Introduction
Idiopathic scoliosis is a three-dimensional spinal deformity which has a reported incidence of 2–4% in a pediatric population aged six to fourteen [
1]. Even though the initial diagnosis of scoliosis is usually made by physical examination, the definitive diagnosis and management is made by standing posteroanterior (PA) and lateral (LAT) radiographs of the full spine [
2‐
4]. This imaging is needed to determine the type of curve and his severity to identify the presence of underlying pathologies, to decide whether treatment is necessary and if necessary repeated studies are required throughout the whole course [
2,
4,
5]. Owing to the increased radiation sensitivity of children, numerous follow-up images and increased life expectancy, the chance of a radiation-induced abnormality is of concern [
4,
6,
7]. The purpose of this study was to optimize our standard full-spine radiography protocol with respect to diagnostic quality and dose. Two studies were conducted to investigate this issue. First, a phantom study was performed where the impact of several technical parameters on image quality and radiation dose was evaluated and the optimal protocol was identified. In the prospective study, we performed this optimized (PA) full-spine radiograph in pediatric patients with idiopathic scoliosis and we compared radiation dose and image quality of these radiographs with earlier studies from our radiology archive.
Discussion
Owing to the increased radiation sensitivity of children, the potential risk of multiple radiographic examinations in the management of scoliosis has led to efforts to reduce exposure on the basis of the ALARA principle [
4,
6,
7,
10‐
12]. This includes limiting the number of radiographs [
13], development of competing digital technologies and optimizing existing protocols [
7,
10,
12]. With this study, an optimization of the existing protocol was performed by adjusting the acquisition parameters together with image quality assessment. This process could be easily performed in a clinical environment. Since the first examination of scoliosis patients requires a high-quality anatomical image, our goal was to optimize our standard protocol without compromising image quality, this obviously within the possibilities of our X-ray system.
Our optimization process included a phantom study to understand the impact of technical parameters on dose and image quality. From this phantom study, we identified a protocol which had similar image quality at the lowest radiation dose. We then applied this protocol on a patient study. The dose gains were achieved by adding a 2-mm Cu filter and lowering relative exposure (protocol 8, Table
3) which resulted in a dose reduction of approximately 70% in our phantom study and 45% for our patient study. The tube potential of 84.8 kV remained and is within the limits of 65–90 kV stated by the Commission of the European Communities as examples of good radiographic technique for a full-spine examination in pediatrics [
14,
15]. From the phantom study, we also observed that the lowest radiation dose could be achieved by removing the anti-scatter grid but this at cost of the subjective IQ (protocol 11 Table
3). Although the loss in image quality was minor (mean total score of 29.5 for this protocol in comparison with a score of 30 for protocol A), and would not necessarily affect the correct diagnosis and/or therapeutic decisions, the aim of our study was to preserve our initial image quality. This protocol was therefore, inevitably rejected to use for our patient study but could be considered in studies where only spinal curve measurements are required. Although our lowered dose images result in higher image noise levels, as expressed by a significant lower signal-to-noise ratio (SNR) compared to the standard protocol images, the visual grading of the images indicated that there was no significant difference in score, and thus image quality between both protocols was perceived as similar (Table
5). Interreader agreement was found to be moderate to almost perfect for both groups.
Previous studies already evaluated the effective doses at scoliosis examination [
12,
14,
16‐
19]. The reported effective doses varied between 0.02 and 0.49 mSv. One study compared conventional film/grid technique (F/G) with air-gap technique using computed radiography (CR) [
12], two studies [
17,
18] used an air-gap technique using conventional films and the other three studies performed the scoliosis examinations by digital-pulsed fluoroscopy [
14,
16,
19]. In four of these studies, the effective doses were determined based on the Monte Carlo calculation using PCXMC or a similar computer program [
12,
14,
16,
18], whereas the estimation in two analyses involved calculating the total energy transfer to the organs, based on air kerma or TLD measurements [
17,
19]. Although there is no accepted standard method for the determination of the effective radiation doses of radiological procedures in children, Geijer H. et al. [
16] found a remarkably good correlation between the measured values of effective dose and the Monte Carlo calculations which indicates that these calculations are a useful tool in the clinical setting for the evaluation of the effective dose. The lowest doses in these studies (0.02 mSV) were achieved by digital fluoroscopy [
17] and were significantly lower than the effective dose from our optimized protocol (Table
2), however, the image quality in this study was described as lower but acceptable for accurate angle measurements, whereas the image quality in our study was preserved (Table
5). Other studies compared radiation dose and image quality between a slot-scanning device (SSD) (EOS imaging) and a digital radiographic system in assessing scoliosis in children [
10,
20]. SSD has the possibility to simultaneously acquire two orthogonal images, which facilitates three-dimensional surface reconstruction of vertebrae what makes this system interesting from a clinical perspective. Significant dose savings with an equivalent or superior image quality were reported with the SSD, however, effective doses are rarely determined which makes comparison with our study more difficult. Yvert et al. [
10] reported mean DAP values of 39.8 ± 11.7 cGy cm
2 for standing whole-spine frontal radiographs. These values are comparable with the DAP values of our optimized protocol (47.0 ± 22.4) taking into account the fact that the mean age in this study (11.4 ± 2.6 years) was somewhat lower compared to our study (13.4 ± 1.7 years). Two studies reported on micro-dose X-ray (EOS) protocols for patients with adolescent idiopathic scoliosis where the DAP values were between 3.9 ± 0.7 cGy cm
2 [
21] and 4.3 ± 1.3 cGy cm
2 [
22]. These dose values are significantly lower than our reported doses, however, the image quality assessment mainly concerned static parameters (e.g., Cobb angle measurements, tilting assessment) and technology (e.g., collimation), whereas in our study, we performed a more detailed radiological image quality assessment including the evaluation of bone sharpness and the visibility or delignation of specific anatomical structures, which again, makes a comparison with our study more difficult. A drawback of a SSD system is its higher cost. In a health economics study that compared SSD to standard X-ray, McKenna et al. demonstrated that an SSD system can be shown only to be cost-effective if the utilization is about double than standard X-ray [
23]. This suggests that an optimized conventional radiographic system can provide a dose- and cost-effective solution for the diagnosis and follow-up of idiopathic scoliosis.
Some limitations of our work have to be considered. Most importantly, these results have only be validated when using the Axiom Luminos dRF from Siemens, and therefore they cannot be directly extrapolated to other types of X-ray systems. Second, this study only included patients without brace who were able to hold a standing position, results could differ for supine or in-brace radiographs.
In summary, with a dose reduction of 45%, image quality remained similar in terms of bone/soft tissue contrast, bone sharpness, visibility of processus spinosus, delignation of the intervertebral spaces, assessment of the Risser grade and assessment of the spinal curve. We conclude that, in the diagnosis and follow-up of idiopathic scoliosis, a dedicated lowered dose full-spine radiograph protocol can replace the standard protocol with similar image quality.