X - RAYS IN DIAGNOSTICS

D. Krilov

22. 10. 2008. .

HISTORY

W.C.Röntgen (1845-1923) 8.11. 1895. - discovered a new type of radiation in experiments on gas discharges; he demonstrated that this radiation:

induces the ionization in the air penetrates through the matter does not deflect in electric and magnetic field foggs the photographic plate induces the burns on skin in January 1896. he produced the first

anatomical X-ray picture of the hand Nature, Jan. 23 1896 Science, Feb.14 1896

The nature of X-rays X - rays are electromagnetic waves (1 pm – 0,1 nm) natural sources of X-rays do not exist; the artificial source

is X-ray tube X-rays are produced by two mechanisms:1. by rapid decceleration of fast electrons in electric fields

generated by heavy nuclei2. by relaxation of heavy atoms in tube target (anode) the medical application is based on specific interactions of

incident X-ray photons with atoms in tissues; the image is

produced from the beams transmitted through the body

X-ray tube

cathode

anode Ua= 30-150 kVIg = 3-5 A

Ie= 20-30 mA

mv2/2= eUa

tungsten disc

Generation of X-rays Brehmsstrahlung (braking radiation)High speed electrons enter the crystal lattice of target atoms and are decceleratedin electric field of atomic nuclei.Energy of emitted photon depends onenergy loss of photon. The photon with highest energy is generated when electron is stopped

Ein

Eout

h

outin EEh

kVU

,nm

Uec

hh

amin

amin

max

241

transmitted beam is composed of photons withdifferent energies - continuous spectrum

Collisions of incident electrons with electrons in target material

Incident electron ejects one of electrons from inner shell of target atom.

The empty state is filled by an electron from higher shell, the energy difference is emitted as X-photon

Only the photons with energies equal to differences of particular atomic levels are emitted - line spectrum reflects the atomic structure of target

The probability of such events is low - the intensity of line spectrum is low

Linear spectrum consists of a number ofnarrow lines transitions into K shell: K,

transitions into L shell: L, L

X-ray spectrum

The spectrum is the plot of spectral radiancy over wavelength

It is composed of continuous and linear part

continuous spectrum has well defined short-wavelength cutoff determined by anode voltage

the highest radiancy is at wavelength

linear spectrum is not important for medical diagnostics

min

I(W/m3)

min3

4

Influence of tube voltage on X-ray spectrum

Beam power is determined by empiric relation:

Ie is the current of electrons in tube which depends on heating current of cathode; Z is atomic number of target atoms

Intensity of beam is the ratio of power and the surface of the window on tube

Increase of voltage enhances the beam intensity.

The spectrum is shifted to shorter wavelengths – the hardness of the beam is higher

I

ZIUkP ea2

dII

spec

tral

rad

ian

cy

API

Influence of heating current and window filter on X-ray spectrum

low heating current

short wavelength cutoff and wavelength of highest radiancy are not influenced by heating current; only the intensity of beam depends on the current

high heating current

with filter

without filter

short wavelength cutoff is the same but wavelength of highest radiancy is shifted to shorter wavelengths. The intensity is lower but the hardness is higher.

I

I

Interaction of X-ray photons with atoms in tissue

the type of interaction depends on photon energy and atomic composition of tissue

photoelectric effectpredominates for the photons with energies lower than 80 keV; it is more probable for heavy atoms intissue which are present in bones (Ca)

Compton scatteringpredominates for the photons with higher energies; it is more probable for lighter atoms in soft tissue (O, N, C, H)

Law of attenuation for X-rays

Intensity decrease of monochromatic beam along its path through the tissue:

() is linear absorption coefficient which depends on tissue and wavelength of radiation

in medical diagnostics is commonly used mass absorption coefficient:

which depends on probabilities for photoelectric effect () i Compton

scattering ()

xe)(I)x(I 0

m

Half value layer

21x

21x

m/

lnx

2

21

the thickness of absorber which attenuates half of the photons

parametparameteer r for determination offor determination of

hardness of hardness of polpolychychromatromatic beamic beam

- - higher higher xx1/21/2 means harder beammeans harder beam

20

21 21

IIxx

/x/

xx

meII 0

x

21x

Plot for attenuation of real polychromatic beam

The analytical expression for the attenuation of polychromatic beam does not exist

The average energy of polychromatic beam is chosen as the energy of corresponding monochromatic beam with equal half value layer.

I

x

rapid decay due to absorptionof low energy photons

along the path through tissue the beam becomes harder due to predominate influence of higher energy photons – penetration power is increased

X-ray diagnostics In classical radiography we get the image obtained from transmitted beams; it

displays the shadows of tissue structures - the image is 2D projection of 3D object; therefore the shadows of bones overlay the shadows of soft tissue

Intensity of transmitted beam depends on absorption coefficient

The images of bones are obtained with high contrast if we apply lower tube voltage - low energy photons; then, the absorption coefficient for photoelectric effect in bones is increased and the absorption in the soft tissue is very low

the good images of soft tissue are obtained if we apply higher tube voltage - high energy photons; in such conditions the absorption coefficient for Compton effect is increased; we can see shadows of soft tissues but also the more pale shadows of the bones due to lower absorption of high energy photons; however, the overall contrast is worse than for low voltage

Computer Assisted Tomography (CT,CAT)

Hounsfield and Cormack – 1972. it is the combination of special way

of recording, accumulation of data and mathematical processing for the image reconstruction

The basic concept of the method The narrow beam propagates

through the layer of the body, its thickness is determined by the beam width.

At the other end is a detector which measures the intensity of transmitted beam.

The layer (perpendicular to the long axis of the body) is divided in volume elements - voxel (10 mm3).The size of one voxel is determined by the cross section of the beam ; the voxel size determines the resolution of the image

To each voxel is attributed its absorption cofficient. The beam propagates throuh the row of voxels and the intensity of transmitted beam is:is: x...t

neII 3210

pixel pixel – – element of 2D image of the layer; one pixel contains the information from one voxel - the number of pixels depends on the number of voxels

By subsequent parallel translation of tube and detector across the layer, the whole layer is recorded. The process is repeated after each rotation of the pair tube-detector for a small angle. In that way we get enough data for the processing of the complex mathematical algorithm for the distribution of attenuation coefficients in the layer. The calculated data are transformed into pixels and displayed in grey scale.

The number of counted photons determines the precision of absorption measurement along one row of voxels.

The contrast depends on absorption differences in particluate tissues.

The described procedure demanded time-consuming recording, so the technological improvement was based on building the devices for much faster recording which became possible after construction of light, small and cheap detectors.

Novel equipments for CT

They enable instantaneous recording for large number of directions in the layer simuntaniously. In that way the interval of patient irradiation is significantly shortened. Application of fan shaped beams and automatic rotation of the tube, makes the recording time even shorter.

CT equipment which is in use nowadays is constructed with immobile detectors arranged in a circle perpendicular to the long axis of the body, while the tube is rotating in that plane. By automatic shift of bed, the new layers are recorded.

Spiral CT

This novel method enables additional shortening of recording time, due to very fast computers. The data are taken and processed while the body is continuously shifted.

The image of the heart can be obtained in 0.1 s.

The computers enable reconstruction of 3D image