# Ch01: Imaging Modalities — X-ray, CT, MRI, Ultrasound & Nuclear Medicine（影像学总论）
Preparatory Mindset
The whole of medical imaging is built on a few physical principles. X-rays are electromagnetic radiation of extremely short wavelength (0.0006–50 nm) discovered by Wilhelm Roentgen in 1895. A modern X-ray machine has three components: the X-ray tube, the high-frequency generator (40 KV; 60–120 KV) and the control console. Electrons are produced by heating the filament, accelerated to high velocity and suddenly stopped at the target anode; >99% of the energy becomes heat and <1% becomes X-rays. When you look at any radiograph, you are really looking at differential absorption — the visibility of normal and abnormal structures depends on how much X-ray each tissue absorbs.
Core Concepts
Basic properties of X-rays used in imaging
- Penetration — basis of radiography
- Differential absorption — basis of tissue differentiation
- Photographic effect — image recording on film/detector
- Ionization of medium — detection/measurement of radiation
- Fluorescent effect — fluoroscopy
- Biological effects — radiotherapy and the need for radiation protection
- Rectilinear propagation — prediction of the path of the beam
The four basic densities (conventional radiography)
| Structure | Appearance on film | Why |
|---|---|---|
| Gas | Black | Least absorbed → most developed → most blackening |
| Fat | Dark grey | Absorbs slightly fewer X-rays than other soft tissues |
| Soft tissue (solid viscera, muscle, blood, fluids, bowel wall) | Grey | Similar absorptive capacity |
| Calcified structures / bone | White | Most absorbed → least blackening |
| Metal (e.g. contrast, prostheses) | Bright white | Highest absorption |
Views and projections
- PA (posteroanterior) — beam passes back-to-front; the standard projection for a routine chest film
- AP (anteroposterior) — beam passes front-to-back
- Frontal — either PA or AP
- Lateral — second view
- A film is two-dimensional: all structures along the beam path overlap. At least two views are required to gain information about the third dimension (e.g. PA + lateral).
Mammography vs conventional radiography
| Conventional radiography | Mammography | |
|---|---|---|
| Target | Tungsten | Molybdenum |
| KV | 60–120 KV | 40 KV |
| X-ray quality | Hard X-ray (0.031–0.008 nm) | Soft X-ray (0.062–0.093 nm) |
Computed Tomography (CT)
- Also relies on X-rays transmitted through the body, but the tube and detectors rotate around the patient; data are manipulated by computer to produce axial sections.
- Spiral/helical CT: tube continuous rotation + table continuous movement → volume data → coronal/sagittal reconstructions.
- CT number / Hounsfield Unit (HU): a numerical value per pixel quantifying X-ray absorption. Water = 0 HU, air = −1000 HU, bone = +1000 HU.
- Key advantage over plain film: very small differences in absorption can be visualized — the density range recorded increases ~ten-fold; fat is distinguished from other soft tissues; brain substance can be separated from CSF.
- Window technique: brightness adjusted by window level; contrast adjusted by window width. Wide window shows all structures but loses fine density detail; narrow window shows variations of a few HU but much of the image is black or white. At least two sets of images are needed (e.g. lung window + mediastinal window; brain window + bone window).
- Contrast enhancement: iodinated contrast media given IV (or orally for bowel) to display vascularity and abnormal tissue.
- Clinical applications: neuroimaging (acute head trauma, acute intracranial hemorrhage; low sensitivity for early ischemic stroke; intracranial metastatic disease; white matter degenerative disease); head & neck (soft tissue, paranasal sinuses, bone); body imaging (chest, abdomen, pelvis); pulmonary nodules; renal calculi (without contrast); acute appendicitis; specialized protocols for liver, pancreatic, renal and adrenal masses; acute abdomen; spine trauma; CT angiography (coronary arteries, 3D CTA).
Magnetic Resonance Imaging (MRI)
- Basic principle: hydrogen nuclei (protons in water and lipids) align with a strong magnetic field; a radiofrequency pulse at the resonant frequency flips them; as they relax back they induce a signal detected by receiver coils.
- Signal depends on proton density, T1 relaxation time and T2 relaxation time.
- T1WI — contrast mainly due to T1 relaxation; T2WI — contrast due to T2 relaxation; PdWI — proton density.
- Common sequences: T1WI, T2WI, FLAIR (removes CSF signal, highlights brain lesions), DWI (diffusion-weighted; acute infarct and abscess show restricted diffusion), fat-saturation, MRA/MRV (flowing blood = signal void; TOF MRA shows vessels bright without contrast).
- Signal intensity table (T1WI / T2WI): water = low/high; fat = high/high; melanin = high/high; subacute hematoma = high/high; calcification/cortical bone = low/low; bone marrow (lipid) = white on T1.
- Flow void effect: rapidly flowing blood excited by RF flows out of the imaging plane before signal is measured → vessels appear dark (no/low signal).
- Advantages: excellent soft tissue contrast (brain, spine, pelvis, MSK, breast); multiplanar; no ionizing radiation. Disadvantages: limited lung imaging (artifacts), cost, time, contraindications (implants).
- Contrast medium: Gadolinium-DTPA (paramagnetic agent) — e.g. meningioma and pituitary adenoma enhance.
Radionuclide imaging / Nuclear medicine
- Diagnostic: radioactive isotopes emit gamma rays as they decay; short half-lives; rapidly excreted.
- SPECT — single photon emission CT; PET — positron emission tomography with F-18 fluorodeoxyglucose (FDG); PET-CT / PET-MRI combine metabolic and structural information.
- Structural alterations → CT/MRI; functional changes → SPECT; metabolic changes → PET.
Ultrasonography (key principles)
- Uses high-energy sound waves >20 kHz (diagnostic range 2–20 MHz); a mechanical wave; the transducer acts as transmitter and receiver.
- Reflection occurs at boundaries with different acoustic impedance; the portion reflected depends on the impedance difference. Air/bone-soft tissue interfaces reflect almost all energy → it is difficult to image lung and behind bones, and impossible across bowel gas; gel is used to remove air between transducer and skin.
- Attenuation = gradual weakening of the beam (reflection, scattering, absorption); penetrance is inversely related to frequency; TGC (time gain compensation) amplifies returning signals by depth.
- Modes: A-mode (one-dimensional peaks), B-mode/2D (brightness modulation — most common), M-mode (motion — cardiac dimensions), Doppler (color, CW, PW) — measures blood flow velocity via the Doppler effect (moving red blood cells; red = toward transducer, blue = away).
- Terminology: anechoic = no internal echoes (fluid/cyst; with posterior acoustic enhancement); hyperechoic = bright, no shadow; strong echo = bright + acoustic shadow (stones); hypoechoic; isoechoic; complex = mixed cystic/solid.
- Preparation: pelvis exam with full bladder (lifts intestine); liver/gallbladder exam after 6-hour fast.
- Applications: abdominal solid organs, echocardiography, obstetrics, superficial organs (neck, breast, eyes, scrotum), MSK (tendons, muscles, nerves), vascular (DVT, venous insufficiency), emergency (trauma, acute abdomen).
High-Yield Points
- Key Point: Four basic densities: gas (black) < fat < soft tissue < bone/metal (white).
- Key Point: Hounsfield scale: water 0 HU, air −1000 HU, bone +1000 HU.
- Key Point: At least two views are needed because a film is 2-D with structure overlap.
- Key Point: FLAIR suppresses CSF; DWI shows restricted diffusion in acute infarct/abscess.
- Key Point: US cannot image through bone or bowel gas; cysts are anechoic with posterior enhancement; stones are strongly echogenic with acoustic shadow.
- Key Point: PET-CT combines metabolic (FDG uptake) + structural imaging; FDG also accumulates in inflammation, so uptake is not specific for tumor.
LMCHK OSCE Practice（OSCE & LMCHK）
- Chest: plain CXR first; CT for detail/staging; CTPA for pulmonary embolism; US for pleural collections (thoracentesis guidance). - Brain: acute head trauma / acute intracranial hemorrhage → plain CT (non-contrast); early ischemic stroke → CT is low-sensitivity, MRI DWI is best; SAH → CT (then LP if negative). - Abdomen: US first-line in children, pregnant women, RUQ pain, gallstones; CT with contrast = gold standard for most non-obstetric acute abdomens; CT is the gold standard for urinary tract stones. - Spine: plain film in trauma; MRI = gold standard for cord/nerve roots; bone scan for bony metastases. - MSK: plain radiographs first ("one view is no view"); MRI for marrow/soft tissue; bone scan for metastases/occult fracture (not myeloma).
- Which modality? A practical "imaging choice" framework for OSCE-style questions:
- In the exam you will be asked: imaging technique used, location of the lesion, imaging features, common causes, and diagnosis/differential diagnosis — always answer in that order.
Topic Summary
Radiology rests on differential absorption of X-rays (four densities), CT's ability to display tiny HU differences with windows, MRI's soft-tissue contrast (T1/T2/FLAIR/DWI), ultrasound's acoustic reflection (echogenicity terms, Doppler), and nuclear medicine's metabolic imaging (SPECT/PET). Choosing the right first-line modality for a clinical problem is a core exam skill: plain film for most chest/bone/acute-abdomen screening, non-contrast CT for acute brain hemorrhage/trauma, US for children/pregnancy/gallbladder, MRI for cord/marrow/soft tissue, and PET-CT for tumor staging.