Ebook Microscopic magnetic resonance imaging: Part 2
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Ebook Microscopic magnetic resonance imaging: Part 2
Chapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2stand long acquisition times. A drawback is that the fixation process can alter the measurements (image SNR and contrast). Alive specimens require perfusion systems adapted to the limited available space and the high magnetic field within the scanner. In this chapter, we describe a number of practic Ebook Microscopic magnetic resonance imaging: Part 2al considerations regarding sample preparation and perfusion system design which should be followed in order to ensure good quality MRM images.6.1 FixEbook Microscopic magnetic resonance imaging: Part 2
ed TissuesFor ex vivo MR measurements, tissue samples are usually chemically fixed, aiming to preserve their in vivo properties as much as possible. SChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2(whole mouse or rat brains) are difficult to fix through immersion as they can begin to deteriorate during the time necessary for the penetration of the fixative and before the fixation process is complete. In such cases, it is recommended to performMicroscopic Magnetic Resonance Imaging: A Practica Ebook Microscopic magnetic resonance imaging: Part 2l PerspectiveI utsa CiobanuCopyright (£' 2017 Pan Stanford Publishing Pte. ltd.ISBN 978-981-4774-71-0 (Paperback), 978-981-4774-42-0 (Hardback), 978-1Ebook Microscopic magnetic resonance imaging: Part 2
-315-10732-5 (eBook)www.panstanford.com64 I Sample Preparationa transcardiac perfusion (perfusion through the left ventricle, for details see Ref. (l)Chapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2aldehyde, or 2% formaldehyde plus 2% glutaraldehyde have also been used. After fixation the samples are typically washed in PBS solution to remove the fixative and then placed in Fluorinert for imaging. The placement of the sample in Fluorinert during imaging is not obligatory but it is recommended Ebook Microscopic magnetic resonance imaging: Part 2as it presents several advantages. First, Fluorinert prevents the sample from drying and, at the same time, does not require a field of view larger thEbook Microscopic magnetic resonance imaging: Part 2
an the sample itself as it is proton free (not visible in '11 MR1). In addition, Fluorinert reduces susceptibility artifacts as it has the magnetic suChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2 in case of copper coils placed close to the sample, as discussed in Section 2.2.1). Moreover, having the density 1.6 times higher than that of water, Fluorinert can help remove air bubbles trapped within the tissue. Alternatively, the sample can be embedded in an agar gel (l)henain, 2006).It is wel Ebook Microscopic magnetic resonance imaging: Part 2l known that chemical fixation alters tissue characteristics and causes shrinkage. The aldehyde fixatives mentioned previously can significantly and dEbook Microscopic magnetic resonance imaging: Part 2
ifferentially impact several MR parameters. It has been shown that the fixation process reduces both the Tỵ and Tz relaxations times of the tissue. PBChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2xation leads to a reduced SNR in spin density-weighted images which, surprisingly, is not recovered by PBS washing despite the increase in 7’2. Diffusion-weighted MR signals arc also affected by the fixation process. Specifically, Sun et al. showed that while the fractional anisotropy remains unchan Ebook Microscopic magnetic resonance imaging: Part 2ged upon formaldehyde fixation, the apparent diffusion coefficient is significantly reduced (Sun, 2005). Shepherd el al. found significant increased mEbook Microscopic magnetic resonance imaging: Part 2
embrane permeability and decreased extracellular space after fixation (Shepherd, 2009).In addition to the changes in tissue properties, improper fixatChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2esthe timing of the fixation protocol are two key factors. Low aldehyde concentrations (<2%) or short fixation times will lead to tissue deformation and even to the inversion of the white-gray matter contrast (Cahill, 2012}. Long fixation periods (>6 months) lead to neuropil destruction giving rise Ebook Microscopic magnetic resonance imaging: Part 2to severe hypointensities in TỊ-weighted images of fixed nervous tissue (van Dujin, 2011).Besides allowing long acquisition times, fixed tissues preseEbook Microscopic magnetic resonance imaging: Part 2
nt the advantage that, after the MR acquisition, they can be histologically examined for correlational studies. However, in the light of the discussioChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2pecimens eliminates the fixation issues discussed in the previous section. It brings in interesting opportunities and, at the same time, presents new technical challenges. Live tissue imaging requires the development of dedicated perfusion chambers capable of maintaining its viability and compatible Ebook Microscopic magnetic resonance imaging: Part 2 with the strict spatial and material constraints imposed by the high magnetic fields used. Besides the ability to mimic the desired physiological conEbook Microscopic magnetic resonance imaging: Part 2
ditions MR compatible perfusion systems should satisfy the following requirements:-1All materials should be MR compatible.-2The sample should not moveChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withs Ebook Microscopic magnetic resonance imaging: Part 2ents minimal susceptibility artifacts even at very high magnetic fields.-4Air bubbles should be eliminated through the insertion of air traps into the system.-5The distance between the sample and the RF coil should be kept small. Live perfused specimens are typically imaged using surface coils as th Ebook Microscopic magnetic resonance imaging: Part 2e solenoidal geometry is most of the time incompatible with the placement of a perfusion chamber.66 Sample PreparationPerfusionChamberEpoxy Glue AirTrEbook Microscopic magnetic resonance imaging: Part 2
aP l"nSample/ Cover SlipPlastic SupportSurface CoilBio-compatible Adhesive (water-proof)Figure 6.1 Schematic diagram of a simple perfusion system desiChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withsChapter 6Sample PreparationTissue samples used in MRM are divided in two categories: fixed and alive, fixed tissues are easier to handle and can withsGọi ngay
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