What Is Tomography?

One-sentence definition

Tomography is a family of imaging techniques that place sensors around the outside of an object, acquire boundary signals, and then use a reconstruction algorithm to recover the internal distribution inside. The internal properties of the object under test — density, conductivity, permittivity, temperature, etc. — are rendered as 2D cross-sectional slices or 3D volume data, and the process is usually non-invasive, non-contact and real-time.

Basic principle: from boundary signals to an internal image

Every tomography system follows a similar workflow:

• Excitation: a sensor array applies a known signal (voltage, current, electromagnetic wave, X-ray…) at the boundary of the object under test
• Response acquisition: the electrodes / probes not involved in excitation measure the boundary response (current, voltage, field, attenuation…)
• Inverse reconstruction: an inverse problem is solved (typically LBP, Landweber, Tikhonov regularisation, iterative algorithms, etc.) to infer the internal medium distribution from the boundary data
• Visualisation: the reconstructed result is presented to the operator as a colour cross-section, 3D volume rendering, time-series animation and so on

Main technology branches

The electrical tomography modalities common in industry and medicine are grouped by which electrical quantity is measured into four families:

ECT — Electrical Capacitance Tomography

An electrode array is mounted on the outer wall of the pipe or reactor under test and the capacitance between electrodes is measured. It suits multiphase scenarios with a large permittivity contrast, such as gas-solid two-phase, gas-oil two-phase, or low-water-cut gas-liquid flow. Typical applications: fluidised beds, pneumatic conveying, oil-gas pipelines.

ERT — Electrical Resistance Tomography

The electrode array contacts the fluid (or the surface of the object) directly and measures the resistance / conductance between electrodes. It suits scenarios with a large conductivity contrast, such as water-based two-phase flow, mineral slurries, and mixing processes with strong conductivity contrast. Typical applications: stirred tanks, slurry pipelines, chemical reactors.

EMT — Electromagnetic Tomography

A coil array excites an alternating magnetic field and the field perturbation is measured. It suits scenarios with a metallic or conductive phase, such as liquid-metal flow or multiphase monitoring with a metal phase present.

EIT — Electrical Impedance Tomography

Same physical basis as ERT, but usually refers to medical applications: electrodes are placed on the surface of the chest, impedance changes are measured, and lung ventilation and perfusion distributions are reconstructed. It is non-invasive and can run continuously at the bedside.

Why use tomography?

• Non-invasive: sensors attach to the outer wall or body surface — they neither damage the object nor disturb the existing process
• Real-time and dynamic: typical acquisition rates exceed 100 frames per second, allowing transient processes to be observed
• Process-level visualisation: not just a single-point measurement — it shows the distribution across the whole cross-section or volume
• Harsh-environment tolerance: no optical window is required, so it works in high-temperature, high-pressure, sealed metal pipes

Where it is not a good fit

Tomography is not a cure-all. Evaluate carefully if your application falls into any of these cases:

• The object is too small (below the millimetre scale) — electrode size and boundary sensing are constrained by physical layout
• Very high spatial resolution is required (CT-grade) — electrical tomography resolution is limited by electrode count
• The electrical contrast between phases is extremely small — reconstructed image contrast is insufficient
• Strong, un-shieldable electromagnetic interference on site — signal-to-noise ratio suffers

Next step

If you already know whether you need ECT or ERT, head straight to the product centre; if you are not yet sure how to choose, read ERT vs ECT; or simply contact us to discuss your specific scenario, and we will assess technical feasibility together.