Pictures with NMR Magnetic Resonance Imaging

As chemists performing spectroscopy, we are usually looking for the highest spectral resolution possible. Generally, one of the factors that limit the spectral resolution is the uniformity of B0 (the homogeneity of the static field) over the volume of the sample. If the field strength differs from point to point within the sample, there will be a similar point-to-point variation of Larmor frequency, and the width of the resonance line will reflect that distribution in frequency. The shim system (Section 3.2), which is used in a spectrometer to improve B0 homogeneity, consists of a set of electrical coils within the 

Let us look at the effect of a magnetic field gradient in more detail. A gradient Gv along the x direction may be expressed in units of gauss per centimeter (a convenient and popular 

If we have an object that extends for a length Ax (the field of view, or FOV) along the x axis and the object occupies a spectral bandwidth Aco (in radians per second) or Av = Aco/27t (in hertz), the relation between these quantities is 

Just as when acquiring a chemical shift spectrum, we would set the acquisition spectral width, or sweep width (SW) (in hertz), to the field of view Av (again in hertz). (Actually, we would set the spectral width a bit larger to avoid having the spectrum or image bump into the upfteld and downfteld Nyquist spectral limits.) The spatial resolution obtained is limited by the spectral resolution of the acquisition. If we 

Equation (16.5) yields the x spatial resolution and the x gradient strength  

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