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Medical physics. Dr.Aida Radwan Assistant Professor of medical physics Radiotherapy Department National Cancer Institute Cairo University. Different types of radiations. Chapter (2) PHYSICS OF DIAGNOSTIC X – RAYS. Production of X – Rays.

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Medical physics

Dr.AidaRadwan Assistant Professor of medical physics Radiotherapy Department National Cancer Institute Cairo University

Different types of radiations

Dr.Aida Radwan Tolba

Chapter (2)


Production of X – Rays

  • When electrons are accelerated to energies in excess of 5 keV and are

  • then directed onto a target surface, X-rays may be emitted.

  • The X-rays originate principally from rapid deceleration يتباطأ مفاجئ of the

  • electrons when they interact with the nucleus of the target atoms.

  • Therefore, electrons have to be speeded up to produce x-rays.

  • Trying to speed up an electron in air is difficult, thus it is necessary to use

  • a glass bulb (x-ray tube) from which most of atoms have been evacuated.

Dr.Aida Radwan Tolba

  • The essential features of المكونات الاساسية a simple X-ray tube:

  • a heated metal filament to provide a copious supply امداد غزيرof electrons by thermionic emission and to act as a cathode;

  • (2) an evacuated chamber across which a potential difference can be

  • applied;

  • (3) a high positive potential to accelerate the negative electrons

  • (4) a metal anode (the target) with a high efficiency for conversion of

  • electron energy into X-ray photons;

  • (5) a thinner window in the chamber wall that will be transparent to

  • most ofthe X-rays.

Dr.Aida Radwan Tolba

  • In modern x-ray tube the number of electrons accelerated toward the

  • anode depends on the temperature of the filament, and the maximum

  • energy of the x-ray photons produced is determined by the accelerating

  • voltage-kilovolt peak (kVp).

  • An x-ray tube operating at 80 kVp will produce x-rays with a spectrum

  • of energies up to a maximum of 80 keV.

Essential features of a simple, stationary anode X-ray tube

Dr.Aida Radwan Tolba

Dr.Aida Radwan Tolba

Components of The X-Ray Tube toward the

1- The Cathode

  • The cathode assembly in a modern x-ray tube consists of

  • a wire filament,a circuit to provide filament current,

  • and a negatively charged focusing cup.

  • The function of the cathode cup is to direct the electrons toward

  • the anode so that they strike the target in a well-defined area, the

  • focal spot.

  • Since size of focal spot depends on filament size, the diagnostic

  • tubes usually have two separate filaments to provide "dual focus"

  • namely one small and one large focal spot.

  • The material of the filament is tungsten which is chosen because of

  • its high melting point.

Dr.Aida Radwan Tolba

2- The Anode toward the

  • The material chosen for the anode should satisfy a number ofrequirements

  • It should have:

  • 1. A high conversion efficiency كفائة تحويل for electrons into X-rays.

  • High atomic numbers are favored since the X-ray intensity is

  • proportional toZ. At 100 keV, lead (Z = 82) converts 1% of

  • the energy into X-rays but aluminium (Z = 13) converts only 0.1%.

  • 2. A high melting point so that the large amount of heat released

  • causes minimal damage to the anode.

  • 3. A high conductivity so that the heat is removed rapidly.

  • 4. A low vapor pressure, even at very high temperatures, so that

  • atoms are not boiled off from the anode.

  • 5. Suitable mechanical properties for anode construction.

Dr.Aida Radwan Tolba

  • In stationary anodes the target area is pure tungsten (Z = 74,

  • melting point 3370 °C) set in a metal of higher conductivity such as

  • copper to conduct heat to the outside of the tube where it is cooled

  • by oil, water or air.

  • Rotating anodes have also been used in diagnostic x-rays to reduce

  • the temperature of the target at any one spot.

  • The heat generated in the rotating anode is radiated to the oil

  • reservoirخزان من الزيت surrounding the tube.

  • It should be mentioned that the function of the oil bath surrounding

  • an x-Ray tube is to insulate the tube housing from high voltage

  • applied to the tube as well as absorb heat from the anode.

Dr.Aida Radwan Tolba

Physics of X – Ray Production 74,

While the energy of most of the electrons striking the target is dissipated تتشتت in the form of heat, the remaining few electrons produce useful x-rays. There are two mechanisms by which x-rays are produced.

One gives rise to bremsstrahlung and the other characteristic x-rays.

(1) Bremsstrahlung (Continuous Spectrum)

  • The process of bremsstrahlung (braking radiation) is the result of radiative

  • "collision" (interaction) between a high-speed electron and a nucleus.

  • The electron while passing near a nucleus may be deflected from its path by

  • the action of Coulomb forces of attraction and lose energy as

  • bremsstrahlung, a phenomenon predicted by Maxwell's general theory of

  • electromagnetic radiation.

Dr.Aida Radwan Tolba

  • According to this theory, energy is propagated through space by

  • electromagnetic fields. As the electron, with its associated electromagnetic

  • field, passes in the vicinity of a nucleus, it suffers a sudden deflection and

  • acceleration.

  • As a result, a part or all of its energy is dissociated ينفصل from it and

  • propagates in space as electromagnetic radiation (x-ray photon )

The bremsstrahlung process

Dr.Aida Radwan Tolba

  • The amount of bremsstrahlung produced for a given number space byof

  • electrons striking the anode depends upon two factors:

  • (1) the Z no. of the target (i.e. the more protons in the nucleus, the

  • greater the acceleration of the electrons),

  • and

  • (2) the kilovolt peak (i.e. the faster the electrons, the more likely

  • they will penetrate into the region of the nucleus).

Dr.Aida Radwan Tolba

(2) Characteristic X-Rays space by

  • Electrons incident on the target also produce characteristic x-rays.

  • An electron, strikes a K electron in a target atom and knocks it out of

  • its orbit and free of the atom.

  • The vacancy in the K shell is filled almost immediately when an electron

  • from an outer shell of the atom falls into it and in the process,

  • a characteristic Kx-ray photon is emitted

  • An x-ray photon emitted when an electron falls from the L level to the

  • K level is called a Kα characteristic x-ray, and that emitted when an

  • electron falls from the M shell to the K shell is called a Kß x-ray.

Dr.Aida Radwan Tolba

Schematic diagram space bytoexplain the production of characteristic x-rays

Dr.Aida Radwan Tolba

Approximate Energies of the Ka x-rays and k-edge for several elements

Dr.Aida Radwan Tolba

The spectrum from

a tungsten target x-ray tube operated at 87 KVp.

Dr.Aida Radwan Tolba

  • How X – Rays Are Absorbed shown

  • X-rays are not absorbed equally well by all materials; if they were,

  • they would not be very useful in diagnosis.

  • Heavy elements such as calcium are much better absorbers of

  • x-rays than light elements such as carbon, oxygen, and hydrogen

  • and

  • as a result, structures containing heavy elements, like the bones,

  • stand out clearly. The soft tissues (fat, muscles, and tumors) all

  • absorb about equally well and are thus difficult to distinguish from

  • each other on an x-ray image.

  • Of course, air is poor absorber of x-rays.

Dr.Aida Radwan Tolba

[A] Photon Beam Attenuation shown

  • The attenuation of an x-ray beam is its reduction due to the

  • absorptionand scatteringof some of the photons out of the beam.

  • A simple method of measuring the attenuation of an x-ray beam

  • is shown in Figure.

  • A narrow beam of x-rays is produced with a collimator (a lead plate

  • with hole in it) and an x-ray detector measures the beam intensity.

  • The unattenuated beam intensity is Io. As sheets of aluminum are

  • introduced into the beam, the intensity I decreases approximately

  • exponentially as shown in Figure.

Dr.Aida Radwan Tolba

Arrangement for measuring the attenuation of an x-ray beam. The x-ray photons are both absorbed and scattered out of the beam.

The lower energy (soft) x-rays are absorbed more readily than the higher energy (hard) x-rays; the greater penetration of the hard x-rays is shown by the flattening of the curve in

Graphs or the transmitted intensity versus the thickness or aluminum attenuator for (A) An x-ray beam and

(B) A monoenergetic x-ray beam.

Dr.Aida Radwan Tolba

  • The intensity of a monoenergetic x-ray beam would decrease The x-ray photons are both absorbed and scattered out of the beam.

  • exponentially as shown by the dashed line in Figure .

  • The exponential equation describing the attenuation curve for

  • a monoenergetic x-ray beam is

  • where e = 2.718, x is the thickness of the attenuator, and μ is the

  • linear attenuation coefficient of the attenuator.

  • The linear attenuation coefficient is dependent on the energy of the

  • x-ray photons;as the beam becomes harder, μ is decreases.

Dr.Aida Radwan Tolba

The half-value layer (HVL) for an x-ray beam The x-ray photons are both absorbed and scattered out of the beam.

  • Is the thickness of a given material that will reduce the beam

  • intensity by one-half.

  • The half-value layer for the x-ray beam in Figure is 2.5 mm Al.

  • Note that another 3.5 mm Al is needed to reduce the intensity in

  • half again; this value is the second half-value layer.

  • The half-value layer is related to the linear attenuation coefficient

  • by

  • In lead the HVL of the x-ray beam would be ~ 0.1 mm. You can see

  • why lead is used for shielding material.

  • A lead sheet 1.5 mm thick would be about 15 HVLs and would reduce

  • the beam intensity by a factor of 215 or about 30,000

Dr.Aida Radwan Tolba

The mass attenuation coefficient The x-ray photons are both absorbed and scattered out of the beam.μm

is used to remove the effect of density when comparing attenuation in several materials.

The mass attenuation coefficient of a material is equal to the linear attenuation coefficient μm divided by the density ρof the material.

The quantity ρx is in gm/cm2 and sometimes called the area density; μm is in cm2/gm. The HVL in area density units (gm/cm2) is given by 0.603 / mm.

Dr.Aida Radwan Tolba

PHYSICS OF DIAGNOSTIC X – RAYS The x-ray photons are both absorbed and scattered out of the beam.

Interactions of Photons with Matter

1. Photoelectric Effect

  • The photoelectric effect is one way x-rays lose energy in the body.

  • It occurs when the incoming x-ray photon transfers all of its energy to an

  • electron which then escapes from the atom .

  • The photoelectrons uses some of its energy (the binding energy) to get

  • away from the positive nucleus and spends the remainder ripping electrons

  • مندفع off (ionizing) surrounding atoms.

  • The photoelectric effect is more aptitude to occur قابلية الحدوث in the intense

  • electric field near the nucleus than in the outer levels of the atom, and it is

  • more common in elements with high Z than in those with low Z.

  • Of course, for a given electron to be liberated its binding energy must be

  • lower than the energy of the x-ray.

Dr.Aida Radwan Tolba

  • The binding energy of a The x-ray photons are both absorbed and scattered out of the beam.K electron in iodine is 33 keV, while that

  • in lead is 88 keV, and from 33 to 88 keV an x-ray photon can

  • release a K electron from iodine but not from lead. When the

  • energy of the x-ray is just slightly greater than the binding energy,

  • the probability that the photoelectric effect will occur increases

  • greatly, and this accounts for the sharp rises in the curve for iodine at

  • 33 keV and in the curve for lead at 88 keV in Figure.

  • These rises are called K-edges.

  • The elements in bone, muscle, and fat

  • have K-edge, but they are at such

  • low energies (~ 6 keV for calcium)

  • that they do not appear in Figure.

Schematic diagram illustrates the interaction of x-rays with matter.

(a) photoelectric effect. (b) Compton scattering. ( c ) pair production.

Dr.Aida Radwan Tolba

2. Compton Scattering matter.

  • Another important way x-rays lose energy in the body is by the

  • Compton effect, in which an x-ray photon can collide with a

  • loosely bound electron much like a billiard ball collides with

  • another billiard ball.

  • At the collision, the electron receives part of the energy and

  • the remainder is given to a Compton (scattered) photon, which

  • then travels in a direction different from that of the original x-ray.

Dr.Aida Radwan Tolba

  • The energy transferred to the electron can be calculated in the

  • same way as the energy transferred during a billiard ball collision by

  • using the laws of conservation of energy and momentum.

  • The x-ray has an effective mass m of E/c2( from Einstein's

  • famous equation E=mc2), and its momentum is E/c .

  • We can also calculate the energy equivalent of the electron mass to

  • be 511 keV , and the Compton effect is most likely to occur when

  • the x-ray has this energy.

Dr.Aida Radwan Tolba

  • The number of Compton collisions in the depends only on the number

  • of electrons per cubic centimeter, which is proportional to the

  • density.

  • A gram of bone has about the same number of electrons as 1 gm

  • of water, and thus the number of Compton collisions will be about

  • the same.

  • Note in Figure that the mass attenuation coefficient for Fat,

  • Muscle, and Bone are essentially identical at 150 keV where the

  • Compton effect is dominantمهيمن . However, since the photoelectric

  • effect is more aptitude to occur in high Z materials than in low Z

  • materials, the fraction of x-rays that lose energy by the Compton

  • effect is greater in low Z elements.

Dr.Aida Radwan Tolba

Mass attenuation coefficient for various tissues, lead, and iodine. Note that on a mass basis, all tissues attenuate about the same

above 100 keV.

  • For example, in water or soft tissue the Compton effect is more probable than the photoelectric effect at energies above about 30 keV.

  • Even in bone the Compton effect is more probable than the

    photoelectric effect at energies above 100 keV.

Dr.Aida Radwan Tolba

3. Pair Production iodine. Note that on a mass basis, all tissues attenuate about the same

  • Pair production is the third way x-rays give up energy. When a very

  • energetic photon enters the intense electric field of the nucleus, it may be

  • converted into two particles: an electron (ß-) and a positron (ß+)(positive

  • electron). Providing the mass for the two particles requires a photon with

  • energy of at least 1.022 MeV, and the remainder of the energy over 1.022

  • MeV is given to the particles as kinetic energy.

  • The positron is a piece of antimatter. After it has spent its kinetic energy

  • in ionization it does a death dance with an electron. Both then vanish, and

  • their mass energy usually appears as two photons of 511 KeV each called

  • annihilation radiation.

Dr.Aida Radwan Tolba

Since a minimum of 1.022 MeV is necessary for pair production, this type of interaction is only important at very high energies. Because the intense electric field of the nucleus is involved, pair production is more aptitude to occur in high Z elements than in low Z elements.

Dr.Aida Radwan Tolba

[C] The Relation between Photon Interactions with Matter and production, this type of interaction is only important

Diagnostic Radiology.

  • It is clear that pair production is of no use in diagnostic radiology

  • because of the high energies needed and that the photoelectric effect is

  • more useful than the Compton effect because if permits يسمح us to see

  • bones and other heavy materials( such as bullets رصاصة in the body).

  • At 30 KeV, bone absorbs x-rays about 8 times better than tissue

  • due to the photoelectric effect.

  • To make further use of the photoelectric effect radiologists often

  • inject high Z materials, or contrast media, into different parts of the

  • body.

Dr.Aida Radwan Tolba

  • For example: production, this type of interaction is only important

  • Compounds containing iodine are often injected into the

  • bloodstream to show the arteries

  • Radiologists give barium compounds orally to see parts of

  • the gastrointestinal tract and also to view the other end

  • of the digestive system.

  • Since gases are poorer absorbers of x-rays than liquids

  • and solids, it is possible to use air as a contrast medium.

  • Air is used to replace some of the fluid in the ventricles

  • of the brain.

Dr.Aida Radwan Tolba

Subtraction Technique production, this type of interaction is only important

  • To obtain additional information from a contrast study of the

  • arteries it is possible to use a subtraction technique in which:

  • an x-ray taken after the injection of a contrast medium is

  • photographically subtracted from an x-ray of the same body part

  • taken before the contrast medium was injected.

  • A subtraction x-ray often contains information that cannot be seen

  • on either of the two conventional x-rays.

  • If the photoelectric effect did not exist and radiologists had to rely

  • on the Compton effect, x-rays would be much less useful because the

  • Compton effect depends only on the density of the materials.

  • Bone is about twice as dense as soft tissue and will still be seen on an

  • x-ray film taken with high-energy x-rays, but the contrast will be low.

  • This production, this type of interaction is only important low contrast is not always undesirable, however; for example,

  • on a chest x-ray the ribs are often of no medical interest and hide

  • some of the lungs, and some medical centers use high potentials

  • (~350 KVp) to make the ribs less obvious.

  • The Compton Effect seriously degrades يخفضx-ray images of thick

  • body parts since the scattered radiation that gets through the

  • patient and strikes the film reduces the useful information by

  • reducing the contrast in the image.

Making An X-Ray Image production, this type of interaction is only important

  • It is relatively easy to make an x-ray image, or roentgenogram ,

  • all that is needed is an xray source and a film wrapped in black

  • paper on which to record the image.

  • However, making a good x-ray image while keeping the x-ray

  • exposure at a minimum requires considerable knowledge and the

  • use of modern technology.

Dr.Aida Radwan Tolba

  • Unfortunately, x-rays cannot be focused to make a picture as

  • with a camera.

  • X-ray images are basically images of the shadows cast on film

  • by the various structures in the body.

  • To better understand the physical problems of recording sharp

  • x-ray shadows; let us consider the problems of casting sharp

  • shadows with visible light.

Dr.Aida Radwan Tolba

  • You can try the experiments illustrated in Figure aswith your hand as

  • the object.

  • A large light bulb produces a blurred shadow because the light

  • from different parts of the bulb casts shadows in different places.

  • The blurred edge of the shadow is called the penumbra, which

  • means "next to the shadow". The width of the penumbra can be

  • calculated from the dimensions of the light bulb and the distances

  • to the object and the paper.

  • The penumbra can be reduced by:

  • using a smaller diameter light bulb or by

  • moving the object closer to the paper.

  • (Moving the light bulb further away also reduces the penumbra).

Dr.Aida Radwan Tolba

The principles involved in casting shadows with visible light.

(a) The shadow of an object some distance from a piece of paper is blurred when a large lightbulb is used. This shadow can be made much Sharper

(b) by using a smallerdiameter lightbulb or

(c) by moving the object closer to the paper,

(d)Cloudy water between the lightbulb and paper absorbs some light and scatters much of the

rest, reducing the contrast of the shadow.

Dr.Aida Radwan Tolba

  • Another problem involved in casting a sharp shadow is light.

  • illustrated in The sediment ترسبات in the water absorbs some

  • of the light and scatters much of the light that is not

  • absorbed.

  • The problems involved in obtaining good x-ray shadows are

  • analogousمشابه , and blurring and this can be reduced by:

  • (1) Using a small focal spot,

  • (2) Positioning the patient as close to the film as possible

  • (and increasing the distance between the x-ray tube and

  • the film as much as possible), and

  • (3) Reducing the amount of scattered radiation striking the

  • film as much as possible.

  • (4) It is also necessary to avoid motion during the exposure, since

  • motion causes blurring.

The width of the penumbra light.P can be calculated from the ratios of sides of similar triangles if we know the diameter D of the light source and the dimensions l and L.

Dr.Aida Radwan Tolba

  • The normal sizes of the focal spots on many x-ray light.

  • units are 1 mm (small focal spot) and 2 mm (large focal

  • spot). While decreasing the size of the focal spot

  • reduces the penumbra, it also necessitates lowering the

  • power to avoid damaging the target.

  • This reduces the intensity of the x-ray beam requiring

  • a longer exposure that usually results in blurring

  • due to patient motion.

Dr.Aida Radwan Tolba

  • While light.the patient is generally placed as close to the film as

  • possible in order to reduce the penumbra, sometimes it is also

  • possible to further reduce the penumbra by increasing the

  • distance from the x-ray tube to the film.

  • Chest films are usually taken from a distance of 180 cm for

  • this reason.

  • Unfortunately, increasing the distance reduces the beam intensity

  • according to the inverse square law, making itimpractical to take many

  • x-rays from a large distance; at 90 cm the beam intensity is four

  • times that at 180cm.

  • To obtain a satisfactory x-ray image of thick body parts such as the

  • abdomen and hips, it is necessary to reduce the scattered radiation

  • at the film.

  • The amount of scattered radiation at the film depends somewhat on

  • the energy of the x-rays, but the thickness of the tissue that the x-

  • ray beam passes through is the most important factor (the thicker

  • the tissue, the greater the scatter).

  • Also, the larger the beam, the greater the scatter, and thus one

  • simple way of reducing scattered radiation is by keeping the x-ray

  • beam as small as possible

  • The most significant way of reducing the amount of such as the

  • scattered radiation striking the film is by using agrid consisting of a series of lead and plastic strips.

  • The strips are aligned so that unscatteredx-rays

  • from the source will go through the plastic strips and

  • strike the film while most of the scattered radiation

  • will strike the lead strips and be absorbed.

Dr.Aida Radwan Tolba

  • The grid was invented in 1915, and in 1919 they such as the

  • improved it by making it move during the exposure

  • so that the lead strips do not produce visible shadows

  • on the film.

  • Some modern grids are stationary but have lead strips

  • so fine (4/mm) that their shadows do not interfere with

  • the image.

Dr.Aida Radwan Tolba

The grid consists of alternating thin lead strips and wide plastic strips. The un-scattered x-rays pass through the plastic strips while

most of the scattered x-rays are absorbed by the lead strips.

Dr.Aida Radwan Tolba

  • If two equivalent x-rays are taken, one with and one plastic strips. The un-scattered x-rays pass through the plastic strips whilewithout

  • a grid, the one taken with the grid will be clearer because of

  • the reduced scatter. However, it will require a longer exposure

  • to the patient.

  • Since reducing the scatter reduces the darkening of the film,

  • it is necessary to increase the exposure in order to obtain an

  • optimum darkness (optical density) of the film.

  • In addition, a higher exposure must be given because the lead

  • strips absorb some of the un-scattered radiation

Dr.Aida Radwan Tolba

  • When you have a chest x-ray taken, the technologist tells you

  • to hold your breath since reducing motion reduces blurring.

  • However, it is not possible to hold your heart motion , and x-rays

  • of the heart are somewhat blurred. This blurring can be reduced

  • by making the exposure as short as possible.

  • The desire for short exposures has led to the development x-ray

  • tube with large current capacities that can produce intense x-ray

  • beams.

Dr.Aida Radwan Tolba