose-optimisation strategies specific to trauma and emergency imaging.
- CTDIvol definition
- CTDIvol (CT Dose Index volume) represents the average radiation dose within the scan volume for a specific protocol, expressed in milligray (mGy); it reflects scanner output but not patient size.
- DLP definition
- Dose-length product (DLP) equals CTDIvol multiplied by scan length, expressed in mGy·cm; DLP estimates total radiation energy delivered and is used to calculate effective dose.
- Effective dose estimation
- Effective dose (mSv) is estimated by multiplying DLP by a body-region conversion factor (k-factor): head 0.0023, chest 0.014, abdomen-pelvis 0.015 mSv/(mGy·cm) for adults.
- Typical trauma CT dose
- A standard whole-body trauma CT (head, neck, chest, abdomen, pelvis) delivers an effective dose of approximately 15-25 mSv depending on protocol, scanner generation and patient habitus.
- Contrast bolus timing
- Arterial phase is typically acquired 20-30 seconds after injection start or triggered by bolus tracking (threshold 100-150 HU in aorta); portal venous phase at 60-80 seconds; delayed phase at 5-10 minutes.
- Bolus tracking in trauma
- Bolus tracking with a region of interest in the ascending aorta or main pulmonary artery triggers acquisition at a predefined HU threshold, ensuring consistent arterial or pulmonary arterial opacification.
- Contrast flow rate in trauma CT
- Trauma CT typically uses 3-5 mL/second flow rate of iodinated contrast (350-400 mgI/mL) through an 18-20 gauge cannula; total volume 80-120 mL depending on body weight and protocol.
- Iterative reconstruction for dose reduction
- Iterative reconstruction algorithms (e.g., ASIR-V, ADMIRE, iDose) reduce image noise, enabling 20-50% dose reduction compared to filtered back projection while maintaining diagnostic image quality.
- Automatic tube current modulation
- ATCM (e.g., Smart mA, CARE Dose4D) adjusts tube current in real time based on patient attenuation, reducing dose in thinner body regions while maintaining image quality in thicker regions.
- Tube voltage selection
- Lowering tube voltage from 120 to 100 or 80 kVp increases iodine contrast-to-noise ratio (due to proximity to iodine k-edge at 33 keV) and reduces dose, but increases noise in larger patients.
- Pitch and scan time
- High pitch (>1) reduces scan time and motion artefact, advantageous in uncooperative trauma patients; however, very high pitch may slightly increase noise depending on reconstruction algorithm.
- Dual-energy CT in trauma
- Dual-energy CT enables virtual non-contrast images (reducing need for true non-contrast phase), iodine maps for active bleeding detection and metal artefact reduction in patients with orthopaedic hardware.
- Non-contrast phase in trauma
- Non-contrast head CT is standard for trauma; non-contrast abdomen-pelvis is increasingly omitted in favour of single portal venous phase with dual-energy virtual non-contrast reconstruction to reduce dose.
- Paediatric dose considerations
- Paediatric CT requires weight-based protocol adjustment (lower kVp and mAs), smaller scan volumes and avoidance of multiphase acquisitions; Image Gently campaign principles apply.
- Contrast-induced nephropathy risk
- In emergency CT, the risk of missing a life-threatening injury outweighs the small risk of contrast-induced nephropathy; current evidence suggests true CI-AKI risk is lower than historically estimated.
Caution
CT dose metrics, contrast volumes and timing parameters vary by scanner manufacturer, institution and patient habitus. Always follow local protocol and current ACR or IAEA dose guidance. Paediatric protocols must be weight-adjusted.
Exam pearl
In viva, always state CTDIvol and DLP when discussing dose, and remember that effective dose is a population-level estimate, not an individual patient measurement. Dual-energy CT virtual non-contrast is an increasingly important dose-reduction tool in trauma.
Viva questions
- What is the difference between CTDIvol and DLP?
- CTDIvol represents the average radiation dose within the scan volume in milligray and reflects scanner output for a given protocol. DLP is CTDIvol multiplied by scan length in centimetres, expressed in mGy·cm, and estimates the total radiation energy delivered for the entire examination.
- How does iterative reconstruction reduce CT dose?
- Iterative reconstruction algorithms model the physics of photon detection and reduce image noise through repeated forward and back projections. This allows diagnostic image quality at lower tube current or voltage settings, enabling 20-50% dose reduction compared to traditional filtered back projection.
- Why is lowering tube voltage beneficial for contrast-enhanced CT?
- Lowering kVp from 120 to 100 or 80 brings the mean photon energy closer to the iodine k-edge at 33 keV, increasing photoelectric absorption and thus iodine contrast-to-noise ratio. This allows either reduced contrast volume or improved vessel conspicuity at lower radiation dose.
- What is the role of dual-energy CT in reducing trauma CT dose?
- Dual-energy CT can generate virtual non-contrast images from a contrast-enhanced acquisition, potentially eliminating the need for a true non-contrast phase. It also provides iodine maps for detecting active haemorrhage and metal artefact reduction, all from a single acquisition.
- How is effective dose estimated from DLP?
- Effective dose in millisieverts is calculated by multiplying DLP by a body-region-specific conversion factor (k-factor). For example, the k-factor for abdomen-pelvis is approximately 0.015 mSv per mGy·cm. This provides a population-level estimate of stochastic risk, not an individual patient dose.