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Practical digital photography for surgeons
TECHNOLOGY FOCUS
Practical Digital Photography for Surgeons William B. Schroder, MD Section of Vascular Surgery, UMKC School of Medicine, Truman Medical Center, Kansas City, Missouri As a surgeon, you know the value of an image. Every clinical journal is filled with diagnostic images, operative photographs, and pathology slides. Over the years, we have collected and badly maintained our collections of 35-mm Ektachromes, expensive custom slides, and off-tone homegrown “pseudodiazo” text slides. I kept an expensive, motor-driven camera body, heavy 100-mm macro lens, and ringlight in its own bag locked in my file drawer waiting for interesting pathology to crop up. I missed some good images because the setup was cumbersome to lug around. I became hooked on computers as tool and toy around 1988, and I observed bemusedly when the first expensive but weak digital cameras hit the market. The thousand-dollar, millionpixel barrier was a long time coming down, but when it did, I was there (mostly out of morbid curiosity). My first digital camera was a Kodak DC-120 (Rochester, New York). This oddly shaped camera was slow and power hungry yet offered 1-megapixel resolution and decent image quality. Although suitable for PowerPoint (Microsoft Corporation, Redmond, Washington), enlargements were grainy and I was not yet ready to abandon film. I ordered a Nikon Coolpix 950 (Nikon Corporation) shortly after its introduction, and had to wait weeks on backorder for it to arrive. With its 2-megapixel resolution and outstanding image quality, it quickly became my everyday camera. Its small size and superior macro capabilities allowed me to use it easily in the operating room, and I soon had a CD-ROM collection of hundreds of surgical images that made even impromptu “Grand Rounds” presentations a snap to assemble. At home, even 8 ⫻ 10 enlargements from the 950 were good, and the ability to manipulate the images in Photoshop (Adobe Systems Incorporated, San Jose, California) and control the output made it superior to film for me. In 1999, I swallowed hard and traveled to Ireland with the 950 as my only still camera. Daily emails of JPEG files “back home” were well worth lugging my clunky laptop across the Atlantic (although international dialup service can be pricey). I ordered (and waited) for a Nikon Coolpix 5000 the day it was announced. This 5-megapixel camera is now my “family” Correspondence: Inquiries to William B. Schroder, MD, Section of Vascular Surgery, UMKC School of Medicine, Truman Medical Center, Surgery Administration, 2301 Holmes, Kansas City, MO 64108; fax: (816) 855-6084; e-mail: william.schroder @tmcmed.org
camera, but the 950 stays at work and gets regular use. My colleagues and I own at least 10 Coolpix 9xx cameras (the 2-megapixel 950 and the similar 3-megapixel 990 and 995), and a growing number have purchased the 5000. Many of our professional journals accept digital images (often preferentially) in lieu of the traditional “5 ⫻ 7 glossies.” Even our hospital is going filmless: I can examine radiographs on my desktop PC only moments after they are done, and dictate vascular ultrasounds from my 21-inch “viewbox.” The viewer software allows image capture without having to take a photograph from x-ray film. Is film-based photography dead? For surgeons and our practices, it is. It is time to embrace the digital world and its tools. This article will cover the concept of digital images, image generation, file formats, image manipulation, image storage and retrieval, and how I use digital photography in my surgical practice.
DIGITAL IMAGES: THE PIXEL AND IMAGE SIZE The pixel (short for picture element or picture cell) is the equivalent of the silver “grains” from film-based photography. Understanding and manipulating the individual pixel is fundamental to working with digital images. Like tiles in mosaic artwork, images are built of pixels. This image of Abraham Lincoln (Fig. 1, courtesy of Jelly Belly Candy Company, Fairfield, California) is a jellybean mosaic. Progressive magnification of the image reveals the individual jellybeans and then the individual square pixels that comprise each bean. Remember the “Lite-Brite” Toy? We created images 1 colored peg at a time. Pixels are generated by a photosensitive chip. The chargecoupled device (CCD, Fig. 2) is the most common photosensor for digital cameras. The photosensors on the chip are arranged in arrays of rows and columns, and each sensor is covered with a microlens and filter to create a single pixel in 1 of the 3 “additive” colors (red, green or blue, “RGB”). New CCDs (not yet readily available) allow each pixel to record full-spectrum color. A discussion of CCD technology is beyond the scope of this paper, but digital photography magazines cover this topic well. Pixel densities (the number of photosensors per square centimeter) are growing with a similar curve as microprocessor
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FIGURE 1. Progressive magnification of this jelly bean mosaic demonstrates the “pixel” nature of a mosaic, and then the pixel nature of the individual beans. ([r]Reg. TMs of Jelly Belly Candy Company. Used with permission from the Jelly Belly Candy Company.)
transistor densities (Moore’s Law states that the number of transistors on a chip will roughly double every 18 months). The size of a digital image is expressed as the number of pixels (megapixels, MP, millions of pixels) or pixel dimensions (as pixel counts in length ⫻ width). Thus, an image that has 1800 rows and 1200 columns has 2,160,000 pixels, or 2.16 MP. It is amazing how few pixels actually comprise a television image (approximately 160,000). Table 1 gives image size information from some well-known sources. The 35-mm slide has been the benchmark to measure digital image progress. Estimates vary, but the amount of useable data that can be obtained from a 35-mm slide is just over 5 MP. The current crop of affordable (sub-$1000) consumer-level cameras is producing images in the
4 to 6 MP range. I believe the era of film-equivalence is here, for most of us. However, super fine-grain films like Kodachrome-25 (Kodak) can probably produce higher pixel counts, and most studio work is done with medium-format or larger. These 6 ⫻ 6 (cm) films have over 4 times the area of a 35-mm negative. That would place the pixel count for mediumformat film around 21 MP.
DIGITAL IMAGES: DATA SIZE AND FILE SIZE To store a pixel electronically, the computer file needs to store more both the location and the color of each pixel. Bits, bytes, and the like are beyond this discussion, but a pixel can be pure
FIGURE 2. The charge-coupled device and its accompanying circuits create the pixels for an image. 582
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TABLE 1. Image Size ⫽ Rows ⫻ Columns, Expressed in Millions of Pixels, Megapixels, MP Source Television HTDV (analog, USA) First Hubble Camera Computer Screen Kodak DC-120 Nikon Coolpix 950 Nikon Coolpix 995 Nikon Coolpix 5000 35-mm Color Slide New Hubble Camera Human Eye
Image
Millions of Pixels
⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻
0.16 MP 0.375 MP 0.48 MP 0.79 MP 1.23 MP 2.16 MP 3.15 MP 4.92 MP 5.22 MP (est.) 16.8 MP (2 ccd’s) 120 MP
320 600 800 1024 1280 1800 2048 2560 2760 4096 11k
525 625 600 768 960 1200 1536 1920 1890 4096 11k
monochrome (either a black dot or a white dot), grayscale, or varying degrees of color. Although a bit simplified, each pixel can be stored as 1 byte if it is monochrome, grayscale or up to 256 colors, 2 bytes if up to 65,000 colors, or 3 bytes if it uses “true-color” (16 million colors). Therefore, the data size of an image is equal to the length ⫻ width ⫻ color depth. For example, a 2.16-megapixel image (1800 ⫻ 1200 pixels), in true-color (3 bytes per pixel), would have a data size of 6,480,000 bytes (6.3 megabytes, MB, remembering that a megabyte is 1024 bytes). The image data are captured and stored in a digital file in one of several file formats. The most important concept of file formats is compression. Mathematic algorithms are applied to the image data to store it as a file. Some algorithms can compress (shrink) the image data into a smaller and more easily stored and transferred file. Extreme compression is possible if some “less important” image data are discarded. This type of compression is called “lossy.” Lossy compression algorithms offer variable compression percentages based on the degree of compression desired (and the amount of image data irretrievably discarded). Table 2 shows some common file formats and the file size that is produced when the image data is stored. Many more file formats are available (try a web search on “file format
information”). For practical reasons, use a file format that meets 3 needs: your software supports it, those you share files with can open and view it, and it suits your storage, archiving, and retrieval capabilities. Professionals tend to work with TIF files, because the format is well supported and no image data are lost with multiple manipulations and file-saves. If the same JPG file is repeatedly “tweaked” and resaved, there can be progressive data loss. This does not happen if you just view the file, just if you keep hitting the “save” button. I work most of the time with high-quality JPG files, and do a few things to combat potential data loss. I take all my images using my camera’s “fine” setting (most cameras let you choose that your images be saved uncompressed (as TIF files, for instance, or with 1 or more levels of JPG compression). The fine JPG setting provides an excellent balance between storage on the memory card and quality. I then burn these images onto a CD-ROM before doing any manipulations. I burn at least 2 copies of important images, and keep 1 copy at work and 1 copy at home. I am then free to manipulate, crop, recolor, and so on these images without fear of losing the original data. I save these “improved” files only once and then burn these corrected files onto another CD-ROM. I then have CDs labeled “Raw” and “Tuned”.
GENERATING IMAGES: CHOOSING A CAMERA There are many fine digital cameras, but for the practicing surgeon, the most important camera characteristics are portability, macro capability, copy-stand use, and the ability to copy (digitize) an existing slide collection. I will focus on the Nikon Coolpix 9xx series (950, 990, and 995) because they fulfill these ideal characteristics so well. I think these are the best “Medical” cameras available. (I have no financial conflict-of-interest regarding Nikon.) I own a 950, but it has been supplanted by the 990 and 995. I will focus on the 995 (Fig. 3), which (like the 950) creates a 3.15 MP image (2048 ⫻ 1538) of excellent color, sharpness, and dynamic range. Multiple automatic modes plus full manual controls are available. On-camera flash is modest
TABLE 2. File Size Produced When Image Data Are Stored. The Original Image Was 2560 ⫻ 1704 Pixels With 16 Million Colors* File Name
Extension
Windows Bitmap Tagged Image File Tagged Image File
BMP TIF, TIFF TIF, TIFF
Portable Network Graphics Macintosh Picture Graphics Interchange Format Joint Photographic Experts Group Joint Photographic Experts Group Joint Photographic Experts Group
PNG, PING PICT GIF JPG, JPEG JPG, JPEG JPG, JPEG
Compression None None Modest (LZW) Lempel-Ziv-Welch Modest Modest Yes Yes, Modest Yes, Moderate Yes, Marked
Data Loss None None None None None Possibly** Yes, not usually noticeable Yes, sometimes noticeable Yes, often noticeable
File Size 12.8 MB 12.8 MB 8.9 MB 8.4 8.2 2.9 1.2 576 357
MB MB MB MB KB KB
* Original photograph taken by a Nikon Coolpix 5000 saved by the camera in its “Fine” JPG mode. File conversion and saving done by Adobe Photoshop Elements (©2001, Adobe Systems Incorporated) ** The Graphics Interchange Format (GIF) while technically “lossless,” is limited to only 256 colors. Therefore, if you save a true-color photograph in GIF format you are discarding color data. CURRENT SURGERY • Volume 59/Number 6 • November/December 2002
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FIGURE 3. The Nikon Coolpix 995.
but very serviceable for operating room photography, and external flashes can be connected. Magazines rate the images highly, although not always the best. The “twist-body” construction allows the camera to fold flat and fit easily in a labcoat pocket, yet still allow enough lens-travel to give good (4⫻ optical) zoom range, and “best of breed” macro capability without the “front-to-back” bulk that makes other cameras difficult to pocket. The lens can focus to approximately 2 cm (0.8 inches) from the lens! This closeup capability produces outstanding operative photographs and allows an inexpensive (approx $70 street) screw-on attachment that copies slides and negatives with ease (more below). The tripod socket on the bottom allows easy copy-stand use.
GENERATING IMAGES: LIVE, SLIDE COPY, COPY STAND Use of a camera in the operating room requires ease-of-use, so that you or an assistant can take photographs quickly with a minimum of disruption to the flow of the surgery. Using an extra pair of sterile gloves, surgeons can hold the camera away from their bodies and twist the lens down to the field while composing the image in the LCD (Fig. 4). Alternatively, assistants can photograph over the surgeons’ shoulders while holding the camera at arms length, see the field through the LCD, and take some shots without any disruption to the procedure. The slide copier attachment (Fig. 5) means you do not have to purchase an expensive, fussy, slow (30 seconds or more per slide) device to do what the slide copier does in a few seconds. Remember that the typical 35-mm slide contains about 5.2 MP of data, so slide copies with the Coolpix 995 (3.15 MP) are close. Use the Coolpix 5000 (4.9 MP), and you lose nothing by copying and tossing your slide collection. (Do not forget that the average projected PowerPoint image needs less than 0.8 MP, see below.) I set the camera on macro, indicate the light source as fluorescent, place a slide in the holder, hold the adapter up to an x-ray view box, and push the shutter release. I 584
FIGURE 4. Use of a digital camera in the operating room: The twistaction of the 9xx series (950 shown) cameras allows holding the camera well away from the face while still composing good images. The macro mode produces superior closeups.
can copy about 6 slides per minute with this technique. Certain poorly exposed slides may do best with some on-camera exposure compensation (the Coolpix 5000 can do auto-bracketing.) Digitizing computed tomography scans, plain films, and angiograms is essential for making talks and discussing cases online, and a copy stand is the best way to go about making them. Although Fig. 6 shows a high-end, pricey copy stand, an old x-ray view box and a $75 copy stand (see http://www.adorama. com) will produce the same images. While on the subject of copy-stand use, I find that photographing textbook images (minding all necessary fair-use statutes) produces better results than does flatbed scanning. Moire´ patterns are a problem when scanning halftone images. Halftoning is a technique for producing continuous-tone-appearing images with only solid ink colors, and it is used in most publications. Optical interference creates bands of light and dark (the Moire´) on the output file. Software manipulation and filters can minimize this problem, but I find that a digital camera is less subject to this problem than is a scanner, and it usually prevents the problem in the first place.
FIGURE 5. The Nikon Slide Adapter is shown here attached to a Coolpix 950 camera. It also attaches to the 990 and 995. With an adapter, it will attach to the Coolpix 5000.
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FIGURE 6. Use of a copy stand to digitize x-rays.
IMAGE MANIPULATION: THE DIGITAL DARKROOM Once you have your raw image file safely tucked-away on CDROM, its time to “tune” the image to your needs. Image-manipulation software abounds: As far as I can tell, every camera ships with one or more software packages: some “lite” or limited versions, some “full” but outdated versions (upgrade available at added cost), and some full-fledged, current packages. There are shrink-wrapped, shareware, freeware, and demo versions aplenty. Some are part of suites that let you burn data, video, image, and music CDs. There is even image manipulation built into most Windows versions (Microsoft Photo Editor in Win-
dows XP). In general, almost any package will let you do the basic functions: Crop, zoom, rotate, and adjust contrast and brightness (gamma). Some packages can automate this for you with “picture-fix” and “red-eye” macros and wizards. The granddaddy of all image packages seems to be Adobe Photoshop (about $590 street). If Photoshop (with or without one of its 3rd-party add-ins, called “filters”) cannot do it, it probably cannot be done. For the lighter of wallet, Adobe Photoshop Elements (under $70 street) has almost everything the parent program contains and some wizards and other “ease of use” features that it does not (Fig. 7). For first-time users, the learning curve for Photoshop products may be a bit daunting, but it is worth the climb. My suggestion is to either use what comes free with your camera or just order Photoshop Elements.
IMAGE OUTPUT: SCREEN, EMAIL, POWERPOINT, PRINTER, PHOTOFINISHERS, JOURNALS Because an image is meant to be used, how you manipulate and save the image should be geared toward how you plan to offer its output. Because you already have the original image tucked away on a CD-ROM (don’t you?), you can tailor your image to the output you plan to use. Many people display their images on
FIGURE 7. Adobe Photoshop Elements has almost all of the features anyone needs for image-editing. CURRENT SURGERY • Volume 59/Number 6 • November/December 2002
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their computer screen, and many users still have screen resolutions of 800 ⫻ 600 or 1024 ⫻ 768. Most computer projectors are likewise limited to 1024 ⫻ 768 pixel resolution. “Power users” with 19- and 21-inch monitors may use screen resolutions of 1280 ⫻ 960. Still, that “high-end” screen resolution uses only 1.2 megapixels, and the newer cameras are taking 4 to 6 megapixel images! Using a higher resolution image on monitors and projectors will result in 1 of 2 things: The monitor will display only part of the image, and the user will have to “pan” the image to see its entirety, or the software will “shrink-to-fit” the image so it can all be seen. The former is fine if you are “tweaking” the image but is a pain if you are trying to view it. The latter results in larger files to move around and slower display times, whereas the software crunches the pixels. A special note on PowerPoint presentations: You are probably best to assume that your presentation will run on a 1024 ⫻ 768 projector, and if you are including some text along with your image, you will probably only need an image that is 800 ⫻ 600. When you insert a large image into PowerPoint, you “drag” the corners to shrink the image to fit. However, PowerPoint stores the entire image, and it has to resize it every time it is displayed. The two consequences of using large images in PowerPoint are that the size of the presentation rapidly enlarges, and the presentation slows down while the host computer (sometimes much, much slower than the one you write the presentation on) reads the large file from the (slow) CDROM and then resizes the image “on the fly.” For images I am going to project with PowerPoint, I first resize them to 800 ⫻ 600, and then increase their brightness and contrast to make them project better. I then save them as mid-grade JPG files to save space, and append “-ppt” to the file name (“myfile.jpg” becomes “myfile-ppt.jpg”). This way I know that the file has been developed and tuned for a PowerPoint presentation. When emailing images, I always assume that the recipient has dialup service, and do not attach too many or too large images. Unless the recipient has requested a high-resolution image, I send lower quality JPG files in resolutions of 640 ⫻ 480. Also, do not forget your upload times because even “fast” non-dialup services like DSL, Cable, and Satellite are asynchronous and upload more slowly than download. Many surgeons belong to “listserv” broadcast mailing lists. These are very effective tools for sharing information and questions rapidly across the country. However, it is considered poor form to attach image files to broadcast mailings. Most listservs have a common website where the moderator will post an image on a web page for interested individuals to view. When it comes to hardcopy output (desktop printing, photofinishers, or journal submission), the working term is “dots per inch” (dpi). The digital image contains rows and columns of dots. How tightly packed together these dots are placed on the output paper determines the size of the final print. For example, an image with a resolution of 1800 ⫻ 1200 pixels will be 6“ ⫻ 4” (1800/300 ⫻ 1200/300) when printed at 300 dpi, (3“ ⫻ 2” if printed at 600 dpi, etc.). The dots-per-inch is sometimes 586
specified within the image, but it is easily changed within the generating program or overridden by the printer’s software driver. Many programs assume screen output (the average monitor has a dpi of 72 to 96 dpi). If the image above was printed at 72 dpi, the output would be 25“ ⫻ 17”! Although 300 dpi was the “holy grail” of laser printers a few years ago, 600 or 1200 dpi is the laser standard today. Most inkjet printers print at 720 dpi or higher, and some claim 1440 dpi in at least the horizontal direction. Personally, I really cannot see any practical difference beyond 600 dpi, and current limits on storage, data transfer, and processor power make handling images beyond 600 dpi impractical. For instance, an 8 ⫻ 10 image, at 600 dpi, in “true color” has a raw data file of over 86 megabytes. (Remember that a 35-mm slide has around 5 MB of data available.) PhotoShop manipulations are slow on images of this size, even on a highend consumer system. What happens when you take a 2.2 MP image (1800 ⫻ 1200) and ask your printer to make an 8 ⫻ 10? At this size, the image needs between 150 and 180 dpi to print, so your fancy 720-dpi printer will wind up using about 16 drops of ink to make a single dot on the page (720/180 ⫽ 4, and 4 ⫻ 4 ⫽ 16). Smart software or printer drivers will vary the colors of these 16 droplets to smooth the transitions between pixels, which increases the perceived quality of the printed image, but adds no data. This technique is called “interpolation.” Well-done interpolation can make a very acceptable 8 ⫻ 10 printout even from a 2.2 MP image. I take my 1800 ⫻ 1200 Coolpix 950 image and carefully hand-tune it with PhotoShop into a 600 dpi, 6000 ⫻ 4800 pixel file, with judicious application of filters to improve brightness, contrast, sharpness, and image coloration (tone). I then either print it on my desktop printer or send it to an online service. Digital photofinishing is now common. What began on the Internet has found its way into the traditional “one-hour” labs. Get them the digital file (either via upload for the online services, or handing them a CD-ROM or memory card for the local laboratories), and they produce the same print they would produce from a film-based image by projecting the digital file onto photosensitive paper. These images have the same quality and durability as those done from film. I rarely print photographs from my desktop printer, as those done by an online photofinisher are better and usually cheaper. If you wonder if digital photography has finally “won” over film-based photography for the average amateur, you need look no further than Kodak. My favorite online digital photofinisher, Ofoto.com, was recently bought by Kodak!. Medical journals have accepted digital manuscripts for years, but many still wanted multiple 5 ⫻ 7 glossies for their printers to use. Some still want the glossies for use as “reviewer’s copies,” but increasing numbers will accept JPG files, allowing digital submission of manuscripts via email with Microsoft Word and JPG files. This is, in fact, how this article got into print! Few journals print images much larger than 2 ⫻ 3 inches, so even at
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600-dpi printing, the file only needs to be 1200 ⫻ 1800 (2.2 MP). Because most digital cameras now exceed 3 MP, it is easy to produce acceptable images for journal submission.
SUMMARY Digital photography is now sufficiently capable of replacing film-based imaging for the average amateur photographer. It
should replace film-based photography for the surgeon. Understanding the pixel and how it is generated, manipulated, saved, and output is essential to successful digital photography and imaging. The surgeon’s camera should be especially capable in the areas of portability, use in the operating room, close-focus abilities, slide-stand use, and 35-mm slide copying. Archive your original images onto CD-ROM, hand-tune them specific to the eventual display medium, and enjoy!
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Practical Digital Photography for Surgeons William B. Schroder, MD Section of Vascular Surgery, UMKC School of Medicine, Truman Medical Center, Kansas City, Missouri As a surgeon, you know the value of an image. Every clinical journal is filled with diagnostic images, operative photographs, and pathology slides. Over the years, we have collected and badly maintained our collections of 35-mm Ektachromes, expensive custom slides, and off-tone homegrown “pseudodiazo” text slides. I kept an expensive, motor-driven camera body, heavy 100-mm macro lens, and ringlight in its own bag locked in my file drawer waiting for interesting pathology to crop up. I missed some good images because the setup was cumbersome to lug around. I became hooked on computers as tool and toy around 1988, and I observed bemusedly when the first expensive but weak digital cameras hit the market. The thousand-dollar, millionpixel barrier was a long time coming down, but when it did, I was there (mostly out of morbid curiosity). My first digital camera was a Kodak DC-120 (Rochester, New York). This oddly shaped camera was slow and power hungry yet offered 1-megapixel resolution and decent image quality. Although suitable for PowerPoint (Microsoft Corporation, Redmond, Washington), enlargements were grainy and I was not yet ready to abandon film. I ordered a Nikon Coolpix 950 (Nikon Corporation) shortly after its introduction, and had to wait weeks on backorder for it to arrive. With its 2-megapixel resolution and outstanding image quality, it quickly became my everyday camera. Its small size and superior macro capabilities allowed me to use it easily in the operating room, and I soon had a CD-ROM collection of hundreds of surgical images that made even impromptu “Grand Rounds” presentations a snap to assemble. At home, even 8 ⫻ 10 enlargements from the 950 were good, and the ability to manipulate the images in Photoshop (Adobe Systems Incorporated, San Jose, California) and control the output made it superior to film for me. In 1999, I swallowed hard and traveled to Ireland with the 950 as my only still camera. Daily emails of JPEG files “back home” were well worth lugging my clunky laptop across the Atlantic (although international dialup service can be pricey). I ordered (and waited) for a Nikon Coolpix 5000 the day it was announced. This 5-megapixel camera is now my “family” Correspondence: Inquiries to William B. Schroder, MD, Section of Vascular Surgery, UMKC School of Medicine, Truman Medical Center, Surgery Administration, 2301 Holmes, Kansas City, MO 64108; fax: (816) 855-6084; e-mail: william.schroder @tmcmed.org
camera, but the 950 stays at work and gets regular use. My colleagues and I own at least 10 Coolpix 9xx cameras (the 2-megapixel 950 and the similar 3-megapixel 990 and 995), and a growing number have purchased the 5000. Many of our professional journals accept digital images (often preferentially) in lieu of the traditional “5 ⫻ 7 glossies.” Even our hospital is going filmless: I can examine radiographs on my desktop PC only moments after they are done, and dictate vascular ultrasounds from my 21-inch “viewbox.” The viewer software allows image capture without having to take a photograph from x-ray film. Is film-based photography dead? For surgeons and our practices, it is. It is time to embrace the digital world and its tools. This article will cover the concept of digital images, image generation, file formats, image manipulation, image storage and retrieval, and how I use digital photography in my surgical practice.
DIGITAL IMAGES: THE PIXEL AND IMAGE SIZE The pixel (short for picture element or picture cell) is the equivalent of the silver “grains” from film-based photography. Understanding and manipulating the individual pixel is fundamental to working with digital images. Like tiles in mosaic artwork, images are built of pixels. This image of Abraham Lincoln (Fig. 1, courtesy of Jelly Belly Candy Company, Fairfield, California) is a jellybean mosaic. Progressive magnification of the image reveals the individual jellybeans and then the individual square pixels that comprise each bean. Remember the “Lite-Brite” Toy? We created images 1 colored peg at a time. Pixels are generated by a photosensitive chip. The chargecoupled device (CCD, Fig. 2) is the most common photosensor for digital cameras. The photosensors on the chip are arranged in arrays of rows and columns, and each sensor is covered with a microlens and filter to create a single pixel in 1 of the 3 “additive” colors (red, green or blue, “RGB”). New CCDs (not yet readily available) allow each pixel to record full-spectrum color. A discussion of CCD technology is beyond the scope of this paper, but digital photography magazines cover this topic well. Pixel densities (the number of photosensors per square centimeter) are growing with a similar curve as microprocessor
CURRENT SURGERY • © 2002 by the Association of Program Directors in Surgery Published by Elsevier Science Inc.
0149-7944/02/$22.00 PII S0149-7944(02)00666-9
581
FIGURE 1. Progressive magnification of this jelly bean mosaic demonstrates the “pixel” nature of a mosaic, and then the pixel nature of the individual beans. ([r]Reg. TMs of Jelly Belly Candy Company. Used with permission from the Jelly Belly Candy Company.)
transistor densities (Moore’s Law states that the number of transistors on a chip will roughly double every 18 months). The size of a digital image is expressed as the number of pixels (megapixels, MP, millions of pixels) or pixel dimensions (as pixel counts in length ⫻ width). Thus, an image that has 1800 rows and 1200 columns has 2,160,000 pixels, or 2.16 MP. It is amazing how few pixels actually comprise a television image (approximately 160,000). Table 1 gives image size information from some well-known sources. The 35-mm slide has been the benchmark to measure digital image progress. Estimates vary, but the amount of useable data that can be obtained from a 35-mm slide is just over 5 MP. The current crop of affordable (sub-$1000) consumer-level cameras is producing images in the
4 to 6 MP range. I believe the era of film-equivalence is here, for most of us. However, super fine-grain films like Kodachrome-25 (Kodak) can probably produce higher pixel counts, and most studio work is done with medium-format or larger. These 6 ⫻ 6 (cm) films have over 4 times the area of a 35-mm negative. That would place the pixel count for mediumformat film around 21 MP.
DIGITAL IMAGES: DATA SIZE AND FILE SIZE To store a pixel electronically, the computer file needs to store more both the location and the color of each pixel. Bits, bytes, and the like are beyond this discussion, but a pixel can be pure
FIGURE 2. The charge-coupled device and its accompanying circuits create the pixels for an image. 582
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TABLE 1. Image Size ⫽ Rows ⫻ Columns, Expressed in Millions of Pixels, Megapixels, MP Source Television HTDV (analog, USA) First Hubble Camera Computer Screen Kodak DC-120 Nikon Coolpix 950 Nikon Coolpix 995 Nikon Coolpix 5000 35-mm Color Slide New Hubble Camera Human Eye
Image
Millions of Pixels
⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻
0.16 MP 0.375 MP 0.48 MP 0.79 MP 1.23 MP 2.16 MP 3.15 MP 4.92 MP 5.22 MP (est.) 16.8 MP (2 ccd’s) 120 MP
320 600 800 1024 1280 1800 2048 2560 2760 4096 11k
525 625 600 768 960 1200 1536 1920 1890 4096 11k
monochrome (either a black dot or a white dot), grayscale, or varying degrees of color. Although a bit simplified, each pixel can be stored as 1 byte if it is monochrome, grayscale or up to 256 colors, 2 bytes if up to 65,000 colors, or 3 bytes if it uses “true-color” (16 million colors). Therefore, the data size of an image is equal to the length ⫻ width ⫻ color depth. For example, a 2.16-megapixel image (1800 ⫻ 1200 pixels), in true-color (3 bytes per pixel), would have a data size of 6,480,000 bytes (6.3 megabytes, MB, remembering that a megabyte is 1024 bytes). The image data are captured and stored in a digital file in one of several file formats. The most important concept of file formats is compression. Mathematic algorithms are applied to the image data to store it as a file. Some algorithms can compress (shrink) the image data into a smaller and more easily stored and transferred file. Extreme compression is possible if some “less important” image data are discarded. This type of compression is called “lossy.” Lossy compression algorithms offer variable compression percentages based on the degree of compression desired (and the amount of image data irretrievably discarded). Table 2 shows some common file formats and the file size that is produced when the image data is stored. Many more file formats are available (try a web search on “file format
information”). For practical reasons, use a file format that meets 3 needs: your software supports it, those you share files with can open and view it, and it suits your storage, archiving, and retrieval capabilities. Professionals tend to work with TIF files, because the format is well supported and no image data are lost with multiple manipulations and file-saves. If the same JPG file is repeatedly “tweaked” and resaved, there can be progressive data loss. This does not happen if you just view the file, just if you keep hitting the “save” button. I work most of the time with high-quality JPG files, and do a few things to combat potential data loss. I take all my images using my camera’s “fine” setting (most cameras let you choose that your images be saved uncompressed (as TIF files, for instance, or with 1 or more levels of JPG compression). The fine JPG setting provides an excellent balance between storage on the memory card and quality. I then burn these images onto a CD-ROM before doing any manipulations. I burn at least 2 copies of important images, and keep 1 copy at work and 1 copy at home. I am then free to manipulate, crop, recolor, and so on these images without fear of losing the original data. I save these “improved” files only once and then burn these corrected files onto another CD-ROM. I then have CDs labeled “Raw” and “Tuned”.
GENERATING IMAGES: CHOOSING A CAMERA There are many fine digital cameras, but for the practicing surgeon, the most important camera characteristics are portability, macro capability, copy-stand use, and the ability to copy (digitize) an existing slide collection. I will focus on the Nikon Coolpix 9xx series (950, 990, and 995) because they fulfill these ideal characteristics so well. I think these are the best “Medical” cameras available. (I have no financial conflict-of-interest regarding Nikon.) I own a 950, but it has been supplanted by the 990 and 995. I will focus on the 995 (Fig. 3), which (like the 950) creates a 3.15 MP image (2048 ⫻ 1538) of excellent color, sharpness, and dynamic range. Multiple automatic modes plus full manual controls are available. On-camera flash is modest
TABLE 2. File Size Produced When Image Data Are Stored. The Original Image Was 2560 ⫻ 1704 Pixels With 16 Million Colors* File Name
Extension
Windows Bitmap Tagged Image File Tagged Image File
BMP TIF, TIFF TIF, TIFF
Portable Network Graphics Macintosh Picture Graphics Interchange Format Joint Photographic Experts Group Joint Photographic Experts Group Joint Photographic Experts Group
PNG, PING PICT GIF JPG, JPEG JPG, JPEG JPG, JPEG
Compression None None Modest (LZW) Lempel-Ziv-Welch Modest Modest Yes Yes, Modest Yes, Moderate Yes, Marked
Data Loss None None None None None Possibly** Yes, not usually noticeable Yes, sometimes noticeable Yes, often noticeable
File Size 12.8 MB 12.8 MB 8.9 MB 8.4 8.2 2.9 1.2 576 357
MB MB MB MB KB KB
* Original photograph taken by a Nikon Coolpix 5000 saved by the camera in its “Fine” JPG mode. File conversion and saving done by Adobe Photoshop Elements (©2001, Adobe Systems Incorporated) ** The Graphics Interchange Format (GIF) while technically “lossless,” is limited to only 256 colors. Therefore, if you save a true-color photograph in GIF format you are discarding color data. CURRENT SURGERY • Volume 59/Number 6 • November/December 2002
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FIGURE 3. The Nikon Coolpix 995.
but very serviceable for operating room photography, and external flashes can be connected. Magazines rate the images highly, although not always the best. The “twist-body” construction allows the camera to fold flat and fit easily in a labcoat pocket, yet still allow enough lens-travel to give good (4⫻ optical) zoom range, and “best of breed” macro capability without the “front-to-back” bulk that makes other cameras difficult to pocket. The lens can focus to approximately 2 cm (0.8 inches) from the lens! This closeup capability produces outstanding operative photographs and allows an inexpensive (approx $70 street) screw-on attachment that copies slides and negatives with ease (more below). The tripod socket on the bottom allows easy copy-stand use.
GENERATING IMAGES: LIVE, SLIDE COPY, COPY STAND Use of a camera in the operating room requires ease-of-use, so that you or an assistant can take photographs quickly with a minimum of disruption to the flow of the surgery. Using an extra pair of sterile gloves, surgeons can hold the camera away from their bodies and twist the lens down to the field while composing the image in the LCD (Fig. 4). Alternatively, assistants can photograph over the surgeons’ shoulders while holding the camera at arms length, see the field through the LCD, and take some shots without any disruption to the procedure. The slide copier attachment (Fig. 5) means you do not have to purchase an expensive, fussy, slow (30 seconds or more per slide) device to do what the slide copier does in a few seconds. Remember that the typical 35-mm slide contains about 5.2 MP of data, so slide copies with the Coolpix 995 (3.15 MP) are close. Use the Coolpix 5000 (4.9 MP), and you lose nothing by copying and tossing your slide collection. (Do not forget that the average projected PowerPoint image needs less than 0.8 MP, see below.) I set the camera on macro, indicate the light source as fluorescent, place a slide in the holder, hold the adapter up to an x-ray view box, and push the shutter release. I 584
FIGURE 4. Use of a digital camera in the operating room: The twistaction of the 9xx series (950 shown) cameras allows holding the camera well away from the face while still composing good images. The macro mode produces superior closeups.
can copy about 6 slides per minute with this technique. Certain poorly exposed slides may do best with some on-camera exposure compensation (the Coolpix 5000 can do auto-bracketing.) Digitizing computed tomography scans, plain films, and angiograms is essential for making talks and discussing cases online, and a copy stand is the best way to go about making them. Although Fig. 6 shows a high-end, pricey copy stand, an old x-ray view box and a $75 copy stand (see http://www.adorama. com) will produce the same images. While on the subject of copy-stand use, I find that photographing textbook images (minding all necessary fair-use statutes) produces better results than does flatbed scanning. Moire´ patterns are a problem when scanning halftone images. Halftoning is a technique for producing continuous-tone-appearing images with only solid ink colors, and it is used in most publications. Optical interference creates bands of light and dark (the Moire´) on the output file. Software manipulation and filters can minimize this problem, but I find that a digital camera is less subject to this problem than is a scanner, and it usually prevents the problem in the first place.
FIGURE 5. The Nikon Slide Adapter is shown here attached to a Coolpix 950 camera. It also attaches to the 990 and 995. With an adapter, it will attach to the Coolpix 5000.
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FIGURE 6. Use of a copy stand to digitize x-rays.
IMAGE MANIPULATION: THE DIGITAL DARKROOM Once you have your raw image file safely tucked-away on CDROM, its time to “tune” the image to your needs. Image-manipulation software abounds: As far as I can tell, every camera ships with one or more software packages: some “lite” or limited versions, some “full” but outdated versions (upgrade available at added cost), and some full-fledged, current packages. There are shrink-wrapped, shareware, freeware, and demo versions aplenty. Some are part of suites that let you burn data, video, image, and music CDs. There is even image manipulation built into most Windows versions (Microsoft Photo Editor in Win-
dows XP). In general, almost any package will let you do the basic functions: Crop, zoom, rotate, and adjust contrast and brightness (gamma). Some packages can automate this for you with “picture-fix” and “red-eye” macros and wizards. The granddaddy of all image packages seems to be Adobe Photoshop (about $590 street). If Photoshop (with or without one of its 3rd-party add-ins, called “filters”) cannot do it, it probably cannot be done. For the lighter of wallet, Adobe Photoshop Elements (under $70 street) has almost everything the parent program contains and some wizards and other “ease of use” features that it does not (Fig. 7). For first-time users, the learning curve for Photoshop products may be a bit daunting, but it is worth the climb. My suggestion is to either use what comes free with your camera or just order Photoshop Elements.
IMAGE OUTPUT: SCREEN, EMAIL, POWERPOINT, PRINTER, PHOTOFINISHERS, JOURNALS Because an image is meant to be used, how you manipulate and save the image should be geared toward how you plan to offer its output. Because you already have the original image tucked away on a CD-ROM (don’t you?), you can tailor your image to the output you plan to use. Many people display their images on
FIGURE 7. Adobe Photoshop Elements has almost all of the features anyone needs for image-editing. CURRENT SURGERY • Volume 59/Number 6 • November/December 2002
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their computer screen, and many users still have screen resolutions of 800 ⫻ 600 or 1024 ⫻ 768. Most computer projectors are likewise limited to 1024 ⫻ 768 pixel resolution. “Power users” with 19- and 21-inch monitors may use screen resolutions of 1280 ⫻ 960. Still, that “high-end” screen resolution uses only 1.2 megapixels, and the newer cameras are taking 4 to 6 megapixel images! Using a higher resolution image on monitors and projectors will result in 1 of 2 things: The monitor will display only part of the image, and the user will have to “pan” the image to see its entirety, or the software will “shrink-to-fit” the image so it can all be seen. The former is fine if you are “tweaking” the image but is a pain if you are trying to view it. The latter results in larger files to move around and slower display times, whereas the software crunches the pixels. A special note on PowerPoint presentations: You are probably best to assume that your presentation will run on a 1024 ⫻ 768 projector, and if you are including some text along with your image, you will probably only need an image that is 800 ⫻ 600. When you insert a large image into PowerPoint, you “drag” the corners to shrink the image to fit. However, PowerPoint stores the entire image, and it has to resize it every time it is displayed. The two consequences of using large images in PowerPoint are that the size of the presentation rapidly enlarges, and the presentation slows down while the host computer (sometimes much, much slower than the one you write the presentation on) reads the large file from the (slow) CDROM and then resizes the image “on the fly.” For images I am going to project with PowerPoint, I first resize them to 800 ⫻ 600, and then increase their brightness and contrast to make them project better. I then save them as mid-grade JPG files to save space, and append “-ppt” to the file name (“myfile.jpg” becomes “myfile-ppt.jpg”). This way I know that the file has been developed and tuned for a PowerPoint presentation. When emailing images, I always assume that the recipient has dialup service, and do not attach too many or too large images. Unless the recipient has requested a high-resolution image, I send lower quality JPG files in resolutions of 640 ⫻ 480. Also, do not forget your upload times because even “fast” non-dialup services like DSL, Cable, and Satellite are asynchronous and upload more slowly than download. Many surgeons belong to “listserv” broadcast mailing lists. These are very effective tools for sharing information and questions rapidly across the country. However, it is considered poor form to attach image files to broadcast mailings. Most listservs have a common website where the moderator will post an image on a web page for interested individuals to view. When it comes to hardcopy output (desktop printing, photofinishers, or journal submission), the working term is “dots per inch” (dpi). The digital image contains rows and columns of dots. How tightly packed together these dots are placed on the output paper determines the size of the final print. For example, an image with a resolution of 1800 ⫻ 1200 pixels will be 6“ ⫻ 4” (1800/300 ⫻ 1200/300) when printed at 300 dpi, (3“ ⫻ 2” if printed at 600 dpi, etc.). The dots-per-inch is sometimes 586
specified within the image, but it is easily changed within the generating program or overridden by the printer’s software driver. Many programs assume screen output (the average monitor has a dpi of 72 to 96 dpi). If the image above was printed at 72 dpi, the output would be 25“ ⫻ 17”! Although 300 dpi was the “holy grail” of laser printers a few years ago, 600 or 1200 dpi is the laser standard today. Most inkjet printers print at 720 dpi or higher, and some claim 1440 dpi in at least the horizontal direction. Personally, I really cannot see any practical difference beyond 600 dpi, and current limits on storage, data transfer, and processor power make handling images beyond 600 dpi impractical. For instance, an 8 ⫻ 10 image, at 600 dpi, in “true color” has a raw data file of over 86 megabytes. (Remember that a 35-mm slide has around 5 MB of data available.) PhotoShop manipulations are slow on images of this size, even on a highend consumer system. What happens when you take a 2.2 MP image (1800 ⫻ 1200) and ask your printer to make an 8 ⫻ 10? At this size, the image needs between 150 and 180 dpi to print, so your fancy 720-dpi printer will wind up using about 16 drops of ink to make a single dot on the page (720/180 ⫽ 4, and 4 ⫻ 4 ⫽ 16). Smart software or printer drivers will vary the colors of these 16 droplets to smooth the transitions between pixels, which increases the perceived quality of the printed image, but adds no data. This technique is called “interpolation.” Well-done interpolation can make a very acceptable 8 ⫻ 10 printout even from a 2.2 MP image. I take my 1800 ⫻ 1200 Coolpix 950 image and carefully hand-tune it with PhotoShop into a 600 dpi, 6000 ⫻ 4800 pixel file, with judicious application of filters to improve brightness, contrast, sharpness, and image coloration (tone). I then either print it on my desktop printer or send it to an online service. Digital photofinishing is now common. What began on the Internet has found its way into the traditional “one-hour” labs. Get them the digital file (either via upload for the online services, or handing them a CD-ROM or memory card for the local laboratories), and they produce the same print they would produce from a film-based image by projecting the digital file onto photosensitive paper. These images have the same quality and durability as those done from film. I rarely print photographs from my desktop printer, as those done by an online photofinisher are better and usually cheaper. If you wonder if digital photography has finally “won” over film-based photography for the average amateur, you need look no further than Kodak. My favorite online digital photofinisher, Ofoto.com, was recently bought by Kodak!. Medical journals have accepted digital manuscripts for years, but many still wanted multiple 5 ⫻ 7 glossies for their printers to use. Some still want the glossies for use as “reviewer’s copies,” but increasing numbers will accept JPG files, allowing digital submission of manuscripts via email with Microsoft Word and JPG files. This is, in fact, how this article got into print! Few journals print images much larger than 2 ⫻ 3 inches, so even at
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600-dpi printing, the file only needs to be 1200 ⫻ 1800 (2.2 MP). Because most digital cameras now exceed 3 MP, it is easy to produce acceptable images for journal submission.
SUMMARY Digital photography is now sufficiently capable of replacing film-based imaging for the average amateur photographer. It
should replace film-based photography for the surgeon. Understanding the pixel and how it is generated, manipulated, saved, and output is essential to successful digital photography and imaging. The surgeon’s camera should be especially capable in the areas of portability, use in the operating room, close-focus abilities, slide-stand use, and 35-mm slide copying. Archive your original images onto CD-ROM, hand-tune them specific to the eventual display medium, and enjoy!
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