Medical imaging has become an indispensable part of healthcare today, as it is used to diagnose diseases and injuries. The rapid growth and expansion of diagnostic imaging over the past two decades is a good thing, as it means that imaging modalities are constantly being developed and used in new ways to benefit patients (Hendee, O’Connor, 2012). However, many of these diagnostic imaging modalities use ionising radiation, and the before
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mentioned expansion of medical imaging has increased radiation exposure to patients worldwide. It is well known that exposure to X-rays poses some risk for the development of cancer or genetic effects, therefore the basic radiation protection concept is ALARA, which means to keep exposure to X-ray examinations As Low As Reasonably Achievable (Aliasgharzadeh et al., 2015). It is important to know and monitor the radiation dose received by patients during radiological examinations in order to estimate the potential harm that may occur to the patients undergoing the examination and use it as an indicator to ensure image quality. With this in mind, radiographers strive to keep radiation doses as low as possible without compromising image quality (Aliasgharzadeh et al., 2015; Mratiy, 2015).
The radiation exposure of patients is estimated using a quantity termed ‘effective dose’
(Zdešar et al., 2000). It is used in radiation protection to calculate the annual radiation exposure of workers and the general public. It does not represent a real radiation dose to an individual, but it is a calculated number that provides an approximate measure of stochastic risk. It is determined by calculating all organ equivalent doses, averaging male and female values, and weighting each organ by a tissue weighting factor. At the end, all double-weighted organ equivalent doses are summed (Fisher, Fahey, 2017). In addition to the effective dose, we also use the skin entrance dose (ESD), which is measured on the patient's skin and is defined as the absorbed dose in the air at the point where the X-ray beam enters the patient's body. It is the sum of the direct X-ray beam and the backscattered radiation from the patient. The contribution due to scattered radiation depends on the imaging technique and the size of the patient - the larger the patient, the higher the scattered radiation (Aliasgharzadeh et al., 2015; Zdešar et al., 2000).
In 2014, the European Union conducted a survey of the population dose from medical imaging, they included 36 countries in their survey, including Slovenia (European Union, 2014). The average effective dose for radiography of the pelvis and hips in Slovenia was 0.52 mSv, while the average for all European countries was 0.71 mSv, with a range between 0.21 and 2 mSv. In addition to comparing the effective doses for the pelvic radiography between countries, the doses for thorax/chest, abdomen, cervical, lumbar and thoracic spine, and mammography were also calculated. The results showed that patients receive the highest doses during radiological examination of the lumbar spine, abdomen and pelvis (European Union, 2014).
Aliasgharzadeh et al. (2015) reached the same conclusion when they measured ESD in four different hospitals for the eight most commonly performed radiographic examinations, including AP pelvic radiography. They measured a relatively high ESD in patients undergoing pelvic radiography. Within their work hospital ESD measurements were 1.21, 2.89, 2.70 and 2.80 mGy. Compared with the ESD values of other radiographic examinations, only abdominal and lumbar spine radiography had higher doses than pelvic radiography. To reduce the absorbed dose, they recommend a large distance between the patient and the X-ray source, a high tube potential and a low tube current, but to this extent to obtain good image quality (Aliasgharzadeh et al., 2015). Kloth et al. (2015) suggest acquiring pelvic X-ray images with a reduced radiation exposure, they compared pelvic images obtained with digital radiography (DR) using a 400 exposure class (recommended for skeletal X-rays) and 800 (reduced dose). Exposure class is similar to speed class, which was traditionally used with film-screen systems. The results showed reduced radiation exposure by 42% in comparison to standard radiation dose, without affecting image quality.
Alzyoud (2019) performed a study in which she compared radiation dose and image quality between supine and erect AP pelvic projections. Within this work AP thickness increased by 13%, 24% and 19% in the upright position for normal, overweight, and obese patients respectively, based on BMI classification groups, when changing supine and erect positions.
The dose-area product (DAP) also increased in the erect position. In the normal-weight group, DAP increased by 42% but was not statistically significant, although there were statistically significant differences in overweight and obese patients, where DAP increased by 55, and 105%, respectively. Changing from the supine to erect position also increased the whole-body absorbed dose by 40%, 50%, and 92% in the normal, overweight, and obese BMI groups, respectively. The effective dose was 38% higher in normal weight patients, 65% higher in overweight patients, and 120% higher in obese patients. The dose results for the normal BMI group of patients were not statistically significant, but for the other two BMI groups they were.
Image quality decreased by 6% in normal-weight patients, 10% in overweight patients, and 15% in obese patients. These results were all statistically significant. Also, the visualisation of the sacrum and sacroiliac joints was worse in the erect position (Alzyoud, 2019). The reason for the decrease in image quality in patients with higher BMI is that as body fat
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increases, the attenuation of the X-ray beam leads to increased noise and lower contrast resolution (Modica et al., 2011).
The study by Alzyoud (2019) also reported statistically significant differences between the supine position and erect AP pelvic position in terms of patient thickness, as AP thickness increased by 16% when moving from the supine to erect position. Based on the patient thickness Metaxas et al. (2018) also reported a lower effective dose in the supine projection during radiography of the abdomen, lumbar spine, and pelvis because in the supine position some of fat tissue shifts laterally, but in the upright position the fat tissue descends and increases AP thickness due to gravity.
Pelvic radiography is commonly used for clinical decision making, including diagnosis, observation, and prediction of developmental dysplasia of the hip (DDH). The position of the patient during this examination is very important in the diagnosis of DDH, as it can influence the information on the image. However, most hospitals tend to take images of the pelvis in the supine position, but these images may differ compared to those taken in the erect position (Fuchs-Winkelmann et al., 2008; Troelsen et al., 2008).
A study by Troelsen and colleagues (2008) was undertaken comparing supine and erect images in patients with DDH. They looked for differences between these two imaging projections in terms of pelvic tilt, joint space width and radiographic parameters of dysplasia, such as centre edge angle and acetabular index. Based on their findings, they recommend AP pelvic images, which should be taken in erect position to best evaluate hip deformities because people, especially young adults with hip deformities, notice symptoms during activities such as running, walking, or jumping. These types of images are most relevant for diagnostic and preoperative evaluation because they provide the best match between hip deformities, symptoms, and functional appearance. Using supine projection in patients with DDH risks misdiagnosing the patient as having acetabular retroversion, which may lead to unnecessary acetabular realignment surgery, resulting in a possible worse outcome for the patient (Troelsen et al., 2008).
Fuchs-Winkelmann et al. (2008) also conducted a study comparing radiographic angle and signs of hip osteoarthrosis (OA) on erect and supine pelvic images in patients with long-term follow-up after closed reduction of the DDH. Their results showed that there was no difference between erect and supine images when evaluating the depth of the acetabulum
(ACM angle), while the acetabular roof slope (AC angle) was greater, the centre-edge angle was smaller, and the minimum joint space width was smaller in images taken in erect position. These results led to the conclusion that erect pelvic radiography is more accurate than supine radiography in assessing minimum joint space width.
Pullen et al. (2014) investigated the variability of acetabular coverage in images of the pelvis in the supine and erect positions in patients with femoroacetabular impingement (FAI). They reported that changing the pelvic tilt when taking AP supine pelvic X-ray just to meet the standards for adequate supine radiography may artificially affect the screening characterization of the acetabular deformity, meaning that supine pelvic images may not represent the true, functional, erect position of the acetabulum when impingement is involved. Other authors also concluded in their studies (Jackson et al., 2016;
Ross et al., 2015) that the functional position of the pelvis during radiography may influence the signs on the image when evaluating pincer impingement, and if these changes are not appreciated when diagnosing and treating patients with FAI, it may lead to choosing the wrong treatment pathway and poor outcomes later on. Jackson et al. (2016) also reported the results of their study showing a decrease in pelvic tilt on images taken in erect position that corresponded with a lower incidence and severity of radiographic signs of pincer impingement, such as a decreased ischial spine sign, the presence of a crossover sign, and even a small increase in acetabular tilt.