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Title: Medical imaging
Description: Notes on the topic of medical imaging for a cellular pathology module taught on the third year of a biomedical science degree. These notes discuss; x-ray, CT, MRI, and ultrasound imaging modalities, covering the principles,application, advantages and disadvantages of each imaging technique.
Description: Notes on the topic of medical imaging for a cellular pathology module taught on the third year of a biomedical science degree. These notes discuss; x-ray, CT, MRI, and ultrasound imaging modalities, covering the principles,application, advantages and disadvantages of each imaging technique.
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Medical imaging
Radiation- the emission or transmission of energy in the form of waves or particles through
space or through a material medium
...
Non-ionizing radiation- any type of electromagnetic radiation that does not carry enough
energy per quantum (photon energy) to ionize atoms or molecules, that is, to completely
remove an electron from an atom or molecule
...
Ionizing radiation can
cause DNA damage, leading to mutations, and ultimately cancer
...
X-ray principles
X-rays result from the conversion of the kinetic energy obtained by electrons accelerated
under a potential difference into electromagnetic radiation, as a result of collisional and
radiative interactions
...
The x-ray tube provides the proper environment and components to produce
x-rays
...
In the x-ray tube, two electrodes, the cathode and anode, are
situated a small distance apart in a vacuum enclosure called the inert
...
A separate, isolated circuit connected the cathode filament to a low voltage power
source
...
The second step involves the application of a high voltage supplied by the x-ray
generator to the cathode and anode
...
The andoe is usually composed of
tungsten
...
When the machine is turned on,
x-rays travel through the body and are absorbed in different amounts by different tissues,
depending on the radiological density of the tissues they pass through
...
For
example, structures such as bone contain calcium, which has a higher atomic number than
most tissues, thus bones readily absorb x-rays and thus produce high contrast on the x-ray
detector
...
Conversely, x-rays travel more easily through less radiologically
dense tissues, such as fat and muscle, as well as through air-filled cavities such as the
lungs
...
X-ray applications
X-ray radiography detects; bone fractures, certain tumours and other abnormal masses,
pneumoniae, calcifications such as kidney stones or vascular calcifications, arthritis in joints,
bone loss such as in osteoporosis, dental issues, heart problems such as congestive heart
failure, blood vessel blockages, digestive problems, and foregin objects such as items
swallowed by children
...
Tumours tend to appear as regular or irregular shaped masses that
are brighter than the background on the radiograph
...
While usually
benign, microcalcification may occasionally indicate the presence of a specific type of
cancer
...
X-ray disadvantages
Disadvantages of x-rays include; the use of ionising radiation which may be potentially
harmful thus limiting the use of x-rays in children and preventing frequent use, low sensitivity
in soft tissues, yields only two dimensional images, yields structural information only not
functional information, failure to determine age of lesions, and the contrast media used may
be dangerous
...
X-rays pass through the patient from multiple projections and are
detected by an electrons detector array that records patterns of densities
...
The filtered back projection method is used to enhance contrast and to increase
resolution of the image
...
Anti-scatter collimators are used in CT
scanners
...
Radiocontrast agents are substances used to enhance the
visibility of internal structures in CT imaging
...
However, due to associated toxicity
at the iodine concentrations necessary for imaging they are not suitable for most clinical
applications
...
CT applications
A study conducted by Lin, et al
...
In this
patient with laboratory-confirmed COVID-19, CT findings preceded symptoms and included
bilateral pleural effusions, previously not reported in association with COVID-19
...
, 2020, reported on the use of chest CT images to diagnose
pneumomediastinum in association with diabteic ketoacidosis in diabteic patients
...
, 2020, reported on the use of CT imaging in the diagnosis of
early-stage lung adenocarcinoma, which os manifested as isolated pulmonary nodules and
ground glass nodules on CT
...
CT scans can be used to
diagnose conditions including; damage to bones, injuries to internal organs, problems with
blood flow, stroke, and cancer
...
1mm which is much higher than the resolution obtained through x-ray or
ultrasound (0
...
CT disadvantages
Disadvantages of CT scans include; the use of ionizing radiation especially in comparison to
x-rays as CT uses a higher dose of radiation, relative expensive equipment required
especially in comparison to ultrasound and x-ray imaging, slower imaging in comparison to
ultrasound (seconds to minutes), lower patient in comparison to ultrasound and x-ray
imaging, and potential toxicity of intravenous contrast agents
...
Certain atomic
nuclei possess a property known as spin dependent on the number of protons
...
Application of a strong, external magnetic field (B0) alings the
nucleus either in parallel with or perpendicular to the external field
...
The absorption of energy by the
nucleus causes a transition from higher to lower energy levels and vice versa on relaxation
...
multiple FR pulses are applied to obtain multiple FIDs, which are then averaged to improve
the signal-to-noise ratio (SNR)
...
It will be
made up of contributions from different nuclei within the environment being studied
...
MRI scanners use cryogenic superconducting magnets 15
...
Relaxation described the process by which a nuclear spin returns to thermal equilibrium after
absorbing RF energy
...
A longitudinal
relaxation occurs, energy is displaced into the lattice, surrounding the nucleus environment
...
Water and cerebrospinal fluid have
long T1 values and thus appear dark on T1 weighted MR images, while fat has a short T1
value and appears bright to T1 weighted MR images
...
Transverse relaxation is the
process of energy redistribution among the nuclei within a spin system
...
These agents shorten the T1 relaxation time,
causing increased signal intensity on T1 weighted images, so the enhanced parts appear
bright on T1 weighted images, or shorten the T2 relaxation time, causing reduced signal
intensity on T2 weighted images, so the enhanced parts appear darker on T2 weighted
images
...
MRI contrast agents may be administered
orally or intravenously
...
Some agents selectively distinguish liver
pathologies
...
This
technique relies on the fact that cerebral blood flow and neural activation are coupled
...
The magnetic environment experienced by each MR sensitive nuclei is different due to the
magnetic fields of the electrons within their immediate chemical environment
...
Thus, the
magnetic resonance frequency spectrum consists of nuclei which resonate at different
frequencies
...
The most important visible peaks on the cerebral proton MR spectrum are
N-acetyl aspartate (a marker of neuronal dysfunction and loss), choline (a marker of
membrane activity, increased in inflammatory and malignant processes, decreased in
hepatic encephalopathy), creatinine ( a marker of internal reference level as it is constant in
health and disease), myo-inositol (elevated in microglial activation and astrogliosis), and
glutamine and glutamate
...
, 2020, reported the use of MRI scans to determine small
vessel disease score and thus predict risk of cognitive decline and dementia
...
, 2020, concluded that baseline diffusion MRI is potentially capable
of predicting two year clinical outcomes in patients with Parkinson’s disease on an individual
basis
...
Ceballos-Ceballos, et al
...
additional
uses of MRI scans include; anomalies of the brain and spinal cord, tumour detection, cyst
detection, breast cancer screening in at risk women, assessment of injuries or abnormalities
of joints, assessment and detection of diseases of the liver and other abdominal organs,
evaluation of pelvic pain in women to assess causes such as endometriosis, and
investigation of uterine anomalies
...
1mm) especially in comparison to ultrasound (0
...
MRI disadvantages
Disadvantages of MRI scans include; the requirement of very expensive equipment, not
widely available, low throughput due to very ling imaging time (minutes to hours) especially
in comparison to CT (minutes) and ultrasound (seconds to minutes), movement artifacts
created by breathing and heartbeat, hazardous for patients with metal implants, low
sensitivity in some instances such as detecting microcalcifications, potential toxicity of
contrast media, and low patient comfort
...
Ultrasound frequencies range from 2-15 MHz
...
The transducer converts energy from one form to another (electrical to
sound/mechanical and vice versa)
...
The returned echoes are converted back into electrical impulses by the transducer crystals
and are further processed to form the ultrasound image presented on the screen
...
5-3
...
0-7
...
Ultrasound waves are
reflected at the surfaces between the tissues of different density, the reflection being
proportional to the different in impedance (how much resistance an ultrasound bea,
encounters as is passess through a tissue, and depends on the density of the tissue, and the
speed of the sound wave)
...
Doppler shift is defined as the change in frequency of sound waves due to a reflector moving
towards or away from an object, which in the case if ultrasound is the transducer
...
Ultrasound applications
The temporal super-resolution if the dynamic ultrasound imaging, a means to observe rapid
heart movements, is important in the medical diagnosis of cardiac conditions
...
Microvascular
ultrasonographic imaging is a doppler ultrasound technique
...
Advantages of microvascular ultrasonography are its superiority in detection and
visualisation of the small blood vessels in tissues
...
Smaller, high frequency probes are used to assess the breast, thyroid,
testes, muscles and tendons
...
Ultrasound advantages
Advantages of ultrasound imaging include; the absence of ionising radiation and so can be
used in pregnancy and can be used repeatedly, inexpensive equipment especially in
comparison to CT and MRI, widely available, the equipment is portable, can yield a three
dimensional image which is an advantage over x-ray, yields real time functional images, high
throughput due to rapid imaging time (seconds to minutes) especially in comparison to CT
(minutes) and MRI (minutes to hours), and has high patient comfort
...
5mm) in comparison to CT (0
...
1mm)
Title: Medical imaging
Description: Notes on the topic of medical imaging for a cellular pathology module taught on the third year of a biomedical science degree. These notes discuss; x-ray, CT, MRI, and ultrasound imaging modalities, covering the principles,application, advantages and disadvantages of each imaging technique.
Description: Notes on the topic of medical imaging for a cellular pathology module taught on the third year of a biomedical science degree. These notes discuss; x-ray, CT, MRI, and ultrasound imaging modalities, covering the principles,application, advantages and disadvantages of each imaging technique.