EE4603 Ch1(Intro)
Transcript of EE4603 Ch1(Intro)
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EE46 3 BiomEdical imaging
SyStEmS
I - INTRODUCTION
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Medical imaging aims to produce images (2D or 3D) of normal and
diseased tissue within the human body.
See into the body with minimal distress/inconvenience to the patient
Started in 1895, when Wilhelm Conrad Roentgen discovered x-rays.
The first radiograph:
Mrs Roentgens left hand
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Over the next few decades, x-raysbecame a widely used diagnostic tool.
X-rays are suitable for
examining bone structure, e.g., fractures and breaks investigating some tissue abnormalities
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A new imaging option, computed tomography(CT), became available in the
early 1970s.
By combining a series of X-rays taken from different angles, computeralgorithms can reconstruct a 3-D image of any part of the body.
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Magnetic resonance imaging(MRI) was developed in the 1980s. It is not
based on X-rays. The patient is subjected to a very strong magnetic field. A
radio signal is then applied, which triggers atoms in the body to send out
signals of their own. These radio signals are collected and processed to give3-D images.
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Ultrasound imagingwas developed from sonar technology used during World
War II.
It obtains images by reflecting sound waves off tissues inside the body.
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In nuclear medicine imaging(NMI), a patient ingests or is injected with a slightly
radioactive substance. The distribution of the substance in the body can be
imaged to give an indication of pathological conditions, e.g., tumours and
increased metabolic activity.
In positron emission tomography
(PET), the radioactive substanceemits positrons.
In single-photon emission
computed tomography (SPECT), thesubstance emits high-energy
photons (gamma rays).
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PET images SPECT images
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PET/CT combines PET and CT imaging in a single gantry system:
- the two sets of images are acquired sequentially and merged into one
- metabolic activity can be correlated with anatomic structures
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Method Parameters Measured Medical Applications
X-ray, CT Attenuation of photons Anatomy, mineral content
MRI Concentration of water,
physical and chemical
environment of the water
molecules
Anatomy, blood flow, chemical
composition
Ultrasound Echoes returning from reflectingsurfaces of tissues
Anatomy, tissue structuralcharacteristics, blood flow
PET, SPECT Concentrations of radioactive
isotopes
Metabolism, receptor site
concentration
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Fundamentals of Image Acquisition
The general imaging process is similar for the various modalities
Image formation
The first step in biomedical image formation occurs when some form of
energy is measured after its passage through and interaction with some
part of the body. The measured signal may be processed to give 2D/3D
images in digital format
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A digital image is an array (or
matrix) whose elements
denote the brightness (orintensity) values. The
individual elements are
often called pixels.
0 000 0 000
0
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0
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0
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0
76 333434 43 3334
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16 15 56 3 5653 7
26 48
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3133 53
8 65
8 32
65 84
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pixels
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The 3D image is comprised of a stack of these 2D images (or slices). A 3D
image element is called a voxel.
image slice
voxel
2D image
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Spatial resolution
This refers to the ability to see fine details. An imaging system
has higher spatial resolution if smaller objects in the image can beviewed. (A quantitative measure of resolution uses the point spread
function, PSF.)
High-
resolution
images
Low-resolution
images
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The limiting spatial resolution is the size of the smallest object
that is visible. This depends on the imaging modality as well as the
quality of the scanner.
Typical values:
X-ray 0.08 mm
CT 0.25 mm
US 0.3 mm
MRI 1 mm
PET 5 mm
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414 x 490
207 x 245
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The spatial resolution may differ for each orthogonal direction represented in a
volume or 3D image (anisotropic) or they may be equal (isotropic). We may
differentiate between the in-plane resolution and the through-plane resolution.
y
z Through-plane res.
x
Volume being
imaged
In-plane res.
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The term spatial resolution is also used to denote the pixel size in a 2D
image (voxel size in a 3D image).
Consider a CT scan that is used to image a volume 500x500x500mm
3
.
If there are 200 slices and each slice is of size 256x256 pixels, then
through-plane resolution = 500200 = 2.5 mm
in-plane resolution = 500256 = 1.95 mm
500 mm500 mm
500 mm256 rows
256 columns
200
slices
One voxel
1.951.95
2.5
Image volume Image slice
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Local contrast
Individual structures are recognized by local differences in signal strength
among adjacent structures. The visibility of a structure is related to itscontrast against the structures surrounding it.
poor contrast good contrast
good contrast
MRI
CT
poor contrast
Signal magnitude
along dashed line
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The difference in intensity between the object of interest (e.g., a tumour
mass) and the surrounding tissue (the background) is measured by the
local contrast:
where
signal at the target
signal at the background
t b
b
t
b
s sC
s
s
s
=
=
=
ts
bs
0
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Temporal resolution
Aperture time tap:
the amount of time it takes to capture the signal information to form oneset of images. A small aperture time will help to reduce motion artifacts.
Image repetition time tr:
The interval of time required to produce successive images. It is the time
needed to rest the imaging system to acquire another set of information
sufficient to form a new image. This limits the ability of the systemto acquire 4-D data sets, that is, 3D volumes through time.
Time
tap
tr
tap = aperture time
tr = image repetition time
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3D Visualisation
Medical visualisation can simplify the task of the radiologist by providing a
3D representation of the patient's anatomy constructed from the set of
image slices.
Conventional view
Blood vessels
Hand
Head