 Vivek Bhargava |
In this article, we will look at how X-ray imaging has evolved over the years, and while we will review all aspects of the X-ray imaging chain, we will focus more on the acquisition aspect and also take a look forward on newer technologies, which are emerging in the market.
X-ray generators range from the simple, single phase and three phase two pulse and 12 pulse types to the more recently offered high frequency types.
High frequency generators which offer the advantage of reduced exposure, minimised soft radiation and significantly lower skin dose are gaining in popularity and are now available virtually across the entire spectrum of X-ray systems (from the smallest portable device to the most sophisticated cath labs). Similarly, X-ray tubes range in capacity and sophistication from the smallest stationary anode types used in portable and mobile units to the so-called ‘zero heating’ high capacity tubes used in modern multi-slice CT scanners.
While one may have the best of X-ray generators and tubes, it is equally important to have an excellent capture system to ensure that the final output reflects the maximum possible information for the radiologist. In recent times, with the advent of digital systems for capture, there has been a paradigm shift in the way images are captured, processed and finally presented.
Before we look at the differences between the conventional (or analog) method of capturing images and the digital capture systems, it is important to understand that as far as the process of acquisition is concerned, there is no difference ie the X-ray generator, tube etc. remain the same in either method of capture. Hence, the difference they make is beyond the purview of the current discussion.
 Block diagram of a typical CR system |
Exposing the storage phosphor screen: In a CR system, the CR cassettes (which contain the storage phosphor screens) are exposed exactly like a conventional cassette (in a bucky or spot film device on the X-ray table or on a chest stand or vertical bucky). When the X-ray photons (having passed through the patient’s body) fall on the storage phosphor screen, close to 50 per cent of their energy is released in the form of fluorescence and the rest of it produces a latent image. Here comes the vital difference between conventional systems and CR systems. In a conventional system, it is the fluorescent image formed on the rare earth screen, which forms a latent image on a film, which has been loaded in the cassette in a dark room.
A digital cassette has no film inside it; the latent image, which has been formed on the phosphor screen, is the one, which will be subsequently ‘read’ to produce the final image. Typically, for up to eight hours after a latent image has been formed on a storage phosphor screen, the data can be ‘read’ from it without significant loss of information.
Stimulating the phosphor: Inside the CR system, there is a high-intensity light that stimulates the phosphor molecules and luminesces them. A laser beam (with a suitable optical system consisting of a lens and a galvanometer) is used to read the luminescent phosphor screen across its length and breadth.
Changing light energy to analog signal: Typically, photomultiplier tubes (PMTs) are used to receive the signals; these are devices that emit an electric signal in proportion to the quantum of light collected by them. Gain adjustment and calibration of the PMTs is an important part of optimising a CR system to produce optimal quality images. Converting analog to digital: This consists of an Analog to Digital Converter (ADC) and most good CR systems typically have a 12-bit output (i.e. 4,096 levels of signal).
Processing the digital signal: Since the signal is now in digital form, various processing techniques can be used to enhance it like edge enhancement, changing the brightness (level) or its contrast (window). Virtually, all reputed manufacturers now offer the final digital output or image in standard DICOM format. Every CR system thus includes a computer with a suitable monitor to allow the radiologist to view the images and process them to allow optimal presentation.
Printing the image: Depending on the needs of the institution, the images can be printed on a suitable printer (like CT or MRI images) or can be burned on a CD or DVD.
Erasure and re-use: After the laser has read the phosphor plate, the plate is exposed to high intensity light to ‘erase’ it, i.e. to bring the phosphor molecules back to an energy state where they can produce a latent image when stimulated by X-ray photons. Thus, the same screen can be used over and over again.
 Multi-purpose DR system |  A prototype mobile X-ray unit with integrated CR reader |
CR system uses digital cassettes with phosphor screens. This means that for every exposure, the radiographer has to place the cassette, position the patient, shoot X-rays and then take the CR cassette back to the digitiser for obtaining the image as outlined in the steps above. When the volume of X-ray investigations is large (say for example more than 100 or 150 chest X-rays to be taken in a couple of hours), one approach is to have multiple X-ray rooms with CRs in each room or a heavy duty, multi-cassette CR system. Another more elegant and productive way is to use Direct Radiography (DR).
Some manufacturers rather erroneously expand DR as Digital Radiography and use the term interchangeably with CR systems, but for the purpose of this article the term DR refers to Direct Radiography. In a DR system, rather than using digital cassettes with phosphor plates, a ‘detector’ is used; this replaces the conventional bucky or spot film device or the chest stand or vertical bucky, as the case may be. This detector array (which is typically at least 17″ x 17″ wide to take care of all examinations) consists of either Charge Coupled Devices (CCD) or Cesium Iodide (flat panel detector).
Internally, the thallium doped cesium iodide phosphor is physically coupled to a large area amorphous silicon flat panel array, which gives the detector its name. The incident X-ray photons cause the cesium iodide layer to produce light whose intensity is measured by the photodiodes, which are formed by the silicon.Each pixel of the detector forms a Thin Film Transistor (TFT) from which the signal is read out and typically a 17″ x 17″ array consists of at least a 3,000 x 3,000 array matrix with a 14 bit output (8,192 levels).Currently, Flat Panel Detectors (FPDs) have become extremely popular and have proved their worth in the most demanding of applications like cath labs, where besides the larger frame rates involved, a good spatial resolution is desired along with stability and durability. The speed and productivity advantage are obvious.
Traditionally, most DR manufacturers integrate a good high-quality X-ray system (typically HF, 50 KW and above) to take full advantage of the excellent acquisition system; but there are also some manufacturers who offer ‘portable’ detectors that can be coupled to existing X-ray systems.
However, since the cost of a good high-quality X-ray system is not very large in comparison with the larger investment on the detection system, usually it does make good sense to go in for an integrated system that combines a good X-ray generator, multi-position arm or table and a FPD. The diagram below shows an example of a multi-purpose DR system where the same detector is used for various radiographic investigations
Pick and Choose
So when should one go in for DR rather than CR? The answer lies in the fact that these are not alternative technologies but rather complimentary solutions for digital radiography.
Typically a busy radiography department will have the DR in their main investigation room where large volumes of x-ray investigations are done and in their other x-ray rooms or out-patient / trauma / other departments could have CRs all of which can of course be networked CR cassettes can also be used in conjunction with DR systems for difficult cross-table angular examinations or on immobile patients. Again, while the CR cassettes must be processed separately, special software allows integration of CR images into the patient’s exam file so that CR and DR images can be viewed together, in much the same manner as some workstations which can display CT and MR images of the same patient at the same time on one console.
Future Trends
Compactness and ease of use will be the themes dominating the world of digital radiography and already a prototype mobile X-ray unit with integrated CR reader was recently displayed at RSNA 2005.
In DR, the use of thinner, flexible substrates and on-substrate magnification amplification and multiplexing circuitry; which can reduce the costs associated with Application Specific Integrated Circuits (ASICs) will help to bring down costs. We can look forward to portable DR detectors with automatic exposure control in-built in the flat panel array and ‘plug and play’ connectivity. As CR and DR technologies evolve, they are expected to expand into new applications, by complementing each other and offer healthcare providers better options for patient care. More power to the digital era!
The writer is General Manager-Digital Capture Medical Imaging Carestream Health
E-mail- vivek.bhargava@carestreamhealth.com
