The basic tenants of abdominal ultrasound scanning for the private practitioner (Proceedings)

Abdominal ultrasound has taken on a "larger-than-life" position in diagnostic imaging in veterinary medicine for several reasons. First and foremost is that ultrasound is a non-invasive technique that can be used by the small animal practitioner for imaging the peritoneum, parenchyma of the abdominal organs and retroperitoneum. Additionally, the best anatomic technique for imaging the abdomen is Computed Tomography (CT). However, CT is not readily available and requires heavy sedation (multi-slice scanners) or general anesthesia.

Questions to Ponder about Abdominal Ultrasound

What Am I Imaging?

Sound. Specifically, ultrasound. Ultrasound is defined as a series of alternating waves of compression and rarefaction that travels through a medium, and is characterized by the length of a compression and rarefaction cycle called a wavelength (measured in mm for ultrasound), velocity within the medium or tissue (defined as 1540 m/sec as an average in soft tissues) and the number of cycles of compression and rarefaction per unit time or frequency (defined in Megahertz [MHz] in ultrasound or 1 x 106 cycles/sec). Ultrasound can be defined then as any sound above the normal human hearing range of 20 kHz.

The ultrasound wave then is transmitted as a mechanical wave through the fat and soft tissues of the dog or cat's abdomen. There are two basic interactions. The first interaction is no interaction and the sound wave is propagated or transmitted through the medium. This is good. This means we can image deeper than the first micrometer beyond the epidermis of the skin. The second interaction is that the sound wave interacts with some medium that has different acoustic impedance and will be reflected, refracted, scattered or absorbed. If one looks at the overall intensity (Watts/cm3) of the ultrasound waves as they are transmitted in the medium, one will find that the intensity decreases with depth of tissue being imaged. This is because of these four basic tissue interactions that are occur and thereby decrease or attenuate the ultrasound beam. The physical density and the velocity of sound within the medium determine the acoustic impedance of the medium. Due to differences in acoustic impedance, there will be a scattering, reflection or refraction of the ultrasound waves. The actual change in acoustic impedance occurs at an acoustic interface. The returning or reflected ultrasound waves are then the specific waves in an image as these are the waves that are that are ultimately digitized at the transducer and form the basis for the gray scale two-dimensional image that is displayed on the US screen. Acoustic interfaces with large differences in acoustic impedance will result in total reflection of the sound waves, such as soft tissue – gas and soft tissue – bone interfaces.

What are the "fatal" assumptions (thereby the basic limitations) of veterinary ultrasound?

There are multiple fatal assumptions made by the ultrasound machine. First, the machine assumes that the "average" speed in tissues is 1540 m/sec or 1.54 mm/μsec. This, however, is not the case. Ultrasound differences in speed in fact occur between liver, gall bladder, renal cortex, renal medulla, gastrointestinal tract and fat. The biggest difference is fat (1480 m/sec). Now this may seem like a relatively small difference (60 m/sec), but this is in fact huge. This results in a propagation speed error and the organ or structure imaged is in fact placed deeper in the tissue or image. So, the deeper one goes in the US image, the more likely there are propagation speed errors and the US machine has placed the structure of interest deeper ("the machine lied") in the image than it truly is.

The second fatal assumption is that the ultrasound wave can only go straight out, be reflected and come straight back. In other words, multi-path artifacts and side lobe artifacts are not possible. In this case, the image contains information on the screen that is totally NOT there!

The third fatal assumption is that the anatomy in the patient has a direct correlation with the anatomy displayed on the screen. This is fact is wrong on several levels. First, we are not imaging anatomy; we are not imaging histology; we are not imaging pathology; but, we are imaging differences in acoustic impedance. This difference in acoustic impedance, in fact, does NOT have a direct correlation with anatomy, histology or pathology. You can have a change in an organ parenchyma (metastatic disease in the liver) and not seen any change on the US image. Second, almost all of the probes used are micro convex curved array probes. The transducer piezoelectric crystal elements line up so that the US beam is a diverging beam at depth. This means that there is a gap, at depth, between adjacent lines. So how does the US machine know what grayscale pixels are to be placed between the US rays on the US image? It guesses that the pixels between the US rays are the "average" signal intensity based on the pixels at the same depths between the known US rays. Third, the intensity of the US grayscale as seen on the image is a direct correlation with the degree of a reflector's scattering strength. Again, we are imaging differences in acoustic impedance and not scattering strength.

A final fatal assumption has to do with you as the practicing veterinarian. Recognize that in a radiology residency it takes a minimum of three years to learn abdominal ultrasound; at least master an intermediate skill set. If you intend to be doing biopsies the first week after purchasing your ultrasound machine, be ready for legal action against you after the death of the first several patients. You cannot expect to know everything there is to know about ultrasound in 1 year or even after 3 weekend short courses. So, know your limitations. Admit and accept those limitations. "Above all, do no harm." You took that oath at graduation. Put it on a piece of paper in big letters and stick it over the ultrasound machine in order to hold yourself accountable.

So.... what are the benefits and limitations of US (as if we have not beaten up US already)

The benefits of abdominal ultrasound are the information that is gained by doing the procedure (otherwise we would not be doing ultrasound). This information is used in veterinary medicine more so than human medicine. We can in fact image wall thickness and specific measurements of various organs such as the gastrointestinal tract and the caudal pole width of the adrenal gland. Ultrasound is good at detecting lymph node abnormalities (enlargement, changes in echogenicity, that are presumed to represent some form of reactivity or pathology). Ultrasound provides a rapid way of helping to stage various tumors; although a negative scan does not rule out diffuse infiltrative disease. The US equipment sold in the veterinary market no longer limits us. Technology changes every four to five years so that one should think in terms of replacing the US machine every five years. The newest ultrasound machines allow even beginners to acquire diagnostic images. One can clearly use US for FNA of organs and abnormal mass lesions, etc. The limitations of US include: user's ability to obtain high quality, diagnostic images; user's understanding of the type(s) of information that US can provide; user's understanding of the physics and knobs associated with the ultrasound machine; US provides a small field of view and in an oblique imaging plane relative to other cross sectional imaging modalities (CT and MR); US is inferior in determining the origin of mass(es) based on the presence of large mass(es) lesions in the abdomen and compression of other structures; US is inferior in determining surgical respectability; US is inaccurate in measurements (large inter-observer differences); and the unrealistic expectations on our part as to what US can do, how quickly I can get at that information and the lack of specificity and sensitivity of the imaging modality itself. It is our responsibility to recognize these limitations as being inherent to US in order know how firmly you can stand on the diagnostic statements made after the scan. Ultrasound can in fact detect changes in an organ prior to biochemical alterations or clinical signs are manifested. At the same time, the US examination may be considered normal, even in very ill patients, with biochemical abnormalities with gross evidence of disease at surgery or necropsy. Above all else, you have to put the probe over the area of abnormality in order to visualize it.

What is the purpose of the abdominal US examination?

Abdominal radiographs are a part of the initial diagnostic work up in regards to an evaluation of the abdominal US examination. Both examinations are complimentary and should be used as such. If we can also have a specific focus, even though we are doing a complete abdominal scan, details within the region specific to the question being answered for the test becomes very important. Additionally, ultrasound is not a test to rule out disease processes and the sensitivity and particularly the specificity of the US as a test are not 100%. US changes are sometimes non-specific and provide the clinician with a list of differentials at best. With a specific search or purpose so that the clinician is less inclined than to be distracted by incidental findings. There can be a number of incidental findings that must be included in the patient's medical record. These findings may be unrelated to the patient's clinical signs, but may be more important in the sense of finding a definitive answer as quickly as possible. Abdominal radiography still remains the best first test for a survey instrument of the abdomen. I you think that you have looked at the entire abdomen of a 70 kg Great Dane or better yet, fat Rottweiler, you are kidding yourself. Incidental findings can be important or you may decide that the finding warrants following the change over time (say 1 to 3 months for the first recheck). A huge benefit for abdominal ultrasound is the ability to collect cytological samples (recognizing that cytology does have limitations as with any other test) or collecting biopsy samples if necessary.

Ultrasound has been shown to be unreliable for the measurement of various structures. A heavy reliance on organ measurements is unreliable at best and can be potentially harmful if a clinical course is taken that is harmful to the patient. The reality is that even though veterinary ultrasound of the abdomen has been around since the early 1980s, there are very few reliable normal values for organ measurement, even fewer organ volumes and because of the breed and size variation in dogs, breed specific measurements have not been pursued. Operator reliability and repeatability is the biggest obstacle when just looking at taking measurements of abdominal structures in general. Even the counting of structures (calculi or fetuses) has been shown to be misleading. Other hindrances to accurate measurements would include: tumor size (hemangiosarcoma masses would be virtually impossible to quantify), lack of specificity for changes that are neoplastic on histology and the inability to three-dimensional reconstruct structures from an accurate volumetric data set.

Ultrasound can have serious limitations when assessing the type of fluid present (echogenic fluid does not equate with sepsis, blood or protein), evaluation of the entire gastrointestinal tract, looking for free abdominal air, distinguishing between focal areas of mineralization or gas accumulation, say within the gall bladder (very different differentials, treatment plans and prognosis), diaphragmatic integrity, presence of an absolute extra-hepatic biliary obstruction at the time of the scan and small areas of pathology such as small ureteral calculi, polyps of the stomach, gastric ulceration, etc. However, ultrasound is great for determining the presence of nodular alterations in parenchymal organs, alterations in the shape of an organ, the detection of small volumes of peritoneal or retroperitoneal effusion, taking samples from abdominal structures or the peritoneal/retroperitoneal space and to a certain extent, re-evaluation after therapeutic intervention (either surgery, radiation or chemotherapy). Ultrasound can also provide significant information about the liver, gall bladder, spleen, urinary system, pancreas, adrenal glands, abdominal lymph nodes and the assessment of tumor staging for mass lesions and parenchymal abnormalities. However, as has been emphasized before, ultrasound does not rule out disease. IF the changes are present that fits with a particular disease, great; but if not present, all bets are off.

Ultrasound guided organ sampling

This clearly is one of the advantages of ultrasound scanning. It provides a quick, non-ionizing radiation methodology for obtained samples of abdominal fluid collections, cells from solid organs or cells from mass lesions. Some points to consider however would include the following.

     1. How do we know the area sampled truly reflects the pathology that is present?

     2. Have we actually sampled the pathology?

     3. Did the near field tissue contaminate the needle while sampling a deeper lesion that would potentially change the cytological conclusions?

     4. For "normal" appearing organs on an ultrasound scan with biochemical alterations, can we be certain that we have in fact sampled an area of disease?

     5. Do we need to worry about the potential for peritoneal seeding in each and every FNA that we collect?

     6. How good is my clinical pathologist?

     7. Is my slide making technique adequate enough to ensure good cytological specimens?

     8. How much blood contamination is too much?

     9. How many aspirates are adequate and when is enough, enough?

Cases of Ultrasound Supremacy

Ultrasound is extremely useful for determining causes of renomegaly, particularly in a cat. Differentials would include: hydronephrosis, lymphoma, Cryptococcus, amyloidosis, perinephric pseudocysts, compensatory hypertrophy, or polycystic kidney disease. Although, we have mentioned the US pitfalls associated with oncological screening, it is still a useful technique and should not be thrown by the way side. One can assess for multi-organ involvement (FNA would be necessary to rule out disease, but if characteristic changes are present, then assumptions can be made for staging purposes), re-evaluation during or following therapy, assessment for tumor margins for radiation therapy, and interventional applications.

Summary

One needs to understand what one is imaging and have a clear grasp of the physics and artifacts associated with the ultrasound image. The practitioner needs to know how to run the knobs of the machine in order to make the best image possible so that an appropriate diagnosis can be made. One needs to re-familiarize yourself with anatomy of the abdomen, but from a three dimensional perspective. Understand and believe that there are numerous limitations to ultrasound, as with any test or imaging technique. The good news is that these limitations do not out weigh the benefits; otherwise no one would be doing ultrasound. Understand your purpose in doing an ultrasound examination and ask very specific questions about a particular organ, area or structure related to the ultrasound examination. At the same time, understand the limitations of ultrasound in answering those questions. Use ultrasound to safely obtain cytological, microbiological and histological samples for a definitive diagnosis. The quest for ultrasound is a lifetime learning experience. In order to engage in this quest, one must be committed to pursuing this quest with a passion that original drove you into veterinary medicine and got you through veterinary school. Live long and prosper!

References

Nyland TG, Mattoon JS. Small Animal Diagnostic Ultrasound, second edition. Philadelphia: WB Saunders, 2002.

Kremkau FW. Diagnostic Ultrasound. Principles, Instruments and Exercises, 6th ed. Philadelphia: WB Saunders, 2002.

Burk RL, Feeney D. Small Animal Radiology and Ultrasonography: A diagnostic atlas and text, third edition. Philadelphia: WB Saunders, 2002.

Pennick D and d'Anjou Marc-Andre, (ed). Atlas of Small Animal Ultrasonography. Blackwell Publishing: 2008.