Digital radiology – The new imaging frontier (Proceedings)


Digital radiology – The new imaging frontier (Proceedings)

Aug 01, 2009

We are going to start with a review of radiologic image acquisition. With conventional film-screens, 49% of X-rays pass through the cassette without interaction at all; 50% interact with the phosphor layer of the screen. The phosphor screen emits light to form a latent image on the film. Only 1% of the x-rays actually interacts with the film directly. (Film is more sensitive to light than to x-rays.) The latent image is the non-visible image that exists on the film after exposure but prior to development. Actually the latent image is grains of silver that are waiting to be developed. With chemical development the latent image is converted into a viewable image. The silver grains are converted into black, metallic silver. The washing clears away excess unexposed silver.

Now onto computed radiology, also known as CR. A reusable detector plate is utilized in place of a film/ screen/ cassette combination. It can be used on with any existing x-ray equipment and on multiple table. It is made up of a storage phosphor system that results in digital output. The storage phosphor system is similar to film screen technology: both use screens to absorb x-rays, both emit light, both can be used thousands of times. CR is different as the screen retains a portion of the x-ray energy which is extracted during the read out process.

In forming the CR latent image, 50% of x-rays interact with the phosphor and an immediately flash of light is released (same as in convention film-screen). The other 50% of x-ray energy is stored in the phosphor plate as trapped "metastable" electrons. This 50% of x-rays make up the latent image. To remove the latent image, the "metastable" electrons are stimulated with a laser at a specific energy (wavelength). The laser light is used to scan the storage phosphor in a rectilinear fashion with a pin point focused laser beam which results in a high quality image with very high resolution. The drawback to CR is the time of laser scanning, it is similar to film processing. During the scanning the trapped electrons are given extra energy allowing them to get out of the "trap". As they move to the lower "stable" energy level they "unload" the energy as visible light. The light is detected by photomultiplier tubes, amplified and converted into an analog electric pulse. The magnitude of this electric pulse is proportional to the intensity of the light emitted by the phosphor screen.

Now onto DR: digital radiography. There are two varieties, direct and indirect digital radiography. With direct, semiconductor material (amorphous selenium) is coated on a plate which converts x-ray energy directly to electrical energy. A thin film transistor (TFT) is directly coupled to the semiconductor detector which detects and reads the electric charge. This process results in the highest quality image. Indirect digital radiography coats scintillation material (GdOxS) on a plate, which converts x-rays to light, then a TFT converts the flash of light to an electric charge.

Indirect digital radiology uses an input phosphor which absorbs the x-rays and converts their energy into visible light similar to the intensifying screen in screen-film radiography. This phosphor forms in long, needle-like crystals which function like light pipes channeling the visible light emitted within them toward the photocathode. This results in increased spatial resolution by limiting the lateral spread of light. A detector array is used to detect the light produced by the phosphor. The array is comprised of a large number of discrete detector elements each of which contain both a light sensitive area and a region that holds the electronic components. This array is comparable to the film emulsion. Sequentially these array elements are read out and the charge from each is digitized, stored, and forms the latent digital image. This read out occurs rapidly with a viewable image available in approximately 4 secs. The fast availability of digital images allows evaluation of positioning immediately, allowing repeat radiographs to be obtained if necessary. In digital radiology there is no film reader needed.

Lets review the advantages/ disadvantages of digital imaging. Our first question is , "What's wrong with conventional film screen imaging?". The answer is NOTHING!!!!!

Film screen is still the "GOLD STANDARD" that all new imaging systems are measured against. So then you ask, "Why should I change to digital?". Digital images offer other advantages. The important questions are what are the advantages and are they real? Real means that they manifest as advantages in your practice. An advantage to a large, multidoctor practice may not apply to a smaller, single doctor practice. First is speed: images can be ready to read in just a few seconds with digital radiography (DR). With computed radiography (CR) images are available in about the same time as with conventional film/ screen systems. The issue of speed is really only important if you perform a large number of radiographs per day. Not necessarily an advantage if you only have 3-4 radiographic exams a day. Second is technique and latitude. Fewer repeat films due to improper technique. Retakes due to improper technique costs both time and money. Positional errors require repeat exams with any system. THIS IS AN ADVANTAGE OF DR AND CR THAT APPLIES TO ALL PRACTICES. With film screen the latitude is completely dependent on kVp and mAs. There is a narrow margin for error. Digital is much less dependent on kVp and mAs, and more dependent on post-processing such as windowing, contrast, etc. There is a wide margin of error. Third is image transfer. Digital offers the ability to send images to other computers in the hospital (intranet) so you can show your client images in the exam room or review them in your office. This also allows multiple individuals to view the same images at the same time in different places. With digital you have the ability to send images to ANY specialist for consultation via the internet in seconds. It is also very easy to duplicate images in digital format for clients, other vets, etc. The fourth factor to consider is digital acquisition. It costs less over time as you don't have to buy film or chemicals, replace broken or old cassettes, maintain a processor, or have down time when your processor is being maintained or is broken. The fifth item to consider is digital storage. No lost films, no film degradation (in 10 years they won't look brown), they are stored forever on a hard drive, and requires less real estate in your practice for storage. Sixth is digital equipment. Both film screen and CR uses cassettes. These are easy to use for special views such as horizontal beams and intra-oral imaging. Cassettes can be used in multiple rooms/ hospitals/ also mobile. Veterinary DR systems are usually fixed in a particular location (NOT a requirement but HIGHLY recommended). The last item to consider is COST. There are upfront costs with all systems. Film screen – initially can be 20K-30K due to the need to purchase cassettes, processor, film, and chemicals. Upkeep of the processor, chemical disposal, and film costs will be between 10K and 40K annually. CR can start at 50K initial purchase price, plus may need computer monitors. There is some upkeep of the cassettes (which last between 5-10 years) as well as service contract on the laser film reader. DR plates are more expensive and may be up to $80K, plus may need computer monitors. Upkeep is the annual cost of the service contract. Don't forget will all three systems you have to purchase an x-ray machine and will have a service contract on it as well.