Physics “echoes” from internal organs [1]. Figure 1.

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
1.

 

 

 

 

 

                                      Figure 1. Diagram of a Transducer
2.

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Transducers:

Ultrasound
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.

A transducer
is a device used to produce an ultrasonic vibration. There are numerous types
of transducer, each characterised by the medium in which the waves are formed
and the source of the vibration. 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 an applied oscillating magnetic field to force
the atoms of a magnetic material towards and then away from each other,
consequently producing a periodic variation in length, which causes a
high-frequency vibration. This type is mainly used in the lesser frequency
ranges and are typically found in ultrasonic machining and ultrasonic cleaners.

But, the
most widely used form of transducer is the piezoelectric crystal transducer.
This produces a mechanical vibration by converting an oscillating electric
field that has been applied to the crystals. Piezoelectric transducers are so
popular as they can be operated at all output levels over the whole frequency
range. Different shapes are applied for certain uses, for example, a concaved
shape provides a focused sound wave, whilst a disc shape will create a planar
sound 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). A real time-motion picture can be displayed as
the crystals are activated several times – in such a way that an entire image
frame is created about 20 times each second.

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
4.

The
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.

There are
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

Sonar (sound
navigation and ranging) has vital marine functions. By emitting ultrasonic
sound pulses and recording the time it takes for the pulses to ‘bounce’ off an
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.

There are
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 track a third body using a method known as triangulation.
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 sound pulse being emitted 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 techniques cannot be
used over great distances because sonar is restricted by the water’s
temperature gradients, which cause the sound beam to create ‘shadow regions’ as
the beam curves away from the surface 9.

Ultrasound
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, small boats are now capable of mapping the depth of the water as
they are often equipped with ultrasound devices 10. This is a massive
help to the navigator when trying to avoid shallow points.

Summary

Overall, in
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.

Sonar is
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.