NMR and MRI - Prizztech Oy · PDF fileis used to get μV level signals. ... School of...

Seppo VahasaloPhilips Medical Systems MR Finland2008-06-17

NMR and MRI

2Philips Medical Systems MR Finland, Seppo Vahasalo, 2008-06-17

Contents

• Some local MRI history

• NMR and MRI Physics

• Magnet technology and electronics

• Recent trends in MRI and magnet technology

• Markets and players

• Magnetic fields. Safety, EMF directive


Philips Medical Systems MR Finland


Company History

30 Years of MRI Expertise– 1978 MRI research program at Helsinki University of Technology

with Instrumentarium Oy– 1981 First 0.17 T superconducting scanner– 1980s Instrumentarium Imaging: Acutscan, Magnaview, Mega4,

Merit MRI systems (Low Field resistive solenoid magnets) – 1993 Joint Venture (50/50) with Picker International Inc.;

Picker Nordstar Oy founded – 1995 First Open MR scanner: Picker Outlook (0.23T)– 1996 Full ownership to Picker International– 2000 Marconi Medical Systems Finland Oy– 2000 OEM agreement with Philips– 2001 PMS MR Technologies Finland Oy– 2002 Panorama 0.6T with superconducting magnet– 2004 Panorama 1.0T coils– 2005 Start Integrated coil development for 1.5T/3T


Product History

Outlook, 1995 – ‘98

Proview, 1998 - 2002

0.02T Acutscan, 1983 – ’870.04T Magnaview, 1987 – ‘89

0.1T Merit, 1989 – ‘94

Panorama 0.23T, 2002 – ‘07

Panorama 0.6T, 2002 – ‘06


NMR and MRI

• Nuclear Magnetic Resonance is physical phenomenon, referring to property of atoms with an odd spin to be able to adsorb energy at the specific frequency, depending on the atom type and strength of the external magnetic field

• NMR spectroscopy is a method to measure chemical composition of material and especially chemical bonds of organic molecules by high resolution RF probing in a strong magnetic field

• Magnetic Resonance Imaging is used for anatomical imaging, typically capable of whole body scans, using a single atom type and frequency, whereas

900MHz, 21.2 T NMR Magnet at HWB-NMR, Birmingham, UK being loaded with a sample


Physics of NMR - Polarization

• Hydrogen nucleus, for example in water behaves like a tiny magnet

• When H1 nuclei are put into a strong external magnetic field, they are aligned according to that and precess around it at the Larmor frequency

• Larmor frequency is directly proportional to the external magnetic field strength by the gyromagnetic ratio 42578100 Hz/T


Physics of NMR - Magnetization

• About half of the nuclei are aligned parallel to the external field and the other half antiparallel

• There is an excess of about 1 ppm of spins pointing to the direction of external field -> measurable macroscopic net magnetization

• How much is the excess for a 2mm x 2mm x 5mm voxel water (= 0.02ml = 0.02g)?

– One mol of water weighs 18g (2H1+O16) and carries 2x 6.02x1023 H1

nuclei– In one voxel there are (0.02/18) x 2x

6.02x1023 = 1.34x1021 H1 nuclei– The excess is 3x1.34x1021 / 2x106 =

2.01x1015 nuclei


Physics of NMR – Excitation of Nuclei

• It is possible to bring in energy to the system at the resonant frequency (Larmor frequency). For a 21.2T NMR Spectrometer this corresponds to irradiating the sample at RF frequency of 902.7 MHz

• In CW spectroscopy the frequency (or B1 field) of excitations is swept, in pulsed NMR a very short RF pulse is applied to excite a broad range of frequencies (Fourier spectrum)

• Macroscopically this corresponds to:

– Forcing the phase of individual spins coherent and

– Tilting the macroscopic magnetization away from B0 direction


Physics of NMR - Relaxation

• When excitation ends, the nuclei start to relax back to the original state. That happens by emitting weak RF signals.

– There are two different relaxation methods: T1 (spin-matrix or loss of transversal magnetization) and T2 relaxation (spin-spin or loss of phase coherence)

• An efficient antenna is needed to collect these signals that are digitized and stored into a computer memory as raw data.

• In case of NMR there are no gradients applied and the received signal is just a FID(Free Induction Decay),


Physics of NMR – Spectral Information

• In the real world the resonance is much more complicated

• When the resonance is measured at high resolution for spectroscopy, in the signal can be seen for example:

– Effect of multiple coupled spins in different bonds

– Local chemical environment (perturbations to the local magnetic field, caused by neighboring atoms


MRI – Some Additional Physics

• In MRI linear magnetic field gradients are applied to acquire spatial information

• By applying a narrow bandwidth excitation pulse with a linear magnetic field gradient on, it is possible to excite only a narrow slice of tissue

• By manipulating the excitedspins during echo formationand signal collection withadditional gradients and RF pulses, signal can beidentified into a single voxel

• By using different TE and TRtimes, it is possible to gettissue contrast


MRI – Technology

• Extremes in electronics: 400A magnet currents and about 400 kW peak power is used to get μV level signals


Nobel Prize in 2003


Paul C. Lauterburg

• University of Illinois, Urbana, IL, USA

• Discovered in 1971 the possibility to create a two-dimensional picture by introducing gradient in the magnetic field. By analysis of the characteristics of the emitted radio waves, he could determine their origin. This made it possible to build up two-dimensional pictures of structures (H2O vsD2O) that could not be visualized with other methods.


Sir Peter Mansfield

• University of Nottingham, School of Physics and Astronomy, Nottingham, United Kingdom

• Further developed the utilization of fast changing gradients in the magnetic field. He showed how the signals could be mathematically analyzed, which made it possible to develop a useful imaging technique. Mansfield also showed how extremely fast imaging (EPI) could be achievable. This became technically possible within medicine a decade later.


Key benefits of MRI

Safe imaging method with good and selectablecontrast between different tissues

Diffusion imaging

MR Angiography: imaging of arteries to detect abnormalflow, stenosis and aneurysms

Functional imaging


MRI Magnets


MRI scanners

Cylindrical, axial field Open, transverse field


Open-type magnets

Field generators on either side of a gap

Field direction transverse to patient


Cylindrical magnets

GE

Oxford/ Siemens

IGC/ Philips

Magnex


Simple Cylindrical Magnet


Simple Cylindrical MagnetRegion of ±1% deviation from central field

On axis:

B(z)=B0+C2z2

Field is not homogeneous


Sectioned MagnetRemove turns from center of coil


Sectioned Magnet

Windings can be concentrated

Region of ±1% deviation from central field


Sectioned Magnet

Windings can be concentrated

Field stays the same if centers of coils don’t move


Sectioned Magnet

Not good enough for MRI!

± 1 % region

Helmholtz Coil

length ≈ 0.5*diam.

B(z)=B0+C4z4

Hermann von Helmholtz

(1821-1894)


Sectioned Magnet – 3 sections


Sectioned Magnet – 3 sectionsMaxwell Coil

length ≈ 0.76*diam.

B(z)=B0+C6 z6

James Clerk Maxwell

(1831-1879)

12

3

45

6

Still not good enough for MRI.



Oxford Instr. 1977 (Aberdeen)

Bruker, 1979 (Philips)


Sectioned Magnet – 6 sectionslength ≈ 1.3*diam.

B(z)=B0+C12 z12

State of the art MRI-magnet

± 1 % region


Cylindrical MRI Magnet

±1000 ppm



± 100 ppm



± 10 ppm



± 1 ppm

B(z)=B0+C12 z12

large gradients at edge of FOV

Magnetic force isproportional to field gradient... Safety issue?



Full range

FOV boundary gradients are invisible.

Strong gradients are located outside magnet.


External fieldMagnetic force hazard

Strong gradients are located outside magnet

Force on piece of iron:

Fx = mdip dB/dx

Typical gradient 10 T/m

Fmag ≈ 50 Fgravity


External fieldPacemaker hazard

Fields > 0.5 mTreach far outside magnet

B(r) ∼ 1/r3

10% of central field


Active shieldingActive shielding

Coils at larger diameter with opposing current

Shield coil


Active shieldingTypical compact actively shielded magnet configuration

0.5 mT boundary

< 4 m axial< 2.5 m radial

B(r) ∼ 1/r5


Shielded Magnet – external field

Magnetic force hazard

Active Shielding does NOT solve the magnetic force problem!


Magnetic dipole flying into magnetic Field

Potential energy = mdip . Bconverted into kinetic energy

Impact velocity independent of field details

Comparable to drop from high building


MRI Magnet – external field The magnet trap

The stronger the field the deeper the hole


Compact cylindrical magnet technology

3 tesla built 2002

0.5 tesla built 1988


Magnet technology details

Helium tank

Coil

Radiation shield

Vacuum can

Helium


Superconducting magnet technology

4.2 K

Patient Space

First generation compact magnet


Operating conditions of superconductor

• Ample margin needed between actual current and critical current

• More field >>more conductor

Current

B con

duct

or Quench



4.2 K

Patient Space

First generation compact magnet



4.2 K

Patient Space

Thinner structures

Reduced clearance

Increased current density

Stronger magnet in same envelope



NbTi/Cu wire1-2 mm200 A/mm2

4.2 K

Patient Space


High temperature superconductors and MRI

• Problems with HTC superconductors in MRI are– Maximum current density, maximum critical flux density– Limited length of available wires-> At the moment not cost effective

• We have done a feasibility study for a 0.6T HTC iron core MRI magnet– Possible to build– With the extrapolated developments (improvements in current/flux

density, reductions in price) will be viable soon, but ..– Trend is towards higher field strengths, so this design does not

make sense commercially


Ultra-high field magnets

Magnex 7T and 8T


9.5T Whole Body Magnet

• University of Illinois in Chicago• Imaging of Na ions• Requires slow motion in

magnetic field


Cost aspect of Ultra-high field magnets

• Rule of thumb: system cost is about $1 million/T up to 7T systems– 1.5T system cost $1.5M

• The rule applies up to 7T, after that exponential growth of cost• A 7T system

– Has some 420 kilometers of superconducting wire– Weighs about 30 tons– Requires about 440 tons of steel around magnet for stray field

shielding• From 7T to 9.4T add about $8M• Above 10T the cost is about $10M/T, so a 15T system would cost

$150 million• In France plans to build a 11.7T whole body magnet


Better Detectors?

• One option to higher magneticfield strength is to use moresensitive detectors

• Ultimately thermal noise from sample and environment sets the limit for SNR in detection

• Best results with SQUIDS and optical atomic magnetometers, still not competitive with traditional resonant antennas with LNAs

30 mT magnetic field, SQUID detector(Vadim Zotev, LANL,NM)

Optical magnetometer – IR laser, Cesiumfilled cell, detector. John Kitching, NIST


Key properties of cylindrical MRI magnets

• Field generation:• Field shaping:• Stray-field containment:

Coils

Coils

Coils

Enclosed in cryostat with liquid heliumL/D ratio > 1.5Common architecture up to 3 Tesla

Unshielded long coils for 7T and more


Open-type magnets

Technology determined by central field

- low field (<0.4T)

- mid field (0.5 – 0.8T)

- high field (>0.8T)


Low-field open magnets (B<0.4T)

Iron magnetic circuit

Air gap with pole shoes

Pole shoe

D>2 x gapsize(Field shaping)

Yoke

(Return flux)


Low-field open magnets + resistive coils

Drive coil

More field >>

more ampere-turns

heavier yoke


Resistive Magnets with iron flux return yoke and pole shoes, B≈0.2 T

Philips Siemens


Low-field open magnets + permanent magnet

Layer of PM material(Nd B Fe blocks)

More field >>

more PM material

heavier yoke


Example of permanent magnet

Hitachi Aperto

0.4 tesla

Magnet Pole piece

Yoke

Column


Limits of yoke/pole configuration

• Power dissipation of resistive magnet too large• Permanent magnet material cost explodes• Magnetic saturation of pole iron

• For B > 0.5T use superconducting drive coils• Field shaping with iron rings


Mid-field open magnets + superconducting coils

Drive coil(in Cryostat)

More field >>

more ampere-turns

heavier yoke

Rings to profile field in gap


Mid-field open magnet

Philips 0.6 T

Drive coil

Iron ring structure

Iron flux return yoke


Examples of Mid-field open magnets with iron flux return and superconducting coils

Philips 0.6T Panorama

Hitachi 0.7T Airis


Iron-less superconducting high-field open magnets• Weight of yoke becomes prohibitive at >1 tesla

(≈40 tons at ≈ 0.7 tesla).

• Fully superconducting coil system for field generation and field shaping

• External field cancelled by active shield coils


Fully superconducting 1.0T open magnet (Philips/IGC)

Main coil

Shield coil


Fully superconducting 1.0T open magnet (Philips/IGC)

Vacuum tank ofcryostat

Total weight 6 tonszero boil-off


Open magnet summary

CoilsIron yokeCoils

Iron yokeFlux return

CoilsCoils + IronPole pieceField shaping

Supercon coilsSupercon coilsResistive coilNd B Fe

Field generation

B>0.8T0.5<B<0.8 TB<0.4 tesla


Trends in MRI and Magnets

• Going for higher field strengths– Initially 0.5T magnet– In 90’s 1.5T system the majority– Now 3.0T magnets are the

fastest growing sector (1.5T still biggest)

• Now biggest frenzy in short wide bore magnets

• Other fashions:– In 90’s gradient strength and

slew rate– In early 00’s number of receive

channels– Today magnetic field strength

and openness

Siemens Magnetom Espree, 1.5TBore diameter 70cm, length 125cm


MRI Markets and players

• Price about 750 – 2000 k€/system

• About 3000 systems sold annually

– Biggest market share: Siemens

– Traditional number one, GE has been trailing, now about the same with Philips

– All the rest (Hitachi, Toshiba, etc) fairly small


Magnetic fields and safety

• Now about 50 years of research and 500.000.000 patient scans-> No adverse health effects caused by magnetic fields

• Movement in strong static field may induce temporary nausea and dizziness, typically affect only engineers and operators, not patients.

• Biggest concern projectile effects and RF burns


Concerns with legislation in Europe

• Directive 2004/40/EC of the European Parliament, originally to be implemented by 30th April 2008 in member countries.

– Protection of workers from exposure to electromagnetic fields– Exposure limit for low frequency 40 mA/m2(rms)

• Order of Magnitude calculation for a person moving at speed of 1.25 m/s in 1.5T magnetic field: Jmove = 375 mA/m2(peak)

– Similar problem for gradient field and RF fields• Because of fierce opposition from

MR community, implementationin law has now been postponedfor 4 years

• Current exposure limits:– IEC: 4T for patients– ICNIRP: 2T for workers– FDA :7T for patients Simulated RF adsorbtion


Thanks

Thanks toJ. Overweg and M. Savelainen

for simulation/magnet pictures and magnet information

Thank you for your attention

Questions?

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