Today I’ll tackle one of the questions I posed in the first post on the blog. If you have an animal with abdominal disease, should you take abdominal radiographs or do an ultrasound? Of course there is no easy answer, but there are two things to keep in mind. First, each modality has its pros and cons, which we will talk about momentarily. Second, in many cases you could choose either and get to the same diagnosis. So a lot of the time, it depends on your level of comfort with the modality, availability, and finances. But let’s take a look at some of the advantages of radiographs and ultrasound.
Abdominal radiographs are the tried and true method of diagnosing abdominal disease. We know a lot about their capabilities and limitations through their use over many years. One of their great advantages is that they are very quick to produce, and your technical staff is trained to take them for you. So in ten minutes, you have two radiographs of the abdomen of your patient, and you have seen an additional patient in your exam room. Radiographs also give you an excellent overview of the whole abdomen. You get the big picture, and can see how the different normal and abnormal portions relate to each other. Things like abdominal contour can tell you if there is peritoneal effusion, and masses can be localized to an abdominal quadrant. Something like a GDV, with an organ in abnormal position, is best diagnosed on radiographs, where you can see the relationship of gastric regions to the rest of the abdomen.
Radiographs are limited in distinguishing the internal structure of organs. The outer contours are visible because of the fat contacting the serosal surfaces, and the inner contours of the GI tract are outlined by gas. Unless there is something of air or mineral density within a soft tissue organ, you won’t be able to see it. For example, a mass in the kidney or liver is soft tissue opacity as well, so unless it distorts the surface of the organ it won’t be visible. Contrast studies such as an upper GI or excretory urogram can help to define organs with a lumen (ureter, bladder, small intestine), but that still leaves solid organs like spleen and liver.
The other major disadvantage of radiographs is that everything in the depth dimension is compressed into 2D for viewing. So the kidneys look like they are at the same depth on a lateral radiograph, when in fact they are separated by several centimeters. The orthagonal projections helps to localize objects in that missing dimension, but it can still be difficult.
Ultrasound is a modality that has seen more and more application in recent years. It’s different from radiographs in that the picture you see on the screen is only a few millimeters in thickness, so that depth dimension compression is negligible. You are actually looking at thin slices of the abdomen one after the other, displayed in “real time”. This is great because you can look at a thin slice of an organ, for example a kidney, without the averaging. Ultrasound physics also allows you to see many more “shades” of echogenicity, while radiographs are limited to air, fat, soft tissue, mineral and metal. So the mass that you couldn’t see in the kidney on a radiograph is very visible on ultrasound. Ultrasound does not always detect focal lesions such as masses though, especially in the liver.
Now most people are thinking that ultrasound is the way to go in all cases. But some of the advantages can be disadvantages too. In ultrasound, you can look at an organ from any imaginable plane, and they look different in each view. With radiographs, you have 2-4 positions, and you get very used to how things should look in each. It’s easy to get confused in ultrasound, or not be able to find the plane you need. This is the major disadvantage; ultrasound is operator dependent. It takes a very long time, and many hours of doing ultrasound exams, to become competent at a complete exam, and to interpret findings. So level of competence can be a disadvantage if you miss or misinterpret a lesion, and it takes 20-40 minutes for a complete exam. Equipment is becoming less of a disadvantage as portable units get better image quality, but it may also be a factor in the quality of your exam.
The narrow field of view of an ultrasound image can be a limitation in some cases as well. It can be difficult to go back and forth between a mass and organ to try to find where it originates. Physical limitations of ultrasound are gas (post-operative abdomen, ileus or gastric dilation) and bone, though bone is less of an issue in the abdomen.
- Advantages of radiographs: “the big picture”, technical ease, familiarity with interpretation, and ability to view the GI tract, diagnosing free gas.
- Advantages of ultrasound: good tissue and lesion discrimination, little depth averaging, sensitive to small amounts of peritoneal fluid, “real time” (see peristalsis, blood flow).
- Disadvantages of radiographs: depth compression, limited density discrimination, insensitive to small amounts of peritoneal fluid.
- Disadvantages of ultrasound: narrow field of view, operator dependent, time consuming, gas, equipment.
So now you are faced with a patient with vomiting. Which should you choose? Radiographs will give you an overview of the GI tract and identify any radiopaque foreign bodies, and patterns of gastric outflow or small intestinal obstruction. If you are not convinced by the radiographs, an upper GI is a good and easy way to check for patency of the GI tract. Mass lesions and generalized infiltrative disease are harder to interpret, and may not be visible. Radiographs are very good at showing free air in the abdomen in case of perforation. Ultrasound can be very sensitive to free air, but only in the hands of a very experienced operator.
If you choose ultrasound, you will probably get a limited evaluation of the stomach because of gas. The small intestine generally can’t be followed from start to finish, so there is the possibility of missing a subtle lesion. You will probably pick up small amounts of peritoneal effusion much better, and can see lesions such as dilated, fluid-filled bowel, bowel masses, foreign bodies, and intessusceptions. If you find a lesion, you may be able to aspirate it with ultrasound guidance.
Each type of abdominal disease has pros and cons when it comes to choosing radiographs or ultrasound. It helps to be familiar with the limitations of each, and make an educated choice depending on the diseases on your initial list of differential diagnoses. Radiographs and ultrasound can both rule things out, add a new finding to your list, and/or make a definitive diagnosis. Of course, you can do both if you need the different information that each can provide. What are your experiences?