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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Flashcards
Absorbed dose
energy deposited per unit mass
Gray = J /kg
KERMA
Kinetic Energy released per unit mass
Gray
Linear energy transfer (LET)
Energy transfer to a tissue per unit length
high LET = more damage to tissue
Equivalent dose
Equivalent dose = absorbed dose *wr (radiation weighting factor)
Sievert = J/kg
effective dose
radiosensitivity of different tissue
effective dose = equivalent dose x wT (tissue weighting factor)
Sievert = J/kg
deterministic effect
-thershold dose
-repair mechanism
stochastic effect
-no thershold
-risk increases linearly with dose
-no rise of severity with dose
-yes or no cancer
deterministic effect dose
Skin erythema = 2-5
Cataract = 5
dose yearly
Radon = ca. 50%
Foetal dose-termination of pregnancy
1. < 1000 mGy : no termination
2. 100-500 mGy : optional
3. > 500 mGy : recommend
X-Ray tube
-1% X-ray
-99% heat
-cathode (-) : focusing cup
-anode (+)
focusing cup
high melting point
improves resolution
thermionic emission
tungsten filament
X-tube V and A
tube voltage = kV = accelerating voltage
tube current = mA
good anode
-high atomic number
-good isolation
-high melting point of target material
-low vapour point
rotating anode
increase the area stuck by electrons
stands more heat
anode heel effect
-reduction in beam intensity towards the side of the anode
-reduced by increasing the focus to image distance
contrast resolution
contrast increase
- structure thickness increase
- kV deceases
spatial resolution
line pairs per mm
quantum noise
- reduces contrast resolution.
- signal-to-noise
SNR = N/squr {N}
ratio of scatter
scattered : primary radiation
4 : 1 =thorax
9 : 1 = pelvis
-Pt thickness
-field size
how to reduce scatter?
-decrease field size
-decrease pt thickness
-decrease kV
-use a grid
-increase object-image distance
antiscatter grid
-improve contrast
-increase pt dose
Grid construction
-lead strip: 0.05 mm wide
-line density : 20-60 strips / cm
-interspace material = carbon fibre = 0.2 mm
Grid ratio = h/w = 8:1
high ratio = high contrast
improve the contrast by the factor of 2-4.
Grid ratio
grid ratio = depth / withd = typical 8 : 1
larger ratio = better contrast
contrast improve factor 2-4
unfocused grid
rays are increasingly attenuated
reduced the effect = increase the FID or decrease the grid ratio
focused grid
- specified distance to the anode
- specified angle of the grid
- tube must be centered
magnification
-increase b
-increase a + b
-decrease a
blurring in magnification
-because of the penumbra
decrease by
-decrease the focal size
-decrease b (object-image-distance)
-increase a+b (focus-image-distance)
CR plate
- electrons trap in phosphor.
- electrons stimulated by red laser light and emitte blue green light.
- photomultiplier
Computed radiography
- lower spatial resolution than film radiography
- Thicker resolution reduces resolution
Digital radiography
- Thin-film-transistor
- transistor stores electric charges
- direct and indirect
- indirect: x-ray to light in carsium iodide, then to electric charge.
- direct: x-ray to electric charge in selenium
DR
indirect versus direct
indirect:
- more x-ray absoprtion
- lower dose
- less noise
direct:
- better spatial resolution
- faster, no cassette
mammography
- lower kV
- lower Z (atomic number)
- 1.3 - 3 mGy per exposure
molybdenum-42
k-edge 20 keV
for small breast
rhodium
k-edge 23 keV
for big breast
mammography
anode heel effect
anode heel effect
- mamma is conical, thinner at the nipple
- the tube is angled, so highest beam intensity is at the chest wall, the lowest at the nipple
mammography: compression
- reduce dose
- reduce unsharpness
- reduce motion artefart
image intensifier
-input screen
-focus
-ouput screen
