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Summary Biomedical imaging - macroscopy
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Samenvatting (Engels) biomedical imaging deel macroscopy
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Biomedical Imaging – MacroscopyLearning objectives
• Familiar with the noninvasive biomedical imaging landscape.
- different techniques and the aproach we can take (how to use them)
- Background + basics of biomedical imaging
• Theoretical and practical knowledge of the different techniques, including translationability
• Knowledge about the different techniques and the information they provide (e.g. blood flow,
molecular binding, brain activity, ….)
• To have knowledge of the mutual complementarity of these techniques (making links within
the course)
• Being able to choose the optimal imaging technique for a given problem and know its major
capabilities and limitations.
• Describe clinical and laboratory animal examples with a neuro focus for each technique.
MRI
1. Noninvasive in vivo biomedical
Medical Imaging Techniques
Different imaging techniques based on ‘waves’
• All biomedical imaging techniques are based on waves
• There is a broad spectrum of waves: visible light, radio waves, X-waves, gamma waves and
ultraviolet
- Molecular imaging is mostly based on gamma rays (prefer a high energy level)
• X-ray: radiography and Computerized Tomography (CT)
- CT (X-ray): bonestructure (anatomy), but also physiology
• Radiowaves: Magnetic Resonance Imaging (MRI)
- MRI: anatomy (specfic organ), physiology and metabolism and to some extent also the
molecular (limited sensitivity)
• γ-waves: Positron Emission Tomography (PET) & Single Photon Emission Computed
Tomography (SPECT) imaging
- very sensitive to molecular processes, metabolism and physiology → limited resolution so
cannot be used for anatomy
• Visible light: Optical imaging & Bioluminescence imaging
- sensitive especially at the molecular level → do not allow looking deeper into the tissue
• Soundwaves: Ultrasound (US)
- similar to MRI (typically for cardiac imaging and prenatal babies), but not as sensitive as
MRI or PET and therefore can also be used for metabolic processes
,For each technique, there is a compromise between molecular sensitivity and spatial resolution. The
lower the better (for spatial resolution) → you can see more detail in a tissue. For molecular sensitivity,
you want to detect the smallest change and therefore you want the highest possible sensitivity. This
means that in an ideal world, you want an imaging technique that offers extremely high spatial
resolution, with extremely low molecular sensitivity (in the pico-molar range) → doesn't exist.
Different imaging techniques visualize different processes
• Looking at different phenomena separately → completeness is important ➔ combining
techniques
Medical Imaging Techniques in the clinic
• CT: anatomy of different organs → gives little information on metabolism and molecular
processes
• PET/CT: CT shows anatomy while PET gives better information/sensitive information on
molecular processes
• SPECT: better resolution than PET → you can see the distribution of radioactive substances
throughout the body
• MRI: high spacial ratio → observe different structures (+ linking with PET → extra information
about molecular processes)
2. Magnetic resonance imaging (=MRI)
• an MRI scanner is a big magnet where the patient or the subject is put into We have
- a patient-table, where the subject can lay on → goes automaticly in and out of the scanner
- a big magnet: some sort of a cylinder to create a magnetic field
- gradient coils inside & a radio frequency coil: this will generate a pulse which is essential
to detect a signal
- there are also different orientation coils, because you want to be able to magnetise
different parts of the body differently to get the maximum signal (this is quite complex)
,Medical imaging techniques in preclinical research
• Big difference between human brain and rat/mouse brain (size) → challenge: you need to
miniaturize the imaging systems for small animal imaging
for humans, we use clinical scanners → for animals, we use preclinical scanners: you increase the
magnetic strength, allowing you to generate an image with a resolution in the micrometer range
MRI = Nuclear magnetic resonance imaging (MRI)
→ So it has a nuclear component, a magnetic component and a resonance component
• Signal is based on atomic nuclei
• Protons (+) and neutrons (0) with spin ½ (rotation)
• spinning protons = positive charged sphere representing an electrical circular loop around the
axis of rotation
- electrical current creates a magnetic field (right hand rule)
- Proton ~ small magnet with magnetic moment µ
→ the spinning of a proton will create a magnetic moment
- vb. hydrogen = proton = positivly charged and when it rotates it creates a magnetic
momentum = specific magnetic field
, → The NMR signal is primarily originated from protons (hydrogen)
• if you take a tissue: contains a large amount of water composed of 1 oxygen atom and 2
hydrogen atoms → you have two protons spinning, creating a magnetic moment
• Protons in tissue: will randomly spin in different orientations → no specific magnetic signal
• Protons in MRI scanner
- MRI creates a specific magnetic field = B0
- protons align themselves according to B0, some also in the opposite direction = high energy
state
• Protons after RF pulse
- RF (radiofrequent) pulse = wave with a specific energy and frequency, that will destabilise
the longitudinal magnetisation (rotate protons)
➢ flipping protons by 90°
- The higher tesla → the more the protons align perpendicular to B0
➢ The more protons in the tissue → the more lateral magnetisation → the more
signal and that is the proton density (= the actual signal of MRI).
➢ In reality, the protons are not going to align perfectly, they will also spin with a
different frequency → Larmor frequency
• After the RF pulse, the protons are in an unstable phase → must be stabilised ("back to
equilibrium") → return to longitudinal (= lowest energy state)
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