Introduction

As a radiologic technologist one of your primary responsibilities is to get X-rays images with satisfactory image quality at a radiation dose that is As Low as Reasonably Achievable (ALARA). With this in mind, you must have a good understanding of the basis concepts in radiation dose measurements and the units for measuring radiation dose. In the Radiation Biology post we described how there are potential health effects of radiation exposure including: carcinogenesis, hereditary  effects,  and acute radiation syndromes.

In the radiation biology post we describe that depending on the physical effect either the probability of harm and/or the severity of harm is strongly dependent on the radiation dose the body receives. Therefore, it is necessary to have a way to measure and report radiation dose levels. For this we need to have standard units for the radiation dose. In this section we will describe the different types of dose measurements and how they are related. We note that these measures are surrogates of the dose that the patient receives as they are not measured doses in individual patients. We will focus on the SI (International System) units in this section.

Exposure

Exposure can be thought of like the concentration of x-ray energy per unit area and it is measured in units of Roentgen or the SI units C/kg (Coulomb per kilogram of air). In the context of an x-ray system there are two major knobs we have to change the exposure. These are described in more detail in our post on X-ray Generation. The first method to increase the exposure is increasing the mA or increasing the quantity of x-rays generated. If the mA is increased there will be more x-rays passing through a region of fixed size (i.e. more x-rays per mm²) If we want to change beam quality, i.e. change the energy of the x-rays, we change the kVp (i.e. the tube potential). If we increase the kVp, there will be an increase in the average energy of photons. If the mA is left fixed while the kVp is increase there will be more photons and on average these photons will have higher energy. Under these conditions more energy will be deposited in the patient (i.e. the patient will receive a higher radiation dose).

Energy of x-rays can be measured by passing photons through an ion chamber which has an air filled region between two plates, one positively and one negatively charged. Thus, in an ion chamber there is a difference in electrical potential between the two plates. This potential will pull any charged particles that are generated within the air. If x-rays pass through the air chamber they can ionize the air within the chamber (i.e. knock out electrons from the air molecules). Since electrons are negatively charged they will be attracted to the

positive plate in the ion chamber. The higher the radiation dose the more electrons will be attracted to the positive plate. These electrons passing through the positive plate will generate an electrical signal (i.e. an increase in the electrical current in the circuit). The exposure is reported in units of Coulombs per kilogram of air. In this way it is fair to compare the measurements made on a small ion chamber to measurements made with a large ion chamber. The electrical charge is measured in Coulombs and mass of air in the chamber in kilograms. We can calculate the exposure after correct calibration of the device. Therefore, typically we just need to read from the ion chamber. The SI units are nice as they are consistent with other measurement units but in practice we don’t use a chamber that is nearly large enough to use a kg of dry air. In this table we provide the traditional unit that is named after Roentgen who discovered x-rays.

Air KERMA

The exposure is measured by measuring the charge that is deposited on plates from ions produced in air. A related quantity is the Air KERMA (Kinetic Energy Released per unit MAss).

The Air KERMA measures how much energy is deposited in the air due to the radiation, rather than how much charge is deposited in the ion chamber.

The SI units for energy are J and again it is normalized to how much air is in the chamber so the SI units for Air KERMA are J/kg.

Air KERMA can be computed from a calibrated ion chamber as well.

In the next section we will introduce the concept of absorbed dose (Gy=J/kg). The air KERMA is actually the absorbed dose just measured in air rather than a tissue like material.

Absorbed Dose

Absorbed dose is a measure of the energy deposited per unit mass of tissue. The SI units are Gray (Gy) which is 1 Joule of energy per kilogram (J/kg). Often, in radiology equipment, we’re looking at doses that are much lower than Gray, so we often talk about units of milliGray for instance of 1/1000 of a Gray.

The absorbed dose is different from the exposure in that it is a measurement in a tissue like material and we are interested in the energy absorbed within the material (whereas exposure measures the charge collected).

The traditional unit for measuring the absorbed dose is the rad. In this table we have the conversion between rads and Gy(mGy).

Depending on the type of radiology equipment different methods for estimating the absorbed dose may be used. It is not feasible to insert ionization chambers into the body during the exams so estimates of the absorbed dose have been developed.

In mammography the practice is to measure the entrance exposure or air kerma, as discussed above, and use that measurement to estimate the absorbed dose to the breast.

On the other hand, for CT the absorbed dose is measured in tissue like phantoms by inserting ion chambers into the phantom itself during the measurements.

Equivalent Dose

The damage caused by radiation to individuals depends of type of radiation that is incident on the body and the anatomy that is irradiated. In this section we will cover how the type of radiation is accounted for in dose measurement and the associated radiation dose units.

Equivalent dose is calculated by multiplying the absorbed radiation dose by a weighting factor specific to each type of radiation.

The need to have these radiation weighting factors is described in the description of LET and RBE. As different types of radiation have varying biological effects even if the radiation dose is the same.

The relative weighting that converts from Absorbed Dose to Effective Dose are given in this table.

Luckily for those of us who are primarily concerned with x-ray radiography and CT the conversion is very easy since the weighting factor is 1.0. So the Absorbed dose and the Equivalent Dose will have the same value but with different units.

When the dose has been converted to Equivalent dose it is measured in Sieverts (Sv) rather than in Gray(Gy).

Patients may be exposed to other types of radiation with different relative bioliogical impact, for example, alpha radiation, will have more sever effects given the same radiation dose. Thus, the need to track the Equivalent Dose in addition to the physical unit of the Absorbed Dose.

One final weighting which will be discussed in the next section is to account for which body parts have been irradiated in reporting the radiation dose.

Effective Dose

Not all organs are equally radiosensitive and a means is needed to account for this varied radio sensitivity across organ and tissue types. For instance hereditary effects [add link to header in radiation biology] are only possible in the gonads when germline cells receive radiation damage so a relatively high weight is given to the gonads.

Additionally, in the somatic (non-germline cells) there is varying radiosensitivity which is directly dependent upon how frequency the different tissue types are reproduced within the body.

For instance bone marrow cells are continuously being reproduced and thus will have a higher sensitivity to radiation. This is also why the a severe Acute Radiation Syndrome is linked to the bone marrow.

The ICRP has determined the effective weighting factors for each organ within the body as given in this table.

If we want to calculate the Effective Dose, we take our Equivalent Dose and then we multiply it by a weighting for each organ that is irradiated. So for each of the organs which is exposed, we have a weighting factor.

Multiplying the Equivalent Dose that each organ receives with weighting factor and adding up all of the contributions gives an Effective Dose.

The effective dose is an important quantity to understand and it is applicable to estimate potential risk to a large population. However, for a given individual, it is difficult to define the likelihood of harm.

Summary

In this section we provide a summary figure that relates the different units described above. Since these units are used frequently in the clinical environment it will be great if you have a solid understanding of these relationships.

For instance, in the figure in this section you should be able to cover any given box with your finger and be able to recite what is under your finger from memory.

We will summarize these important relationships here.

Exposure measurements are point-like while Absorbed Dose is an estimate of the energy absorbed in the patient, normalized by the mass of the patient.

The SI units of Absorbed Dose, are Gy (Gray) or mGy (milliGray).

To get Equivalent Dose from Absorbed Dose, we need a weighting factor, which is 1 for x-rays. Effective Dose can be calculated from Equivalent Dose taking into account weighting factors of different particles and different organs of the body.

Thus, in the end we have a measure in Sv (Sieverts) or mSv which takes into account the physical energy imparted (normalized by mass), the relative biological effect of the radiation type and the relative radiosensitivities of the different organs irradiated.