Remote Sensing • 29. Remote sensing • Content • 29.1 Production and use of X-rays • 29.2 Production and use of ultrasound • 29.3 Use of magnetic resonance as an imaging technique • Learning outcomes • Candidates should be able to: • (a) explain in simple terms the need for remote sensing (non-invasive techniques of diagnosis) in medicine • (b) explain the principles of the production of X-rays by electron bombardment of a metal target • (c) describe the main features of a modern X-ray tube, including control of the intensity and hardness of the X-ray beam • (d) show an understanding of the use of X-rays in imaging internal body structures, including a simple analysis of the causes of sharpness and contrast in X-ray imaging
(e) show an understanding of the purpose of computed tomography or CT scanning • (f) show an understanding of the principles of CT scanning • (g) show an understanding of how the image of an 8-voxel cube can be developed using CT scanning • (h) explain the principles of the generation and detection of ultrasonic waves using piezo-electric transducers • (i) explain the main principles behind the use of ultrasound to obtain diagnostic information about internal structures • (j) show an understanding of the meaning of acoustic impedance and its importance to the intensity reflection coefficient at a boundary • (k) recall and solve problems by using the equation I = I0e–μxfor the attenuation of X-rays and of ultrasound in matter • (l) explain the main principles behind the use of magnetic resonance to obtain diagnostic information about internal structures • (m) show an understanding of the function of the non-uniform magnetic field, superimposed on the large constant magnetic field, in diagnosis using magnetic resonance.
Remote Sensing • Remote sensing is the investigation of an object using equipment that has no direct contact with the object being investigated • e.g. an orbiting satellite may be designed so that it can detect small changes in mean sea level. These small changes can then be interpreted to determine the nature of the rocks under the sea-bed. This investigation enables information to be gathered without actually drilling into the sea-bed • Medical diagnosis for over 100 years used 2 risky techniques • Observe the patient externally for fever, pulse-rate, breathing, vomiting, skin condition etc. This was part science and part art • Carry out investigative invasive surgery which involved a high risk causing many patients to die either from trauma of surgery or infection • Although there is some risk versus benefit with any procedure, now diagnostic imaging techniques have been developed that enable externally placed equipment to obtain detailed information about internal body structures without surgery i.e. non-invasive, from under the skin i.e. a form of remote sensing • Some of the techniques make use of • X-rays • Ultrasound • Magnetic resonance imaging(MRI)
THE DUCK IS DEAD - diagnosis A woman brought a very limp duck into a veterinary surgeon. As she laid her pet on the table, the vet pulled out his stethoscope and listened to the bird's chest. After a moment or two, the vet shook his head sadly and said, "I'm sorry, your duck, Cuddles, has passed away."The distressed woman wailed, "Are you sure?""Yes, I am sure. The duck is dead," replied the vet."How can you be so sure?" she protested.. "I mean you haven't done any testing on him or anything. He might just be in a coma or something." The vet rolled his eyes, turned around and left the room. ?????????????
cont… He returned a few minutes later with a black Labrador Retriever. As the duck's owner looked on in amazement, the dog stood on his hind legs, put his front paws on the examination table and sniffed the duck from top to bottom. He then looked up at the vet with sad eyes and shook his head.The vet patted the dog on the head and took it out of the room. !!!!!!!!!!!!!!!!! A few minutes later he returned with a cat. The cat jumped on the table and also delicately sniffed the bird from head to foot. The cat sat back on its haunches, shook its head, meowed softly and strolled out of the room.
Verdict The vet looked at the woman and said, "I'm sorry, but as I said, this is most definitely, 100% certifiably, a dead duck."The vet turned to his computer terminal, hit a few keys and produced a bill, which he handed to the woman. The duck's owner, still in shock, took the bill. "$250?" she cried, "$250 just to tell me my duck is dead?"
Verdict justified • The vet shrugged, "I'm sorry. If you had just taken my word for it, the bill would have been $20, ….
Properties of X-rays Produce fluorescence in materials e.g. zinc sulphide Produce latent image (developed to give visible image) Penetrate substances opaque to light Ionize and excite atoms and molecules Have biological effects in living organism
The production of X-rays • Whenever a charged particle is accelerated, electromagnetic radiation is emitted • The frequency of the radiation or emitted photon is proportional to the magnitude of the acceleration • Whenever high speed electrons are stopped in a metal target, X-ray photons are emitted • X-rays production process: • A metal filament(cathode) in an evacuated tube made of material that is opaque to X-rays, is heated using a low voltage supply causing electrons to be emitted through the thermionic effect(the opaque material also reduces background radiation) • The electrons are then accelerated through a potential difference of between 20 – 90 kV so that they have high energy and high speed, but this acceleration is insufficient to cause X-ray radiation to be emitted • These high energy and high speed electrons are then accelerated towards the target(anode), constructed of metal with a high melting point and high atomic number • When these electrons are bombarded and strike or are stopped by the target metal, large decelerations are involved, causing the electrons to lose kinetic energy very rapidly giving rise to the emission of X-ray photons known as Bremmstrahlung radiation or ‘braking radiation’(slowing down) • This X-ray beam is made to pass out of the tube through a window that is transparent to X-rays • Not all of the energy of the electrons is emitted as X-rays as the majority is transferred as thermal energy in the target rotating metal anode and cooling is necessary
X-ray spectrum • A typical X-ray spectrum of the variation with wavelength of the intensity has 2 distinct components: • A continuous distribution of wavelengths with a sharp cut-off at the shortest wavelength, λ0 • Sharp peaks may be observed corresponding to the emission line spectra of the target material and therefore a characteristic of the target Continuous X-rays Characteristic X-rays λ0
Continuous spectrum The continuous distribution comes about because the electrons when incident on the metal target, will not all have the same decelerations but will instead have a wide range of values. Since the wavelength of the emitted spectrum is dependent on the deceleration, there will be a distribution of wavelengths The cut-off wavelength corresponds to an electron that is stopped in one collision in the target so that all of its kinetic energy is given up as one X-ray photon KE energy is lost in the form of X-ray photons Energy of photon depends on how much KE lost – hence a continuous range Max energy of photon occurs when all KE of electrons is converted to X-rays.
Characteristic/discrete X-rays • Superimposed on the continuous spectrum • Produced when an incident electron knocks electrons out of the K-shell(lowest shell) of the target atom • An electron from the L or M shell may move into the vacancy in the K-shell, emitting characteristic X-rays
Discrete X-rays M shell L Kβ L shell K K shell
Difference between continuous and characteristic spectrum ContinuousCharacteristic (i) A continuous range of wavelength A discrete wavelength (ii) Produced by loss of KE of incident Produced by electron electron transition from higher shell to inner shell
Quality of beam Quality of beam describes how penetrating the beam is For monochromatic radiation, the quality is completely described by the wavelength High quality refers to a very penetrating beam. High quality beam is also known as hard X-ray while low quality is called soft X-ray. Quality is specified/measured by HVT (half-value thickness)
Control of the X-ray beam • In order that the optimum X-ray image may be obtained, there are 2 factors that need to be controlled • Hardness – is the penetration of the X-ray beam, which determines the fraction of the intensity of the incident beam that can penetrate the part of the body being X-rayed. In general the shorter the wavelength of the X-rays, the greater their penetration • Intensity – this is the wave power per unit area, and this affects the degree of blackening of the image
Hardness/penetration • The kinetic energy Ek of an electron is equal to the energy gained by the electron when it is accelerated from the cathode to the anode i.e. Ek = eV where e is the charge of an electron and V the accelerating potential difference • Using E = hc/λ at the cut-off wavelength, eV = hc/λ0 hence λ0 = hc/(eV) i.e.the accelerating potential V thus determines the cut-off wavelength • The larger the potential difference, the shorter the wavelength • Therefore the hardness(penetration) of the X-ray beam is controlled by variation of the accelerating potential difference between the cathode and the anode • A continuous distribution of wavelengths implies that there will be X-ray photons of long wavelengths that would not penetrate the person being investigated and hence would not contribute towards the X-ray image • Such X-rays would add to the radiation dose received by the person without serving any purpose, so the X-ray beam emerging from the tube frequently passes through aluminium filters that absorb these long-wavelength photons
Intensity The intensity of the beam depends on the number of photons emitted per unit time and hence the number of electrons hitting the metal target per unit time Since the electrons are produced by thermionic emission, increasing the heater or filament current in the cathode will increase the rate of production of electrons and hence increase the intensity of the X-ray beam
Example The accelerating potential difference between the cathode and the anode of an X-ray tube is 30 kV. Given that the Planck constant is 6.6 x 10-34 J s, the charge on the electron is 1.6 x 10-19 C and the speed of light in free space is 3.0 x 108 m s-1, calculate the minimum wavelength of photons in the X-ray beam. Solution For the minimum wavelength, Energy gained by electron = energy of photon eV = hc/λ0 Therefore λ0 = 4.1 x 10-11 m
The X-ray image • The image is based on the penetration of body parts by X-rays. • The transmitted X-rays produce a latent image on a photographic film. The latent image is developed to give a visible image of the internal organ scanned. • The image is not really an image in the sense of the real image produced by a lens • X-ray is an ionising radiation. It loses energy mainly due to (a) absorption and (b) scattering • The reduction in energy is an exponential decay to the distance traveled. • When an X-ray beam is incident on the body part of the patient, it can penetrate soft tissues(skin, fat, muscle etc) with little loss of intensity • A photographic film after development will show a dark area corresponding to these soft tissues • Bone however, causes a greater attenuation(reduces the intensity by a greater extent) than soft tissues and therefore the photographic film will be lighter in colour in areas corresponding to the positions of bones • The quality of the shadow image produced depends on its sharpness and contrast
Quality of imaging Quality of an X-ray image is described by sharpness and contrast Sharpness refers to a clear boundary between different tissues or the ease with which the edges of structures can be determined A shadow image where the bones and other organs are clearly outlined is said to be a ‘sharp image’ But although an image may be sharp, it may still not be clearly visible because there is little difference in the degree of blackening between e.g. bone and surrounding tissue An image having a wide range of degrees of blackening is said to have good contrast Contrast refers to different intensity (brightness) in the image of various parts of the internal organ.
Sharpness Image is sharp if the boundary is clearly visible A sharp image requires a parallel X-ray beam which can be achieved by (1) reducing the area of the target anode in the X-ray tube (2) limiting the size of the aperture through which the X-ray beam passes (3) reducing scattering of the emergent beam
Sharpness – (1) reducing the area of the target anode in the X-ray tube Secondary/partial shadows/penumbra can cause images to be blur The full shadow produces an area that is white on the film Where there is no shadow, the image will be black In the region of partial shadow or greyness, the image gradually changes from white to black If the image is to be sharp, this area of greyness must be reduced as much as possible The area of the target anode should be kept to a minimum
Sharpness – (2) limiting the size of the aperture through which the X-ray beam passes A reduction in the grey area at the edge of the image can also be achieved by limiting the size of the aperture through which the X-ray beam passes This is achieved by using overlapping metal sheet plates, through which the X-ray beam passes after leaving the tube
Sharpness - (3) reducing scattering of the emergent beam As a result of interactions between photons and any substance through which the beam passes(even air), some photons will be scattered resulting in loss of sharpness These scattered or stray photons may be absorbed in a metal lead grid placed in front of the photographic film Scattering can also be reduced by reducing the distance between patient and film
Contrast Good contrast is achieved when neighbouring body organs and tissues absorb the X-ray photons to very different extents e.g. bone and muscle Not the case e.g. if stomach or blood vessels are being investigated In such a case to improve contrast, especially for soft tissues, a contrasting medium is used The patient is asked to swallow a solution of barium sulphate(barium meal taken orally) which is a good absorber of X-ray photons, causing the outline of the stomach to show up clearly Blood vessels can be made to show up visibly by injecting a radio-opaque dye into the bloodstream
Contrast Contrast also depends on other factors such as increasing exposure time and the use of intensifying cassettes or backing the film with fluorescent materials
The attenuation of X-rays When a parallel beam of X-ray photons passes through a medium, absorption processes occur that reduce the intensity of the beam The intensity is reduced(attenuation) by the same fraction each time the beam passes through equal thicknesses of the medium no matter what the starting point is chosen. This thickness of medium is called the half-value-thickness(HVT) and denoted by the symbol x½ The decrease in transmitted intensity is an exponential decrease
Linear attenuation coefficient • Consider a parallel beam having an incident intensity I0 and the medium(absorber) having thickness x and the transmitted energy is I • The transmitted energy is given by the expression I = I0 e-xor I = I0 exp(-μx) where is a constant depending on the medium and on the energy of the X-ray photons, known as the linear attenuation coefficient or the linear absorption coefficient of the medium. The unit is mm-1 or cm-1
Linear attenuation coefficient and HVT • For a thickness x½ (the half-value-thickness) of the medium, the intensity I will be equal to ½I0 hence ½ I0 = I0 e-xor I0 exp(-μx½) which gives μx½ = ln 2 where x½ is HVT • In practice, x½ does not have a precise value as it is constant only when the beam has photons of one energy only. • Approximate values of linear absorption coefficient for some substances are: Substance μ/cm-1 copper 7 water 0.3 fat 0.9 bone 3
Example The linear absorption coefficient of copper is 0.693 mm-1. Calculate: (a) the thickness of copper required to reduce the incident intensity by 50% (b) the fraction of the incident intensity of a parallel beam that is transmitted through a copper plate of thickness 1.2 cm Solution (a) using I = I0 e-xor I = I0 exp(-μx½) I/I0 = 0.50 = exp(-0.693x½) ln 0.50 = - 0.693x½ therefore x½ = 1.0 mm (b) I/I0 = exp(-0.693 x 12) I/I0 = 2.4 x 10-4
Computed tomography(CT scanning) The image produced on a photographic plate or film is a ‘shadow’ or ‘flat’ image, and there is little or no indication of ‘depth’ i.e. the position within the body is not apparent. This is a 2-dimensional view Soft tissues lying behind structures that are very dense also cannot be detected Tomography is a technique whereby a 3-dimensional image is obtained or constructed by ‘slicing’ or sectioning the body using a CT scanner through different angles using computer technology and techniques Data from each individual X-ray image and angle of viewing is fed into a powerful computer enabling a 3-D image of the entire object to be reconstructed, which can then be viewed from any angle
Basic principles of CT or CAT scan Can be illustrated using a simple cube with the aim of producing an image of a slice or section through the body from measurements made about its axis The section or cube is divided into a series of smaller units called voxels which absorbs the X-ray beam to different extents due to its structure The intensity transmitted through each voxel alone is given a number referred to as a pixel, and these various pixels are built up from measurements of the X-ray intensity along different directions through the slice or section Operated by using a moving X-ray emitter and detector and a powerful computer to store data, reconstruction of these pixels in their correct positions is done to display the 3D image of the internal organ being scanned
Re-construction of the slice or section Suppose a simple cube showing a four-voxel section The image of each voxel would have a particular intensity, known as a pixel. The pixels are built up from measurements of X-ray intensity made along a series of different directions around the section of the body The number on each voxel is the pixel intensity that is to be reproduced Pixel(picture element) is actually a two dimensional unit based on the matrix size and the field of view. When the CT slice thickness is also factored in, the unit is known as a Voxel, which is a three dimensional unit.
Final pattern • 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 from each pixel. - 2. After deduction of the ‘background’, the result must be divided by three to allow for the duplication of the views of the section since 4 sets of readings were taken
CT cont… • In practice, the image of each section is built up from many small pixels, each viewed from many different angles. • The greater the number of voxels, the better the definition, similar to a digital camera • In order to build up an image of the whole body, the procedure would be repeated for further sections through the body. • All the data for all the sections can be stored in the computer memory to create a three-dimensional image. Views of the body from different angles may constructed • The collection of the data and its construction into a display on a screen requires a powerful computer and complicated software programming and programs. In fact, the reconstruction of each pixel intensity value requires more than one million computations. • The computer allows for the contrast and brightness of the image to be varied so that an optimum image can be obtained