ELSEVIER l Original Contribution Magnetic Resonance Imaging, Vol. 14, No. 4, pp. 403-411, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved 0730-725X/96 $15.00 + .OO 0730-725X( 96) 00019-g AN INTERLEAVED SEQUENCE FOR ACCURATE AND REPRODUCIBLE CLINICAL MEASUREMENT OF MAGNETIZATION TRANSFER RATIO G.J. BARKER, P.S. TOFTS, AND A. GASS Institute of Neurology, London, UK We demonstrate an interleaved dual spin echo-based sequence for quantitative measurement of Magnetisa- tion Transfer Ratio (MTR) in a clinical environment that overcomes the problems of patient motion between scans faced by noninterleaved methods. The sequence also provides proton density and Tz-weighted images, allowing direct comparison among the three contrast regimes. Phantom studies and in vivo measurements on normal controls show the sequence to be robust in normal use. The values of MTR calculated from the sequence are shown to be precise and reproducible enough to allow regional variations to be identified within and between white matter and other brain tissues. Keywords: Magnetization transfer ; Spin echo; Interleaving. INTRODUCTION The dependence of MR (Magnetic Resonance) image intensity on the proton density, T1 and T2 of the object being imaged is well known; pulse sequences that ex- ploit differences in these parameters between tissue types (and between normal and abnormal tissue) are the basis of most current clinical MR imaging proto- cols. Other mechanisms for manipulating contrast are available, however, and recently magnetization trans- fer (MT) imaging has become a focus of interest for many sites. This technique depends on the existence, in many tissues, of two ‘pools’ of protons: a ‘free’ pool (which consists of mobile protons such as those in water), and a ‘bound’ pool, which consists of the less mobile protons in proteins and other large macro- molecules. The bound pool does not usually contribute directly to the MR signal, but can influence the free pool’s relaxation properties. Applying RF (Radio Fre- quency) pulses at a suitable off-resonance frequency can partially saturate the bound pool (without directly affecting the free pool) and the subsequent exchange of magnetisation results both in a decrease in the T1 of the free pool and a reduction of its equilibrium magnetisation. The latter typically predominates, lead- ing to a reduction in the MR visible signal. Because the magnitude of the effect depends on the presence (and size) of the bound pool, the change is tissue specific, allowing differentiation of tissues by the de- gree and complexity of their macromolecular structure. The theory of magnetisation transfer has been de- scribed in detail by several authors l-5 (and references therein). Several pulse sequences have been described based on either direct off-resonance saturation,6-s or the use of binomial pulses; 9X1o the clinical implementa- tion and optimisation of a typical sequence have been discussed by Hajnal et al.” Applications of MT have been reported in diverse anatomical regions ranging from the CNS to the heart, with particular interest in angiography and contrast enhanced studies. For a re- view of many of these areas see Balaban and Ceckler.” Most of these studies have been qualitative rather than quantitative, with MT being applied to suppress un- wanted tissue or to increase contrast between patho- logic and normal tissue. Quantitative results are usually presented as the per- centage difference between two sets of measurements, one collected with MT saturation pulses applied, the other without MT. If such results are to be reliable, then it is vital that the two image sets from which they are derived are accurately registered with each other. For in vitro studies this poses few problems, and in RECEIVED 8128195; ACCEPTED l/28/96. rology, Queen Square, London WClN 3BG, UK. Address correspondence to G.J. Barker, Institute of Neu- 403