"How much radiation does this CT give me?" — one of the most common questions from patients and clinicians. The common language of the answer is effective dose, in millisieverts (mSv). But effective dose is subtler than it first looks: it is a protection quantity designed to compare different exams, not to "calculate" the exact risk to one patient. Below we give both the numbers and the correct way to read them, grounded in primary sources.
Gy, Sv and effective dose
Three quantities must be separated (see our Radiation Units article):
- Absorbed dose — gray (Gy): energy deposited per unit mass in a tissue (J/kg). A physical, measurable quantity.
- Equivalent dose — sievert (Sv): absorbed dose scaled by the biological effectiveness of the radiation type (radiation weighting factor wR). In diagnostic x-ray the radiation is photons/electrons, so wR = 1; the organ equivalent dose (Sv) is numerically equal to the absorbed dose (Gy).3
- Effective dose — sievert (Sv/mSv): each organ's equivalent dose is multiplied by a tissue weighting factor (wT) reflecting that organ's sensitivity to stochastic (cancer/heritable) harm, and summed over the whole body. A partial exposure is thus reduced to a single "whole-body-equivalent" number.3
Example ICRP 103 tissue weighting factors: 0.12 for red bone marrow, colon, lung, stomach, breast and the remainder tissues; 0.08 for gonads; 0.04 for bladder, liver, oesophagus, thyroid; 0.01 for bone surface, brain, skin. They sum to 1.3 This is why the same absorbed dose delivered to the chest (breast, lung) yields a higher effective dose than to the hand.
What effective dose really is (and isn't)
Effective dose is a quantity built to compare different exams and body regions on a common stochastic-risk scale. It is ideal for setting limits, comparing protocols and monitoring dose at the population level.
But there is a crucial boundary here: effective dose is not designed to predict the cancer risk of a single individual. The ICRP explicitly states it should not be used for epidemiological studies or for detailed retrospective assessment of individual exposure and risk; those require organ/tissue doses and risk data appropriate to that person.3 So "your CT was 10 mSv, therefore your cancer risk rose by X" is a burden effective dose cannot bear. Effective dose is a comparison and awareness tool.
Typical doses per exam
The values below are typical for an adult; the real dose can easily vary by a factor of 2–3 with equipment, protocol, patient size and country. The sources are the widely cited Mettler catalog and the RadiologyInfo (RSNA/ACR) tables.14
| Exam | Typical effective dose | ~Equivalent background |
|---|---|---|
| Dental (bitewing) | 0.005 mSv | ~half a day |
| Chest x-ray (PA) | 0.02–0.1 mSv | ~a few days |
| Mammography (2 breasts, 2 views each) | 0.4 mSv | ~7 weeks |
| Abdominal x-ray | 0.7 mSv | ~3 months |
| Lumbar spine x-ray | 1.5 mSv | ~6 months |
| Head CT | 2 mSv | ~8 months |
| Chest CT | 7 mSv | ~2–3 years |
| Abdomen–pelvis CT | 10 mSv | ~3 years |
| Coronary CT angiography | ~12 mSv | ~4 years |
The key message of the table: radiographs are very low and CT is markedly higher in effective dose. An abdomen–pelvis CT can correspond to about 100 chest x-rays. This does not mean "CT is bad" — a well-indicated CT saves lives. But it makes concrete why avoiding unnecessary repeat CTs matters.
Compared to background
The best way to put the numbers in context is the natural background radiation we are all already exposed to. The world average is about 2.4 mSv/year (UNSCEAR), rising to ~3.1 mSv/year in some countries such as the US due to radon (NCRP 160).5 That is roughly 0.007–0.008 mSv per day.
From this come the "equivalent time" comparisons (on a ~3 mSv/year basis): a chest x-ray ≈ a few days of background; a mammogram ≈ ~7 weeks; a head CT ≈ ~8 months; a chest CT ≈ ~2.5 years.4 Another anchor: a single coast-to-coast flight adds roughly 0.03–0.04 mSv from cosmic rays at altitude.5
What about risk?
At low doses, radiation protection cautiously estimates risk with the linear no-threshold (LNT) model. ICRP 103 gives a "detriment-adjusted nominal risk coefficient" of about 5% per sievert for the whole population (cancer + heritable effects).3 But turning that number into an individual prophecy is wrong, because:
- The coefficient is a population average; it varies greatly with age, sex and tissue (children are more sensitive; in the elderly there is less remaining lifetime for a cancer to appear).
- Effective dose, as noted, is not designed for individual risk.3
- Most importantly: the diagnostic benefit of a well-indicated exam far outweighs this small probabilistic risk. For example, the population-average added lifetime cancer risk from a 10 mSv abdomen CT is very small next to a person's baseline (radiation-independent) lifetime cancer risk.
The right frame is justification and optimization. Is the exam truly needed (justification)? If so, is it done at the lowest dose adequate for diagnosis (optimization)? These two questions matter more than "how many mSv."
Common misunderstandings
- "MRI also gives radiation." No. MRI and ultrasound use no ionizing radiation; this table is only for x-ray-based exams (radiography, CT, mammography, fluoroscopy) and nuclear medicine.
- "I can read effective dose as my cancer risk." No; effective dose is a comparison quantity, not an individual risk calculator.3
- "Every exam is the same." No; a dental x-ray and an abdomen CT can differ by thousands of times.
- "I had one scan, now it builds up and harms me." Stochastic risk is dose-cumulative but the levels are very low; the real issue is avoiding unnecessary repeats, not fearing a single needed exam.
References
- Mettler FA Jr, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology 248(1):254–263, 2008. Tetkik başına tipik efektif dozların en çok atıf alan derlemesi. rsna
- Mettler FA Jr, Mahesh M, Bhargavan-Chatfield M, et al. Patient Exposure from Radiologic and Nuclear Medicine Procedures in the United States and Worldwide: Dose Assessment. Radiology 295(2):418–427, 2020. 2008 kataloğunun güncellenmiş sürümü.
- ICRP Publication 103. The 2007 Recommendations of the International Commission on Radiological Protection. Ann. ICRP 37(2–4), 2007. Efektif dozun tanımı, doku ağırlık faktörleri (wT), radyasyon ağırlık faktörleri (wR) ve efektif dozun bireysel epidemiyolojik risk için kullanılmaması gerektiği uyarısı.
- RadiologyInfo.org (RSNA/ACR). Radiation Dose in X-Ray and CT Exams. Yaygın tetkiklerin tipik efektif dozları ve doğal fona 'eşdeğer süre' karşılaştırmaları. radiologyinfo.org
- UNSCEAR 2008 Report, Vol. I, Annex B. Exposures of the public and workers from various sources of radiation. Dünya ortalaması doğal fon ~2,4 mSv/yıl. Ayrıca NCRP Report No. 160 (2009), ABD nüfusu için ortalama ~3,1 mSv/yıl doğal fon.
- İlişkili DoseSave yazıları: Radyasyon Birimleri: Gy, Sv, kerma · Radyasyon ve Sağlık (stokastik/deterministik, LNT) · Fon (Doğal) Radyasyon · BT'de Doz: CTDIvol, DLP, SSDE