--- module: 075-03 language: en chapter: 75 title: "Anatomical Language, Body Plans, Fascia, Compartments, and Imaging Anatomy" module_title: "Sectional anatomy, imaging conventions, and three-dimensional localisation" source_sha256: a2615ac5997d7aab83c798d79cdced7d1b7263db651bee0a5433cef7f241b1a9 --- # Sectional and imaging anatomy ## Imaging principles ### Sampled representations from physical signals ### Check identity, indication, orientation, quality ### Modalities show tissue properties, not anatomy ## Three dimensions from orthogonal planes ### Follow structures across contiguous slices #### Curved vessel: circle, oval, or tube ### Multiplanar and curved reformats ### Maximum-intensity projection hides overlap ### Volume rendering depends on threshold ### Partial-volume averaging #### Obscures small lesions ### Slice thickness: detail versus noise ### Artefacts mimic or hide disease ## Projection radiography ### Attenuation projected onto a detector #### Dense material white, air dark #### Depth collapsed, soft tissues overlap ### Posteroanterior view reduces cardiac magnification ### Assess technical adequacy first ### Two perpendicular views localise ### Silhouette sign #### Right heart border: right middle lobe #### Hemidiaphragm: lower lobe ## CT signal, windows, and contrast ### Hounsfield units: air about minus 1000, water 0 #### Values vary with scanner and tissue mixture ### Window width sets range, level sets centre #### A finding may show on one window only ### Contrast phases answer different questions #### Enhancement reflects flow, leakage, timing #### Non-contrast first for haemorrhage, stones ### CT angiography needs a timed bolus ### Ionising radiation #### Deterministic: threshold, severity rises #### Stochastic: probability rises #### Effective dose is a population comparison #### Children: size-adjusted protocols #### Justify and optimise exposure ## MRI contrast ### Hydrogen nuclei, radiofrequency, relaxation #### Brightness has no universal meaning ### T1: fat bright, fluid dark ### T2: free water bright, shows oedema ### FLAIR suppresses cerebrospinal fluid ### Diffusion: restricted water motion #### ADC separates true restriction from shine-through ### Susceptibility: blood, iron, calcification, air ### Gadolinium enhancement is not malignancy ### Safety screening: implants, heating, projectiles ## Ultrasound as real-time anatomy ### Fluid anechoic, bone and air reflect ### Frequency: penetration versus resolution ### Probe markers define the plane #### Confirm local conventions ### Enhancement, shadowing, reverberation ### Anisotropy darkens tendons and nerves ### Doppler measures motion by frequency shift #### Colour is not artery versus vein #### Angle, aliasing, scale affect reading ## Nuclear and hybrid imaging ### Tracers linked to physiological processes ### SPECT gamma rays, PET paired photons ### Lower resolution, high functional sensitivity ### High uptake is not synonymous with cancer ### Low uptake: necrosis, size, timing ### Hybrid registration can be imperfect ### Standardised uptake values are not absolute ## Reproducible search pattern ### Confirm patient, side, phase, comparisons ### Do not search only for the suspected diagnosis ### Review the whole field in consistent order ### Describe features systematically #### Separate observation from interpretation ### Variants and postoperative change mimic disease ### Incidental findings need a responsible clinician ## Images and living anatomy ### Supine position changes relations ### Dynamic imaging shows reflux and motion ### Findings must fit symptoms and examination #### Early disease occult, age changes silent ### The patient is the reference anatomy