Number Of Phase-encoding Steps Research Articles

Cardiac T 1 Imaging

T(1) mapping of the heart has evolved into a valuable tool to evaluate myocardial tissue properties, with or without contrast injection, including assessment of myocardial edema and free water content, extracellular volume (expansion), and most recently cardiomyocyte hypertrophy. The magnetic resonance imaging pulse sequence techniques developed for these applications have had to address at least 2 important considerations for cardiac applications: measure magnetization-inversion recoveries during cardiac motion with sufficient temporal resolution for the shortest expected T(1) values, and, second, obtain these measurements within a time during which a patient can comfortably suspend breathing. So-called Look-Locker techniques, and variants thereof, which all sample multiple points of a magnetization recovery after each magnetization preparation, have therefore become a mainstay in this field. The rapid pace of advances and new findings based on cardiac T(1) mapping for assessment of diffuse fibrosis or myocardial edema show that these techniques enrich the capabilities of magnetic resonance imaging for myocardial tissue profiling, which is arguably unmatched by other cardiac imaging modalities.

Phase-encoding strategies for optimal spatial resolution and T1 accuracy in 3D Look–Locker imaging

The Look-Locker (LL) imaging method provides an accurate and efficient approach for mapping the spin-lattice relaxation time, T(1). However, the same recovery of signal during LL image acquisition required to estimate T(1) also results in unwanted modulation of k-space. This is particularly problematic with 3D LL imaging as the number of phase-encoding steps during the recovery interval (e.g., 16) increases in an effort to reduce imaging times. This modulation of k-space has the effect of introducing a point spread function (PSF), which can lead to either image blurring (if the earlier tip angles are assigned to the centre of k-space) or edge enhancement (if the earlier tip angles are assigned to the edges of k-space), thus corrupting T(1) estimation, particularly for small objects. In this study, the PSF and its effect on the acquired images for four different interleaved phase-encode schemes (centric-in, centric-out, sequential and hybrid-sequential) are simulated for a range of T(1), tip angle and 3D LL acquisition parameters expected in practice. It is shown by simulation and confirmed experimentally in phantoms that a hybrid sequential phase-encoding scheme reduces image blurring while maintaining T(1) accuracy ( approximately 2%) and precision (2%) over a range of object sizes down to 2 pixels (2 mm).

High-resolution, spin-echo BOLD, and CBF fMRI at 4 and 7 T.

With growing interest in noninvasive mapping of columnar organization and other small functional structures in the brain, achieving high spatial resolution and specificity in fMRI is of critical importance. We implemented a simple method for BOLD and perfusion fMRI with high spatial resolution and specificity. Increased spatial resolution was achieved by selectively exciting a slab of interest along the phase-encoding direction for EPI, resulting in a reduced FOV and number of phase-encoding steps. Improved spatial specificity was achieved by using SE EPI acquisition at high fields, where it is predominantly sensitive to signal changes in the microvasculature. Robust SE BOLD and perfusion fMRI were obtained with a nominal in-plane resolution up to 0.5 x 0.5 mm(2) at 7 and 4 Tesla, and were highly reproducible under repeated measures. This methodology enables high-resolution and high-specificity studies of functional topography in the millimeter to submillimeter spatial scales of the human brain.

Scan time reduction in proton magnetic resonance spectroscopic imaging of the human brain

A simple technique is described for scan time reductions in proton magnetic resonance spectroscopic imaging (MRSI) of the human brain. Scan time is reduced by approximately 35% while preserving spatial resolution by reducing the field of view (FOV) and number of phase-encoding steps in the transverse direction of the brain. A multislice MRSI of the brain is demonstrated which takes approximately 20 min with a square FOV, and 13 min with a reduced FOV. The signal-to-noise ratio (SNR) in the reduced FOV scan was measured to be 15% lower than that of the full FOV scan, which is close to the expected theoretical value of 19% based on the square root of the scan time. The method can be applied with any sequence, and requires minimal software and no hardware modifications. Scan time in MRSI is minimized in this method by using FOVs no larger than the dimensions of the object to be imaged. The method may also be combined with other fast MRSI techniques to provide further scan time reductions.

Fast kappa-space sample selection in MRSI with a limited region of support.

One of the primary drawbacks in the application of magnetic resonance spectroscopic imaging is the long acquisition times required to obtained the desired resolution. When region of support information is available, the number of phase-encoding steps and thus time can be reduced without loss of information if the kappa-space locations are chosen well. We propose to select locations using a rectangular sampling array that is shifted to various positions in kappa-space to obtain the necessary sampling density. This method allows multiple samples to be selected simultaneously and reduces the computation required to evaluate the selection criterion. We present an efficient forward selection algorithm for optimizing the shift pattern so that the image can be reconstructed as reliably as possible from a periodic nonuniform set of samples. The proposed algorithm has important practical potential in that it can finish the selection in less than half a minute for typical image sizes and can reconstruct the image with fewer samples than regular sampling. With appropriate imaging hardware, this new algorithm makes selective sampling possible in a real-time image acquisition setting.

Optimized outer volume suppression for single-shot fast spin-echo cardiac imaging.

Among the ultrafast MRI techniques, the single-shot fast spin-echo sequence offers a robust alternative to echo planar imaging, essentially because of a much reduced sensitivity to B0 inhomogeneity. This property is particularly appealing in situations in which B0 inhomogeneities can be severe and difficult to correct, such as in cardiac imaging. With single-shot cardiac imaging, however, achieving high resolution over the necessarily large field of views without introducing back-folding artifacts is problematic. One option is to use multishot sequences. However, then issues related to cardiac gating arise. Another solution is to use, optimized presaturation slabs with quadratic phase pulses generated by the Shinnar-LeRoux algorithm. These can be set to reduce the field of view in the phase-encoding direction, resulting in a reduction in the number of phase-encoding steps. For instance, for a 1 x 2-mm spatial resolution, over a rectangular, 250 x 125-mm field of view, and using a half Fourier acquisition, an echo-train length of only 40 is required. With a 4.5-msec echo spacing, the total imaging time is approximately 180 msec. The efficacy of this solution on phantoms and volunteers is demonstrated. Multislice short-axis examinations of the whole heart, realized within a single short breath-hold of approximately 10 seconds, are shown. The possibility of investigating not only cardiac anatomy but also both contractility and myocardial perfusion is discussed.

Fast Keyhole MR Imaging Using Optimized k-Space Data Acquisition

In some dynamic magnetic resonance imaging (MRI) applications, the sample is still, and only the signal intensity changes with time. For such cases, the keyhole imaging principle can be used. In standard keyhole imaging, a low-frequency image signal is acquired, using a limited number of phase-encoding steps, which correspond to the rectangular sampling region in the k-space center. However, such a region practically never coincides with the position of the k-space points, which carry the most relevant low-frequency image information. In this paper we propose an improved keyhole method, which allows dynamic acquisition of a low-frequency image signal from selected most relevant k-space points via fast imaging mechanisms. Dynamic data acquisition is executed in the presence of time-varying magnetic-field (MF) gradients after single sample excitation. Special care has been taken in the design of the gradient sequence to minimize gradient load. This improved keyhole imaging method has been considered theoret...

"Crisscross" MR imaging: improved resolution by averaging signals with swapped phase-encoding axes.

At conventional magnetic resonance imaging, the number of phase-encoding steps is often chosen to be less than the number of readout samples, resulting in reduced spatial resolution in the phase-encoding direction. Signal averaging improves the signal-to-noise ratio of the image but does not affect this asymmetry in resolution. By swapping the phase- and frequency-encoding directions between excitations and averaging the resultant data sets ("crisscross" imaging), equal spatial resolution can be obtained in both principal directions. Increased spatial resolution can be obtained with this method with no increase in imaging time, and the method can potentially be applied in echo-planar and three-dimensional Fourier transform imaging.

Signal strength in subsecond FLASH magnetic resonance imaging: The dynamic approach to steady state

Subsecond fast low-angle shot (FLASH) magnetic resonance imaging (MRI) allows single shot studies of the human heart within measuring times of about 100-300 ms depending on the data matrix. In contrast to conventional FLASH MRI subsecond applications acquire data during the approach to steady state. A detailed analysis of the saturation behavior of the signal is given for the ideal case of a rectangular slice profile. In a second step, realistic slice profiles assuming Gaussian-shaped excitation pulses were taken into account by means of a numerical solution of the Bloch equations. It turns out that the signal strength and the resulting image intensity is considerably higher than may be expected from steady-state considerations. Correspondingly optimized flip angles depend on the number of phase-encoding steps. Assuming long T1 relaxation times as, for example, encountered in muscle and brain tissue and repetition times of 5 ms or less, optimum flip angles are 12 degrees-16 degrees. The use of even higher flip angles (greater than or equal to 20 degrees) causes heavily distorted slice profiles and a dynamic increase of the effective slice thickness. Flip angles of the order of the Ernst angle (6 degrees) correspond to steady-state conditions and lead to considerable signal losses. The theoretical results are confirmed by subsecond FLASH MRI studies of the human heart using a 2.0 T whole-body system (Siemens Magnetom).

Predictability of SNR and reader preference in clinical MR imaging

Fifty-four independent scans were performed in two volunteers covering one anatomic region in each (the brain and knee) with the purpose of ascertaining the agreement between predicted and measured signal-to-noise ratios (SNR). Systematically varied parameters were number of excitations (NEX), field of view (FOV), section thickness ( d z ), and the number of phase-encoding steps ( N y ). Correlation coefficients of measured versus predicted SNR were 0.82 and 0.86, respectively, in the anatomies studied. Significantly improved correlations were found for data subpopulations in which NEX was held constant. To assess the criteria guiding reader preference, a blinded study was performed in which radiologists were asked to rate images from least to most desirable. In order to quantitatively determine the criteria for reader preference, plots of mean rating versus SNR, voxel volume, and an image quality index [ IQI = SNR/(voxel volume)] were performed. The latter was found to be a better predictor of reader preference than either SNR or spatial resolution alone. The data suggests T 1-weighted scan protocols yielding SNR of approximately 20 are preferable with any excess SNR being traded for smaller voxel size or shorter scan times.