Free Medical Books Radiology

MRI Techniques PDF

MRI Techniques PDF Free Download
MRI Techniques PDF
Preface:

The flow phenomena which we are going to describe in this section enable us to spontaneously view blood vessels (without the injection of a contrast-enhancing product); the effect they have on the image depends on the sequence used (spin echo (SE) or gradient echo (GE)), on the parameters of that sequence (TR, TE…) but also on the particular parameters of the flow itself: its velocity, the orientation of the vessel in relation to the slice, etc. Thus, we can construct a form of imaging relating to the flow: magnetic resonance angiography, or MRA. 1.2.1. Time of Flight (TOF) When the vessel runs through the slice, the intensity of the flow signal depends on the time-of-flight of the protons Tt, i.e. the time taken to traverse the thickness Δz of the plane being imaged at velocity V. 1.2.1.1. Phenomenon of flow void in a spin-echo sequence We work on a spin-echo sequence where the 90° and 180° pulses are selective in the given slice. With stationary protons: they experience both pulses and are therefore able to generate a signal. With moving protons (in the bloodstream), two scenarios may arise: – Either they remain in the slice (Tt < TE/2) and are therefore struck by both pulses to generate a signal. For a vessel perpendicular to the slice, these protons have a velocity of V < Δz/(TE/2): this is qualified as a “slow” flow. – Or they leave the slice entirely before the transmission of the 180° pulse (i.e. Tt < TE/2). They are then replaced within the slice by protons which were not subjected to the initial 90° pulse and which therefore do not generate a signal. (The extreme case, Tt = TE/2, is represented in Figure 1.3). For a vessel perpendicular to the slice, these protons have a velocity of V > Δz/(TE/2): this is qualified as a “fast” flow.

More Info:
Keywords:
MRI Techniques Free Download ,MRI Techniques Ebook ,MRI Techniques Free