Biological Effects of Radiation Part 1: SI units of measurement (NCRP #160)

Radiation has long been both a marvel and a menace in the medical world, powering life-saving diagnostic imaging while also carrying risks that must be meticulously managed. At the heart of understanding radiation’s biological effects lies the precision of measurement. That’s where SI (International System of Units) comes in—a globally adopted framework that enables radiologists, physicists, and health professionals to communicate and calculate radiation doses with consistency and clarity.

In this first segment, we’ll explore the SI units defined for radiation measurements, shedding light on what they mean, how they differ, and why each one matters in clinical practice and radiation protection.

1. Exposure (Coulomb per Kilogram - C/kg)

Before radiation penetrates the human body, it first travels through air. The earliest and most foundational measurement of this interaction is exposure. Exposure refers to the amount of ionization that occurs in a certain volume of air due to radiation. Specifically, it quantifies the amount of electrical charge liberated by ionizing photons in air, per unit mass.

The SI unit for exposure is the coulomb per kilogram (C/kg). This replaces the older unit, the roentgen (R). One roentgen is equal to 2.58 × 10⁻⁴ C/kg, a conversion that reflects a shift toward more accurate, reproducible measurements.

Even though we do not image the air, the ionization effects in air are directly analogous to those in human tissue. Air has an effective atomic number close to that of soft tissue (approximately 7.8 for air vs. 7.6 for muscle), making air ionization a reliable proxy for tissue response in radiologic measurements.

2. Air Kerma (Gray - Gy)

A more precise modern unit that builds upon exposure is air kerma, short for Kinetic Energy Released per unit MAss. Air kerma measures the kinetic energy transferred from ionizing radiation (typically X-ray photons) to electrons in air. These liberated electrons then impart their energy to the surrounding air molecules.

The unit of air kerma is the gray (Gy), which in this context refers to joules per kilogram (J/kg). To avoid confusion with other applications of the gray, it’s often denoted as Gyₐ (with a subscript ‘a’ for air). Air kerma represents the energy actually deposited in a small volume of air and is considered more direct and accurate than the traditional exposure metric.

Measurement is typically performed using ionization chambers, and while the method is similar to exposure measurement, air kerma involves calibration that reflects energy transfer rather than just ion pair creation.

3. Absorbed Dose (Gray - Gy)

As radiation leaves the air and enters the body, it begins to deposit energy into tissues. This brings us to the absorbed dose, which measures the amount of energy deposited by ionizing radiation in a unit mass of biological tissue.

Again, the SI unit used is the gray (Gy)—1 gray equals 1 joule of energy absorbed per kilogram of tissue. This measure is particularly significant because it is directly associated with the physical damage radiation may cause at the cellular level.

In medical imaging and therapy, absorbed doses are typically in milligray (mGy) for diagnostics and up to several gray for treatments like radiation therapy. Historical units such as the rad are still encountered, where 1 Gy = 100 rad.

The absorbed dose is foundational, not only for calculating more nuanced biological effects, but also for directly assessing patient safety and equipment calibration in radiology and nuclear medicine.

4. Dose Equivalent (Sievert - Sv)

While the absorbed dose tells us how much energy is deposited into tissue, it doesn’t account for what kind of radiation is doing the damage. That’s where the dose equivalent comes into play.

The dose equivalent adjusts the absorbed dose based on the type and biological impact of the radiation involved. Different forms of ionizing radiation (like alpha particles, beta particles, X-rays, neutrons) vary in how much biological harm they cause—even if the absorbed dose is the same.

To capture this difference, we apply a radiation weighting factor (also called a quality factor or QF) to the absorbed dose. The formula is:

Dose Equivalent (Sv) = Absorbed Dose (Gy) × Quality Factor

In this context, the sievert (Sv) is the SI unit, replacing the older rem, where 1 Sv = 100 rem.

Here's how quality factors vary for different types of radiation:

• X-rays, gamma rays, and beta particles: QF = 1

• Neutrons: QF = 10

• Alpha particles: QF = 20

Example:

If a worker receives:

• 2 rads from X-rays (QF = 1)

• 1 rad from fast neutrons (QF = 10)

• 1 rad from alpha particles (QF = 20)

The dose equivalents would be:

• X-rays: 2 × 1 = 2 rem

• Neutrons: 1 × 10 = 10 rem

• Alpha particles: 1 × 20 = 20 rem
  Total Dose Equivalent = 32 rem = 0.32 Sv

This calculation highlights how important the type of radiation is in determining the actual biological risk—not just the amount absorbed.

5. Effective Dose (Sievert - Sv)

The concept of effective dose takes the biological evaluation even further by incorporating the sensitivity of specific organs and tissues to radiation. Not all tissues respond to radiation equally: some are more susceptible to damage (like bone marrow, breast, and lungs), while others are more resistant (like skin or bone surfaces).

The effective dose is a weighted sum that reflects:

• The absorbed dose in each organ or tissue,

• The radiation type (via the quality factor),

• And the tissue weighting factor, which reflects the relative sensitivity of each tissue to radiation-induced stochastic effects (like cancer).

Thus, the effective dose is calculated as:

Effective Dose (Sv) = Σ (Absorbed Dose × Radiation Weighting Factor × Tissue Weighting Factor)

This composite measurement provides a single risk estimate for non-uniform exposures, such as when a CT scan irradiates the abdomen but spares the chest and limbs. It allows practitioners and regulators to compare risks across different procedures and exposure types using one standardized value.

Because of its comprehensiveness, effective dose is the gold standard for radiation protection, especially when assessing risk for occupational exposure or population studies.

For example:

• A dental X-ray may deliver an effective dose of 0.005 mSv

• A chest CT might yield 7 mSv

• Background radiation from natural sources averages about 3 mSv per year in the U.S.

Each of these values allows direct comparison of the overall biological risk, even though the types and amounts of exposure differ dramatically.

Connecting the Doses to Real-World Practice

With a clear understanding of the SI units—coulomb per kilogram (C/kg), gray (Gy), and sievert (Sv)—we can now look at how these measurements play out in clinical settings and why they’re vital for both radiation protection and patient care.

Diagnostic Imaging: Tracking Exposure and Dose

In diagnostic radiology, exposure (C/kg) and air kerma (Gy) provide immediate feedback about the amount of radiation leaving the X-ray tube and entering the patient’s body. Tools like ionization chambers are placed in air or on phantom models to measure this radiation in real-time, which is then converted into air kerma values.

These measurements help radiologic technologists:

• Confirm that machines are functioning correctly

• Optimize technical factors like kVp and mAs

• Reduce unnecessary repeat exposures

• Stay within established dose limits for patients

It’s worth noting that while these values are measured in air, they form the baseline for estimating absorbed doses in the body.

Absorbed Dose and Skin Dose

Once radiation enters the body, the focus shifts to absorbed dose (Gy). In medical contexts, specific absorbed doses may be reported for the skin, organs, or even small volumes of tissue. For example, mammography protocols often use entrance skin dose to assess the radiation a patient’s breast tissue receives during an exam.

Absorbed dose is essential for evaluating:

• Radiation therapy plans

• Risks of deterministic effects like skin burns

• Cumulative doses in serial imaging (e.g., multiple CT scans)

Effective Dose: Communicating Risk

When it comes to conveying overall radiation risk—especially to patients or non-specialists—effective dose (Sv) is the preferred metric. It simplifies a complex interaction of radiation, tissue types, and biological effects into one number that correlates with cancer risk and genetic damage.

For instance, a technologist might explain:

• A dental X-ray: ~0.005 mSv

• A chest X-ray: ~0.1 mSv

• A full-body CT scan: ~10 mSv

This helps patients understand not only how much radiation they’re receiving but also how it compares to everyday exposures like natural background radiation or airplane travel.

Radiation Protection and Occupational Safety

In occupational settings, effective dose and dose equivalent (both in sieverts) are critical for ensuring the safety of radiologic workers. Dosimeters worn by staff record cumulative radiation exposure, and safety protocols aim to keep this well below annual limits established by regulatory bodies (e.g., 50 mSv/year for radiation workers in the U.S.).

Because different forms of radiation are encountered—especially in interventional radiology, nuclear medicine, or radiotherapy—dose equivalent is especially important. It ensures that biologically more harmful types of radiation (like alpha or neutron particles) are not underestimated.

The Path Forward: Why Precision Matters

As medical technology advances and imaging becomes more integrated into routine care, the need for precise, universally understood measurements only grows. The SI units defined by the NCRP and global scientific consensus allow professionals to:

• Compare doses across procedures and equipment

• Track and limit exposure to safeguard long-term health

• Develop policies and best practices rooted in evidence

In summary, understanding radiation’s biological effects starts with understanding how we measure it. Whether it’s the ion pairs in a cubic centimeter of air or the composite risk from a CT scan, every unit—C/kg, Gy, Sv—tells part of the story. Together, they form the language of radiologic science: clear, accurate, and indispensable.

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