Talin A. Tasciyan

Two-dimensional pulsatile hemodynamic analysis in the magnetic resonance angiography interpretation of a stenosed carotid arterial bifurcation

A two-dimensional pulsatile hemodynamic analysis based on the finite-element technique was performed on a minimally stenosed carotid artery to identify the possible explanation for the differences in the x-ray and magnetic resonance carotid angiograms of a patient. The magnetic resonance angiogram was obtained by applying the maximum intensity projection algorithm to axial slices, acquired using the time-of-flight principle. The differences in the x-ray and magnetic resonance depictions were interpreted based on velocity profile, wall shear stress, and streamline data provided by the hemodynamic analysis. The specific contribution of the stenosis was further isolated from that of the bifurcation by comparing the flow patterns within the stenotic artery with those of its normal counterpart. The Doppler spectral velocity wave form of the patient constituted the basis for the pulsatile flow velocity specification. The analysis took into consideration the non-Newtonian viscosity of blood. The numerical procedure was validated through different convergence criteria and through shear stress comparisons. The importance of hemodynamic analyses in relation to magnetic resonance angiography was further discussed along with possible shortcomings of the technique.

MR subtraction angiography with a matched filter.

The technique of matched filtering (MF) has been used in the past with X-ray digital subtraction angiography as a method of improving signal-to-noise ratio (SNR) in subtraction angiographic images. In this work we describe how MF can be applied to a series of images produced by cinematographic magnetic resonance (cine MR) to produce angiographic images. Likewise, a simple subtraction image can be formed by subtracting an image in which flow is not well visualized from an image at the same location but with flow visualization. Theory predicts that a subtraction image resulting from the MF technique will yield typical SNR improvements of 60% over results from simple subtraction. Twenty-one studies of the human popliteal, canine aorta, and canine carotid artery were undertaken in which MF was compared with simple subtraction. It was determined that cine MR can be used to produce subtraction angiographic images and that MF can produce a modest improvement in SNR over simple subtraction.

Partial angle inversion recovery (pair) MR imaging: Spin-echo and snapshot implementation

The effects of varying the inversion or excitation RF pulse flip angles on image contrast and imaging time have been investigated in IR imaging theoretically, with phantoms and with normal volunteers. Signal intensity in an IR pulse sequence as a function of excitation, inversion and refocusing pulse flip angles was calculated from the solution to the Bloch equations and was utilized to determine the contrast behavior of a lesion/liver model. Theoretical and experimental results were consistent with each other. With the TI chosen to suppress the fat signal, optimization of the excitation pulse flip angle results in an increase in lesion/liver contrast or allows reduction in imaging time which, in turn, can be traded for an increased number of averages. This, in normal volunteers, improved spleen/liver contrast-to-noise ratio (9.0 vs. 5.7, n = 8, p less than 0.01) and suppressed respiratory ghosts by 33% (p less than 0.01). Reducing or increasing the inversion pulse from 180 degrees results in shorter TI needed to null the signal from the tissue of interest. Although this decreases the contrast-to-noise ratio, it can substantially increase the number of sections which can be imaged per given TR in conventional IR imaging or during breathold in the snapshot IR (turboFLASH) technique. Thus, the optimization of RF pulses is useful in obtaining faster IR images, increasing the contrast and/or increasing the number of imaging planes.

Optimization Of The Excitation RF Pulse In Combined Inversion Recovery Gradient And Spin-echo (ir-crease) Technique

We designed a pulse sequence based on the simultaneous acquisition of gradient and spinecho inversion recovery images. The gradient echo image was obtained by a rapid readout gradient reversal immediately after the RF excitation pulse within a multi-echo inversion recovery technique. By analyzing the solution to the Bloch equations, we were able to maximize the image contrast and/or obtain images faster through short TR’s and optimization of the RF pulse excitation. The optimum excitation flip angle a will be small for an odd number of Inversion, Excitation and Refocusing RF pulses and conversely large (180°-a) for an even number. The in vivo results of the gradient and spin-echo inversion recovery technique were in full accordance with theoretical results. Clinical trials of this technique have begun. - ri

An Evaluation Of Geometrical Spoilers In Fast MR imaging

In fast MR sequences, the repetition time TR is usually shorter than T2, leading to the accumulation of steady-state transverse magnetization which in turn causes imaging artifacts or contaminates T1 contrast. This study shovs, through theoretical considerations as well as on images, that to restore T1 contrast and eliminate imaging artifacts, the geometrically incremented spoilers should be preferred, especially in multislice imaging. The results will be presented on brain and abdominal images.