Many of you, whether you are presenting a case for grand rounds, or for a meeting or more informal lecture, will be using images in your Powerpoint (or Keynote) presentations. It’s hard to know what size, resolution and format to choose to end up with a quality image that is not so big that your computer freezes. We all want to show the audience what to look for in the most clear and efficient way possible. Here are the things I do to optimize my images.
File types
Although most people with digital imaging systems will have images that are in DICOM format, they are not readable by most slide presentation software. You need to select the images you like, and export them in a more generic format such as .jpg or .tif. Tif files preserve the original image size and quality, while .jpgs compress the image to a more reasonable size. I like to save my images as .tif format, then crop and resize to .jpg. That way I still have the original size file in case I ever want to zoom in on a particular lesion. If you save .jpg files several times, they get successively more compressed with loss of detail, and you can end up with a file that is pixellated or too small. Keep the .tif original if you can.
Exporting Images
Depending on what image viewing software you have, you will need to save or export the image to a .tif or .jpg. Before you do, make sure you have adjusted the window and level so that the image looks as good as possible. Also, turn off the patient information that is displayed on the screen. In eFilm, you can do this by clicking on the capital “A” button, and in Osirix go to 2D viewer/Annotations/none. Then save your image to a specified location. If you are photographing with a digital camera, use high resolution and a tripod to hold the camera steady.
Grayscale
Many people have some version of Photoshop for adjusting their images. It has features that are very helpful for adjusting images for presentations, though I am sure other software can do the same job. The first thing is to adjust your image mode to grayscale. Both Osirix and eFilm sometimes save a file in RGB mode, even though it is only shades of gray. Digital cameras also typically operate in color mode. The color channels make your image larger without adding data, and can also make the overall tone a green or purple color. Change them to grayscale right away, unless you have an image with color in it such as Doppler ultrasound.
Crop
The next thing to do is crop your image to the right size. Include enough anatomy that your reader can orient themselves to where they are looking, but eliminate extra dead space. For example, in a lateral radiograph of an abdomen that includes most of the thorax, crop to just cranial to the diaphragm. The important parts of your image will be displayed larger and with more pixels. Cropping too small, for an extreme closeup, can be difficult for the reader to orient to.
Image resolution
Depending on how you exported the image, your radiograph could be set at 1 pixel per inch, or 300 pixels per inch. For displaying on a screen 120 pixels per inch is fine. 72 is a common setting, and may not be enough especially if you compress it as a .jpg. In Photoshop, go to Image/Image size and change the resolution first.
Actual size
The other thing you can change under Image Size is the actual width and height of the image. I usually make the width 7 inches for a radiograph. The standard Powerpoint slide is 8.5×11 inches, so that’s enough width so everything looks good. 7 inches works well for a lateral radiograph, and 4 inches in width is good for a v/d projection.
Save a copy
Once I am happy with the cropping, contrast, size and resolution, I save the file as a .jpg. I admit this is partially through laziness, because then I don’t have to rename the file. It saves as a default the same file name in the same place, but smaller size and with a .jpg extension.
Automate this sequence with Actions
The best thing I learned recently was that you can save this sequence of commands in Photoshop using the Actions palette. Cropping is user defined, so you have to do that yourself, but the grayscale, resolution, size and save actions can all be done automatically. Look under “Actions” in Photoshop help for detailed instructions. All you need to do is find the Actions palette, click “new action”, and record the series of steps we discussed. You can then save it, and use it over and over by “playing” the action, or assigning it to a function key. I just learned about this feature, and can’t believe the time it’s going to save me! Thanks to John Doval for clarifying many of these points today.
File types
Tif format is primarily develop to store printable files, because it contains at most information for printer. It’s better to export dicom images as a bmp file, because this format is very simple and free (without patent protection). This is the reason why bmp is readable by most graphic editors. Other good advantage is, that bmp isn’t compressed (but some editors can make also compressed form of bmp). The size of bmp file is similar, if not the same, as tif file (its my own experience, with our DR system).
Image resolution
This feature is necessary only if you want to print your image in better quality. The information “how many dots per inch” must be displayed, doesn’t influence the image on screen. It says only how many dots of ink must be in 1 inch on paper.
Additional information see on: http://en.wikipedia.org/wiki/TIF ; http://en.wikipedia.org/wiki/Windows_and_OS/2_bitmap ; http://en.wikipedia.org/wiki/Dots_per_inch
Thanks for your input and references!