MRI Flashcards
What is MRI?
Uses non-ionising radiation
Magnetic resonance imaging uses a strong magnetic field and radiofrequency pulses. All nuclei spin and have a magnetic charge so when a magnetic field is applied, most of the hydrogen nuclei (protons) spin in tissues and body water align with the magnetic field. Since most of the protons’ spin align themselves with the magnetic field, the net force is in the direction of the magnetic field which is longitudinal magnetisation (due to difference in the number of spins in parallel and anti-parallel state).
When a radiofrequency pulse is passed through the patient (by radiofrequency coils), the protons are stimulated and move against the pull of the magnetic field (misalignment) by either 90 degrees or 180 degrees. When we transmit a radiofrequency pulse, the protons absorb the energy and get excited, flipping some of their spins into higher energy state (bear in mind only protons that spin with the same frequency as the radiofrequency pulse will respond to the radiofrequency pulse). If more than 50% of the protons are pushed into the high energy state, the longitudinal magnetisation decreases and a transverse magnetisation occurs. Transverse magnetisation occurs as the energy pushes all the protons to spin together; if we add up all the moments, we get a net magnetisation at 90 degrees to the longitudinal axis which is the transverse magnetisation. As the protons are creating the transverse magnetisation spin, they will produce a small alternating current in a coil or wire.
Once the radiofrequency pulse is removed, the protons’ spins realigns with the magnetic field, releasing electromagnetic energy. The MRI detects this electromagnetic energy to differentiate between different tissues and gives us an image. The time it takes to realign with the magnetic field and the amount of electromagnetic energy released depends on the environment and the chemical nature of the molecules. We can tell the difference between various types of tissues based on the magnetic properties.
-Physics point: protons that align with the magnetic field are in a low energy state; protons that align against the magnetic field are in a high energy state (before the radiofrequency pulse is aligned).
What happens after?
-When the spin of nuclei aligns with the magnetic field, they carry out a wobbling motion called ‘precession’ (essentially rotation). The rate of rotation (precession frequency) is directly proportional to the strength of the local magnetic field. Since most of the protons’ spin align themselves with the magnetic field, the net force is in the direction of the magnetic field which is longitudinal magnetisation. However, if we transmit a radiofrequency pulse, the protons absorb the energy and get excited, flipping some of their spins into higher energy state (bear in mind only protons that spin with the same frequency as the radiofrequency pulse will respond to the radiofrequency pulse). If more than 50% of the protons are pushed into the high energy state, the longitudinal magnetisation decreases and a transverse magnetisation occurs. Transverse magnetisation occurs as the energy pushes all the protons to spin together; if we add up all the moments, we get a net magnetisation at 90 degrees to the longitudinal axis which is the transverse magnetisation. As the protons are creating the transverse magnetisation spin, they will produce a small alternating current in a coil or wire.
-Longitudinal magnetisation = due to difference in the number of spins in parallel and anti-parallel state (parallel = spins aligning with magnetic field; anti-parallel = spins not aligning with magnetic field)
We lose longitudinal magnetisation when radiofrequency pulse is added because there will be equal proportions of parallel and anti-parallel spins
-When the radiofrequency is removed, the protons relax back to their original positions which will temporarily stop the precession. The positive charges of the nuclei repel each other causing them to move apart. As they move apart, the transverse magnetisation is lost which is known as T2 or ‘spin-spin’ relaxation. As the high energy protons relax back to low energy state, the absorbed energy is dissipated into surrounding tissue as heat. Longitudinal magnetisation is restored – which is T1 magnetisation.
What is T2 relaxation?
loss/decay of transverse magnetisation. Spins return to original out of phase arrangement. Transverse decay occurs as spins move together (spin-spin interaction), magnetic fields interact, modifying precession frequency slightly. T2 is tissue specific and always shorter than T1. T2 is unrelated to field strength – instead, it is related to how protons move. Solid tissues have short T2 times as protons can’t move far from original position; fluid tissues have long T2 as protons can move far away. Longer T2 times give brighter signal on T2 weighted image.
What is T1 relaxation?
restoration of longitudinal magnetisation. After T1, 63% of the longitudinal magnetisation is restored. T1 varies dependent on the field strength – the higher the strength, the longer the T1
What are the proton characteristics?
- The protons in our body have different characteristics e.g. some are associated with free-flowing water; some are fixed into energy storing molecules (e.g carbs). As a result, they have differences in their T1 and T2 relaxations. These differences can be measured by changing the repetition time of the radiofrequency pulse and echo time – this is the pulse sequence.
- The protons in free-flowing water are able to hold onto their absorbed energy much longer, so they continue to spin in phase, maintaining the transverse magnetisation. This means that if we turn on the coil to measure the transverse magnetisation at this point, the water will have a large transverse magnetisation, while the fat will have a low one i.e., water will give a stronger signal. The water has a stronger signal at this point, so will be assigned white; the weaker signal from the fat will appear grey or black. To accentuate the difference between the T2 relaxations of water and fat, we would wait a long time between radiopulses (a long repetition time) and a long time to listen to the return signal. This allows the differences to be measured and an image to be produced in the T2 weighting. The long repetition time reduces the T1 relaxation effects; the long echo time accentuates the T2 relaxation effects.
What is repetition time and echo time?
- Repetition time = amount of time between successive pulse sequences applied
- Echo time = time between the radiofrequency pulse and receipt of the echo signal
- The duration of radiofrequency pulses can be altered to control how far the net magnetisation moves towards the transverse plane. Different pulse sequences use different combinations of radiofrequency pulses.
- Initial excitation pulse flips magnetisation 90 degrees into transverse plane; doubling the pulse length will flip it 180 degrees – larger flip angles means more energy can be measured before spins of protons align with the magnetic field again.
How is the signal generated?
- Signal generation: Changing the electrical charge will generate a magnetic force; changing the magnetic force will generate a current as the precessing protons move towards and away from it. Magnet with bore for patient is used; main magnet is a magnet between 1.5 and 3 Tesla; coils are bathed in H2 and N2 – this means the coil has zero resistance, and the current passing through creates magnetism but no heat. It works via electromagnetism. T1 recovery and T2 decay are used to create different image weightings.
- T1 = water is dark, fat is bright. Privileging T1 data gives T1 image (anatomical scan)
- T2 = water is bright, fat is dark. Privileging T2 data gives T2 weight (pathological scan)
- You can add gadolinium contrast agents that enhance contrast by altering local magnetic field
- It produces 3D images. Each voxel (3D version of pixel) corresponds to a volume in the body. The MRI machine detects the electromagnetic energy released from each voxel, localises them in 3D space to make a visible image.
How do you generate a visible image?
-Generate a static field (B0): Magnet with bore for patient is used; main magnet is a magnet between 1.5 and 3 Tesla; coils are bathed in H2 and N2 – this means the coil has zero resistance, and the current passing through creates magnetism but no heat. It works via electromagnetism. Patient lies on sliding couch; the head enters the machine first; patient is at isocentre of magnetic field. There is an alarm button if patient gets distressed
- Localising signals using gradients: Incrementally changing field strength from one end of the field to another allows us to localise signal within patient. Field strength changes precessional frequency; to change to transverse magnetism, we need the radiofrequency pulses at matching frequency. Gradients are created in X, Y and Z planes. Thin slice and thick slices can be produced based on high and low enhancement of the magnetic field and the adjacent tissues.
- Excite a signal many times to complete the image
- Transmit signals to computer for reconstruction
-The frequency of the spin of the proton is determined by the strength of the local magnetic field. When we first get into an MRI machine, a superconducting magnet produces a near homogenous magnetic field from one side to another. This determines the strength of the MR machine. Common systems are 1, 1.5 and 3 Tesla. There are 3 sets of gradient magnets in the MR machine used to localise locations in 3D space. The Z-axis runs in the sagittal plane; the X-axis in the coronal plane; Y-axis in the axial plane. To select a particular slice of tissue in the body. We can turn on the magnets in the Z-axis. These magnets create a magnetic field from head to toe. We then put in a radiofrequency pulse to cause the desired area to resonate as previously described. Since the local magnetic gradient is homogenous, all the net magnetic moments in the slice are in phase so cannot be distinguished from each other. To further localise these moments and their signal strengths, we have 2 more gradients that can be used to isolate the source of the signals. The first gradient is called the phase encoding gradient, which is briefly turned on to create a gradient along the Y-axis (for example). This causes the magnetic moments at the bottom of the gradient to slow down and the ones at the top to speed up. The gradient is quickly turned off and the moments return to spinning at their base frequency, but there has been a phase shift in the Y-axis. This phase shift can be used to localise the moments in the Y-direction. We then tune the system to focus on a particular phase in the matrix and use the 3rd gradient (in the X-direction) to definitively localise each signal within the selected row. The gradient works the same at last, just in a different direction. It is called the frequency encoding gradient. It is left on whilst the signals are recorded. Now, each signal has a unique phase and frequency which can be localised in 3D space. This is repeated for each row to localise each signal. Each signal (now localised to a voxel) is assigned a greyscale value depending on the signal strength. This is used to generate a 3D image.
An MRI sequence is a particular setting of radiofrequency pulses and gradients, resulting in a particular image appearance.
Protons and T1 and T2 signals?
- The protons in fatty acids are relatively fixed, hence the transverse magnetisation decays rapidly and the protons release their absorbed energy more rapidly too (T1 relaxation)
- The protons in free-flowing water are able to hold onto their absorbed energy much longer, so they continue to spin in phase, maintaining the transverse magnetisation. This means that if we turn on the coil to measure the transverse magnetisation at this point, the water will have a large transverse magnetisation, while the fat will have a low one i.e., water will give a stronger signal. The water has a stronger signal at this point, so will be assigned white; the weaker signal from the fat will appear grey or black. To accentuate the difference between the T2 relaxations of water and fat, we would wait a long time between radio pulses (a long repetition time) and a long time to listen to the return signal. This allows the differences to be measured and an image to be produced in the T2 weighting. The long repetition time reduces the T1 relaxation effects; the long echo time accentuates the T2 relaxation effects.
-Using the same tissues in their baseline state, to accentuate the T1 relaxation differences, we can apply a radiofrequency pulse to flip the protons into the high energy state, pushing their spins into phase, producing a transverse magnetisation. T2 relaxation occurs as the proton move apart faster in fat than in water. The protons in fat then fall back to the low energy state, releasing the absorbed energy as they do so (T1 relaxation) and restoring the longitudinal magnetisation. In water, the protons are able to hold the absorbed energy longer, so the T2 relaxation occurs much slower; as a result, the T1 relaxation occurs much later, so the transverse magnetisation is maintained for longer, with restoration of the longitudinal magnetisation occurring later on. If another radiofrequency pulse is put in in quick succession after the initial one (a short repetition time), the fully recovered fat protons will produce a large transverse magnetisation and strong measured signal which can be recorded with a short echo time. Since the protons in water will still have a strong transverse magnetisation from the energy, they’ve held onto from the energy they’ve held onto from the initial pulse, the second pulse will push even more of the proton in the water into the high energy state, reversing the net longitudinal magnetisation, which will be measured as a low signal
What is proton density MRI?
Proton density:
- A long repetition time minimises the T1 effects
- A short echo time minimises the T2 effect
- This sequences gives us an idea of the absolute number of protons (proton density) in the region
Can be used for:
Joint disease and injury
• High signal from meniscus tears
What can you see in a T1-weighted image?
o provides the most anatomically-relevant images
o brightness depends on tissue properties.
o fluid (in CSF and orbits) is dark
o grey matter is darker than the white matter
o uses shorter repetition times and echo times. If T1WIs did not have short TRs, then all the protons would recover their alignment with the main magnetic field and the image would be uniformly intense. Selecting a TR shorter than the tissues’ recovery time allows one to differentiate them (i.e. tissue contrast).
o Provides good contrast between gray matter (dark gray) and white matter (lighter gray) tissues, while CSF is void of signal (black). Water, such as CSF, as well as dense bone and air appear dark. Fat, such as lipids in the myelinated white matter, appears bright.
o Useful for vascular changes and disruptions of blood brain barrier if gadolinium is used. High signal can mean slow blood, subacute blood (haemorrhage).
• Lower signal for more water content,[48] as in edema, tumor, infarction, inflammation, infection, hyperacute or chronic hemorrhage.[49]
• High signal for fat
What can you see in a T2-weighted image?
o standard sequence
o fluid is bright
o white matter is darker than grey
o cannot differentiate white and grey matter well
o long repetition time and echo time
o Blood flow appears dark hence vessels appear as shadows
o Occluded areas appear bright as there is no flow
• Higher signal for more water content[48]
• Low signal for fat
What is FLAIR?
• FLAIR (fluid attenuation inversion recovery)
o commonly used sequence
o similar to T2, but the fluid is darker or “suppressed”
o useful for areas of oedema or inflammation
o used to identify plaques in multiple sclerosis (especially periventricular)
What are the risks of MRI?
- Metals should not be taken into the scanner as they can experience a force from the scanner’s magnetic field and can induce currents
- Loud noise e.g beeping and clicking so some patients may require special ear protection
What is MRI used for?
-MRI can be used for detecting diffuse cerebral disease, multiple sclerosis, HIV related disease, spinal cord imaging, orthopaedic imaging, tumour staging, imaging of biliary tree and cardiac MRI.
MRI gives better resolution and contrast between structures, making it especially useful in tumour staging, e.g., pelvis.
-MRI can be T1 weighted (water is dark) or T2 (water is bright)
