Diagnostic Imaging
Modality choices
Plain radiography for bone and lung; ultrasound for abdomen, thyroid and image-guided biopsy; CT for staging of lung, liver and bone; MRI for soft tissue, brain, spine, bone marrow, liver, prostate and pelvis; PET/CT with 18F-FDG for metabolically active tumours, occult metastasis and equivocal findings. Diffusion-weighted imaging improves lesion detection without contrast.
Reading and reporting
Measure lesions on the same modality, same window and same plane as baseline, in the plane of greatest dimension. Report incidental findings and recognise false positives: FDG uptake in inflammation and infection, physiological uptake in brain, myocardium and urinary tract, and brown fat.
Radiation protection
ALARA (as low as reasonably achievable), justification and optimisation. Cumulative dose matters most in young patients and in those needing repeated surveillance.
Screening imaging
Mammography reported with BI-RADS, with ultrasound or MRI as adjuncts in dense breasts; low-dose CT for high-risk smokers.
Practical pitfalls
Pseudoprogression after radiosurgery or immunotherapy, fibrous residual masses after treatment (PET helps), artefacts from implants, motion and contrast timing, and small-volume peritoneal disease that CT systematically underestimates.
Traps
Mixing modalities between baseline and follow-up invalidates response assessment, and calling a lesion new on a different technique is a classic error.
Rule of thumb
Choose the modality by the question asked: CT for lung, bone and overall staging; MRI for brain, spine, liver, prostate and pelvis; PET for occult metastasis and equivocal findings.