Pregnancy scanning and foetal bone
The publication date of the book ‘Ultrasound for Canine Pregnancy Scanning’ is fast approaching, and with that, some final touches and triple-checking research into some of the remaining puzzles of ultrasound imaging.
Those with scanning experience already will know very well how bone appears on ultrasound. Not only does it return very strong reflections, making it appear bright white, but solidified bone also affects imaging of surrounding tissue, casting a shadow behind it. This is due to both its high reflectivity (and why it’s so reflective is explained thoroughly in the book) but also the fact that it is a very strong absorber of ultrasound energy. This means that effectively no ultrasound energy makes it through, and this is why areas beneath solidified bone are black, and in ‘shadow,’ just like a tree blocking the evening sunlight.
Nobody knows precisely why bone is such a strong absorber of ultrasound. A popular theory is that bone molecules reasonate strongly with ultrasound frequencies. In other words, they vibrate so much that by the time the next ultrasound wave comes along, the molecules are nowhere near having returned to their neutral position. They are totally out of sync with the passing wave and the energy is lost in this friction and resistance, being converted into heat rather than passed back to the traveling wave.
My personal theory is that this same phenomenon occurs not only because of resonant frequencies but also because the speed of sound in bone is so great. Our ultrasound pulse literally doubles in speed when it travels through it. We already know that increasing ultrasound frequency results in greater attenuation, which is why we use lower frequencies for larger animals. Just as increasing frequency results in tighter-packed wave fronts and less time for molecules to return to their neutral position, so, surely, must a much faster speed of sound propagation. Combined with the fact that bone’s physical properties in comparison with its surrounding tissues makes it a very strong reflector, and you end up with almost total attenuation of ultrasound energy by hardened bone. Luckily for us, rib spaces and gaps between vertebrae still allow ultrasound energy through and, as it diverges, the ‘gaps’ begin to be filled again. In addition, bone which is still forming is softer and seems to be less attenuating, so our images thankfully don’t stop at the spine of the first foetus we find!
Is absorption a problem?
The fact that bone absorbs ultrasound energy so strongly and converts it into heat is unproblematic when using correctly calibrated ultrasound equipment in the right way. Professional ultrasound equipment from a reputable supplier will have been specifically created for safe use on small animals, and with high quality ultrasound training from an accredited sonographer, you will be taught how to use this safely. You will also notice that your thermal index (TI), displayed at the top of your screen, will always be at very safe, low levels (i.e. below 1). In the example image above, it is at 0.1, but note that this is assuming that the user is scanning soft tissue (hence TIS: Tissue Index Soft Tissue).
Potential problems occur when using equipment in a way it was not intended, or without the education required to understand what is happening when you are sending ultrasound energy into the body. From an ultrasound machine point of view, be wary of any equipment that does not display the Tissue Index, as it is for the animal’s safety, and your protection (it’s a legal requirement when scanning within the United Kingdom, Europe, the United States or Canada). From a practice point of view, remember that absorption is the conversion of ultrasound energy into heat. Knowing that bone is so absorbent, avoid scanning over bone very close to the transducer for extended periods of time, and avoid scanning sick animals with low cardiac output or poor blood perfusion (this should warrant immediate referral to a veterinarian, anyway), because they will not have sufficient blood flow to dissipate the heat. In normal, healthy, adult animals, blood flow will rapidly dissipate any heat in tissues close to heated bone.
The latter point is the main reason why scanning your pregnant friend or relative probably isn’t the brightest idea. At key developmental stages, blood flow within the foetus is poor. Scanning over foetal cranial bone may cause heating of areas of the brain, for example. Obstetric sonographers use very specific settings – frequencies and power output levels – according to the stage of development of the child. They also use ultrasound machines and transducers which have been made with human pregnancy scanning in mind.
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