MRI Flashcards
What are 3 advantages of MRI?
Is non-invasive
Is non-destructive
Uses no ionising radiation, making serial studies more ethically acceptable
what are 4 disadvantages of MRI?
Time - takes longer. therefore patient motion a greater problem
Contraindications - More patients have contraindications for MRI compared to CT. Such contraindications arise because of metal in the patient in the form of passive medical implants or metallic fragments in their eyes, or because of active electronic implants such as cardiac pacemakers
Cost - expensive
Difficult theory
will muscle produce a higher signal than cortical bone?
Yes
how is it that the presence of bone can be inferred from MR images?
(1) cortical bone shows up as dark against the high signal from soft tissue and (2) marrow (i.e. fat) in the trabecular bone also produces high signal (Fig 1).
Very briefly how does MRI work?
(1) place the patient in a strong external magnetic field to align the protons; (2) apply a pulse of RF radiation at the correct frequency to produce resonance; (3) detect the RF radiation emitted as resonating protons relax to give an NMR signal
What quantity is the static magnetic field and how do we refer to it?
The static magnetic field (B0) is a vector quantity (i.e. it has direction as well as magnitude)
What effect does B0 have on the patient?
exerts a force on hydrogen nuclei (protons) within the body and this causes them to align themselves in the direction of the field or in the opposite direction.
What does the RF pulse do?
The RF pulse causes the nuclei to change their alignment but only if the radio waves within it have a particular frequency (the Larmor frequency).
What happens after the RF pulse is switched off?
After the pulse is switched off, the patient emits RF radiation at the same frequency as the protons revert (relax) to their original alignments in the magnetic field. This emission is detected by a receiver to create a measurable signal (an electrical voltage that varies with time).
How is the strong static magnetic field created?
by the flow of direct current in coils of electrically conducting material
Why is the static magnetic field always on and present?
In the large majority of MRI scanners, the coil material is maintained at a very low temperature (that of liquid helium) such that it is superconducting i.e. it has no electrical resistance. For practical reasons, this means that the current is always on and that the field is always present.
How is the RF pulse created?
The RF pulse (often called the RF magnetic field) is created by the flow of alternating current in a separate coil (that is not superconducting). The same coil may be used to detect the emitted RF radiation at a later time, or a separate coil may be used for this purpose.
What are gradient coils and why do we have them?
The MRI scanner also has a further set of three non-superconducting coils (the gradient coils) that are used to locate the source of the emitted RF radiation within the patient.
In these gradient coils, direct current is rapidly switched on and off to produce gradient magnetic fields in three mutually perpendicular directions (X, Y and Z).
What does ‘proton’ refer to in MRI?
the nucleus of a hydrogen atom (hydrogen-1) and does not refer to a proton in the nucleus of atoms of other elements that exist in the body
What are 4 key pieces of information about the proton to understand MRI?
Protons have mass (1.7 × 10-27 kg), positive electric charge (1.6 × 10-19 coulomb) and ‘spin’
Spinning charges produce a magnetic field
Hydrogen nuclei therefore act like tiny magnets or magnetic dipoles (which have ‘north’ and ‘south’ magnetic poles separated by a short distance)
The human body is about 80% water and therefore contains lots of hydrogen nuclei
What property does spin give to a proton?
angular momentum
What is angular momentum?
loosely defined as the quantity of rotation possessed by an object. Angular momentum is a vector quantity
How is angular momentum quantised on an atomic scale and what is it in a proton?
it can only have certain discrete values. The angular momentum of a nucleus is Ih/2π,where I is a quantum number that can only be zero, an integer (whole number such as 1, 2 etc.) or a multiple of ½ (such as ½, ³⁄₂, ⁵⁄₂ etc.). For the proton, I=½
What is magnetic dipole moment?
The MDM is an important property of a magnet; it can be considered as the characteristic of the magnet that indicates how quickly it will align itself with an external magnetic field. The MDM is also a vector quantity.
How do you calculate MDM?
the product of the spin angular momentum and γ, the gyromagnetic ratio
What does the gyromagnetic ratio depend on?
γ is the MDM divided by the spin angular momentum. Its value depends on the type of nucleus; for the proton it is 2.67 x 108 rad s^-1T^-1. Here ‘rad’ means radian, the SI unit of angle, with 2π radians (a full circle) being equal to 360°.
What happens to the protons in a body when in a high magnetic field?
protons align either with the field (in a relatively low-energy state) or against the field, i.e. in the opposite direction to it (in a higher-energy state). A slightly greater number of hydrogen nuclei align with the field than against the field
What can happen to a proton in a magnetic field if applied with an oscillating magnetic field?
An oscillating magnetic field at the correct frequency can make protons change from the low- to the high-energy state and in the opposite direction. An oscillating field of this type is equivalent to electromagnetic radiation at the same frequency
What is the equation for Larmor frequency?
ν = γB0/2π
Why does a spinning proton create a magnetic field?
A moving electric charge produces a magnetic field, and so the proton may be regarded as a tiny magnet because it is a charged particle that is spinning rapidly about its axis
Does the proton align exactly with B0?
No - a quantum mechanical rule prevents exact alignment and its MDM μ is inclined at an angle to the external field B0, which causes it to behave in a rather different way.
What is a precessional orbit?
the proton also has spin angular momentum and this combination of spin and torque causes μ to precess around the direction of B0. Precession is a type of rotational motion that is completely different from the spinning of the proton about its own axis. During precession, the proton MDMs travel on the surface of two cones whose axes are in the direction of the external magnetic field.
What is precessional frequency?
The rate at which the gyroscope precesses about the direction of the gravitational field G
What is v in the larmor equation?
proton/Larmor precessional frequency -When the gyromagnetic ratio and the magnetic field strength are expressed in their SI units, the unit of the Larmor frequency is the hertz; it expresses the number of precessional rotations per second.
What is the larmor processional frequency directly related to?
The Larmor precessional frequency is directly proportional to the strength of the magnetic field.
What is magnetisation?
Magnetisation is the net MDM density (MDM per unit volume) and it is a property of the material (in this case human tissue), whereas MDM is a property of the magnet (in this case proton).
How can the MDM vector μ be decomposed further?
μ2 in the direction of B0 and a component μ1 perpendicular to B0
the μ2 components generate a longitudinal magnetisation M along the direction of B0
the μ1 components are in random phases (they do not have phase coherence) and they are rotating due to precession (dashed arrows) - they cancel out and so there is zero transverse magnetisation in the plane perpendicular to B0
How do you increase NMR signal strength?
increase magnetisation
the only practical way to do this is increase the size of the static field - ie increase the T of the coil
What is the net magnetisation?
Their net magnetisation (M) is in the direction of the main field and is sometimes called the ‘equilibrium magnetisation’. The amount of magnetisation in the direction of the main magnetic field (the B0 direction) is 100% in this equilibrium condition and so we say the spins have 100% longitudinal magnetisation
Why can’t we measure M in the z axis/
we cannot measure longitudinal magnetisation directly because M is in the same direction as B0 (the Z direction) and many orders of magnitude smaller than B0
What is excitation?
tip over the magnetisation by typically 90° to convert it into transverse magnetisation and then measure that. This is called excitation.
What is resonance?
We can tip or ‘flip’ the magnetisation over by applying a rapidly oscillating magnetic field at 90° to the B0 field (i.e. in the X or Y directions). However, the varying field will only flip the spins if it oscillates at the same frequency as the precession of the spins (i.e. the Larmor precessional frequency); this is what we mean by ‘resonance’.
What is the RF field normally referred to as?
B1
What trajectory does the magnetisation follow when in B1 field?
can be thought of as alternately pushing and pulling at the magnetisation, progressively tilting it away from the B0 direction. The magnetisation follows a spiral trajectory until it is precessing (or rotating) in the plane at right angles to the B0 field (in the transverse plane (the XY plane))
What is the loss of transverse magnestisation called?
T2 relaxation
T or F:
Immediately after a 90° pulse-
A. 100% of equilibrium magnetisation is recovered
B. Longitudinal magnetisation falls to 0%
C. Transverse magnetisation falls to 0%
D. Transverse magnetisation increases to 100%
A. F
B. T
C. F
D. T
What are the 5 steps of relaxation after a 90 degree RF pulse?
- The longitudinal equilibrium magnetisation has now been flipped into the transverse plane. Immediately after the RF pulse, all the spins that contribute to the magnetisation are precessing at the same frequency and are in phase
- Some spins immediately start to precess more slowly than others - The spins are no longer in phase. This causes the transverse magnetisation to decrease.
- As time goes on, there is a greater and greater phase dispersion of the transverse components of the spin MDMs and the transverse magnetisation continues to decrease
- Finally, there is no net phase coherence and therefore no net magnetisation in the transverse plane
- At the same time as the spins precess and lose phase coherence, they also begin to reorient themselves along the longitudinal (Z) direction. Eventually the full longitudinal magnetisation is recovered, i.e. the spins return to their equilibrium condition.
What happens to the received magnetisation signal after 90 degree RF pulse?
Immediately after the 90° pulse there is 100% transverse magnetisation. This high-amplitude magnetic field that is precessing (rotating) at the Larmor frequency will induce a high-amplitude oscillating voltage in the receiver coil at the same frequency.
However, as phase coherence is lost, the strength of this transverse magnetisation will diminish and so too will the high amplitude of the induced voltage in the receiver coil.
What is free induction decay?
the high-amplitude oscillating voltage in the receiver coil at the same frequency. Varying voltage signals such as the FID are the raw data from which all MR images are reconstructed.
Why do spins dephase?
each hydrogen nucleus itself acts as a tiny magnet because of its electric charge and spin. It therefore produces a tiny magnetic field which will affect the field experienced by adjacent nuclei. This tiny field may add to, or subtract from, the field produced by the MRI scanner and this changes the larmor frequency of the proton.
What is spin-spin relaxation time?
spin-spin relaxation time is the time constant of the FID fall-off. Also called the transverse relaxation time or T2
What is T2* and why does this occur?
The real situation, in which the envelope of the FID is a steeper decreasing exponential curve with time constant T2*. The reason for this is imperfections in the static B0 field produced by the MRI scanner.
How do we overcome the effects of T2*?
by using spin echo sequences
How does a spin echo sequence work?
Stage 1: immediately after the 90° pulse the spins start to dephase due to spin-spin interaction and the field imperfections
Stage 2: some spins therefore gain phase compared to others and so the phase angle between them increases.
Stage 3: A second RF pulse is applied. this is a 180° pulse.
Stage 4: this has the effect of flipping the whole transverse plane through 180°. The spins are still precessing counter-clockwise, but now the slower precessing spins have gained phase compared to the faster spins
Stage 5: at a certain time referred to as TE (the echo time, or time to echo) the two spins will be back in phase creating a detectable signal (the echo) whose amplitude is unaffected by main magnetic field inhomogeneities. and any loss of transverse magnetisation over the time period TE will be due to the ‘pure’ T2 of the spins.
T or F:
Immediately after the 180° refocusing pulse -
A. Equilibrium magnetisation is fully restored
B. 100% transverse magnetisation is restored
C. The transverse magnetisation is greater than immediately after the 90° pulse
D. The transverse magnetisation is greater than at the echo time (TE)
A. F
B. F
C. F
D. F
What is spin-lattice relaxation time?
recover of longitudinal magnetisation over time described by an increasing exponential curve that approaches a limiting value, the equilibrium magnetisation M, with a time constant T1, the spin-lattice relaxation time.
Which is always bigger t1 or t2?
T1 is always greater than T2.
How can we measure variance in T1 time?
we allow the longitudinal magnetisation to partially recover before repeating our spin echo sequence. The second spin echo converts the partially recovered longitudinal magnetisation into measurable transverse magnetisation. If, for example, the longitudinal magnetisation has recovered by only 45% of its equilibrium value, this will be converted to 45% of maximum transverse magnetisation.
The time between the first 90° pulse and the second in the repeated sequence is called the repetition time (TR).
What is the difference in lesions in T1 and T2?
images based on T2, lesions tend to appear brighter than normal tissues, whilst in images based on T1 they will appear darker.
Why do we weight towards T1 or T2 rather than equally weighting?
if T1 and T2 effects contribute equally to the signal that lesions will be isointense with normal tissue and therefore undetectable.
How do we weight to T1?
Reduce TR to maximise T1 contrast (i.e. TR about 500 ms)
Reduce TE to the lowest achievable on the MRI scanner in order to minimise T2 contributions.
The nomenclature for a typical T1W spin echo (SE) sequence would be: SE 500/5 ms (TR/TE)
How do we weight to T2?
Increase TR to minimise T1 contribution. In an ideal world we could make this 15 seconds or more, by which time even CSF with its very long T1 of 3 seconds would have recovered nearly all its longitudinal magnetisation. However, as we will see in later sessions, we need to repeat these sequences perhaps many hundreds of times in order to build up image information. Generally speaking, patients do not stay still for more than about 8 minutes. With a ‘conventional’ SE pulse sequence, in order to obtain an image resolution of 256 pixels we need to make 256 repetitions; this means a maximum repetition time of about 2 seconds
Increase TE to maximise T2 contribution
A typical T2W spin echo sequence would have the nomenclature:
SE 2000/80 ms (TR/TE)
How do we weight for proton density?
Increase TR to minimise T1 contributions (up to a practical limit of about 2000 ms in a conventional SE sequence)
Reduce TE as much as practicable to minimise T2 contribution (i.e. a TE of about 5 ms)
A PD SE sequence would therefore be:
SE 2000/5 ms (TR/TE)
What does proton density sequences measure
an index of how many hydrogen nuclei there are per unit volume of tissue.
Why are relaxation times for tumours normally higher than those of normal tissue?
because they usually contain more unbound water.
Why can fat be a problem in MRI?
fat has an unusually short T1 but a reasonably high T2. This means that it produces high signal in both T1 and T2-weighted images. It is for this reason that a number of strategies have been developed to reduce or eliminate signal from fat as the high signal can otherwise obscure pathology.
What makes a T1 long or short?
In a pure liquid, molecules are tumbling rapidly and freely:
Molecules have little time to interact with each other
T1 is long
In viscous liquids, water binds to less mobile macromolecules (this is the nearest to soft tissues in the body):
It is easier to transfer energy
T1 is short
In solids, molecules are relatively fixed, so there is a reduced chance of their coming close enough for an energy exchange:
T1 is long again
What makes a T2 long or short?
In liquids, spins are tumbling end over end, so the magnetic fields they produce tend to even out, and therefore cause less perturbation to other spins:
There is little net change in local magnetic field
There is reduced spin-spin interaction
T2 is long
In solids, molecules are fixed. Therefore, the local field is fixed:
Local field variations are therefore significant
Spins dephase
T2 is short
If an SE 15 000/1 ms sequence was used to image the brain, which of the following weightings would it most likely represent?
PD
What are the 5 steps in acquiring a gradient-echo sequence?
- RF pulse
- Slice selection
- phase encoding
- frequency encoding
- sequence repetition
What is involved in slice selection?
To excite protons in a particular slice, it is necessary to apply the RF(read a full definition of this term) pulse in the presence of a slice-select gradient. Note that the area of the negative lobe of the gradient is equal to half the area of the positive lobe - this ensures that phase coherence is maintained within the image slice.
What is involved in Phase encoding?
it is necessary to repeat the pulse sequence a number of times (typically 256 or 512) in order to build up an image with that equivalent number of data lines, or pixels. Each time the sequence is repeated, a different phase-encode gradient strength is used. For example, the strength of this gradient may commence with a large positive value and gradually be decremented each time the sequence is repeated until a large negative value is reached. Each ‘rung on the ladder’ represents a phase-encoding gradient of a particular strength, and this is shown schematically in the pulse sequence.
What is involved in frequency encoding?
The frequency-encoding gradient is applied after each step of the phase-encode gradient. and it is during the application of the frequency-encode gradient that the MRI signal is ‘read’. In fact the frequency-encode gradient is often referred to as ‘read’ gradient. Again, the area of the negative lobe of the gradient is equal to half of the area of the positive lobe - this ensures that phase coherence is optimised at the central point of the frequency encoding
What is the Echo time of a sequence?
The echo time (TE) is the time taken from the RF(read a full definition of this term) pulse to the MRI signal, or echo.
What is the TR time?
The repetition time (TR) is the length of time between one RF(read a full definition of this term) excitation pulse and the next one.
How can the length of time for a scan be calculated?
If the TR is known, then the length of time taken to complete the sequence can be calculated. The scan time is equal to the TR value (typically 500-3000 ms) multiplied by the number of phase encode steps required (typically 256 or 512).
Also, if more than one signal average is required then the scan time will increase. Two signal averages will take twice as long to acquire, three signal averages will take three times as long to acquire, and so on.
In a spin echo sequence where is the 180 degree pulse positioned?
The 180° pulse is positioned exactly half way between the initial RF pulse and the time at which the signal is acquired, in other words the 180° pulse is applied at a time equal to TE/2.
What is the problem with SE sequences?
generally take longer to implement than GE sequences.
What is an inversion recovery sequence?
An IR pulse sequence is really very similar to a GE or an SE sequence, except that a 180° ‘inversion pulse’ is applied at the start of the sequence. The effect of this 180° pulse is to flip (invert) the equilibrium magnetisation in the z direction. As soon as this has occurred, T1 recovery processes begin. In the sequence, a delay time TI (time from inversion) is built in to allow the scanner operator to control the resulting contrast that is based on the T1 recovery rates of tissues. Following the application of an inversion pulse and an appropriate TI delay, a GE(read a full definition of this term) or an SE(read a full definition of this term) sequence is run.
What is a STIR sequence?
Short Tau inversion recovery - Looking more closely at the exponential recovery of the longitudinal magnetisation for two example tissues, e.g. fat and water, it can be seen that the (short T1) fat magnetisation passes across the TI axis – known as the null point - earlier than the (long T1) water magnetisation. At this specific TI time (approximately 130 ms at 1.5 T) there will be no longitudinal fat magnetisation available for the subsequent sequence and the resulting images will contain no fat signal. This is known as a STIR sequence.
What is a FLAIR sequence?
Fluid attenuation inversion recovery - water magnetisation passes across the TI axis (null point) later than the fat magnetisation. At this specific TI time (approximately 2500 ms at 1.5 T) there will be no longitudinal water magnetisation available for the subsequent sequence and the resulting images will contain no water signal.
T or F Regarding a GE pulse sequence:
A. RF pulses are separated by a time interval TE
B. An echo signal is acquired when a frequency-encoding gradient is applied
C. One phase-encoding gradient is used
D. The slice-selection gradient has two positive lobes
E. TR is the rise time of the RF pulse
A. F. RF(read a full definition of this term) pulses are separated by a time interval TR(read a full definition of this term).
B. T. Because it accompanies signal acquisition, the frequency-encoding gradient is also called the read or read-out gradient.
C. F. Typically, 256 or 512 phase-encoding gradients are used to produce one image.
D. F. The slice-selection gradient has a positive and a negative lobe; the area of the negative lobe is equal to half that of the positive lobe to preserve phase coherence.
E. F. TR(read a full definition of this term) is the interval between successive RF(read a full definition of this term) pulses.
T or F Regarding an SE pulse sequence: A. A 180° RF pulse is used to invert the longitudinal magnetisation
B. The image acquisition time is shorter than that for a GE sequence
C. An echo is formed after a time interval TI
D. TR is twice as long as TE
E. The image acquisition time is the product of TR and the number of phase-encoding gradients
A. False. The longitudinal magnetisation has already been tipped into the transverse plane by a 90° RFpulse; the 180° RFpulse re-phases the magnetisation in the transverse plane.
B.False. The use of a re-phasing RF pulse means that, in general, the acquisition time is longer than that of a GE sequence.
C.False. An echo is formed at a time TE after the 90° RF pulse and a time TE/2 after the 180° RF pulse.
D. False . TR and TE are set independently by the MRI scanner operator; it is possible that TR could be twice as long as TE but, in general, it is longer than 2TE.
E. Correct. With no image averaging, the acquisition time is TR multiplied by the number of phase-encoding gradients that are used.
What are the metal related sources of hazard with MRI?
The ferromagnetic missile effect on extraneous metal
Migration/rotation of metal implants or fragments in the body
Current induction and heating of extraneous metal
Current induction and heating of implanted devices
What measures can help to guard against the ferromagnetic missile effect in an MR unit?
All patients and staff must be checked (‘sifted’) for ferromagnetic material before entering the Controlled Access Area
In some centres all patients are required to change out of street clothes into gowns or surgical ‘greens’ in order to ensure no clips or pins or coins are hidden in their clothing
What measures can help to guard against the migration/rotation of metal implants in an MR unit?
the ‘Safety Checklist’
to ascertain if the patient, and any attending staff member, have potentially hazardous implants or fragments of metal in their bodies.
How can burns come about in MRI?
the resonant antenna effect, whereby the MR’s radiofrequency (RF) pulses set up a standing voltage wave in the metal if the length is equivalent to the half wavelength of the RF radiation.
If this happens, the tips of wires eg ecg/pulse oximetry can undergo rapid heating and burn the patients
What are the risks with cardiac pacemakers in MRI fields?
cardiac pacemakers or implanted defibrillators can malfunction when exposed to the fields of MR.
The pacemaker, its leads and the myocardium itself form a single conductive circuit. If this system happens to ‘tune’ to the frequency of the RF radiation, currents will be induced in the circuit. This may cause the heart to contract. With the MR often firing scores of RF pulses a second, there will not be sufficient time between contractions and little blood will be pumped
The resonant antenna effect may be responsible, with induced currents in the tips of the wires heating up where they are in contact with the myocardium and thus causing burns and thermal shock
Batteries and/or implant casings may be ferromagnetic and so may undergo migration and torque
The electromagnetic fields may cause electronic circuitry to malfunction
If the circuitry controls a drug reservoir in an implanted pump then there might be the danger of the whole reservoir being rapidly discharged into the patient
Who publishes guidlenes for MRI safety?
Medicines and Healthcare products Regulatory Agency (MHRA)
What do the MHRA MRI safety guidelines include?
Setting up a Controlled Access Area
Designation of staff who may be involved with MR and their different training requirements
The use of a checklist or questionnaire for those who enter the high magnetic field environment
Writing Local Rules detailing safe working practices in the high magnetic field environment including the control of equipment
The safety of different types of medical implant
Procedures in the event of emergency situations
What are the 2 different lines or zones within the MRI controlled areas?
the 5 gauss line - the boundary outside which it is considered safe for people with implants such as cardiac pacemakers.
MR Projectile Zone -In the MR Environment, the volume within the 3 mT magnetic field contour, where the risk of the ferromagnetic missile effect is considerable
What is important to clarify with implanted devices for MRI safety?
important to establish the make and model of the implant, as well as the manufacturer. The MR safety of any implant should be established by consulting the manufacturer about the particular make and model and its MR compatibility or otherwise.
Are orthopaedic implants MRI safe?
generally made of a sufficiently high quality surgical steel that they have no significant ferromagnetic component. Nevertheless heat can be generated by the RF field of the MRI scanner. For this reason, patients need to be carefully monitored throughout their time in the magnet room, and need to be warned to press the emergency ‘call’ button if they feel the slightest discomfort.
Why can cerebral aneurysm clips be unsafe in MRI?
although “non-ferromagnetic” they undergo a sterilisation process which ay encourage the build-up of ferromagnetic ‘domains’ in the metal.
WHat must happen if the patient arrests in MRI?
the patient must always be removed from the magnet room, and preferably from the Controlled Area, before being made accessible to the crash team. Often they may bring a ferromagnetic trolley carrying many ferromagnetic objects and devices.
How do you turn off the magnetic field in an emergency?
Superconducting quench - If the coil temperature rises above the superconductivity threshold, the windings suddenly develop a finite resistance. The several-dozen amperes of circulating current passing through this elevated coil resistance create heat. This heat causes a sudden, explosive boil-off of liquid helium.
What can be dangerous about quenching an MRI machine?
Quenches present certain hazards to staff. Though not toxic, the gaseous helium will displace oxygen and so there is the danger of asphyxia. The gases are also very cold. As such the area should be evacuated
What are the three modes of operation for an MRI machine?
Normal mode, which encompasses routine procedures in which the risk to the individual is minimal
Controlled mode, in which the exposure is greater and imaging performance improved.
Research/experimental mode, for which exposure is usually greater still and needs to be restricted to prevent harmful effects.
Why is the effect of B0 (static magnetic field) normally very low on humans?
tissues of the body are primarily diamagnetic in nature and this means they are only very weakly ‘magnetisable’. Effects in strong fields is still possible though
What can humans experience in fields of over 2T?
At fields of 2T or greater, humans can experience certain sensations including vertigo, dizziness, nausea and a metallic taste in the mouth; sensitivity to these effects varies between individuals. The effects are usually associated with head motion and may be caused by currents being induced in the semi-circular canals as they move through the magnetic field, thereby ‘cutting’ the magnetic lines of force and so inducing electric currents.
What effects can the gradient fields cause?
Though the gradients produce maximum fields that are not as strong as the static field, perhaps a fiftieth of the value, their rapidly-changing nature means that they will induce electrical fields and hence currents in conductive materials, such as the tissues of the human body.
it is possible for the induced currents to cause PNS and muscle stimulation. This usually manifests itself as muscular twitching. Cardiac stimulation or epileptic fits might possibly result.
What is the minimum threshold for inducing PNS symptoms?
20 Ts-1 is regarded as the minimum threshold
What are the effects of the RF wave on tissues in the body?
the effect of the RF is power dissipation in the tissues of the body and their subsequent heating. Heat generated in tissue is usually compensated by thermoregulation, where dilation of the vasculature increases blood flow and so allows the heat to be carried away and dissipated via the skin and exhaled breath.
Which tissues in the body deal less well with increase in temperature and are heat sensitive?
These include the eyes, which have little blood flow and the testes which should be at a lower temperature than the rest of the body.
What measures heat energy transfer to the patient in MR?
In MRI, the specific absorption rate (SAR) is used as a measure of the amount of energy per unit time (power) deposited per unit mass of tissue in the patient or subject by the RF field. The usual units are watt per kilogram (Wkg-1).
What are the limits for temperature rise for the whole body with MR?
Normal - 0.5 for whole body
controlled - 1
research - 2
In normal mode what are the max temperatures for the head, trunk and limbs?
head 38
trunk 39
limbs 40
What are the patient whole body SAR limits with MR?
Normal 2
controlled 4
research >4
