Monday, 8 February 2010

Submarines to sprogs

Dr Mark Lythgoe gave me another generous helping of food for thought the other morning, with episode three of his Images That Changed the World series, considering the foetal ultrasound scan.


Ultrasound imaging (also known as sonography) is an incredible tool for safe and non-invasive investigation of the soft tissues of the body. It can reveal the presence of an embryo at a very early stage in development, as well as show the foetus in sufficient detail not only to let its digits be counted, but to look further inside and resolve abnormalities in the internal organs.  Dr Joseph Woo has put together a fantastic site on the usage and history of ultrasound in obstetrics, including an amazing gallery of images.

Twin pregnancy at 10 weeks from Obstetric Ultrasound - A Comprehensive guide
Ultrasound images are such a routine part of obstetrics now that it's hard to imagine a time without them. Historically, however, the 9 month journey from bundle of cells to wriggling sprog was shrouded in mystery, punctuated by unseen movement and illustrated only by the unsettling sight of cold, grotesque pickled embryos and foetuses in jars. The programme explored the impact on parents of having even a scruffy little black and white outline of their offspring-in-progress, but what caught my ear was the history of the development of ultrasound imaging technology.

Towards the end of the 18th Century, Italian experimental biologist Lazzaro Spallanzani concluded that the uncanny ability of bats to orient themselves without the use of sight is related to their hearing. 140 years later Donald Griffin recorded their inaudible cries using the first ultrasound microphone and completed the picture. Bats emit sharp sounds above the range of human hearing ('ultrasound' begins just beyond what we can hear, about 20 kilohertz), and listen for the echoes, using the timing and volume of the echo to guage the distance and nature of obstacles and prey. The resolution of echolocation is dependent on the frequency of the sound used, so the extremely high-pitched squeaks of bats allows them to detect tiny prey insects on the wing. I found a great page from the Western Ecological Research Center, with modified recordings of the sounds various species of bat use to investigate their surroundings – there's even one which is low enough for humans to hear without modification.

Bat in flight by Arnold Song at Brown University, who was researching the aerodynamics of their flight

Other species also use echolocation, from cave-dwelling swiftlets to the tiny shrew, but perhaps the most well-known are the toothed whales, including dolphins. Greater sound transmission in the liquid medium apparently allows a dolphin to resolve a golfball a football field away, but that's far too sporting a measure for me. Let's just say that it beats a bat's ~17 meter range.

It wasn't until the Titanic was rusting on the sea bed, however, that humans really took up the act.  In 1913 Alexander Behm patented the echo sounder, a device intended to prevent future disasters by detecing icebergs.  It proved much more effective at locating the sea-bed though, and has been a huge aid to shipping since.

During WWI the threat of enemy submarines drove further research into the potential of echoes, culminating in the invention of ASDIC (later renamed 'SONAR' by the Americans), the piercing 'ping' of which is a familiar staple of tense, dank aquatic thrillers. Reflected sound was also used to detect far smaller dangers, when engineers used ultrasound to examine the internal structures of aircraft wings for stress-induced cracks.

Submarine Sonar Operator's Manual - a fantastic piece of online archive material, training operators of the US's state-of-the-art sonar equiment at the end of WWII - from the Maritime Park Association

In the wake of WWII, obstetrician Professor Ian Donald returned home and began to wonder about the application of this technology to his own profession.  Armed with an ultrasound transducer, he began experimenting on tumours and cysts removed from patients, to see whether they could be differentiated from other tissues, such as muscles (represented by slices of beef). In 1958 he published his work, and so began the application of ultrasound imaging to medicine.

While it has used the same basic principle of reflected sound waves for the past 50-odd years, sonography has evolved into a vital medical tool.  Imaging resolution has improved hugely, structures can be viewed in realtime (cue videos of waving foetal hands and wrinkling noses) and shown 3D rather than cross-section.  Ultrasound can even reveal blood flow in the arteries by analysing the doppler shift of the echoes.  It can also be used in more active ways, such as in controlled powerful bursts to break up kidney stones into easily passed fragments. 

From submarines in the watery deeps to foetuses immersed in amniotic fluid - not a bad bit of lateral thinking, and one that has had a huge impact on medicine and pregnancy.

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