Types Of Pulse Sequences Research Articles

Technical review, types of imaging, part 4--magnetic resonance imaging.

Technical review, types of imaging, part 4--magnetic resonance imaging.

A comparison of five standard methods for evaluating image intensity uniformity in partially parallel imaging MRI

To investigate the utility of five different standard measurement methods for determining image uniformity for partially parallel imaging (PPI) acquisitions in terms of consistency across a variety of pulse sequences and reconstruction strategies. Images were produced with a phantom using a 12-channel head matrix coil in a 3T MRI system (TIM TRIO, Siemens Medical Solutions, Erlangen, Germany). Images produced using echo-planar, fast spin echo, gradient echo, and balanced steady state free precession pulse sequences were evaluated. Two different PPI reconstruction methods were investigated, generalized autocalibrating partially parallel acquisition algorithm (GRAPPA) and modified sensitivity-encoding (mSENSE) with acceleration factors (R) of 2, 3, and 4. Additionally images were acquired with conventional, two-dimensional Fourier imaging methods (R=1). Five measurement methods of uniformity, recommended by the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) were considered. The methods investigated were (1) an ACR method and a (2) NEMA method for calculating the peak deviation nonuniformity, (3) a modification of a NEMA method used to produce a gray scale uniformity map, (4) determining the normalized absolute average deviation uniformity, and (5) a NEMA method that focused on 17 areas of the image to measure uniformity. Changes in uniformity as a function of reconstruction method at the same R-value were also investigated. Two-way analysis of variance (ANOVA) was used to determine whether R-value or reconstruction method had a greater influence on signal intensity uniformity measurements for partially parallel MRI. Two of the methods studied had consistently negative slopes when signal intensity uniformity was plotted against R-value. The results obtained comparing mSENSE against GRAPPA found no consistent difference between GRAPPA and mSENSE with regard to signal intensity uniformity. The results of the two-way ANOVA analysis suggest that R-value and pulse sequence type produce the largest influences on uniformity and PPI reconstruction method had relatively little effect. Two of the methods of measuring signal intensity uniformity, described by the (NEMA) MRI standards, consistently indicated a decrease in uniformity with an increase in R-value. Other methods investigated did not demonstrate consistent results for evaluating signal uniformity in MR images obtained by partially parallel methods. However, because the spatial distribution of noise affects uniformity, it is recommended that additional uniformity quality metrics be investigated for partially parallel MR images.

How MRI Compatible is “MRI Compatible”? A Systematic Comparison of Artifacts Caused by Biopsy Needles at 3.0 and 1.5 T

This study was designed to systematically investigate artifacts caused by interventional needles recommended for use in MRI, with focus on field strength, needle/mandrin type, orientation and sequence. Eight different MRI compatible needles were placed in porcine tissue and examined at 1.5 and 3.0 T with balanced-steady-state-free-precession (B-SSFP) and T1-weighted-spoiled-gradient-echo (T1-SPGR) sequences in different orientations to B0. Artifact diameters with regards to the primary, inner, and secondary, outer artifacts were assessed and statistically evaluated. The types and degree of artifacts varied considerably, especially between different mandrin types even for the same needles. Orientation of the needle in the magnetic field was another main contributor to the artifact dimensions. Less important factors were the type of pulse sequence and field strength. Artifacts ranged from 0.7 mm (steel, 0°, B-SSFP, 3.0 T, inner) to 71.4 mm (nitinol, 90°, B-SSFP, 1.5 T, outer). Inner artifact diameters in B-SSFP were slightly larger (8.2 ± 5.7 mm) than those in T1-SPGR (7.6 ± 5.4 mm) and comparable between 1.5 and 3.0 T (e.g., 8.0 vs. 8.4 mm, B-SSFP). Although all were sold as "MR compatible," the artifacts differed greatly between needle types, and even more so for different mandrins. The results suggest an empirical approach to the needle choice based on lesion type and approach angle.

Assessment of bilateral filter on low NEX open MRI views

Purpose: A bilateral filter (BF) has the property of de-noising digital images while preserving edges. This paper investigates the impact of selecting the BF variables in order to diminish the deterioration of SNR due to reducing the number of excitations (NEX) per image in magnetic resonance imaging (MRI) technique. The paper studies the behavior of the BF on the axial, sagittal, and coronal MRI orientations acquired by spin echo (SE), turbo spin echo (TSE), and the fluid attenuated inversion recovery (FLAIR) pulse sequences from an open MRI device. Method: a set of MRI head images where acquired at different NEXs. The BF was applied at various values of window sizes, filter domains, and filter ranges. The comparison between the high NEX and the BF low NEX MR images was calculated using the peak signal-to-noise ratio (PSNR), the mean-square error (MSE), and the normalized absolute error (NAE). Results: Numerically, the BF is able to achieve up to 29, 44, and 59 % noise suppression from the T2 axial, T1 sagittal, and coronal MR views, respectively. Conclusions: The BF can successfully compensate a part of the SNR decrease due to reduce NEX. The behavior of the BF varies with the type of MR view and the type of pulse sequence. The BF is a successful tool to enhance the appearance of low NEX MR images acquired from open MR devices. This means that BF can contribute in shortening the MRI scan time.

Broadband homodecoupled heteronuclear multiple bond correlation spectroscopy

A general concept for removing proton–proton scalar J couplings in 2D NMR spectroscopy is proposed. The idea is based on introducing an additional J resolved dimension into the pulse sequence of a conventional 2D experiment to design a pseudo 3D NMR experiment. The practical demonstration is exemplified on the widely used gradient coherence selected heteronuclear long-range correlation spectroscopy (HMBC). We refer to this type of pulse sequence as tilt HMBC experiment. For every 13C chemical shift evolution increment, a homonuclear J resolved experiment is recorded. The long-range defocusing delay of the HMBC pulse sequence is exploited to implement this building block. The J resolved evolution period is incremented in a way very similar to ACCORDION spectroscopy to accommodate the buildup of heteronuclear long-range antiphase magnetisation as well. After Fourier transformation in all dimensions the spectra are tilted in the J resolved dimension. Finally, a projection along the J resolved dimension is calculated leading to almost disappearance of proton–proton spin multiplicities in the 2D tilt HMBC spectrum. The tilt HMBC experiment combines sensitivity with simple experimental setup and can be recorded with short recycle delays, when combined with Ernst angle excitation. The recorded spectra display singlet proton signals for long-range correlation peaks making an unambiguous signal assignment much easier. In addition to the new experiment a simple processing technique is applied to efficiently suppress the noise originating from forward linear prediction in the indirect evolution dimensions. In case of issues with fast repetition times, probe heating and RF power handling most of the RF pulses can be replaced by broadband, frequency swept pulses operating at much lower power.

Fast-brain MRI in children is quick, without sedation, and radiation-free, but beware of limitations

Fast-brain MRI is a promising technique for young children who require anesthesia for conventional MRI; however, poor contrast resolution and the use of one type of pulse sequence only has limitations. We aimed to review and document pitfalls of fast-brain MRI in non-sedated children. Fifty fast-brain MRI studies (Fast Imaging Employing sTeady State Acquistion [FIESTA] protocol; 1.5-Tesla Signa Excite HD, GE HealthCare, Milwaukee, WI, USA) performed between January 2008 and August 2010 in 30 non-sedated patients aged 1day to 5years of age (mean: 18months) were reviewed retrospectively and compared to the most recent MRI or CT scan. The indications were: ventriculoperitoneal (VP) shunt insertion or revision or follow-up (20/50, 40%), postoperative follow-up (9/50, 18%), macrocephaly, ventriculomegaly or congenital malformation (15/50, 30%), complications of prematurity (6/50, 12%). The VP shunt position and size of fluid-filled structures were satisfactorily assessed in all patients. Undetected findings in 7/50 studies (14%) were: venous sinus thrombosis (one patient), subdural hematoma (three), failure to differentiate blood products (two), and limited evaluation of extra-axial collections (one). FIESTA fast-brain MRI provides satisfactory assessment of shunt position and the size of fluid-filled structures, but radiologists should be aware of limitations for depiction of venous sinus thrombosis, and bleeding. Modification of fast-brain protocols appears to be indicated.

Magnetic Resonance Imaging of the Musculoskeletal System

Farjoodi, Payam MD1; Mesfin, Addisu MD1; Carrino, John A. MD, MPH1; Khanna, A. Jay MD, MBA1 Author Information

Magnetic resonance tissue quantification using optimal bSSFP pulse-sequence design

We propose a merit function for the expected contrast to noise ratio in tissue quantifications, and formulate a nonlinear, nonconvex semidefinite optimization problem to select locally-optimal balanced steady-state free precession (bSSFP) pulse-sequence design variables. The method could be applied to other pulse sequence types, arbitrary numbers of tissues, and numbers of images. To solve the problem we use a mixture of a grid search to get good starting points, and a sequential, semidefinite, trust-region method, where the subproblems contain only linear and semidefinite constraints. We give the results of numerical experiments for the case of three tissues and three, four or six images, in which we observe a better increase in contrast to noise than would be obtained by averaging the results of repeated experiments. As an illustration, we show how the pulse sequences designed numerically could be applied to the problem of quantifying intraluminal lipid deposits in the carotid artery.

Overcoming Artifacts from Metallic Orthopedic Implants at High-Field-Strength MR Imaging and Multi-detector CT

At magnetic resonance (MR) imaging and multidetector computed tomography (CT), artifacts arising from metallic orthopedic hardware are an obstacle to obtaining optimal images. Although various techniques for reducing such artifacts have been developed and corroborated by previous researchers, a new era of more powerful MR imaging and multidetector CT modalities has renewed the importance of a systematic consideration of methods for artifact reduction. Knowledge of the factors that contribute to artifacts, of related theories, and of artifact reduction techniques has become mandatory for radiologists. Factors that affect artifacts on MR images include the composition of the metallic hardware, the orientation of the hardware in relation to the direction of the main magnetic field, the strength of the magnetic field, the pulse sequence type, and other MR imaging parameters (mainly voxel size, which is determined by the field of view, image matrix, section thickness, and echo train length). At multidetector CT, the factors that affect artifacts include the composition of the hardware, orientation of the hardware, acquisition parameters (peak voltage, tube charge, collimation, and acquired section thickness), and reconstruction parameters (reconstructed section thickness, reconstruction algorithm used, and whether an extended CT scale was used). A comparison of images obtained with different hardware and different acquisition and reconstruction parameters facilitates an understanding of methods for reducing or overcoming artifacts related to metallic implants.

Technical parameters affecting image characteristics in in vivo MR microscopy of the mouse.

The aim of the study was to assess the effects of changing acquisition parameters used for high-resolution in vivo magnetic resonance (MR) microscopy on image quality and scan time. The head or abdomen of 11 normal and 1 glioblastoma-bearing anesthetized BALB/c mice were imaged using a high-resolution 7.0-Tesla magnet. Scan parameters such as matrix size (MTX), slice thickness (ST), number of excitations (NEX), pulse sequence type including repetition time (TR) and echo time (TE), respiratory gating, and intraperitoneal contrast medium administration were altered to assess their actual effect on signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) as compared to calculated effects. As expected, SNR increased with increasing ST or NEX and with decreasing MTX. However, although the empirical increase in SNR was similar to that expected for increased ST, it was less than that anticipated for increasing NEX or decreasing MTX. Increasing NEX and applying respiratory gating both increased SNR and reduced the image degradation associated with respiratory motion in images of the abdomen. Intraperitoneal contrast medium administration produced a marked increase in CNR in the subject with the implanted glioblastoma, suggesting that this route is satisfactory for the enhancement of lesions disrupting the blood-brain barrier. The consequence of improving image quality in terms of spatial and contrast resolution is increased scan time. However, the actual increase in SNR when altering acquisition parameters may not be as much as predicted by theory.

Estimating cerebral atrophy in multiple sclerosis patients from various MR pulse sequences.

The purpose of this study was to determine how measures reflecting cerebral atrophy (CA) are influenced by pulse sequence (PS) and segmentation algorithm (SA) used in multiple sclerosis (MS) patients and healthy control (HC)s. Magnetic resonance imaging (MRI) scans from 10 relapsing-remitting MS (RRMS) patients and five HCs were used to determine the change in brain fractional volume (BFV) over a two-year period. T1-weighted, fluid-attenuated inversion recovery (FLAIR), and proton density (PD)/T2-weighted sequences were analysed Image segmentation to determine brain volume was performed using the following a histogram SA, an adaptive fuzzy c-means algorithm (AFCM), and an adaptive Bayesian segmentation with a K-means clustering. Combinations of the SA and PS in MS patents demonstrated significant differences in the per cent change in BFV from baseline. For the combination of PS and SA the per cent change in BFV for year one and year two varied from +2.05% to - 1.6% and +0.79% to -3.11%, respectively. Analysis of the HCs data revealed fluctuations in BFV varying from +0.26% to -0.29%. MRI estimates of CA are dependent on both the type of PS and SA; therefore, the choice of SA technique and PS should be consistent during an MS treatment trial. The progression of CA in MS should only be used as a secondary or tertiary outcome measure in treatment trials until a better understanding of how this measurement is affected by the disease, the image acquisition and analysis techniques.

Voltage shift phenomena in a heteroepitaxial BaTiO3 thin film capacitor

Voltage shift phenomena of the hysteresis loop were characterized for a c-axis oriented heteroepitaxial BaTiO3 film by means of switching current measurements using various types of pulse sequences. During application of voltage, the hysteresis loop gradually shifted along the voltage axis according to the polarity of the voltage. Even after the application of voltage, while the top and bottom electrodes were short-circuited, the hysteresis loop continued to move. Under certain conditions, a part of the hysteresis loop shifted back, whereas the rest shifted forward. These results were explained, assuming that there is a nonswitching layer between the ferroelectric layer and the bottom electrode, and that the discontinuity of polarization can be compensated by injection of negative charges from the electrode. It was suggested that the nonswitching layer is possibly formed by relaxation of lattice misfit strain in the heteroepitaxial ferroelectric thin film.

5512825 Method of minimizing dead-periods in magnetic resonance imaging pulse sequences and associated apparatus

A method of minimizing dead-periods in magnetic resonance imaging pulse sequences employs a specimen disposed within magnetic field, a source of RF signals, a receiver for receiving signals from the specimen responsive to RF pulses and emitting respective output signals. A computer is provided for receiving the output signals from the receiver and establishing image information which may be displayed. For the dead-period, the minimum and maximum phase encoding step, the scan plane gradient pulse for the slice, phase encoding and readout directions are determined and the moments contained within the dead-period waveform is determined. The values are transformed into gradient amplifier coordinates and the minimum dead-period based on a dead-period waveform is determined. The dead-period is employed in establishing a hardware optimized waveform which may be trapezoidal. The trapezoidal waveform is preferably established by for each phase encoding step determining the starting and ending gradient levels and the moments contained within the dead-period waveform and employing the waveform to design a trapezoidal waveform using the calculated minimum dead-period. The method is particularly advantageous when employed in oblique magnetic resonance imaging. The method may also be employed with velocity-encoded or flow-compensated pulse sequences by employing first gradient moments in the information processing, in addition to the starting and ending gradient levels and the zeroeth moments. For other types of pulse sequences, the zeroeth moment and other higher moments may be employed. Associated apparatus is also disclosed.

Reduction of the water hump using a composite refocusing pulse

.B, field inhomogeneity gives rise to a broad hump in a NMR spectrum. This hump is ;a particular nuisance in recording the proton NMR spectrum of dilute samples dissolved in Hz0 because the intense hump from H20, referred to as a “water hump,” cannot be saturated with a monochromatic RF field so that it remains as an intense an’d broad signal in the spectrum, which makes it difficult to observe weak solute signals. Dykstra has demonstrated that the modification of a NMR probe by shielding the leard wire of RF coil can significantly reduce the water hump ( 1) . Many recent NMR probes have been modified according to the procedure of Dykstra, and we also use thi.s kind of a modified NMR probe on our spectrometer in the present study. The use of the modified probe efficiently reduces the water hump by 90-95% (1). However, for dilute solutions, especially those with high ionic strength, the remaining water hnmp is still a severe obstacle. Under the condition of high ionic strength, RF power in the sample solution is reduced due to dielectric loss and then the resulting inhomogeneous B, field becomes significant, which gives a larger water hump. Several pulse sequences have been devised for obtaining the spatial selectivity of signals in in vivo spectroscopy. Bendall and Gordon (2) have proposed a pulse sequence which can selectively observe signals from the region of homogeneous B, field. Their pulse sequence is based on a (90”-180” -Acq.) scheme, where the phases of 180” pulse and receiver are cycled in a Exorcycle manner as described by Bodenhausen (3). To improve spatial selectivity further, both Shaka and Freeman (4) and Tycko and Pines (5 ) have developed composite narrow-band inversion pulses that are very sensitive to B1 field strength. The replacement of a regular 180” pulse in the (90°18;O”-Acq.) scheme with these narrow-band composite pulses makes for more efficient spatially selective observation. In a different type of pulse sequence, Bax has shown a spatially selective composite 90” pulse (6). Recently, Zagorski has demonstrated that the wideband inversion pulse GROPE-16, by Shaka and Freeman ( 7), is applicable to solving the problem of B, inhomogeneity (8). The requirement of the pulse scheme for the water hump suppression in highresolution NMR spectroscopy is to eliminate only the water hump efficiently without attenuating signals over a relatively wide frequency range. However, these requirements

Broadband homonuclear cross polarization using flip-flop spectroscopy

We have developed a new type of pulse sequence for broadband homonuclear cross polarization (l-10), known as TOCSY or HOHAHA in the literature. Our new sequences, called FLOPSY-8 and FLOPSY16 (flip-flop spectroscopy), offer a noticeable improvement in polarization transfer efficiency for a given expenditure of RF power, and outperform commonly used windowless sequences like WALTZ-16, DIPSI-2, DIPSI-3, and MLEV-16 or MLEV-17. In this Communication we describe how the FLOPSY sequences work by a geometrical analogy with the firmly established area of composite 180” pulses, and we compare FLOPSY with the other sequences. In Fig. 1 we show the familiar rotating-frame geometrical picture for an ensemble of isolated spin-f nuclei, in which the orthogonal axes are labeled with the linear spin operators I,, I,,, and I,. The state of the spins is represented as a vector from the origin to a point on the surface of the unit sphere, and evolution under an RF field is represented as a rotation of the spin vector about a vector representing the effective field. Spin inversion is accomplished on resonance by 180” pulse, mapping 1, onto 1,. Off resonance the effective field has a nonzero z component, and complete inversion is not possible with a single elementary rotation. On the right we show an analogous picture for the zero-quantum (ZQ) manifold of a two-spin system. Here it is the average d@erence in z magnetization II, Izz which labels the z component, and the transverse components in the picture label the x and y components of ZQ coherence. The x component is familiar as the transverse part of the scalar spin-spin coupling; the y component does not normally appear as part of the high-resolution spin Hamiltonian. It is formally related to the x component by a zero-quantum phase shift, that is, a unitary transformation by the longitudinal component in the ZQ frame. The three components in the ZQ frame are easily derived by considering the single-transition (II) or fictitious spin-4 (12) operators for the flip-flop transition I+-) I -+). Provided the dynamics remain within the ZQ manifold, our geometrical picture makes it easy to understand broadband cross polarization. An on-resonance 180” pulse in the ZQ frame will invert the average difference in z magnetization. The total z magnetization remains unaffected by the ZQ rotations, so we can write

Optimal Pulse Sequences for Magnetic Resonance Imaging-Computing Accurate T1, T2, and Proton Density Images

Proton nuclear magnetic resonance images (proton MRI) are functions of not only proton density (rho), spin lattice relaxation time (T1), and spin-spin relaxation time (T2) but also MRI scan parameters, so the images differ for different pulse sequences and scan parameters. Systematic measuring errors also vary depending on the type of pulse sequence and the scan parameters. "Pure," objective, T1, T2, and proton density images can be computed from several different Proton MRI, however, systematic measuring errors can cause large deviations between "pure" images obtained using different methods. We tested several scan sequences, and investigated methods to improve the resolution and reduce the errors over the range of T1, T2, and proton density values encountered in representative human tissues. We found that, for given scan time, the combination of inversion recovery 3 spin echo (IR3SE) and saturation recovery 4 spin echo (SR4SE) sequences gave more accurate computed images than other comparable methods tested. It follows, then, that adopting such a sequence as "standard" allows meaningful comparison of clinical results obtained by different researchers.

Strategies and tactics in NMR imaging relaxation time measurements. I. Minimizing relaxation time errors due to image noise--the ideal case.

The effect of NMR image noise on errors in calculated values of relaxation times is quantitatively assessed by use of relaxation time noise figures, which are derived on the basis of statistical principles as functions of pulse delay, repetition, and recovery intervals for several types of pulse sequences. Two strategies for determining relaxation times are considered: two point (ratio of intensities for two experiments) and multipoint (least-squares fit of intensities to pulse-sequence functions for n experiments). For given total measurement times, values of pulse interval times are found which give minimum relaxation time noise figures. A comparison of ratio methods shows that the best is a combination saturation-recovery, inversion-recovery (SR/IR) technique. For short measurement times (less than about 10T1) this optimized SR/IR ratio determination is also superior to the best multipoint method, a series of inversion-recovery experiments with equally incremented inversion-recovery times. An examination of the effect of signal averaging on the relaxation time noise shows that up to a measurement time characteristic of the particular method used (e.g., for times up to about 5T1 for the SR/IR ratio determination, 100T1 for the multipoint inversion-recovery method), increased measurement time is more effectively allotted to longer pulse intervals than to signal averaging. Numerical examples are tabulated which can help one to set optimum values for pulse intervals, given a rough estimate of the relaxation time to be determined.

Contrast manipulation in NMR imaging

The past few years have shown rapid growth of NMR imaging in both image quality and diagnostic usefulness. It has become apparent, as the images have been published, that both inter- and intra-group imaging of the same underlying pathology produces images which can have vastly differing appearance. This effect is mainly due to imaging techniques which use different pulse sequence types and timings thus varying the relative contribution of the protpn density, T 1, and T 2 properties of the tissues. In this paper we investigate the contrast manipulation effects and methods for SNR optimization for the saturation recovery, inversion recovery, spin echo, and inversion recovery spin echo pulse sequences when applied to three clinically relevant imaging tasks.