Medical Compact Backscatter Imaging

Designing and prototyping a compact handheld x-ray 

Spring 2023 MAE 156B Senior Design Project

University of California, San Diego

Current X-ray imaging devices are based on “transmission mode imaging” which is a process where photons are sent through an object and are detected on the other side in order to produce an image. This practice requires high energy photons in order for the photons to completely pass through the object.  

However an alternative imaging method, backscatter imaging, instead acquires an image by detecting the photons that reflect back from the object. Backscatter imaging utilizes lower energy photons, and therefore is less dangerous in terms of human radiation exposure, as well as in terms of high voltage through the device, as less energy is required to generate an image. This technique is commonly used in a variety of engineering and R&D applications, but it is not currently used in medicine as acquired images are not as clear as traditional imaging. The objective of this project is to not only design such a device, but also to determine the photon energy that yields an optimal image quality. Such a device has potential for many different medical situations, particularly for spinal or breast cancer applications.  

Traditional transmission x-ray is not optimal for the imaging of shallow tissues as photons must penetrate through the body of a patient in order to be detected. This results in the introduction of extraneous information from tissues and structures behind the target, distracting from or obscuring important elements in the radiological image. Furthermore, penetration throughout the entire body requires higher energy photons as photons must be capable of passing through more material, as well as more overall photons, as photons may scatter away from the detector when they contact patient tissue and bones. This has the potential to expose patients to more radiation than may be necessary and to cause health issues, such as cancer, down the line for the patient.

  In contrast, a backscatter imaging would only require direct access to the tissues being imaged, as the detector and device would be on the same side, resulting in an increased access to and ease of radiological imaging in situations where patient mobility is restricted, such as an operating room. 

Additionally, such a device would generate an image using fewer overall photons, as photons would not have to penetrate throughout the entire body structures of the patient, reducing the number of opportunities for photons to be absorbed by tissue or otherwise scattered away from the detector. Furthermore, as photons do not need to be able to penetrate through large amounts of dense objects (such as bone), the photons required for imaging are not required to carry large amounts of energy as is the case with transmission imaging. Therefore, imaging through the backscatter modality has the potential to expose patients to significantly lower amounts of radiation. 

A simulation is required to determine the optimal energy of a backscatter imaging device due to safety considerations. Given the complex interactions between photons and the medium, a Monte Carlo simulation is appropriate for this project.  Varying energy levels of photons can results to a different outcome.

• An x-ray source is placed at the lower half of the housing unit (Large round hole)

• A leaded plastic shield with an aperture surrounds the x-ray source to block leakage radiation and allow x-rays to emit only through the aperture

• A collimator is placed close to the x-ray aperture to focus the x-ray emission. 

• Two stepper motors are next to the x-ray source to control a dual axis rotation system which moves the collimator across a 4x4 cm scanning area

• The high voltage terminal is at the top of the housing to prevent arcing between the electrical components due to working with high voltages.

• Completely battery powered

  • 12V lead acid battery input

• Scanning mechanism resolution up to 20 data points per centimeter squared

• Power output up to 20kV

• Leakage radiation of between 3 and 16 millirads

• Radiation shielding considered effective

• Radiation values at the scanning area around 45 millirads