Imaging scanners that

detect s igna ls f rom magnet ic

nuc le i in l iv ing t issues are

revolut ion is ing b iomedica l

knowledge and opening the

way to ear l ier d iagnosis and

safer, less invas ive surger y

VISIONS18B R I E F I N G P A P E R S F O R

P O L I C Y M A K E R S

M a g n e t i cr e s o n a n c ei m a g i n g

any people today will havehad a magnetic resonance

imaging (MRI) scan to diagnosea medical condition. In the past 20 years, MRI has evolvedfrom an ingenious idea into aroutine method for looking atstructures inside the body. Asinstrumentation and methods of acquiring the image continueto improve, new remarkableapplications are still emerging,making MRI one of the mostsignificant technologies forimproving human health.

The development of MRI is an extraordinary story in that itemerged in the field of esotericnuclear physics – clearlyillustrating the benefits offundamental research toeveryday life. Seventy years ago,physicists explored how atomicnuclei with magnetic moments,when aligned by an externalmagnetic field, absorbed pulsesof radio waves. The nuclei twist

out of alignment at particular radiofrequencies (theresonant frequency),and then re-emit theenergy as they 'relax'back to their originalstate. The signaldetected depends notonly on the type of

nucleus and magnetic fieldstrength but also on its physicaland chemical environment – forexample, the molecular structurein which the host atom sits. Anymolecule containing magneticnuclei, such as those of hydrogen(single protons), thereforeproduces a characteristicspectrum of nuclear magneticresonance (NMR) frequencieswhen subjected to a magneticfield and a radio-frequency (RF)pulse; NMR spectroscopy isnow one of the most importantways of analysing chemicalstructure, particularly of organicand bio-molecules.

Physicists also noted that thewater (which contains two singleprotons in hydrogen atoms) inbiological tissues gave an NMRsignal that relates to itsdistribution and environment.This suggested that livingstructures such as blood vesselsand muscles could be imaged byscanning the water contentacross an organ. In the 1970s,the American scientist PaulLauterbur and the Nottingham-based physicist Peter Mansfield(now Sir Peter) thought of usingadditional field gradients indifferent directions to distinguishthe spatial position of nuclei. Thesignal obtained could be broken

down into its basic componentsusing complex mathematicalmethods and then processed to create a three-dimensionalimage.The first coarse imagestook a long time to acquire, butgradually more rapid methods of imaging were proposed, andnow they can be obtained in lessthan a second. Lauterbur andMansfield are among severalscientists who have won NobelPrizes for breakthroughs in thefield of NMR.

By the 1980s, instrumentsthat could image inside a patientwere being built for clinical use,and in this context the name ofthe technique was changed tothe more reassuring 'magneticresonance imaging'. Today, thereare about 20,000 MRI scannersworldwide, with more than 70million scans performed everyyear. Typically, they employ apowerful magnet with a fieldstrength of about 1.5 tesla(20,000 times the Earth'smagnetic field). The patient liesin a channel along the centralaxis of the magnet. Unlike CATscans, the procedure producesno harmful side-effects, and canimage soft tissues that aretransparent to X-rays. Theimages are three-dimensional,

Images of theheart taken in

different planes

A typical MRI scanner

MRI pioneer SirPeter Mansfieldof the Universityof Nottingham

A p o w e r f u l i m a g i n g t o o l f o r m e d i c i n e

THANKS GO TO PENNY GOWLAND AND LUCA MARCIANI OF THE UNIVERSITY OF NOTTINGHAM, STEPHEN KEEVIL OF G

M

UY'S CAMPUS, KING'S COLLEGE LONDON AND DONALD MCROBBIE OF CHARING CROSS HOSPITAL FOR HELP WITH THIS PAPER

and the latest MR systems offer resolutions down to thesubmillimetre-scale.

The technique has benefitedenormously from collaborationsbetween physicists andclinicians. For example, doctorscan see more if the image'contrast' is improved. This canbe done by manipulating the RFpulses and magnetic fields so as to spread out the time takenfor nuclei in different tissues torelax back to their initial state. Or chemical contrast agents may be injected to highlighttarget structures. Another new possibility is to introduce MR-sensitive molecules thatbind to specific receptors in the target tissue.

What MRI can do

MRI distinguishes betweennormal and diseased tissuethrough differences in watercontent and how it is boundchemically. It is particularlyeffective for diagnosing tumoursand other abnormalities in thebrain. The technique can alsoimage blood flow in arteries andthrough the heart, as well asmuscle and bone injuries.

Applications continue toadvance rapidly, and UK researchgroups at many of the majorhospitals are at the forefront ofdevelopments. One growingarea is to use MRI to studyphysiological changes in realtime. For example, scientists at Nottingham are trying tounderstand appetite by followingwhat happens to fatty food inthe stomach, and also bymonitoring how the brainresponds to taste. They havealso studied the brain activity ofthe unborn child in response tovarious stimuli.

Another approach is to look at chemical changes in brainmetabolism directly using

spectroscopic signals frommolecules of interest – forexample, glucose labelled withcarbon-13 (which has a magneticmoment). Signals from otherelements such as sodium couldalso be used to followneurological changes. Heliumgas, prepared in a highlypolarised (aligned) state, can beemployed to image lungs, whichcontain little water.

The area that most excitesresearchers is the application ofMRI in surgery. This is a newfield but it is developing fast. At the moment, several Londonhospitals are employing MRI asa planning tool, to aid, forexample, breast and prostatebiopsies. A team at Guy'sHospital is pioneering MRI incardiac intervention surgery.Although X-rays are used toguide a catheter into the heart,say, to insert a device to seal adefect, MRI is able to monitorthe success of the procedure by providing a detailed image ofthe heart before and after theintervention.

In the case of childrenneeding several operations, theaccumulated X-ray dose maysignificantly increase the risk ofcancer. So the eventual aim is torely solely on MRI to guidesurgery, particularly on the heart

and brain. However, theequipment is expensive and theapproach is currently limited bythe fact that metal instrumentscannot be used (as their intrinsicmagnetism distorts the image);MRI-compatible surgicalimplements still need to bedeveloped. MRI is also very noisy.

Technological developmentsthat will enhance MRI are comingalong, including smaller and moreopen scanners. New methods ofcollecting data using arrays of RFdetectors are being developed,as are computational techniquesthat select only the data ofinterest. These developmentswill improve patient comfort as well as speed up image-acquisition and improveresolution. We can look forwardto an exciting future for MRI.

S O M E M R I A P P L I C AT I O N S Diagnosis of:TumoursMultiple sclerosisStrokesParkinson's diseaseSchizophreniaCoronary heart diseaseCongenital heart problems Torn ligaments and slipped discsFoetal development

In surgery to: Plan surgical proceduresGuide and monitor operations

Medical and biochemical studies of:Physiological processesBrain activity and cognitive neuroscienceBody biochemistryDrug metabolism

MRI scans showingchanges in blood flow inthe brain due to a stroke

The abdominalcavity and digestivesystem as seen witha 1.5-tesla scanner

Front cover:

A high resolution image of the brain taken with

Nottingham's new 7-tesla'Ultra High Field Magnetic

Resonance Facility'

Inset:

The world's first MRimages of catheter

manipulation during aheart operation. They

were taken at a rate of 10 images per second

Below left:

MR images of lungs usinghyperpolarised helium-3

Credits:

Brain, abdominal and lung images supplied bythe Sir Peter Mansfield

Resonance Centre,University of Nottingham.Cardiac images are from

Guy's Hospital, King'sCollege London.

MR images oflungs usinghyperpolarisedhelium-3

Visions is a ser ies of papers which

highl ight exci t ing new areas of research

in physics, and their theoret ical and

technological impl icat ions.

AV A I L A B L E V I S I O N PA P E R S :

1 H i g h i n t e n s i t y l a s e r s2 Q u a n t u m i n f o r m a t i o n3 E x o t i c n u c l e a r b e a m s4 P h y s i c s a n d f i n a n c e5 S p i n t r o n i c s6 T h e L a r g e H a d r o n C o l l i d e r7 Pa r t i c l e a c c e l e r a t o r s – t h e n e x t f r o n t i e r8 F l a t s c r e e n d i s p l a y s9 S u p e r c o n d u c t i v i t y

10 G r a v i t y w a v e s11 E - s c i e n c e12 P h o t o n i c s13 M e g a - T e l e s c o p e s14 T e c h n o l o g i c a l p l a s m a s15 S e e i n g w i t h n e u t r o n s16 B o s e - E i n s t e i n c o n d e n s a t e s17 F r e e e l e c t r o n l a s e r s

A B O U T T H E I N S T I T U T E O F P H Y S I C S

The Institute of Physics is an international learned society and

professional body for physicists. The Institute has more than 35,000

individual members.

F O R F U R T H E R I N F O R M AT I O N C O N TA C T :

Department of Higher Education and Research

The Institute of Physics

76 Portland Place, London W1B 1NT, UK

e-mail: tajinder.panesor@iop.org

Institute website: http://www.iop.org

Further information on MRI can be found on the following websites:

http://electronics.howstuffworks.com/mri.htm

http://en.wikipedia.org/wiki/Magnetic_resonance_imaging

www.magres.nottingham.ac.uk