Production and use of X-rays
Section: Medical Physics | Syllabus: Cambridge AS Level Physics 9702
Production and Use of X-rays X-rays are high-energy electromagnetic radiation with wavelengths typically between 10^-8 m and 10^-13 m. They are widely used in medical imaging due to their ability to penetrate soft tissue while being absorbed by denser materials like bone.
Production of X-rays X-rays are produced in an X-ray tube by the bombardment of a metal target with high-speed electrons. How X-rays are Produced A heated cathode emits electrons by thermionic emission Electrons are accelerated towards a metal anode (target) by a high potential difference (typically 30-150 kV) When electrons strike the metal target (usually tungsten), X-rays are produced X-rays are produced by two mechanisms: Bremsstrahlung (braking radiation): Electrons decelerate rapidly when they interact with target atoms, emitting X-ray photons with a continuous range of energies Characteristic X-rays: Electrons knock out inner-shell electrons from target atoms; when outer electrons fall to fill the vacancy, X-rays of specific wavelengths are emitted Figure: X-ray Tube Cross-section of an X-ray tube showing: Cathode: Heated filament that emits electrons by thermionic emission Anode (target): Metal target (usually tungsten) attached to rotating anode Evacuated glass tube: Contains cathode and anode in vacuum High voltage supply: Accelerates electrons from cathode to anode (30-150 kV) Lead shielding: Absorbs stray X-rays except through exit window Oil reservoir: Cooling system (less than 1% of electron energy becomes X-rays; rest becomes heat) Figure: X-ray Spectrum Graph of relative intensity vs wavelength showing: Continuous spectrum (Bremsstrahlung): Smooth curve starting at minimum wavelength λ_min, rising to a peak, then decreasing Characteristic peaks: Sharp spikes superimposed on continuous spectrum at specific wavelengths (depend on target material) Minimum wavelength: Cut-off at λ_min = hc/eV (when electron loses all KE in one collision) Higher accelerating voltage shifts the curve left (shorter λ_min) and increases intensity Minimum Wavelength of X-rays The maximum energy of an X-ray photon equals the kinetic energy gained by an electron accelerated through the p.d.
Maximum photon energy: E_max = eV Since E = hf = hc/λ, the minimum wavelength is: λ_min = hc/eV where e = electron charge, V = accelerating p.d., h = Planck constant, c = speed of light Effect of Changing Tube Parameters Increasing accelerating p.d.: decreases minimum wavelength, increases maximum photon energy, shifts spectrum to higher energies Increasing tube current: increases number of X-ray photons (intensity) but does not change minimum wavelength X-ray Imaging and Contrast Contrast The difference in visibility (degree of darkening) between two areas in an X-ray image.
Good contrast allows different structures to be distinguished clearly. Contrast depends on the relative absorption of X-rays by different tissues. Bone absorbs X-rays strongly (appears white), air absorbs very little (appears black), and soft tissues appear as shades of grey.
Contrast Enhancement Soft tissues often have similar absorption properties, making them difficult to distinguish. A contrast medium can be used to improve visibility: Barium meal: Barium sulfate suspension swallowed to image the digestive tract Iodine compounds: Injected intravenously to image blood vessels These substances have high linear attenuation coefficients, making the structures they fill appear clearly on the X-ray image.
Attenuation of X-rays As X-rays pass through matter, their intensity decreases exponentially due to absorption and scattering. I = I_0 e^-μ x where I = transmitted intensity, I_0 = incident intensity, μ = linear attenuation coefficient (cm^-1), x = thickness of material The linear attenuation coefficient μ depends on: The density of the material (denser materials absorb more) The atomic number of the material (higher Z absorbs more) The energy of the X-ray photons (lower energy photons are absorbed more easily) Half-Value Thickness (x_1/2) The thickness of material required to reduce the X-ray intensity to half its original value.
x_1/2 = 2/μ = 0.693/μ Computed Tomography (CT) Scanning Conventional X-ray images are 2D projections of 3D structures, providing no depth information. Computed Tomography (CT) overcomes this limitation by producing 3D images.
CT Scanning A technique that produces a 3D image of internal structures by combining multiple X-ray images taken from different angles. How CT Scanning Works An X-ray tube rotates around the patient, taking images from many different angles in the same plane (section) Detectors measure the transmitted X-ray intensity at each angle A computer combines these measurements to create a 2D cross-sectional image (slice) of that section The patient moves through the scanner and the process is repeated for multiple adjacent sections The multiple 2D slices are combined to form a 3D image of the internal structure Figure: CT Scanner Setup Cross-section o…
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