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)