What Does ALARA Mean?

ALARA stands for As Low As Reasonably Achievable. It is the guiding principle of radiation protection in medical imaging worldwide, enshrined in the guidelines of organisations including the International Commission on Radiological Protection (ICRP) and adopted by regulatory bodies in virtually every country.

In the context of CT scanning, ALARA means that every scan should use the minimum radiation dose necessary to produce images of diagnostic quality — no more. The principle recognises that ionising radiation carries a small but real theoretical risk, and that this risk should never be accepted without a genuine clinical benefit that outweighs it.

Why CT Radiation Deserves Attention

A single chest CT delivers roughly 5–7 millisieverts (mSv) of effective radiation dose — equivalent to approximately two years of natural background radiation exposure. While this is not alarming in the context of a necessary diagnosis, the cumulative effect of multiple scans over a lifetime, particularly in younger patients or those with chronic conditions requiring regular imaging, deserves careful management.

It is important to note that the risk from any single, clinically justified CT scan is very small and almost always dwarfed by the risk of missing the condition being investigated. ALARA is not a reason to avoid necessary imaging — it is a reason to perform it thoughtfully.

Dose Optimisation Strategies Used in Modern CT

1. Automatic Exposure Control (AEC)

Modern CT scanners continuously modulate the X-ray tube current (mA) based on the patient's body size and the density of the anatomy being scanned at each moment. Thicker regions receive more photons; thinner regions receive fewer. This prevents the "one setting fits all" over-irradiation of smaller patients.

2. Low-kVp Protocols

Reducing the peak X-ray tube voltage (kVp) from the standard 120 kVp to 80–100 kVp in appropriately sized patients can reduce dose significantly while actually improving iodine contrast visibility — a genuine win-win for contrast-enhanced studies.

3. Iterative Reconstruction Algorithms

Traditional CT image reconstruction (filtered back-projection) requires a certain noise level to produce readable images. Advanced iterative reconstruction software intelligently reduces image noise mathematically, allowing diagnostic quality images to be obtained from lower-dose raw data.

4. Bismuth Shielding and Organ-Based Tube Current Modulation

Radiosensitive organs such as the lens of the eye, the thyroid, and breast tissue can be partially shielded with bismuth-impregnated shields or protected using organ-based tube current modulation, which reduces the dose at the start of rotation when the beam passes over sensitive surface anatomy.

5. Protocol Optimisation and Scan Length

Every CT protocol should be tailored to answer the specific clinical question. Scanning an unnecessarily large anatomical region or performing extra phases without clear clinical indication violates the ALARA principle. Regular audit of protocols ensures they remain fit for purpose.

Special Populations: Extra Care Required

  • Children: Children are more radiosensitive than adults and have more years ahead of them for any radiation effect to manifest. Paediatric-specific protocols (often called "size-appropriate" or "paeds-dose" protocols) are essential.
  • Pregnant patients: The developing foetus should be protected wherever possible. CT of the abdomen/pelvis in pregnancy is used only when the clinical need is urgent and alternative imaging (ultrasound, MRI) cannot answer the question.
  • Patients requiring repeated imaging: Those with chronic conditions (e.g., Crohn's disease, cancer follow-up) may accumulate significant lifetime dose. Low-dose surveillance protocols and alternative modalities should be considered whenever clinically appropriate.

What You Can Do as a Patient

  1. Keep a record of your imaging history and share it with every new clinician involved in your care.
  2. Ask questions: "Is a CT scan the best option here, or would an ultrasound or MRI be equally useful?"
  3. Avoid requesting scans that have not been recommended by a qualified clinician — self-referral for screening CT without medical indication is rarely beneficial and not risk-free.

Summary

ALARA is not merely a technical guideline — it is a culture of responsibility shared by radiographers, radiologists, clinicians, and equipment manufacturers. Through dose modulation technology, advanced reconstruction algorithms, protocol optimisation, and informed clinical decision-making, modern CT departments work continuously to ensure every patient receives the diagnostic benefit of the scan with the least possible radiation burden.