Estimation of Organ Equivalent and Effective Doses From Diagnostic X-ray
CHAPTER ONE
OBJECTIVE OF STUDY
The aim of this work is to study the estimation of organ equivalent and effective doses from diagnostic x-ray.
CHAPTER TWO
LITERATURE REVIEW
Diagnostic radiology is one of the most common fields to use the benefits of X-rays. Minimizing the disadvantages of ionizing radiation is the driving force of radiation protection. A basic principle in radiation protection is the ALARA principle, which is an acronym for as low as reasonably achievable (ICRP 2007). It summarizes the approach to ionizing radiation according to the current knowledge; in the region of low doses, the probability of radiation detriment increases as a function of increasing radiation dose (ICRP 2007).
According to the most recent report, the number of X-ray examinations performed in Finland in 2011 was approximately 3.7 million, excluding dental surgery X-ray examinations (Helasvuo 2013). Of this number, approximately 9 % were computed tomography (CT) examinations and 89 % were conventional and contrast media X-ray examinations. The most common CT examinations were head, whole body, abdomen and thorax scans and the most common conventional X-ray examinations were thorax and mammography. Additionally, the number of dental radiographs taken annually in Finland is approximately 2.7 million. In 2014, the number of intraoral, panoramic, cephalometric and cone beam computed tomography (CBCT) examinations in Finland was 2.4 million, 300 000, 35 000 and 7 500, respectively (T Helasvuo, June 11, 2015, personal communication). Even though the number of X-ray examinations has decreased from 4.6 million in 1984 to 3.7 million in 2011, the number of CT examinations has increased together with the collective radiation dose accumulated from CT examinations (Helasvuo 2013; Brenner 2010).
In X-ray imaging, the patient receives a certain amount of radiation energy through absorption processes. The radiation that passes through the patient is attenuated according to the properties of the organs and tissues. The radiation sensitivities of different organs and tissues vary. The radiation energy absorbed in a tissue or an organ divided by the tissue or organ mass is the organ dose. Organ dose determination is most fundamental part of estimating the radiation risk of an individual. Organ doses cannot be measured directly and the selected method for organ dose determination has a marked impact on the uncertainty related to the organ doses.
The radiation detriment effects can be either deterministic or stochastic. Deterministic effects of radiation are related to high dose levels and cause injury and loss in populations of cells. Deterministic effects have a threshold dose below which no clinically visible effects are observed. The severity of the effect increases as a function of dose. Stochastic effects consist of cancer risk and hereditary effects, and the probability of an effect, but not its severity, increases as a function of dose without a threshold (ICRP 2007). The radiation effect classification, dose limitation concepts, and the definition of detriment and threshold have undergone several changes in the past decades (Hamada & Fujimichi 2014).
The International Commission on Radiological Protection (ICRP) has defined and introduced the concept of effective dose for risk management purposes, and the tissue weighting factors for the calculation of the effective dose. The tissue weighting factors are based on epidemiological cancer incidence studies and risk estimation of hereditary diseases (ICRP 2007). Initially, the tissue weighting factors are based on atomic bomb survivor data with whole-body exposures, but in X-ray imaging organs and tissues receive partial or heterogeneous exposure. The tissue weighting factors are intended to apply to a population of both genders and all ages, not for an individual.
CHAPTER THREE
MATERIALS AND METHODS
AREA OF STUDY
Keffi is a Local Government Area in Nassarawa State, Nigeria. Its headquarters are in the town of Keffi. It has an area of 138km2 and a population of 92,664 at the 2006 census.
Patient Dose Survey
The survey method in this work was based on the guidelines established by the NBIRR protocols. Federal Medical Centre, Keffi was used in this survey. In the x- ray machine used, specific data such as type, model, waveform, filtration, film-screen combination and output were recorded. In this paper, we only considered the posterior anterior (PA) view. For each patient and x-ray unit, the following parameters were recorded: sex, age, weight, height, body mass index, focus-to-film distance (FFD), film size, chest thickness, kVp and mAs.
The diagnostic x-ray machine used in this project work is anode rotated with tungsten target. Chest radiograph examinations of some patient were considered. The x-ray examinations of patients were carried out using posterior-anterior radiographic view using grade. The x-ray exposure to patients was given from a control room where exposure parameters of kVp and mAs were selected for each patient. The source to skin distance (SSD) was obtained for each patient from the focal – film distance (FFD) and the thickness of the patient’s chest.
SSD = FFD – thickness of the patient’s chest (4)
The radiograph of the chest involves two major viewings of positioning of patients during medical examinations; the posterior anterior (PA) view and the lateral view but in this investigation, we only considered the posterior anterior (PA) view.
A loaded x-ray films cassette (35cm x 42cm) is fixed on an erect, Bucky or chest stand.
CHAPTER FOUR
RESULTS AND DISCUSSION
Table 1: Patient information and x-ray machine parameters.
With the chest in contact with the Bucky, the arms placed on both side of the hips with the shoulder rolled forward (this is to displace the scapulae from a lungs field). A horizontal x – ray beam 90cm to the films is centered to the lower border of the scapulae. The beam is collimated to the area of interest. The exposure factors are carefully selected with an FFD of 150cm – 180cm.
The data collected mere based on the exposure parameters and the total filtration of the beam from the machines. Entrance skin dose is the maximum amount of x-radiation absorbed by living tissues during medical examinations. The skin dose to patients was determined by calculation from the x-ray tube parameters and exposure radiographic parameters using Edmonds (1984) skin dose formula which is given as:
Skin dose (µGy) = 418(KVp)1.74 mAs
Where kVp is the peak voltage responsible for the quality of penetration mA is the tube current, responsible for quantity of electrons from the filament, T is the total filtration of the beams always a constant (2.9mmAl), and SSD = FFD – thickness.
CHAPTER FIVE
CONCLUSION AND RECOMMENDATION
CONCLUSIONS
The results presented in this research work indicate that the ESDs received by patients undergoing x-ray examination in medical centre Keffi were within the guidance levels of 0.4mGy set by radiation protection bodies. Considering the condition of the machine used for this examination, a reference skin dose of 1mGy can still be compared with the skin doses evaluated in this research. It is possible that the x-ray machine is properly maintained, although all x-ray machines have to pass through quality control evaluations after each three months before it be allowed to operate. Owing to the fact that the maximum absorbed dose recorded in this research is 0.851mGy which is a bit higher than the guidance level, we therefore have to make the following recommendations.
RECOMMENDATION
Under age patients undergoing radiological examinations should be given special considerations by cutting down doses using high kilovolt technique and lower mAs. The ALARA (as low as reasonably achievable) principle should be used when carrying out x-ray activities. Consistency in Quality Assurance program and training of personnel will go a long way in reducing the radiation doses received by the patients. The estimation of stochastic risk to Keffi population as a result of X-ray diagnostic procedures and the establishment of dose reference levels in Keffi Medical Centre Radiological Unit are guide for future monitoring and studies.
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