Draft:Biplanar X-ray Radiography

Introduction

Medical imaging is an invaluable tool for diagnosis, giving healthcare workers insight into what is going on inside of a patient’s body. There are several different types of medical imaging technology that have various advantages that provide better information about a patient’s condition depending on their disease. Magnetic resonance imaging (MRI) is great for capturing soft tissues. MRI gives incredible resolution and detail for anatomic structures with emphasized contrast for soft tissues. Xray and CT are used for imaging anatomic structures with enhanced contrast in calcified structures, with CT giving three dimensional images. Although X-ray radiographs are quicker and readily accessible, they are limited in producing uniplanar radiographs. X-ray biplanar radiography overcomes this limitation while maintaining a low radiation dose for targets.

Science of the Technology  

Energy Source

X-rays are generated by applying an electrical voltage to a cathode, a negatively charged metal filament, causing the electrons to be released [1]. The electrons are hurtled towards an anode, a positively charged material, in order to cause the necessary electron interactions that convert the kinetic energy of the discharged electrons into heat and x-rays [2]. To maximize the collision of the electrons with the anode, a vacuum is used in the collimator to prevent collision with other gas molecules that would negatively impact photon production [3]. The most common interaction that occurs when the electrons hit the anode are collisional transfers, where the invading electron interacts with outer orbit electrons. Collisional transfer electron interactions produce heat but not x-ray photons. Due to the amount of heat generated and the need for an electron-rich material for photon generation, Tungsten is the preferred material for the anode. Tungsten is a heat-resistant material with substantial electrical conductivity [4]. The other two main interactions that occur as the electrons interact with the tungsten anode are characteristic radiation and Bremsstrahlung. While characteristic radiation depends on the invading electron interacting with an inner shell electron, Bremsstrahlung radiation requires the invading electron to interact with the nucleus. Bremsstrahlung radiation occurs less often compared to collisional transfers but are mainly responsible for x-ray photon production [5]. With biplanar x-ray radiography, there are two sources, or collimators, that are used to produce the final image.

Detection

As X-rays travel through the object, they are absorbed or scattered. The remaining photons are received by an image receptor. There are typically two types of detection mechanisms used in biplanar x-ray systems. In an image intensifier, photons that passed through the target hit a fluorescent screen that converts x-ray photons into visible light. Electrons that are in proportion to the light intensity are generated when visible light photons strike the photocathode. These electrons are amplified and focused through a vacuum generated by an electric field. Finally, the electrons hit the output phosphor screen and are converted back to a visible light signal that can be captured through a camera.

Flat-panel-detectors (FPDs) are also used as detectors in more advanced biplanar X-ray systems. In an FPD, the incoming X-ray photons are converted into visible light as they hit the scintillation layers. The photo detector array then captures this light and converts it into digital signals that can be captured and processed into clear, high-resolution images.

3D Image Reconstruction

The final image is reconstructed from the two x-ray sources that are positioned to obtain an anteroposterior and lateral view of the target. The 3D images are not developed simultaneously during capture. The 3D images are reconstructed using a software DSX Studio or ReVerteR. Biplanar radiography setups can include cameras that image calibration plates for correction during image processing [3]. 3D software can create the images of the target’s anatomy by aligning the image data from the calibration plates, proving a digital marker for the target’s anatomy, with the data gathered from the two x-ray sources positioned orthogonally to one another.

Application

Biplanar X-ray system has diverse applications in biomechanics research. In orthopedic surgery and sports medicine, it offers precise analyses of joint mechanics. Data obtained from multiple planes are critical for injury diagnosis and treatment planning. For musculoskeletal assessment, combined with other imaging modalities such as MRI, it captures detailed 3D dynamic measurements of bones and ligaments, advancing our understanding of skeletal motion during dynamic activities (Englander 2018). This technique is also valuable in identifying skeletal deformities and abnormalities, such as scoliosis or joint misalignment. Furthermore, it supports posture and alignment studies by providing accurate evaluations of joint positioning, which are essential for rehabilitation and ergonomic assessments.

References

[1] C. H. McCollough, "The AAPM/RSNA physics tutorial for residents. X-ray production," (in English), Radiographics, vol. 17, no. 4, pp. 967-84, Jul-Aug 1997, doi: 10.1148/radiographics.17.4.9225393.

[2] C. T. Badea, "Chapter 4 - Principles of Micro X-ray Computed Tomography," in Molecular Imaging (Second Edition), B. D. Ross and S. S. Gambhir Eds.: Academic Press, 2021, pp. 47-64.

[3] D. Tafti and C. V. Maani, "X-ray Production," in StatPearls. Treasure Island (FL): StatPearls 2024, StatPearls Publishing LLC., 2024.

[4] C. Li and R. M. German, "The properties of tungsten processed by chemically activated sintering," Metallurgical Transactions A, vol. 14, no. 10, pp. 2031-2041, 1983/10/01 1983, doi: 10.1007/BF02662370.

[5] Maier A, Steidl S, Christlein V, et al., editors. Cham (CH): Springer; 2018.

[6] Z. A. Englander, J. T. Martin, P. K. Ganapathy, W. E. Garrett, and L. E. DeFrate, "Automatic registration of MRI-based joint models to high-speed biplanar radiographs for precise quantification of in vivo anterior cruciate ligament deformation during gait," Journal of Biomechanics, vol. 81, pp. 36-44, 2018/11/16/ 2018, doi: https://doi.org/10.1016/j.jbiomech.2018.09.010.