Is a further reduction of radiation exposure in scoliosis monitoring achievable using at detector and pulsed uoroscopy technology? - A feasibility study on human specimen

Background: In the case of scoliosis therapy, frequent radiological monitoring of the spine is necessary. However, x-ray requires high radiation doses; therefore digital pulsed uoroscopy with at detector technique can be used alternatively. The latest developments in this technology lead to further dose reduction with an improved image quality. To evaluate the new system, we asked if there is a difference in dose area product (DAP) concerning the opening area (OA) and image quality settings (IQS). Further we wanted to investigate the inter-observer reliability using an established scoring system and correlate the DAP with the point value. Methods Therefore, we examined 4 cadaver spines (T1 to sacrum) with the at detector technique using digital pulsed uoroscopy and simulated the abdominal soft tissues with water bags. The images were merged calculating whole spine images from several digital single images and evaluated by 3 different investigators (spine surgeon, experienced pediatric radiologist, assistant physician) using an established scoring system. For comparison and validation of our model, we used digital radiography images of the cadaver spines. Results The values for the DAP increased from the small OA (33%; 0.56 µGy·m²) to the maximum OA (100%; 0.82 µGy·m²) by 45% (p = .003) and from low IQS (0.57 µGy·m²) to high IQS (0.84 µGy·m²) by 48% (p = .028). The inter-observer reliability was strong (3 vs. 1: ρ = .818; 3 vs. 2: ρ = .742; 2 vs. 1: ρ = .586; p <.001), but there was no correlation between DAP and point value (ρ = -.053, p = .588). Despite the low DAP, the setting 33% OA achieved the best point values, therefore this setting is preferred. Conclusions Using a digital uoroscopy system allows a signicant reduction of radiation exposure for whole spine images by a factor of 7.5 (3.88 µGy·m² to 0.5 µGy·m²) compared to slot-scanning x-ray (EOS).


Introduction
Adolescent idiopathic scoliosis (AIS) is a common spinal deformity, de ned as a curvature of more than 10° of the spine in the coronal plane accompanied by a rotation component (1). Conservative therapy with bracing and physiotherapy are common treatment options all over the world (2). AIS usually requires annual full spine radiography during conservative treatment, because the deformity can progress until skeletal maturity is reached (3). This monitoring leads to elevated radiation dose during childhood and especially during adolescence, when the bones are growing at an accelerated rate.
In earlier days, before the introduction of digital x-ray devices, cumulative doses of 140 mGy could be observed on the female breast. This radiation exposure even led to an increased breast cancer rate in scoliosis patients (4,5). Thanks to a number of technical developments such as digital x-ray using multiwire proportional chambers (6), radiation exposure has been reduced in recent decades by the ratio 13.1 for spinal examination (7). Digital x-ray diagnostics represent the standard of investigation to date, but this requires quit high radiation doses. In a previous study, using a conventional screen/ lm system with ampli cation factor 800, an average dose are product of 94.9 µGy x m² was achieved.. The average dose area product (DAP) of the conventional, digital lm-foil combination was 94.9 µGy x m² (8).
Alternatively, using the at detector technique with the whole spine image being calculated from several digital single images, a DAP of 9.37 µGy x m² was achieved (9). A variant of the described technique is the storage of the pulsed uoroscopy during the scanning process, yielding a DAP of 7.8 µGy x m² (8).
The Multi Diagnost Eleva system (Philips Medical Systems; Netherlands) with a new at detector, gridcontrolled uoroscopy (GCF) and Dose Wise technology (10) was originally developed for pediatric applications such as cardiac and vascular interventions. Adapted to musculoskeletal examinations, this system provides the technical opportunity to further reduce the radiation exposure for the mostly young scoliosis patients.
Aim of the study was to evaluate the at detector technique for whole spine examinations with pulsed uoroscopy and determine the examination parameters for clinical use. Therefore, we investigated 4 human spinal specimens and evaluated the images by three investigators. First, we asked if differences in DAP depending on the opening area and image quality setting are assessable. Further we wanted to investigate the inter-observer reliability using an established scoring system and correlate the DAP with the point value. Finally, we wanted to establish the optimal settings for further human studies.

Specimens
We used four human spinal specimens for the tests, anonymously donated by body donors after a corresponding declaration of consent. The ethics vote with the number 891/2019B2 of the Ethics Committee of Tuebingen was available. In order to simulate the abdominal soft parts, a water bag of approximately seven liters was xed on the specimens.

Digital uoroscopy:
All examinations were carried out on a C-arc-equipped at detector unit (Philips MultiDiagnost Eleva, Philips Medical Systems; Netherlands) with technical optimization of the unit for paediatric uoroscopy purposes, including ltration and automatic customization of the X-ray beam spectrum, shape, and pulse frequency (10). The specimens described above were placed on the examination table in a plastic box including a reference ruler in the back of the specimen and examined by an experienced pediatric radiologist. During a synchronized scan with a C-arm speed of 4 cm/sec uoroscopic images were achieved with a pulsed frequency of 7.5 images per second. The images were merged by the software ViewForum R6.3 "Spine Measurements" (Philips Medical Systems, Netherlands) and an overall image was calculated based on the reference ruler (see Fig. 1A.). In preliminary tests, the manufacturer's recommendations of 7.5 images per second and minimum 50% opening area were tried to reduce. However, with a reduced pulsation of only 4 images per second or an opening area of 25%, the image quality was no longer su cient for the automatic image calculation.
Therefore we de ned the following parameters for testing within the study: Opening area 100%, 50% and 33% and image quality setting -(low), o (medium), + (high). Each OA setting was combined with every image quality setting, resulting in 9 settings per specimen. All other parameters have been adopted identically from previous studies (8,9): Frame rate 7.5 images/sec; tube voltage 60 kV, tube current 250 mA, lters: 1 mm Aluminum (Al), 0.1 mm copper (Cu), 0.3 mm copper. During the investigation, the dose area product was determined using a dose area product meter (Diamentor CD, PTW Dosimetry, Freiburg, Germany). Digital X-ray: For comparison, standardized whole spine radiographs of the spinal specimens were taken (see Fig 1 B), with a digital X-ray unit Digital Diagnost (Philips Medical Systems; Netherlands) on a dedicated grid stand for whole spine recordings, using a digital x-ray detector Pixium 4600 (Trixell, Moirans, France) with the standard examination protocol used in daily routine (tube voltage 85 kVp, automatic tube current modulation and an Exposure Index of 800).

Image evaluation:
The evaluation was carried out by three independent investigators (spine surgeon, experienced pediatric radiologist, assistant physician) on a PACS workstation (Centricity PACS®, GE Healthcare, Chicago, Illinois, United States), who were blinded to the settings of the uoroscopic system. An established scoring system was utilized to evaluate the images, as described by Kloth et. al (11). In this system four evaluation criteria (endplates, pedicle, spinous process and lateral alignment) are each rated with a score from 1 (fully assessable) to 4 (not assessable) (see Tab (11).

Model validation:
An important factor before the evaluation of the data was the validation of our model with cadaver specimen using digital x-ray: On average we found a DAP of 30.3 µGy·m for digital X-ray imaging in our study, corresponding to the data in patients´ x-ray described in the literature (Kluba et al.: 31.5 µGy·m ) (12).

Data analysis
For data analysis, a sum score was calculated from the determined score points, leading to a minimum score value of 4 points (optimal assessment of the image) and a maximum score value of 16 points  Table 2a).   In conclusion, we can state that there is an increase in DAP from small to large aperture area and from low to high image quality, even if not all differences have become signi cant.
Finally, we wanted to establish the optimal settings for further human studies. We therefore analyzed the

Discussion
Our study shows a very interesting result: A low OA of 33% leads to a reduction in radiation exposure compared with a large OA (100%) of 45% and at the same time to an improved assessment of the image, meaning 33% OA setting can simultaneously improve the image assessment and reduce the radiation dose. Looking at the image quality setting, we found the expected correlation of improved image assessment from -to + with increasing radiation exposure.
Three radiological applications are currently available for monitoring the course of scoliosis patients: First, the uoroscopic system presented here, which is rarely used in clinical routine. Second, digital x-ray examinations, representing the standard examination tool in most clinics. Third, the slot-scanning x-ray (EOS), which is increasingly used due to the three-dimensional images. In the following, these examination systems will be compared with the Philips MultiDiagnost Eleva and our results.
Comparing our uoroscopic system with digital x-ray diagnostics, uoroscopy shows a signi cantly reduced radiation exposure. In recent years the development and spread of digital image processing systems and at detectors, which have higher detectable quantum e ciency (DQE) led to a lower mean DAP compared to conventional lm-foil systems. Nevertheless, in previous studies, uoroscopy was clearly superior concerning radiation exposure compared to x-ray diagnostics. In a previous study in our departments, we could show signi cantly reduced DAP values for uoroscopy (7.8 µGy·m²) compared to x-ray imaging (94.9 µGy·m²) (8). However, other working groups were able to reduce the DAP of anteriorposterior x-ray images using a digital at panel detector (FPD) and an image stitching system (ISS) in full-spine radiography to 16.8 µGy·m² (13). Comparing conventional lms (DAP: 97.0 µGy·m²), digital xray (31.5 µGy·m²) and digital uoroscopy (5.0 µGy·m²), Kluba et al. found similar results (12). Compared to this information from the literature, our examination technique reveals enormous possibilities of reducing the radiation exposure by a factor of 33 (16.8 µGy·m² to 0.5 µGy·m²) compared to the latest xray diagnostics.
First advantage for x-ray examinations is the standardized feasibility by the radiology assistant according to generally accepted guidelines and settings (14). To date, there is no automated protocol for the scanning process with the uoroscopic system and so far has to been carried out by the radiologist.
The second advantage of the x-ray examinations is a better assessability with higher score points compared to the uoroscopic technique. However, the end plates as well as the spinous processes are particularly relevant for the assessment of a deformity on the spine. These are already su ciently displayed at a uoroscopic image and do not require the high assessability and radiation dose of x-ray such as traumatic or oncological issues. From the authors' point of view, an x-ray examination would be helpful at the initial presentation to rule out malformations or dysplasia. Further monitoring of the deformity could be carried out with the uoroscopic system.
Another method that has become increasingly popular in recent years is slot-scanning x-ray (EOS), which is used for examinations in scoliosis patients (15). First advantage of EOS is a software called sterEOS 3D, which offers the opportunity of visualizing the spinal deformity in all three planes (16). Second advantage is that it is even easier to use than X-rays. However, compared with our results, high radiation exposure was described in DAP (59.9 µGy·m² ap in females and 18.2 µGy·m² in children in an anteriorposterior view, regarding a Phantom Study (17). In accordance with the ALARA principle (as low as reasonably achievable), a reduction in radiation exposure was already sought for the EOS system and DAP of 3.88 µGy·m² in female 4.42 µGy·m² in male was achieved (18).
Despite the signi cant improvements and further reduction of DAP in EOS technology, our system can reduce DAP again by a factor of 7.5 (3.88 µGy·m² to 0.5 µGy·m²).

Study Limitations
A limitation of our study is certainly the use of cadaver and an evaluation with patients is still pending. However, we were able to show that our model achieved absolutely similar values compared to data from routine clinical practice (x-ray) and from literature (Our results: 30.3 µGy·m² for digital X-ray imaging vs. Kluba et al.: 31.5 µGy·m²) [12]. Nevertheless, the age and height of the patient are relevant in uencing factors on the DAP, which cannot be fully mapped with the specimen of the body donor.

Conclusion
From the authors' point of view, the uoroscopic system presented in this paper shows excellent radiation exposure values with good image assessability in the setting 33% OA / + image quality, leading to great bene t especially for young scoliosis patients. However, the practical feasibility must be improved e.g. by automated scanning processes, comparable with the EOS system.   Scatterplots of different observers with LOESS smoothing. The inter-observer reliability was strong (3 vs.