Ultrasound from Past to Present

Shane Kessel CRGS, CRVS, RDMS, RVT

Sonography has had a relatively short history in the world of imaging.  The ultrasound exam is dynamic and utilizes high frequency sound waves to produce an image.  These frequencies are any above the audible sound range of humans.  This range on average is 20 – 20,000 Hz or cycles per second.  Diagnostic medical sonography uses frequencies in the millions of cycles per second or Megahertz (MHz) to produce images of anatomy6. A Sonographer evaluates the rapid production of images in real-time and records still images, or clips of images, for interpretation by a physician.  How do we achieve sight through sound?  What evolution of technology resulted in the present day application of sound and where is the future potentially?

The earliest record of the notion of echolocation was in 1794.  A scientist by the name of Lazzaro Spallanzani studied bats.  In his work, he discovered that a blind bat could function but a deaf bat could not.  Although, at the time, he did not understand the reason, this formed the basis for ultrasound physics1.

In 1877, brothers Pierre and Jacques Curie discovered piezoelectricity1. This is how ultrasound transducers convert electrical energy into the mechanical sound waves sent into the patient.  This is also how the sound waves convert back into the electrical signal for processing by the computer and finally, into an image on the screen.  Simply stated, certain materials contain properties that when a mechanical force is applied, they produce an electrical current.  The reverse of this is true as well.  When one applies a voltage to these piezoelectric materials, they will vibrate, thus emitting a mechanical wave or sound.

There are many historical uses of ultrasound for medical purposes performed over the last century.  It would be difficult to list the names and contributions in this article but most would agree the work of Ian Donald, Tom Brown and colleagues in Glasgow, in the mid-1950s, led to the wider use of ultrasound in the world of medicine2.  Ian Donald and Tom Brown are credited with the development of the first compound ultrasound scanner3.

Before we delve into compound scanning, we need to go back to the earliest form of ultrasound used in medicine.  Amplitude mode, or A-mode, was the first type of ultrasound.  A-mode works by sending a single pulse of sound into a medium and records the returning echoes on a graph.  The echoes are displayed on an oscilloscope in the form of spikes.  Each spike on the display represents an interface where sound reflects back to the source and the position on the X-axis is representative of the time it took for the echo to return.  Knowing the speed of sound in the medium allows the computer to calculate the depth of the reflector from the source.  In the field of non-destructive testing, A-mode detects flaws in metal, which pose potential safety risks in aircraft or ships to this day.  The illustration below outlines the process (Figure 1).

 

 Figure 1.  A transducer produces ultrasound that travels through the metal object.  When the sound hits the front and back walls a large spike is produced from the reflected sound.  Smaller spikes are seen originating from the front and back of the defect present in the object.

From here, we move to brightness mode scanning, or B-mode.  Like A-mode, a single pulse of sound propagates through a medium.  With B-mode, we replace the amplitude spikes with shades of gray.  The more intense the echo returned, the brighter the shade of gray on the display.  If no echo returns from a specific depth, black appears on the display.  Using a single pulse has virtually no benefit.  This is where compounding comes into play.  By sending several pulses, in a sweeping motion, across a region of interest, the echoes from the individual pulses can be added together to form an image.  Thousands of little dots of varying shades of gray come together to form a representation of the anatomy surveyed.

In the earliest days of compound imaging, the images were bi-stable in nature.  In other words, they consisted of black and white shades only.  Although, the contrast was high, there was very little tissue differentiation (Figure 2)5.

 

 Figure 2. (a) shows a cross-section of an intact spine (small circle) and (b) an image of a bifid spine.

A short history of sonography in obstetrics and gynaecology. Facts Views Vis Obgyn. 2013;5(3):213-29.4

Eventually the technology progressed to include more shades of gray, improving tissue differentiation.  Further technological advancements allowed the production of many image frames in rapid succession.  This allowed for “real-time” assessments of anatomy, adding to the amount of diagnostic information obtained from the scan.

During this evolution, many technologies, such as Doppler, expanded the scope of ultrasound as an imaging modality.  Doppler provided the sonographer the ability to assess the hemodynamics of the vascular system.  Doppler technology gives both quantitative and qualitative information of blood flow in the medium to large vessels of the body as well as the heart.  The sonographer can quickly assess the presence, direction and quality of flow with both colour images and spectral tracings.  Spectral analysis of the waveforms obtained with Doppler can yield measurable velocities, pulsatility and resistivity indices and acceleration of flow.

Presently, there is much study in the use of contrast in ultrasound and elastography.  Both of these technologies are not new, but appear to be a big part of the future of ultrasound imaging.  For example, contrast enhanced ultrasound (CEUS) can give information which is limited by current Doppler technologies.  Although Doppler can give information about flow in medium to large vessels, it is limited in demonstrating perfusion5.  The addition of contrast and specialized imaging techniques can both demonstrate perfusion and suppress the background signal from surrounding tissue5.  The resultant images are similar to what is demonstrated with contrast enhanced CT and MRI when imaging focal liver lesions as an example6.

Elastography is the use of ultrasound to evaluate the elastic properties of tissue.  In other words. Is the tissue hard or soft?  This is synonymous to the manual palpation of tissue but done with imaging6. Elastography is used clinically for cancer detection, characterization of small parts, viability of the myocardium and more6.  Assessments can be both qualitative and quantitative depending of the method used.  Quantitative analysis uses a colour overlay on top of the 2D image, dependent on the manufacturer.  Typically, blue represents stiff tissue and red softer tissue7.

In a relatively short time, sonography has evolved, like so many other technologies, to become an important tool in patient diagnoses and treatment.  Not only has it expanded in practice in the world of imaging, the energy itself has become of interest for several medical applications.  Ultrasound energy paired with MRI imaging is one example of current and future practice.  Ultrasound energy thermally ablates tissue, removing tumors without the need of a surgical incision8.

Ultrasound’s future in imaging and medicine is definitely an exciting one. Examples cited in this article are just the “tip of the iceberg” with many new advancements on the horizon.  It is a thrilling time to be involved in the world of imaging.


References:

  1. Ultrasound Schools Info. History of Ultrasound. Retrieved from https://www.ultrasoundschoolsinfo.com/history/
  2. Woo, J. (1998-2002) A short History of the development of Ultrasound in Obstetrics and Gynecology. Retrieved from http://www.ob-ultrasound.net/history1.html
  3. The British Medical Ultrasound Society. Retrieved from https://www.bmus.org/about-ultrasound/history-of-ultrasound/
  4. Campbell, S. (2013; 5(3): 213-29). A Short History of Sonography in Obstetrics and Gynaecology. Facts Views Vis Obgyn. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3987368/
  5. Wilson, S.R., Greenbaum, L.D., Goldberg, B.B. (2009, July). Retrieved from https://www.ajronline.org/doi/full/10.2214/AJR.09.2553
  6. Kremkau, F.W. (2016). Sonography, Principles and Instrumentation, 9th St. Louis, Missouri: Elsevier.
  7. Sigrist, R.M.S., Liau, J., Kaffas, A.E., Chammas, M.C., Willmann, J.K. (2017, March 7). Ultrasound Elastography: Review of Techniques and Clinical Applications. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5399595/
  8. Jolesz, F.A. (2014, April 30). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4005559/

 


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