Practical considerations for image management.
by Ekram Kahn
Although the use of charting software in operatories is one of the primary reasons dentists choose to computerize their offices, the implementation of digital imaging hardware and software is the hottest trend in dental technology today. Film photography and intraoral cameras have been the foundation upon which the concepts of imaging in dentistry were developed. With the introduction of computers in the operatories, still photography has gone digital and intraoral cameras are increasingly mated to video-capture devices that integrate practice-management software with digital imaging.
The realistic implementation of the "paperless office" hinges on the successful application of a variety of digital imaging devices and software. Beginning with intraoral cameras and digital still cameras, dentists are utilizing digital X-ray systems and scanners to complete their tool set for imaging in dentistry.
This plethora of devices that generate images creates a distinct problem of acquiring and managing those images. The key to addressing this problem is to achieve seamless integration of hardware and software technologies. In so doing, it will not only facilitate image acquisition, but also allow dentists to easily blend all of this high-tech gadgetry into the busy workflow of their operatories. This is easier said than done. With the absence of standards that enforce device and software interoperability, the dentist is left with the daunting task of navigating through a technology jungle. No matter what category of imaging devices you consider, the manufacturers have different methods of interfacing with computers.
Types, options, and solutions
To gain an understanding of the problem and its solutions, let's take a look at the types of images that need to be acquired and the various options that are available. There are two types of images that are commonly generated in dentistry: color and grayscale. Intraoral and digital still cameras typically generate color images; digital X-ray systems generate grayscale. The video signal from intraoral cameras has a color depth that translates to 24-bit color (16.7 million colors or "True Color") when digitized. Depending on the manufacturer, digital X-ray sensors generate 10-bit and 12-bit grayscale images.
The size of these images in their digital form depends on the resolution in which they are generated and acquired. Whether they are of diagnostic quality depends on the video card in the computer and the capabilities of the display monitor. As you can see, there are many factors that determine the characteristics of an image as it travels through the stages of generation, acquisition, manipulation, and storage.
Intraoral cameras have become the foundation of imaging in modern dental practices. They generate a live, analog, full-motion (30 frames per second) color-video stream traditionally shown on a television display or printed by a dye sublimation video printer. Using an analog video network for the intraoral camera system created the need for managing printed images and didn't provide a viable means to archive or query images for presentation.
This problem is easily addressed by implementing a video-capture solution that would digitize and capture still-frame images from a live analog video source. Typically, a video-capture card such as the ATI All in Wonder was used to capture video from intraoral cameras and endodontic microscopes.
Recently, the ATI capture cards had compatibility problems with capturing video into several practice-management software programs that were installed on computers with the Windows 2000 operating system. Apparently, the software drivers for the ATI card were the issue. If you plan to use Windows 98, then you will not experience these problems. On the contrary, if you plan a new installation of computers with the Windows 2000 operating system, I recommend that you do not use ATI cards and instead use cards from Hauppauge, Winnov, or Osprey. All of these cards are compatible with the major practice-management software programs installed on a Windows 2000 system.
Let me address the ongoing debate about the image quality of analog vs. digital intraoral cameras. Regardless of whether an intraoral camera is analog or digital, the S-Video signal will have to go through a video-capture card. Any video signal that goes through a capture card will be reduced to a resolution of 640 x 480 pixels at a 24-bit color depth. Although some high-end capture cards can capture at higher resolutions, the software that typically is used in the dental industry often has a maximum still-capture resolution of 640 x 480 pixels. The only method by which to transfer a pure digital video stream to a computer is to use a camera that generates digital video and outputs that video via an IEEE 1394 port (a.k.a. Firewire). Most intraoral cameras currently do not have this capability. Also, dental imaging/charting software can't capture video from devices interfacing with a Firewire port. The dental technology industry may evolve in this direction, but your decision about an intraoral camera should be based on the quality of the optics and the durability of the camera. After all, the intraoral camera will have an S-Video output, which will connect to the S-Video input of the video-capture card. Therefore, choose the camera that generates the best image rather than entertaining the analog vs. digital controversy.
Film cans to flash cards
Still cameras for film photography have always been popular, and now their digital counterparts are part of the hottest trend in dentistry — digital photography. Film is out, memory chips are in! The first digital cameras on the market didn't have the required resolution to yield images comparable to film. But now, with cameras capable of three megapixels and higher, there is no reason to refrain from using one in your practice.
Although resolution, optics, and battery life are important features, a significant consideration is transferring the digital images from the camera to the computer. It involves removing the memory chip from the camera and then placing it into a reader that interfaces with the computer. Here lies the dilemma. There is a dizzying array of storage formats used by digital cameras, each having its own interface. Compact Flash (CF), Smartmedia, Memory Stick, and Secure Digital (SD) are the commonly used types of nonvolatile memory chips. A nonvolatile memory chip doesn't require power to retain its contents. Therefore, you can have several memory chips that can be used during the day just by swapping them out when full.
With CF cards, camera users can easily eject their "film" and transport the images via the PCMCIA Type II adapter card or a CF reader to either obtain prints or move the photos to another digital system such as a computer, Pocket PC, etc. One 16MB CF card can be used repeatedly for more than 100 years to take millions of pictures, and will store 30 or more digitally compressed images or photos. Higher capacity Compact Flash cards (currently up to 512MB) and Microdrives up to 1GB can store large numbers of images even with today's highest resolution digital cameras. Photos can be "developed" in seconds and transmitted or printed using high-resolution color printers. Color printers equipped with PC card slots or CF memory readers are now available. Although these printers will allow you to print directly from the digital camera without the use of a computer, they eliminate the benefits of storing your images on a computer.
SmartMedia chips are about one-third the size of a conventional PC card and only 0.76 mm thick. This new storage card is expected to help electronic devices — including the digital still camera and various forms of portable information equipment — become even smaller in size. The distinguishing features of the SmartMedia card are its negligible size and weight.
Choices, choices ...
While the majority of the market share belongs to Compact Flash and SmartMedia chips, new formats, such as Secure Digital and Memory Stick (SONY), look promising as future market leaders in nonvolatile storage because of their security features and obvious size advantages. With all of these choices, the question that begs to be answered is, "Which format is the best?" The answer for now would be Compact Flash and SmartMedia. Both formats are commonly available and their cost per megabyte is attractive. As soon as the market migrates to Secure Digital and Memory Stick format, their availability and cost per megabyte will put them at the head of the pack.
To leave your options open, you can use a universal memory chip reader/writer that connects to your computer via the USB (Universal Serial Bus) interface. Imation manufactures one that reads and writes to all of the major formats and is one of the smallest on the market. With this device, you will essentially be free to choose any digital camera on the market and not be concerned about the process of transferring images from the camera to the computer.
The S-Video output from intraoral cameras will continue to be the industry standard. Therefore, acquiring images will involve a video-capture card. All other peripherals like digital X-ray sensors, scanners, and input devices will incorporate the USB and, to a lesser extent, the IEEE 1394 "Firewire" interfaces. You might be wondering about the differences between the two.
Almost everyone who works with these various buses considers the 1394 a complement to USB, since it offers much higher speeds (up to 1.2 gigabit/sec.) and is designed for isochronous and asynch video/audio/data transfer. While USB is ideal for computer peripherals at speeds in the neighborhood of 12 Mbps, 1394 has a different mission. Many new PCs now include ports for both of these standards. Both USB and 1394 bus standards are upgrading to new enhanced versions that will allow significantly higher performance. The USB II standard specifies an effective speed of 480 Mbs — a 40-fold increase from the current specification. Also, 1394b is a significant enhancement to the basic 1394 specification that enables speed increases to 3.2 gigabits/sec., supports distances of 100 meters on UTP-5, can use plastic and glass optical fiber, and significantly reduces latency times by using arbitration pipelining. It is fully backwards compatible with the current 1394a specifications.
1394b is an important step forward in increasing the performance and simplifying the implementation of 1394 on PCs. With its long-haul capabilities, 1394b makes 1394 the convergence bus between PC products, CE systems, and home networking. The 1394 bus will support the high speeds necessary for digital video applications, but its adoption by most intraoral camera manufacturers is questionable. To date, Cygnus Technologies is the only intraoral camera manufacturer that offers a camera with a Firewire interface.
Digital X-ray image acquisition and management
In the past, digital X-ray systems presented the most difficult image-acquisition problems. Each manufacturer had a different method of capturing the image from the sensor. The CCD or CMOS chip in the sensor produces an analog signal that is digitized by the capture board that interfaces with the computer. Compatibility issues were common because these capture boards were proprietary and the process of bridging to practice-management programs was not stable. The major manufacturers addressed this issue by introducing sensors with USB interfaces. This greatly reduces the integration problems and positions digital X-ray technology as a viable long-term technology investment.
As opposed to color images generated by intraoral and digital still cameras, digital X-rays are grayscale images. Digital X-ray systems generate images that are between 10 and 12 bits. While a 24-bit color image has 16.7 million colors (2 raised to the 24th), a 10-bit grayscale image has 1,024 shades of gray (2 raised to the 10th). A controversy exists regarding what constitutes a diagnostic-quality digital X-ray. Some argue that it is the line pair per millimeter resolution of a sensor, while others say grayscale reproduction and dynamic range are more significant factors. No one factor is the determinant of diagnostic-quality digital X-rays. The following matrix can be used as a general reference when evaluating digital X-ray systems.
Many factors, such as the active area of the sensor and image readout rate, influence the real-world performance of a particular system. Often, we delve too deeply into the technical specifications of the product itself and ignore the complementary systems that also have an impact. An image can only be as good as the integrated computer/software systems that acquire the image from the sensor. The video display controller card, the manipulation software, and the actual monitor used will determine the perceived quality of an image. The sensor may generate an excellent image, and the computer and software will acquire that image, but the final determinant is the display.
Video display characteristics
Although much attention is given to absolute brightness in a display system, other factors have a great effect on perceived image quality. Among these factors are: contrast ratio, which defines the dynamic range of the display system; and resolution, which is the system's ability to display fine detail. Contrast ratio is often under-emphasized, even though it has a tremendous effect on perceived image quality.
Poor contrast ratio usually means the system exhibits a poor black-level performance. This is characterized as either an inability to display dark enough blacks — resulting in "washed out" images — or a loss of fine detail in the dark areas of the image.
Resolution of a digital display is specified as the number of horizontal and vertical pixels. For example, an SVGA imaging device is 800 pixels wide and 600 pixels tall. A larger number of pixels means the system can display images with finer detail. This also is referred to as the spatial frequency of an image. Areas of fine detail have high spatial frequency content.
Brightness, contrast, and resolution are interrelated. The human ability to resolve fine detail is affected by both the brightness and the contrast of the image. Simply selecting the brightest display with the highest native resolution is not necessarily the best choice for overall perceived image quality. While brightness is important to capture attention, contrast performance is most critical for conveying information.
Manipulation and storage
Image generation and acquisition naturally lead us to the topics of image manipulation and storage. The charting component of most practice-management systems can perform basic image capturing from cameras, but it doesn't have any manipulation functions.
You will need to integrate an imaging package such as ImageFX, DICOM Imaging Suite, or Vipersoft in order to manipulate and archive your images. These imaging packages are now adding the ability to capture digital X-ray images directly from the respective sensors. All of these programs can acquire images from a scanner, allowing you to become as paperless as possible. Therefore, you will no longer need to juggle several different software programs to seamlessly automate the process of generating, acquiring, manipulating, and storing digital images in dentistry.