Digital Radiography

Dawn of the Digital Era

Digital Radiography is witnessing rapid innovations in hardware as well as its software applications. Most of the imaging devices in a radiology department like ultrasound, CT, MRI, DSA already use digital imaging technology.

Dr Bhujang Pai Head of Radiology PD Hinduja Hospital, Mumbai

Rapid advancement in the field of medical imaging has been possible due to the use of computers as they can process digital data very fast and efficiently. To use computers in medical imaging, analog data first needs to be converted to digital data for processing and then converted back to analog images for viewing and interpretation. This is done by Analog-to-Digital Converters (ADC) and Digital-to-Analog Converters (DAC), respectively. Most of the imaging devices in a radiology department like ultrasound, CT, MRI, DSA already use digital imaging technology.

Radiography

Radiography is recording of information about an object using X-ray transmission. The intensity of X-rays is nearly uniform before entering an object being radiographed. After passing through the object, the spatial distribution of transmitted X-ray intensities carries all the radiographic information about the object. This information can be detected by means of something that is sensitive to radiation. Conventionally, this is done by Film-Screen Radiography (FSR). It can also be done by some digital detectors. When digital detectors are used to capture this information, the process is termed as digital radiography.

Conventional Radiography

In FSR, the absorbed X-rays are first converted into light by a pair of intensifying screens. Films sandwiched between these screens records a latent image that becomes visible after chemical processing. Since 100 years, conventional radiography has been found to be very useful. Intensifying screens, introduced over 60 years ago and rare earth screens in recent years, have greatly reduced the radiation dose required for producing good quality images. Advancements in FSR technology have almost reached the limit of possible improvements. Only a completely new technology will be able to provide substantial advantage over the current FSR techniques.

Digital Radiography Systems

A digital detector replaces film and screens in digital radiography. There are two basic types of digital radiography systems depending upon the types of detectors used to capture radiographic information:

• Computed Radiography (CR) systems use a Photo-Stimulable Phosphor (PSP) plate enclosed in a light tight cassette. CR utilises a two-stage process with the image capture and image readout done separately.
• Direct Digital Radiography (DR) systems use detectors that have a combined image capture and image readout process.

Computed Radiography System

CR cassettes use PSP plates in place of film and screens. These plates are coated with europium-activated Barium Fluoro-Halide (BaFX: Eu 2+). The Halide used may be bromide, iodide or a combination of both. CR cassettes are used just like conventional cassettes on normal radiographic equipment and are available in similar sizes. X-ray information is stored in PSP imaging plates as electrons, in semi-stable higher energy states, in sinks or 'F' centres. The number of such trapped electrons is directly proportional to the absorbed X-ray dose. The imaging plate comes out or is exposed by opening the CR cassette within the CR reader. Image information is acquired by scanning the plate by a laser beam. Red laser light excites these trapped electrons during scanning. Electrons eject from the higher energy sinks and comes down to the base level. They emit a higher energy blue light during this process. This light is captured by a light guide, converted into electrical signals, amplified, digitised and used to form the image. The imaging plate is ready for re-use after exposure to white light. Patient information and cassette ID needs to be linked in a CR system , as there is no direct electrical connection between the CR reader and the cassette. A bar code reader or a chip embedded on the CR cassette is used for this purpose

Direct Digital Radiographic Systems

Cassettes form an important component in both FSR and CR. To improve workflow and to avoid the use of cassettes, a new class of detectors was manufactured that combined the processes of image capture and image read-out. This formed the evolutionary basis of DR systems.

There are four different types of DR Systems available depending on the type of detectors used in them

• Flat Panel Detector (FPD) based systems
• 2D or 'Area' Charge Coupled Device (CCD) array based systems
• Slot scanning type and
• Photon counting type

Flat panel detector (FPD) based systems

FPD based DR systems are the most popular. In these, Thin Film Transistor (TFT) arrays are used, which are made of amorphous silicon (a-Si).

2D or 'Area' CCD array-based systems

In these DR systems the X-rays are absorbed and converted into visible light in large scintillators or phosphors.

Slot-scanning types

These systems use a narrow fan beam that moves across the anatomical region. Two precisely aligned moving slit collimators, one on either side of the patient are used in such systems.

Photon counting type DR system

Photon counting type of DR system has construction similar to the slot scanning type described above but uses a different type of detector. These systems use a multi-slit detector made of crystalline Silicon (Si) as a scintillator.

Innovations and Newer Applications in DR

DR is witnessing rapid innovations in hardware as well as its software applications. Few of the exciting applications are mentioned below. Clinical utility and the true potential of these applications will be understood better in the years ahead.

Tomosynthesis: In this technique, multiple low dose exposures are given from various angles while the X-ray tube moves in an arc and the detector remains stationary. Multiple images with different focal zones are possible to be created by addition of these low dose images after pixel shift.

Dual-energy imaging: By using a high and low kilo-voltage technique, two datasets are created. Soft tissues and bones can be separately depicted by this method. Dual-energy techniques are most effective when both images are acquired simultaneously.

Computer Aided Diagnosis (CAD) software programs: These are important in early detection of cancer of the lung and breast. The suspicious areas are marked by the software for review by the radiologist. The efficiency of CAD software program is related to its sensitivity and specificity profile. These programs are gradually improving with newer generations/ editions having better sensitivity and specificity profile. The main advantage of CAD is that it permits a radiologist to avoid overlooking diagnostically significant findings.

Automatic image stitching: This is a feature that is useful in determining precise measurements in lengthy anatomical regions like the spine or lower limbs.

Mobile DR: The use of mobile DR systems is hampered by the fragility of the FPDs and the high-costs. A mobile DR system, when compared with an FSR system, avoids problems related to the availability, storage, transportation and disposal of films and chemicals.

Wireless FPDs: With the introduction of the model Pixium 3543 from Thales, wireless portable DR is now a reality. It wirelessly transfers image data to the DR system. Alternatively, the image data can be transferred to DR console via an ethernet cable. It has no cables and does not interfere with surrounding machines.

Fluoroscopy and radiography: Real-time digital imaging in DR is possible with the Pixium RF 4343, from Thales. It facilitates high-quality radiography and fluoroscopy (up to 30 images/s). The fluoroscopy feature is of use in gastroenterology, urology, and vascular applications.

Newer Innovations and Applications in CR

Some of the drawbacks of CR systems, namely cassette handling, long read out time of PSP plates, low DQE and poor resolution have been addressed by newer innovations and technological advances.

Automated CR systems with fast readout: CR systems efficiency has been recently improved by reducing the read out time and by removing the step of cassette handling. Automated CR systems achieve this by line-scan lasers and photodiode detectors that reduce the readout time of a PSP plate to less than 10 seconds.

Newer phosphors for PSP plates: Commercially available PSP plates have unstructured phosphor like rubidium chloride or barium fluorohalides doped with europium. These are scanned in a raster pattern.

Mobile CR systems

Impact of DR on Departmental Workflow

Plain radiography accounts for 50 to 70 per cent of the total workload in a large radiology department even today. Study of the workflow in radiography is therefore important in daily practice. Let us briefly examine the issues that govern workflow in radiography.

The process of FSR workflow ranges from patient registration to the availability of dried radiographic film. Analysis of these individual steps may help in finding ways to increase the workflow. The process consists of the following key steps: (a) entering patient information into the register/ console, (b) setting exposure parameters, (c) getting and positioning the radiographic cassette, (d) positioning the patient, (e) radiographic exposure, (f) film processing, (g) cassette reloading, and (h) image quality check before the patient goes away.

It is evident from above that a technological move from FSR to CR does not eliminate or reduce the duration of any of the described steps. As a result, workflow does not significantly improve by introducing digital radiography in the form of CR. However, in DR, the steps (c), (f), and (g) are totally eliminated, significantly improving the workflow. Availability of HIS/ RIS to automatically populate the patient information in equipment consoles enhances the workflow significantly in both CR and DR systems.

Conclusion

Conventional Radiography is evidently the last of the radiology modalities to embrace and incorporate digital technology. By their tremendous impact on the image quality and the workflow, digital radiography systems have become practicable alternatives. CR is a simple and cost-effective technology that permits use of existing radiographic equipment. It has been suggested that for moderate workload (upto 50-60 films per day), a CR system is adequate. High cost of a DR system is justified only when the workload is much beyond this level.

The current scenario in CR and DR is one of relentless technological advancement and expansion. CR systems now have features that traditionally had been associated with DR. Similarly conventional X-ray machines can now be equipped with a DR detector as a retrofit, saving greatly in costs as compared to purchase of a new DR systems. As fallout of these developments, the distinction between the CR and DR technologies is blurred.

Lastly, a change over to digital technology is essential to create a fully digital 'filmless' radiology department and fully reap the benefits of implementing RIS and PACS programs.

bupai@hotmail.com