Feb 18, 2018
I want to start this episode with a quote from Robert Hooke, a British natural philosopher, architect and polymath who lived and worked during the years 1635 to 1703. Hooke’s personal story as a hardworking and honest, but ultimately irascible and enigmatic character is one I will save for another time but as I was working on this episode I found a quote attributed to him relevant to our topic for this episode. Robert Hooke wrote the following in the late 1600s:
“It may be possible to discover the motions of the internal parts of bodies, whether animal, vegetable, or mineral, by the sound they make; that one may discover the works performed in the several offices and shops of a man’s body, and thereby (sic) discover what instrument or engine is out of order…[I could proceed further, but methinks I can hardly forbear to blush when I consider how the most part of men will look upon this: but, yet again,] I have this encouragement, not to think all these things utterly impossible.”
Today we will explore how Robert Hooke’s prediction from the late 1600s has come true.
In this episode, I’m going to be discussing a type of radiologic imaging used often in urology called ultrasonography or, more commonly, ultrasound.
I recently had a moment to reflect on ultrasonography as one of my young partners and his wife are currently pregnant with their first child. Just to be technical here, she is doing the work of the pregnancy and he is taking on the responsibility of being anxious and excited. As only a nervous future father can do he was showing me the ultrasound that had been done of the 20-week-old baby in utero. The pictures, of course, showed the baby’s development in striking detail. facial structures, small hands and feet, a beating heart, and last but not least, the picture that my partner and I examined with the most scrutiny…wait for it…the developing genitalia. That’s right. It’s a boy!
According to one website describing ultrasound technology, ultrasound is marvelous for its ability to “peer inside patients with nary a needle or knife to be seen.” Although ultrasound lacks the resolution of CT scans or MRI scan, it also is easier and less costly to perform, doesn’t have the ionizing radiation associated with CT imaging, and the results can be seen right away. No needle, no knife. Ultrasound technology has allowed us to identify distinct characteristics of a child in utero.
We talked in episode number 41 with Dr. Chris Atalla about how we use the ultrasound technology during a robotic partial nephrectomy. Ultrasound is critical in defining, intraoperatively, the anatomic extent of endophytic masses during complex partial nephrectomy cases. Placing an ultrasound probe through a laparoscopic port directly on to the kidney during the surgery allows the surgeon to determine the tumor’s location on the kidney, its depth within the parenchyma, and size and location relative to the renal vasculature and/or ureter to avoid injury to those structures during the procedure.
The first reported use of ultrasound technology in urology was for characterization of renal masses into either cystic or solid. In 1970, Dr. Barry Goldberg and Dr. Howard Pollack presented at a meeting of the AUA in Philadelphia a report characterizing 150 renal masses into either cystic or solid based on A-mode ultrasound technology. In 144 of the 150 cases (96 per cent), the physical state of the mass, that is cystic, solid or complex in nature, was correctly predicted. This was a major breakthrough because, at that time, characterization of masses into the typically benign cyst or often cancerous solid mass typically required invasive procedures such as arteriography, aspiration, biopsy or surgical. The immediate advantage of being able to characterize a mass with a high degree of certainty without an invasive procedure was immediately apparent.
The study that I am referencing can, at the time of this recording, still be found online along with a number of other groundbreaking articles from the Journal of Urology over the last hundred years at JU100.org.
Ultrasound has become an important part of my nearly every day existence in the clinic where the determination of a renal lesion as being cystic or solid comes up nearly daily. Ultrasound is used for almost every organ that we deal with in urology as well.
Let’s looks at the different organs individually
What is ultrasonography or ultrasound? I want to take brief couple minutes to explain how an ultrasound creates the pictures that we see.
Sound travels in waves through the air, the ground, and various other things such our body tissues as a vibration or wave. The number of vibrations per second is called frequency. Frequency varies for each pitch and is measured in hertz. One hertz is equal to one vibration per second. A sound with a low frequency will have a low pitch. A sound with a high frequency will have a high pitch. For reference, the piano’s 88 keys span the frequencies 27.5 Hz (A0) to 4186 Hz (C8). A piano tuned to standard concert pitch puts middle C at a frequency around 261.63
A healthy human ear is said to be able to hear frequencies that range from 20 to 20,000 Hertz. Most bats can detect frequencies as high as 100,000 Hz. Elephants can hear sounds at 14–16 Hz, while some whales can hear sounds as low as 7 Hz (in water).
Ultrasound refers to sound waves whose frequency is more than 20,000 cycles per second, any sound above human hearing. Any frequency that is below the human ear of 20 hz is known as infrasound.
Bats use ultrasound for navigation, called echolocation. Bats send out an ultrasound signal while in flight. As sound travels if something gets in the way sound is reflected back in the form of an echo. The sound waves from the bat bounce of off structures in the bat’s flight path and the bats receive the signal back. The bat has the ability to alter its flight to avoid hitting things based on the echoes it receives.
Ultrasonography utilizes the principles of sound propagation and reflection to create the pictures that we see when we do an ultrasound
Let’s go back to my partner’s wife. When she is having her ultrasound done, the ultra-sonographer will place a probe called a transducer on her abdomen. Within the transducer ultrasound waves are produced by applying an electrical current to a piezo-electric crystal contained inside. When the transducer is applied to the skin, the waves are transmitted through the contiguous tissues of the body. Any change, however slight, in the nature of the tissue causes some sound to be reflected toward the emitting transducer which also serves as a receiver.
Got it? The transducer is sending out sound waves but also recording them as the sound waves are reflected back by the body tissues. The transmission of sound through tissue is determined by the specific acoustical impedance of that tissue. The junction of 2 tissues of different acoustical impedance or density is known as an acoustical interface, and the reflection of sound at an interface is known as an echo.
The common form of ultrasound imaging that you and I know is called B-mode ultrasound which displays a dot of light depending on echo depth and intensity from the transducer. All of those dots of light, varying in intensity on a grid, creates a picture when contrasting dots of light and dark create an image on the screen.
And those images can show striking detail, not only differentiating a renal mass into something cystic or solid but also allowing us to see, in utero, a baby in development.
In the 1600s Robert Hooke predicted we would determine the motions of the internal parts of the body by the sounds that they make. And he was right. When sound waves travel through our bodies we each echo back a different chorus based on our internal structure.
My mother always said I marched to the sound of my own rhythm. I guess she was right all along.