Remote Sensing

X-rays

Computed tomography scanning (CT)

Ultrasonic waves

Magnetic resonance imaging

1. X-rays

Production of X-ray

Main features of a modern X-ray tube

Use of X-rays in imaging

Solve problems using equation I = I0e–µx

Production of X-rays

A typical spectrum of X-rays

Spectrum has 2 components. There is continuous distribution and series of high-intensity spikes

When a charged particle is accelerated, electromagnetic radiation is emitted. (Bremmstrahlung radiation)

Greater the acceleration, the shorter the wavelength.

When high speed electrons strike a metal target, large accelerations occur and the radiation produce is in the X-ray region.

There is a minimum wavelength where the whole energy of electron converted into energy of photon.

kinetic energy of electron = eV = hc / λ

Peaks correspond to the emission line spectrum of the atoms of target.Electron that bombard the target excite orbital electrons(target) in the lower energy level.Subsequent de-excitation of electrons gives rise to line spectrum

Main features of X-ray tube

Intensity of X-ray beam is determined by the rate of arrival of electrons at the metal target(tube current)

Tube current is controlled by heater current

Hardness of X-ray beam(penetration) is controlled by the accelerating voltage.

More penetrating X-rays have higher photon energy and large potential is needed.

Longer wavelength X-rays(softer X-rays) are always produced.

They would not be able to pass through patient

But, these soft X-rays would contribute to the total radiation dose.

So, an aluminium filter is frequently fitted across the window

Use of X-ray in imaging

X-ray radiation affects photographic plate. Once exposed, will appear blackened after developed. X-ray beams are used to obtain ‘shadow’ pictures of the inside of body.Ex: absorption by bone of X-ray photon is greater than the absorption by muscle or tissues.

Quality of image depends on sharpness and contrast.

Sharpness is concerned with the ease with which the edges of structures can be determined.

Contrast: degree of blackening of the image between one organ and another

Sharp image is possible if X-ray have minimum width.

Factors in the design of the X-ray that may affect sharpness

The area of the target anode

The size of the aperture

The use of lead grid in front of the photographic film to absorb scattered X-ray photons

The area of target anode

Bigger focal spot will increase the area of partial shadow, causing a greater blurring of the edge of image.

A small anode is desirable for a sharp image

The size of the aperture

Random scatter increases with the beam size and causes a blurring of the image.

A small aperture is preferred for sharp image with good constrast.

Use of lead grid

Use of lead grid in front of the photographic film will absorb scattered X-ray photons

In order to improve contrast, a ‘contrast medium’ is used.

Stomach: patient given a drink containing barium sulphate

Blood vessels: a contrast medium that absorbs X-radiation would be injected into bloodstream

Solve problems using equation I = I0e–µx

Intensity decreases in proportional to the inverse of the square of the distance from source

Thus, in a vacuum, I = I0/r2

In a medium where absorption processes are occurring, the intensity I of a parallel beam decreases by a constant fraction

The transmitted intensity I is given by I = I0e-µx

The thickness of the medium required to reduce the transmitted intensity to one half of its value is a constant known as half-value thickness x ½

The half value thickness x ½ is related to the linear absorption coefficient µ by

x ½ . µ = ln 2

2. Computed Tomography scanning

Purpose of CT scanning

Principles of CT scanning

how image of a 8-voxel can be developed using CT scanning

Purpose of CT Scanning

The image of an X-ray plate is a ‘flat image’ and does not give any impression of depth.Tomography is a technique by which an image of a slice, or plane, of the object may be obtained.In this technique, a series of X-ray images are obtained from different angles through a slice but in the plane of the slice

Images of successive slices can be combined to give a three-dimensional image

•Principles of CT scanning

The section through the body is divided up into a series of small units called voxels

The image of each voxel have a particular intensity-pixel

Pixel are made up of measurements of X ray intensity

Suppose a section consists of four voxels with intensities as below.The number on each voxel is the pixel intensity that is to be reproduced

If a beam of X-rays is directed from the left, then detectors will give readings of 5 and 9This allows the four voxels to be reconstructed

The X-ray tube and detectors are now rotated through 45o

The procedure is repeated and the X-ray tube and detectors rotated a further 45o

Final images are taken after rotating a further 45o

In order to obtain the original pattern of pixels, two operations must be performed

1. The background intensity must be removed. The background intensity is the total of each set of detector readings. In this case, 14 is deducted

2. The result must be divided by three

This processes are

Image of each section is built up from many small pixels, each viewed from many different angles.

Needs a powerful computer and complicated programmes

Generation and detection of ultrasonic waves using piezo-electric transducers

Ultrasonic waves- produced using a piezo-electric transducer such as quartz.

Two opposite sides of the crystal are coated with thin layers of silver to act as electrical contacts

Position of the oxygen links are not rigid fixed.

Movement can be encouraged for oxygen ion by applying an electric field.

When the crystal is unstressed, their effects are neutralised

If a constant voltage is applied across the electrodes, the positive silicon ions are attracted towards the cathode and the negative oxygen ions towards the anode.

Depending on the polarity of the applied voltage, the crystal becomes either thinner or thicker

An alternating voltage applied will set up mechanical vibrations in the crystal

If frequency of voltage is same as natural frequency of vibration, resonance occurs.

The oscillations are in ultrasonic range(>20 kHz) producing ultrasonic waves

Ultrasonic transducers can also be used as receivers

How it can be used as receiver

An ultrasonic wave which is incident an unstressed piezo-electric crystal, the pressure variations alter the positive and negative ions within the crystal.This induces opposite charges on the silver electrodes, producing a potential difference between them.This p.d. can be processed.

Main principles behind the use of ultrasound

When an ultrasound wave meets the boundary between two media, some of the wave energy is reflected and some is transmitted

From energy considerations,

I = IR + IT.

The relative magnitudes of the reflected and transmitted intensities depend not only on the angle of incidence but also on the two media.For any medium, a quantity known as specific acoustic impedance is

Z = ρc ρ=density of medium c= speed of light

When a wave is incident normally on a boundary between two media having specific acoustic impedance of Z1 and Z2, the ratio IR / I is

The ratio IR / I is known as intensity reflection coefficient,

Some approximate values of specific acoustic

impedance Z

Intensity coefficient is very large for ultrasound entering of leaving the human body(a boundary between air and soft tissue)

It is important to ensure there is no air trapped between the transducer and skin.

This is achieved by using medium as gel.

A second factor that effects the intensity of ultrasonic waves through a medium is absorption.

As wave travels through medium, energy is absorbed by the medium and the intensity of beam decreases exponentially.

The temperature of medium rise.

The heating effect of suitable frequencies, is used in physiotherapy to assist with recovery

The intensity I of the beam after passing through the medium is

I = I0 e-kx,

k =absorption coefficient

Values for ultrasound of frequency 1MHz

A-scanA-scan measures the distance of different boundaries from the transducerA short burst of ultrasound is transmitted, and some ultrasound is reflected and some is transmitted.The reflected pulse is picked up by the transducer. Echoes from deeper in the body tend to be of lower intensity

Example of A-scan

B-scan

Combines a series of A-Scan, taken from different angles to for a 2 dimensional picture

Consists of a series of small crystals, each having a slightly different orientation

Advantage of Ultrasonic wave

Minimal health risk than X-ray diagnosis

Equipment is portable and simple to use

Small features within the body can be identified with(high frequencies)

Principle behind the use of magnetic resonance

Atomic nuclei behave as they posses a ‘spin’This ‘spin’ causes the nuclei to behave as tiny magnetNuclei rotate about the direction of the field as they spin.Similar to motion of a top spinning in a gravitational field

The frequency(Lamour Frequency) depends on the nature of the nature of the nucleus and the strength of magnetic field.

Lamor frequency lie in the radio-frequency region of the electromagnetic spectrum

The atoms will resonate if a short pulse of radio wave of frequency equal to to Lamor frequency is applied

When the pulse ends, the atoms will return to their original equilibrium position and emit photons with frequency of radio wave after a short period of time(relaxation time)

Hydrogen nuclei within the person have the same Lamour frequency because this frequency is dependent on the magnetic field strengthA non-uniform magnetic field strength is applied to locate a particular position of hydrogen atomsThere is a unique value of magnetic field strength at each point in the a personCoils transmit RF pulses and also detect RF emissions from patient.