Demystifying radiation units: mGy vs. mSv
To grasp the significance of 1 mGy, it is crucial to understand the different units used to measure radiation. Ionizing radiation is a form of energy, and its measurement involves several key concepts, particularly absorbed dose versus effective dose.
Absorbed dose (Gray and mGy)
The milligray (mGy) is a unit of absorbed dose, which measures the amount of energy deposited by ionizing radiation per unit mass of tissue. The base unit is the gray (Gy), and 1 mGy is one-thousandth of a Gy. In simpler terms, it quantifies the physical energy absorbed by a person's body or a specific organ. For example, a computed tomography (CT) scan delivers a certain absorbed dose to the organs within the scanned area, which can be measured in mGy.
Effective dose (Sievert and mSv)
However, simply knowing the absorbed dose is not enough to assess biological risk. This is because different types of radiation (e.g., X-rays, alpha particles) have varying biological effects for the same amount of absorbed energy. The Sievert (Sv) and its subunit, the millisievert (mSv), are used to measure the effective dose, which accounts for these differences. It incorporates a weighting factor that reflects the biological harmfulness of the radiation type.
For most medical imaging procedures, such as X-rays and CT scans, which primarily use X-rays, the weighting factor is 1. This means that for these specific types of radiation, the absorbed dose in mGy is numerically equal to the effective dose in mSv (i.e., 1 mGy $\approx$ 1 mSv).
How 1 mGy compares to everyday radiation exposure
Putting 1 mGy into perspective is the best way to understand its magnitude. We are constantly exposed to radiation from natural sources, known as background radiation, which comes from cosmic rays, radon in the air, and naturally occurring radioactive materials in the soil.
- Annual background radiation: On average, natural background radiation exposes a person's thyroid to approximately 1 mGy per year. However, regional variations exist. For the entire body, the average annual background effective dose in the US is around 3.1 mSv.
- Transcontinental flight: A single round-trip transcontinental flight exposes the thyroid to a dose of about 0.05 mGy due to increased cosmic radiation at high altitudes.
- Chest X-ray: A standard chest X-ray delivers a very low dose, approximately 0.07 mGy to the thyroid, far less than the typical annual background exposure.
Comparison of 1 mGy to common medical imaging doses
Medical imaging procedures, especially CT scans, use significantly higher doses than a single chest X-ray but are still within safe limits for diagnostic purposes. The following table provides a comparison based on absorbed doses to specific organs.
| Radiation Source | Absorbed Dose (mGy) | Notes |
|---|---|---|
| 1 mGy (baseline) | 1 mGy | Equivalent to the average annual background dose to the thyroid. |
| Chest X-ray | $\approx$ 0.07 mGy | Dose to the thyroid, much lower than 1 mGy. |
| Chest CT scan | $\approx$ 10 mGy | Dose to the thyroid, significantly higher than 1 mGy. |
| Brain CT scan | $\approx$ 60 mGy | Dose to the eye lens, a much higher localized dose. |
| Multi-image CT scan | Up to 1000+ mGy | Can occur with extensive or specialized CT procedures. |
Health risks associated with 1 mGy
Radiation risks are classified into two main types: deterministic and stochastic. Deterministic effects have a threshold dose, above which the severity of the effect increases with dose (e.g., radiation burns). These occur at very high doses, often in the hundreds or thousands of mSv. At 1 mGy, there is no risk of deterministic effects.
Stochastic effects are based on probability and have no known threshold. This includes the risk of developing cancer. The probability of a stochastic effect increases with the dose, but there is no certainty that any effect will occur. While 1 mGy is a small dose, even small doses contribute to a person's cumulative lifetime exposure, which is linked to a very small, incremental risk of cancer.
Recently, studies have brought attention to the cumulative effects of medical imaging, particularly in pediatric patients. A recent study found an increased risk of hematologic cancers in children with cumulative doses, noting that compared to no exposure, the relative risk was 1.41 for doses between 1 mGy and less than 5 mGy. This does not mean a single exposure of 1 mGy is dangerous, but it reinforces the need for careful justification of medical imaging, especially in children, to minimize cumulative dose. The increase in cancer risk from a single 1 mGy exposure is tiny when compared to the natural incidence of cancer in the general population.
Minimizing radiation exposure: The ALARA principle
Medical imaging, though a source of radiation, is often life-saving and medically necessary. The guiding principle for radiation safety is ALARA—As Low As Reasonably Achievable. This means medical professionals are obligated to minimize a patient's radiation exposure while still obtaining necessary diagnostic information.
This is achieved through several methods:
- Justification: Ensuring the medical procedure is necessary and that no alternative, non-ionizing imaging technique (like MRI or ultrasound) is suitable.
- Optimization: Using the lowest possible dose to get a high-quality diagnostic image. For example, modern CT scanners have dose-reduction features.
- Time, distance, and shielding: These are standard radiation protection measures. While more relevant for medical staff or radiation workers, they help contain and manage radiation in medical settings.
Conclusion
How much radiation is 1 mGy is a question best answered with context. It represents a very small absorbed dose of radiation, roughly equivalent to the annual natural background exposure to the thyroid. While doses from modern medical imaging are generally safe, they are higher than 1 mGy, and cumulative dose is a factor to consider, particularly for children. Ultimately, medical imaging is a powerful and necessary tool, and its benefits often far outweigh the minimal risks associated with the low doses used. Patients should discuss any concerns with their healthcare provider to ensure that any exposure is both justified and optimized for their care.
Further reading
- FDA: What are the Radiation Risks from CT? https://www.fda.gov/radiation-emitting-products/medical-x-ray-imaging/what-are-radiation-risks-ct
Minimizing radiation exposure: A proactive approach
While radiation from necessary medical procedures is managed by professionals, patients can also be proactive in minimizing their total exposure:
- Keep a record: Keep a list of all your radiation-based medical scans (CT scans, X-rays) so doctors have a complete picture of your exposure history.
- Talk to your doctor: Always discuss the necessity of any recommended imaging procedure with your doctor. In some cases, a non-radiation alternative may be suitable.
- Ask about dose: It is your right to ask medical staff about the radiation dose for procedures like CT scans.
Understanding the units
- mGy: MilliGray, a measure of absorbed dose (energy deposited per kg of tissue).
- mSv: MilliSievert, a measure of effective dose (accounts for biological effects).
- Equivalent: For X-rays, 1 mGy $\approx$ 1 mSv.
By being informed and communicating with your healthcare team, you can manage your radiation exposure responsibly while still receiving the best medical care possible.
