LIMITED FLIP ANGLE MR IMAGING OF BLOOD VESSELS

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We propose to study in detail the technical characteristics, advantages, and limitations of limited flip angle (LFA) magnetic resonance imaging and apply it to the specific task of projection imaging of blood vessels. Expected signal levels of LFA imaging will be predicted from theoretical grounds and measured experimentally. Repetition times and flip angles will be optimized for distinguishing prescribed pairs of materials. Algorithms will be devised for determining images of the T1 relaxation time from measurements acquired at several different flip angles. Contrast-to-noise ratios (CNR) and efficiency (CNR divided by the square root of time) of LFA imaging will be compared rigorously with conventional spin-warp methods. Limited flip angle methods will also be applied to the different imaging of blood vessels in projection format. The basis of the signal will be the randomization in phase and hence reduction in signal caused by the motion of flowing blood along a magnetic gradient. Data for a high velocity (systolic) and low velocity (diastolic) image will be acquired in interleaved fashion over approximately ten cardiac cycles and assigned respectively to the phase destructive and phase coherent images. Such assignment will be determined from an additional simultaneous measurement of blood pressure pulses. Velocity profiles in vessels at various phases of the cardiac cycle will be measured in dogs with Doppler ultrasound. Such measurements will be used to optimize the MR technique. LFA MR angiographic images of dogs will be compared with results from IV-DSA.