Medical Imaging
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104 Terms
What produces X-rays?
Rapid deceleration or acceleration of charged particles
How is the kinetic energy of electrons transformed in X-ray production?
Into high-frequency electromagnetic radiation
What distinguishes X-rays from gamma rays?
Their origin
How are gamma rays produced?
Radioactive decay or particle interactions with mass defect
How are X-rays produced?
Bremsstrahlung from accelerated charged particles
What is Bremsstrahlung radiation?
Radiation emitted due to acceleration or deceleration of charged particles
Why are medical X-rays called soft X-rays?
They have lower energies than typical gamma rays
What is the function of an X-ray tube?
To accelerate electrons and rapidly decelerate them to produce X-rays
What material is commonly used as the anode in X-ray tubes?
Tungsten
How are electrons produced in an X-ray tube?
By thermionic emission
What is thermionic emission?
The release of electrons from a heated metal
Why is the X-ray tube evacuated?
To prevent electrons colliding with air molecules
What accelerates electrons in an X-ray tube?
A high potential difference
What is the maximum kinetic energy gained by electrons in an X-ray tube?
eV
What fraction of electron energy is converted into X-rays?
About 1%
What happens to the remaining electron energy?
It is converted into thermal energy
Why is the anode rotated or cooled?
To prevent overheating
In what directions are X-rays emitted from the anode?
In all directions
Why is collimation of X-rays necessary?
To direct the beam and minimise patient exposure
What is the window in an X-ray tube?
A thin section allowing X-rays to exit
What is a collimator?
A set of parallel metal tubes that absorb non-parallel rays
What type of spectrum does Bremsstrahlung produce?
A continuous spectrum
Why does Bremsstrahlung produce a continuous spectrum?
Electrons lose varying amounts of energy
What are characteristic X-rays?
Sharp spectral lines from electron transitions in anode atoms
How are characteristic X-rays produced?
Incident electrons eject inner-shell electrons
Why do characteristic X-rays have fixed wavelengths?
They depend on atomic energy levels
Why do characteristic peaks appear in X-ray spectra?
Increased photon emission at specific energies
Why are X-rays ionising radiation?
They have enough energy to remove electrons from atoms
How can X-rays damage living tissue?
By ionising DNA and cells
How is X-ray damage minimised in medicine?
Low intensity and short exposure times
How are X-rays used in cancer treatment?
To destroy cancer cells
What is X-ray attenuation?
The reduction in intensity as X-rays pass through matter
Why do different tissues attenuate X-rays differently?
Different absorption coefficients
Why does bone appear white on X-ray images?
It absorbs more X-rays than soft tissue
What is X-ray beam intensity?
Power per unit area
How does intensity change in a collimated X-ray beam?
It decreases exponentially
State the X-ray attenuation equation
I = I₀e⁻ᵘˣ
What does μ represent in attenuation?
The attenuation coefficient
What are the units of the attenuation coefficient?
m⁻¹
What does a larger attenuation coefficient indicate?
Greater absorption
What is simple scattering?
Elastic scattering of low-energy X-rays
At what energies does simple scattering occur?
1–20 keV
What is the photoelectric effect in X-ray absorption?
Photon is absorbed and an electron is ejected
At what energies does the photoelectric effect dominate?
Below about 100 keV
What is Compton scattering?
Inelastic scattering between photon and electron
What happens to photon energy in Compton scattering?
It decreases
What is pair production?
Photon produces an electron–positron pair
What is the minimum energy for pair production?
1.02 MeV
Why is pair production unimportant in medical X-rays?
X-ray energies are too low
What are contrast media?
High-attenuation materials used to improve image contrast
Why do contrast media have high attenuation?
High proton number
Give examples of contrast media
Iodine and barium sulphate
Why are contrast media effective in soft tissue?
Soft tissue has low proton number
How does attenuation coefficient depend on proton number?
It is proportional to Z³
Why is iodine used in blood imaging?
It highlights blood flow
Why is barium sulphate used in digestive imaging?
It outlines the intestines
What is a CAT scan?
A 3D imaging technique using X-rays
How does a CAT scan improve on conventional X-rays?
It distinguishes overlapping tissues
How are CAT images produced?
From many 2D slices reconstructed by computer
What shape beam is used in CAT scanning?
Fan-shaped X-ray beam
What are CAT scan detectors used for?
Measuring X-ray intensity after attenuation
What is a disadvantage of CAT scans?
Higher radiation dose
What are medical tracers?
Radioactive compounds that target specific organs
Why are gamma emitters used in tracers?
They are penetrating and weakly ionising
Why are short half-lives desirable in tracers?
To minimise radiation dose
Why are tracers often produced on-site?
They decay rapidly
What type of decay does fluorine-18 undergo?
Beta-plus decay
What is detected in PET scans from F-18 decay?
Gamma photons from annihilation
What is the half-life of fluorine-18?
About 110 minutes
What is technetium-99m used for?
Imaging major organs
What does metastable mean?
Long-lived excited nuclear state
What radiation does technetium-99m emit?
Gamma radiation
What is the half-life of technetium-99m?
About 6 hours
Why is Tc-99 formed after decay acceptable?
It has a very long half-life
What is a gamma camera used for?
Detecting gamma photons from tracers
Why is a collimator used in gamma cameras?
To ensure directional detection
What material is used for scintillators?
Sodium iodide
What happens in a scintillator?
Gamma photons produce visible photons
What is the role of a photomultiplier tube?
Amplify the electron signal
How is the emission position determined in a gamma camera?
From scintillator impact location
What is a PET scanner?
A ring of gamma detectors for 3D imaging
Why are two gamma photons detected in PET?
They are emitted in opposite directions
Why can PET locate annihilation sites accurately?
Photon speed and timing are known
What tracer is commonly used in PET?
Fluorodeoxyglucose
Why is PET useful in cancer detection?
Cancer has high glucose uptake
What is ultrasound?
Longitudinal sound waves above 20 kHz
What frequencies are used in medical ultrasound?
About 5 MHz
Why is ultrasound safe for imaging?
It is non-ionising
What device produces ultrasound?
A piezoelectric transducer
What is the piezoelectric effect?
Voltage causes crystal deformation and vice versa
Why is ultrasound pulsed?
To receive reflected waves
What is an A-scan ultrasound?
Single-point distance measurement
What is a B-scan ultrasound?
2D image from multiple A-scans
Why do smaller wavelengths improve image resolution?
They diffract around smaller features
What is acoustic impedance?
Product of density and sound speed
State the equation for acoustic impedance
Z = ρc
What happens when ultrasound meets a boundary?
Partial reflection and transmission
What determines reflection intensity?
Difference in acoustic impedance
Why is coupling gel used in ultrasound?
To reduce reflection at skin boundary
What is the Doppler effect?
Frequency change due to motion