Cellular Pathology: MRI & MRS Flashcards
What is meant by the term “contrast”?
- The ability to see differences in signal between different anatomical or pathological regions of a particular part of the body, e.g. brain.
In a Computed tomography (CT) scan what is the signal intensity proportional to and what does this mean?
- Signal intensity is proportional to how much X-rays are absorbed
- This means that brighter areas of a CT scan are regions where the X-rays are absorbed strongly
What is the main structure/s in the body that CT scans are used to image?
- Bones
- Can however be adjusted to image soft tissue
What is the main structure/s in the body that Magnetic resonance imaging (MRI) scans are used to image?
- MRI scans are used to image soft tissue and NOT bone
Where does the signal for an MRI scan come from?
- Signal for MRI is picked up from the protons in the water and fats in the soft tissue
For a T2-weighted MRI scan where would the signal intensity be highest? Why is this?
- Signal intensity would be highest in fluid structures such as CSF and sinous mucus
- This is because T2 is highest in areas of high fluid content
In a CT scan what is the signal intensity depndent on?
- Signal intensity dependent on Hounsfield number which is a measure of how much x-rays are attenuated by a particular structure
Why does bone have a high hounsfield number?
- Bone is able to highly absorb X-rays which means they are hightly attenuated (they lose a lot of their intensity) when X-rays go through bone
Why does water have a lower hounsfield number compared to bone?
- Water is less able to absorb X-rays comapred to bone which means the X-rays are attenuated less when they go through water and so lose less of their intensity
On a CT scan why would a haemorrhage show up as lighter than an oedema?
- Oedema is area of increased water content so shows up as darker as X-rays aren’t highly absorbed by water and so have lower attenuation
- Products produced by breakdown of blood produced via haemorrhage absorb X-rays to a higher degree than water and so the X-rays will be attenuated more and so that area of haemorrhage will show up as lighter
What are the 2 different types of MRI scan?
- T1-weighted MRI scan
- T2-weighted MRI scan
What are some of the differences between T2-weighted and T1-weighted MRI images?
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T2-weighted image:
- CSF appears very bright
- Fatty tissue shows up dark
- Really good at detecting pathological changes in the brain that generate lesions, e.g. Stroke or Brain tumour
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T1-weighted image:
- CSF appears dark
- Better shows contrast between white and gray matter compared to T2
- Really good at picking up anatomical changes that relate to changes in volume of grey matter, e.g. ageing or alzheimer’s disease
MRI contrast is very sensitive to changes in a large variety of the physical properties of tissue water and blood. List some of these physical properties and what things may cause them to change
- T1 relaxation: paramagnetic blood breakdown products
- T2 relaxation: tissue fluidity – oedema, deoxyhemoglobin
- Water diffusion (micro): cell membrane integrity, cell size
- Flow (macro): blood flow
- Perfusion: blood flow, blood vessel density
MRI imging can be used to cretate a perfusion map, what is a perfusion map?
- A quantitative map that shows the blood volume throughout the brain
On the perfusion map shown below why does the frontal meningioma show up as bright red?
- Meningioma is a tumour that develops on the meninges of the brain and so as a result of this it’s highly vascularised which is why it shows up as bright red
What colour does grey matter show up on the perfusion map? Why is this?
- Grey matter shows up green/yellow because it has a greater blood volume than white matter
What is a T2 map?
- A map that is a quantitative measure of the T2 relaxation time time related to changes in fluidity of the tissues
What is a diffusion map?
- A map that measures how free water is to diffuse in different areas
What is a diffusion anisotropy map?
- It’s a map that measures the ability of water molecules to diffuse in different directions
Define the term anisotropy
- Anisotropy is the ability to diffuse in different directions in non uniform manner
On the diffusion anisotropy map why is there a low signal intensity in the middle of the meningioma?
- Water within the meningioma can diffuse equally so there’s no anisotropy
Explain why MRI is able to produce and detect signals when scanning the human brain
- It’s because our brains are composed of 75% water and the variety of signals detected come from water
- Water is a H20 and the 2 hydrogen atoms within a water molecule each have a proton at their cores
- These protons spin on their axises each creating a magnetic moment
- When someone goes into a scanner all the magnetic momnets produced by the protons align with the magnetic field in the scanner
- This can then be manipulated to then produce a signal
- These signals are then used to produce an MR image
Define the term magnestism
- The positive charge of a spinning proton produces a magnetic moment (μ)
Define the term resonance
- In a magnetic field Bo the magnetic moment of a proton precesses at the Larmor frequency VL
What is the larmour equation and what does it mean?
- Larmour equation: VL = 2π γ B0
- This equation means that the resonance frequency of a magnetic nucleus, e.g. proton, is directly proportional to the strength of the magnetic field
Name the different components of an MR scanner and explain how each componenet works
- Magnet that the patient lies in containing a strong uniform magnetic field
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Loops of wire called imaging coils which surrond any part of the patients body
- A radiofrequency excitation pulse is sent through the imaging coils to the part of the body that is being imaged
- The part of body being imaged is imaged in slices in any orientation
- These slices produce signals which are picked up by the imaging coils and sent to the radiofrequency amplifier/reciever
- Radiofrequency amplifier/receiver receives signals from imaging coils and sends them to a computer
- Computer turns signals received from RF amplifier/receiver into an MR image
- There are also magnetic field gradients which can create spacial variation in the magnetic field
- This changes the freuency of signals going to the RF amplifier/receiver and thses chnages allow you to work out where in space that signal comes from.
Explain the difference between T1 relaxation and T2 relaxation
- In the equilibrium situation the magnetisation produced by the magnetic moments of the protons is aligned with the strong magnetic field (B0) - magnetisation in Z-axis
- Then a radiofrequency pulse is given which causes the magnetisation to un-align with the magnetic field and go from the Z-axis to the XY-axis producing a signal
- After this 2 processes occur:
- The signal produced from the magnetisation dies over time - This is called T2 signal decay/relaxation
- There’s also a slow increase in magnetisation back along the Z-axis until it reaches back to equilibrium levels - This is called T1 relaxation/T1 signal recovery along Z
How is the radiofrequency pulse and the MR signal produced from the tissue used to create an MR image?
- Upon introduction of a radiofrequency pulse, an MRI signal is generated from tissue. This is repeated several hundred times to create an MR image (TR).
When producing an MR image there’s a time between the radiofrequency pulse being introduced and the MR signal being produced by the tissue. What is this time called?
- This time is called the echo time (TE)
When is the signal picked up by the tissue strongest?
- Signal picked up is strongest straight after the radio frequency pulse and it exponentially decays with time (T2 relaxation/decay).
Describe the process of T2 relaxation in tissues
- Signal is produced by all the protons of all the hydrogen atoms in the body
- In a cell there is:
- Highly mobile intracellular and extracellular water
- Bound water found in the hydration complexes in cellular structures such as membranes
- Protons found in macromolecules of cell membranes which are also bound
- Bound water decays quickly
- Free water decays quite slowly
Describe the effect the length of the echo time has on the signal intensity produced by bound and free water within a cell
- If there’s a short echo time after the radiofrequency (RF) pulse then a strong signal will be produced by protons found in bound water and free water
- If there’s a longer echo time after the RF pulse you’ll still get a strong signal from protons found in free water but will get a weak signal from protons found in bound water
- This is because the signals from protons in bound water will have decayed before it can be detected
Why is an MR image taken at a short echo time called a proton density image?
- Because the contrast shown between tissues in the image is mostly proportional the amount of fat and water, and therefore protons, in the tissue
Why is there a strong signal produced by the lesion in this MR image?
- Lesion in first image shows up brighter due to the oedema
- In an Oedema there’s increased water content which means more protons and that means a stronger signal is produced
What type of MR image is taken at after very long echo time?
- T2-weighted image
Contrast on a T2-weighted image can be very low. How can the contrast on a T2-weighted image be improved?
- T2-weighted image can be adjusted by increasing the brightness which will increase the contrast
What effect does repetition time have on the signal intensity produced by different tissues when producing a T1-weighted MR image?
- A short repetition time means that the protons within a tissue with a short T1 relaxation time will be able to re-align their magnetisation with the magnetic field quickly enough to produce a strong signal
- Protons within tissues with long T1 relaxation time, e.g. CSF, aren’t able to re-align their magnetisation with the magnetic field quickly enough so those tissues produce weak signal
- A long repetition time will mean tissues with both short and long T1 relaxation times will produce strong signals as protons will have enough time to re-align their magnetisation with magnetic field
What are contrast agents?
- Substances that are used to chnage T1 and T2 relaxation times
- They can be paramagnetic (unpaired electrons) or superparamagnetic (ferrites)
- They are chelated (bonded to ions/molecules) to reduce toxicity
Explain how T1 and T2 weighted imaging can be used to elp with the treatment of brain cancers?
- T1-weighted image, alongside contrast agent such as Gd-DTPA, can be used to distinguish tumour core from rest of brain tissue allowing you to identify wherr to surgically remove the tumour
- T2-weighted image is used dentify where to give the radiotherapy dose to destroy any remaining tumour cells
Why is the use of a contrast agent and the T1-weighted image able to distinguish the tumour from the rest of the brain tissue?
- In normal brain contrast agent isn’t able to get out of intravascular spaces due to blood brain barrier
- This means normal brain tissue shows up as darker as T1-relaxation time isn’t affceted by contrast agent
- In the tumour blood brain barrier is broken so contrast agent can enter tumour tissue and reduce T1-relaxation time within it
- This makes tumour tissue appear brigther than areas of normal brain tissue
Would a water molecule have a higher or lower peak on the magnetic resonance spectrum compared to a lipid molecule, why is this?
- A water molecule would have a higher peak on the spectrum compared to a lipid molecule.
- This is because the electrons within the water molecule spend more time away from the protons in the hydrogen atoms compared to the lipid molecule which means the protons in the water molecule are less shielded from the external magentic field
- This means the MR signal produced by the water molecule will be at a higher frequency meaning the peak will be higher than the peak produced by the lipid molecule
Explain how MRS can be used along with MRI to aid diagnosis/treatment of someone with a disease that produces brain lesions
- MR imaging (T2-weighted) can be used to distinguish area of lesion from normal tissue
- MRS can be used to produced 1H spectrum of area of brain lesion to see what chemicals are reduced in that area which could show neuronal loss/inflammation
- MRS can also be taken from brain areas with no lesions to see if there’s a reduction in particular chmeicals in seemingly normal regions which would indiate neuronal loss/inflammation in these regions as well
How does the 1H spectrum of a low grade tumour differ to that of normal brain tissue?
- There’s No NAA peak in low grade tumour spectrum because there’s no neuronal tissue
- There’s an Elevated Choline/Creatine peak which is a marker of tumours
How does the 1H spectrum of a high grade tumour differ to that of normal brain tissue?
- There’s no NAA peak which indicates there’s no neuronal tissue
- Elevated Cho/Cr indicates tumour
- High lipid peak indicates necrotic high-grade tumour
