DPI stands for dots per inch. However, in this explanation, I will refer to PPI (pixels per inch) when it suits me and when you are likely to see information on the screen or in a request as DPI, I will still use DPI to avoid confusion. The reason for the change in terminology is that an image on your computer is formed of pixels (short for picture elements). Typical pixel dimensions of a CT scan are 512 pixels wide and 512 pixels high. If it were printed with a specified number of pixels per inch, it will be a certain size. However, there may be many more dots (e.g. ink dots coming from the nozzles on the print head) used by the printer to make up each pixel. So dots per inch and pixels per inch can mean different things.

Two of three pieces of information are needed to ensure conformance to the desired quality when printed:

1. Pixel dimensions

2. PPI

3. Size to be printed at

Let us assume that you have an image that is 100 by 100 pixels in size. If you print at 300 PPI (pixels per inch) the image will be 1/3 of an inch by 1/3 of an inch. If you try to print at 10 inches by 10 inches (25.4cm by 25.4cm), there is no way you will achieve 300 PPI as you only have 100 pixels to spread over 10 inches.

Important point 1: if you are asked to supply images at 300 DPI (meaning PPI), you must find out what size they are to be printed at. This is easily done through the publisher or the journal's website. If they are to print at 1 inch by 1 inch at 300 DPI (PPI!), then all you need is an image of pixel dimensions 300 x 300. If the print is to be 2 inches by 2 inches, you need 600 x 600 pixels for the same DPI (remember I am calling it PPI)

Here are typical pixel dimensions of images we deal with:

Using the examples above, you can use simple maths to work out that if you were to print a plain film at 300 PPI, it would measure approximately 20 cm x 22 cm (an inch is 2.54cm). That is big!

If you print a nuclear medicine image at 300 PPI, the image will be 1.1 x 1.1 cm. That is small!

Important point 2: If you have an image with sufficient pixel dimensions, you will be fine. If you have an image with insufficient pixel dimensions, you need to do something extra.

Pitfall : With a plain film the original pixel dimensions are as above and the file you are looking at on the PACS monitors is called a DICOM file. If you save a version of this as a jpg file or bmp file by copying and pasting to paint, you are not getting the original file. Make sure that what you get has sufficient pixel dimensions for your needs.

To obtain details of the file you want to work with, right click on the file and choose properties. Then go to the details tab and you will see pixel dimensions and DPI.

What to do if you do not have a picture of sufficient pixel dimensions or if you have a suitable picture in terms of pixel dimensions, but the DPI is set to a value less than that which is required:

The latter is easy. In some cases you need do nothing. The picture will be of appropriate quality, but a low setting of DPI means the print will be very large. If, however, the printing process specifies a smaller size the DPI will usually be adjusted to a higher value. If you wish to change the setting of the DPI, this is easy – see below.

To work with your images, download Irfanview. It is free, easy to use and very widely recommended. The link is here: http://www.irfanview.com/

Follow the links from the main page and where you see 'Download File Now', click on that. You may have to click on the notification bar, as shown below, in order to download the file.

When asked whether you wish to save or run the file, click run. Once it is installed, download the plugins (via the same link above). Again, choose to run the file. You will now have Irfanview installed and with the plugins you can open DICOM files.

Steps to change the DPI or size:

1. Start Irfanview

2. Open you image file:

3. Navigate to the folder or drive containing your file and open your file:

4. Choose Image>Resize/Resample:

5. In the dialogue box shown below, set the DPI to 300 (red circle). Notice that my file is only 800x600 pixels (blue circle). These are the dimensions of the original DICOM file (and are small because it is an US picture as opposed to a plain film which would be much larger). The green circle shows how big the file will be when printed at the DPI selected and the pixel dimensions shown. Important: I selected the DPI first – so should you, only then may you click on the cm or inches radio button in the green circled area to see how big the file will be.

If the size indicated is not large enough, you will need to change it to the size you want to print at. For this, you must have resample selected (yellow). What happens here is that the new size becomes larger than the current size shown in the blue circled area. You need larger pixel dimensions to print at a larger size for the same DPI and the program is simply making up the the extra pixels using, in this case, the Lanczos filter.

If the size is too large or just right, make no further changes (so all you will have done is firstly changed the DPI, then clicked 'cm' to check the size and then clicked 'OK'). In this case the blue area will show that current size = new size. Only in this case, you can select resize (yellow), though in this version of Irfanview this step is not necessary.

For Photoshop the instructions would be different. Do not resample an image if it is of appropriate pixel dimensions in the first place. Resampling an image changes it.