Foreword

Paul Tofts has succeeded brilliantly in capturing the essence of what needs to become the future of radiology in particular, and medicine in general - quantitative measurements of disease. This is a critical notion. The discipline of radiology started with the ability to discern shadows that were abnormal. On chest x-rays one could see the 'white in the right' and that was correlated to the clinical diagnosis of pneumonia. It is truly amazing how long such descriptions were adequate and indeed,...

Relaxation

The absorption of energy by the system makes it unstable, and it immediately starts to move, or relax, back to a state of equilibrium. The absorbed energy is dispersed by a number of processes, known collectively as relaxation mechanisms. These mechanisms can be grouped into those that transfer energy away from the spins to 'the lattice',1 and those that redistribute it within the spin system (Figure 6.7) (Goldman, 1988 Harris, 1986 Gadian, 1982 Abragam, 1961 Slichter, 1963 Callaghan, 1991...

Exchange Of Magnetization

The observed relaxation times are affected by a number of variables including viscosity, temperature, molecular weight and the presence of dissolved ions and molecules. Water is a small molecule that tumbles rapidly in the liquid state at room temperature, with a correlation time of about 10-12 s relaxation times resulting in long (T2 > 1s and T1 > 4 s)1, for reasons as discussed in Section 6.3.3. However the water in biological tissue is in close contact with a variety of different...

Info

Alternatively, it may be assumed that a continuous distribution of relaxation times exists (Larsson et al., 1998 Fenrich et al., 2001 Whittal et al., 1997 Graham et al., 1996 Lawson and Hanson, 1974 Whittal and Mackay, 1989 Whittal et al, 1991) so that where A(T2) is the amplitude of the relaxing component at transverse relaxation time T2. Whittal suggests the use of the nonnegative least square method (Lawson and Hanson, 1974) to solve Equation (6.24), which can be...

References

Anders KH, Guerra WF, Tomiyasu U, Verity MA and Vinters HV. The neuropathology of AIDS. UCLA experience and review. Am. J. Pathol. 124 537-558, 1986. Anderson C and Jensen FT. Precision, accuracy and image plane uniformity in NMR relaxation time imaging on a 1.5 T whole-body MR imaging system. Magn. Reson. Imag. 12 772-784, 1994. Armspach J, Gounot D, Rumbach L and Chambron J. In vivo determination of multiexponential T2 relaxation times in the brains of patients with multiple sclerosis. Magn....

Measurement Science and MRI Come Together

Measurement science has been around a long time MRI2 has been around for about 20 years. This book is about the blending of the two paradigms. We have come to expect to be able to measure certain quantities with great accuracy, precision and convenience. Instruments for mass, length and 1 Sections of this chapter were reviewed by Jeffrey T. Yap PhD, Associate Professor, University of Tenessee Medical Center, Knoxville, TN, USA. time are all conveniently available, and we expect the results to...

Measuring PC or PD

Measuring PC or PD is usually carried out using a sequence with minimal T1 or T2 weighting the residual weighting is removed using an analytic model for the signal intensity. The signal intensity of a spin echo sequence is usually modelled by where g is the signal from tissue with unit PD in the absence of T1 or T2 losses (i.e. long repetition time, TR, short echo time, TE) g can be thought of as the gain of the receiver system. It defines the relationship between PD and the resulting signal,...

T

T1 -losses in a gradient echo or spin echo sequence. For a PD sequence, a combination of TR and FA can be chosen to give T -losses less than a prescribed amount, for particular tissues. Thus normal white matter (Ti 600 ms), imaged with a sequence with TR 1000 ms (i.e. T1 0.6 TR), needs a FA of no more than 17 for the T1 losses not to exceed 1 . At this TR and FA, a lesion with T1 2 s would suffer 6 signal loss. The signal losses were calculated using the equation S S0 1 exp(-TR T1)...

Psychometric Measures Sensitivity Validity and Reliability

From a clinical point of view, a potential new quantity to characterize brain tissue can be evaluated by considering Sensitivity - does the quantity alter with disease Is the false negative rate low Validity - is it relevant to the biological changes that are taking place Reliability - is it reproducible Is the false positive rate low Thus the concept of validity (which is absent from a judgement based merely on accuracy and precision) enables the relevance of a metric to be considered. For...

Uncertainty in Measurement Type A and Type B Errors

The scientific measurement community has moved to refine the traditional concepts of random and systematic error, and instead uses a different (though closely related) method of specifying errors.8 Type A errors are those estimated by 8 From Estimating Uncertainties in Testing, Measurement Good Practice Guide no. 36, by Keith Birch, of the British Measurement and Testing Association, on behalf of the National Physical Laboratory, UK. NIST technical note TN1297, Guidelines for Evaluating and...

Water Content

Pathologists and surgeons have measured water content for many decades, using biopsy or postmortem tissue. Although water content is intrinsically different from proton density, its measurement gives some confirmation that changes in PD measured in disease are approximately correct, and values are remarkably similar, particularly in white matter (see Tables 4.1 and 4.2). Nonetheless, there are two principle reasons why the value of water content can, in principle, differ from free PD (i.e. PD...

Valerie L Stevenson

Consultant Neurologist, The National Hospital for Neurology and Neurosurgery, Queen Square, London, WC1N 3BG, UK 5.2 Physical Principles 113 5.3 Biological Basis of 71 113 5.5 General Methods for Measuring T1 and Potential Pitfalls 116 5.6 Review of T1 Measurement Sequences 123 5.7 Clinical Applications of T1 Measurement 131

Contributors

Barker Philip A. Boulby Bruno Brochet Wellcome Department of Imaging Science, Institute of Neurology, University College London, Queen Square, London, WC1N 3BG, UK NMR Research Unit, Department of Neuroinflammation, Institute of Neurology, University College London, Queen Square, London, WC1N 3BG, UK MRI Unit, National Society for Epilepsy, Chalfont Centre for Epilepsy, Chalfont St Peter, Gerrard's Cross, Buckinghamshire, SL9 0RJ, UK EA 2966 (Neurobiology of Myelin...

Rephasing The Signal

Transverse magnetization is generated by the application of a radio frequency pulse in the presence of longitudinal magnetization. If a sample is completely relaxed, a maximum signal will be generated when a 90 pulse is applied. At this point, the sample is considered saturated, because the two energy levels are equally populated. It would appear that, once the transverse magnetization has dephased, the only way to generate another FID is to wait until the longitudinal magnetization has...

Temperature Dependence

The temperature dependence of phantom properties can be a major limitation to their usefulness. Typical changes in T1, T2 and D are 1-3 C. The Eurospin gels have a T1 temperature coefficient of +2.6 C.13 The best temperature performance T1 and T2 phantoms are obtained using Ni or a Gd polymer pair in agarose. Ni has a minimum in its relaxation rate, fortunately at room temperature (Kraft et al., 1987), allowing a brain-equivalent Ni-DTPA agarose gel to have a flat temperature response (Figure...

Subject Positioning and the Prescan Procedure

The subject is positioned on the scanner couch by the radiographer (technologist). The subject should be comfortable, to reduce movement during the scan as much as possible. The radiographer should use their insight into the subject's emotional state to reduce anxiety if necessary preparation on a separate couch may be helpful. A cushion * The statistical material was reviewed by Dan R. Altmann. under the knees can reduce cramp. Occasionally it is desirable to place the subject prone. Prone...

Withinsubject Standard Deviation and Repeatability

The simplest and most useful approach to characterizing measurement error (plus any biological variation over time) is that of Bland and Alt-man, which uses pairs of repeated measurements in a range of subjects (Bland and Altman, 1986, 1996a Wei et al, 2002 Galbraith et al, 2002 Pad-hani et al., 2002). For repeated measurements on the same subject (who is assumed to be unchanging during this process), the measurement values are assumed to be samples from a normal distribution with standard...

Does Free PD Equal Water Content

The effect of the two factors discussed above seems to be small in practice, and the normal values for water content and PD are very close, at least in white matter (Tables 4.1 and 4.2). Two of the PD values should probably be raised by about 3 pu to account for the temperature of the concentration standard. Thus the mean PD value is about 73 pu in white matter. The water content values have to be multiplied by the specific gravity to give comparable volume concentrations, which average 72.5...

Limits to Progress

It may appear that qMR research proceeds under its own impetus. However the current state and rate of progress in developing reliable qMR methodology 7 However, pathologists do report cellular abnormalities in normal-appearing white matter in multiple sclerosis. are determined by several factors MRI manufacturers, research institutions, pharmaceutical companies, computer technology and publicly funded research councils. MRI machine manufacturers (vendors) will take on some of the measurement...

Multiexponential T2 Behaviour

The proton signal will be partly attenuated by T2 losses, depending on the minimum TE used, the interecho interval and T2. Determining a mono-exponential T2 and then correcting the shortest echo time dataset with this T2 value will partly correct the attenuation. Fitting a multiexponential model will recover the myelin water signal (T2 10 - 50 ms) more fully. However in the absence of full knowledge of the T2 values and T2 distribution for the myelin water, we must assume that the PD estimate...

Introduction

Proton density PD refers to the concentration of protons in tissue, protons being the hydrogen nuclei that resonate and give rise to the NMR signal. Since most visible tissue protons are resident in water, it is often seen as a short-hand way of looking at water content, although the proton 1 Reviewed by Dr Alex MacKay Professor in the Departments of Radiology and of Physics and Astronomy, University of British Columbia, Vancouver, Canada. concentration in lipid, where present, is also high. PD...