Basic physics and principles of making a great image: Part 1 (Proceedings)

Aside from ultrasound, a digital radiography suite is going to be the most expensive upgrade for the private practitioner in the realm of diagnostic imaging. However, this expense will be well worth the investment from a diagnostic imaging standpoint. After more than a century of film and film screen imaging, the backdrop of diagnostic radiology has changed from hanging films on view boxes to LCD monitors.

X-ray Film (analog)

X-ray film has been the mainstay for diagnostic imaging for over 100 years. It provides a wide range of latitude (gray scale presentation), contrast, speed and detail options and is a durable, portable storage medium. However, it degrades over time, is not easily duplicated and requires physical storage. So why go digital? The digital era is upon us and the veterinary market is ultimately going to be dictated by what is happening in the human market. This trend is not going to go away and film screen combinations will be more difficult to get. A major disadvantage of the film era is the developmental process, which can totally destroy great radiographs (even that had the proper exposure factors and anatomic positioning). In the analog film system, the image capture, processing and the storage of the image are all contained on the film. In digital radiography, these processes are separated. Raw data can be re-processed at any time and reviewed in a variety of formats based on the viewing software and stations used.

What are the digital advantages?

Although multiple advantages of digital radiography (DR) and computed radiography (CR) have been sited, each of these advantages may not be directly applicable in the veterinary market and may not make sense for return on the investment for a given practice. Digital radiography is any digital image acquisition technique where the images are captured, reviewed, processed and stored in a digital environment (computer based). The advantages of digital radiography include: rapid acquisition (with review of images in 4 to 6 seconds for digital radiography plate systems), greater case through put; fewer repeat radiographic exposures (in theory) due to incorrect exposure (although positioning and centering errors still must be corrected for by repeating the radiograph). Additionally, there is a greater dynamic range (more latitude than a gray scale film), advanced processing algorithms (edge enhancement techniques) and the ability manipulate the images. Less storage space is needed, no more lost films (hard drive or optical media storage), no image deterioration over time and no more film chemistry is required. Digital images are easy to duplicate and move around in the cyber world and can be used as a suitable medium for easy transfer of images (such as electronic film interpretation).

Image Formats

Image acquisition consists of an acquisition station within the radiology suite. Pre-processing algorithms are used that have been established by the manufacturer such that the initial raw data is "filtered" and the image presented for initial review rapidly. It is silly to think that all of the quality control goes away with the digital era. In fact, if one does not quality control their radiographs currently, the image quality in digital era will be just as bad. The basic image format is called DICOM and has a .dcm tag associated with the file. DICOM stands for digital imaging and communication in medicine. This was an industry agreed upon standard and currently the American College of Radiology is using DICOM 3.0. Within each image, all patient data is saved along with the image data. These images are not secure, encrypted or protected (that is why it is an industry standard). Do not send images in .jpg (photography file format that allows for lossy compression) but in DICOM format for review in teleradiology or if saving images to a CD, etc. There are several freeware software packages for review DICOM images. These include Dicom works (PC) and Osirix (Mac).

Digital radiography systems

There are currently three basic types of digital radiography systems. Digital cameras or digitizing images of films will not be discussed as this is not digital radiography! These systems include: hybrid systems, computed radiography systems, digital radiography systems (direct and indirect).

Hybrid radiography systems

In hybrid systems, the camera set-up is a CCD system where a CCD chip takes the place of the film screen combination in image detection and capture. A phosphor plate is under the table at the level of the grid and converts the x-rays to light information. The light is then transferred via optical coupling (mirror and lens system so that the image is minified) to a CCD chip that converts the light image to an electronic image. The image is then pre-processed according to the body part (thorax vs. abdomen vs. musculoskeletal) and displayed on the LCD monitor in the radiology suite. The image is then quality controlled and patient released. The images can then be sent to the PACS system (Picture Archival and Computer System) for final display, review, interpretation, and storage. Several hybrid systems exist in the veterinary market, but the FOVEA® system appears to be a good option (Fovea 9X; 17" x 17" FOV; 4.0 lp/mm; 9 MP) for just under 70K. Since this is a CCD system under the table, the complete radiology room is changed out for the above price. A similar system is available through Sedecal (Vet Ray).

Computed Radiography (CR)

CR systems were developed in the 1980's and the film screen combination is replaced by a photostimulable storage phosphor (PSP) that is loaded in a protective cassette. The cassettes are the same size as the older film/screen cassettes so there is no table modifications or retrofitting that needs to take place. The x-ray information is then converted to electron energy stored in the PSP in electron traps. The CR cassette is then fed into a readout machine where the PSP plate is pulled out of the cassette and laser optics are used to scan and read the PSP while resetting the entire PSP for the next exposure. The electronic signal is captured so that the output is directly correlated to the x-ray input that was captured by the electron traps of the PSP. The work station then consists of an image that can be manipulated and sent to PACS. The read out time for a 14" x 17" cassette is 20 seconds. The throughput is on the order of 50 to 70 plates per hour. CR is the cheapest of the Digital era alternatives (40 to 60K) and there are several systems worth evaluating today. These include: Orex/Idexx, Fuji, Konica, 3M and Kodak film systems. CR has a broad dynamic range, excellent resolution and image manipulation algorithms that are equivalent to DR.

Digital Radiography

Digital radiography systems can be divided into direct and indirect systems. Currently, Orex/Idexx is the only direct DR system available in the veterinary market today and is priced around 100K. In direct DR the x-rays are directly converted to an electronic signal without a light intermediate. Semiconductors detect the x-rays and create a small charge or current that is detected by thin film transistors (TFT) converting the charge or current to an electrical signal. Excellent image quality, resolution is the trade-off for an expensive system. Indirect DR systems use a scintillation x-ray detector plate intermediate where the x-ray information results in a light flash from the scintillation plate that is then registered by the TFT layer from a light flash to an electrical signal. The electrical signals are then processed into an image. In DR, the imaging plate is placed within the old cassette tray cabinet with some modifications being required depending on the different DR system and the type of x-ray cassette holders. DR images have excellent dynamic range, resolution and are ideal for the impatient (image available within 4 to 6 seconds). This does still not get rid of quality control as DR image artifacts do occur and there is still a range of receptor exposures that are not acceptable (noisy images vs. overexposed or saturated imaging receptors). Common indirect DR systems available today include: Sound-Eklin (Canon Plate; 65K) and TruDR® (Varian Plate; 55K). Both of these systems use a GdOxS and amorphouse silicon based indirect scintillation plate. Vet Ray also offers complete imaging systems with Canon detectors for DR units.

What is PACS and what does it take to get running?

What happens once the image is made? PACS is a picture archival and communication (computer) system that allows for retrieval of image data, review, manipulation, interpretation and storage. A good PACS system has three key components. The first is a central storage server that is on site with dual redundant off-site back up for when the hard drive crashes. Notice the operative word is not IF but WHEN! The second component is a good review workstation that includes several high grade, high resolution (1 to 2 MP) gray scale monitors. The final component is a high quality, dependable viewing software for evaluation, review and manipulation of images. Use of ExamPacs® from CoActiv is software that has been reliable and provides a PACS alternative to the standard veterinary vendors. There are free PACS viewers and solutions such as K-Pacs (www.k-pacs.net) and Clear Canvas (www.clearcanvas.ca). Teleradiology, using companies such as Antech Imaging Services (www.antechimagingservices.com) or PetRays (www.petrays.com), can provide one access to "instant" interpretations and are based on having a good 10/100 GBit infrastructure with at least a DSL or cable modem high speed connection of the hospital to the outside world.

CCD vs. CR vs. DR?

What is right for your practice depends on the throughput, the quality control issues, the service for the equipment in the area and number of cases you are doing on a daily basis. DR and CCD systems in the human market are hard to justify if one is not taking over 50 studies a day in a given room. In an emergency practice setting, a DR or CCD system may make a lot of sense. CR systems are reasonable alternatives that are cheaper. Beware of any DR system less than 40K and any CR system less than 35 K. Although prices have been dropping, digital systems for less than 20K are not proven and you get what you pay for.