Fundamentos físicos de los Rayos X - uv.es · En cambio, un fotón de luz azul (λ = 400nm) tiene...

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Departament dEnginyeria Electrnica

Intelligent Data Analysis Laboratoryh!p://idal.uv.es

Fundamentos fsicos de los Rayos XSistemas e Imgenes Mdicas Mster en Ingeniera Biomdica

Diagnstico por la imagen [SIM Mster IB] Joan Vila Francs

Rayos X

Descripcin

Produccin de los Rayos X

Interaccin de los Rayos X con la materia

Dosis y riesgo


Rayos XLos Rayos X son una forma de radiacin electromagntica ionizante.

La radiacin electromagntica tiene comportamientos a la vez de partcula y de onda.

Como partcula, la radiacin EM se representa como un flujo de paquetes o cuntos de energa llamados fotones que viajan en lnea recta.Como onda, la radiacin EM se representa como dos campos -elctrico y magntico- trasversales que varan sinusoidalmente.


Rayos XLas ondas electromagnticas viajan a una velocidad c con una frecuencia de oscilacin f.

Cada ciclo de onda atraviesan una distancia .

E

Bkk+q

-q

c = fc = 3108 m/s

en el vaco


Rayos X

espectro de radiacin electromagntica


Rayos XLa energa de una onda EM depende de su longitud de onda :

E = h f, h = 4.135710-18 keVs (6.626110-34 m2 kg / s)Por ejemplo, un fotn de rayos X de = 0.1nm tiene una energa de 12,4keV. En cambio, un fotn de luz azul ( = 400nm) tiene una energa de unos 3eV.

Energa de ionizacinH 12.6 keVC 9.3 keV12 30 eV


Estructura del tomoUn tomo est formado por un ncleo atmico rodeado de una nube de electrones.

El ncleo est compuesto por protones y neutrones.El nmero de protones o nmero atmico, Z, determina su elemento qumico.El nmero de neutrones determina su istopo.La suma del nmero de protones y neutrones es el nmero msico del tomo, A.

Los electrones orbitan alrededor del ncleo atmico.La fuerza que mantiene unido cada e- al ncleo se denomina energa de enlace (binding energy)


Estructura del tomoModelo de Bohr:


Estructura del tomoCapa de electrones:


Produccin de rayos XSe producen al colisionar partculas cargadas (electrones) con la materia:

2 Fundamentals of X-ray Physics

sources, the radiation is generated by the deceleration of fast electrons enteringa solid metal anode, and consists of waves with a range of wavelengths roughlybetween m and m. Thus, the radiation energy depends on the electronvelocity, , which in turn depends on the acceleration voltage, Ua, between cathodeand anode so that with the simple conservation of energy

eUa = me (.)the electron velocity can be determined.

2.2.1X-ray Cathode

In medical diagnostics acceleration voltages are chosen between kV and kV,for radiation therapy they lie between kV and kV, and for material testingthey can reach up to kV. Figure . shows a schematic drawing of an X-ray

Fig. .. Schematic drawing of an X-ray tube.Thermal electrons escape from a cathode fila-ment that is directly heated to approximately ,K. The electrons are accelerated in theelectric field between cathode and anode. X-ray radiation emerges from the deceleration ofthe fast electrons following their entry into the anode material

Charge of electrons: e = . C; mass of electrons:me = . kg. There is no clear definition of the X-ray wavelength interval. The range overlaps withultraviolet and -radiation.

Acceleration energy is measured in units called electron volts (eV). eV is the energy thatan electronwill gain if it is acceleratedby an electrical potential of one volt.The same unitis used to measure X-ray photon energy.


Produccin de rayos XLos rayos X se producen al interaccionar los e- con la materia mediante dos mecanismos:

Radiacin de frenado (bremstrahlung)Radiacin caracterstica


Interacciones de los e- con la materia

Radiacin de frenado (bremstrahlung)

50120 kVd.c.110V

a.c.Glass envelope

ElectronsAnode Stator

RotorHeater/Cathode

Glass WindowFilter

CollimatorX-rays

+

Cathode cupSIDE VIEW

FilamentBroad beam

Negative potential

Beam cut-off

Narrow beam

(i) (ii)

Figure 3.1 (i) A standard x-ray tube; (ii) focusing the electron beam using a negative voltage to thecathode cup. (Part (ii) after Dowsett et al., 2006, p. 76. Reproduced by permission of EdwardArnold (Publishers) Ltd.)

Target atom

Electron beam

Bremmstrahlung

e

e

e

(i)

Ionization/Excitation K-shell Vacancy CharacteristicX-ray

(ii)

e

e

Figure 3.2 Production of (i) bremstrahlung x-rays, when electrons are deflected by a heavy nucleus, and(ii) characteristic x-rays, resulting from the excess energy of an electron falling into the vacancycaused by an ejected inner shell electron. (Part (ii) after Wolbarst (1993), p. 57.)

3.2 Images from x-rays 49



Radiacin caracterstica

50120 kVd.c.110V

a.c.Glass envelope


RotorHeater/Cathode

Glass WindowFilter

CollimatorX-rays

+


FilamentBroad beam

Negative potential

Beam cut-off

Narrow beam

(i) (ii)


Target atom

Electron beam

Bremmstrahlung

e

e

e

(i)


(ii)

e

e





Espectro de emisin de la radiacin de frenado



Espectro de emisin total



Ejemplo: espectro de emisin de un tubo de Tungsteno excitado a 100 keV

Characteristic x-rays are characteristic of the target material, since they depend onthe energy level differences of the atoms of the target. In regular radiography, usinga tungsten target, most of the x-ray energy produced is bremmstrahlung radiation(Fig. 3.3); whereas in mammography, using a molybdenum target, most of the x-raysproduced are (molybdenum) characteristic x-rays, with precise energies.

The high voltage to the anode is switched on for only a fraction of a second to producea pulse of x-rays, which exit through a glass window in the metal housing of the x-raytube. The x-rays are filtered by a thin sheet of aluminum to remove low-energy x-rayswhich would be unable to penetrate body parts. If these were not filtered out, they wouldbe absorbed by the body and increase the dose to the patient.

The radiographer can control the exposure in two ways. Changing the voltage (kV)across the tube changes the energy of the electrons, and hence the maximum energy of thex-ray photons and their penetrating ability. Changing the filament current (mA) changesthe number of electrons produced, and therefore the number of x-ray photons, i.e. theintensity of the x-ray beam.

X-rays and visible light are both part of the electromagnetic spectrum (Fig. 1.2), and,as such, have both wave and particle properties. As particles, the energy of a singlephoton, E, is given by

E hf (3:1)

where h is Plancks constant (6.626 1034 J s) and f is the frequency of the correspond-ing wave. For a 60 keV x-ray photon, the frequency of the x-ray wave is 1.45 1019Hz.

All waves obey the following relationship:

f l c (3:2)

0.2Rel

ativ

e in

tens

ity

0.4

0.6

0.8

1.0

Energy (keV)

K shell characteristicradiation

High energycut-off

Low energycut-off

20 40 60 80 100 120

Figure 3.3 Spectrum of x-rays from an x-ray tube, with a tungsten target, operating at 100 kVp. Thehigh-energy cut-off is determined by the electron energy, which in turn is determined by the kVpacross the tube; the low-energy cut-off is determined by the thickness of an aluminum windowon the tube, which stops very low energy x-rays. Characteristic K-shell radiation is shownsuperimposed on the bremmstrahlung spectrum. (Copyright (1999). From Physics forDiagnostic Radiology, 2nd Edition, by Dendy & Heaton. Reproduced by permission of Taylorand Francis Group, LLC, a division of Informa plc.)

50 Medical images obtained with ionizing radiation



Ejemplo: espectro de emisin de un tubo de Molibdeno (mamografa)



Eficiencia de la produccin de Rayos X:Aproximadamente slo un 1% de la E. cintica se convierte en Rayos X.En el rango de 60-120keV, la intensidad emitida es aproximadamente proporcional a kV2 x mA.La radiacin de frenado es proporcional a kV x Z.


El tubo de rayos X

50120 kVd.c.110V

a.c.Glass envelope


RotorHeater/Cathode

Glass WindowFilter

CollimatorX-rays

+


FilamentBroad beam

Negative potential

Beam cut-off

Narrow beam

(i) (ii)


Target atom

Electron beam

Bremmstrahlung

e

e

e

(i)


(ii)

e

e




Interaccin de los RX con la materia

Cuando un haz de Rayos X atraviesa la materia, a cada fotn le puede ocurrir uno de los siguientes casos:

Transmisin: el fotn atraviesa la materia como radiacin directa.Absorcin: el fotn transfiere toda su energa a la materia y desaparece.Dispersin: el fotn se desva de direccin, pudiendo perder parte de su energa, y saliendo del material como radiacin dispersa (scattered) o secundaria.



haz incidente haz atenuado

rayo transmitido

rayo absorbido

rayo dispersado

rayo dispersado



Tipos de interaccin de los Rayos X con la materia:Dispersin Rayleigh Efecto fotoelctrico Dispersin Compton Produccin de pares


Interaccin de los RX con la materiaDispersin Rayleigh (coherente)

2.3 PhotonMatter Interaction

Fig. .. Principles of photonmatter interaction. For the Rayleigh process the dipole an-tenna characteristic is illustrated. For pair production the successive process of pair annihi-lation is illustrated as well

the model does not take into account the quantum aspects of light and, therefore,for energies being considered here, this classical scattering model yields results indisagreement to what is found by experiments.

pZ

Ecoherenteff8 3

2

/


Interaccin de los RX con la materiaEfecto fotoelctrico





There are three further mechanisms of X-ray attenuation that take the quantummechanical interaction principles into account. These are either pure photon ab-sorption processes (photoelectric absorption and pair production) or a mixture ofscattering and photo energy absorption (Compton scattering).

2.3.2.2Photoelectric Absorption

The entire energy of anX-ray photon, h, can be absorbed by an atom, if the bindingenergies of atomic electrons are smaller than h.The interacting electron is raisedto a state of the continuous spectrum, i.e., the electron of a lower shell is kickedoff the atom and travels through the material as a free photoelectron. The energybalance of this ionizing process is given by

h (+ Atom) ! Eion(Atom+) + Ekin(e) . (.)(.) means that the electron leaves the atom with a kinetic energy equal to thedifference between the quantum energy of the incident photon and the bindingenergy of the electron. This energy difference is removed from the primary beamand transferred locally to the lattice in the form of heat. The vacancy left by theelectron that was kicked out is filled by electrons from outer shells or, in the caseof solids, by electrons from the band. As a result of recombination, characteristicX-ray fluorescence lines can be measured. If the radiation energy of the successiverecombination process for the electron vacancy described by (.) is large enoughto kick out another electron in the more outlying shells, the new free electron iscalled an Auger electron. As mentioned already in " Sect. .., Auger electrons aremonoenergetic particles.This process is sometimes called radiation-free transitionor internal conversion.

The linear absorption coefficient depends on the energy, h, of the incidentX-ray beam and the atomic number, Z, of the absorber material. It has been demon-strated experimentally that a useful rule of thumb is given by

= k A

Z

(h) , (.)where k is a constant that depends on the shell involved, is the density, and A isthe atomic weight of the material.Thus, the absorption is given by

# Z . (.)This strong Z dependence of the absorption coefficient is utilized within the choiceof radio-opaque contrast media based on for example iodine (I, Z = ) or bar-ium (Ba, Z = ) (Lauenberger and Lauenberger ). In Fig. . the attenuation For recovering the nature of the photoelectric absorption in , Albert Einstein receivedthe Nobel Prize for physics in .

p ZEpe

3

3


Interaccin de los RX con la materiaEfecto fotoelctrico

Coeficiente de atenuacin debido al efecto fotoelctrico

en funcin de la energa del haz


Interaccin de los RX con la materiaDispersin o efecto Compton





K-shell, more than % of the photoelectric absorption takes place in the emissionof K-shell electrons.

2.3.2.3Compton Scattering

An X-ray photon with energy h sometimes collides with a quasi-free electron.The energy balance of this collision process is given by

h(+ e)! Ekin(e) + h . (.)In contrast to the photoelectric absorption, the X-ray photon loses only a part of itsenergy during the Compton collision.Thus, the scattered photon is of lower energywhen it continues its travel through the matter. The energy level of the scatteredphoton can be measured under the scatter angle , via the shift of wavelength

= hmec( cos()) . (.)

The complementary part of the energy is carried by the kinetic energy of the recoilelectron that is kicked off the atom. This Compton electron is also called a sec-ondary electron. Both the scattered photon and the Compton electron may haveenough energy to undergo further ionizing interactions. (.) can be derived byconsidering the conservation of energy and momentum. It says that in the forwardscattering direction no energy is transferred, whereas in the backward scatteringdirection most of the quantum energy is transferred. However, even at # of de-flection the scattered X-ray quanta retain at least two-thirds of their initial energy(Bushong ). Sometimes Compton backscattering produces ghost images of thedetector back board.

Overall, the probability of Compton scattering depends on the electron den-sity n and not on the atomic number of the scattering medium. Since the electrondensity difference between distinct tissues is small, Compton scattering provideslow-contrast information. The total cross-section for Compton scattering can bederived from the KleinNishina equation (Leroy and Rancoita )

Compton = re #$ + EE %&( + E) + E

ln( + E)E ' +

ln( + E)E

+ E( + E) ( ,

(.)

where E = h)(me c) is the reduced energy of the incoming photon. Substituting(.) into (.) leads to

Compton = n Compton , (.)the desired (material-independent) energy dependence of the attenuation coeffi-cient due to Compton scattering. For recovering the nature of incoherent scattering Arthur Holly Compton received theNobel Prize for physics in .

For instance a weakly bound valence electron in the outer shell.

pEc


Interaccin de los RX con la materiaDispersin o efecto Compton


Interaccin de los RX con la materiaProduccin de pares






Porcentaje de absorcin debido a efecto fotoelctrico frente a Compton en funcin de la energa del haz para

distintos materiales


Transmisin de los rayos X


2.3.1LambertBeers Law

As illustrated in Fig. ., all physical mechanisms that lead to the attenuation ofradiation intensity (reduction of photons) measured by a detector behind a homo-geneous object are usually to be subsumed by a single attenuation coefficient, .Within this simple model the total attenuation of a monochromatic X-ray beamcan be calculated in the following way. The radiation intensity which is propor-tional to the number of photons after passing a distance through an object isdetermined by

I( + ) = I() ()I() . (.)By simple reordering of (.) the quotient

I( + ) I()

= ()I() (.)can be obtained. Taking the limit of (.) leads to the differential quotient

lim!

I( + ) I()

= dId= ()I() . (.)

In a first step, the medium is assumed to be homogeneous, i.e., in (.) the ob-ject can be described by a single attenuation constant () " along the entirelength of penetration. This leads to an ordinary linear and homogeneous, first-order differential equation with constant coefficients. The solution is obtained byseparation of variables. Consider the right-hand side of (.), which can be sepa-rated to

dII() = d . (.)

Fig. ..Mathematical model of monochromatic X-ray attenuation.The photons are run-ning through an object of thicknesswith a constant attenuation coefficient, . Equal partsof the same absorbing medium attenuate equal fractions of the radiation

: coeficiente de atenuacin lineal (1/cm)


Transmisin de los rayos X


Fig. ..Mass attenuation coefficient, !, versus incident photon energy for lead (top) andwater (bottom). For the diagnostic energy window of CT, E = [keVkeV], photoelec-tric absorption is dominant for lead and Compton scattering is dominant for water. Pairproduction is possible for quantum energies of E " MeV (compiled with data from the webdatabase XCOM [Berger et al. ])

Radiation by Compton scattering is a significant contribution to the patientstotal dose, as well as to the radiologist, in interventional imaging procedures.There-fore, the radiologist must wear a shielding lead apron. In Fig. .a, it is schematic-ally shown that the patient becomes a source of radiation himself. For that reason,it is important to know the spatial iso-dose lines of a CT system to be obtained bya standard scattering phantom. In Fig. .b,c, so-called scatter diagrams of a PhilipsTOMOSCAN CT system are shown in top and side view.


Integration of both sides of (.)

dII() = d (.)gives

ln "I" = + C . (.)Due to the physical fact that the intensity is defined as a positive quantity, the abso-lute sign, " ", is obsolete so that subsequent exponentiation leads to

I() = e+C . (.)With the initial condition I() = I, the special solution of the differential equa-tion (.)

I() = I e , (.)is obtained, known as Beers law of attenuation . The linear attenuation coeffi-cient, , of (.) is an additive combination of a scatter coefficient, s, and an ab-sorption coefficient, , i.e.,

= s + . (.)The attenuation coefficient is measured in units of [] = m. Beers law holds forpencil-beam geometry only, where the scattered radiation is completely removedfrom the main beam. With a wide beam, much of the scattered radiation carries onwith a forward component to its direction (Barrett and Swindell ). Generally,the linear attenuation coefficient is given by

= NAA tot = n tot , (.)

where is again the density, A is the atomic weight of the material and NA is theAvogadro constant. That means the attenuation coefficient is given by the numberof target atoms per unit volume, n, times the total photon atomic cross-section, tot,for either scattering or absorption.

By introducing the absorber density, , the mass attenuation coefficient

= $ (.)can be defined. The mass attenuation coefficient is measured in units of [] =m$kg. This coefficient is useful for estimating the mass of a material required toattenuate the primary beam in the design of X-ray shielding.

This observation holds for monoenergetic X-ray. In the case of polychromatic X-ray onemust integrate over all energies.

Si se normaliza respecto a la densidad del tejido , se obtiene el coeficiente de atenuacin msico:

coef. de atenuacin msico para el agua


Transmisin de los rayos XLa atenuacin de los rayos X tambin se puede caracterizar mediante el half-value layer (HVL):

Grosor de tejido que reduce la intensidad del haz de RX a la mitad. Est relacionado con por:

HVL = ln2


Transmisin de los rayos XDependencias de la atenuacin de los RX

2.4 Problems with LambertBeers Law

Fig. .. Attenuation of an X-ray beam while running through a tissue that is modeled bya sequence of discontinuous partitions

If the discretization of the tissue is very fine, i.e., is chosen to be small, the factorterms in parentheses can be interpreted as the Taylor expansion of the exponentialfunction. With

e ! (.)for small , the approximation

In ! I e e e# ei# en (.)can be obtained. By taking the limit $ the exact equation

In = I en"

i = i $ I e ()d (.)can be obtained. This view on the modeling of X-ray attenuation is closely relatedto a statistical view of photon number reduction by photonmatter interaction thatwill be introduced at the end of this chapter. To summarize the facts above, it canbe said that the attenuation of X-ray through matter is well understood and thus,

Fig. .. Causes of attenuation: Wavelength of the incident beam, atomic number, massdensity, and thickness of the medium (after Laubenberger and Laubenberger [])


Produccin de imgenes radiolgicas

El ojo humano no detecta los rayos X.

Hay que convertir la distribucin de los rayos X transmitidos en una forma visible mediante:

Exposicin de una pelcula fotogrfica sensible a los RX.Estimacin de la densidad de fotones midiendo la ionizacin de una cmara de gas.Conversin de los fotones de RX en luz visible.Utilizacin de un detector de estado slido que genere una corriente proporcional a la densidad de fotones incidente.



as elect/E. Thus materials such as bone have a higher value of attenuation coefficientand attenuate x-rays more than soft tissues, as a consequence of their larger Zeff, and(photoelectric) absorption is the dominant interaction. Materials with a high effectiveatomic number, such as iodine (Zeff = 53) or barium (Zeff = 56), can be used to increaseattenuation. They can be injected or swallowed to change the attenuation of soft tissuesfilled with the material compared to other soft tissues, and are known as contrast agents.For higher-energy x-rays, the overall attenuation is smaller with very much smallerphotoelectric absorption and Compton scattering becoming dominant.

In projection or planar x-ray radiography the image is a simple two-dimensionalprojection or shadowgram of a three-dimensional object, the part of the patient in thefield of view (Fig. 3.5); x-ray film is the detector. Projection radiography includes

! film-screen radiography, including chest radiography, abdominal radiography, angio-graphy (studies of blood vessels) and mammography (Activity 3.1 has examples of theresulting radiographs and their visualization);

! fluoroscopy, in which images are produced in real time using an image intensifier tubeto detect the x-rays;

! computed radiography, in which a re-usable imaging plate containing storage phos-phors replaces the film as the detector;

! digital radiography, which uses semiconductor sensors.

Images of the human body can be acquired or displayed in three main orientations(Fig. 3.6). The coronal plane divides the body into front and back. This is the orientationdisplayed in the common posterioranterior (PA) chest radiograph, where the x-raysenter from the patients back (posterior) and are collected by a film placed at hisfront (anterior). The acquired image is a superposition of many coronal images atdifferent depths within the body. The sagittal plane is a side view, dividing the bodyinto right and left, and the transverse plane, sometimes referred to as the axial or trans-axial plane, is a plane perpendicular to the long axis of the body, dividing it into top andbottom planes.

Ribs

LungX-raytube

Optical density (OD)of developed film

0 1

Detectionby film

Image

2

Figure 3.5 Basic components of x-ray imaging. Fewer x-rays pass through the ribs, due to their highattenuation, resulting in less blackening of the film, i.e. the ribs appear whiter. (After Wolbarst(1993), p. 16.)


(i) (ii)

Figure 3.7 Posterioranterior chest radiographs of (i) a normal patient and (ii) a patient suffering fromtuberculosis.

Focal spot size

Focusobjectdistance

Patient

Objectimagedistance

A B

P

C

Detector plane

Feature

Blur Blur

D

S

Figure 3.8 Blurring at the detector plane results from the x-ray focal spot size and placement of the patient.The closer the patient is to the detectors, the smaller the blurring.




Mejora del contraste:para baja energa (kV pequeo):

Domina efecto fotoelctrico contraste proporcional a la diferencia entre Z

para alta energa (kV grande):Domina dispersin Compton contraste proporcional a la diferencia entre



Endurecimiento del haz de RX

los rayos X menos energticos son absorbidos por los tejidos y no aportan informacin a la imagen



Energa del haz adecuada:A bajas energas predomina el efecto fotoelctrico:

mayor contraste entre distintos tejidos (p. ej. hueso/msculo)mayor absorcin del haz (incremento de la dosis)

A altas energas predomina el efecto Compton:menor contraste entre tejidosmayor dispersin (emborronamiento de la imagen)

No hay un valor ptimo, hay que llegar a un compromiso en funcin del grosor a observar

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Departament dEnginyeria Electrnica

Intelligent Data Analysis Laboratoryh!p://idal.uv.es

Dosis y riesgoImgenes por rayos X


Exposicin radiolgicaLa exposicin es la magnitud fsica que caracteriza el efecto de las radiaciones ionizantes.

Se mide como la cantidad de carga elctrica que produce un haz en una unidad de masa de aire seco en condiciones estndar de presin y temperatura (1 atm y 20C).

Unidades:roentgens [R] (unidad tradicional)[CKg-1] (unidad del S.I.)

1 R = 2.5810-4 CKg-1


Dosis de radiacinLa dosis absorbida mide la cantidad de radiacin ionizante recibida por un material.

Se mide como la energa depositada en un medio por unidad de masa.

Unidades:gray [Gy] (unidad del S.I.)

1 Gy = 1 Jkg-1 de material

rad (radiation absorbed dose, unidad tradicional)1 rad = 0.01 Gy


KermaEl kerma (Kinetic Energy Released per unit MAss) mide la energa transferida por la radiacin ionizante a los electrones del material irradiado.

Para radiaciones en el rango del diagnstico mdico (por debajo de 1MeV), dosis absorbida y kerma son equivalentes.Su unidad es J/Kg ( = Gy, unidad de dosis absorbida)El Air kerma equivale a la dosis absorbida por el aire.


Relacin entre exposicin y dosis

El ratio, ffactor, entre dosis absorbida (rad) y exposicin (R) depende del tipo de tejido y la energa de la radiacin


Dosis de radiacin equivalenteLa dosis equivalente tiene en cuenta el riesgo de dao producido en los tejidos humanos segn el tipo de radiacin.

Este riesgo se calcula multiplicando cada tipo de radiacin por una constante especfica wR.

Unidades:sievert [Sv] (unidades S.I.)

1 Sv = 1 J Kg-1 wR

rem (unidades tradicionales)1 rem = 0.01 Sv


Dosis de radiacin equivalenteLa constante o factor de peso de la radiacin wR es:

11.3 Definition of Specific CT Dose Measures

Table ..Weighting factors (wR) for different radiation types (ICRP )Radiation type and energy range Radiation weighting factorwRPhotons (all energies) Electrons und muons (all energies) Neutrons E < keV

keV < E ! keV keV < E ! MeV MeV < E ! MeV E " MeV

Protons (other than recoil protons) E " MeV -Particles, fission fragments, heavy nuclei

Table .. Tissue weighting factors to be used for calculation of the effective dose (ICRP)

Tissue or organ Tissue weighting factor (wT)Gonads .Bone marrow (red) .Colon .Lung .Stomach .Bladder .Breast .Liver .Esophagus .Thyroid .Skin .Bone surface .Remainder .

11.3Definition of Specific CT Dose Measures

The spatial dose distribution in CT fundamentally differs from the one observedin traditional projection radiography. Figure .a provides an illustration of the re-sulting relative iso-dose lines as seen in an anterior posterior projection radiographof a skull. Since tissue attenuates the penetrating X-ray beam, the dose decreasescontinuously along the y-axis, i.e., from the tube-adjacent side toward its oppositeside. As a result of BeerLamberts law the applied dose decreases exponentially.In contrast, Fig. .b provides the iso-dose lines resulting from a CT of the sameskull. Here, the object is X-rayed from all sides, so that the resulting dose distri-


Dosis de radiacin

dosis de radiacin promedio para algunas situaciones tipo


Dosis de radiacin

Dove & Physics of Medical Imaging 9/22/2003

Table 4.2 Maximum permitted doses from ICRP

Condition Dose Radiation worker 50 mSv = 5 rem (5-yr average < 20 mSv = 2 rem) Public 1 mSv = 0.1 rem over 5 years

The US Nuclear Regulatory Commission has adopted standards that limit maximum exposure for the general public to 0.5 rem per year. Limits for occupational exposure are 1.25 rem/3 months for the whole body, and 18.75 rem/3 months for the extremities. Routine personal monitoring is usually done with film badges and ring-type finger badges.

The maximum permitted dose levels have been reduced over the last 70 years. In 1931, the maximum permitted level was 15 mSv (1.5 rem) PER WEEK. It is possible that further reductions will be made. The reason is that even small doses have long-term effects, and it is these effects that are the cause of continuing controversy in setting XsafeY settings. The biological effects can only be expressed in statistical terms as the chance that a generic change, or a leukemia (or other cancer) might develop over a given period of time. The assessment of risk is complicated because there are also natural causes of these changes. The existence of long-term effects is the reason why young people, and particular the unborn fetus, are subject to the greatest risk for ionizing radiation, and thus are subject to their own specific maximum radiation exposure levels. For example, it is recommended that, under the X10 day rule,Y women are only exposed to diagnostic X-ray procedures during the 10 days following menstruation when pregnancy is unlikely.

4.3 Environmental dose

We are exposed to radiation from many different sources during our lives. Some of the sources are natural, some are man-made. quantifies the body doses of various of these sources. You should compare these with the maximum allowed dose given above.

Table 4.3

Table 4.3 The doses correspond to six different situations (values are approximate).

Cosmic radiation 200 Sv (20 mrem) over 1 year Natural radioactive materials (e.g., 238U) 300 Sv (30 mrem) over 1 year Naturally occurring radioactive materials in the body (e.g., 40K)

300 Sv (30 mrem) over 1 year

Chest X-ray 500 Sv (50 mrem) skin dose for one X-rayCoronary angiogram 20 mSv (2 rem) skin dose for one

procedure Nuclear power station < 1mSv (100 mrem) over 1 year 1 km from

the station

Page 23 of 53

XrayF03.doc

dosis de radiacin promedio para algunas situaciones tipo


Dosis de radiacin

dosis de radiacin promedio para algunas exploraciones mdicasProtection (ICRP), 1991)). Using the figure for fatal cancer, if 100 000 people were toreceive uniform whole-body doses of 1mSv each, then about five of them would dieprematurely of radiation-induced cancer.

When a medical exposure is made, there should always be the expectation thatsome benefit will come of it, and that the dose to everyone involved, patient and staff,should be as low as reasonably achievable, the so-called ALARA principle. Minimizingexposure time, maximizing the distance from the radiation source, and establishing

Table 3.4 Typical effective dose equivalents for variousdiagnostic procedures.

Examination Range (Sv)

Dental x-ray 1020Chest 1050Skull 100200Pelvis 7001400Abdomen 6001700Mammogram (each image) 10002000Lumbar Spine 13002700Barium meal 19004800IVU (intra-venous urography) 25005100Head CT scan 20004000Body CT scan 500015 000Nuclear medicine 200010 000

Table 3.3 Relative radiosensitivity of the organs of the humanbody.

Organ/tissue Relative radiosensitivity

Gonads 0.20Bone marrow (red) 0.12Colon 0.12Lung 0.12Stomach 0.12Bladder 0.12Breast 0.05Liver 0.05Esophagus 0.05Thyroid 0.05Skin 0.01Bone surface 0.01Other organs 0.05

Based on the International Commission on RadiologicalProtection (1991).

3.4 Dose and risk 85

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