9.1 H spectroscopy Flashcards
(25 cards)
Why does hydrogen have a nuclear spin?
because it contains only one proton -> has an odd number of protons and neutron in nucleus -> has a nonzero spin
What characteristics does the nuclei of atoms with a zero spin have?
even no. of protons + even no. of neutrons
How do nonzero nuclei cause an MRI signal
only nonzero atoms can interact with the magnetic field in MRI. they can absorb and release energy at a resonance frequency.
they generate a signal that MRI scanners can detect.
In 1H NMR, how does the electron cloud change results?
the local electron cloud around each hydrogen nucleus shields it slightly from the magnetic field B0. ->
a more shielded proton experiences a weaker field, so it resonates at a lower frequency
What is NMR?
nuclear magnetic resonance
What resonance does a hydrogen atom that is not very shielded by electron clouds emit?
low shielding -> experiences a stronger effective field -> higher resonance emitted -> higher chemical shift value
What causes the amount of shielding of a hydrogen (from the electron cloud) to change?
-hydrogen is bonded in lots of different molecules -> the different atoms hydrogen is bonded to changes the amount of shielding of hydrogen
Using 1H NMR spectroscopy, why is it hard to see/get the chemicl shift values of brain metabolites?
because of the high concentration of water in the brain compared to metabolites, there is a huge water peak of chemical shift -> causing the nearby metabolite signals to be very small and overlapping -> you can’t differentiate between metabolites
What can you do to alleviate the water dominant effect when measuring brain metabolites for 1H NMR?
increase the strength of magnetic field to at least 3 Teslas
What happens to the brain metabolite peaks when you increase the magnetic field strength ?
no more water dominance: better spectral resolution (separation of the peaks) and signal-to-noise ratio
Give some examples of brain metabolites?
creatine, glutamate
aas and NTs
What does 1H spectroscopic imaging of the brain measure?
Does it use voxels?
What scan do you need to do first before you do a 1H spectroscopic MRI scan?
-the chemical shifts of brain metabolites.
-yes, each voxel has its unique spectrum
-a structural MRI scan eg T1 for reference
What is the magnetic field strength of 1H spectroscopic imaging? why?
at least 3Teslas usually 3-7T because any lower would bring about the issue of water dominance
What is the voxel size of a 1H spectroscopic image ? How does this compare to other structural and fMRI voxel sizes?
0.5–2 CM3 which is way larger than structural: 1mm3, fMRI:2–3 mm3
How does a 1H spectroscopic image detect tumours?
voxels including the tumour have a completely different spectrum compared to healthy tissues -> tumour metabolism is very different compare to healthy tissues
What is the issues with the voxel size?
voxels need to be very large 1CM3 to capture the chemical shifts of metabolites but this reduces spatial details/lower resolution of images
What is unique about 1H spectroscopy imaging?
you can create an image for each individual metabolites
What does the resonance frequency of a nucleus depend on?
What equation does this come from? state equation? what does each variable mean?
-What is the gyromagnetic ratio of 1H?
-Why is the gyromagnetic ratio and resonance relevant to 1H spectroscopic imaging?
-the gyromagnetic ratio and the strength of magnetic field B0
-Lamour Equation: f=γ * B0
where f=resonant frequency, γ=gyromagnetic ratio
- γ≈42.58MHz/T
-gyromagnetic ratio determines how fast a nucleus precesses around an external magnetic field B0 and therefore what frequency it resonates when exposed to radio waves.
If the gyromagnetic ratio is γ≈42.58MHz/T and B0= 3Teslas what is the resonance frequency of 1H?
3 x 42.58 = 128 MHz
In summary, what variables affect the resonance frequency of a nucleus/H atom? how do they affect?
-strength of magnetic field B0 increases -> faster precessing -> higher resonance emitted
-electron cloud shield effects of the B0, more shielding -> lower resonance
-gyromagnetic ratio of an atom as this ratio changes with different atoms due to the different nuclear properties
Why do other nuclei, not 1H, need a different RF coil?
different nuclei resonate at different frequencies (due gyromagnetic ratio, shielding) -> need separate RF coil
What is heavy water imaging?
How is it put into the body??
What is this technique a current hot research topic and why is better than 1H imaging?
What are the limitations?
-2H spectroscopic imaging: 1 proton and 1 neutron in deuterium.
-deuterium is infused with deuterated water/glucose
-better because it only measures the deuterium infused (no water dominance issue) -> higher sensitivity to tracking metabolism activity, easier to track individual metabolite, more spectral peak separation
-has a lower gyromagnetic ratio than 1H, therefore a lower resonance frequency -> lower sensitivity
What are the benefits of 2H spectroscopic imaging?
-can directly track actual biochemical pathways -> can see from spectroscopy when glucose is converted into lactate
-high specific signal as there is little deuterium naturally in human body
What is 13Carbon or 13C?
How abundant is it in the human body?
Which spectroscopic techniques uses 13C?
How does this technique work?
What effect does the hyperpolarisation have? why is this beneficial?
-a stable, NON-radioactive isotope
-about 1.1% prevalence
-Hyperpolarized ¹³C
-inject pyruvate into body and image
- hyperpolarisation boosts signal by 10,000–100,000x allowing the real-time imaging of fast metabolic reactions
(dramatically higher MR signal than standard MRI)
