Neuroradiology 2000) 42: 852±855 Ó Springer-Verlag 2000 HISTORICAL NOTE
N.Tubridy Neuroradiological history: Sir Joseph C.S.McKinstry Larmor and the basis of MRI physics
Abstract The origin of many of the Key words Larmor, Joseph × Received: 23 July 1999 Accepted: 14 February 2000 eponymous terms in modern medi- Magnetic resonance imaging × cine is unknown to many people Neurology history who utter them daily. As a contribu- tion to understanding the historical N.Tubridy )) NMR Unit, Institute of Neurology, backgroundof MRI, we providea Queen Square, London WC1N 3BG, UK brief account of the life andwork of e-mail: [email protected] Joseph Larmor, the Irish scientist, Tel.: + 44-171-8373611 whose name is frequently usedby Fax: + 44-171-278 5616 chemists, physicists andradiologists alike. C.S.McKinstry Department of Neurology, Royal Victoria Hospital, Grosvenor Road, Belfast BT12 6BA, UK
Joseph Larmor was born in Magheragall in Co. Antrim 2 years before Einstein publishedhis papers on light on 11 July 1857 Fig. 1), andwas one of the last of a quantum andrelativity. group of nineteenth century `Scoto-Irish' physicists In 1909 Joseph Larmor was knightedandwas the whose work influencedheavily the experiments and Unionist Member of Parliament for Cambridge Uni- discoveries of the early 20th century. He attended school versity from 1911 to 1922. He hadintense feelings over at the Royal Belfast Academical Institution, and was ªthe Irish questionº andin his maidenspeech in 1912 he brought up in Belfast by his uncle Fig. 2). He was a thin, defended the Unionist position in the debate on Irish delicate, black-haired boy of precocious ability in both home rule. He recalledhow he was involvedin the de- mathematics andclassics. He obtaineda doublefirst in feat of the alternative vote, which he claimedto have the scholarship examination at Queen's College Belfast, secured by a long speech, leading the bewildered House andultimately graduatedwith the highest honours. At deeper and deeper into mathematics until the Whip Queen's College he obtaineda BA andan MA. gave him the signal that the wantedabsentees hadar- He went to St.John's College in Cambridge; a severe rived[1]. However, his major concern was for education illness forcedhim to lose a year but he became senior andthe universities. He hada strong attachment to his wrangler andwon Smith's Prize. He took the mathe- native country, andgenerally spent part of his summer matical tripos in 1880, securing first place in the list, and vacation in Ireland. He never married, and retired aged was electeda Fellow of the College the same year. He 75 years in 1932. His last years were spent at Holywood, was appointedProfessor of Natural Philosophy in Co. Down until his death on 19 May 1942 Fig.4). Queen's College, Galway, where he workedfrom 1880 Larmor's scientific work was wide-ranging. By 1892, to 1885 Fig.3). In 1885, he returnedto St.John's as a he hadpublishedsome 30 papers on a variety of subjects lecturer and, on the death of Sir George Stoke, Larmor in appliedmathematics, andhe was electedto the Royal was made the Lucasian professor at Cambridge in 1903. Society of London that year. Between 1894 and 1897 he At this time, the atom was not yet understood and it was published, in the Philosophical Transactions, his great 853
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Fig.1 The house in which Joseph Larmor was born in Magher- the heavens were composedof aether also referredto agall, County Antrim, Northern Ireland as quintessence), andthis was postulatedto explain the Fig.2 The house in Antrim Road, Belfast, where Larmor was workings of the universe which surrounded the earth; it brought up by his uncle now a solicitor's office) was supposed to be a sort of medium inside which the Fig.3 Joseph Larmor reprintedwith courtesy of the National universe existed. The nature of aether was, however, University Hospital, Galway) unclear but it was generally agreedthat it existed.Ae- ther was believed to be divided into spheres, naturally perfect, rotating without friction. The planets were be- memoir `A dynamical theory of the electric and lumini- lievedto be regions of high densitywithin this trans- ferous medium' in three parts. The theory was re- parent) aether. The only changes in these general beliefs submitteda year later as an essay on the theory of the from the time of Aristotle to medieval times were in the aberration of light andwon the Adams prize in the character of aether and the addition of a quantitative University of Cambridge. The essay was published as a approach to its analysis. book ± Aether and matter ± in 1900. His writing style has By Larmor's time, aether was the mechanism usedto been described as `difficult' and `lacking lucidity' [1] and explain how light waves travelled, and could be con- his lectures as ill-ordered and obscure. He was decidedly sidered in the same way as water, which also provides a conservative in his scientific views anda difficult col- medium for waves to travel. Larmor could not discover league on syndicates and in examining dissertations. His direct evidence of the motion of the earth through ae- fellow workers described him as ªdetermined but not ther but he did provide a partial explanation, which led persuasiveº, andin the wordsof D'Arcy Thompson, to the theories that were part of Albert Einstein's The- ªLarmor made few friends, perhaps; but while he lived, ory of Relativity [1]. andthey lived,he lost none.º [2] Larmor also made major contributions to electro- Throughout Larmor's work runs an underlying dynamics and electron theory. He proposed that the theme: the principle of least action, or Hamilton's Prin- electrical charge of an electron might be a measure of its ciple. This states that the motion of a particle over short mass andelectrons are now measuredin electron volts intervals will minimise that action integral. William eV). He effecteda new concept calledthe electron Rowan Hamilton 1805±1865) was Ireland's most fa- theory of matter anddevelopedits radicalimplication mous 19th century mathematician andHamilton qua- regarding the nature of mass, the structure of the atom ternions sets of vectors involving imaginary numbers), andits relation to radiation, the kinetic theory of heat are regularly usedin computer graphics andspace-craft equipartition), andthe electrodynamics of moving guidance systems today. A child prodigy, Hamilton bodies. He also contributed to the progress of what is spoke 13 different languages by the age of 13 years [3]. now calledthe Lorentz transformation. This transfor- In the latter half of the 19th century, the existence of mation, namedafter the Dutch physicist HendrikAn- the aether was still acceptedandideasabout such things toon Lorentz, was first written down in 1887 by Voigt, as electrons were not common; there was little under- andin 1889 another Irish physicist ± George Fitzgerald standing of matter and energy. Aristotle explained that from Monkstown in Dublin ± hadpublishedsimilar 854
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Fig.4 Sir Joseph Larmor's gravestone at, HolywoodCounty the Larmor precession, the Larmor frequency, andLar- Down mor's theorem. Fig.5 Sir Joseph Larmor courtesy of The Royal Society of Med- Larmor's precession explains the triplication and icine) polarisation of spectral lines which was discovered by Fig.6 Sir Larmor at the unveiling of LordKelvin's statue in 1913 Pietar Zeeman with relatedwork by H.H.Lorentz) when a strong magnetic fieldwas appliedto sodium. Zeeman later won the Nobel Prize) The Larmor fre- quency is given by the equation: Omega = e/2m). B ideas in a paper in Science entitled`The ether andthe where Omega = the angular momentum of an electron; earth's atmosphere'. e = electron; B = magnetic fieldstrength). This theory Larmor was secretary to the Royal Society from holds that, with several orbiting electrons, some orbits 1901 to 1912 andwas the Royal Society's Bakerian wouldbe acceleratedandsome sloweddown,while Lecturer in 1909. In 1915 he was awarded the Royal some wouldremain unaffectedby a magnetic field,so Medal and in 1921 the Copley Medal. He was elected to that there wouldbe a nett effect of zero or a very small the London Mathematical Society in 1884 and held amount). However, spectral lines wouldbe separate for several offices: council member 1887±1912; vice-pre- different speeds. Larmor postulated that electrons orb- sident1890 and1891; treasurer 1892±1914; andpre- itedaroundsome centre. sident 1914±1916. He received the De Morgan Medal of The Larmor theorem was an extension of the above the London Mathematical Society in 1914. He received andstates that: Omega = e/2m). B, i.e., that for any many honorary degrees from universities in Britain, in- chargedparticle with the same e/m ratio, subject to cluding Glasgow, Aberdeen, Birmingham, St.Andrews, electric andmagnetic fields, the effect of the magnetic Edinburgh and Durham, as well as from Academies in fieldcan be negatedby a transformation of the frame of Washington, Boston, Philadelphia, Rome, Bologna and reference with this formula. Larmor's formula described Paris. He also receivedthe freedomof the city of Bel- ways to calculate the power P) radiated by an elec- fast [4]. tron's acceleration in terms of charge e), speedof light Larmor's proposal of an electrical theory of matter c), andthe acceleration a): P = 2/3 e 2 × a2 × c3.Thisis was overshadowed by Einstein's papers on relativity still applicable at speeds of less than close to the speed andthe development of quantum theory. He is re- of light but is no longer vieweduseful in terms of re- memberedprincipally as the first person to give the lativity. formula for the radiation of energy from an accelerated In the fieldof magnetic resonance physics Larmor is electron andhe was also the first to explain the effect of best known for his equation describing the effect of an a magnetic fieldin splitting the spectrum into multiple external magnetic fieldon the precession frequency of lines. Larmor was one of the last physicists to really be- protons placedin that field:Omega wo)=g × Bo where lieve in strict Newtonian mechanics, as physics was wo = the precession frequency of the spinning protons in being taken over aroundthis time by the theories of re- the magnetic field; g = the gyromagnetic ratio; and lativity andquantum mechanics. His name is given to Bo = the strength of the external magnetic field in Te- 855 sla)). The equation states that the precession frequency Four years after Larmor's death in 1942, in the USA, of a spinning proton becomes higher when the magnetic Bloch andPurcell publishedthe results of their work on fieldstrength in which it is placedincreases andit is the magnetic resonance which eventually leadto the devel- basic physical equation underling magnetic resonance opment of MRI as we know it today. imaging. Larmor rejectedEinstein's work, andin the 1920s his Acknowledgements We wouldlike to thank Mary Sampson, Ar- own work, andthat of his studentscame to a close chivist, Royal Society, London; Marie Bohan at the National Uni- versity Hospital, Galway; Janice Goldblum, Archivist at the Na- Fig. 5). He finishededitingwritings of George Stokes tional Academy of Sciences; Dr. Elizabeth Leedham-Green, Dep- andLordKelvin Fig.6) andrevisedsome of the work uty Keeper, Cambridge University Archives; Jennifer McKee at of Henry Cavendish. He also wrote substantial obitu- Queen's University Belfast; andGarrett Tubridyfor work in Tri- aries of Josiah Gibbs, George Stokes andLordKelvin nity College Dublin. [5].
References 1. Eddington AS 1942) Sir Joseph Larmor 2. Thompson DW 1943) Sir Joseph Lar- 4. McKinstry CS, Conn S 1995) Who was 1857±1942). In: Obituitary Notices of mor: a great Irishman. In: Kendall J ed) Larmor? abstract) Presentedat Roent- Fellows of the Royal Society of London, Yearbook of the Royal Society of Edin- gen Centenary Congress, Birmingham, vol IV, 11 1942±1943). Morrison and burgh, vol II 1941±1942). Oliver and p 1145 Gibb, London, Edinburgh, pp 97±207 Boyd, Edinburgh, London, pp 11±13 5. Boylan H 1988) Joseph Larmor. In: 3. Woodruff AE 1973) Joseph Larmor. In: Boylan H ed) A Dictionary of Irish Gillispie CC ed) Dictionary of Scientific Biography. Gill andMacMillan, Dublin, Biography, vol VIII. Scribner, New York, pp 193±194 pp 39±41