As a minimum requirement, a digital radiography system must be able to provide you with radiographic images that are at least as good as film images. A digital radiography system is useless if the images are crummy. As I tell people I consult with “you wouldn’t buy a refrigerator that does not keep your food cold would you?”

There are many determinants of image quality. The hardware and software used to generate the images such as the type of imaging detector, image processing, and monitor used to view the image will have a dramatic effect on image quality.

Image quality will also depend on the user using the system. Some systems are very easy to use other systems are very (….extremely) user dependant. Some systems kick out an image with little user input while some systems require the user to do a lot of fiddling with the image to make a good image. Some systems are very tolerant of exposure errors while others are intolerant of even small errors of exposure.

In this tutorial, I will discuss how you should evaluate images from a digital radiography vendor and I will introduce the concept of image repeatability as a measure of system performance because it is the only measure I know of that tests the inherent imaging ability of the system, tolerance for user error AND the ability of the company selling you the machine to train your technical staff in using the equipment.

Evaluating images – Before you begin

Always compare digital systems to digital systems. Too many veterinarians have been taking horrible radiographs for so long that they are amazed by the image quality of even the worst digital radiography system. These veterinarians often purchase the first (or cheapest) digital radiography system they encounter. Some are satisfied with the purchase but others find themselves wanting more from the system within a few weeks after the purchase.

The black, overexposed, radiographs that you are obtaining with your diptank cannot be the litmus test to which you compare digital image quality. You must compare the quality of one digital system to another digital system.

Evaluating images - STEP 1: Get some images and look at them

As a student of this website, I forbid you to obtain images directly from a vendor; use images shown to you by a vendor at a veterinary show; or obtain images from a practice that a vendor sends you to.

I say this because in all of those cases, the images are chosen by the vendor to make their system look good. One vendor sent me images that looked fantastic. However, in all of the systems that the vendor installed, the images were crummy. A few years later, (after the product was removed from the market) the vendor told me that “they could never reproduce the image quality they obtained in the lab.”

Do your research and contact practices that have the system you are interested in. If you don’t know of a system, contact your local radiologist. They may be able to direct you to a practice in your area.

Evaluating images – STEP 2: Get a good monitor and turn off the lights

This step is essential. You would not go shopping for a new house with mud caked all over your glasses would you? Of course not. The same applies to digital radiography systems.

Ideally, you should evaluate the images you obtain on a medical grade grayscale monitor. If you don’t have one, you should contact your local referral hospital or veterinary school. They may be able to let you use theirs.

A good quality monitor will allow you to view the images at full resolution and help you see the differences between the digital radiography systems you are evaluating.

Evaluating images – STEP 3: Evaluate image sharpness

Your digital radiography images should be sharp and have good anatomic detail. Fuzzy images are unacceptable. There are a number of causes for unsharpness in an image. These include motion during radiography, limitations of the imaging sensor, and post processing. The following is an image from one of the “fuzzier” digital radiography systems on the market.

The image above is pretty terrible and it is easy to see that it is fuzzy. In order to fully evaluate your images, you will need to zoom up the image. Be sure to zoom the image to at least twice the normal anatomic size. The image should maintain sharpness at this level of zoom. If it does not, look for another system. The following is a zoomed image from a system that generates non-fuzzy images. This is a beagle carpus.

I was evaluating a system once where the vendor actually turned off the zoom feature for the demo. The images looked great as little thumbnails but when I asked him to export the images so I could look at them on a different viewer and zoom them up, he refused. Demo over!

Evaluating images – STEP 4: Evaluate the exposure latitude of the system.

One of the benefits of digital radiography is its ability to provide you with greater soft tissue visualization and decrease retakes for exposure problems. Some systems do this well. Other systems…not so much.

To see what I mean by exposure latitude take a look at the following images.

In the previous image, you will note that the spine is properly exposed as is the caudal abdomen. However, the right stifle joint is non-existent on this image. This system cannot effectively display the stifle joint and the abdomen and the spine on a single image. NOTE: when evaluating images for exposure latitude, you must be sure that you don't evaluate an image that is over exposed. All overexposed images will have poor exposure latitude from plate saturation.

In contrast to the previous image, the following image has a very wide exposure latitude.

As you can see, the second image has a vastly superior exposure latitude.

Veterinarians in the San Diego area looking to purchase a digital radiography system often visit with us and read films for a day. This gives them the opportunity to see images from many different systems. In many cases, at the beginning of the day, veterinarians seem to prefer the very contrasty first image (with a narrow exposure latitude.). At the beginning of the day, most vets even state that they don’t like the second image because it is too "busy." I agree. The second image is very busy but that is because it contains a significantly greater amount of diagnostic information.

This wide exposure latitude also means that you wont have to repeat this image to evaluate the stifle joint in this pet. That may not seem like a big deal but look what happens to the lungs in a system with poor exposure latitude. In the following image, the image is properly exposed but the lungs are burned out and lack detail. In fact, the lungs cranial to the heart contain no diagnostic information and a pulmonary nodule in that lung field will be missed. Look closely, although the image appears to be properly exposed on first glance, this image needs to get repeated.

For comparison, the following is an thoracic image with a wide exposure latitude.

Evaluating images – STEP 5: Evaluate the repeatability of the system

Repeatability is a new determinant of image and system quality that I would like veterinarians to consider. I believe that it is one of the most important aspects of image and system quality but it is something that vendors and veterinarians never consider when selling or purchasing a system.

Image repeatability can be thought of as the consistency of an imaging system. Some digital radiography systems consistently produce images that are of similar quality. Other systems can produce a good image but you only get a good image in maybe one out of every 6 exposures.

I don’t have a good handle on all of the factors that determine image repeatability but I do know that they are related to two main areas:

• Image processing and exposure latitude: some systems just seem to be able to deal with errors in exposure better than others. Systems with a wide exposure latitude have very consistent image quality
• User effects: The influence that the user has on the system cannot be overstated. In the wrong hands, any digital radiography system will give you crummy images.

It is hard to determine which of the above factors have a greater effect on image consistency but it is my impression from reading radiographs from many different hospitals, with many different digital radiography systems, who employ large numbers of technicians of varying skill levels that image processing and exposure latitude have an equal if not greater effect on image repeatability than user effects. I say this because the radiographs we receive from some hospitals are always consistent while the radiographs from other hospitals are "hit and miss." The interesting thing however, is that the only common link between the consistent hospitals and the inconsistent hospitals is the digital radiography system.

We must not, however, discount the user with regard to image quality. It is also my impression that some systems are much easier to use and give the technical staff the advantage. Others are very user dependant. It also seems that some digital radiography companies do a much better job at educating customers on how to use their system

It is for this reason that I say image repeatability may be the number one determinant of image quality. It is the only measure that considers:

• -how effectively a digital radiography system can process images

• -how forgiving the imaging system is to errors in technique
• -how easy a system is to use
• -the ability of a digital radiography company to train end users.