Academe Learn
CBME Curriculum Preview

Page 6 of 14

RD1.1 | Core Principles of X-ray, Fluoroscopy, CT, Ultrasound, Nuclear Medicine and MRI — Summary & Reflection

KEY TAKEAWAYS

Core Principles of Imaging Modalities — Key Points

  • Six modalities are grouped into ionising (X-ray, fluoroscopy, CT, nuclear medicine) and non-ionising (USG, MRI). Non-ionising is preferred when diagnostically equivalent.
  • X-ray (~0.02 mSv): projection image; first-line for chest and bone. CT (~8–10 mSv for abdomen): cross-sectional, fast, superior contrast; first-line for trauma and acute haemorrhage. Fluoroscopy: real-time X-ray; dynamic studies. Nuclear medicine: functional imaging with radiopharmaceuticals; bone scan, V/Q, PET-CT.
  • USG: piezoelectric, real-time, no ionising radiation — first-line in biliary, renal, obstetric, paediatric and vascular imaging. MRI: T1/T2 tissue contrast via NMR, superior soft-tissue detail — first-line for CNS, spinal cord, musculoskeletal. MRI has contraindications — screen every patient (pacemaker/ICD, cochlear implant, ferromagnetic intra-ocular foreign body, aneurysm clips).
  • ALARA (As Low As Reasonably Achievable): time, distance, shielding. AERB is India's radiation regulator (Atomic Energy Act 1962) — not the NRC or ICRP.
  • Iodinated contrast: caution at eGFR <30 (CA-AKI risk); withhold metformin at time of contrast and for 48 h if eGFR <60 or large volume. Gadolinium: avoid in eGFR <30 (NSF risk); use macrocyclic agents if unavoidable.
  • Modality choice rule: USG first (biliary/renal/obstetric/paeds/pregnancy); CT for acute trauma/haemorrhage; MRI for CNS/cord/soft tissue; X-ray for chest/bone first-line.

REFLECT

Consider a patient in your next clinical posting who undergoes a CT scan. Before the scan report arrives, ask yourself: (1) What was the clinical question — and could it have been answered by ultrasound or MRI without ionising radiation? (2) Was contrast given — and was the patient's renal function and metformin status reviewed beforehand? (3) Who gave the clinical justification for this examination, and was ALARA applied in the protocol (dose modulation, lowest feasible kV/mAs)? Reflecting on real imaging decisions is the bridge between understanding modality principles and applying them as a clinical habit.