MRI Physics: Equipment & Safety Anna Beaumont FRCR part I Physics.
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Transcript of MRI Physics: Equipment & Safety Anna Beaumont FRCR part I Physics.
MRI Physics: Equipment & Safety
Anna Beaumont
FRCR part I Physics
1.5 T GE Signa (1992)1.5 T Philips Intera (2001)
1.5 T Philips Achieva (2005)
3.0 T GE 750 (2008)
1.5 T GE Optima (2010)
Scanners in Hull
• From 2 to 5 systems in 5 years
• 1.5 T to 3.0 T including latest wide bore
The MRI Controlled Area
Scan RoomCabinet Room
Control Room 5 Gauss Line
Patient Bore
detachable table
music
panic button&intercom
short & wide bore
Some terminology• In the presence of an externally applied magnetic
field:• Ferromagnetic materials
– Strongly attracted to magnetic fields– Induced magnetisation may persist after removal of field– E.g. iron, nickel, cobalt
• Paramagnetic materials– Weakly attracted to magnetic field, – No permanent magnetism persisting after field removal– E.g. magnesium, molybdenum, lithium, gadolinium contrast
• Diamagnetic materials– Repelled by magnetic field
Some concepts• When a current is run through a loop of wire, it induces a magnetic field.
• The strength of the magnetic field is:– Proportional to the current flowing;– Inversely proportional to the distance from the wire.
• The greater the current, the stronger the field.• The further away from the wire, the weaker the field.
– Principle of coils used in MRI
• Also, if a coil of wire is placed in a changing magnetic field, a current will be induced in the wire.– Principle of how we collect the signal.
MRI Equipment: Overview
• Magnet• Shims
• Gradient Coils• RF Coils• RF Cage
1. Main Magnet & Static Field
Magnet
• Application– Whole body & peripheral
systems• Type
– Permanent, resistive, superconducting
• Orientation– Horizontal, vertical field
• Design– Tunnel-short & wide bore– Open
1987: Elscint’s Gyrex System
Today: Philips’ vertical HFO System
Types of Magnet
Permanent– Two opposing flat-faced magnetised poles
(ferromagnetic materials)– Iron/ Alloys of aluminium, nickel & cobalt– Low power consumption– Low operating cost– No cryogen – Small fringe field
• But– Very heavy ~80 tonnes. – Low field strength (0.064T ~ 0.3T)
www.mri-q.com
Types of Magnet
Resistive• Large electromagnets (like the ones in scrap
yards)• Magnetic field generated by running a current
through loops of wire• Field strength ~ 0.3T• Produce a lot of heat, so water cooling• Light weight• Can be shut off• Low cost• High power consumption• Large fringe field
www.mri-q.com
Types of Magnet
Superconducting• Magnetic field generated by running current
through loop of wire, (niobium-titanium).• Wire surrounded by liquid helium to cool it.• Cooling reduces electrical resistance; no
energy required to maintain current flow.• Much larger currents possible, so larger
magnetic field possible.• At -269ºC (4K) wire loses its resistance and
will sustain magnetic field.
Types of Magnet
– High field strength (needed for low sensitivity)– High field homogeneity– Low power consumption– High SNR– Fast scanning
• But– High capital costs– High cryogen costs– Technical complexity
Superconductors
• Niobium-Titanium• Cryostat
– Liquid helium– Cryoshielding helium
only (to prevent boil off)– Cryoshielding consists of
cooling a metal cylinder surrounding the He vessel
• Cryogens– Replenish due to boil off
Quench PipeCryostat
Superconductors
Quench• When magnet loses
superconductivity & cryogen (liquid helium) boils off rapidly
• Gas should leave room by quench pipe
• Failure to vent the gas could result in asphyxia and frostbite
• Oxygen monitor needed in room to give warning
Quench PipeCryostat
Homogeneity
• Uniformity of B0 field– crucial in MRS (magnetic
resonance spectroscopy) for good resolution of spectral peaks.
• DSV specification (Diameter of a Spherical Volume)– Quantifies B0 homogeneity
over given distance
isocentre
e.g. DSV40cm
=0.2 ppm
40 cm
Homogeneity• To obtain most homogeneous magnetic field, magnet must be finely
tuned (“shimmed”) by adding small corrective fields:– Passively (using movable pieces of ferromagnetic material)
• Or– Actively, using small electromagnetic coils distributed within the
magnet– Active shim correction can be made after patient is in scanner as they
will cause inhomogeneities
• Scanners are active or passively shielded to reduce fringe field- 11 T scanner requires 1400 Tons of steel- 7 T unshielded has a 23 m 5 G line
• Active shielding makes the fall-off very rapid (gradient of B)
Fringe (stray) Field
• Scanner ‘footprint’– Credit cards erased at 10 G– Safety limit is ‘five Gauss line’– Pacemakers not allowed within 5
G• 7 Tesla scanner has 23 m 5 G
line• Shielding (to reduce stray field)
– Passive (metallic)– Active (outer superconducting
coil whose field opposes that of the inner coil)
• May be measured with handheld Gaussmeter
> 30 G Stainless steel, non-ferromagnetic objects
< 30 G ECG monitors, unrestrained ferromagnetic objects
< 10 G Credit cards, x-ray tubes
< 5 G Pacemakers, general public
< 3 G Moving cars etc
< 1 G TVs, CT & PET scanners
< 0.5 G Railways, gamma cameras
Adjacent Scanners
1a. SafetyStatic Field Effects
B0 Effect
Three forces associated with exposure to static magnetic fields:1) Translational force (projectile effect)
2) Displacement of intra-corporeal metallic foreign objects: intraocular foreign body (metal worker, history of ballistic orbit trauma, old intra-cranial aneurysm clips)
3) Perturbed functioning of certain devices: cardiac pacemaker, neurostimulators, cochlear implant, derivatic valves
Peak field, gradient & force product
• Translational Force B dB/dz• Torque B2
• Peak areas around the bore ends
Remember we are talking static field gradients not imaging gradients
Projectile Effect & Issues with Implants
• July 31, 2001 New York: Death of 6 year-old boy due to oxygen tank pulled into bore
• This is thought to be the first ever death due to projectiles
• September 15, 2000 New York: MRI ‘Disarms’ Police Officer• -3 hrs to ramp down scanner and remove it
• 1 April, 2000 Australia: Patient with pacemaker scanned and died as a result of malfunction
• Another accident left a patient blinded from a minute metal fragment in his eye
• Ex-vivo testing of devices required at appropriate field strength
• Non ferromagnetic materials with no electrical activity (titanium and its alloys, tantalum) carry no particular risks in relation to magnetic fields
Projectile Effect
Examples of the incompatibilityproblem of some implants and devices.
No injury caused but resulted In severe image artefacts.
‘Hair bobble not on screening form’
Solutions
• Operate controlled area• Screening
patients/helpers• Orbit x-ray if required• Check compatibility of
device (Field, gradient and force product)
If in doubt DO NOT take the chance
1b: SafetyCryogens
Quench• Loss of superconductivity
• Cryogens rapidly boil
• Temperature in room drops
• Frost bite, asphyxiation risk to patient
• Oxygen monitor in room
• Door should open outwards• http://mri-q.com/what-is-a-quench.html
cryostat
quench pipe
2. Gradients
Gradients (db/dt)
• 3 orthogonal or in combination
isocentre
B0
0
B0+BB0-B 0+ 0-
y
x
z
Gz = dB0/dz
Gradient Coils
Often used as z gradient coil
Often used as x & y gradient coils
By running current in opposite directions in the two halves of the gradient coil, the magnetic field is made stronger near one and weaker near the other
Images: radiopaedia.org
Gradient Coil
• Z gradient • X&Y gradient
Images: mri-q.com
GradientsGradient CharacteristicsPerformance linked to:• Maximal amplitude (magnetic field variation in
mT/m)
• Slew rate– High slew rates & low rise time required to switch gradients quickly
& allow ultra-fast imaging sequences such as Echo Planar Imaging (EPI)
• Linearity: must be as perfect as possible within the scanning area
Gradients
• Gradient waveform trapezoidal
• These values are different for each system:
• Amplitude, 10-50 mT/m• Rise time, 200 s • Slew rate 20-150 T/m/s
Slew Rate (T/m/s) = Amplitude (mT/m) Rise Time (s)
Rise time
Max amplitude plateau
Gradients
• Rapid switching of gradients induces currents in nearby conducting materials (electric wires, homogenisation coils)
• Called “Eddy currents”
• Oppose gradient fields and cause decay in profile
• To reduce eddy currents:– Active gradient shielding– Optimising current profile sent to gradient coils to
compensate for eddy currents
2a. Safety:Gradient Stimulation
‘db/dt’ Effects• Refers to change in field due to gradient switching.
• Gradients encode image information; faster gradients speed up scans, stronger gradients improve resolution.
• Electrical stimulation can occur (Peripheral Nerve Stimulation, muscular) at about 60 T/s.
• Cardiac stimulation is theoretically well above this.
• Actual threshold depends on rise time.
• Echo planar sequences are those most likely to cause this type of effect .
2b. Acoustic Noise
Acoustic Noise
• Lorentz force on gradient coils
• Gradient coils vibrate and this is transmitted to other parts of scanner and patient
• Type of noise is ‘stressful’ (low frequency and periodic)
• Ear plugs must be worn by patient• Certain scans worse than others
seCond
thuMb
FirstField
Motion
Current
Noise Levels
Manufacturer
Field Strength (T)
SPL (dB(A))
Philips 1.5 112
Siemens 1.5 106
GE 1.5 110
Varian 3.0 118
Bruker 3.0 113
Acoustic trauma threshold is 140 dB
3:RF
RF Coils
• Needed to transmit and receive RF waves
• Volume coils and Surface coils
• Volume coils– Usually saddle shaped to
guarantee uniform field inside
– Area of examination needs to be inside the coil, e.g. head coil
RF Coils
Surface coils• Placed close to the area
under examination• Consisting of a single or
double loop of wire• High Signal to Noise
Ratio (SNR)• High resolution • However; signal
uniformity falls off quickly away from the coil
RF Coils: Signal Characteristics
Distance
SNR
a
Theoretical cylinder coils
surface coils
Quadrature Coils
• Quadrature or “circularly polarised” coils generally have a saddle shape
• Contain two loops of wire placed at right angles to each other (and orthogonal to B0 axis)
• Produces more signal than single loop coils
• Most volume coils are quadrature coils
Phased Array Coils• Consist of multiple
surface coils
• Surface coils have high SNR but limited sensitive area.
• Combining 4-6 surface coils creates a coil with a large sensitive area.
RF Coils
• Typical Scanner Configuration:– Integrated body coil– Head coils – Torso Coil– Surface coil– Specialist coils e.g. wrist, breast
B1 Uniformity
• Surface coil uniformity problematic• Commercial software correction methods (e.g.
SCIC, PURE)
original corrected
3: SafetyRF Heating
3: RF Heating• RF energy transmitted through free space from transmit coil
to patient. – When conducting materials are placed within the RF field, a
concentration of electrical currents sufficient to cause excessive heating and tissue damage may occur.
• RF power deposition expressed as Specific Absorption Rate (SAR in W/Kg)– Up to 3.0 T, SAR B2– SAR of 1 Wkg-1 applied for an hour would result in a temperature rise
of about 1 °C.– Can lead to heat stress (Testis, Foetus etc)– Will affect implants/devices too
• Some sequences use more RF than others, e.g. SE vs. GRE• At higher fields the body is more conductive and leads to
weaker penetration– RF power needs to therefore be increased
RF Burns
• Burns are most common adverse incident
• Unknown below 1.0 Tesla, becoming more common
• Usually high SAR scan and other contributory factor (position of patient and wires)
Solutions• Solutions include low or multiple-small flip angles, fewer
slices• Need to avoid formation of current loops formed by
external conductors placed next to skin, e.g. ECG leads.• Coil cables should not come into direct contact with the
skin: use padding. This includes the transmit RF coil
• Avoid crossed cables
• Avoid closed current loops formed by touching extremities, e.g. clasping hands, knees touching.– Position of highest electrical resistance is skin-skin contact– All the energy of the current will be released as heat at that point
RF Cage
• MRI inherently low (RF) signal technique
• RF (Faraday) cage– Distributes EM radiation around cage’s exterior and
cancels it within– All 6 sides enclosed in copper– Electromagnetic shielding– Integrity must be maintained– Penetration Panel (contains filters & waveguides for
pipes, ducts, cables)– Mesh window– Closed scan room door
RF Cage Construction
Waveguides
Mesh Window
PenetrationPanel
Door surround
4: Other Safety considerations
4a: Claustrophobia
• Affects between 1 -10 % of patients, depending on patient entry
‘The patient was experiencing intense anxiety about an upcoming MRI test, which involves the person remaining motionless inside a very cramped tube for hours as the machine takes pictures of the brain. The medical team reported that no one who was even moderately claustrophobic had ever completed the MRI test. After three weeks of working with the tapes the day for the MRI test came. She successfully entered into a very relaxed state and eventually went to sleep during the three-hour procedure.’
Solutions
• Ventilation, music, lighting, sedation• More open designs, shorter & wider
bores
4b: Pregnancy
• No harmful effects, better than ionising radiation
• However, pregnant women excluded in first trimester– Foetus expected to be more
susceptible to effects– Contrast agents can pass
placenta
4c: Contrast Agents
• Gadolinium agents better tolerated than iodinated (CT) agents
• Gd-DTPA very long safety record– Adverse events include nausea/vomiting, local
warmth/pain– 5 million uses, 1,234 AEs (1992)– Anaphylactic shock and death in 1 case
• Some concern re: nephrogenic systemic fibrosis in kidney dysfunction (transmetallation)
• Record any reaction to MHRA
MHRA (Medicines and Healthcare products Regulatory Agency)
• Latest guidelines November 2014
• Summarises HPA (ex NRPB), IEC & ICNIRP recommendations
• Stratify operation into 3 modes:– Normal
• No effects– Controlled
• Transient/mild effects– Research
• Unrestricted, requires monitoring
MHRA
• Free poster available to download
Summary
• “An MR scanner is a coil within a coil within a coil within a coil….”
Graphics from mri-q.com
Summary
• Main field (B0) Coils (principal magnet windings plus superconducting shim and shield coils)– Shim coils (to improve homogeneity)
• Gradient coils (for imaging, including their active shields)–Radiofrequency (RF) Body Coil
(transmits B1 field)
»Patient coils (primarily to detect MR signal, some are transmit/ receive)