A radiograph is, in essence, a shadow play: as the beam from the tube crosses the patient, different tissues absorb it to different degrees, and the pattern of radiation reaching the detector carries that "shadow." The animation below summarizes the whole journey; then we unpack each step.
1 · X-rays are born in the tube
It all begins in the X-ray tube. The hot filament at the cathode emits electrons; the 20–150 kV voltage applied between cathode and anode accelerates them toward the anode.1 When they strike the anode, most of their kinetic energy becomes heat and only a small fraction becomes X-rays — which is why anode heat management is critical.1
Two mechanisms produce X-rays: bremsstrahlung (a continuous spectrum from the braking of electrons near the nucleus) and characteristic radiation (from an inner-shell vacancy being filled).1
Before leaving the tube the beam is filtered: low-energy photons that add skin dose without helping the image are removed, "hardening" the beam. The collimator then limits the beam to the needed area only.
2 · Through the patient: the image is "written" here
The real information forms inside the patient. As the beam crosses tissue it is attenuated — but not every tissue attenuates equally. The probability of photoelectric absorption is roughly proportional to the cube of the atomic number and inversely to the cube of energy (≈ Z³/E³).2 So high-atomic-number bone (calcium) absorbs a lot; soft tissue transmits most of the beam. This differential absorption between tissues is the source of contrast.2
This attenuation is mathematically exponential: the number of photons passing through a tissue of thickness x without interacting falls off exponentially from the initial count (the Beer–Lambert law):
Here μ is the tissue's linear attenuation coefficient — high in bone, low in soft tissue; the larger μ, the fewer photons get through. The pattern of photons reaching the detector arises precisely because this exponential attenuation differs from tissue to tissue.2
Just before reaching the detector the beam carries a not-yet-visible pattern — the "aerial image." The biggest threat to that pattern is the next step: scatter.
3 · Scatter and the grid
Compton scattering is the predominant interaction in soft tissue across the diagnostic range; the photon collides with an electron and continues in a changed direction.2 These scattered photons reach the detector at random angles and fog the image, lowering contrast. Scatter grows with field size and patient thickness — far worse in the abdomen than in an extremity; collimating to a smaller field reduces it.3
The most widely used tool against scatter is the anti-scatter grid: thin lead septa that pass the perpendicular primary photons and absorb the oblique scattered ones.3
4 · The detector: from beam to signal
The beam has now reached the detector — but to "see" the X-rays we must first turn them into a measurable signal. There are two main routes:3
- Indirect conversion: the X-ray is first turned into visible light in a scintillator (cesium iodide, CsI; or Gd₂O₂S); a photodiode converts that light into electric charge, stored in the dexel capacitor. Because the light spreads a little in this two-step process, sharpness drops slightly.3
- Direct conversion: an amorphous selenium (a-Se) photoconductor converts the X-ray directly into charge — no light step. The applied field draws the charge straight to the collection electrode; since charge does not spread laterally, spatial resolution is better.3
5 · From signal to screen
The detector surface is a matrix of millions of tiny detector elements — dexels. Each dexel has a TFT switch, a charge-collection electrode and a storage capacitor. Throughout the exposure each dexel accumulates its own charge; when the exposure ends, the array is read out row by row.3
The analog charge signal then passes through an analog-to-digital converter (ADC) and becomes a number — a gray value for each pixel. Image processing (windowing, edge enhancement) is applied and the result appears on the screen. The system also produces an exposure index (EI) that feeds back the dose level reaching the detector.
The whole chain
So the moment we casually call "press the button, get an image" hides a tight physics chain running in the background: production in the tube → filtration/collimation → differential attenuation in the patient → scatter and grid → beam-to-signal conversion in the detector → ADC and image processing → screen. At every link the balance between dose and image quality is re-struck — and measuring and optimizing each step of this chain is the job of medical physics.
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Bölüm 6 (X-ışını üretimi, s.167; bremsstrahlung s.37; karakteristik s.32). Atıflardaki sayfa numaraları bu baskıya aittir.
- Bushberg JT, et al., a.g.e., Bölüm 3 — etkileşimler: fotoelektrik (≈Z³/E³, s.42), Compton saçılması (s.39) ve üstel zayıflama (N = N₀e−μx, Denklem 3-5).
- Bushberg JT, et al., a.g.e., Bölüm 7 — radyografi ve dedektörler: anti-saçılma grid (s.231), sintilatör/indirekt algılama (s.209), bilgisayarlı radyografi/depo fosforu (s.213), düz panel TFT dizileri ve direkt/indirekt dönüşüm (s.220–221).