Beyond the patient · Scans explained
When the patient is an animal
The physics of a scan does not change with the species on the table. The paperwork does, because the regulations this site covers are written for human patients.
Do the same machines scan animals?
Yes, and the physics does not change with the species. X-rays, computed tomography, ultrasound and magnetic resonance all produce pictures by the same measurements whether the subject is a person or a dog. What changes is the paperwork: the regulations this site covers, the Ionising Radiation (Medical Exposure) Regulations 2017, govern medical exposures of people in a healthcare setting, as the Care Quality Commission's summary of the framework states. Veterinary imaging sits outside that patient-facing framework, run under the radiation protection rules that govern workplaces generally rather than the duties written for human patients.
The everyday side of that boundary is easy to picture. Most households meet imaging through their own GP or hospital; the households that also keep animals meet it a second time through the vet. A guide about the ordinary life of those households, The Companion Edit, covers living with dogs and cats from room checks and feeding routines to the questions to prepare before a veterinary visit. The appointment on the veterinary side is a different legal object even when the scanner looks identical.
Which rules apply to which exposure?
Two statutory instruments split the territory. IR(ME)R 2017, SI 2017 No. 1322, defines its four duty holders in regulation 2: the referrer, "a registered health care professional" entitled to refer individuals for exposure; the practitioner, who takes responsibility for an individual exposure; the operator, who carries out practical aspects; and the employer, in whose radiological installation the exposures happen. The regulation's world is the human patient, and its Schedule 2 duties, from identity checks to pregnancy enquiries, are written around that world.
The second text is the Ionising Radiations Regulations 2017, SI 2017 No. 1075, which protects workers and the public wherever radiation is used at work. Regulation 9 requires the employer to "take all necessary steps to restrict so far as is reasonably practicable" the exposure of employees and others, "by means of engineering controls, design features and by the provision and use of safety features and warning devices". Schedule 3 of the same instrument sets the annual limits: 20 mSv for employees aged 18 and over, 6 mSv for trainees under 18, and 1 mSv for any other person. A veterinary practice is a workplace like any other for those purposes.
What does the UKHSA framework say about the methods?
The UK Health Security Agency divides imaging into the methods that use ionising radiation and those that do not. X-ray imaging in its forms, general radiography, fluoroscopy, interventional radiology and CT, plus nuclear medicine, sits in the ionising family; ultrasound and MRI are listed as "types of imaging that do not use X-rays or radioactive substances". The division is physical, not clinical, which is why it holds regardless of who or what is in front of the detector.
The dose figures in the UKHSA comparison table of 18 March 2011 are human figures: a chest X-ray at 0.014 mSv, a CT scan of the head at 1.4 mSv, and the UK average annual background dose of 2.7 mSv. They describe the human side of the framework because the entire regulatory edifice described on this site, from justification to dose limits, is built for people. The machine does not know the difference; the law does.
| Text | What it covers | Key instrument |
|---|---|---|
| IR(ME)R 2017 | Medical exposures of people: justification, optimisation, the four duty holders, employer procedures | SI 2017 No. 1322 |
| IRR 2017 | Workers and the public wherever radiation is used: dose limits, controlled areas, engineering controls | SI 2017 No. 1075 |
| UKHSA guidance | Public information on methods and dose comparisons for human imaging | Publications of 18 March 2011 and 22 September 2022 |
| Veterinary imaging | Outside the patient duties of IR(ME)R; the workplace rules of IRR 2017 apply to the people involved | SI 2017 No. 1075 |
Why the same picture is a different event
Because a medical exposure is defined by its patient. Under IR(ME)R the referrer must supply "sufficient medical data" for the practitioner to decide justification under regulation 11, and the employer must record an evaluation of each exposure under Schedule 2. None of that machinery can engage without a human patient at the centre of it. A dog on the same table produces the same grey values and none of the same statutory file. The scanner is indifferent; the law is specific.
Checked against the source
- IR(ME)R 2017 protects "people against the dangers from being exposed to ionising radiation in a healthcare setting" (Care Quality Commission, IR(ME)R annual report 2020/21, updated 10 August 2022).
- The four duty holders, referrer, practitioner, operator and employer, are defined in regulation 2 (legislation.gov.uk, SI 2017 No. 1322).
- IRR 2017 regulation 9 requires exposure of employees and the public to be restricted "so far as is reasonably practicable" (legislation.gov.uk, SI 2017 No. 1075).
- Ultrasound and MRI are "types of imaging that do not use X-rays or radioactive substances" (UKHSA, 22 September 2022).
- The UKHSA dose table gives 0.014 mSv for a chest X-ray, 1.4 mSv for a head CT and 2.7 mSv as the UK annual average (18 March 2011).
Same physics, different paperwork
The examination and the legal event are not the same thing. A CT of a person and a CT of a dog are the same physical measurement, but only the first triggers the referral, justification and recorded evaluation duties of IR(ME)R 2017. The second happens under the workplace regime of IRR 2017, which governs the staff in the room rather than the patient on the table.
Ultrasound adds a second confusion: because it sits outside the ionising family in the UKHSA split, neither regulation's dose machinery applies to it for anyone. The physics decides which framework could ever apply; the species on the table decides which one actually does.