Beyond the eye

Nuclear Magnetic Resonance and Relaxation
November 7, 1997

Nuclear Magnetic Resonance (NMR) is a subject of major importance in a number of fields of science. Initial discoveries were by physicists such as Rabi, Block and Purcell, but (as the author states in his introduction) these rapidly became the cherished tool of chemists. Additionally, over the past ten years, NMR has been extensively exploited by the medical field for both spectroscopy and imaging. There are many books in the subject area, normally addressed to one particular community of readers. They rarely make easy reading. Revealing remarks by the author about other writers in the field are: "These models were first considered by ... A readable account is given by ...", which I can well believe.

This book is clearly intended for, and readable by, physicists. I suspect it will not interest much the other two groups of chemists and clinicians. As the author says, it is impossible to cover the whole field of NMR in a single text book; indeed the recently published Encyclopedia of NMR took eight large volumes over such an attempt. Brian Cowan states that although one might see this book as no more than another assortment of topics, it is his collection of topics. By and large, he has made a good job of it.

The book is clearly addressed to physicists and has many interesting insights. An obvious problem for any book on the subject of NMR is how to present the mathematics and in particular the quantum mechanical aspects of the subject. Indeed many books skip these aspects completely, just relying on classical derivations of the various results.

While this is usually adequate for many of the users of NMR instruments, it is certainly unsatisfactory for a physicist studying this subject to any depth. On the other hand there are a number of classical works in the field which are essentially impenetrable to all but a very well (mathematically speaking) educated reader. This book is well designed in that all the mathematics is worked out carefully in sensible steps, and, for the target audience, leads gently from the simpler classical approach towards the conceptually more difficult but important quantum-mechanical approach. The author provides a lot of guidance, for example explaining the difference between the Heisenberg and Schrodinger views of quantum mechanics well, plus many useful mathematical digressions. I enjoyed many of his remarks. For example, after deriving a particular result he comments that "The observant reader will notice that this equation is impossible", and then explains why and where the approximations were made that resulted in this paradox. In fact the book is full of the use of the standard physics method of approximation and simplification: take an equation, then expand it, noting that the higher-order terms can be "safely" ignored in order to achieve the desired results. This contrasts very significantly with a comparable book written for chemists.

Cowan notes that, while it would be impossible to manage "a system with 1023 coordinates, one is unlikely to need more than, say, ten variables to describe its observable properties". Thus, much of the book is about methods whereby one can reduce the unsolvable many-body problem to relatively simple statements about overall properties, not the least of which is that of relaxation, or the changes that occur with time after some event.

The book presents continuous and pulsed acquisition methods, and has a good section about the associated electronics. There is however a chapter on NMR imaging methods, which is rather poor, and there is almost no discussion of the various often intricate pulse sequences as used for example in the many clever spectroscopy analysis techniques.

An unusual and interesting section is about the author's speciality - solid helium-3 - which has very odd properties in that it behaves rather like a liquid as a result of quantum mechanics. There is also a section on "Dilute solids in n dimensions", which might intrigue a chemist.

There are a few mistakes such as suggesting that a typical NMR image is of size 64x64. There is also a title presenting "The equality of T2 and T2" (which is perhaps unsurprising), but in general, especially given the extensive mathematics in the book, the proof reading seems to have been very carefully done. There is a good index and a reasonable list of references. More diagrams and illustrations would probably have been helpful. This book is a good read, maybe not as the title suggests, for relaxation but very suitable for its target audience, final year and graduate physicists, for whom it is recommended.

Andrew Todd-Pokropek is professor of medical physics, University College, London.

Nuclear Magnetic Resonance and Relaxation

Author - Brian Cowan
ISBN - 0 521 30393 1
Publisher - Cambridge University Press
Price - £60.00
Pages - 434

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