General methods

Images were acquired in a 2.35 T magnet with a 120 mm clear bore. RF pulses were transmitted by a volume saddle coil and signals were received by a single loop

surface coil, of diameter 3 cm. For in-vivo studies, the surface coil for signal reception

was placed beneath the brain of an animal, as it lay prone in a Perspex holder. The head was held by ear bars to minimise motion and the holder was positioned within the magnet, such that the brain lay at the centre of the magnetic field. Details of anaesthesia

and physiological monitoring are given in the methods section for each in-vivo study.

CASL was implemented with a 3 second, continuous RF pulse for labelling, applied at the same time as a low amplitude gradient along the bore of the magnet. A labelling plane was selected at a position where blood flow through the carotid arteries was predominantly parallel to the labelling gradient, close to the brain. The control pulse for multiple slice experiments was implemented by modulating the amplitude of B] by sin 27ift to produce two inversion planes, proximal to the imaging plane(s)

Chapter 5: Im plem entation o f the A M control for m u ltip le-slice C A S L

The amplitude of the RF pulse, Bi, experienced by a particular spin at time, t, is given by the expression:

Bi = Bio sin (27tft + (])) equation [5.1]

where Biq is the peak amplitude of B]

f is the modulation frequency

([) is the phase of the modulation when the spin passes through the labelling plane (half way between the two inversion planes for the AM control)

CASL images were also acquired using the standard control, i.e. by reversing the frequency offset applied for labelling to select a control plane distal to the imaging slice

{Figure 5.1). It is possible to implement the standard control by reversing the polarity of

the labelling gradient, while maintaining the same frequency offset for the label and the control. This approach is preferable in theory because there is evidence that the resonance of protons bound to macromolecules may be asymmetric (Pekar J et al. 1996; Barbier EL et al. 1999), leading to differences in MT unless the labelling and control pulse are applied at exactly the same frequency offset. However, CASL images that were acquired by reversing the gradient polarity produced artifacts in subtraction images due to translation of the object within the field of view in the control image relative to the label image. This is most likely to be a result of eddy currents produced by the labelling gradient, which affect the imaging sequence. Therefore reversal of the frequency offset of B% was employed for the standard CASL control in all studies reported here.

A post-labelling delay, w, was inserted after the labelling or control period and before image acquisition. During the delay, w, labelled blood in the macrovasculature washes out of the imaging slices, leaving signal only from spins that perfuse the tissue. In addition, Ti relaxation of the tissue restores the magnetisation from Mosat (its equilibrium level when the magnetisation of macromolecular protons is saturated) to Mo (the equilibrium level in the absence of the labelling or control pulse). Values of w were selected according to the objectives of each study. Long post-tagging delays were employed to study perfusion signals and short w to observe the maximum effects of MT. Slices were acquired consecutively, and therefore w was greater in slices that were acquired later.

Chapter 5: Implementation o f the A M control for m ultiple-slice C A SL

A multiple-slice, spin-echo echo planar imaging (EPI) sequence was employed for acquisition of CASL images. The images had a 64 x 128 matrix, TE = 35.2 ms, TR = 4 s (labelling time + 1 s). Up to 7 slices were acquired with slice thickness 2 mm, 2.1 mm separation (centre to centre) and a delay of 78.2 ms between slice acquisitions.

T1 was calculated from inversion recovery images that were acquired at a range

o f inversion times (TI) as part of most in-vivo experiments. Differences in signal

between label and control were generally normalised to the signal intensity in the inversion recovery image with the longest inversion time, to approximate the signal intensity at Mq. If no inversion recovery data were acquired, difference signals were nonnalised to the signal intensity in the control image.

su p erior (n o se ) A standard control — Im age AM control labelling right 2 order of 3 im a g e s for 4 multiple slic e 3 acq u isitio n s inferior (tail) fig u re 5.1

Positions o f labelling and control planes for CASL in rat brain.

Proton spins in arterial blood are labelled by AFP as they pass through the labelling plane at the back of the brain.

For the standard control, a single control plane lies superior to the image, at the same distance from the image as the labelling plane. For the AM control two inversion planes produce a double inversion o f moving protons, one inversion on either side of the labelling plane.

Chapter 5: Im plem entation o f the A M control for m u ltip le-slice C A SL