Linear slot-scanning radiography (LSSR) is the method of taking X-ray images whereby the X-ray source and detector move across the patient, or region of interest, during image acquisition. This contrasts with conventional or digital radiography systems, where a static source and detector are used, with a limited maximum field of view.
Fan beam versus Cone beam Geometry
In a conventional X-ray system, the X-ray beam emitted from the source has a conical shape, i.e. it spreads out in both width and length. The beam can be collimated to cover only the region of interest (ROI). The entire ROI is imaged in one exposure; hence, these systems are also referred to as full-field radiography systems.
In LSSR the beam is shaped as it exits the source by passing it through a very narrow slot. The resulting beam has a fan shape which spreads out only across the width of the detector, but is narrow in the scan direction. A further, tungsten collimator is used to trim the beam further in the scan direction. Typically, the thickness of the fan beam in the scan direction is about 6 mm when it reaches the detector. This has the effect of limiting the number of secondary, poorer quality, photons that form the X-ray beam. This means that only the higher-quality, primary photons are used to form the X-ray image, and that there are far fewer scattered photons throughout the surrounding area.
Image Acquisition and Detector Configuraton
Once the X-rays have passed through the object being imaged, they are converted to light using Gadox (Rarex Green Fast) scintillators, and detected by an array of Charge-Coupled Device (CCD) cameras.
The image acquisition procedure makes use of a process called line scanning. In this process, the image is acquired by building up the complete image from successive rows of data that are acquired as the C-arm (X-ray source and detector) scans along the object being imaged. The data are processed using a technique called time-delay integration (TDI). This is the same process used in a fax machine or a flat-bed scanner, in which repeated ‘binning’ of image data during the scan increases the signal to noise ratio, and therefore the quality of the detected signal.
Furthermore, the detector is housed in a unique, scatter-rejecting housing, so that the few scattered X-rays that do traverse through the object of interest are not able to reach the detector, and therefore do not reduce image quality. This further improves the signal to noise ratio.
These varied scatter-reduction techniques mean that a post-patient anti-scatter grid is never required, even for larger patients, and the associated increased radiation exposure required to penetrate the grid is also not needed. This lower initial exposure contributes to the significantly decreased dose of the system.
The Inverse Square Law
A further advantage of using a fan beam over a cone beam is that the fan beam breaks the inverse square law. The inverse square law states that as an output from a source is measured, its intensity decreases by the square of the distance from the source. Mathematically,
Where is the intensity at a given distance r from the source, and is the intensity at the source.
In the case of a fan beam, such as the Lodox system, the intensity decreases in direct proportion to the distance from the source, i.e.
This means that the intensity of the X-ray fan beam decreases much less between the patient and the source than a conventional cone beam’s intensity decreases. This again allows a smaller dose to be generated at the source to start with, resulting in a lower dose to the patient.
8 Reasons that make Lodox X-ray Scanning Technology Effective
- A narrow fan beam consisting of mostly primary X-rays is emitted
- The scattered radiation is very low
- There is no need for an anti-scatter grid so higher power/dose at the source unnecessary
- LSSR breaks the inverse square law, i.e., 1/r vs 1/r2 geometry
- Beam hardening with aluminium and copper filters reduces dose to the patient
- The narrow beam scans over an area for a very short time (between 22 ms and 88 ms, depending on scan speed), resulting in a very low instantaneous X-ray exposure to any one part
- A high signal-to-noise ratio (SNR) is achieved with multiple resampling using TDI scanning
- Sensitive electronic detectors (CCD cameras) can detect lower power X-rays
Technical papers on Statscan and linear slit scanning radiography (LSSR)
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