Radiology Study Guide

📖 Core Concepts Radiologist – MD who interprets images, performs image‑guided procedures, and obtains informed consent. Imaging modalities – Different technologies (plain radiography, fluoroscopy, CT, US, MRI, nuclear medicine) each with unique physics, strengths, and limitations. Ionizing vs. non‑ionizing radiation – X‑rays & gamma rays damage DNA (cancer risk); ultrasound & MRI use sound or magnetic fields → no ionizing damage. Radiation protection – Time, distance, and shielding (lead apron, thyroid shield) follow the inverse‑square law:  \(I \propto \frac{1}{d^{2}}\). Interventional radiology (IR) – Minimally invasive therapeutic procedures performed under real‑time imaging guidance. Teleradiology – Remote transmission of images for off‑site interpretation, enabling rapid after‑hours coverage. --- 📌 Must Remember First‑line plain radiography – arthritis, pneumonia, fractures, kidney stones, bone tumors. CT of choice – cerebral hemorrhage, PE, aortic dissection, appendicitis, diverticulitis, obstructing stones. MRI advantage – best soft‑tissue contrast; contraindicated with pacemakers, most metallic implants. Ultrasound limits – cannot cross air or bone; image quality ↓ with excess subcutaneous fat. Radiation safety hierarchy – Shield → Distance → Time (lead aprons, maximize distance, shorten exposure). DXA – low‑energy dual‑energy X‑ray absorptiometry = osteoporosis screening. Mammography – low‑energy projection radiography for early breast cancer detection. Common contrast agents – barium sulfate (GI tract), iodine‑based (vascular, GU, GI). Radiopharmaceuticals – Tc‑99m, I‑123, F‑18 FDG are the workhorses of nuclear medicine. --- 🔄 Key Processes Plain Radiography workflow Patient positioned → X‑ray tube emits photons → beam passes through body → detector (film or digital sensor) captures image → image processed/displayed. CT image acquisition Rotate X‑ray tube & detectors → acquire multiple projections → computer reconstructs axial slices → reformat to coronal/sagittal → optional 3‑D rendering. Ultrasound scanning Transducer emits high‑frequency sound → echoes return from tissue interfaces → time‑delay converted to grayscale image → Doppler adds color flow info. MRI pulse sequence (simplified) Align H⁺ nuclei in strong B₀ field → apply RF pulse → nuclei emit signal → gradient fields encode spatial location → signal collected by coils → Fourier transform → image. Radiation protection steps Time: limit exposure seconds. Distance: step back (e.g., 2 m vs. 1 m halves dose by factor 4). Shielding: place lead apron/thyroid shield, use collimation to restrict beam. --- 🔍 Key Comparisons CT vs. Plain Radiography – CT: cross‑sectional, higher contrast resolution, higher dose. Plain: 2‑D, lower dose, quick first‑line. MRI vs. CT – MRI: superb soft‑tissue contrast, no ionizing radiation, longer exam, contraindicated with many metals. CT: faster, better for bone and acute hemorrhage, uses ionizing radiation. Ultrasound vs. MRI – US: real‑time, portable, operator‑dependent, limited by bone/air. MRI: high‑resolution static images, no operator dependence, longer, higher cost. Barium sulfate vs. Iodine‑based contrast – Barium: oral/rectal, GI lumen imaging, not absorbed. Iodine: IV/IA, enhances vasculature and organ parenchyma, can cause allergic reactions. Digital radiography vs. Film‑screen – Digital: immediate image, lower dose, post‑processing possible. Film: slower, higher dose, no post‑processing. --- ⚠️ Common Misunderstandings “MRI uses radiation” – false; MRI uses magnetic fields & radiofrequency pulses, no ionizing radiation. “Ultrasound can see through bone” – false; bone blocks sound, so US cannot assess intracranial structures. “All contrast agents are the same” – false; barium stays in GI tract, iodine is systemic, each has specific indications and contraindications. “Higher dose always yields better images” – not always; optimal dose balances image quality with ALARA (As Low As Reasonably Achievable) principle. --- 🧠 Mental Models / Intuition “Layer cake” for modality selection – think of imaging as layers: Surface/air → start with plain X‑ray. Soft tissue/vascular → CT (fast) or MRI (detail). Functional/metabolic → Nuclear medicine (PET/SPECT). Radiation protection “inverse square” – doubling your distance quarters the dose; visualize a sphere expanding around the source. --- 🚩 Exceptions & Edge Cases MRI with certain metallic implants – some newer “MRI‑conditional” devices are safe; always verify labeling. CT in pregnant patients – limit to essential studies; consider MRI or ultrasound when feasible. DXA in patients with metal implants – metal can artifactually increase BMD; alternative sites or modalities may be needed. Fluoroscopy distortion – peripheral image may be geometrically distorted; flat‑panel detectors reduce this issue. --- 📍 When to Use Which Suspected fracture → start with plain radiography; if occult, consider CT. Acute neurologic deficit → non‑contrast CT for hemorrhage; MRI if ischemia suspected and time permits. Pregnant patient with abdominal pain → ultrasound first; avoid CT unless life‑threatening. Evaluation of suspected PE → CT pulmonary angiography (contrast‑enhanced CT). Screening for osteoporosis → DXA (dual‑energy X‑ray absorptiometry). Assessing renal artery stenosis – CT angiography or MR angiography; IR for therapeutic angioplasty. --- 👀 Patterns to Recognize “High‑contrast, low‑dose” → plain radiography and DXA. “Cross‑sectional + contrast” → CT angiography for vascular mapping. “Real‑time + needle guidance” → fluoroscopy or ultrasound for IR procedures. “Functional tracer uptake” → nuclear medicine (e.g., focal FDG uptake → malignancy). --- 🗂️ Exam Traps Choosing MRI for acute bleed – many students pick MRI for “best soft‑tissue”, but CT is the gold standard for acute hemorrhage. Assuming all contrast agents are nephrotoxic – iodine‑based agents can be nephrotoxic; barium is not absorbed, but can cause obstruction if perforation exists. Confusing “image intensifier” with “flat‑panel detector” – modern fluoroscopy often uses flat‑panel; image intensifiers have peripheral distortion. Selecting ultrasound for lung parenchyma – US cannot penetrate aerated lung; chest X‑ray or CT required. Over‑relying on “high dose = better image” – violates ALARA; exam questions may test dose‑optimization concepts. ---