Optimization of signal‐to‐noise ratio in calculated T₁ images derived from two spin‐echo images

Abstract

A simplified model relating signal intensity in an MR image to spin-lattice relaxation time (T1), repetition time (TR), number of signal averages and the average tip angle (-alpha) of the protons within the slice has been developed. This model has been used to select the optimal repetition times of two spin-echo images for a fixed total imaging time to maximize signal to noise in calculated T1 images. Theoretical predictions of T1 are virtually identical to spectroscopically measured values, and the relative noise (delta T1) in T1 images calculated from two measured spin-echo images is in good agreement with the theoretically predicted values of delta T1/T1. This model predicts that: (a) for a T1 of approximately 500 ms, the least T1 image noise is obtained with one of the spin-echo images collected with a TR of 400-500 ms. The longer the TR of the other spin-echo image, the lower the T1 image noise, but past a TR of approximately 1400 ms, T1 image signal/noise is optimized for the same total imaging time by increasing the number of averages in the shorter TR spin-echo image rather than increasing the TR of the second spin-echo image. (b) The error is reduced and the optimum TR1 is reduced as -alpha is increased from 63 to 90 degrees. (c) For a range of T1, optimal selection of TR1 and TR2 based on an intermediate value of T1, results in relatively little increase over optimal values in delta T1/T1 for the entire T1 range.

Fast mapping of the T2 relaxation time of cerebral metabolites using proton echo‐planar spectroscopic imaging (PEPSI)

“…For each subject, five PEPSI data sets were collected at TEs of 50, 100, 160, 220, and 300 ms. The TR was chosen to approximate the T 1 values of metabolites for optimization of the acquisition efficiency for signal‐to‐noise ratio (SNR) (28). Given previously reported T 1 values ranging between 1000 ms and 1500 ms at 3T (2), we chose a TR of 1200 ms for this study.…”

Quantification of myocardial blood flow and extracellular volumes using a bolus injection of Gd‐DTPA: Kinetic modeling in canine ischemic disease

“…Two interleaved sets of three sagittal slices were obtained at each of three TR/TE values (in units of ms) equal to 150/35, 600/35, and 645/80. These images gave TI-weighted and T2-weighted images and also allowed the computation of T1 and T2 maps (20,21).…”

Comparison of four magnetic resonance methods for mapping small temperature changes

“…Conventional methods, which determine T 1 by fitting a recovery curve to a series of measurements, such as the multipoint inversion recovery (IR) or the repeated saturation recovery (SR) methods, are the most accurate but also the most time-consuming approaches (Crawley and Henkelman 1988). Methods based on only few measurements, such as the spin echo (SE) with two different repetition times (T R ) or the gradient echo (GE) with different flip angles, produce inaccurate T 1 estimates (for example Prato et al 1986). The recently reported rapid T 1 mapping techniques, known as 'snapshot' or 'turboflash' significantly speed up the acquisition process, but they either suffer from a large uncertainty of measured T 1 (Haase 1990) or need excessive post-processing (Tong and Prato 1994).…”