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
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.