Q1. What is effective dose?
A1. Effective dose means the radiation dose that is evaluated damage to whole body, taking into account the risk of hereditary disease and the incidence rate from radiation-induced cancer for each organ exposed.

Q2. What is ICRP?
A2. ICRP is an abbreviation of the International Commission on Radiological Protection.
The ICRP is one of the advisory bodies of World Health Organization (WHO) and it issues recommendations (*1) on radiation protection.
These ICRP recommendations are the most important reference regarding radiation protection. They have been adopted as the basics of laws on radiation around the world, including Japan.
(*1)International Commission on Radiological Protection. The 2007 Recommendations of the International Commission on Radiological Protection, ICRP Publication 103.

Q3. Is effective dose expressed as an estimated value, not a measured value?
A3. Effective dose cannot be measured as you measure length with a ruler.
As described in Q1, effective dose is not an absolute amount of radiation. It is an index of the dose that is evaluated as causing radiation-induced damage to the whole body (including the future effects of radiation such as cancer). The degree of radiation damage caused varies depending on an individual’s age and physique, so different individuals will be affected differently. In other words, even with the same X-ray imaging conditions, the effective dose varies depending on the person having their X-ray taken. In addition, the effective dose varies depending on the position between the radiation field and the area of the body exposed by X-rays. It is not possible to measure the effective dose that is correct for all of these conditions. We evaluated the effective dose under assumption of a specific situation. For these reasons, the measurement results are not applied to all cases, and so the effective dose is generally expressed as an estimate value (see Q4, Q5)

Q4. How is the effective dose estimated?
A4. In Morita Mfg. Corp. the effective dose is estimated by referring to a paper published by Iwai et al. (*2–5). We use a human phantom of the whole body, a small dosimeter, and the weighting factor stated in the ICRP Pub.103 2007, which is the latest ICRP recommendation.

Details of measurement equipment:
human phantom of whole body: Rando phantom, female model (assumed height: 163 cm; assumed weight 54 kg). Manufactured by The Phantom Laboratory Inc.
Small dosimeter: Fluoroglass dosimeter manufactured by Asahi Techno Glass Corporation.

(*2) K. Iwai. Estimation of an Effective Dose of the Patient by Multifunctions Cone Beam X-ray Computed Tomography. Oral Radiology, 2008; 48: 61-67.
(*3) K. Iwai. Estimation of Organ or Tissue Dose and the Effective Dose of Patient by Dental Cone Beam X-ray Computed Tomography. Oral Radiology, 2008; 48: 68-74.
(*4) K. Iwai. Estimation of Effective Dose from Limited Cone Beam X-ray CT Examination. Oral Radiology, 2000; 40: 251-259.
(*5) T. Okano. Absorbed and effective doses from cone beam volumetric imaging for implant planning. Dentomaxillofacial Radiology 2009; 38: 79-85.

Q5. Is it possible to simply compare with other companies’ published values?
A5. It is possible to compare if the estimated conditions are the same. However, there are many cases that different conditions are applied and so a simple comparison cannot be made. For example, the results should change when the following conditions change. Hence, just comparing values would lead to a misunderstanding of the situation.
· X-ray irradiation conditions (for details see Q6)
· The difference between the year of the ICRP recommendation used in the calculation (for details see Q7)
· The assumed physique of the human body (for details see Q8)
If you want to compare values, you will firstly need to check the estimated condition of the effective dose is the same.

Q6. Does the effective dose change depending on the X-ray irradiation conditions?
A6. Yes, it changes.
If there are changes in the X-ray irradiation conditions, such as the tube current, tube voltage, exposure time and radiation field, then the effective dose changes.
· Effect of different values for tube current, tube voltage, and irradiation time
Reducing the set values of these will mean that the amount of X-rays generated will be reduced. Therefore, the effective dose will be lower. However, the resulting image quality will be poorer. The effects can be simply expressed as follows.
A small effective dose means a poor image quality.
A high effective dose means a good image quality.
Setting the X-ray conditions to their lowest values will reduce the effective dose, however the poor image are useless.
Therefore, to compare the effective dose while considering actual X-ray imaging conditions, it is important to use X-rays conditions that produce the same image quality.

· Effects of different irradiation conditions
Reducing the radiation field can reduce the effective dose. In addition, differences in the radiation field have little effect on image quality.

Q7. Does the effective dose change depending on the year of the ICRP’s recommendation?
A7. Yes, it changes.
The effects that radiation has on every tissue or organ (tissue weighting factor) are updated in each recommendation of the ICRP. Therefore, the effective dose depends on the year of the ICRP recommendation used to calculate it.
In addition, it is not easy to convert from the previous recommendation (ICRP Pub.60 1990) to the latest recommendation (ICRP Pub.103 2007).
For reference, in the case of Veraviewepocs 3D, the estimated effective dose stated in the latest recommendation is about twice that of the previous recommendation.

Q8. Does the effective dose change depending on the assumed body physique?
A8. Yes, it does.
Effective dose depends on the assumed physique of the body.
In Morita Mfg. Corp., we estimate effective dose by using a human phantom of the whole body with a height of 163 cm and a weight of 54 kg.
(For details, see Q4.)