Farhad Farzaneh

Improved precision in calculated T1 MR images using multiple spin-echo acquisition.

Calculated T1 images require that magnetic resonance signals be detected at several inversion or repetition times (TR). Multiple spin-echo (SE) acquisitions provide several measurements of the magnetization at each TR, the signal size diminishing according to T2 decay. In this work we review one method (Case 1) for estimating T1 from single echoes and present four new methods (Cases 2-5) in which multiple acquired echoes are used. For Case 2 a fit is performed using the first echo at each TR, repeated using second echoes, etc., and the final T1 estimate is the simple average of the individual fits at each echo time (TE). For Case 3 the optimum weighted average is performed. For Cases 4 and 5 synthetic SE images are generated at each TR prior to the T1 fit, Case 4 using a synthetic TE of zero, and Case 5 using a TE providing maximum signal-to-noise ratio in the synthetic image. The relative precision in T1 provided by each method is calculated rigorously. It is proven that Cases 3 and 5 are optimum and equivalent and can theoretically reduce the noise in T1 images by as much as 40% over Case 1 with no increase in scanning time. Approximations are proposed that enable the optimum methods to be implemented in a practical fashion. Experimental images are presented that verify the relative predicted behavior.

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.

The precision of TR extrapolation in magnetic resonance image synthesis

We present a model of noise propagation from acquired magnetic resonance (MR) images to TR-extrapolated synthetic images. This model assumes that images acquired at two repetition times TR1 and TR2 are used to generate synthetic images at arbitrary repetition times TR. The predictions of the model are compared with experimentally acquired phantom data, and show excellent agreement. The model is utilized in an analysis of two applications of MR image synthesis: scan time reduction and multiple-image synthesis. Scan time is reduced by acquiring data at two short repetition times, and synthesizing at a longer repetition time, with TR1 + TR2 less than TR. For T1 = 800 ms, a reduction of 20% in scan time results in a 45% reduction in signal-to-noise ratio SNR, when compared to direct acquisition. Reducing scan time by much more than 20% produces large noise levels in the synthetic image, and is unlikely to be useful. In multiple-image synthesis, images are synthesized at any repetition time in the range 0 to TR1 + TR2, for contrast optimization. If T1 = 800 ms, and TR1 + TR2 = 2000 ms, the optimum combination of TR1, TR2 results in synthetic images whose SNR is at worst 22% less than the SNR of directly acquired images. For many values of TR, the synthetic images have SNR superior to that obtainable by direct acquisition.

Two New Pulse Sequences for Efficient Determination of Tissue Parameters in MRI

In clinical magnetic resonance (MR) imaging, results at several different repetition times (TR) are often desired. Short-TR short-TE pulse sequences are used to generate “T1-weighted” images, while long-TR long-TE sequences are generally used to derive “T2-weighted” images. Often it is necessary to have both sets, as the multiple images acquired of a given slice are used together to delineate morphology, detect pathology, and differentiate materials. In addition to acquiring both T1- and T2-related information, another consideration is the need to acquire image data quickly. In particular it is useful to generate images which can be used for T1 estimation with reduced scanning times. We have addressed these two issues of multiple-TR images and short scan time T1-weighted images with the development of two pulse sequences. The first we call an asymmetric-TR (ATR) multislice pulse sequence (Farzaneh et al. 1988), and the second is a spoiled FLASH pulse sequence (Wang and Riederer, in press). We discuss some of the details of these two sequences in this paper.

High-Speed Techniques For Estimating Ti And T2 Images

Relaxation times T1 and T2 and proton density NM) are intrinsic tissue properties which dictate in part the appearance of magnetic resonance images. In this work methods are presented for estimating these quantities using high speed fitting algorithms. A technique is described which uses a digital video processor for generating a computed T1 image from two acquired images in approximately one second. Comparable results are possible with T2 fitting as well.