"Imaging" is not one thing. Each modality rests on a different physical language: some read the attenuation of X-rays, some the echo of sound, some the magnetic behavior of nuclei. Below we take seven modalities one by one, each working principle shown with an animation.
Radiography — a single shadow
The most basic method: the X-ray beam crosses the patient in one pass and forms a single 2D projection (shadow) on a flat detector. Fast and cheap, ideal for bone and lung — but depth information is limited; all structures overlap.
CT — a slice from hundreds of angles
CT breaks past the limit of radiography: the tube and detector rotate around the patient (rotate–rotate geometry) and collect projections from hundreds of angles; a computer reconstructs a cross-section from this data.1 In modern systems the gantry rotates several times per second.1
Fluoroscopy — a real-time movie
Where radiography takes a single frame, fluoroscopy produces a moving image with continuous or pulsed X-rays — for dynamic procedures like advancing a catheter or watching a swallow.5 The cost is long exposure time and the dose that accumulates with it.
Mammography — low energy, finest detail
To catch the breast's low-contrast, very small structures (microcalcifications), mammography uses low-energy X-rays and compression; it has the highest spatial resolution in radiology.5 Compression both improves detail and lowers dose.
Ultrasound — the echo of sound
Ultrasound uses no ionizing radiation. The probe sends a short sound pulse into tissue; the pulse travels at ~1540 m/s in soft tissue and part of it returns as an echo from every interface where the acoustic impedance changes.2 The round-trip time of the echo gives depth, its amplitude gives brightness (pulse–echo).2
MRI — nuclei spinning in a magnetic field
MRI also uses no ionizing radiation. In a strong magnetic field, hydrogen nuclei (protons) in the body precess like a spinning top; the precession frequency is proportional to the field strength (the Larmor relation) — about 64 MHz at 1.5 T.3 A radiofrequency (RF) pulse "tips" these protons; the signal they emit as they relax forms the image. This is why MRI's soft-tissue contrast is superb.
Nuclear medicine / PET — radiation emitted from within
Where the others place the source outside, in nuclear medicine the source is inside the patient: the administered radiopharmaceutical shows function/metabolism, not anatomy. In PET the tracer emits a positron; when it annihilates with an electron, two 511 keV photons are born about 180° apart.4 A ring-shaped detector catches the two photons simultaneously (in coincidence) to locate the source.4
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Bölüm 10 (Computed Tomography) — tüp ve dedektörün hasta etrafında dönmesi (rotate–rotate geometri), gantry rotasyonu.
- Bushberg JT, et al., a.g.e., Bölüm 14 (Ultrasound) — pulse–echo görüntüleme, yumuşak dokuda ses hızı 1540 m/s, akustik empedans farkları.
- Bushberg JT, et al., a.g.e., Bölüm 12 (Magnetic Resonance Basics) — protonun manyetik alanda presesyonu, Larmor bağıntısı; 1,5 T'de proton rezonans frekansı ~64 MHz.
- Bushberg JT, et al., a.g.e., Bölüm 15 ve 19 (Radyoaktivite ve PET) — pozitron–elektron anihilasyonu sonucu yaklaşık 180° zıt yönde yayılan iki 511 keV foton; koinsidans algılama.
- Bushberg JT, et al., a.g.e., Bölüm 8 (Mammography) ve Bölüm 9 (Fluoroscopy) — düşük enerjili X-ışını ve kompresyon; gerçek zamanlı/pulslu floroskopi.