1. Radiation quantities and units
1.1. Types of radiation quantities
1.1.1. Total radiation delivered
1.1.1.1. Total Photons
1.1.1.1.1. Count of photons / Counts per minute (CPM) converted into units: Curies or becquerels
1.1.1.2. Total Energy
1.1.1.2.1. Integral dose
1.1.1.3. Exposure
1.1.1.3.1. Surface integral exposure
1.1.1.4. Dose area product
1.1.1.4.1. similar in concept to surface integral exposure and exposure area product in that they all express total radiation delivered to a patient. The principle difference is in the units used.
1.1.1.4.2. in dose units, such as Gy-cm2
1.1.1.4.3. For a uniformly exposed area, the DAP is just the product of the air kerma ,in Gy or mGy, and the exposed area in cm2.
1.1.1.4.4. DAP provides a good estimation of the total radiation ENERGY delivered to a patient during a procedure.
1.1.1.4.5. Both radiographic and fluoroscopic machines can be equipped with devices (DAP meters) or computer programs that measure or calculate the DAP for each procedure. It is the most practical quantity for monitoring the radiation delivered to patients.
1.1.1.5. Dose Length Product
1.1.1.6. Effective dose
1.1.2. Concentration of radiation While each of these quantities have useful applications, they are very limited in that they do not give information on the total radiation delivered to a body.
1.1.2.1. Photon concentration (Fluence)
1.1.2.1.1. Concentration of photons absorbed in the image forming process
1.1.2.2. Energy concentration (Fluence)
1.1.2.2.1. Absorbed Dose - Concentration absorbed in tissue
1.1.2.3. Exposure
1.1.2.3.1. Expresses only the concentration at some specified point. Knowing the exposure tells us nothing about the total radiation imparted to a body.
1.1.2.4. Air Kerma (Kinetic Energy Released per unit MAss [of air])
1.1.2.4.1. Air kerma is just the Absorbed Dose in air
1.1.2.5. Absorbed Dose
1.1.2.5.1. The radiation quantity used to express the concentration of radiation energy actually absorbed in a specific tissue.
1.1.2.5.2. This is the quantity that is most directly related to biological effects.
1.1.2.6. Mean Glandular Dose (Mammography)
1.1.2.7. Equivalent Dose
2. Measures of biological effect
2.1. Because various types of radiation might not produce the same biological impact, even when the dose or energy delivered to the tissue is the same. In other words, just knowing the (physical) dose does not tell the whole story.
2.1.1. Dose equivalent
2.1.1.1. The relationship is: Dose Equivalent (Sv) = Dose (Gy) x wR Unit, sievert (Sv)
2.1.1.1.1. The value of the radiation weighting factor (wR) is a characteristic of each specific type of radiation. What makes it easy is that the radiations we use for medical imaging (x-ray, gamma, beta, positron) all have radiation weighting factor (wR) values of one (1). Therefore, for our types of radiations: Dose Equivalent (Sv) = Dose (Gy)
2.1.1.2. a quantity that expresses the relative biological impact of the radiation by including a radiation weighting factor (wR).
2.1.1.3. The relationship is: Dose Equivalent (Sv) = Dose (Gy) x wR
2.1.2. Effective Dose
2.1.2.1. Effective dose is becoming a very useful radiation quantity for expressing relative risk to humans, both patients and other personnel. It is actually a simple and very logical concept. It takes into account the specific organs and areas of the body that are exposed. The point is that all parts of the body and organs are not equally sensitive to the possible adverse effects of radiation, such as cancer induction and mutations.
2.1.2.2. Onenote image
2.1.2.3. For the purpose of determining effective dose, the different areas and organs have been assigned tissue weighting factor (wT) values. For a specific organ or body area the effective dose is: Effective Dose (Gy) = Absorbed Dose (Gy) x wT
2.1.2.4. If more than one area has been exposed, then the total body effective dose is just the sum of the effective doses for each exposed area. It is a simple as that. Now let's see why effective dose is such a useful quantity. There is often a need to compare the amount of radiation received by patients for different types of x-ray procedures, for example, a chest radiograph and a CT scan. The effective dose is the most appropriate quantity for doing this. Also, by using effective dose it is possible to put the radiation received from diagnostic procedures into perspective with other exposures, especially natural background radiation. It is generally assumed that the exposure to natural background radiation is somewhat uniformly distributed over the body. Since the tissue weighting factor for the total body has the value of one (1), the effective dose is equal to the absorbed dose. This is assumed to be 300 mrad in the illustration. Let's look at an illustration. If the the dose to the breast ,MGD, is 300 mrad for two views, the effective dose is 45 mrad because the tissue weighting factor for the breast is 0.15. What this means is that the radiation received from one mammography procedure is less than the typical background exposure for a period of two months. Tissue Weighting Factors Tissue Weighting Factor Gonads 0.25 Breast 0.15 Red Bone Marrow 0.12 Lung 0.12 Thyroid 0.03 Bone Surface 0.03 Remainder 0.3 (For the remaining organs a value of 0.06 is used for each of the five organs receiving the highest dose.) Total Body 1.0