Most surgeons have attempted photography of an interesting case at one time or another. Typically, 35mm slides are shot through a camera coupled with the operating microscope. Occasionally, just one print from each roll of film may be worth showing. Most often, picking up the dark, blurred, and often featureless results from the photography department brings a sense of frustration.
The digital revolution of the past five years has provided a multitude of consumer digital cameras with capabilities once the province only of professional photographers. The neurosurgeon with a working knowledge of computers, employing one of these modest cameras, can obtain spectacular intraoperative images far beyond the typically frustrating results obtained with film. This article will attempt to provide a basic guide to intraoperative digital imaging using today’s commercial digital cameras and the operating microscope. It is hoped that it will convey the simplicity of operation of today’s affordable equipment and the excellent results that can be achieved using it.
Anatomy of the Digital Camera
The heart of the digital camera is the CCD or charged-coupled device. This solid-state semiconductor consists of a thin matrix of silicon which has been sparsed into a grid of picture elements or pixels. These pixels act as light sensors. As photons of light strike a pixel, an electric charge is created. This charge is proportional in magnitude to the intensity of the sensed light. The resulting electrical charges are then transferred in sequence to an analog-to-digital converter which outputs an electronic rendition of the image which can then be stored or displayed with a computer.
Important Camera Features
There are a number of excellent cameras available on the market and the list grows longer each day with the addition of more sophisticated and higher resolution capabilities. Important features to consider when selecting a camera for intraoperative imaging include the CCD format or resolution, optical zoom capabilities, threaded lens, storage and transfer method and an LCD.
CCD resolution is critical for sharp, crisp output of the image to a monitor or when printing to slide or film. The more pixels present on a CCD chip, the higher the resolution. This is achieved by subdividing a chip into smaller and smaller pixels. The multi-megapixel cameras with greater than 2 million pixels (non-interpolated) are preferable.
Optical zoom refers to the ability of the lens to change focal lengths, thereby enlarging the subject image. Care must be taken in discerning optical zoom and digital zoom. Digital zoom refers to a camera’s ability to further enlarge the image by interpolating and adding new pixels to the image. The result is significantly inferior and has little usefulness for intraoperative imaging. The minimal optical zoom capability should be at least 3X.
Another important feature regarding the lens is the ability to take macro images, which are extreme “close-up” shots, usually within one foot or less. This feature is needed for through the microscope shots as well as for framing individual pictures of CTs or MRI films.
Lastly, the presence of threads on the camera’s lens allows significant flexibility in using alternative lens converters, filters, and coupling the camera to the microscope. A variety of accessories are available which screw onto these threads.
The method of image storage and ease of download refers to how the camera stores and downloads its pictures to the computer or printer. Various methods are employed by different camera manufacturers. These include fixed storage within the camera, 3.5 inch floppy discs, removable memory cards and, in some newer cameras, recordable CDs or miniature hard drives such as the IBM Microdrive. For ease of use and maximum flexibility as well as economic concerns, removable type media is preferred. Downloading to a computer can be accomplished via parallel, SCSI, orserial cable and in newer cameras via USB cable. USB is preferable due to the faster communication with the computer, but it requires a USB equipped PC or MAC. Most cameras that offer USB also offer serial and/or parallel connection options.
Most digital cameras offer a preview screen apart from the optical viewfinder. Preview screens are small LCD color displays built right into the camera. The small LCDs are essential in framing and composing intraoperative images. It allows you to preview what the camera will see before the picture is taken, as well as allow for fine adjustments during coupling of the camera to the microscope. In addition, they allow review of taken images to ensure the expected result. The optical viewfinder although useful for most general photography, has no value during intraoperative imaging.
Camera Recommendations
The ideal camera for intraoperative imaging should contain a high resolution megapixel CCD, optical zoom and macro capabilities, removable type storage media, threaded lens, color LCD, and remote capabilities. There are a number of excellent consumer digital cameras on the market for less than $1,000 which meet these capabilities. These include the Olympus 3000Z and 3030Z, Epson photo PC 3000Z, Cannon Powershot G1, and the Nikon family of cameras.
The Nikon family includes the 800, 950 and 990 models. The newer 880, although an excellent camera, requires an adapter in place of standard lens threads. The author prefers the Nikon 990 camera. This 3.34 megapixel camera can take images with a maximal size of 2038 x 1536. It has both full automatic and full manual settings. Its lens is a nine-element 3X optical zoom lens that contains 28mm threads and boasts the closest macro capability on the market. The camera uses CompactFlash cards and has a 1.8-inch LCD. It has USB and serial capabilities as well as a TV video output. Lastly, it can be controlled remotely with an optional wire remote control.
Unlike 35mm SLR cameras, commercial digital cameras are equipped with lenses that are not removable. In order to photograph through the microscope, one must employ a configuration termed afocal coupling. Afocal coupling is a method of photography frequently used by amateur astronomers for imaging the planets; the camera’s lens is lined up with the eyepiece of the telescope or, as in our case, the microscope.
Photographing afocally can by done by simply holding the camera up to the eyepiece. Take care to hold the camera as close as possible to the eyepiece and center the image on the LCD view screen. Use the zoom capability of the camera to zoom in on the image to enlarge the field of view and minimize vignetting (when the image from the eyepiece does not fill the cameras field of view, resulting in a circular image that does not reach the corners of the picture frame).
Although this method generally works, it is a less than ideal setup. A more stable configuration involves threading an appropriate adapter to the camera lens allowing it to be mounted on the microscope eyepiece throughout the case. The specific thread size must be known for the type of camera used, and the adapters can be purchased from photography or astronomy vendors.
or the Nikon 990, the thread size is 28mm, and the adapters required are a 28-to-T thread step up ring, and an eyepiece projection adapter. The step up ring is connected to the camera lens followed by the eyepiece projection adapter. The entire assembly is then secured to an unused eyepiece on the microscope via three thumb-screws on the eyepiece projection adapter. A remote control cable can be hooked up to the camera and secured near the microscopes control arm, where the surgeon or assistant can reach it during the case.
File Types and Storage
There are several standard formats for storing images on a computer. Many cameras can be set by the user to different levels of quality and resolution. The degree of resolution needed depends on the finaal output desired. Some rules of thumb are listed in the Resolution Table. The greater the degree of resolution, the larger the file that must be stored on the computer. In order to store images more compactly, various file compression formats are in use. The two most common are the tiff (tagged image file format) and jpeg (joint photographic experts group). When taking images, it is recommended that the highest resolution be used. This will allow flexibility if at a later date one wishes to output the images for print, 35mm slide, or reproduction for publication. TIFF format is also preferable to JPEG.
Summary
Intraoperative imaging through the microscope need not be restricted to 35mm film. The digital revolution has placed in the hands of consumers a variety of equipment with capabilities of a professional level. As this article has briefly outlined, using a relatively simple setup and modest equipment, spectacular images can be recorded with a minimum of effort.
Carlos A. David, MD, is a neurosurgeon at Lahey Clinic Medical Center, Burlington, Massachusetts