Developments in Linear Detector Arrays for CT Imaging and X-Ray DR

Table of Content

Introduction
LDA Applications and Challenges
Detector Solutions - Common Platform
     Overview
     Digital Detector Cards
     Control Unit
     System Topology
Pilot Applications and Reference Cases
Summary and Future Developments

Introduction

Various industrial and security X-ray inspections systems are using linear detector arrays (LDAs). Concept of the operations and the basic technology of LDAs are well-known within the industry, where they have been a key component of the inspection systems.

Also, LDAs are gradually being used on new application areas, where the earlier inspection techniques had limitations or X-rays were not used at all. This also pushes existing LDA products to operate at their limits or requires completely new products to meet the required performance and fit into the new applications.

X-ray image quality is typically the dominating performance measure, defined principally by contrast and resolution. However, various other performance measurement parameters and other requirements exist, including operating environment, detector size and shape, material discrimination, penetration, scanning speed, and much more.

The need for different functions in the LDAs and the related software is also ever increasing. Data pre-processing needs to be included to LDAs to improve image quality. Pixel-to-pixel variations in X-ray response gain and dark offset must be normalized, but it is also necessary to have more sophisticated data corrections, such as compensation of temperature related drift and mechanical tolerance variations.

Additionally, the LDAs are required to monitor the operating environment and to diagnose their own condition. LDAs need to be field upgradable without specific tools for rapid development cycles and changes in the inspection applications.

LDA Applications and Challenges

LDAs are particularly suitable for systems where a conveyor belt or other methods are used to continuously move the inspected material or object. Such applications are common in the manufacturing industry, including quality control of raw materials, processed parts and finished products.

The food processing industry is one of the largest users of X-ray based scanning systems. Figure 1 illustrates a typical food scanning system and the related LDA.

Figure 1. Food scanning X-ray system and LDA

In industrial inspection applications, high scanning speed is required with increasing requirements for X-ray imaging quality (higher resolution and contrast). Material discrimination is required more often to improve detection capabilities.

Additionally, end users are becoming more familiar with the capabilities of the X-ray inspection, and at the same time, the manufacturing lines’ capacity is being increased, making it necessary to reduce the time and cost of inspection. This leads to a challenging set of requirements for the whole inspection concept and for the LDAs, which are a key part of these systems.

There are many industrial applications where the imaging system is moved past an object or a single object is moved past the detector during inspection. These systems include a number of NDT applications, where LDAs are employed either for a simple DR imaging or as a detector of a CT system.

The tire manufacturing industry is one with increasing quality controls and more demanding requirements for the X-ray detector. Figure 2 displays a tire inspection system, where an X-ray inspects a tire. In this setup, a panoramic X-ray source is located inside of the tire and a U-shaped LDA outside of the tire.

The tire is rotated for a full coverage within a few seconds. The X-Ray scanner used for an off-the-road vehicle tire is a huge construction with over 3 meters of detector array, which needs nearly 10,000 pixels at 0.4 mm pitch. During scans, such systems generate high data rates up to 350 Mbits of payload data per second. Excellent image quality is required to distinguish actual defects from possible image artifacts.

Figure 2. Tire inspection system (Image by courtesy of Mesnac)

There is a wide range of grading and sorting applications where X-ray imaging is utilized. Many of these systems are leveraging the capability of dual energy X-ray to distinguish materials, which is not possible with the traditional absorption-based gray scale images.

Many of these systems also run on relatively low X-ray energies to effectively determine thin layers of low atomic weight materials. Such applications need different detector concepts compared to conventional sandwich detector. It is expected that LDAs will provide well registered, high-resolution X-ray image data on high and low energy ranges.

Additionally, such X-ray detectors are usually a component of processing lines that are running on relatively high speed. In such cases, a high scanning rate is required on the dual energy LDA, which leads to very high data rates.

Security screening systems largely rely on X-ray technology and there is no clear alternative technology available. There are some similarities between the requirements for new security X-ray detectors and the requirements set by industrial applications. Many systems require high speed operation and high pixel quantities.

In particular, systems with stationary 3D-imaging capabilities (non-rotating CT imaging) or multiple views are setting new requirements for the detectors. Detector sub-systems are expected to be scalable and to operate on line integration periods down to 100 microsecond level.

Industrial and security applications demand very robust image data transfer between the detectors and the host computer. Specific image data interface solutions have been used traditionally, such as frame grabber or camera link. The data link is now expected to be based on standard cost efficient interfaces. It is necessary to provide related software libraries and development kit for rapidly integrating LDA to new applications.

Detector Solutions - Common Platform

Overview

LDAs used for various applications have several common design features. Therefore, a common platform provides an efficient solution for designing and implementing various LDA models. Targets for such a platform include:

  • High throughput data processing
  • Compact size
  • Modularity
  • Cost efficiency
  • Intelligent features, e.g. remote upgrades, self-diagnostics
  • Easy product integration
  • Utilization of standard computer interfaces
  • Possibility to run various data processing functions
  • Pre-compliance with EMC and other standards

The following sections discuss some key design selections and characteristics of a platform, which has been employed to respond the requirements above.

Digital Detector Cards

The X-ray sensitive sensor element is the key component for LDAs and all digital X-ray imaging systems. The new platform provides a digital front end (DFE) board, and can be utilized with a variety of preamplifier boards, which are detecting the X-rays and generating voltage in proportion to the X-ray flux on each pixel.

Detector cards based on the platform are shown in Figure 3. The following are the key characteristics of digital detector cards:

  • Compact mechanical size, two options: 60 x 48 mm or 60 x 96 mm
  • Convert X-ray radiation onto A/D count values
  • Short integration time, down to 100 µs
  • Local ADC and reference voltage for low noise and high uniformity: 16 or 18-bits, 250..1000 ksps
  • High throughput digital data link for cascading (250 Mps)
  • 16 bit local A/D conversion
  • Remote firmware upgrade
  • Local diagnostics: Test pattern, temperature, and voltage monitoring
  • Several pixel size and scintillator options with single and dual energy configurations

Figure 3. Digital X-ray detector cards

Control Unit

The digital X-ray data is interfaced into the host computer by the control unit (Figure 4), which also performs certain processing functions. A control unit has the following characteristics:

  • High throughput bandwidth (600 Mps)
  • Connection for up to 80 pieces of 64-ch dual energy or 128-ch single energy cards, over 10,000 pixels
  • Host computer link via Gigabit Ethernet (Cat 5e copper or fiber optic). Camera link as option.
  • Image data pre-processing functions, such as:
    • Pixel averaging/summing
    • Normalization
    • Drift and alignment corrections

High scan rate X-ray imaging with a high resolution LDA generates a large amount of data. Special image data processing solutions have been required traditionally, including camera link and frame grabbers. Today’s computers are equipped with high bit rate standard interfaces, which are now capable of handling data rates for most of the applications.

Gigabit Ethernet is an obvious choice for the physical layer of the interface. In most of the applications, it can work with cost efficient Cat 5e cabling, but the ideal option for operation with long distances or in noisy conditions is fiber optic cabling.

Protocol for the data communication and transfer to X-ray LDA is based on TCP/IP standard. UDP protocol is used for actual data streaming, which is optimized for X-ray imaging purposes. Interfacing library and software development kit has been implemented too. Therefore, the platform is capable of sustaining continuous bit rates of 600 Mbps and providing developers with straightforward access to control parameters and operation modes of the LDAs.

Figure 4. Control unit for the common platform

System Topology

Figure 5 shows typical system topology with the new platform. Here, the data from digital detector cards is read by the control unit via a robust and fast digital serial link. The control unit is physically based on cost efficient ribbon cables on small LDA configurations.

Large systems use Cat 5e or Cat 6 twisted pair cabling. Power is distributed via the same cabling on a high voltage bus and regulated locally to the required levels. The amount of power consumed by the detectors is very small, around a watt per detector card and about five watts in the controller. This enables LDAs to be built that do not need special environmental controls in most of the applications.

Figure 5. Topology of a system, which is based on common platform

Pilot Applications and Reference Cases

LDAs and other X-ray detector products based on the new platform are integrated or in beta testing for industrial applications, which are discussed in this section.

Tire inspection is one of the most demanding industrial applications. The new platform is used to implement detector arrays for these systems. Advances in mechanical construction and data processing have resulted in high quality images and scanning rates of up to 2 m/s. As a result, large OTR tire sizes can be scanned within a few seconds with 0.4 mm pixel pitch.

Figure 6 shows an example of image quality improvement. Earlier constructions experienced data discontinuity on certain detector array joints, which might affect analysis of tire carcass and belt structure integrity. This limitation is now eliminated.

Figure 6. Example of high resolution tire X-ray scan without and with pixel discontinuity correction

Another reference case is a high-throughput system for raw material inspection, where a 1.6 meter long LDA with 0.4 mm pitch and scan rates of down to 200 µs per line is required. This results in around 400 Mbps data rates, which is reliably maintained by the new platform.

The new platform is successfulled used for industrial dual energy LDA constructions to detect contaminants and sort materials, which do not create large enough differences in contrast when using traditional single energy LDAs.

For security screening applications, the new platform is being implemented on systems where earlier generation detectors could not satisfy the requirements or suffered a cumbersome system construction. A baggage screening system featuring a fast switching multifocal X-ray source is one such application. Detector array must be synchronized with the source and offer certain additional markers with the image data. These functions can be efficiently provided by new detector cards and control unit.

The new platform is also utilized for a large size, high speed cargo and vehicle screening system, where the common digital front end is being utilized by the X-ray detectors but the actual sensor part is designed to effectively detect 6 MeV pulsed radiation. For this security application, simple and standard cabling can be used as the distance between the controller and detectors is more and the system consists of large amount of detectors.

Summary and Future Developments

Applications that are driving development requirements for LDAs are presented in this article. Here, a common platform was proposed as a solution to serve development of LDAs and other X-ray detectors. It has been a challenging task to develop this new platform solution, where detectors need to fulfill the most challenging requirement set and still stay cost effective.

The new platform also offers other benefits to users, including low development costs, generally improved product lifecycle management, short time-to-market with new LDA models, simplified production and logistics, and straightforward compliance with standards.

Further development potential is anticipated in the following areas: Multi-row and multi-energy detectors, detection performance, different pitch and scintillator options, electronics integration level, and development of imaging standards and interfaces (10 Gb Ethernet, USB3, Wireless interfaces).

This information has been sourced, reviewed and adapted from materials provided by Detection Technology, Inc.

For more information on this source, please visit Detection Technology, Inc.

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit