Physics : Magnetic Resonance Angiography ( Mra )

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Magnetic Resonance Angiography (MRA) is a technique in which magnetic pulses are used to image pictures of blood vessels inside the body. A MRA can be achieved using two techniques phase contrast and time of flight. A phase contrast can be done using complex subtraction and phase subtraction. A time of flight (TOF) can be done using dark blood and bright blood. A dark blood imaging is visualized using inversion recovery, saturation and dephasing. A bright blood imaging is analyzed using rephased /dephased, in-flow, use of contrast agents. Once a required MRA is processed, it is envisioned using a maximum intensity projection (MIP). A phase contrast works on idea of introducing two bipolar gradient pulses for flow compensation. Phase can…show more content…
A phase unwrapping can be done using manual segmentation or automatic segmentation. A simple image processing region growing algorithm can be used to achieve that. A manual boundary drawing can be used to unwrap phase. In such a manual segmentation a phase or a magnitude image is preferred to be worked with. Flow uncertainty can be pictured in flow quantification and sources of error needs to be figured out and those should be resolved. The main sources are partial volume effect, poor segmentation and noise. Parameters like vessel size (actual vessel size, resolution), signal-to-noise ratio (SNR) (Scan parameters, blood velocity, velocity threshold), vessel segmentation accuracy (Segmentation method, SNR ) would rule the impact of these errors. Certain pixels that comprise of both still and moving spins would affect the phase value which does not reflect the correct/true velocity. Consider the flow is laminar and the assuming the magnitude signal behavior, this error can be demonstrated as a function of a vessel size. This would be the partial volume effect. A poor segmentation would be to miss some of the pixel that contains blood flow. Considering partial volume pixels were missed, this error can also be defined as function of the vessel. Noise can be denoted by considering the pug flow and ignoring the phase wrapping as σF/Fv ∝ (ve/vp)(1/SNR)(1/λ) where ve and vp are venc and peak velocity respectively, while σF/Fv is the standard deviation of flow relative
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