effect_of_long_component_in_SS

effect_of_long_component_in_SS - Phys. Med. Bioi., 1987,...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Phys. Med. Bioi., 1987, Vol. 32, No 10, 1345-1353. Printed in the UK Scientijic note A SAFE (saturate after the echo) sequence to eliminate an artefact in measurement of T2 in partially saturated NMR systems Paul S Tofts and G Johnson Institute of Neurology, Queen Square, London WClN 3BG, Received 18 December 1986, in final form 14 April 1987 1. Introduction 'H relaxation times tissues are influenced by the physicochemical environment of the protons. There is, therefore, potential for tissue characterisation times. However, most pathological show elevated (Beal et al 1984) simple analyses which assume mono-exponential decay are often unable to demonstrate any differences between various pathological tissues. Many tissues, however, display multi-exponential T2 (Barnes et a1 1986), accurate determination of multi-exponential behaviours may give the specificity necessary for characterisation. To study multi-exponential transverse magnetisation decay with the optimum sensi- tivity, it is important to follow the decay up to long echo times TE, until the signal has disappeared into noise (i.e. TE= 5TJ. The naive equation often used (Mills et 1984) to describe image intensity in a single-echo experiment is (for a single T2 component perfect radiofrequency ( RF) pulses) IN = GN exp( - TE/ T2)[ 1 - - TR/ T1)] (1) where G is a constant describing instrument gain, N is the spin density, TE is the echo time TR is the repetition time. The slope of ln(Z,) against TE gives T2. However, fuller analysis reveals that the exact value of the signal intensity is (Young et al 1982) ~E=GN~X~(-TE/T~){~-~~~~[-(TR-TE/~)/T']+~~~(-TR/ T (2) in which case the slope of h(IE) against TE is dependent on Tl , T2, TE and TR, and T2 cannot be determined without making a separate determination of and recourse computer fitting techniques. could be left as a free parameter the fit, but this increase number of degrees of freedom would reduce precision with which T2 couldbedetermined.Measurements of T2 are usually made usingequation (1). The relative error in intensity, eI, caused by assuming data follow when in fact they are described by (2), is given by 0031-9155/87/101345+09$02.50 @ 1987 IOP Publishing Ltd 1345
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
1346 P S Tofts and G Johnson is always negative (i.e. real data lie below the line indicated by (1)). The exact equation(2)doesreducetothe naive equation (1) (i.e. E,+O) underthe following conditions: TEcc TI i.e. short echo time or TR >> i.e. fully relaxed. Since TE may be as long 5 T2, and T2 may be as high 0.7T1 for oedema in biological systems at 0.5 T, then condition (4a) may not be met, in which case is only correct under the fully relaxed condition (4b). We investigated this artefact further, and have produced a modified spin-echo pulse sequence (SAFE-saturate after the echo) which eliminates the artefact. It retains simplicity of equation (l), whilst working under almost any conditions of TR. The behaviour of the sequence has been verified experimentally.
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 02/27/2008 for the course EE 591 taught by Professor Nayak during the Fall '07 term at USC.

Page1 / 9

effect_of_long_component_in_SS - Phys. Med. Bioi., 1987,...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online