Date on Master's Thesis/Doctoral Dissertation

5-2014

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Electrical and Computer Engineering

Degree Program

Electrical Engineering, PhD

Committee Chair

Amini, Amir A.

Committee Member

Zurada, Jacek M.

Committee Member

Stoddard, Marcus F.

Committee Member

Giridharan, Guruprasad A.

Committee Member

Inanc, Tamer

Abstract

4D flow Phase Contrast MRI is a relatively new technique in MRI which is capable of deriving time-resolved three-dimensional velocity fields in a 3D volume noninvasively. 4D flow imaging is a 3D k-space acquisition where for the third dimension, an additional phase-encoding step is required. The velocity field can then be used to obtain flow waveform, wall shear stress, vascular compliance, blood pressure, and other hemodynamic information. A significant limitation of 4D flow methods has been the requisite long scan times, requiring the patient to remain motionless at times on the order of 10-20 minutes, depending on scan parameters. The scan times may become more prohibitive in case of 4D cardiovascular studies where respiratory gating with navigator echoes is required. In this thesis the feasibility of using a reduced TE stack of spirals k-space acquisition for 4D flow imaging are investigated. Starting with fundamentals of MRI, the basics of Phase contrast and 4D flow MRI are thoroughly discussed in Chapter 1-3 and subsequently experimental phantom results are reported in Chapter 4, pointing to the feasibility of performing highly accurate 4D velocity and flow measurement with the proposed pulse sequence under a variety of flow conditions and with substantial reductions in scan time when compared to conventional 4D flow. In phantom studies, results based on the root mean square error criterion indicate that 4D Reduced TE (RTE) Spiral PC MRI is capable of providing the same level of accuracy as conventional 4D conventional PC MRI but in a much shorter scan time (30% reduction in scan time when imaging an FOV of 100*100*60 mm3 and spatial resolution of 1.5*1.5*3 mm3). Moreover, the proposed method has the added advantage of achieving the shorter echo time of 2 ms versus 3.6 ms for conventional 4D flow at Q=50ml/s and 1.57 ms versus 3.2 ms at the higher flow rate of Q=150 ml/s leading to more accurate assessment of flow distal to narrowings. Statistical results indicate that at low flow rates performance of both methods are similar. At higher flow rates, however, 4D RTE spiral flow achieves better accuracy. Qualitative results in phantom studies also revealed that at higher flow rates, better flow visualization was achieved with4D RTE spiral flow compared with conventional 4D flow. In the second part of Chapter 4, we also report on application of the proposed sequence, in-vivo, to 5 healthy volunteers and 5 subjects with mild to moderate Aortic Stenosis (AS) disease. Results from the proposed method were statistically correlated with velocity profiles derived from conventional 4D flow and Doppler Ultrasound. Results indicate that 4D RTE Spiral is capable of providing the same level of accuracy in flow measurement as Conventional 4D flow MRI for imaging of the aortic valve, but on average resulted in a 30% reduction in scan time and 45% reduction in echo time. 4D RTE Spiral was also able to achieve an echo time of 1.68 ms versus 2.9 ms for that of conventional 4D flow MRI, permitting less signal dephasing in the presence of jet flows distal to occlusions. With Doppler Ultrasound adopted as the reference method, 4D RTE Spiral flow measured peak velocity and maximum pressure gradient with a higher level of accuracy when compared to Conventional 4D flow MRI. Both methods measured left-ventricular out flow tract (LVOT) diameter, Aortic Valve (AV) eject time and time to AV peak with same accuracy. It is concluded that 4D RTE Spiral flow MRI is an excellent technique for flow measurement in cardiac patients who are unable to tolerate longer scan times, currently required by conventional 4D flow methods.

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