X-ray imaging basics

In short

X-ray imaging (radiography) uses a beam of high-energy photons produced in an X-ray tube that passes through the body, is attenuated differently by bone, soft tissue, fat and air, and is recorded on a detector to give a two-dimensional shadow image. It remains the first-line and highest-volume imaging investigation in India - chest, limb, spine, abdomen, dental and mobile ward radiographs - because it is quick, cheap and widely available. For B.Sc Radiology and Imaging Technology students it carries the largest share of marks in "Radiographic Physics", "Radiographic Techniques" and "Equipment" papers, and it is the core of practical and viva assessment. Recruiters - Apollo, Fortis, Max, Manipal, AIIMS and state medical colleges, Medall, Dr Lal PathLabs, Krsnaa and SRL diagnostics, plus equipment companies such as Siemens Healthineers, GE HealthCare, Philips, Wipro GE and Allengers - test exposure factors, positioning, image quality and quality-assurance knowledge in technical rounds.

Explained

Nature and production of X-rays

• X-rays are electromagnetic radiation, wavelength roughly 0.01-10 nm, energy in the keV range; diagnostic X-rays are about 20-150 keV. They travel in straight lines at the speed of light, are uncharged, cannot be deflected by magnets, penetrate matter, cause ionisation, fluorescence and biological effects, and expose a detector.
• Requirements for production: a source of electrons (heated filament - thermionic emission), a high potential difference to accelerate them, and a target to stop them suddenly inside an evacuated envelope.
• Bremsstrahlung ("braking") radiation: a fast electron passing near a tungsten nucleus is decelerated and deflected; the lost kinetic energy appears as an X-ray photon. This gives a continuous spectrum from near zero up to a maximum energy numerically equal to the applied kVp, and accounts for most of the diagnostic beam.
• Characteristic radiation: an incident electron ejects an inner-shell (K-shell) electron of tungsten; an outer electron fills the vacancy and the energy difference is emitted as a photon of fixed energy. For tungsten the K-characteristic lines are about 59 and 67 keV, so they appear only above about 70 kVp.
• Efficiency is very low: under about 1% of the electron energy becomes X-rays at diagnostic kVp; over 99% becomes heat - which is why tube cooling and heat-unit limits dominate equipment design.

The X-ray tube and its components

• Cathode: a coiled tungsten filament (often dual - broad and fine focus) inside a focusing cup of nickel or molybdenum held at negative potential to narrow the electron stream. Filament current (a few amperes at about 10 V) sets the tube current (mA).
• Anode: a tungsten-rhenium target on a molybdenum/graphite disc. Tungsten is chosen for high atomic number (Z = 74, efficient X-ray production), high melting point (about 3,400 °C) and good thermal conductivity. Mammography tubes use molybdenum or rhodium targets for a low-energy beam.
• Stationary vs rotating anode: rotating anodes (about 3,000-10,000 rpm) spread heat over a large focal track, permitting far higher loading; stationary anodes survive only in dental and small mobile units.
• Line focus principle: the target face is angled (about 7-20°) so a large *actual* focal spot (heat spread over a big area) projects as a small *effective* focal spot (good geometric sharpness). Effective size = actual size × sin θ.
• Anode heel effect: beam intensity is lower on the anode side because photons produced inside the target are absorbed by the target itself. Practical use - place the thicker body part (shoulder in a humerus, lumbar region in a thoracolumbar spine) under the cathode side.
• Glass or metal-ceramic envelope, evacuated to prevent electron collision with gas; oil-filled housing for cooling and electrical insulation, with a lead-lined shield so radiation leaves only through the port/window.
• Space charge effect: the electron cloud around the filament repels further emission; at low kVp the tube is space-charge limited, at high kVp emission (saturation) limited.
• Tube rating and heat units: single-phase HU = kVp × mA × s; three-phase/high-frequency HU = kVp × mA × s × 1.35. Anode heat capacity and cooling curves limit rapid repeat exposures.

Beam quality, quantity and exposure factors

• Quantity (number of photons) ∝ mAs, ∝ kVp² approximately, ∝ 1/d². Quality (penetrating power) is governed by kVp and filtration, expressed as half-value layer (HVL) in mm of aluminium.
• kVp: controls maximum photon energy, penetration and (inversely) subject contrast. Increasing kVp raises both quantity and quality and increases scatter. The 15% rule: a 15% rise in kVp roughly doubles receptor exposure - so it is offset by halving mAs.
• mA and time (s): their product mAs controls quantity and hence density/signal only. Short exposure times reduce motion blur - essential for chest and paediatric work.
• Filtration: inherent (tube window, oil, housing - about 0.5-1.0 mm Al equivalent) plus added aluminium. Total filtration must be at least 2.5 mm Al equivalent above 70 kVp (typically 1.5 mm Al for 50-70 kVp and 0.5 mm below 50 kVp). It removes soft, non-image-forming photons that would only raise skin dose - "beam hardening", which raises mean energy while reducing total output.
• Automatic exposure control (AEC): ionisation chambers placed in front of the image receptor (or solid-state phototimer cells behind it) terminate exposure when the preset signal is reached; back-up timer and correct chamber selection are mandatory.
• Generators: single-phase (high ripple), three-phase 6- and 12-pulse, and modern high-frequency generators (several kHz, ripple under 4%) that give near-constant potential, shorter exposures and lower patient dose.
• Rectification: self-rectified, half-wave, full-wave; high-frequency inverter circuits now standard.

Interaction with the patient and image formation

• Photoelectric absorption: the photon is wholly absorbed, ejecting an inner electron; probability ∝ Z³/E³ approximately. It creates subject contrast (bone vs soft tissue) but delivers dose. Dominant at low kVp.
• Compton scatter: the photon interacts with a loosely bound outer electron, loses part of its energy and changes direction; the scattered photon carries no useful positional information, so it adds a uniform fog that lowers contrast and is the main hazard to staff. Dominant in soft tissue above about 30 keV and increasingly so with rising kVp.
• Coherent (Rayleigh) scatter contributes a small fraction; pair production needs 1.02 MeV and does not occur diagnostically.
• Attenuation follows an exponential law for a narrow monoenergetic beam: I = I₀e^−μx. The differential attenuation between tissues - driven by density, effective atomic number and thickness - is what forms the image.
• Five natural radiographic densities, most to least radiolucent: gas/air, fat, water/soft tissue, bone/calcium, metal/contrast media.
• Scatter control: close collimation (the single most effective measure, and a legal requirement that the field be no larger than the receptor), anti-scatter grid, air gap technique, and compression.
• Grids: lead strips separated by interspace material. Grid ratio = h/D (height of lead strip ÷ interspace width), commonly 5:1 to 16:1; higher ratio removes more scatter but needs more exposure. Types - linear, crossed, focused, parallel; stationary (fine-line) or moving (Potter-Bucky reciprocating). Generally used above about 10 cm part thickness or 70 kVp. Grid factor/Bucky factor = exposure with grid ÷ exposure without. Errors - off-centring, off-focus (outside focal range), upside-down focused grid, and grid cut-off.

Image receptors: from film to digital

• Film-screen: a double-emulsion film between two intensifying screens of rare-earth phosphor (gadolinium oxysulphide, lanthanum oxybromide) in a light-tight cassette; screens convert X-rays to light and amplify the effect (intensification factor), cutting dose greatly at some cost in sharpness. Faster (coarser, thicker) screens = more quantum mottle and blur. Processing - developer, fixer, wash, dry - in an automatic processor; largely historical in Indian hospitals now.
• Computed radiography (CR): a photostimulable storage phosphor plate (barium fluorohalide) holds a latent image that a laser scanner reads as emitted light; the plate is then erased and reused. Wide latitude, same cassette workflow.
• Digital radiography (DR) flat panels: *indirect* - caesium iodide or gadolinium oxysulphide scintillator over an amorphous silicon TFT photodiode array; *direct* - amorphous selenium converts X-rays straight to charge. Immediate images, highest detective quantum efficiency (DQE), lowest dose for a given image quality, and PACS/DICOM integration.
• Digital image terms: matrix and pixel size, bit depth (10-16 bit), spatial resolution in lp/mm, DQE and MTF (modulation transfer function), window width/level for contrast and brightness, look-up tables, post-processing, edge enhancement, dual-energy subtraction.
• Exposure indicator (EI, with target EI and deviation index DI): digital systems rebuild a diagnostic-looking image over a very wide exposure range, so density no longer warns of overexposure - dose creep must be watched using EI/DI.
• Fluoroscopy gives real-time imaging with an image intensifier or flat-panel detector, used for barium studies, IVP, HSG and C-arm work in theatre.

Image quality, geometry and radiographic technique

• Density/brightness (overall blackening - mAs, kVp, distance), contrast (difference between adjacent areas - kVp, scatter, grid, tissue composition, windowing), spatial resolution/sharpness, noise (quantum mottle - too few photons), and distortion (size and shape).
• Geometric unsharpness (penumbra) = focal spot size × OID / SOD; minimised by a small focal spot, long SID and the part close to the receptor.
• Magnification = SID/SOD = image size/object size; distortion arises from object-receptor non-parallelism or beam angulation. Elongation and foreshortening follow from tube, part and receptor alignment.
• Motion unsharpness - the largest avoidable blur; countered by short exposure time, immobilisation, suspended respiration, and clear instructions in the patient's language.
• Standard SID: about 100 cm (40 in) for most table/erect work, 180 cm (72 in) for PA chest to reduce heart magnification and beam divergence.
• Common projections: chest PA (and lateral, lordotic, decubitus), abdomen AP erect and supine for obstruction/perforation, skull and paranasal sinus views (Caldwell, Water's, lateral, submentovertex), cervical/thoracic/lumbar spine AP and lateral with obliques, pelvis AP, extremity AP/lateral/oblique, Towne's, Schuller's; standard terms - AP, PA, lateral, oblique, axial, tangential, decubitus, erect, supine, prone.
• Markers and identification: anatomical side markers, patient ID, date; unmarked or wrongly marked films are a serious reporting hazard.
• Criteria for an acceptable PA chest: full inspiration (at least 10 posterior ribs visible above the diaphragm), medial scapular borders clear of the lung fields, symmetrical clavicles/sternoclavicular joints, visible vertebral outlines and posterior ribs through the heart shadow, and apices to below both costophrenic angles included.
• Artefacts: patient motion, jewellery/clothing/hairpins, grid lines and cut-off, double exposure, CR plate ghost images, dead pixels and panel calibration faults, backscatter, processing streaks on film.
• Quality assurance (routinely examined): kVp accuracy, timer accuracy, mA/mAs linearity and reproducibility, HVL/total filtration, beam-light field congruence, focal spot size (star or slit test), tube leakage, collimator and SID accuracy, grid alignment, AEC performance, image-receptor and monitor (SMPTE/TG18 pattern) checks, plus repeat/reject analysis.

Indian regulation and practice (overview only - the detail belongs to radiation protection)

• Diagnostic X-ray equipment and its use are regulated by the Atomic Energy Regulatory Board (AERB) under the SHANTI Act, 2025 (which replaced the Atomic Energy Act, 1962) and the Atomic Energy (Radiation Protection) Rules, 2004 (framed under the 1962 Act; check for rules notified under the new Act); registration/licensing of X-ray installations is done through AERB's e-LORA online portal, with room layout approval, a Radiological Safety Officer where required, TLD personnel monitoring, and QA testing at prescribed intervals.
• PNDT/PC-PNDT Act, 1994 (amended 2003) applies to ultrasound and other sex-determination-capable equipment, not to plain radiography, but the mandatory notice against sex determination is displayed in Indian imaging departments.
• Pregnancy enquiry ("10-day/28-day rule" practice), gonad and thyroid shielding, use of lead aprons of at least 0.25-0.5 mm Pb equivalent, and the ALARA principle govern daily practice. Dose reference levels and specific numeric limits are revised periodically by AERB/ICRP - check the current AERB safety code rather than relying on an older figure.

Key points

• Bremsstrahlung - continuous spectrum from electron deceleration; the bulk of the diagnostic beam.
• Characteristic radiation - discrete tungsten K-lines at about 59 and 67 keV, present only above roughly 70 kVp.
• Efficiency - under 1% of input energy becomes X-rays; the rest is heat, hence the rotating anode.
• Line focus principle - an angled target gives a small effective focal spot with a large heat-loading area.
• Heel effect - lower intensity on the anode side; put the thicker part towards the cathode.
• kVp - controls penetration, beam quality, scatter and subject contrast; quantity ∝ kVp² roughly.
• mAs - controls photon quantity and image density only, not penetration.
• 15% rule - +15% kVp doubles receptor exposure; halve the mAs to compensate.
• Inverse square law - intensity falls as 1/d²; exposure must rise as d² when SID increases.
• Filtration - at least 2.5 mm Al equivalent total above 70 kVp; hardens the beam and cuts skin dose.
• Photoelectric effect - creates contrast (∝ Z³/E³); Compton scatter - creates fog and staff dose.
• Collimation - the cheapest and most effective scatter and dose control; field never larger than the receptor.
• Grid ratio = lead strip height ÷ interspace width; higher ratio = better contrast, more patient dose.
• Geometric unsharpness = focal spot × OID/SOD; small focus, long SID, part close to receptor.
• Chest PA at 180 cm SID, high kVp, short time, full inspiration - reduces heart magnification and motion blur.
• DR flat panels give the highest DQE and lowest dose; CR uses a reusable storage phosphor plate.
• Exposure index / deviation index - the only reliable guard against dose creep in digital radiography.
• AERB and e-LORA - registration, licensing and QA of every X-ray installation in India.

Formulas and examples

Symbols: kVp = peak kilovoltage (kV), mA = tube current, s = exposure time, SID = source-to-image distance, SOD = source-to-object distance, OID = object-to-image distance, HVL = half-value layer (mm Al), μ = linear attenuation coefficient (cm⁻¹ or mm⁻¹), I = intensity (mGy/min or similar), h and D in the same length unit.

Example 1 - Distance change (direct square law). A knee AP is taken at 100 cm SID with 10 mAs. The tube cannot be lowered and the exposure must be made at 120 cm. What mAs keeps the same receptor exposure?

• mAs₂ = 10 × (120/100)² = 10 × 1.44 = 14.4 mAs (set 14 or 15 mAs on the console).
• Check with the inverse square law: intensity at 120 cm = (100/120)² = 0.694 of that at 100 cm, so exposure must rise by 1/0.694 = 1.44 times. Consistent.

Example 2 - 15% rule and reciprocity. A lumbar spine lateral uses 80 kVp, 200 mA, 0.4 s.

• mAs = 200 × 0.4 = 80 mAs.
• Patient motion is a problem, so raise kVp by 15%: kVp₂ = 80 × 1.15 = 92 kVp, mAs₂ = 80/2 = 40 mAs.
• At 400 mA, time = 40/400 = 0.1 s - exposure time cut to one-quarter of the original, so motion blur falls sharply. Contrast will be slightly lower and scatter higher, which a grid controls.

Example 3 - Attenuation and HVL. A 70 kVp beam has an HVL of 3 mm Al. What fraction is transmitted through 9 mm Al, and what is μ?

• 9 mm = 3 HVLs → transmission = (1/2)³ = 1/8 = 0.125, i.e. 12.5% (87.5% attenuated).
• μ = 0.693/3 = 0.231 mm⁻¹. Verify: I/I₀ = e^{−0.231 × 9} = e^−2.079 = 0.125. Correct.

Example 4 - Magnification and geometric unsharpness. A chest is radiographed at SID 180 cm; a nodule lies 10 cm from the receptor. Effective focal spot = 1.2 mm.

• SOD = 180 − 10 = 170 cm → M = 180/170 = 1.059, i.e. 5.9% magnification.
• Ug = 1.2 × 10/170 = 0.071 mm.
• Repeat at SID 100 cm with the same 10 cm OID: SOD = 90 cm → M = 100/90 = 1.111 (11.1%) and Ug = 1.2 × 10/90 = 0.133 mm - nearly double the blur. This is exactly why the chest PA is taken at 180 cm.

Example 5 - Heat units and focal spot. A three-phase unit exposes at 80 kVp, 300 mA, 0.2 s. The target angle is 12° and the actual focal spot is 5 mm wide.

• HU = 1.35 × 80 × 300 × 0.2 = 6,480 HU per exposure. Ten such exposures in quick succession deposit 64,800 HU - well within a 300 kHU anode but the per-minute cooling curve must still be respected.
• Effective focal spot = 5 × sin 12° = 5 × 0.2079 = 1.04 mm.

Where it is used

• Hospital radiology departments and standalone diagnostic centres: general radiography, OPD chest and limb films, ward and ICU mobile radiographs, emergency trauma series - the bulk of the daily workload at Apollo, Fortis, Max, Manipal, AIIMS, PGIMER and district hospitals.
• Public health programmes: chest radiography for TB screening under India's National TB Elimination Programme (including AI-assisted reading of digital chest X-rays on portable units), pre-employment and insurance medical checks, pneumoconiosis screening in mining and foundry industries.
• Operating theatre and interventional work: mobile C-arm fluoroscopy for orthopaedic nailing and plating, spine surgery, urology and cardiac cath labs - a major employment area for B.Sc technologists.
• Dental and specialist imaging: intraoral periapical and bitewing radiographs, OPG/panoramic units, cephalometrics, mammography units in breast clinics and screening camps.
• Equipment industry and service roles: application specialists, installation and service engineers, and QA/physics support with Siemens Healthineers, GE HealthCare, Philips, Allengers, Wipro GE and Trivitron; AERB-related licensing, QA testing and layout documentation.
• Veterinary, industrial and security radiography: non-destructive testing of welds and castings, and baggage screening - the same physics applied outside medicine.

Interview questions

• Q: What are the two processes that produce X-rays in a tube, and which dominates? A: Bremsstrahlung (continuous spectrum, from electron deceleration near the nucleus) and characteristic radiation (discrete K-lines from inner-shell vacancies); bremsstrahlung produces most of the diagnostic beam.
• Q: Why is tungsten used as the anode target? A: High atomic number for efficient X-ray production, a very high melting point of about 3,400 °C, and good thermal conductivity to carry away the large heat load.
• Q: Difference between kVp and mAs in terms of the image? A: kVp controls beam quality, penetration, scatter and subject contrast; mAs controls only the quantity of photons and so the density or signal level.
• Q: What is the heel effect and how do you use it clinically? A: Beam intensity is lower on the anode side because of self-absorption in the target; position the thicker end of the body part towards the cathode side for even density.
• Q: Why is a PA chest taken at 180 cm instead of 100 cm? A: The longer distance reduces magnification of the heart and mediastinum and reduces geometric unsharpness from beam divergence, giving a truer cardiac silhouette.
• Q: When would you add a grid, and what do you pay for it? A: Above roughly 10 cm part thickness or 70 kVp, where scatter is significant; it improves contrast but needs several times the exposure, so patient dose rises and alignment errors cause cut-off.
• Q: State the 15% rule with a number. A: Raising kVp by 15% doubles receptor exposure - so 70 kVp at 20 mAs gives roughly the same density as 80 kVp at 10 mAs.
• Q: Why is filtration added when it reduces output? A: It removes low-energy photons that would be absorbed in the skin without reaching the detector, lowering patient dose for essentially the same image; at least 2.5 mm Al equivalent total is required above 70 kVp.
• Q: Which interaction gives contrast and which gives fog? A: Photoelectric absorption gives contrast because it depends strongly on atomic number; Compton scatter gives fog and is the main source of staff dose.
• Q: How do you reduce motion blur on a restless patient? A: Shorten the exposure time by using a higher mA and higher kVp with correspondingly lower mAs, immobilise the part, give clear breathing instructions, and use the fine focus only if the loading permits.
• Q: In digital radiography, how do you know the patient was overexposed? A: The displayed image looks normal because of automatic rescaling, so you must check the exposure index and deviation index against the system's target value - the usual safeguard against dose creep.
• Q: Who regulates X-ray installations in India and what approval is needed? A: The AERB, under the SHANTI Act, 2025 (which replaced the Atomic Energy Act, 1962) and the Radiation Protection Rules, 2004; registration/licence through the e-LORA portal with approved room layout, QA testing and personnel monitoring.
• Q: A focused grid is placed upside down - what do you see? A: Severe grid cut-off with a correctly exposed narrow central strip and marked loss of density towards both edges; the image must be repeated.

Common mistakes

• Treating mAs as a control of penetration, or kVp as a mere brightness control - they are not interchangeable despite the 15% rule.
• Mixing up the inverse square law (intensity falls as 1/d²) with the direct square law for maintaining density (mAs rises as d²), and inverting the ratio.
• Forgetting to square the distance ratio, or using SOD where SID is required in magnification problems.
• Saying X-ray production is efficient - over 99% of the energy is heat, which is the whole reason for rotating anodes and heat-unit limits.
• Claiming characteristic radiation is present at any kVp; the tungsten K-lines need about 70 kVp or more.
• Opening the collimator wider than the receptor "to be safe", which raises scatter and patient dose and is a regulatory violation.
• Relying on image appearance for exposure adequacy in CR/DR instead of the exposure index, and omitting anatomical side markers.
• Quoting old film-screen exposure charts, grid factors or AERB dose figures without noting that they vary with equipment and are periodically revised.

Related topics

CT and MRI basics · Ultrasound basics · Radiation safety and AERB · Radiographic positioning and anatomy · Contrast media and special radiographic procedures · PACS, DICOM and image post-processing