This unit introduces the sources of variable phase in the presence of motion. The sources of these phase changes can come from the read gradient, from the slice select gradient and from the phase encoding gradient. Motion during the gradient moves the spin from one location to another and, therefore, changes the effective field it sees. This field change leads to a frequency change in time during either the dephasing or rephasing gradient or both. As a result, the spins do not refocus at the expected time.
The bulk magnetization represents the equilibrium value of the magnetization when all spins have aligned themselves parallel to the main field. When this magnetization is tipped into the transverse plane, it precesses about the main field. This precession generates a changing flux through the face of an appropriately placed coil inducing an electromotive force (emf) in the coil. The measured signal is proportional to the induced emf. This unit discusses the basic physics behind detecting MR signal. The process of calculating induced emf based on Faradays law of induction is explained.
When imaging patients with headaches, the goals are: (1) to rule out a more ominous etiology for their headaches and (2) to assess the degree of hemodynamic and parenchymal involvement when atypical or complicated migraine is suspected. The standard images allow other etiologies, such as mass lesions, to be excluded and an MR venogram can rule out venous sinus thrombosis. Contrast may be helpful in differentiating multiple small metastases, demylinating lesions with inflammatory components, and subcortical infarcts from the nonspecific foci of increased T2 associated with migraines that do not enhance. Perfusion-weighted imaging allows detection assessment of hemodynamic compromise. Diffusion-weighted imaging, in combination with the standard MR images, allows assessment of parenchymal involvement. This unit contains a basic protocol which can be used for the evaluation of patients as well as an alternative protocol for the assessment of cerebral perfusion in patients with visual auras.
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