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Unit 21 LA: A&B (Ionising techniques and non - ionising techniques)

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  • Unit 21 - Medical Physics Applications
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  • PEARSON (PEARSON)

In overall, this unit has achieved DISTINCTION grade and contains all of the necessary contents. This assignment consists of two assignments as A&B assignment is merged together as the distinction criteria is required to compare the ionising techniques from LA:A and non ionising techniques from LA:...

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  • September 14, 2023
  • 33
  • 2023/2024
  • Essay
  • Unknown
  • A+

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  • unit 21 medical physics
  • la ab
  • science
  • coursework
  • essay
  • 2nd year coursework
  • ionising and non ionising techniques
  • assignment

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  • BTEC
  • PEARSON (PEARSON)
  • Applied Science 2016 NQF
  • Unit 21 - Medical Physics Applications
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Mohammed Salam Unit 21 LA: AB


Non – ionising techniques

MRI Scan

Principles:

Magnetic Resonance Imaging (MRI) is a technique used to create a picture of the patient’s body. It is
commonly used in clinics to produce detailed images of the patient’s body by employing a strong
magnetic field and radio waves. It is possible to perform MRI scans because the human body has its
unique make-up. Human cells are composed completely of water, mostly hydrogen ions. The magnet
from the MRI attracts the Hydrogen ions to produce an image.



How is it produced?

To produce an image from an MRI it is done by exposing hydrogens (protons) to a large magnetic
field, which partially polarises their nuclear spins, proton nuclear magnetic resonance (NMR) can
detect the presence of hydrogens. The patient must lie under the magnet where the sample tables
are in order to produce an image. The spins are excited with radio frequency radiation that is
specifically tuned, and after they relax from this magnetic interaction, the radio frequency radiation
from these spins is detected. In the relaxation process, the frequency of a proton's “signal” is
proportional to the magnetic field it is exposed to. It is possible to assign a specific location in the
tissue to the proton signal frequency since it is proportional to the magnetic field. In this way, the
protons present in the tissue can be mapped in terms of the density of them. In order to image
organs and other variations in the tissue, a certain amount of contrast is achieved in the subject
tissue due to the fact that proton density varies with tissue type.

The human cells are composed up of water molecules, the protons from the cells are excited and the
MRI uses proton NMR to view the concentrations of protons. MRI are eligible for imaging soft tissues
from organs like eyes, brains, and liver. However, since MRI scanner is based on the concentrations
of protons there aren’t enough of protons in the bone so when an image is produced the bones from
the patients will appear dark which means the bones will not show in the image. Linear positioning
data can be collected in many of directions when a rotating field gradient is used. A two-dimensional
map of proton density can be constructed from these data. Distinct types of tissues exhibit different
proton content, and the proton NMR signals are extremely sensitive to these differences.

Energy levels and Larmor frequency:

The energy of a photon is closely related to its frequency when it moves from a low energy state to a
high energy state, this is referred to as the Larmor frequency. In MRI, the resonance frequency for
hydrogen scans ranges from 12 to 85 MHz.

All nuclei have a spin and have magnetic moments (dipoles) when neutrons and protons combine in
the nucleus, where their separate angular momentum cancels out. In the case of a small bar magnet,
the magnetic moment is defined as its alignment with a field that is equal to an external magnetic
field. Similarly, to a bar magnet, these nuclei also have two poles, which is why they are called
dipoles. Unpaired proton nuclei can be found in hydrogen and phosphorus; unpaired proton and
neutron nuclei can be found in nitrogen.

An external magnetic field aligns nuclei with a magnetic moment and causes them to wobble around
the static magnetic field, generating a secondary spin (precession). The precessional path of the
magnetic field is circular, like the path of a spinning top. MRI uses Larmor frequency as a measure of

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,Mohammed Salam Unit 21 LA: AB


how quickly a proton's magnetic moment precesses around its external magnetic field. The
precession frequency is proportional to the strength of the magnetic field in an MRI image, B 0.

Here is the Larmor frequency equation which is used to calculate the precessional frequency of
nuclei in a substance placed in a static magnetic field, B 0:

ω = γB

ω = Larmor frequency which is in MHz

γ = gyromagnetic ratio in MHz

B = the strength of the static magnetic field

Relaxation time:

When particles gain energy they move from a lower energy to a higher energy state, this is because
radio wave knocks off their alignment and therefore, they move to a higher energy state, but this is
only temporary because when the radio waves are turned off the particles move back to their lower
energy state. The relaxation time is the average amount of time that the nucleus spends in a high
energy state - this is the time it takes for the particle to return to its low energy state. In tissues,
there are two distinct by two relaxation times T1 (longitudinal relaxation time) controls the rate at
which stimulated protons return to equilibrium. A transverse relaxation time (T2) determines how
rapidly excited protons reach equilibrium in phase or fall out of phase when they fall out of phase
with each other. It is the time needed for spinning protons to realign themselves with the external
magnetic field. When protons spin perpendicular to the main field, nuclei lose phase coherence.



Factors affecting signal intensity:

there are several factors that affect the signal intensity of the MRI such as:

 Gradient coils field strength.
 The contrast of tissue kinds in the body.
 The magnetic field strength created by the main magnet.
 Proton density in a volume of tissue sample.
 The relaxation time in the tissue sample.
 Photons that are excited by the sequence of the radiofrequency.
 The requirement for the introduction of a contrast medium for comparison.

These factors determine the signal intensity and if some of them are lacking it can decrease the
signal intensity and therefore may not produce clear image.



Increasing contrast:

The contrast can of an MRI scan can be increased by using a contrast agent. Gadolinium is a
contrasting agent which is used in one third of MRI scans. An MRI scan with gadolinium contrast
medium greatly enhances diagnostic accuracy by improving images or pictures of the body's internal
structures. By virtue of this, blood vessels, inflammation, tumours, and blood supply to some organs
can be seen clearly. There are also other ways that can increase the contrast the MRI image like



2

,Mohammed Salam Unit 21 LA: AB


increasing the relaxation time. The relaxation time is altered when these contrast agents are
introduced to the body, they can be consumed or inserted in the body by an injection.



Increasing resolution:

During phase and frequency encoding, the field of vison is the area that is covered. Field of view is
divided by matrix size to determine the in-plane voxel size, so expanding the field of view increases
the voxel size and decreases the resolution, and reducing the field of view increases the resolution.
There are also other options that can increase the resolution of an MRI scan like the ratio of signal –
to signal which should be decreased to increase the resolution, cross section slice thickness can be
reduced to increase the resolution and the signal sample speed increased can also increase the
resolution of MRI scan.



Annotated diagram of MRI:




Magnet – Magnets are major components of an MRI machine, and they are responsible for
producing high-quality images. These magnets are connected by horizontal tubes called bores.

Gradient Coils – In the MRI machine, three gradient coils are situated within the main magnet, each
of which generates a weaker magnetic field compared to the main one. Using gradient coils, the
main magnetic field can be adjusted to scan specific and different parts of the body. The gradient
coils create a variable field (x, y, and z) that can be increased or decreased as required.

Radiofrequency coils – Radio frequency coils transmit radio frequency waves from the MRI scanner
to various parts of the patient's body. Several coils are located inside the scanner to transmit radio
waves to these parts. An accurate scan is generally achieved by focusing all of the radio frequency
coils on a specific area of the body, allowing all of the coils to be focused on this area.

Sample table – Sample table is a component from an MRI scan that allows to slide the patient into
the MRI machine. When patients lie down on the sample table, their position may vary depending
on which part of their body is being scanned. An isocentre is the exact centre of the magnetic field,
where the part of the body under examination begins the scanning process.

Bore – Bores are connected to the magnets which helps to produce high quality images.



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, Mohammed Salam Unit 21 LA: AB


The medical uses for diagnosis:

The MRI scan is useful for a variety of reasons where other tests such as X-rays which are
insufficient. Because it produces detailed images of virtually any part of the body, an MRI scan can
be used for almost anything. For detecting abnormalities and tumours in the brain and spinal cord,
detailed images are obtained using this technique. The brain and spinal cord tumours can be
detected by the MRI scan because the area that is scanned is very concentrated of protons the MRI
can detect these protons and produce an image which would be very detailed. This enables the
health professionals to evaluate the condition, plans and treatment options can be available because
of the MRI scan. Because such of high-quality image can be produced the MRI can even detect torn
ligaments around the joints. The health professionals prefers to use the MRI scan to view the torn
ligaments rather than x – rays because they cannot view the torn ligaments so MRI scans are used
instead to assess the issues. The MRI scans view the torn ligaments because the concentrations of
protons in that particular area can show the abnormalities. The health professionals use the image
produced from the MRI scan to detect the abnormalities in the patient’s body, which also is used to
provide medical diagnosis to the patient.



The medical uses for treatment:

MRI function is to provide an image from the patient’s body parts to detect any abnormalities and
diseases that could occur in the body. “The results of an MRI scan can be used to help diagnose
conditions, plan treatments, and assess how effective previous treatment has been.” (NHS, 2022).
Without the use of MRI scan further procedure like treatments cannot be done on the patients.



Ultrasound

Principles:

A medical ultrasound scan produces a live image from inside a patient's body using high-frequency
sound waves. Ultrasound scanners use a similar technology to sonar and radar for detecting aircraft
and ships. A health professional can use ultrasound as a medical viewing technique to assess issues
with vessels, organs, and tissues without having to undergo invasive surgery. The use of ultrasound
does not require the use of radiation, which is not the case with other imaging techniques. It is
therefore the most preferred technique for viewing a foetus during pregnancy since ultrasound
produces no radiation and therefore it is safe for the baby.



How is it produced?

By electrically stimulating a piezoelectric crystal, ultrasonic waves are created which can be directed
at specific areas of the body and are above the audible range of humans, humans can hear up to
20KHz the ultrasound uses high frequency sound waves. The frequency that are used in clinics are
between 2 and 20 MHz, the ultrasound travels faster in solids than liquid and gases this makes it
particularly useful for medical use, the greater the stiffness the faster the sound wave will travel. The
Ultrasonic waves are created by stimulating a piezoelectric crystal electrically. Waves travel through
bodies and are reflected back anywhere there is a change in tissue density, like at the border
between two organs. Defining the intensity level of the echoes as well as the position of the tissue


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