45 Questions
What is the process by which X-rays are produced when charged particles are rapidly decelerated?
Bremsstrahlung or braking radiation
How do X-ray tubes produce X-rays?
By accelerating electrons in a high-voltage electric field and then rapidly decelerating them via collisions with a hard metal anode.
What is the purpose of the vacuum tube in an X-ray tube?
To prevent electrons from colliding with air molecules before they have acquired enough energy to emit X-rays.
What is the maximum kinetic energy gained by electrons in an X-ray tube?
Up to 200 keV
What is the process by which electrons are emitted from a heated source in an X-ray tube?
Thermionic emission
What type of X-rays are used in medical imaging?
Soft X-rays
What is the main difference between gamma rays and X-rays?
Their history
What is the purpose of the anode in an X-ray tube?
To rapidly decelerate the electrons
What is the source of electrons in an X-ray tube?
A heater or filament (cathode or negative electrode)
What is the formula used to evaluate the attenuation of an X-ray beam in a medium?
I = I0 e^(-μx)
What is the name of the process where X-rays are absorbed by electrons in a material, resulting in the emission of a scattered photon?
Photoelectric effect
What is the energy range of X-rays that undergo simple scattering?
1-20 keV
What is the name of the process where X-rays lose energy to electrons in a material, resulting in a scattered photon with a longer wavelength?
Compton effect
What is the minimum energy required for pair production to occur?
1.02 MeV
What is the name of the coefficient that describes how well a medium absorbs X-rays?
Attenuation or absorption coefficient (μ)
What is the result of the Compton effect on the wavelength of the scattered X-ray photon?
The wavelength of the scattered photon increases
What is the name of the process where X-rays reflect off layers of atoms or molecules in a material?
Simple scattering
What is the fate of the positron produced in pair production?
It collides with another electron and annihilates, producing photons
What is the principle behind calculating the exact location of the annihilation event in PET scanners?
The exact location of the annihilation event can be calculated based on the arrival times of the photons at the detectors and the known speed of the photons.
What is the role of fluorodeoxyglucose (F-18) in PET scanners?
Fluorodeoxyglucose (F-18) is used as a tracer to locate areas in the body with high rates of respiration, such as cancerous tumours or active parts of the brain.
What is the advantage of ultrasound over PET scanning?
Ultrasound is non-ionising, non-invasive, quick, and affordable.
What is the frequency range of ultrasound used in medical diagnosis?
The frequency range of ultrasound used in medical diagnosis is typically around 5MHz.
What is the principle behind using diffraction in ultrasound imaging?
Diffraction is used to identify apertures or features of a few millimetres in size.
What is the role of the transducer in an ultrasound device?
The transducer produces an electrical signal from the sound waves, which can then be analyzed by computer software to generate an image.
What is the main application of ultrasound in medical diagnosis?
Ultrasound is particularly useful for finding the boundary between two media.
What is the limitation of PET scanning?
PET scanning is expensive and requires tracers to be synthesized on-site.
What is the advantage of PET scanning over other imaging techniques?
PET scanning can accurately demonstrate organ function and observe the effects of various medications.
What percentage of the kinetic energy of the decelerating electrons is emitted as X-rays?
1%
What is the purpose of the collimator in an X-ray tube?
To further collimate the X-ray beam by absorbing non-parallel rays
What is the origin of the characteristic radiation lines in an X-ray spectrum?
Incident electrons knocking out bound low-energy level electrons in the anode atoms
What is the effect of X-rays on living cells?
Ionization of DNA and other tissues, causing damage to the organism and possible harmful mutations
Why is the anode in an X-ray tube either rotated or cooled?
To prevent overheating due to the thermal energy generated by the decelerating electrons
What is the shape of the intensity profile of the braking radiation in an X-ray spectrum?
Hump-shaped
What is the purpose of the window in an X-ray tube?
To allow X-rays to emerge only in one place outside the tube
Why are X-rays, gamma-rays, and UV rays considered ionizing radiation?
Because they have sufficient energy to ionize matter, causing electrons to be emitted from atoms
What is the result of higher energy electrons transitioning to an unoccupied shell in the anode atoms?
Emitted radiation with specific wavelengths
What is the main purpose of using an impedance matching gel between the transducer and the skin in ultrasound imaging?
To maximise the transmission of ultrasound into the patient and maximise the reflected intensities and level of detail.
What is the principle behind the Doppler Effect in ultrasound imaging?
The change in frequency of a wave when it is reflected or produced by a moving source.
What is the role of iron in the blood in Doppler imaging?
Iron reflects the ultrasound wave back to the transducer.
What is the formula used to calculate the observed frequency shift in Doppler imaging?
Δ𝑓𝑓 = $2𝑓𝑓𝑓𝑓 cos 𝜃𝜃 / 𝑐𝑐$
What is the typical frequency shift observed in Doppler imaging using a 5-15 MHz ultrasound?
Around 3 kHz
What is one of the applications of Doppler imaging in medical diagnosis?
Revealing blood clots, narrowing artery walls and calculating the volume of blood flow.
What is the purpose of the angle 𝜃𝜃 in the formula for Doppler imaging?
It is the angle between the probe and the direction of blood flow.
What is the significance of the speed of ultrasound in blood (𝑐𝑐) in Doppler imaging?
It is used to calculate the observed frequency shift.
What is the advantage of using Doppler imaging in medical diagnosis?
It is a non-invasive technique that provides valuable information about blood flow.
Study Notes
X-ray Production
- X-rays are produced when charged particles are rapidly decelerated or accelerated, transforming their kinetic energy into high-frequency photons of electromagnetic radiation.
- X-rays and gamma rays have overlapping frequency spectra, with the only distinction being their history.
- X-rays are produced by Bremsstrahlung or braking radiation, which is when radiation is given off by charged particles due to their acceleration.
X-ray Tubes
- X-ray tubes produce X-rays by accelerating electrons in a high-voltage electric field and then rapidly decelerating them via collisions with a hard metal anode.
- Electrons are emitted from a heated source (cathode) via thermionic emission into a vacuum tube.
- The vacuum tube prevents electrons from colliding with air molecules before acquiring enough energy to emit X-rays.
- The electrons gain a kinetic energy of up to 200 keV, and upon collision, they decelerate rapidly, emitting X-rays.
X-ray Spectra
- Braking radiation produces a broad range of X-ray wavelengths with a hump-shaped intensity profile.
- There are also sharp lines of characteristic radiation due to incident electrons knocking out bound low-energy level electrons in the anode atoms.
- Photons produced via this process have specific wavelengths, resulting in higher intensity at these energies.
Ionising Radiation
- X-rays have high energies that can ionise matter, causing electrons to be emitted from atoms and potentially leading to DNA damage and mutations.
Attenuation of X-rays
- The attenuation of an X-ray beam in a medium can be evaluated by the equation: 𝐼𝐼 = 𝐼𝐼0 𝑒𝑒 −𝜇𝜇𝜇𝜇
- 𝜇𝜇 is the attenuation or absorption coefficient, which depends on the material.
Absorption Mechanisms
- Simple Scattering: X-rays of energy 1-20 keV reflect off layers of atoms or molecules in the material.
- Photoelectric Effect: X-rays of energy less than 100 keV can be absorbed by electrons in the material, releasing a photoelectron.
- Compton Effect: X-rays of 0.5 to 5 MeV lose energy to electrons in the absorbing material, resulting in a scattered photon with lower energy.
- Pair Production: X-rays with energy greater than 1.02 MeV produce an electron-positron pair via the mass-energy relation.
Positron Emission Tomography (PET)
- PET is a non-invasive technique that uses a tracer (e.g., fluorodeoxyglucose) to locate areas with high rates of respiration, such as cancerous tumors.
- Annihilation events produce photons that are detected to calculate the location of the tracer.
Ultrasound
- Ultrasound is a longitudinal sound wave with a frequency greater than 20 kHz.
- Properties of the medium can be determined from measuring the waves after they have interacted with the medium.
- Ultrasound is a non-ionising and non-invasive technique that is quick and affordable.
- Doppler imaging measures the speed of blood flow by detecting the frequency shift of ultrasound waves reflected by moving blood.
Doppler Effect
- The Doppler Effect is the change in frequency of a wave when it is reflected or produced by a moving source.
- The observed frequency shift, Δ𝑓𝑓, is related to the speed of flow, 𝑣𝑣, and the angle between the probe and the direction of blood flow, 𝜃𝜃.
Explore the world of medical imaging with this OCR A Level Physics quiz on X-ray production and its applications. Learn about the production of X-rays and their role in medical diagnostics.
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