Radiology
X-ray Imaging
Radiographic principles — differential X-ray attenuation produces 2D projections used in chest, bone, and contrast studies
X-rays are high-energy electromagnetic photons (10-150 keV) generated in an X-ray tube by accelerating electrons into a tungsten anode. As they pass through the body, tissues attenuate them differentially based on density and atomic number — bone (calcium, Z=20) absorbs strongly and appears white; air absorbs little and appears black; soft tissue and fat are intermediate. The transmitted X-rays expose a digital detector forming a 2D projection. Radiographs are excellent for skeletal trauma, pneumonia, foreign bodies, bowel obstruction, and pneumoperitoneum. Modalities derived from X-rays include fluoroscopy (real-time), CT (cross-sectional via reconstruction), mammography (low-energy contrast), and angiography (with iodinated contrast). Radiation doses are quantified in mSv; chest X-ray ~0.1 mSv vs CT chest ~7 mSv vs background ~3 mSv/year.
- X-ray photon energy10-150 keV
- Tube anodeTungsten (high atomic number, high melting point)
- Bone attenuationHigh (calcium, Z=20) — appears white
- Air attenuationLow — appears black
- Chest X-ray dose~0.1 mSv (~10 days of background)
- CT chest dose~7 mSv (~2 years of background)
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Why X-ray imaging matters
- Trauma evaluation. Plain films screen for fractures; CT provides definitive characterization in major trauma.
- Pulmonary disease. Chest X-ray detects pneumonia, pneumothorax, effusion, edema, masses; CT for staging and equivocal cases.
- Acute abdomen. Upright film reveals free air (pneumoperitoneum) and bowel obstruction patterns.
- Cancer screening. Mammography reduces breast cancer mortality; lung cancer screening with low-dose CT in heavy smokers.
- Interventional procedures. Fluoroscopy guides catheter placement, angioplasty, embolization, biopsy.
- Stroke imaging. Non-contrast CT excludes hemorrhage; CT angiography identifies large vessel occlusion for thrombectomy.
- Bone pathology. Fractures, arthritis, metastases, osteomyelitis, tumors all show characteristic radiographic features.
Common misconceptions
- X-rays show all pathology. Plain films miss many soft-tissue abnormalities visible on CT or MRI.
- One chest X-ray is dangerous. Single CXR adds ~10 days of natural background — minimal individual risk.
- CT and X-ray detect the same things. CT removes superposition and detects subtle findings invisible on plain film.
- Negative X-ray rules out fracture. Scaphoid, hip, and stress fractures often missed initially; repeat imaging or MRI may be needed.
- Iodinated contrast and gadolinium are interchangeable. Iodinated for X-ray/CT; gadolinium for MRI; allergies and renal toxicities differ.
- Lead aprons block all radiation. They reduce scatter exposure significantly but don't shield the primary beam.
Frequently asked questions
How are X-rays produced?
An X-ray tube heats a cathode filament to release electrons by thermionic emission. A high voltage (40-150 kV) accelerates electrons toward a tungsten anode. Two interactions produce X-rays. Bremsstrahlung (braking radiation): electrons decelerate near nuclei, emitting a continuous spectrum. Characteristic radiation: ejected inner-shell electrons are replaced from outer shells, releasing photons at element-specific energies. ~99% of energy becomes heat (anode rotation dissipates it); ~1% becomes X-rays. Tube current (mA) controls X-ray quantity; voltage (kVp) controls photon energy and tissue penetration.
What are the five chest X-ray densities?
From most to least radiolucent (black to white). Air: lung, gas in bowel — black. Fat: subcutaneous, retroperitoneal — dark gray. Soft tissue/water: muscle, blood, organs — gray. Bone: calcium-rich — white. Metal: foreign bodies, prostheses, contrast — bright white. Mnemonic: A-Fat-S-B-M. Reading a CXR: airway (trachea midline?), bones (ribs, clavicles), cardiac silhouette, diaphragm, effusions/edema, fields (parenchyma), gastric bubble, hilum. Compare with prior films and confirm penetration adequate.
How does CT improve on plain X-ray?
CT uses a rotating X-ray source and detector to acquire projections from many angles, reconstructed into thin axial slices (typically 1-5 mm). Eliminates superposition that obscures plain film findings. Hounsfield units quantify density on a scale (water = 0, air = -1000, dense bone +1000). Multi-detector helical CT acquires whole chest in seconds. Contrast (iodinated, IV) opacifies vessels, organs, and pathology. Trade-off: higher radiation dose. Indications include trauma, suspected PE (CT-PA), abdominal pain, stroke (CT, then CT angio).
What's mammography and why is it different?
Mammography uses lower-energy X-rays (~25-30 kVp) and molybdenum or rhodium targets to maximize contrast between breast soft tissues. Two views per breast: craniocaudal (CC) and mediolateral oblique (MLO). Compression flattens breast tissue, reduces motion, lowers required dose, and unfolds overlapping tissue. Tomosynthesis (3D mammography) acquires multiple low-dose projections to reduce overlap. Ultrasound and MRI supplement screening in dense breasts. BI-RADS scoring guides follow-up. Screening recommended for average-risk women starting age 40-50.
How does fluoroscopy work?
Fluoroscopy provides real-time X-ray imaging. Used in barium studies (esophagram, upper GI, small bowel follow-through, barium enema) to assess motility, strictures, fistulas. In angiography, a catheter is advanced into vessels under fluoroscopy and iodinated contrast injected to visualize lumens; coronary angiography, peripheral vascular interventions, neurointerventional procedures (mechanical thrombectomy in stroke) all rely on fluoroscopy. Doses can be high during prolonged procedures — operator and patient dose monitoring matters.
What are the radiation safety concerns?
X-rays are ionizing radiation; can damage DNA and induce cancer (linear-no-threshold model assumes no safe dose). Doses: chest X-ray ~0.1 mSv, abdominal X-ray ~1 mSv, head CT ~2 mSv, chest CT ~7 mSv, abdominal CT ~10 mSv; background radiation ~3 mSv/year. Pediatric tissues are more radiosensitive — protocols use lowest dose achievable (ALARA principle). Pregnancy: avoid abdominal/pelvic X-ray and CT in first trimester unless emergency; chest X-ray with abdominal shielding is acceptable. Lifetime cumulative dose increasingly tracked.
What does iodinated contrast do?
IV iodinated contrast (iohexol, iopamidol) opacifies vessels and well-perfused tissues, distinguishes lesions from normal parenchyma, and detects active bleeding. Contraindications and cautions: iodine allergy (rare anaphylaxis ~1 in 10,000), renal insufficiency (contrast-induced nephropathy — pre-hydrate, consider gadolinium MRI alternative), thyrotoxicosis (iodine load can precipitate storm), metformin (hold for 48 hours after if eGFR <30 to reduce lactic acidosis risk). Newer iso-osmolar non-ionic contrast has lower nephrotoxicity than ionic agents.