European Journal of Radiology 82 (2013) 719–727 Contents lists available at SciVerse ScienceDirect European Journal of Radiology jo ur n al hom epage: www.elsevier.com/locate/ejrad Optimisation of T ∗ 2 -weighted MRI for the detection of small veins in multiple sclerosis at 3 T and 7 T Jennifer Elizabeth Dixon a,1 , Ashley Simpson b,2 , Niraj Mistry b,2 , Nikos Evangelou b,2 , Peter Gordon Morris a,∗ a Sir Peter Mansfield Magnetic Resonance Centre, University of Nottingham, Nottingham, NG7 2RD, UK b Academic Division of Clinical Neurology, University of Nottingham, Queen’s Medical Centre, Nottingham, NG7 2UH, UK a r t i c l e i n f o Article history: Received 24 August 2011 Accepted 19 September 2011 Keywords: Multiple sclerosis MRI Magnetic susceptibility Small veins Diagnosis 7 T a b s t r a c t T ∗ 2 -weighted magnetic resonance imaging at 7 T has recently been shown to allow differentiation between white-matter multiple sclerosis lesions and asymptomatic white-matter lesions, by the presence or absence of a detectable central blood vessel. The aim of the present work is to improve the technique by increasing the sensitivity to veins at both 3 T and 7 T, and to assess the benefit of ultra-high-field imaging. Signal-to-noise ratio (SNR) measurements and simulations are used to compare the sensitivity of magnitude T ∗ 2 -weighted and susceptibility-weighted images for the detection of small veins (<1 pixel in diameter), both with and without the use of gadolinium. The simulations are used to predict the opti- mal scanning parameters in order to increase the sensitivity to these veins at both field strengths, and to reduce the inherent dependence on vessel orientation. The sensitivities of the sequences at both field strengths are compared, theoretically and experimentally, in order to quantify the benefit of imaging at ultra-high-field. Subjects with multiple sclerosis (MS) are scanned at both field strengths, using the optimised sequence parameters, as well as those used in previously published work, and the optimisation is shown to improve the detection of veins within lesions. © 2011 Elsevier Ireland Ltd. All rights reserved. 1. Introduction There has been a great deal of recent interest in magnetic resonance imaging (MRI) at ultra-high-field (7 T). The higher signal-to-noise ratio (SNR) available at 7 T permits scanning at much higher spatial resolution than is possible at conventional field strengths (i.e. 1.5 T, 3 T), and the different relaxation times lead to enhanced contrast. In particular, the decrease in T ′ 2 relative to T 2 at 7 T greatly improves contrast in T ∗ 2 -weighted imaging (T ′ 2 is the refocusable component of the transverse relaxation, T ∗ 2 , while T 2 denotes the random, unrefocusable component). T ∗ 2 -weighting, using gradient-echo acquisitions with long echo times to exploit differences in magnetic susceptibility, enables clear visualisation of iron-rich structures and small veins in the brain. The depiction of veins with diameters smaller than a pixel can be achieved, without the need for exogenous contrast agent, at field ∗ Corresponding author. Tel.: +44 115 9514747; fax: +44 115 9515166. E-mail addresses: jennifer.dixon@nottingham.ac.uk (J.E. Dixon), ashley.simpson@nhs.net (A. Simpson), niraj.mistry@nottingham.ac.uk (N. Mistry), nikos.evangelou@nuh.nhs.uk (N. Evangelou), peter.morris@nottingham.ac.uk (P.G. Morris). 1 Tel.: +44 115 9514747; fax: +44 115 9515166. 2 Tel.: +44 115 9709735; fax: +44 115 9709738. strengths as low as 1.5 T and relies on the paramagnetic nature of deoxyhaemoglobin [1]. In addition to the magnitude informa- tion, the phase of the MR signal in a T ∗ 2 -weighted acquisition can be filtered to create an image, and the contrast in the magnitude image can be increased by multiplying it by a positive phase mask to create what are known as susceptibility-weighted images (SWI) [1–3], and by creating minimum intensity projections (mIPs) [1]. T ∗ 2 -weighted imaging, and specifically the ability to visualise small veins, is of particular interest in the study of multiple sclero- sis (MS). White-matter (WM) MS lesions are easily visible on MRI at conventional field strength, and so MRI is the main diagnostic test used in MS. However, although MRI is very sensitive to these WM abnormalities, its inherent specificity for MS lesions is poor. This is suggested to be due, in part, to the difficulties in differentiating between MS lesions (caused by demyelination) and lesions associ- ated with ageing, vascular disease or hypertension, using current imaging methods [4]. Consequently, diagnostic criteria are neces- sary to refine the specificity of MRI findings by requiring lesions to be disseminated in space (according to anatomical locations) and time, often requiring follow-up scans to demonstrate a new lesion. Histological studies of MS have long since shown a close spatial relationship between MS lesions and small parenchymal veins [5] (in contrast, it is possible that ischaemic lesions, caused by lack of oxygenation, could be centred on narrow or blocked small arteries). 0720-048X/$ – see front matter © 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2011.09.023