medical imaging Flashcards

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1
Q

X-rays are produced when

A

charged particles are rapidly decelerated (or accelerated)
and their kinetic energy is transformed into high frequency photons of electromagnetic
radiation.

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2
Q

Gamma rays are
produced via

A

radioactive decay or during particle collisions with a mass defect e.g.
electron-positron annihilation or nuclear fission of uranium-235

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3
Q

X-rays are produced by

A

Bremsstrahlung or braking radiation which
is when radiation is given off by charged particles due to their acceleration. X-rays used
in medical imaging are often referred to as soft X-rays as they have energies generally
lower than gamma rays.

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4
Q

X-ray tubes produce X-rays by

A

accelerating electrons in a high-voltage electric field then
rapidly decelerating them via collisions with a hard metal anode (positive electrode) e.g.
tungsten. Electrons are first emitted from a heater or filament (cathode or negative
electrode) into a vacuum tube via thermionic emission. The vacuum tube is needed to prevent the electrons from
colliding with air molecules before they have acquired enough energy to emit X-rays. external power supply produces a potential difference between the cathode and the
anode of up to 200kV. Therefore, the electrons gain a kinetic energy of up to 200keV
(see Nuclear and Particle Physics 6.4). Upon collision, the electrons decelerate rapidly
and some of their kinetic energy (~1%) is emitted as X-rays.

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5
Q

collimator

A

that further
collimate the beam by absorbing any rays that are not parallel to the axis of the tubes.

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6
Q

Braking radiation

A

broad range X-ray wavelengths with a hump-shaped
intensity profile as seen below. However, there are also a few sharp lines of
characteristic radiation that are not due to decelerating electrons. These lines are
instead caused by incident electrons knocking out bound low energy level electrons in
the anode atoms. Higher energy electrons will then transition down to the unoccupied
shell and their excess energy will be emitted as radiation

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7
Q

X-ray Attenuation Mechanisms

A

Different materials attenuate X-rays to a different extent so
tissues can be contrasted by measuring the intensity of the attenuated beam once it has
passed through the patient. For example, bone attenuates X-rays to a greater extent
than flesh or other soft tissues. Therefore, when a limb is exposed to an X-ray beam the
X-rays that collide with bone are more likely to be absorbed and the beam shows more
attenuation directly behind the bone. If a photographic film is held behind the patient’s
limb then it will be blackened less if it is in the direct path of X-rays that passed through
bone. This is clearly seen as a white outline of the patient’s skeleton. Nowadays, digital
detectors are used as the images are easier to process, store and transfer.

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8
Q

The intensity of a collimated beam of X-rays (collimated so there is no intensity
decrease due to the spreading out of the beam) decreases exponentially.

A

𝐼= 𝐼 𝑒^−𝜇x
where 𝐼𝐼0 is the initial intensity before entering the medium, 𝐼𝐼 is the attenuated intensity
after passing through a thickness 𝑥𝑥 (m) of the medium and 𝜇𝜇 (m-1) is a property of the
material known as the attenuation or absorption coefficient

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9
Q

Simple Scattering:

A

X-rays of energy 1-20 keV will reflect off layers of atoms or
molecules in the material as they do not have enough energy to undergo more
complex processes.

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10
Q

Photoelectric Effect

A

X-rays of energy less than 100 keV can be absorbed by
electrons in the material as they have the same energy as the ionisation energies
of the atoms. When an X-ray is absorbed by an atom, a photoelectron is released

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11
Q

Compton Effect

A

X-rays of 0.5 to 5 MeV lose only a fraction of their energy to
electrons in the absorbing materials. This is due to an inelastic interaction
between the photon and the electron. The scattered X-ray photon will have less
energy than before, and so its wavelength will be greater. The Compton electron
will be scattered in a different direction as momentum must be conserved.

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12
Q

Pair Production

A

When X-ray energy is greater than 1.02 MeV passes through the
electric field of an atom it will spontaneously produce an electron-positron pair
via the mass-energy relation. The positron will then go on to collide with another
electron and annihilate producing photons. This process is not very important in
medical X-rays as the photon energies are usually not high enough to produce an
electron-positron pair.

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13
Q

Contrast media

A

high attenuation coefficient materials that have heavy atoms with a
large proton number and so a large number of electrons.hese materials, such as barium,, or iodine

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14
Q

Computerised Axial Tomography (CAT)

A

examining the
internal three-dimensional structure of a patient using X-ray imaging. The CAT scanner
records a large number of 2D X-ray images then assembles them into a 3D image with
the help of computer software. The resolution of the image is greater than the
conventional X-ray and the CAT scan can distinguish between differing soft tissues.
However, CAT scans take a significantly longer time and so expose the patient to a far
greater dose of ionising radiation.

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15
Q

CAT scanners contain an

A

X-ray tube that generates a fan-shaped beam. This is directed
onto the patient whilst lying on their back. A ring of electronic detectors opposite detect the X-ray beam intensity. This information is then converted into electrical signals and
processed to reconstruct the tissues that the beam has passed through. The X-ray tube
and the detectors can then rotate about the patient and move up and down their length
to create a full 3D image of the patient’s body when all images of each slice are stitched
together. The image can then be displayed on a computer monitor and analysed.

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16
Q

Medical Tracers

A

radioactive isotopes are combined with specific elements to form
compounds that collect in particular locations in the body

17
Q

n non-invasive diagnosis

A

these sources have to be placed inside the patient’s body and
their emissions detected from the outside. This makes gamma-emitters most useful as
they are least ionising and most penetrative. Beta and alpha emitters are more ionising
so would cause significant damage

18
Q

Radioisotopes used in medicine

A

high activities and short half-lives so
that the imaging can be achieved quickly, the patient is exposed to a minimal dosage of
harmful radiation and only small amounts of the radioactive source are needed

19
Q

Positron Emission Tomography (PET) scans

A

undergoes beta plus decay releasing a positron from a proton and forming a neutron in
the nucleus (see Nuclear and Particle Physics 6.4). The positron then annihilates with an
electron in the patient’s body to form a pair of gamma photons which are detected to
locate the F-18 source in the patient’s tissue. F-18 has a half-life of approximately 110
minutes and is produced through the nuclear transformation of oxygen-18

20
Q

Gamma Cameras

A

detect gamma photons emitted from medical tracers within the body.
When gamma rays are emitted from the body they travel in every direction. This makes
tracing the location of their emission difficult. Therefore, a collimator is used so that
only photons travelling in one direction are detected.

21
Q

collimator is made of

A

a mesh
of parallel honeycomb-shaped tubes so that photons travelling in any direction other
than that of the axis of the tubes is incident upon the walls of the collimator and
absorbed. The collimator has to be made of a high density metal to ensure that the
gamma is absorbed

22
Q

what happens to the photons after they pass through the collimator in a gamma camera

A

scintillation crystal (e.g. sodium iodide)
which is a material that will emit many photons when a high energy photon is incident
upon it. Approximately a tenth of the gamma photons are absorbed onto the scintillator
but each photon produces thousands of visible photons. The visible photons are then
directed onto a photocathode which produces an electron for each visible photon
detected

23
Q

photomultiplier tube

A

Photomultipliers contain a set of
dynodes (intermediate electrodes which emit additional electrons) which are kept at
high voltage so that as the initial electron hits them a cascade of electrons is generated
amplifying the signal. The position of the impact in the scintillator is used to locate the
emission site of the original gamma photon. This signal is finally detected by a
computer and displayed on a screen

24
Q

Positron Emission Tomography (PET)

A

A PET scanner is a ring of gamma cameras
placed around the patient so that an accurate 3D image can be generated from the
emission site of the gamma photons

25
Q

how does pet use arrival times

A

. Each of the photons is
detected at one of two diametrically opposed detectors in the ring. Their arrival times
are recorded. Based on these arrival times, the exact location of the annihilation event
can be calculated as the speed of the photons is known. As annihilation occurs soon after
beta emission, the site of the tracer can be estimated.

26
Q

Ultrasound

A

longitudinal sound wave with a frequency greater than human hearing
range i.e. greater than 20 kHz non-ionising and non-invasive technique that is quick and affordable. It is
particularly useful for finding the boundary between two media

27
Q

transducer in the
ultrasound device

A

is used to produce an electrical signals from the soundwaves. This can
then be analysed by computer software and an image can be generated and displayed on
a screen. n alternating potential difference that causes repetitive
compression and stretching of the crystal. A resonant frequency of the crystal is chosen
to increase the intensity. Once the ultrasound has been created the potential difference
is turned off and the reflected signal is read.

28
Q

Piezoelectric Effect

A

piezoelectric material generates a voltage when it is contracted or expanded or will
contract and expand if a voltage is applied. Therefore, by applying a voltage to a
piezoelectric crystal we can produce ultrasound vibrations and a piezoelectric crystal
absorbing ultrasound will produce an alternating voltage.

29
Q

A scan ultrasound

A

uses a single transducer to emit a signal and
then later receive the reflected signal back. It is used to determine distances from
the ultrasound device to the point of reflection (usually the boundary between
two media). This is achieved by measuring the time delay between generating
and receiving the signal and using the speed of sound in the media to
approximate the distance.

30
Q

B scan ultrasound

A

A more complex scan that produces a 2D image. This is accomplished by
moving the transducer over the patient’s skin. At each position, the scan produces
a measure of the time interval and so the distance to the reflection point between
signal production and reception. The B scan is a series of A scans that are stitched
together to form an image.

31
Q

acoustic impedance

A

the product of its density, 𝜌𝜌, and the speed of sound in that medium, 𝑐𝑐.

32
Q

impedance
matching gel

A

To maximise the transmission of the ultrasound into the
patient

33
Q

. Doppler imaging

A

non-invasive technique to measure the
speed of blood flow. Ultrasound waves are sent into a blood vessel. The blood flowing
past the transducer contains iron that reflects the wave back to the transducer.
Depending on the direction and speed of flow, the ultrasound frequency is either shifted
up or down