Farr-_s Physics for Medical Imaging - Ebook download as PDF File .pdf), Text File .txt) or read book online. Get this from a library! Farr's physics for medical imaging. [P J Allisy-Roberts; J R Williams; R F Farr] -- This title is directed primarily towards health care. FARR'S PHYSICS FOR MEDICAL IMAGING. Table Electromagnetic spectrum . Radiation. Wavelength. Radiowaves. Microwaves. Infrared. Visible light.
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Get Instant Access to PDF File: #d Farr's Physics For Medical Imaging By Penelope J. Allisy Roberts EBOOK EPUB KINDLE PDF. Farr's Physics for Medical Imaging by Penelope J. Allisy-Roberts, , available at Book Depository with free delivery worldwide. This title is directed primarily towards health care professionals outside of the United States. The new edition has been fully updated to reflect.
Rutherford and Bohr model an atom is a massive positively charged nucleus surrounded by electrons in orbits of specific diameters. The proton has a positive electric charge numerically equal to the charge of the electron, while the neutron has zero electrical charge.
The neutron and proton have about the same mass 1. ISOTOPES have the same number of protons in the nucleus "same atomic number" but have different numbers of neutrons "different mass numbers". It is called a Radionuclide. A neutral atom contains an equal number of protons and electrons.
The atomic system allows 2 electrons in the first orbit, 8 in the second, 18 in the third, 32 in the fourth, and 50 in the fifth 2N2. An electron in the K shell is called a K electron. L electrons are in the L shell. Valence shell: Outermost shell. Can't have more than 8 electrons called 'free electrons'. X-rays involve the inner shells, and radioactivity concerns the nucleus.
The diameters of the electronic shells are determined by the nuclear force on the electron, and by the angular momentum and energy of the electron.
The binding force is inversely proportional to the square of the distance between the nucleus and electron K electron has a larger binding force than an L electron. Never greater than keV. The binding energy depends on 1. The energy may take the form of an x ray photon. Electron movement to a higher energy e. When it falls back energy is re-emitted as a single 'packet' of energy or light photon.
Named according to the way of production and the special properties they possess. They are transverse waves; with the electric and magnetic field vectors point at right angles to each other and to the direction of travel of the wave. Field strength versus a time and b distance. Only radiations at the ends of the spectrum penetrate the human body sufficiently to be used in imaging radio waves and X- or gamma rays. The two sine waves in Fig.
They have the same period or frequency but the dashed curve lags behind the solid curve "i.
In single-phase mains supply the current rises and falls as a single sine wave. In a three-phase supply the current rises and falls as three sine waves having phase differences of Wave and quantum theories combined Photon energy is proportional to the frequency.
The constant of proportionality is called Planck's constant h. Intensity is proportional to the square of the amplitude A , see Fig.
Energy fluence and intensity are not easy to measure directly. Instead, an easier indirect measurement is made: 'Air kerma' instead of energy fluence.
Only the intensity of the x-ray will decrease with distance but the energy of the photons not change. Two sources of electrical energy are required and are derived from the alternating current AC mains by means of transformers.
Figure l. The accelerating voltage kV between the anode and cathode 'high tension', 'kilovoltage', or 'kV' , produced by a high-voltage transformer. The mA is controlled by varying the filament temperature.
Using an alternative current for X-ray tube makes electrons moves in one half of the cycle from the cathode to anode, in the other half of cycle the electrons with move in opposite direction undesirable, because: 1.
Figure 1. Alternating voltage 'self-rectification'. Pulsating direct current DC 'full wave rectification, single-phase'. High frequency which is steady DC with negligible ripple. The connecting wires must be sealed into the glass wall of the x-ray tube. Special alloys, having approximately the same coefficients of linear expansion as Pyrex glass, are generally used in x-ray tubes.
Composition of the cathode: The filament, which is the source of electrons for the x-ray tube. Tungsten wire, 0. A metallic focusing cup. The number quantity of x rays produced depends entirely on the number of electrons that flow from the filament to the target anode of the tube.
Where these electrons come from? A pure tungsten filament must be heated to a temperature of at least C to emit a useful number of electrons thermions. Tungsten is not as efficient an emitting material as alloys of tungsten, for example.
But, it is chosen for use in x-ray tubes, because: 1. It can be drawn into a thin wire that is quite strong. Has a high melting point C. Has little tendency to vaporize. Tungsten filament has a reasonably long life expectancy.
When current flows through tungsten wire heated its atoms absorb thermal energy some of the electrons acquire energy move small distance from the metal surface form a small cloud in the vicinity of the filament "the space charge". The electron cloud, produced by thermionic emission, also termed "Edison effect". Figure Saturation voltage Below the saturation point, The tube current is limited by the space charge effect space-charge-limited. Above the saturation voltage, The space charge effect has no influence on the x-ray tube current.
The tube current is determined by the number of electrons made available by the heated filament emission-limited or temperature-limited. Therefore, an x-ray tube current of mA 0. Electron current across an x-ray tube is in one direction only always cathode to anode.
So, its electrical forces cause the electron stream to converge onto the target anode in the required size and shape prevent bombardment of a large area on the anode caused by mutual repulsion of the electrons Figs.
The focusing cup is made of nickel. Modern x-ray tubes may be supplied with a single or, more commonly, a double filament Fig. Vaporization of the filament when it is heated: Filament becomes too thin break up "acts to shorten the life of an x-ray tube". Tungsten is deposited on the inner surface of the glass wall of the x-ray tube produces bronze-color "sunburn".
This tungsten coat has two effects: 1. Filter the x-ray beam. Increases the possibility of arcing between the glass and the electrodes at higher kilovoltage kVp values tube puncture.
Figure Stationary Anode: Consists of a small plate of tungsten "target" embedded in a large mass of copper. The anode angle is usually 15 to Tungsten is chosen as the target material for several reasons. It has a high atomic number 74 more efficient for the production of x rays.
However, it cannot withstand the heat of repeated exposures the massive copper anode acts to the total thermal capacity of anode and to speed its rate of cooling. Tungsten and copper have different expansion coefficient on heating needs 10 satisfactory bonding otherwise the tungsten target would peel away from copper anode.
The anode assembly, seen in cross-section, consists of: An anode disk, cm or more in diameter. A thin molybdenum stem. A blackened copper rotor "part of the induction motor which rotates the target stem". Bearings, lubricated with a soft metal such as silver. An axle, sealed into the glass envelope, which supports the target assembly. Figure The rotating anode x-ray tube The anode of a rotating anode tube consists of a large disc of tungsten or an alloy of tungsten "tungsten-rhenium alloy" better thermal characteristics than pure tungsten and does not roughen with use as quickly.
Typical disc diameters measure 75, , or mm. The diameter of the tungsten disc determines the total length of the target track affects the maximum permissible loading of the anode. The anode rotates at a speed of about revolutions per minute rpm using singlephase mains supply.
The tungsten disc has a beveled edge. The angle of the bevel may vary from 6 to The bevel is used to take advantage of the line focus principle. The purpose of the rotating anode is to spread the heat produced during an exposure over a large area of the anode while the apparent or effective focal spot size has remained the same. Lack of durable bearings Metallic lubricants e.
Heat dissipation Molybdenum stem. How the anode cools Heat produced on the focal track conducted quickly and stored temporarily in the anode disk transferred by radiation to the insulating oil stored temporarily then transferred by convection to the housing lost by radiation and by fan-assisted convection through the surrounding air. The molybdenum stem is sufficiently long and narrow to control the amount of heat that is conducted to the rotor so that it is not in danger of overheating.
Heat radiation is promoted by blackening the anode assembly. The grid-controlled tube has a 3rd electrode control the flow of electrons from the filament to the target. The third electrode is the focusing cup that surrounds the filament. In conventional x-ray tubes a focusing cup is electrically connected to the filament. In the grid-controlled tube, the focusing cup is electrically negative relative to the filament.
The voltage across the filament-grid produces an electric field along the path of the electron beam pushes the electrons even closer together. In addition, the x rays are scattered in all directions following collisions with various structures in and around the tube. The effectiveness of the tube housing in limiting leakage radiation must meet the specifications listed in The National Council of Radiation Protection and Measurements Report No. The high tension and filament transformers are contained in oil-filled earthed metal tank and connected to the tube housing by a pair of highly insulated flexible cables.
Dental X-ray tube: low-powered - small - stationary anode tube. The dose of radiation delivered to the patient, and 2. The heat which inevitably accompanies the production of X-rays.
If heat accumulate in X-ray tube shorten or damage the tube. The size and shape of the focal spot are determined by the size and shape of the electron stream when it hits the anode. The size and shape of the electron stream are determined by 1 the dimensions of the filament tungsten wire coil, 2 the construction of the focusing cup "also called electron lens", and 3 the position of the filament in the focusing cup.
The problems posed by 1 The need for a large focal spot to allow greater heat loading. The line focus principle Figure the surface of target is inclined so that it forms an angle with the plane to incident beam. If it is 17 and the effective focal spot is 1 x 1 mm, the actual focal spot must be 4x1 mm. This makes the focal spot blurring small and fixed whatever the orientation of a structure. Angle the angle between the central ray and the target face The size of the projected focal spot is directly related to the sine of the angle of the anode.
The smaller the angle of the anode, the smaller the apparent focal spot TAKE CARE: The steeper the target for the same actual focal spot and target heat rating the smaller the effective focal spot.
The steeper the target for the same effective focal spot the larger the actual focal spot and target heat rating. The steeper the target the narrower the useful X-ray beam and the smaller the field covered. Some newer 0. There is a limit to which the anode angle can be decreased as dictated by the heel effect the point of anode cutoff. MCQ: For general diagnostic radiography done at a inches focus-film distance 1 m , the anode angle is usually no smaller than Focal spot size is expressed in terms of the apparent or projected focal spot; sizes of 0.
Usually, an X-ray tube has two filaments and two focal spots of different sizes which are selected from the control panel. Table 2. Must be positioned half way between the focal spot and the film. The pinhole is may be positioned closer to the tube anode than to the cassette magnified image of the effective focal spot knowing the magnification enables the true size of the effective focal spot to be calculated.
It is important to align the pinhole to central beam of the X-ray tube accurately. The pinhole must be several times smaller than the focal spot e. Although X-rays diverge in all directions from each point on the target, only one of them passes through the pinhole, and it produces a dot of blackening on the film. Notice the following: Pinhole size of 0.
Intensity is more towards periphery of the focal spot edge band distribution commonest pattern, though undesired. The resulting image Fig. Reveals any extra-focal X-radiation "which degrades image. This is mounted partway between the film and the tube not in contact with film. It occurs particularly at low kV values and with small focal spots. Regarding focal spot: The focal spot size can limit the spatial resolution "geometric unsharpness", depends on the location of the object in the source-to-detector direction.
The resolution impact of the focal spot increases with geometric magnification, i. Thus, a small focal spot is desired in order to optimize spatial resolution. The focal spot size also sets the upper limit on X-ray tube current or output rate heat loading.
If an X-ray tube is operating at its instantaneous power limit, decreasing the size of the focal spot will require a decrease in the tube current radiation output.
B in Fig. It is usually pointed toward the center of the area of interest in the body. Toward the anode edge of the field, the beam A is cut off by the face of the target. Toward the cathode edge, the beam C is cut-off by the edge aperture in the lead shield. Thus, the X-ray field is made symmetrical around the central ray B, A and C are the limits of the useful beam. Mechanism: Electrons penetrate a few micrometers into the target before being stopped by a nucleus so; the X-rays produced are attenuated and filtered by the target material on their way out.
X-rays traveling toward the anode edge of the field have more target material to cross attenuated more than those traveling toward the cathode edge the intensity of the beam decreases toward the anode end of the fields Less importantly, the HVL increases because of the filtration effect.
Factors affecting the heel effect: 1. Anode angle: the steeper the target heel effect. Film size: film size heel effect "with fixed FFD". In radiographs of body parts of different thicknesses the thicker parts should be placed toward the cathode filament side of the x-ray tube.
Any combination of kV, mA, and exposure time should be such that, at end of the exposure, the temperature of the anode does not exceed its safe value, i.
The rating is usually stated as the allowable mA, and this: Decreases as the exposure time is increased. Decreases as the kV is increased. Increases with the effective focal spot size because increase effective focal spot means increase actual focal spot for a fixed anode angle and, Increases with smaller target angles for a fixed effective focal spot, Drawing above explain it, because the actual focal spot then larger.
Is greater for a rotating than a stationary anode. Is greater for a 10 cm disk than a 7 cm disk. Is greater for a high-speed anode. Is greater for a three-phase constant potential than for a single-phase pulsating potential - because the former produces heat more evenly throughout the exposure. The foregoing information is stored on a microprocessor in the control circuit which prevents any exposure being made which would exceed the rating of the tube.
Repeated radiographic exposures To display movement, e. For repeated exposures depends also on the ability of the anode assembly and the oil to accumulate heat both not allowed to exceed its maximum safe temperature. Microprocessor in control circuit calculates the max.
If the anode heat capacity typically 0. The rating depends only on the cooling rate and whether or not the fan is on and NOT at all on the focal spot size or the type of generator. Updated to reflect changes to FRCR examination.
More medically orientated. Covers new legislation concerning radiological safety etc. Completely new design. Product details Format Paperback pages Dimensions x x Table of contents Radiation physics. Radiation hazards and protection. Imaging with X-rays. Film-screen radiography. Digital radiolography. Computed tomography. Gamma imaging. Imaging with ultrasound. Magnetic resonance imaging. Review quote "Not only is it an essential text for the trainee radiologist, and any physics department participating in their teaching, but it is also a useful reference book for the trainee physicist as it provides an introductory overview of diagnostic imaging.
About Penelope J. She went on to study for the MSc in Radiobiology, achieving the degree in She has served on many influential National and International committees dealing with radiation protection affairs and in was awarded the Founders' Prize by the Society for Radiological Protection. Rating details.