Medical imaging Study Guide

📖 Core Concepts Medical imaging: creates visual representations of internal body structures for diagnosis, treatment planning, and research. Non‑invasive: most imaging techniques do not require instruments to be placed inside the patient. Tomography: obtains cross‑sectional (or 3‑D) images by collecting data from many angles and reconstructing them computationally. Major modalities Radiography (X‑ray) – 2‑D projection; first modern imaging method. Computed Tomography (CT) – X‑ray‑based tomographic reconstruction; invented by Godfrey Hounsfield (1972). Magnetic Resonance Imaging (MRI) – Uses static magnetic fields (1.5–3 T) and RF pulses to excite hydrogen nuclei; no ionizing radiation. Nuclear Medicine – Emits radiation from administered radiopharmaceuticals; functional imaging (SPECT, PET). Ultrasound – High‑frequency sound waves (MHz); real‑time imaging, Doppler for blood flow. Elastography & Echocardiography – Specialized ultrasound techniques for tissue stiffness and cardiac assessment. Safety considerations: ionizing radiation (X‑ray, CT, nuclear), magnetic field interactions (MRI), contrast agent reactions. Legal/Privacy: DICOM standard for data exchange; HIPAA‑type privacy rules; anonymization required; US Copyright Office treats automatically generated scans as non‑copyrightable. --- 📌 Must Remember 1972 – Hounsfield invents the first CT scanner. 1979 Nobel Prize – Awarded to Hounsfield & Cormack for CT development. MRI field strength – Typically 1.5 T or 3 T; higher fields increase signal but also heating risk. Radiation‑free modalities – MRI, ultrasound, most nuclear‑medicine scans (except radiation dose from tracer). Pregnancy‑safe imaging – MRI (no contrast) & obstetric ultrasound; X‑ray, CT, nuclear medicine are generally avoided. Contrast risks – Iodinated agents → allergic reactions & nephrotoxicity. DICOM – Universal standard for storing, transmitting, and compressing medical images. Copyright – Pure diagnostic scans (X‑ray, MRI, CT, US) are not copyrightable in the U.S. because they lack human creative input. --- 🔄 Key Processes CT Reconstruction X‑rays pass through patient from multiple angles → detectors record attenuation → computer applies filtered back‑projection (or iterative algorithms) → cross‑sectional image. MRI Signal Generation Static magnetic field aligns hydrogen spins → RF pulse at Larmor frequency excites spins → pulse off → spins relax (T1, T2) emitting RF signal → gradients encode spatial location → signal digitized → image. PET Imaging Inject positron‑emitting tracer (e.g., $^{18}$F‑FDG). Positron annihilates with electron → two 511 keV photons emitted 180° apart → coincidence detection → tomographic reconstruction of metabolic activity. Ultrasound Doppler Transmit sound wave → receive reflected echo → calculate frequency shift (Doppler) → derive blood‑flow velocity and direction. Image Anonymization (DICOM) Remove patient identifiers from header fields (name, ID, DOB) → replace with generic codes → ensure no facial features or tattoos that could reveal identity. --- 🔍 Key Comparisons CT vs. MRI Radiation: CT uses ionizing X‑rays; MRI uses none. Best for: CT – bone, acute hemorrhage; MRI – soft‑tissue contrast, CNS, ligaments. SPECT vs. PET Isotope half‑life: SPECT isotopes longer (e.g., $^{99m}$Tc); PET isotopes short (e.g., $^{18}$F). Resolution: PET generally higher spatial resolution. Fluoroscopy vs. Ultrasound (real‑time) Radiation: Fluoroscopy uses X‑rays; ultrasound does not. Typical use: Fluoroscopy – catheter guidance; ultrasound – obstetrics, cardiac echo. Contrast‑enhanced MRI vs. Non‑contrast MRI Contrast: Gadolinium improves vascular and lesion delineation; not needed for many brain protocols. US vs. MRI in Pregnancy Safety: Both lack ionizing radiation; US is first‑line, MRI reserved for detailed anatomy when needed. --- ⚠️ Common Misunderstandings “MRI is safe for any implanted device.” → False; pacemakers, certain neurostimulators are contraindicated. “All X‑ray images are automatically copyrighted.” → US Copyright Office says purely diagnostic scans lack human authorship, thus not copyrightable. “Ultrasound uses ionizing radiation.” → Incorrect; it uses acoustic energy. “Any imaging can be published without consent if anonymized.” → Consent is still required for patient images in most journals, even after anonymization. --- 🧠 Mental Models / Intuition Anatomy vs. Function: Think of X‑ray/CT/MRI as “structural photography,” while SPECT/PET are “functional PET‑scans of metabolism.” Tomography = “Slice the cake from many angles.” Collect many 2‑D projections → recombine into 3‑D slice. MRI spin‑echo: Visualize spins as tiny compass needles; the RF pulse tips them, and their wobble (relaxation) is the signal we record. --- 🚩 Exceptions & Edge Cases MRI contraindications: Certain metallic implants, ferromagnetic foreign bodies, some tattoos with metallic ink. Contrast allergy: Pre‑screen for iodine allergy; use non‑ionic agents or pre‑medicate if necessary. Radiation dose in pregnancy: Very low doses may be acceptable, but high doses can cause miscarriage or teratogenic effects. Copyright nuance: The US Office interpretation is not binding law; courts may treat it differently, especially for derivative works (annotated images). --- 📍 When to Use Which Suspected fracture or chest pathology → Radiography (quick, low cost). Complex bony anatomy or acute trauma → CT (high spatial resolution, fast). Soft‑tissue detail, CNS lesions, spinal cord → MRI (excellent contrast, no radiation). Functional/metabolic assessment (oncology, neurology) → PET (glucose metabolism) or SPECT (perfusion). Bedside, fetal, or vascular flow evaluation → Ultrasound (real‑time, safe). Pregnant patient needing detailed anatomy → MRI without contrast or obstetric ultrasound. --- 👀 Patterns to Recognize Presence of ionizing radiation → look for safety statements, shielding, contrast‑risk discussion. “Tracer” or “radiopharmaceutical” → functional imaging (SPECT/PET). “Real‑time” + “continuous beam” → fluoroscopy or Doppler ultrasound. “Gradient magnetic fields” → MRI sequences, spatial encoding. “Hybrid” (PET/CT, PET/MRI) → combine anatomy (CT/MRI) with function (PET). --- 🗂️ Exam Traps Confusing SPECT with PET: Remember PET uses positron emitters (511 keV photons) and generally higher resolution. Assuming all MRI scanners are 3 T: Clinical scanners range from 0.5 T to 3 T; field strength affects signal and safety. Believing any image can be published without consent: Consent is required even after anonymization for most journals. Thinking “non‑invasive” means “no safety concerns.” Non‑invasive modalities can still pose radiation or magnetic hazards. Attributing copyright to the radiologist: Pure diagnostic scans lack creative authorship under US law. ---