medical equipment lec 1 x-rays

1. Medical Equipment III
X-ray Imaging
Shereen M. El-Metwally
Associate Professor,
Systems and Biomedical Engineering Department,
Faculty of Engineering - Cairo University
sh.elmetwally@eng1.cu.edu.eg

2. Course Content
 Imaging
 X-Ray Imaging
 Computer Tomography (CT)
 Nuclear Medicine
 Positron Emission Tomography (PET)
 Single Positron Emission Computer Tomography
(SPECT)
 Radiotherapy
 X-ray
 LINear ACcelerator (LINAC)
 Cobalt 2

3. Student Evaluation and
grading
 30% Class work
 Midterm
 Homeworks
 Attendance
 70% Term Exam
3

4. Reference Books:
 Russell K. Hobbie, Bradley J. Roth,
"Intermediate Physics and Biology (Chapter
14, 15), Springer, 2007."
 N. Smith et. Al., “Introduction to Medical
Imaging Physics, Engineering and Clinical
Applications”, 2011.
 P. Mayles et. Al., “Handbook of
Radiotherapy Physics: Theory and
Practice”, 2007.

5. Introduction to Atoms and
Light
5

6.  Light travels in vacuum with a velocity c = 3
×108 m/s
 When light travels through matter, its speed is
less than this and is given by
n is index of refraction of substance
 depends on both the composition of substance and
color of light
n
c
cn 
Nature of light
Light as an electromagnetic wave:

7.  A traveling wave of light can be described by:
f(x − cnt)
represents a disturbance traveling along the x-axis in the
positive direction
 As a sinusoidal wave, the period T, frequency v , and
wavelength λ, are related by
 As light moves from one medium to another,
wavelength changes as the speed changes, while
frequency remains the same.
  nc
T
,
1
Nature of light
Light as an electromagnetic wave:

8. Nature of light

9. Nature of light
Particle concept
 Each particle of light or “photon” has an energy E
given by:
where h is the Planck’s constant
h= 6.63 x 10-34 Js = 4.14x10-15 eVs

10. Atomic Energy Levels and Atomic
Spectra Energy levels:
Isolated atoms have specific discrete internal energies.
The internal energy of the atom is the sum of the energies
of each electron.
 Photon emission or absorption:
Let the energy of the ith state be labeled Ei.
If Ei is greater than the lowest possible energy,
then the atom can lose energy by emitting a photon of energy (Ei
−Ef ) and exist in a lower energy
state Ef
 At the lowest possible internal energy: in this state no
further energy loss can take place.
ph i fE E E 

11. Atomic Energy Levels and Atomic
Spectra
 Ionization energy is the smallest amount of energy
required to remove an electron from the atom when
the atom is in its ground state.
Radiatio
n
Ionizing
Non-
Ionizing
Direct
Indirec
t
Microwaves
Radiofrequen
cy
Ultrasound
Infrared

12. Atomic Energy Levels and Atomic
Spectra
 Energy levels are defined as

13. Photon Interaction with
matter
 Photons in a vacuum travel in a straight line.
 When they travel through matter
 Passing through (Transmission)
 Scattering
 Absorption
 Loss of photons due to scattering and absorption is
called “attenuation”.

14. Photon Interaction with matter
 Beer-Lambert law:
The number of photons transmitted decreases with
distance:
( )
s
a
( )
where : total linear attenuation coefficient
:linear scattering coefficient
:linear absorption coefficient
z:distance
s a zz
o oN z N e N e  



 
 

15. Imaging Tool: Full Definition
 Effect produced
 Interaction with the body
 Effect to be detected
 Image construction

16. X-Ray Imaging
16

17. X-ray Imaging
 The tube – X-rays are produced
 The body – X-rays interact with the body
 The image – X-rays interact with film, detectors
 Film processing, Signal analysis
X-ray Tube
Body
Detectors
Film
processing,
Signal
analysis

18. X-rays
 Made of photons
 Travel at speed of light
 Travel in a straight line
 Has no mass nor charge (cannot be focused by
magnets)
 X-ray beam has a mix of energies
 Diagnostic X-ray range 20-150 keV

19. Diagnostic and Therapeutic X-
Ray
 Diagnostic radiology
 Uses low energy X-rays for imaging
 No tissue damage.
 Radiotherapy
 Uses high energy X-rays to treat tumor.
 Ionizes the water in tumor cells and induces the
formation of free radicals which may cause damage of
genetic material (DNA).
 Normal cells are also affected adversely by radiation but
have the capability to repair.

20. The set-up for planar
radiography
 The basis of X-ray imaging is the differential absorption of X-rays
by various tissues
20

21. X-ray Imaging
 The tube – X-rays are produced
 The body – X-rays interact with the body
 The image – X-rays interact with film, detectors
 Film processing, Signal analysis
X-ray Tube
Body
Detectors
Film
processing,
Signal
analysis

22. The X-ray tube

23. X-ray tube

24. X-ray production
 A current of a few amperes heats the filament. Electrons
are liberated at a rate that increases with the filament
current. This is called “thermionic emission”.
 The released electrons are accelerated across a high
voltage onto a target.
 X-rays are produced as the electrons interact in the
target.
 Vacuum is maintained inside the glass envelope of the
x-ray tube to prevent the electrons from interacting with
gas molecules.
24

25. X-ray production
 Push the “rotor” or
“prep” button
 Charges the filament –
causes thermionic emission
(e- cloud)
 Begins rotating the anode.
 Push the “exposure” or
“x-ray” button
 Accelerating voltage is
applied, so e-’s move
towards anode target to
produce x-rays

26. Focal spot size and Coverage
calculation The “focal spot” is the volume of target within which electrons
are absorbed and x-rays are produced.
 Most diagnostic x-ray tubes use a target angle between 6 and 17
degrees wrt the e- beam direction. The smaller the angle, the
smaller the effective focal spot size (f):
where θ is the target angle and
F is the width of the electron beam.
 Coverage:

27. Focal spot size
 A finite effective spot size
as well as the tube–
patient (S0) and tube–
detector (S1) distances
determine the spatial
resolution of the image.
 To improve the image
spatial resolution, S0 should
be as large, and the value of
f as small, as possible, with
the patient placed directly
on top of the detector.
where P is size of the
‘penumbra’ region, or
geometric unsharpness. 27

28. Dual-focus tubes
 Many X-ray tubes contain two
cathode filaments of different
lengths, called “dual-focus”
tubes.
 A “small” or “fine” filament
produces a narrower e-beam, so
achieves a small focal spot for
radiographs of greater detail.
 A “larger” or “coarse” filament
produces a wider e-beam. Used
for x-ray exposures of high
intensity and short duration in
order to limit the blurring effectsCathode assembly of a dual-focus x-ray tu

29. Energy Spectrum of X-ray
Many of the very low energy X-rays
are absorbed by the housing of the X-
ray tube itself.
The distribution of photon energies
produced by a typical x-ray tube is
referred to as an emission
spectrum. It is composed of:
 A wide spectrum of X-ray
energies, termed ‘general
radiation’ or ‘bremsstrahlung’
(braking radiation in German).
 Sharp peaks whose energy is
characteristic of the particular
target material, hence the name
‘characteristic radiation’.

30. Anode Heel Effect
 The X-ray beam has a higher
intensity at the ‘cathode-end’ than at
the ‘anode-end’, a phenomenon
known as the “Anode Heel” effect.
 This effect is due to X-rays on the
anode side have to travel further
through the anode itself before
leaving the tube, and are therefore
more highly attenuated.

31. Exposure Factors:
 Accelerating voltage (kVp)
 Tube current (mA)
 Exposure time (s)
 mAs – product of mA and s
Exposure factors are set by
radiographer