Partial angle inversion recovery (PAIR) MR imaging: spin-echo and snapshot implementation

Heavily T1-weighted images without respiratory artifacts: partial angle inversion recovery fast spin-echo imaging (PAIR-FSE)

We obtained T1-weighted images in the abdominal region using the partial angle inversion recovery fast spin echo (PAIR-FSE) with the respiratory triggering (RT) method and compared the image quality with that of conventional SE (TR/TE 500/10 msec) with the partial angle inversion recovery (PEAR) method. The signal difference to noise ratio of the PAIR-FSE was 1.6 times higher (6.94 +/- 3.08) than that of SE (4.30 +/- 1.88). Respiratory motion-induced ghost artifacts were reduced by half in PAIR-FSE with RT (1.01 +/- 0.47) in comparison with SE with PEAR (2.24 +/- 0.70). J. Magn. Reson. Imaging 2000;12:960-964.

Correction of errors caused by imperfect inversion pulses in MR imaging measurement of T1 relaxation times

Spin-lattice (T1) relaxation times were measured by an inversion-recovery magnetic resonance imaging method with a slice-selective inversion pulse (SIP), a non-selective rectangular inversion pulse (RIP), or a B1-insensitive adiabatic inversion pulse (AIP). Data analysis either assumed perfect inversion (two-parameter fit) or allowed for imperfect inversion (three-parameter fit). Imperfect inversion pulses caused low T1 values in phantoms with a two-parameter fit, while three-parameter T1 estimates were accurate over the range 430-2670 ms. A difference of approximately 10% between two-parameter and three-parameter T1 values in normal human brain tissue was attributed to B1 inhomogeneity with the slice-selective inversion pulse and rectangular inversion pulse, to the slice profile with the slice-selective inversion pulse, and to T2 effects for the adiabatic inversion pulse. Any T1 method that relies on accurate flip angles may have a significant systematic error in vivo. Phantom accuracy does not ensure accuracy in vivo, because phantoms may have a more homogeneous B1 field and a longer T2 than do biological samples.

Optimization of Signal Intensity and T1-Dependent Contrast with Nonstandard Flip Angles in Spin-Echo and Inversion-Recovery MR Imaging

Theoretical expressions of flip angle values maximizing signal intensity and T1-dependent contrast are derived for spin-echo and inversion-recovery sequences. Experimental data and theoretical predictions are closely correlated for experiments carried out on phantoms, despite the nonideal shape of the RF refocusing pulse used for slice selection. The use of nonstandard angles is justified when rapid MR acquisitions are needed and/or large T1 species must be imaged with refocused sequences. Copyright 1998 Academic Press. Copyright 1998 Academic Press

Comparison between conventional and fast spin-echo stir sequences

PURPOSE

To evaluate the common characteristics and differences in contrast behavior of short-TI-inversion-recovery (STIR) and short-TI-inversion-recovery fast spin-echo (TurboSTIR) sequences.

MATERIAL AND METHODS

Phantoms doped with increasing doses of Gd-DTPA and a pork-fat phantom were used to evaluate the dependence of the STIR and TurboSTIR signals on the T1 relaxation time. Clinical TurboSTIR images were obtained from 30 patients with musculoskeletal abnormalities and compared to conventional STIR images in 15 cases and to postcontrast TurboSTIR images in another 15 cases.

RESULTS

In the phantom measurements, a significantly shorter inversion time (TI) was needed to achieve fat suppression on TurboSTIR images, and, with an identical number of signal averages, contrast-to-noise ratios were lower on TurboSTIR images. These differences between STIR and TurboSTIR can be attributed to the contribution of stimulated echoes to overall TurboSTIR signal and can be compensated by a shorter TI and a higher number of signal averages for TurboSTIR, respectively. With these adaptations, clinical TurboSTIR and STIR images showed an identical contrast behavior with fat suppression and a high sensitivity to pathological lesions but TurboSTIR saved a significant amount of scan time and reduced some types of artifacts. Contrast uptake impaired lesion conspicuity on TurboSTIR images.

CONCLUSION

TurboSTIR sequences should replace conventional STIR sequences and should be performed before contrast administration.

Optimization of parameter values for complex pulse sequences by simulated annealing: application to 3D MP-RAGE imaging of the brain

A number of pulse sequence techniques, including magnetization-prepared gradient echo (MP-GRE), segmented GRE, and hybrid RARE, employ a relatively large number of variable pulse sequence parameters and acquire the image data during a transient signal evolution. These sequences have recently been proposed and/or used for clinical applications in the brain, spine, liver, and coronary arteries. Thus, the need for a method of deriving optimal pulse sequence parameter values for this class of sequences now exists. Due to the complexity of these sequences, conventional optimization approaches, such as applying differential calculus to signal difference equations, are inadequate. We have developed a general framework for adapting the simulated annealing algorithm to pulse sequence parameter value optimization, and applied this framework to the specific case of optimizing the white matter-gray matter signal difference for a T1-weighted variable flip angle 3D MP-RAGE sequence. Using our algorithm, the values of 35 sequence parameters, including the magnetization-preparation RF pulse flip angle and delay time, 32 flip angles in the variable flip angle gradient-echo acquisition sequence, and the magnetization recovery time, were derived. Optimized 3D MP-RAGE achieved up to a 130% increase in white matter-gray matter signal difference compared with optimized 3D RF-spoiled FLASH with the same total acquisition time. The simulated annealing approach was effective at deriving optimal parameter values for a specific 3D MP-RAGE imaging objective, and may be useful for other imaging objectives and sequences in this general class.