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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

0

0

0

0

0

0

0

0

76 333434 43 3334

27

53

16 15 56 3 5653 7

26 48

36

28

33 8

562342

23

44

34 4443 8

3133 53

8 65

8 32

65 84

68

66 52354 4

89756

5623

11

0

5

6

8

9

22

0

0

0

0

0

0

0 43

64

55

54

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