Year 12 Physics_2014- nknkn 2015_remote Sensing

PENABUR International Kelapa Gading Academic Year 2014-2015 Grade 12

Yanoar Sarwono & Fielly Budiman Page 25

REMOTE SENSING X-RAYS X-rays are produced by bombarding metal targets with high-speed electrons. A typical spectrum of the X-rays

The spectrum consists of two components: 1. the continuous distribution with a sharp cut-off at short wavelength

2. a series of spikes which are characteristic of the target material

Explanation: Bremmstrahlung radiation is the emitted radiation because of the acceleration of a charged particle.

Continuous distribution of accelerations gives a continuous distribution of wavelength of X-rays.

A cut-off wavelength is the minimum wavelength where the whole energy of the electron is converted into the energy of one photon, which is

kinetic energy of electron= hceV where e is the electron charge, V is the potential difference, h is the Planck constant, c is the speed of light, and is the wavelength of the emitted X-ray photon.

Explanation: Sharp peaks correspond to the emission line spectrum of the target atoms. The electrons excite the target electrons into lower energy levels. This process gives rise to the line spectrum.



A simplified X-ray tube is shown below

Electrons are emitted from the heated cathode (by thermionic effect), accelerated through a large potential difference (20 kV- 100 kV), bombard a metal anode. The anode is water cooled or spin rapidly to increase the target area. The anode is held at earth potential. The intensity of the X-ray beam is determined by the rate of arrival of electrons at the metal target (the tube current). The hotter the cathode, the greater the tube current is. The hardness of the X-ray beam (the penetration of the X-rays) is controlled by the accelerating voltage between the cathode and the anode. The longer the wavelength, the softer X-ray is. The effect of X-ray radiation: blackened the photographic plates, fluorescence in certain materials, The quality of the image produced depends on the sharpness and contrast of X-rays. Sharpness is the edges of organs are clearly defined. Factors that affect the sharpness are: 1. the area of the target anode (the smaller, the sharper)



2. the size of the aperture through which the X-ray beam passes after leaving the tube

3. A lead grip in front of the photographic film to absorb scattered X-ray photons

Factors affecting the contrast are: the exposure time, X-ray penetration, scattering of the X-ray beam within the patients body. In vacuum, the intensity of an X-ray beam, I is decreasing in proportional to the inverse of the square of the distance from the source 2r , thus 20 /I I r . In a medium, the transmitted intensity I is decreasing exponentially, given by

0xI I e

where is the linear absorption coefficient (m-1), 0I is the initial intensity, x is the thickness of the medium (m). The graph of the percentage transmission versus the thickness x is illustrated below

The half-value thickness 1/2x or HVT is the thickness of the medium required to reduce the transmitted intensity to one half of its initial value. The relation of HVT 1/2x to the linear absorption coefficient is given by



1/2 ln 2x CT SCANNING CT scanning is the computer techniques to get the three-dimensional image or slice through a body. The aim of CT scanning is to make an image of a section through the body from measurements made about its axis, as illustrated below

Voxels are series of small units of the section through the body. Pixel is a particular intensity of the image of each voxel. Below is a section of four voxels and the number on each voxel is the pixel:

If a beam of X-rays is from the left, the detectors will give readings of 5 and 9, and the reconstructed voxels will be below

If the detectors and the X-ray tube are rotated through 45o, new detectors readings will be found and will be added to the previous readings



The same procedure is repeated after rotating the X-ray tube and the detectors through a further 45o,



The final images are taken after rotating them further 45o,

The final pattern is shown below

In order to obtain the original pattern of pixels, two operations must be performed: 1. remove the background intensity. The background intensity is the total of each set of detector readings, in this case 14 is deducted from each pixel 2. divide by three to allow for duplication of the views of the section

ULTRASOUND Ultrasonic waves may be produced using a piezo-electric transducer. The basis of the transducer is a piezo-electric crystal such as quartz. Two opposite sides of the crystal are coated with thin layers of silver to act as electrical contacts as illustrated below



Quartz has a complex structure made up of a large number of repeating tetrahedral silicate units, as illustrated below

The positions of the oxygen links are not rigidly fixed in these units or lattices. Movement of them can be encouraged by applying an electric field. The three conditions of the crystal are shown below

unstressed compressed extended

the centers of the positive and the negative ions coincide, so their effects are neutralized.

the positive silicon ions are attracted towards the cathode and the negative oxygen ions towards the anode. This causes distortion of the silicate units. The crystal may be thinner or thicker

The action and reaction of the piezo-electric crystal is shown below

input output an alternating voltage across the silver electrodes

mechanical vibrations. In resonance, the oscillations are in the ultrasonic range

an ultrasonic wave (pressure variations) alter the positions of positive and negative ions thus result in a varying potential difference

A diagram of a typical piezo-electric transducer/receiver is illustrated below



Such devices operate in the MHz frequency range, up to 600 MHz. When an ultrasound wave meets the boundary between two media, some of the wave energy is reflected and some is transmitted, as shown below

For an incident intensity I, reflected intensity RI and transmitted intensity TI , the energy conservation says

R TI I I Reflection Transmitted

For any medium, the specific acoustic impedance Z is defined as

Z c where c is the speed of the wave in the medium of density . When a wave is incident normally on a boundary between two media having specific acoustic impedances of 1 2 and Z Z , the ratio R /I I of the reflected intensity to the incident intensity is given by

22 1R

22 1

Z ZII Z Z

The ratio R /I I is also known as the intensity

The absorbed ultrasonic waves give rise the temperature of the medium. This is used in physiotherapy. Illustration of a parallel beam of ultrasound waves of intensity 0I incident on a medium of thickness x

The intensity of the beam after passing through the medium is related to the incident intensity by

0kxI I e



reflection coefficient or . The intensity reflection coefficient is very large from air into soft tissue

26

6

430 1.63 10 1430 1.63 10

In order that the ultrasound waves may be transmitted from the transducer into the body, a gel is used to fill any space between the transducer and the skin.

where k is the absorption coefficient.

The followings are how to diagnose internal body structures with the ultrasound: 1. the transducer is in contact with the skin, with a gel as a coupling medium 2. short pulses of ultrasound are transmitted into the body 3. the reflected pulses return to the transducer and are detected and are transformed into voltage pulses and are processes to give an image on an oscilloscope screen. 4. echoes from a deeper body which tend to be of lower intensity will be more amplified 5. The time-base on the X-plates is adjusted so that all of the reflections are seen on the screen for one scan. There are two techniques to display an ultrasound scan: 1. A-scan: the transducer is held in one position to measure the distance of different boundaries from the transducer

2. B-scan: combines a series of A-scans, taken from a range of different angles, to form a two-dimensional picture. The main advantage of ultrasonic scanning is that the health risk is lower, the portable and easy to-use equipment, detection between soft tissues as well as between hard and soft tissues.



NUCLEAR MAGNETIC RESONANCE AND MAGNETIC RESONANCE IMAGING Why is there precession in nuclei when there is an applied magnetic field? 1. Many atomic nuclei behave as if they possess a spin because they have an odd number of protons and/or an odd number of neutrons. 2. Their spin causes the nuclei of these atoms to behave as tiny magnets. 3. If an external magnetic field is applied to these atoms, they tend to line up in the same direction as the magnetic field. 4. The alignment is not perfect and the nuclei rotate about the direction of the field as they spin. The rotation about an axis while they are spinning is called precession . The precession frequency, known as the Larmor frequency depends on the nucleus and the strength of the magnetic field. The Larmor frequency is in the order of radio-frequency (RF) region of electromagnetic spectrum. The principles of the nuclear magnetic resonance (NMR) are below: 1. a short pulse of radio waves of frequency equal to the Larmor frequency is applied 2. the atoms will resonate, absorbing energy 3. the atoms will return to their original equilibrium state after a short period of time, called the relaxation time, when the pulse ends. 4. the RF radiation is emitted by the atoms when they return to the equilibrium state A diagram of a magnetic resonance scanner is

The principles of magnetic resonance imaging (MRI): 1. the person is placed between the poles of a very large magnet producing a uniform magnetic field in excess of 1 Tesla so that all hydrogen nuclei within the person would have the same Larmor frequency 2. a non-uniform magnetic field is applied to locate a particular position of hydrogen atoms 3. radio-frequency pulses are both transmitted to the person and detected by means of suitable coils



NMR applet:

MRI applet:



PROBLEMS 1. 9702_w08_qp4#11 (a) Distinguish between the images produced by CT scanning and X-ray imaging. (b) By reference to the principles of CT scanning, suggest why CT scanning could not be developed before powerful computers were available. Solution: (a) CT image: (thin) slice (through structure), any further detail e.g. built up from many slices / 3-D image, X-ray image: shadow image (of whole structure) / 2-D image; (b) X-ray image of slice taken from many different angles, these images are combined (and processed), repeated for many different slices, to build up a 3-D image, 3-D image can be rotated, computer required to store and process huge quantity of data 2. 9702_w07_qp4#9 (a) State what is meant by acoustic impedance. (b) Explain why acoustic impedance is important when considering reflection of ultrasound at the boundary between two media. (c) Explain the principles behind the use of ultrasound to obtain diagnostic information about structures within the body. Solution: (a) product of density (of medium) and speed of sound (in medium); (b) difference in acoustic impedance determines fraction of incident intensity that is reflected/amount of reflection; (c) pulse of ultrasound (directed into body), reflected at boundary (between tissues), (reflected pulse is) detected and processed, time for return of echo gives (information on) depth, amount of reflection gives information on tissue structures 3. 9702_s09_qp4#11 (a) Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. (b) Data for the acoustic impedances and absorption (attenuation) coefficients of muscle and bone are given in Fig. 11.1. acoustic impedance/ kg m2 s1 absorption coefficient/ m1 muscle bone

1.7 106 6.3 106

23 130

Fig. 11.1 The intensity reflection coefficient is given by the expression

22 1

22 1

Z ZZ Z

The attenuation of ultrasound in muscle follows a similar relation to the attenuation of X-rays in matter.



A parallel beam of ultrasound of intensity I enters the surface of a layer of muscle of thickness 4.1 cm as shown in Fig. 11.2.

Fig. 11.2

The ultrasound is reflected at a muscle-bone boundary and returns to the surface of the muscle. Calculate (i) the intensity reflection coefficient at the muscle-bone boundary, (ii) the fraction of the incident intensity that is transmitted from the surface of the muscle to the surface of the bone, (iii) the intensity, in terms of I, that is received back at the surface of the muscle. Solution: (a) pulse of ultrasound, reflected at boundaries / boundary, received / detected (at surface) by transducer, signal processed and displayed, time between transmission and receipt of pulse gives (information about) depth of boundary, reflected intensity gives information as to nature of boundary; (b) (i) coefficient = (Z2 Z1)2 / (Z2 + Z1)2= (6.3 1.7)2 / (6.3 + 1.7)2= 0.33, (ii) fraction = exp(x)= exp(23 4.1 102)= 0.39, (iii) intensity = 0.33 0.392 I= 0.050 I 4. 9702_s08_qp4#10 Outline briefly the main principles of the use of magnetic resonance to obtain information about internal body structures. Solution: large / strong (constant) magnetic field, nuclei rotate about direction of field / precess, radio frequency / r.f. pulse causes resonance in nuclei , nuclei absorb energy, (pulse) is at the Larmor frequency, on relaxation / nuclei de-excite emit (pulse of) r.f., detected and processed, non-uniform field (superimposed), allows for position of nuclei to be determined, and for location of detection to be changed 5. 9702_s07_qp4#9 (a) Explain the principles behind the use of X-rays for imaging internal body structures. (b) Describe how the image produced during CT scanning differs from that produced by X-ray imaging. Solution: (a) X-ray beam directed through body onto detector (plate), different tissues absorb/attenuate beam by different amounts, giving shadow image of structures, any other detail e.g. comment re sharpness or contrast; (b) X-ray image is flat OR 2-dimensional, CT scan takes many images of a slice at different angles, these build up an image of a slice through the body, series of images of slices is made, so that 3D image can be built up, image can then be rotated

Year 12 Physics_2014- nknkn 2015_remote Sensing

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