Medical Imaging.ppt

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Medical imaging
Medical imaging is a term used to describe any method that
allows us to view the inside of the human body without
invasive surgery. It includes the use of ultrasound and X-rays,
as well as more specialized techniques like magnetic
resonance imaging (MRI).
ultrasound
(fetus)
X-ray
(chest)
MRI
(brain)

Echolocation
Ultrasound imaging has been used in the natural world for
millions of years. Bats and dolphins echolocate by emitting
ultrasound and listening for its reflection off nearby objects.
Many species of bat use it to locate their prey.
Ultrasound was first used for a
medical purpose in the USA in the
late 1940s. Today it is most well
known as the technique used to
view a fetus during pregnancy,
but it has many other uses.
Ultrasound is useful because it is harmless, and it produces
real-time images of the soft tissues of the body. Its main
drawback is the low resolution of the images produced.

Ultrasound in medicine
The use of ultrasound in medical imaging carries no known
risks to the patient or the practitioner. This makes it the
method of choice for antenatal scanning, as there is no risk
to the unborn child.
One problem with ultrasound
is its low resolution. This
makes it difficult to interpret,
but it is useful for monitoring
the rate of development of a
fetus, and estimating how
advanced the pregnancy is.
A scan in the second half of a pregnancy (20+ weeks) can
show up any abnormalities in the limbs and vital organs.

Acoustic impedance
Where r is the density of the material, and v is the speed of
sound within it.
The amount of reflection depends on the acoustic impedance
of each material. Acoustic impedance, Z, is given by:
When ultrasound passes through an object and meets a
boundary between two materials, some of it is reflected.
Think of what you see when you look at your reflection in a
shop window – the effect is similar.
Q. What is the acoustic impedance of wood, if its density
is 650kgm–3 and sound travels through it at 3400ms–1?
Z = 650 × 3400 = 2.2 × 106 kgm–2 s–1
Z = ρv

Reflection and transmission
What happens when a sound wave meets a boundary
between two materials of different acoustic impedances?
The amount of the wave that is reflected is the reflection
coefficient, α, and is given by the following equation:
Where Ir is the intensity of the
reflected beam and Ii is the
intensity of the incident
(original) beam, and Z1 and Z2
are the acoustic impedances
of the two materials.
α = Ir / Ii = (Z2 – Z1)2 / (Z2 + Z1)2
Z1
Z2
Ir
Ii

Q2. A gel is placed between the transducer and the skin.
How much is reflected now? [Zgel = 1.65 × 106 kgm–2 s–1]
The air-skin boundary
Q1. What proportion of ultrasound is reflected by an air-skin
boundary?
[Zair = 4.29 × 102 kgm–2 s–1, Zskin = 1.70 × 106kgm–2 s–1]
α = (Z2 – Z1)2 / (Z2 + Z1)2
= (1.70 × 106 – 4.29 × 102)2 / (1.70 × 106 + 4.29 × 102)2
α = 1.00
α = (Z2 – Z1)2 / (Z2 + Z1)2
= (1.70 × 106 – 1.65 × 106)2 /(1.70 × 106 + 1.65 × 106)2
α = 2.2 × 10–4

Acoustic impedance in medical imaging
When sound meets a boundary between air and another
medium, most of the wave is reflected. What does this mean
for ultrasound as a medical imaging technique?
 A coupling medium (gel) is needed to
ensure most of the sound produced by
the machine is transmitted into the body.
 Ultrasound cannot be used to take
images of the lungs or any other part
of the body that contains an air cavity.
Ultrasound is also absorbed by bone, which means it cannot
be used to image the inside of the skull, but it has many
other uses, especially in the diagnosis of heart problems.

Ultrasound transmission

Pros and cons of ultrasound imaging

How is ultrasound produced?

Piezoelectric transmitters
At the right voltage and frequency, a piezoelectric crystal
emits an ultrasound wave: it is an ultrasound transmitter.
Correspondingly, it will turn an incoming sound wave into
an alternating voltage, acting as a receiver.
The crystal in an ultrasound scanner is made of lead zirconate
titanate (PZT). It has a thickness of half the machine’s working
wavelength. This allows the crystal to resonate, producing a
large signal.
transmitter receiver

Ultrasound: A-scan

Ultrasound: B-scan
An A-scan only gives information about tissue boundaries
along a one-dimensional line. This is useful for very specific
measurements, such as the depth of the eye or the diameter
of the head of a growing fetus, and can provide enough
information to indicate an abnormality.
A brightness scan (B-scan) is a
more complex imaging technique. It
uses an array of transducers, each
measuring amplitudes, and plots each
returning signal as a shaded pixel on
a screen, based on its amplitude.
The B-scan is the standard technique used for producing a
two-dimensional image of a growing fetus.

Understanding ultrasound

Radiography
X-ray imaging, or radiography, is best
known for identifying broken bones, but
it can also be used to view soft tissues.
The patient is positioned between an
X-ray generator and a photographic
plate. The X-rays expose the film
wherever they have passed through
the patient, leaving a shadow wherever
they have been absorbed by the body.
Bone absorbs more X-rays than tissue.
X-ray radiation is ionizing. It can damage cells and DNA.
It is therefore important that patients and doctors keep
their exposure to a minimum.

Rotating anode X-ray tube

Labelling a rotating anode X-ray tube

Creating a sharp image
X-rays cannot be focused – they create shadow images.
There are various ways of ensuring that this shadow is as
sharp as possible:
 Ensure that the photographic film is as close as possible
to the patient.
 Use a lead grid to absorb scattered X-rays.
 Keep the X-ray source as far as possible from the
patient to create parallel rays.
 Collect the image in a computer instead of a
photographic plate so that it can be enhanced
(fluoroscopic image intensification).

Exponential attenuation
The attenuation of X-rays through a material follows the
inverse square law: their intensity drops exponentially.
The half value thickness (x½) of a material is the thickness of
the material that cuts the X-ray intensity to 50%. Double this
thickness, and the intensity goes down to 25%. The half value
thickness is the equivalent of half life in radioactivity.
As X-rays pass through a material they are gradually
absorbed. This leads to attenuation (reduction) of the signal.
I = I0 e–µx
x½ =
ln2
µ
µ = linear attenuation
coefficient

Linear attenuation coefficient

Questions

Differential tissue absorption
When X-rays penetrate tissue, they are not uniformly
absorbed. If they were, the resulting image would show little
contrast. Some tissues absorb X-rays more efficiently than
others. Bone and metal absorb X-rays well so they show up
clearly in white on the image.
Air absorbs little radiation, so X-rays pass through the lungs
easily. They appear black on the film. Soft tissues are
composed mainly of water, and appear as shades of grey.
air in lungs
appears dark
bone
soft tissues

Enhancing the image contrast
X-ray imaging can be used to image soft tissues as well as
bones, but the attenuation coefficients of soft tissues are all
very similar. To make the relevant tissue as visible as
possible, an artificial contrast medium can be used.
To study the digestive system, the
patient is given a barium meal –
a meal containing barium sulfate.
The barium salt absorbs X-ray radiation,
creating a shadow on the image in the
same way that bone does on a normal
X-ray, and allowing the shape of the
digestive tract to be seen clearly.

Computer tomography
Computer tomography (CT or CAT scanning) is a form of X-
ray imaging that produces a much more detailed result than
a standard radiograph.
There is much debate about the risks
of exposing patients to the high levels
of radiation involved in this technique.
The CT scanner consists of a
rotating X-ray source and a ring of
detectors surrounding the patient.
The data produced builds up a
detailed cross-sectional image.
MRI produces just as much detail,
and carries no known risks.

Understanding X-ray imaging

θ
θ
Fibre optics
When light hits a boundary between two transparent media,
some is reflected and some transmitted. If the light hits the
boundary at more than the critical angle (θc) for the two
media, all the light is reflected. This is total internal reflection.
θ < θc θ > θc
Light can be transported along a thin flexible glass fibre by a
series of these internal reflections. The application of this
principle is called fibre optics.

Snell’s law and the critical angle
If the refractive indices of both materials are known, the
critical angle can be calculated using Snell’s law:
sinθc =
n2
n1
n1
n2
θc
This equation only has solutions when n2 < n1. If n2 is close
to n1, then sinθc is close to 1, and θc is close to 90˚, making
total internal reflection very unlikely.
An optical fibre consists of a glass core surrounded by
cladding with a lower refractive index. This is coated in
one or two reinforcing layers of resin or plastic.

Imaging with fibre optics
A bundle of optical fibres can be used to transmit an image. The
bundle is manufactured so that the relative positions of the
fibres are the same at either end. This is a coherent bundle.
Each fibre is like a pixel in a digital camera. The more fibres
that are packed into a small bundle, the higher the resolution
of the image, but the more expensive it is to manufacture.
coherent bundle
optical fibre

Endoscopy
Endoscopy can produce detailed
images in full colour and real time,
which makes it a very useful technique.
An endoscope is a device that uses fibre optic bundles to
see inside the body. It contains coherent bundles of optical
fibres to carry the image, and non-coherent bundles to carry
light into the area being examined.
Endoscopes can be used to examine
many parts of the body, such as the
digestive and respiratory systems.
If necessary, a small incision can be
made to allow access to the area.

Keyhole surgery
Only very small incisions are required, which means that
compared with conventional surgery:
It is used for procedures such as gall bladder or appendix
removal, middle ear investigations, and joint operations.
Keyhole surgery is a form of surgery that uses an
endoscope with surgical attachments to perform an operation
through small incisions in the skin.
 the rate of recovery is much greater
 the risk of infection is lowered
 the cost of the procedure is reduced
 the cosmetic result is better.

Magnetic resonance imaging

Positron emission tomography

Other techniques: true or false?

Glossary

What’s the keyword?

Multiple-choice quiz

Medical Imaging.ppt

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