X-rays
By Kiwi 🥝
• explain that X-rays are
produced by electron
bombardment of a metal
target and calculate the
minimum wavelength of X-rays
produced from the
accelerating p.d.
X- Ray tube
The cathode is heated up by an electric current
The electrons are then emitted because of thermionic emission
The electrons are accelerated and when they hit a metal target, X- rays can
be produced in two ways
When they decelerate, they emit radiation, and this radiation is within XRay range
When an electron loses its kinetic energy, an X-ray photon with maximum
photon energy is produced, causing X-rays to have minimum wavelength.
There is a continuous range of possible wavelengths above this, as
electrons have a range of decelerations, causing a continuous range of
possible photon energies
If the electrons hitting the target have enough energy, they eject
electrons from lower energy levels of atoms in the target
The excited electrons in higher energy levels drop to fill the vacancy
causing emission of X-Ray with a specific energy
The wavelength emitted depend on the spacing between the energy levels of
the target
A line spectrum characteristic of the target is superimposed on the
continuous bremsstrahlung spectrum
How to calculate
minimum wavelength of
X-Rays produced from
the accelerating P.D.
Minimum wavelength = c/f
= hc/E = hc/eV
c= 3*10^8
f = frequency of proton
h = 6.63*10^-34(Planck’s
constant )
E = photon energy
eV = electronvolt
X-rays in imaging
internal body
structures, including
an understanding of the
term contrast in X-ray
imaging
This is done to reduce exposure time in order to prevent cancer
X-ray shadows directly convert X-Ray photons to an electrical
signal
X-Ray image contrast is the difference in darkening between images
of different parts of the body
The more the contrast, the easier it is to distinguish between
different features
It depends on the relative number of X-Ray photons transmitted
through dissimilar parts of the body and the response of the
detector
We can use a contrast medium to adjust the contrast. It absorbs the
X- ray more strongly than the tissue , barium sulfate as the areas
absorbed by it will appear white thus easier to see
recall and use I = I0e–μx for
the attenuation of X-rays in
matter
Bones absorb X-ray radiation
This is why they appear white on the X-ray photograph
When the collimated beam of X-rays passes through the patient’s body, some of the
photons are absorbed and scattered
The attenuation of X-rays can be calculated using the equation:
Where:
I0 = the intensity of the incident beam (W m-2)
I = the intensity of the emergent beam (W m-2)
μ = the linear absorption coefficient (m-1)
x = distance travelled through the material (m)
The attenuation coefficient also depends on the energy of the X-ray photons
The intensity of the X-ray decays exponentially
The thickness of the material that will reduce the X-ray beam or a particular frequency to
half its original value is known as the half thickness
understand that computed
tomography (CT) scanning
produces a 3D image of an
internal structureby first
combining multiple X-ray
images taken in the same
section from different angles
to obtain a 2Dimage of the
section, then repeating this
process along an axis and
combining 2D images of
multiplesections
patient, and it emits a narrow,
monochromatic
X-ray beam which passes through the body
at different orientations.
There is an array of detectors arranged
outside the path of the X-ray tube, which
detect the intensity of the X-ray beam
after it passes through the body. The
detectors only register the intensity of
beam sources placed directly opposite to
them.
The recorded intensities are sent to a
computer, where they're processed and an
image of the cross-section can be formed.