MedPhys
Physik für FCRC
Physik für FCRC
piezoelectric effect
- quartz produces electric when compressed or expand
- lose the prperties when heated
acustic impedance
- Impedance = density x speed of sound
- Z = p x c
- independant to frequency f
- large Z = more reflexion
- small Z = less reflexion
Acustic impredance
- Total reflexion at air, because Zair is 0, and the difference of Z is large.
transducer
- linear
- curvilinear
- phased
- annular
ultrasound: A-mode
- Amplitude Mode
B-Mode
- Brightness mode
ultrasound quality
spatial resolution
- high frequency = high axial resolution (along the axis of the beam)
- focus the beam, more scan lines = high lateral resolution
ultrasound: doppler effect
- louder, when the object is moving towards
- less louder, when object is moving away
power doppler
- amplitude is the amount of the red blood calls, not the velocity
- quantity of the blood flow
Doppler with colors
- red = flow towards the transducer
- blue = flow away the transducer
- turbulence = green / yellow
harmonic frequency
- transducer frequency is the the 1st
- it changes through the tissue, so the harmonic frequencies are produced
ultrasound m-mode
- motion mode
ultrasound artefarct: speckle
- many small structures
- textured appearance
ultrasound artefarct: reverberation
- strong the reflector near the surface
- series of delayed echoes
ultrasound artefarct: double reflection
- diaphragma as a mirror
ultrasound artefarct:shadowing
- bowel gas, stone, bone
ultrasound artefarct: enhancement
- cyst, bladder
ultrasound artefarct:refraction
different velocity through the tissus
at the border
ultrasound artefarct: aliasing
- caused by under sampling
- colour change, and wrapped around
ultrasound contrast agent
- air-filledd (vascular, thrombosis)
- perfluorocarbon (liver, spleen, metastase)
- gold (combined with antibodies)
Nucleus
protons and neutrons
Atomic mass (A)
protons + neutrons
Atomic number
number of protons
nuclides
same protons = same element
Isotope
same chemical properties, but different physical properties
electron orbit
K: 2
outer orbit shell
valence
determines the chemical, electrical and thermal properties of the element
ion
electrons <> protons
electron excitation
gains energy and moves to higher energy outer shell
Binding energy E and atomic number Z of
Iodine?
Barium?
Tungsten?
Lead?
energy to remove the electron from the an atom
high, as Z=atomic number is high
high for electron in the inner shell, closer to the nucleus
Binding energy = E
energy to remove an electron from the shel
E increase = atomic numbre Z increase
E increase = K shell, near to the nucleus
Electromagnetic radiation (EMR)
speed of light in vacuum
3 x 108 m/s
magnetic and electric components
wave and particle component
X-Ray
EMR from the electron
Gamma ray
from the nucleus
EMR
the wave model
f = 1 Hz = 1/s
in different media f constant----- speed changes
EMR
the particle model
photon
Wave equation
v = f x λ
velocity = frequency x wavelength
Planck-Einstein equation
E = h x f
Photon energy = Planck constant x frequency
E=h x c /λ = 1,24 /λ
photon fluence
number of photons in a slice of the beam
energy fluence
energy of all the photons in a fluence
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- 1 / 172
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Cartes-fiches
image intensifier: input screen
- outer surface: phosphor
- inner surface: photocathode
image intensifier: electron focus
- rings are positive
- voltage gradient
flux gain
gain = output brightness : input brightness = 50
minification gain
intensification from big input to small output
flat panel detector
- better contrast resolution
- better spatial resolution
- higher dynamic range
- magnification does not improve the spatial resolution, because the pixels are fixed.
ion chamber
- photon to electron to charge
thermoluminescent dosimeter
- lithium fluoride
- reading = heat to 250°C
Dose area product
Gy/cm2
factors affecting dose
mAs
- double the mAs = double the dose
kV
- increasing the kV = decrease the dose
- increase kV = harden the beam = more penetrating = less scatter = less dose
- entrance surface dose ≈ kV2
focus-image distance (FID)
- increase FID = decrease the ESD, trotz increasing mAs to maintain the picture quality
- because of larger surface area (1)
protection
wall
- 1mm of lead = 120 mm of solid brick
- 1-2 mm is normal
- door and glas with lead
beam
- collimation
- undercoach reduces the dose
scatter
- Compton effect
- main source of radiation to the staff
protecting clothes
- lead aprons reduce transmission: 0.25 mm = 5%, 0.35 mm = 3%, 0.5 mm = 1.5%
- thyroid collars 0,5 mm lead
protection of the patient
- collimation
- magnification the picture in DL reduce the dose
- removal of the grid
Alpha decay
- heavy nuclei
- stopped by few centimeters of air
- Z decreases by 2 and A decreases by 4 (Z=Proton, A = Proton + Neutron)
beta decay
- nuclide with more neutrons
- neutron decay
- neutron = proton + electron + electron anineutrino
- Z increases by 1 = + 1 proton
- A stays the same = + 1 proton - 1 neutron
- different element
positron decay
- nuclides with neutron deficit
- proton = neutron + positron
- Z decreases by 1
- A stays the same = - 1 proton + neutron
- different element
K-electron capture
- nucletide with neutron deficit
- proton + electron = neutron
- emits characteristic ray, eg. gamma ray, when outer electron fills the K-shell
Activity
number of atoms decay in 1 sec
1 Bequerel (Bq) = 1 atom / sec
physical half life
- time of decay to half of the original value
- unaffected by nothing
effective half life
effective half life = 1 / tbiol + 1 / tphys
ideal radionuclide
- short physical half life
- emits only gamma rays for the image
- energy of 150 keV
- single energy
technetium-99m
- gamma rays 140 keV
- 6 hours half life
- parent product is molybdenum 99Mo
- 1 week of generation
cyclotron
- acceleration of ions in a magentic field
- ions steered to a target
- 18O bombing with protons = 18F
gamma camera
- colimator = lead disc, absorps oblique gamma rays
- crystal = thallium-sodium iodide, hight Z, fragile, hygroscopic
- 1 gamma ray = 5000 light photon
- photomultiplier: 1 photoelectron = 1 million electron
pulse heights spectrum: gamma camera
- scattter compton tail on the left
- just photopeak are useful
pulse height analyser
- just pulse with +- 10% of the photopeak = count
collimator of the gamma camera
high resolution
- many but smaller holes
high sensitivity
- fewer but bigger holes
spect ct
- rotation for 3° with a pause of 30 sec = 120 time for 360°
- signal : gamma rays
- pixel matrix 64 x 64 oder 128 x 128
- low spatial resolution
- fusion of spect and CT
positron emission tomography PET
- signal : positron
- 18F =Fluorine-18
- travel 2 mm, before annihilate with an electron
- positron + electron = 2 gamma electrons with 511 keV in opposite direction
Fluorine-18
- in cyclotron
- half life is 2 h
- FDG
- decays to 18O (oxygen)
PET scanner scintillator
- bismuth germanate
- short scintillation decay time
- high energy resolution
coincidence
- true
- scatter
- random
PET quality
contrast
-better than x-ray
spatial resolution
-better in the center
-smaller detector, better the resolution
PET patient dose
between 1-5 mSv
3 princples of ICRP
- Justification: radiation is risky and just for serious illness
- optimisation: ALARA
- dose limitation: just for staff and the public, not for the patient
annual dose limits
only for staff and public
not for patients
IRR 99 (ionising radiations regulations 1999)
-responsibility on the employer
-employer can delegate tasks to employees, but not the responsibilties
- design protection area and local rules
- dose limitation,
- design protection supervisor: almost a medical physisist, acrredited to HSE for 5 years
- radiation incidents
protection work areas
controlled area:
- > 3/10th of any dose limit
- >7,5 microSv/hr external dose
supervised area
- > 1 mSv /yr effective dose
- > 1/10th of any dose limit
classified person
- >3/10th of any dose limits
- must >18 years
- medically fit
dose monitoring
- approved by ADS of HSE
- records sent annually to HSE
- records kept for 50 years
