Transcript Imaging Basics

Imaging Basics
A medical image is a 2D or 3D distribution of signals which
represent properties of an object.
The purpose of medical images include:
Structure e.g. chest X-ray
Organ function e.g. renal scan
A combination of the above e.g. functional MR
The applications of medical images include:
Screening
Diagnosis
Treatment planning
Intervention
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Image Production
There are few natural signals emitted by the human body
which are useful to create medical images. It is therefore
necessary to use external (X-rays, ultrasound etc.) or internal
(radiopharmaceuticals) probes to image. Images are created
by measuring the transmitted or emitted signals, after the
signal has interacted with the tissue. The signal needs to be
strong enough to penetrate the tissue, but not so strong that
its interactions with tissue cannot be detected. Also the image
quality versus dose to patient must be optimised.
Resonant probes have wavelengths similar to that of tissue and
the probe is attenuated and transmitted (X-rays, γ-rays, RF,
infrared).
Non-resonant probes do not have similar to tissue. The signals
are reflected and refracted (ultrasound).
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Imaging Modalities
Imaging
Modality
Probe
Interaction
Property
Image
Radiography
80 kV X-ray
Photoelectric,
Compton
Attenuation
map
X-ray
transmission
2D projection
Computed
Tomography
120 kV X-ray
Photoelectric,
Compton
Attenuation
map
3D
reconstruction
from multiple
tomographs
Ultrasound
Megahertz
sound pulses
Reflection,
refraction
Tissue density,
boundaries
2D sector,
reflection
Magnetic
Resonance
Electromagnetic
pulses
Resonance of
nuclear spins
Proton density
3D
reconstruction,
EM signal map
Nuclear
Medicine
γ-ray emitting
isotope
Uptake of
radiopharmaceu
tical
Concentration
of
pharmaceutical
Distribution of
emissions
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Image Display
It is important that the probe is sampled frequently enough so
that no information is lost. If it is under-sampled aliasing can
occur and image information lost.
Image contrast is very important. With analogue images it is
necessary to choose optimal parameters before image
acquisition as post exposure processing is not possible. With
digital images windowing is possible to create maximum
contrast.
With film images it is necessary to use a dark room to allow
maximum image contrast for the human eye.
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Image Noise
All medical images are subject to noise. The sources of noise
include:
• Scattering
• Detector inaccuracies
• Electronic interference
• Signal processing
The effect of noise on the image is to decrease contrast and
detail. The important quantity in any image is the signal to
noise ratio SNR. The higher this value the better the quality of
the image.
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