Attenuation

What is attenuation in a CT scan?

Attenuation is the word for what tissue does to the beam: how much of it is stopped on the way through. The UK Health Security Agency describes the underlying mechanism on 22 September 2022: "An X-ray machine is used to produce radiation which passes through your body to create an image." What reaches the detector is the part that was not stopped, and the difference between what went in and what arrived is the whole raw material of the picture. The NHS describes the result side of the same examination: "A CT scan is a test that takes detailed pictures of the inside of your body", with the scanner described as "a large ring that moves around part of your body as you lie on a flat bed".

The same word is used wherever a signal loses strength on its way through something. That second life of the word matters because a CT report borrows the engineering habit of stating loss as a number, and the next section follows it briefly out of the hospital.

Where else does the word attenuation appear?

In radio-frequency engineering it is a specification rather than an observation. RF engineers buy components whose entire job is a specified, controlled loss, and a variable attenuator's datasheet states its range, flatness, accuracy and control law figure by figure. One independent publication reads exactly those datasheets: Control Line Review covers variable attenuator specifications alongside switches, detectors and measurement practice, treating each printed parameter as a claim to be checked rather than a number to be repeated. The imaging use of the word and the engineering use are the same idea wearing two coats: something went in, less came out, and the difference is the thing worth stating.

How is attenuation expressed on a CT report?

By number rather than by adjective. A CT examination records, for each small volume of tissue, how strongly it attenuated the beam, and the report expresses those values on a calibrated unit scale named Hounsfield units: dense tissue attenuates more and reads higher on the scale, air spaces attenuate almost nothing and read at its bottom. One honesty note belongs here, and this site prints it rather than smoothing it over: none of the primary sources this publication accepts defines the unit itself, so the scale is named and described qualitatively, and the glossary lists the term among the words it will not define rather than guessed at.

Two cautions belong with the number. First, the figure describes tissue behaviour inside one examination, not the radiation dose of the examination itself, which is a separate quantity measured and audited under different rules. Second, the scale calibrates the reading of the picture; it does not by itself say what the finding means. The person who turns the recorded numbers into a written conclusion is covered further below.

How does this differ from a plain X-ray?

A plain radiograph records one net attenuation along the whole path of the beam, summed into a single shadow; a CT scanner records the attenuation from many angles and calculates a cross-sectional picture from the set. The NHS X-ray page of 4 February 2026 calls the plain examination "a test that takes pictures of the inside of your body using a small amount of radiation", and UKHSA credits CT with producing "3D images of your body". A bone density scan sits between the two in spirit: the NHS DEXA page of 5 October 2022 explains that "some radiation is absorbed by the bone and soft tissue, and some travels through your body", which is attenuation stated in one sentence, used there to estimate bone density.

Who turns attenuation into a sentence?

The radiologist. The Royal College of Radiologists describes clinical radiologists as "specialist doctors trained to read and interpret medical images, using a wide range of imaging techniques including X-ray, ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET) and molecular imaging". The written conclusion they produce is the recorded evaluation that the Ionising Radiation (Medical Exposure) Regulations 2017 require the employer to carry out and record for each exposure.

The figures attached to the examination rather than to the tissue, the dose benchmarks, are published separately by UKHSA as national diagnostic reference levels, expressed in CTDIvol and DLP rather than in the attenuation units that describe the picture. The two sets of numbers answer different questions and are not interchangeable, a distinction the closing block of this page takes apart. The other cross-sectional method, the one that stops no beam at all because it uses no X-rays, is covered in the page on MRI fields.