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Unit 21A - Medical Physics Applications
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Unit 21A - Medical Physics Applications . Radiation use in medical diagnosis and treatment. Achieved distinction.
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Unit 21 Assignment 1 Explain how an MRI scanner works and produces an image.
The primary coil is a strong permanent superconducting electromagnetic that is cooled by liquid helium.
The primary coil makes the spin axis of the hydrogen nuclei in the body point in the same direction. The
radio frequency coils produce a magnetic field that pulses at radio wave frequencies and is directed at 90
degrees to the field of the primary coil. This causes the resonance in the precision of the hydrogen nuclei
as they try to realign with the field of the primary coils and causes the nuclei to emit radio waves. The
gradient coil makes the overall magnetic field stronger which changes the precession frequency of the
hydrogen nuclei and hence the frequency of the radio-waves they emit. Greater contrast can be seen
between tissues due to the different gyromagnetic ratios that allow the hydrogen nuclei to realign with the
field of the primary coil at slightly different rates. This changes the resonant frequencies and the radio
waves emitted.
Explain how an ultrasound transducer works and produces a sonogram.
An ultrasound wave is produced when an alternating current flows down a coaxial cable at a frequency of
2MHz. This is then applied to a quartz piezoelectric crystal which means pressing electricity that is inside
the transducer. The molecules in the piezoelectric undertake resonance due to the natural frequency of
the quartz molecule that matches the driving frequency of the electric current. This means large amplitude
oscillations are observed in the quart molecules producing an ultrasound wave. An image is produced from
an ultrasound by the outgoing waves from the transducer which will pass into the body and reflect at the
boundary between two materials. The transducer acts as a detector and will listen for the reflections. This
is because the piezoelectric crystal will turn the sound wave that was reflected into an electric signal. The
reflections are then used to build an image that will show the different tissues in the body. There are many
treatments uses for ultrasounds, for example killing benign and malignant tumors. Ultrasound waves focus
on the tumour which will heat it up and cause it to die. Another treatment is breaking kidney stones by
matching the frequency of the ultrasound to the resonance which causes the kidney stone to shatter.
There are many diagnostics uses for ultrasounds for example narrowing of the blood vessels, strokes and
heart attacks and prenatal health.
Explain how infrared thermographic camera works and produces a thermogram.
An IRT camera produces thermal images which can show hot spots on the skin that are from a high blood
flow. The infrared thermographic camera works by focusing infrared radiation through a lens onto a
photodetector sensor array. Electrons are liberated within each section of the photodetector array. This
will depend on the intensity of the light. Each charge is gathered by the processor which builds an image.
IRT are used for cardiovascular disorders, Tumour location and respiratory problems. IRT are fast at
producing images, they can map body surface temperature remotely and they are non-invasive. The cons
of this are the hotspot detected is blurred so it's not fully accurate and we can't determine the depth of
the hotspot.
Explain how ultrasound can be used in medical treatment.
Doctors commonly use ultrasound to study a developing foetus, a person's abdominal and pelvic organs,
muscles and tendons, or their heart and blood vessels. External ultrasound is used for prenatal scanning in
order to check the heartbeat and look for any early indications of birth complications. Endoscopic
ultrasound uses a long, flexible endoscopic tube that enters through the mouth to examine problems that
develop in the stomach, lymph nodes in the chest, or gullet. Malignant and benign tumours can both be
, killed with ultrasound; this is done by heating the tumours, which then perish as the ultrasound waves are
focused on them.
kidney stones are broken using this. The kidney stones are broken by them because the ultrasound
frequency coincides with the calcium's resonant frequency, which causes the kidney stones to shatter.
Explain how a laser beam is produced and is used in surgery.
Electrons in a low energy level of an atom absorb a photon from the flash lamp and this will excite to a high
energy level. A laser beam is created when there are lots of electrons that are in excited states. When one
electron de-excites with a certain frequency, the emitted photon passes through the laser cavity, and this
will cause surrounding electrons to de-excite and emit light. The emitted light is coherent and
monochromatic which means they contain the same colour. When there are lots of electrons they go from
a low to high excitation, so a population inversion is created. As the higher energy levels are unstable an
electron will deexcite to a lower energy level and emit a photon. The emitted light reflects and forth from
the reflector at the ends of the gain medium. A standing wave is created, where large amplitude
oscillations are seen due to constructive interference. The frequency of the light matches the cavity which
is the resonance. The photons are emitted whereas the electrons deexcite and give off the light. The
emitted light then passes out of the gain medium, through the 95% reflective reflector, delivering a high
energy which is the laser beam
Explain how a gamma ray imaging (PET scan - medical tracing using radiopharmaceuticals) occurs and
produces an image.
A PET scanner is a ring of collimators with sodium iodide crystals and photomultiplier tubes. The scanner
has a ring arrangement so it can produce a 3D image from gamma rays. Gamma rays have a low ionizing
force and leave the body to be detected by a gamma camera. Isotopes emit gamma rays, which are like X-
rays. The radiation does not stay in your body for very long, as the isotopes used decay within a few hours
which produces an image.
Electrons are a form of matter and have an antimatter counterpart called the positron. A positron has a +1
charge but has all other characteristics of an electron. Positrons are very short-lived and less common than
electrons. When an antimatter particle collides with its corresponding matter, it annihilates very quickly,
turning the particle into two gamma waves travelling in opposite directions. This means we can detect
emitted gamma rays.
A gamma ray is used to image gamma radiation–emitting radioisotopes. There are 3 main components of
gamma rays. A collimator is a block of lead with many holes drilled through it that uses gamma rays from a
patient to produce an image. The collimator has holes in them to allow the gamma rays to travel down the
holes to produce a coherent image. Sodium Iodide crystal is a fluorescent material which means we can
see a faint glow. Fluorescence appears when a high-energy invisible EM wave is absorbed by a material and
re-emitted as different low-energy EM waves in the visible piece of the spectrum. A photomultiplier tube
is used to multiply the signal. The energy of the visible light wave from the NaI crystal causes a single
electron to be released from the first metal plate. Electrons are then accelerated to the next plate by
voltage. This accelerated electron absorbs energy from the accelerating voltage, so it emits a few more
electrons. This effect continues throughout the tube until a very weak light signal is converted into a strong
electrical current that is interpreted by a computer.
Explain how X-rays are produced and form an image.
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