5.1 Principle of Fluoroscopy Machine

Fluoroscopy is a imaging method that uses a continuous or pulsed beam of X-rays passing through a patient to produce a moving, real-time image of internal structures on a monitor. It works because different tissues absorb X-rays by different amounts, and because the transmitted radiation can be converted into visible light and then into an electronic video signal. To understand how a fluoroscopy machine operates, a biomedical technician must understand two sets of principles: the physical behaviour of X-rays and matter, and the electrical and electronic circuits that generate, control and process the radiation and the image signal.

5.1.1 Physical properties

The physical properties of fluoroscopy describe how X-rays are produced, how they travel, how they interact with body tissue and how they create a visible image. These properties explain why a barium swallow at a county referral hospital shows the outline of the oesophagus while the surrounding soft tissue is barely visible, and why the image changes instantly as the patient moves.

Production of X-rays

X-rays are produced inside an evacuated glass or metal-enclosed tube when fast-moving electrons are suddenly stopped by a metal target (the anode). Only about 1% of the electrical energy supplied is converted to X-rays; the remaining 99% appears as heat in the anode. The process has the following stages:

• Electron release: A heated tungsten filament (the cathode) releases electrons by thermionic emission. The hotter the filament, the more electrons are released.
• Electron acceleration: A high voltage between cathode and anode accelerates the electrons towards the target. The higher the voltage, the greater the kinetic energy of each electron on arrival.
• Bremsstrahlung (braking) radiation: Electrons that pass close to the nuclei of target atoms are slowed and deflected, losing energy as X-ray photons. This produces a continuous spectrum of photon energies and makes up most of the beam.
• Characteristic radiation: An incoming electron may knock out an inner-shell electron of a target atom. When an outer-shell electron falls into the gap, a photon with an energy specific to the target metal is emitted. This produces sharp peaks on the spectrum.
• Heat production: Most electron energy is converted to heat at the focal spot. This is why anodes are made of tungsten, which has a very high melting point, and why heat control is critical in fluoroscopy, where exposures last minutes rather than fractions of a second.

Properties of X-rays used in fluoroscopy

The following properties of X-rays make fluoroscopy possible:

• Electromagnetic radiation: X-rays are photons of electromagnetic energy with very short wavelengths, far shorter than visible light. They travel at the speed of light in a vacuum.
• Invisible and unfelt: The human senses cannot detect X-rays, so a technician depends on indicators, dosimeters and the image itself to know that radiation is being produced.
• Straight-line travel: X-rays travel in straight lines from the focal spot and diverge as a cone. This allows the beam to be shaped by collimators and gives the image a geometric relationship to the patient.
• Penetration: X-rays pass through matter that is opaque to visible light. Higher-energy photons penetrate more deeply than lower-energy photons.
• Differential absorption: Dense materials and materials of high atomic number, such as bone and contrast media containing barium or iodine, absorb more X-rays than air or soft tissue. This difference forms the image contrast.
• Fluorescence: X-rays cause certain materials to emit visible light. This property converts the invisible X-ray pattern leaving the patient into a visible image.
• Ionisation: X-rays remove electrons from atoms they pass through. This is the basis of their biological harm and the reason radiation dose must be kept as low as reasonably achievable.
• Not deflected by fields: X-rays carry no charge, so electric and magnetic fields do not bend the beam.

Attenuation and image contrast

Attenuation is the reduction in the intensity of an X-ray beam as it passes through matter. It results from two main interactions, absorption and scattering, and it determines how much radiation reaches the detector behind each part of the patient.

For a narrow monoenergetic beam, the transmitted intensity is given by:

I = I₀ × e^(−μx)

where I₀ is the initial intensity, μ is the linear attenuation coefficient of the material (cm⁻¹), x is the thickness (cm) and I is the transmitted intensity.

Worked example: a beam of intensity 1000 units passes through 10 cm of tissue with μ = 0.2 cm⁻¹.

Formula: I = I₀ × e^(−μx)

Substitution: I = 1000 × e^(−0.2 × 10)

Working: I = 1000 × e^(−2) = 1000 × 0.135

Result: I = 135 units, so about 13.5% of the beam is transmitted.

The attenuation coefficient increases with tissue density and atomic number, and decreases as the beam energy (kVp) rises. A technician therefore understands why raising kVp gives a more penetrating beam with lower contrast, while lowering kVp gives higher contrast but a higher dose to the patient's skin.

Inverse square law

The intensity of X-rays from a small source falls in proportion to the square of the distance from the source, because the same photons spread over a larger area:

I₂ = I₁ × (d₁ ÷ d₂)²

Worked example: at 50 cm from the focal spot the intensity is 400 units. Find the intensity at 100 cm.

Formula: I₂ = I₁ × (d₁ ÷ d₂)²

Substitution: I₂ = 400 × (50 ÷ 100)²

Working: I₂ = 400 × 0.25

Result: I₂ = 100 units

Doubling the distance reduces the intensity to one quarter. This law governs patient entrance dose, which is why the X-ray tube is kept at a minimum distance from the patient's skin set by the manufacturer, and why staff gain protection by standing further from the beam.

Fluorescence and real-time imaging

Radiography records the X-ray pattern on a single film or detector, whereas fluoroscopy converts the pattern leaving the patient into light continuously. The brightness of each point of the light image is proportional to the intensity of X-rays reaching that point, so thin or low-density regions appear bright and dense regions appear dark. Because the conversion is continuous, the image follows movement such as swallowing, the beating heart or the flow of contrast through a vessel. The light produced by the direct X-ray beam is very faint, so practical fluoroscopy relies on electronic brightening and capture of the image, which is described under the parts of the machine.

5.1.2 Electrical electronic properties

The electrical and electronic properties of a fluoroscopy machine determine how mains electricity is converted into a stable, controllable X-ray beam and how the resulting image is converted into an electronic signal for display. A fault in any of these circuits shows up as a dark image, poor contrast, flicker or no radiation at all, so a technician at a mission hospital or a private imaging centre must understand each stage to diagnose problems.

High-voltage generation

The X-ray tube needs a voltage of tens of thousands of volts, but the supply is ordinary mains electricity. The generator changes the supply in these stages:

• Step-up transformer: The high-voltage transformer raises the supply voltage according to the turns ratio: Vs ÷ Vp = Ns ÷ Np. A large number of secondary turns gives a very high output voltage.
• Rectification: The transformer output is alternating, but the tube conducts X-ray-producing current only when the anode is positive. Rectifiers (high-voltage diodes) convert the alternating voltage to a direct, unidirectional voltage.
• Smoothing: Full-wave, three-phase and high-frequency generators reduce the voltage ripple, so the beam has a more constant energy and a higher average output than a single-phase half-wave circuit.
• High-frequency inverter: Modern generators rectify the mains, convert it to a high-frequency alternating supply, step it up with a small transformer and rectify it again. This gives a nearly constant voltage, a compact unit and fast, accurate control of exposure.
• Voltage selection: The operator or the automatic control selects the tube voltage in kilovolts (kVp), which sets the maximum photon energy and therefore the penetration of the beam.

Worked example: a supply of 230 V is applied to a transformer with a turns ratio Ns ÷ Np = 300.

Formula: Vs = Vp × (Ns ÷ Np)

Substitution: Vs = 230 × 300

Result: Vs = 69 000 V = 69 kV

Tube voltage and tube current

Two separate electrical quantities control the X-ray beam:

• Tube voltage (kVp): This is the potential difference between cathode and anode. It sets the energy and penetrating power of the photons, and it affects contrast and patient dose.
• Tube current (mA): This is the flow of electrons from filament to anode. It determines the number of photons produced. Fluoroscopy uses a continuous tube current of only a few milliamperes, far lower than the hundreds of milliamperes used for a single radiographic exposure, because the beam stays on for minutes.
• Filament circuit: A separate low-voltage, high-current step-down circuit heats the filament. Small changes in filament current produce large changes in filament temperature and therefore in tube current, so this circuit is carefully stabilised.
• Space-charge effect: At low kVp, electrons form a cloud around the filament and limit the current. At higher kVp the cloud is swept away and the current depends mainly on filament temperature.
• Independence of control: Because kVp and mA are controlled by different circuits, they can be adjusted separately to balance image quality against dose.

Electrical power and heat

The electrical power in the tube is the product of voltage and current:

P = V × I

Worked example: a tube operates at 80 kV and 2 mA for 5 minutes during a swallow study.

Formula: P = V × I

Substitution: P = 80 000 V × 0.002 A

Result: P = 160 W

Formula: E = P × t

Substitution: E = 160 W × 300 s

Result: E = 48 000 J

Since about 99% of this energy becomes heat in the anode, the tube housing relies on oil, cooling and thermal overload protection to prevent damage. The generator therefore includes circuits that limit exposure time and power if the anode or housing temperature becomes too high.

Exposure control and feedback

Fluoroscopy machines use electronic control so that the image brightness stays constant while the dose is kept low:

• Automatic brightness control: A sensor measures the brightness of the image or the radiation reaching the detector. A feedback circuit raises or lowers kVp and mA so that brightness stays constant as the beam moves from thin to thick regions of the patient.
• Foot switch: The operator holds down a foot switch to produce radiation. Releasing it opens the circuit and stops the beam immediately.
• Pulsed fluoroscopy: The generator switches the beam on in short pulses, for example several per second, instead of continuously. This lowers dose and reduces motion blur, and the pulse rate is selectable.
• Exposure timer: A cumulative timer sounds an alarm or interrupts the beam after a preset fluoroscopy time, which helps the operator to monitor total exposure.
• Interlocks: Electrical interlocks prevent exposure unless conditions such as tube position, door closure or valid settings are satisfied.

Electronic image signal

After the X-ray image has been converted to light, electronic circuits convert it to an electrical signal and finally to a picture. A light-sensitive camera or digital detector changes the brightness at each point into a small electrical charge, which is amplified into a video signal. The signal is processed by electronics that adjust brightness and contrast, may average successive frames to reduce noise, and can store images or sequences digitally. The processed signal is then sent to the display monitor, where the operator watches the moving image in real time. Weak signals are very sensitive to noise, so shielded cables, stable power supplies and good connections are essential to avoid flicker, lines or a grainy picture.