Physics is a
very important aspect of the modern day medical field, and without it, the
diagnosis of medical problems would be challenging to say the least. In
particular, the world of medical imaging has benefited enormously from physics-based
diagnostic techniques, such as Ultrasound. Ultrasound (or Ultrasonic) is
defined as a mechanical, longitudinal sound wave with a frequency exceeding the
upper limit of human hearing. However, despite the term referring to any sound
wave with a frequency greater than 20kHz, ultrasound generally becomes useful
at much greater frequencies, in the range of 1-50MHz. Higher frequencies tend
to be used for scanning areas close to the surface of the body as high frequency
waves are easily absorbed, whereas, lower frequencies waves are used to scan areas
deeper down in the body because they are more penetrating. In this high
frequency range, the sound waves can be used to scan over the human body via a
transducer (as shown in Figure 1),
and an internal image can be formed using the “echoes” from internal organs
Figure 1. Diagram of a Transducer
is produced and subsequently detected by the ultrasound transducer, illustrated
in Figure 1. Transducers can send and
receive high frequency signals and later convert them into electrical signals
that can be diagnosed.
is a device used to convert some other form of energy into an ultrasonic
vibration. There are numerous types of transducer, each characterised by the
energy source and the medium into which the waves are being produced. There are
forms of mechanical devices, including gas-driven or pneumatic transducers,
however, electromechanical transducers are far more useful. The two most common
forms of electromechanical transducer are the piezoelectric and
magnetostrictive devices. The magnetostrictive transducer uses a magnetic material
in which an applied oscillating magnetic field forces the atoms of the material
towards and then away from each other, consequently producing a periodic
variation in length of the material, which causes a high-frequency vibration. This
form of transducer is mainly used in the lesser frequency ranges and are
typically found in ultrasonic machining and ultrasonic cleaners.
most widely used and versatile type transducer is the piezoelectric crystal
transducer. This produces a mechanical vibration by converting an oscillating
electric field that has been applied to the crystals (can be quartz, Rochelle
salt and certain types of ceramic). Piezoelectric transducers are so popular as
they can be operated at all output levels over the whole frequency range.
Different shapes are chosen for certain applications, for example, a concaved
shape provides a focused ultrasonic wave, whilst a disc shape will create a
plane ultrasonic wave 3.
The process of Ultrasound Imaging:
A voltage is
rapidly applied and removed across the transducer repeatedly so ultrasound
waves can be produced by the piezoelectric crystals expanding and relaxing. The
transducer is applied to the skin with a gel and it directs ultrasound waves
into the internal anatomy. As the ultrasound waves encounter tissues with
different characteristics and densities, they produce ‘echoes’ that reflect
back to the transducer. This occurs more than 1000 times per second. Echoes
won’t be produced if there’s no difference in tissue or between tissues, e.g.
blood and bile. When the reflected waves reach the transducer, the
piezoelectric crystals compress and relax, consequently generating a voltage
that corresponds to the intensity of the ultrasound wave that hits them. The
information gathered by the crystals is then processed by a computer to display
an image on a screen (sonogram). The crystals are repeatedly activated many
times in such a way that a complete image frame is formed around 20 times per
second, so that real-time motion is displayed in the ultrasound image.
Any time the
ultrasound waves reaches an interface (change in medium), such as an organ in
the body, part of the wave will be reflected, and the rest will be transmitted
through the medium. The respective intensities of the reflected and transmitted
waves will clearly be less than that of the original wave, and this could
become an issue when detecting reflected waves from deep in the body tissue due
to their very low intensity. The ratio of the intensity of the reflected beam
to the intensity of the incident beam is given by a relationship of the
acoustic impedance, z, of the two materials either side of the interface (Figure 2).
, where .
Figure 2. Equation
relating the ratio of the intensity of the reflected wave to the intensity of
the incident wave
difference in density, and therefore impedance between tissues in the body is
quite small, and so there isn’t an immediate problem inside the body. However,
there is a very large difference in acoustic impedance between air and the
body, and therefore an Aquasonic gel with a density like that of body tissue is
used when applying the transducer for ultrasound imaging. It must also be
recognised that the further the sound ‘pulse’ travels, the more likely it is to
be attenuated, so, to compensate, the signals from deeper tissues are amplified
to give similar intensities to waves from other boundaries. This creates a
clearer signal and therefore, a better image.
Figure 3. An ultrasound image
obtained of the heart’s left ventricle 5.
numerous other uses for ultrasound outside of the medical field, one of the
most notable is for marine ranging and navigating 6.
Ultrasound in Navigation
navigation and ranging) has extensive marine applications. By emitting
ultrasonic sound pulses and recording the time it takes for the pulses to
reflect off a distant object, the location of the object can be determined and
its motion can be tracked. Ultrasonic waves are used as opposed to sound waves
because higher frequency sound waves travel much greater distances with less
diffraction underwater 7.
two forms of Sonar, active and passive. Passive sonar is primarily used to
detect noise from other marine objects (such as submarines, ships or marine
animals). Passive sonar is particularly useful for military vessels that want
to stay undetected as it does not involve emitting a signal. However, passive
sonar is unable to measure the range of an object unless it’s used
simultaneously with other passive devices. For example, two passive bodies at
known locations can also use triangulation to locate and track a third boat or
submarine. Active sonar on the other hand, uses transducers to emit a signal or
pulse of sound into the water. If there is an object in the path of the sound
wave, the wave will ‘bounce’ off the object and return as a signal to the sonar
transducer. The transducer, if equipped appropriately, can subsequently
determine the orientation and range of the object, by measuring the time
between the emission of the sound pulse and the detection of the ‘echo’ signal 8.
It is worth
noting, as the ultrasonic wave reflects off a moving object, the frequency of
the reflected wave will either increase or decrease depending on whether the
object is moving towards or away from the signal (the Doppler effect). The
amount of frequency shift can be used to determine the speed of a moving
submarine for example, a very useful tool in marine military vessels. There are
some limitations with these techniques. For example, the distance over which
these techniques can be used is restricted by temperature gradients in the
water, which cause the sound beam to bend away from the surface and create
‘shadow regions’ 9.
is also used in ranging, to map the bottom of the ocean, producing charts of
depth that are used for navigation, specifically in shallow waters. In the
modern day, even small boats are equipped with sonic ranging devices that map
the depth of the water so the navigator can keep the boat away from shallow
points to avoid beaching 10.
the medical field, ultrasound is a very effective technique for imaging and
diagnosis. It is a process that can be performed in real-time, and there is no
delay between the clinical picture and imaging. Also, there are no real health
risks associated with ultrasound imaging, whereas x-ray imaging can be a health
risk overtime due to exposure to radiation. However, there are limitations to
ultrasound, for example, ultrasound is not ideal for imaging an air-filled
bowel or organs the bowel is obscuring. This is because gas interferes with
ultrasound waves 11.
also a useful technique in the modern day and can massively aid military and
scientific vessels when navigating underwater. The downsides of sonar are
comparable to that of ultrasound imaging. The ultrasonic waves can be easily
disrupted by any external sound waves, including surface noises, other ships
and sea life. Unfortunately, most of these problems are unavoidable and limit
ultrasound as an imaging technique in the long run 12.