MR of the spine with a fast T1-weighted fluid-attenuated inversion recovery sequence.

PURPOSETo optimize a T1-weighted fast fluid-attenuated inversion recovery (FLAIR) sequence using computer-simulated data and to study its clinical utility for imaging the spine.METHODSRelative signal intensities and contrast of relevant normal and pathologic tissues in the spine were computed using an inversion recovery equation modified to account for a hybrid RARE (rapid acquisition with relaxation enhancement) readout. A range of inversion time (TI) and repetition time (TR) pairs that null the signal from CSF was generated. A contrast-optimized heavily T1-weighted fast FLAIR sequence, based on the generated data, was qualitatively compared with conventional T1-weighted spin-echo sequences for imaging various spinal abnormalities.RESULTSA T1/TR pair of approximately 862/2000 was extracted from the computer-generated data to produce effective nulling of CSF signal, to achieve heavy T1 weighting, and to optimize contrast between abnormal tissues and cord/bone marrow. Clinical implementation of the optimized T1-weighted fast FLAIR sequence revealed superior contrast at the CSF-cord interface, better conspicuity of lesions of the spinal cord and bone marrow, and reduced hardware-related artifacts as compared with conventional T1-weighted spin-echo sequences.CONCLUSIONThe optimized T1-weighted fast FLAIR technique has definite advantages over spin-echo sequences for imaging the spine. Comparable acquisition times render the FLAIR sequence the method of choice for T1-weighted imaging of the spine.     

Multislice T1-weighted hybrid rare in CNS imaging: Assessment of magnetization transfer effects and artifacts

Using a T1-weighted hybrid rapid acquisition with relaxation enhancement (RARE) MR sequence that implements an echo-to-view mapping scheme termed “low-high profile order,” we evaluated signal intensity changes in different brain tissues as a function of number of slices, interslice gap, and echo train length (ETL). We also measured phase-encode and frequency-encode noise as well as blurring artifacts along the phase-encode direction as a function of ETL. Off-resonance magnetization transfer effects were demonstrated to be responsible for signal intensities changes in white matter and gray matter when using multislice techniques. These effects are amplified by increasing the number of slices and ETL. Due to the nature of the implemented echo-to-view mapping scheme, no on-resonance magnetization transfer effects were observed from the intraslice echo train. Selective background (white matter and gray matter) suppression in multislice T1-weighted hybrid RARE, secondary to off-resonance magnetization transfer effects, may provide better contrast resolution of enhancing central nervous system (CNS) lesions at much shorter scan time as compared to conventional spin-echo T1-weighted sequences. This improvement in contrast resolution as a function of ETL may be limited by worsening phase-encode noise and blurring artifacts.     

Hybrid RARE: Implementations for abdominal MR imaging

Hybrid RARE (rapid acquisition with relaxation enhancement) is a family of magnetic resonance (MR) imaging techniques whereby a set of images is phase encoded with more than one spin echo per excitation pulse. This increases the efficiency of obtaining T2-weighted images, allowing greater flexibility regarding acquisition time, resolution, signal-to-noise ratio, and tissue contrast. Hybrid RARE techniques involve several important new user-selectable parameters such as effective TE, echo train length, and echo spacing. Choices of other parameters, such as TR, sampling bandwidth, and acquisition matrix, may be different from those of comparable conventional T2-weighted spin-echo images. Different hybrid RARE implementations can be used for abdominal screening, with T2-weighted or T2-weighted and inversion-recovery contrast, or for characterizing liver lesions or imaging the biliary system with an extremely long TE. High-resolution images may be obtained by averaging multiple signals during quiet breathing, or images may be acquired more rapidly during suspended respiration. In this review, the authors discuss the basic principles of hybrid RARE techniques and how various imaging parameters can be manipulated to increase the quality and flexibility of abdominal T2-weighted MR imaging.     

Magnetization transfer imaging of the head and neck: normative data.

PURPOSETo determine magnetization transfer ratios for normal head and neck structures so that evaluation of disease will be possible.METHODSTwo-dimensional magnetization transfer imaging was performed in 12 healthy volunteers and 20 patients. We used a repetition time of 500, echo time of 12, 20 degrees flip angle, and a magnetization transfer pulse offset from the resonance frequency of water by 2000 Hz (pulse duration 19 milliseconds, waveform area approximately 10 times greater than that of a 90 degree pulse). Magnetization transfer ratios (1 - [intensity after suppression/intensity before suppression]) were calculated for normal structures.RESULTSThe magnetization transfer ratio of facial muscles (0.54) was equivalent to that of tongue muscles (0.54). These values exceeded those of parotid (0.39) and submandibular glands (0.41). Fat (0.07) and cerebrospinal fluid (0.05) had negligible transfer.CONCLUSIONMagnetization transfer imaging is a simple and effective means of studying the contribution of macromolecular protons to the MR image. Normal neck structures show a wide range of magnetization transfer rates, maximal for muscle and minimal for cerebrospinal fluid and fat.     

CSF-gated MR imaging of the spine: theory and clinical implementation

A spine phantom and cervical spines of seven volunteers were studied with cerebrospinal fluid (CSF)-gated magnetic resonance imaging to optimize acquisition factors reducing CSF flow artifacts. Peripheral gating was performed with either an infrared reflectance photoplethysmograph or peripheral arterial Doppler signal. The effects of effective repetition time, echo train, trigger delay, number of sections, and imaging plane on image quality were evaluated. Gated imaging of oscillatory CSF motion simulated constant-velocity flow and reduced CSF flow artifacts caused by cardiac-dependent temporal phase-shift effects. Velocity compensation on sagittal even-echo images with a symmetric short-echo time echo train reduced the remaining CSF flow artifacts caused by spatial phase-shift effects. Overall gated imaging time was not increased compared with nongated imaging and was reduced when improved image quality permitted the use of fewer excitations. These results suggest that the combination of CSF gating and flow compensation is clinically useful and efficient because it improves image quality without prolonging imaging time.     

Extraneous lipid contamination in single-volume proton MR spectroscopy: phantom and human studies.

PURPOSETo determine the degree of extraneous lipid contamination in defined volumes of interest studied with single-volume proton MR spectroscopy.METHODSSingle-volume proton MR spectroscopy was performed on a fat/water phantom and in three volunteers using the stimulated-echo acquisition mode (STEAM) and point-resolved spectroscopy (PRESS) localization methods. Three different volumes of interest (8, 27, and 64 cm3) were examined at echo times of 20, 135, and 270 for the STEAM sequences and 135 and 270 for the PRESS acquisitions in both the phantom and the volunteers (volumes of interest were placed adjacent to but not encompassing fat-containing structures, such as the scalp and retroorbital fat). The degree of lipid contamination was then correlated with measurements of the section profiles.RESULTSThe PRESS method resulted in less extraneous lipid contamination in both phantom and volunteer studies. The STEAM method had the highest level of lipid contamination signal in phantom and human studies. In the volunteers, volumes of interest abutting fat-containing structures obtained with PRESS or STEAM sequences showed no lipid contamination. However, the STEAM sequences showed lipid signal in the volume of interest adjacent to orbital fat whereas the PRESS sequences did not. These observations are supported by the section profile studies, which showed that the actual volume excited by the STEAM sequence was 7% to 32% larger than that originally selected, while with PRESS the actual excited volume was 12% to 16% smaller than that originally selected.CONCLUSIONIn our MR unit, short-echo-time STEAM sequences (< or = 135 milliseconds) resulted in extraneous lipid contamination in phantom and human studies adjacent to the orbits. PRESS sequences showed no lipid contamination in volumes abutting fat structures in phantoms or humans. These results correlated closely with the configuration of the section profiles. Although these findings might be dependent on the MR unit used, our study could help determine extraneous lipid contamination for other MR units.     

Effective blood signal suppression using double inversion-recovery and slice reordering for multislice fast spin-echo MRI and its application in simultaneous proton density and T2 weighted imaging

PURPOSE: To design a multislice double inversion-recovery fast spin-echo (FSE) sequence, with k-space reordered by inversion time at slice position (KRISP) technique, to produce black-blood vessel wall magnetic resonance imaging (MRI). MATERIALS AND METHODS: In this sequence, central k-space sampling for each slice is required at inversion time (TI) of the blood signal. To fill the entire k-space, the peripheral lines are obtained less or greater the TI and using a rotating slice order. Blood flow signal suppression was first evaluated using a phantom. Simulation studies were used to investigate FSE image quality. The final sequence was then applied to the rabbit abdominal aorta MRI at 4.7 T. RESULTS: In the flow phantom study, artifacts from slow-flowing water were substantially reduced by the KRISP technique; residual water spins were dephased by the strong phase-encoding gradient required for peripheral k-space. These dephased spins flowed into the slice plane where the center of k-space was being acquired at the TI of the flowing water signal. Multislice black-blood MR images were successfully obtained in the rabbit abdomen using the sequence with the k-trajectory optimized by the simulation study. CONCLUSION: The KRISP technique was effective both in multislice double inversion-recovery FSE and in blood signal suppression.     

Improved fat suppression in STIR MR imaging: selecting inversion time through spectral display

By observing the fat-signal peak on the spectral display of a magnetic resonance (MR) imager while varying inversion time (TI), the authors determined the TI that produced the lowest fat peak for the best suppression of fat signal in subsequent short-TI inversion-recovery (STIR) MR imaging. In 25 volunteers who underwent imaging at multiple TIs, the TI that produced the lowest measured fat signal intensity was the same as that selected by means of TI tuning in 60% of cases and was within 5 msec in the remaining 40%.     

Pulse sequence extrapolation with MR image synthesis

Previous reports have presented validation studies of magnetic resonance (MR) image synthesis in which multiple spin-echo (MSE) source data were used to generate spin-echo images for various echo times and repetition times (TRs). A new method-"pulse sequence extrapolation" -synthesizes images for pulse sequences different from that of the acquisition. MSE data acquired in a time equivalent to a TR of 2,000 msec can be used to generate inversion-recovery (IR) images for arbitrarily chosen TI inversion times. Other combinations of pulse sequences were also studied, and synthetic images were compared visually and quantitatively to directly acquired images with corresponding parameters. Synthetic IR signals of the brain parenchyma consistently matched directly acquired signals to within 6%, with respect to the full magnetization signal. The noise level of synthetic signals was generally no more than twice that of direct acquisition signals, as predicted. This method can achieve selective fat suppression and enhancement in IR imaging.     

Dual inversion recovery,ultrashort echo time (DIR UTE) imaging: Creating high contrast for short‐T2 species

Imaging of short‐T₂ species requires not only a short echo time but also efficient suppression of long‐T₂ species in order to maximize the short‐T₂ contrast and dynamic range. This paper introduces a method of long‐T₂ suppression using two long adiabatic inversion pulses. The first adiabatic inversion pulse inverts the magnetization of long‐T₂ water and the second one inverts that of fat. Short‐T₂ species experience a significant transverse relaxation during the long adiabatic inversion process and are minimally affected by the inversion pulses. Data acquisition with a short echo time of 8 μs starts following a time delay of inversion time (TI1) for the inverted water magnetization to reach a null point and a time delay of TI2 for the inverted fat magnetization to reach a null point. The suppression of long‐T₂ species depends on proper combination of TI1, TI2, and pulse repetition time. It is insensitive to radiofrequency inhomogeneities because of the adiabatic inversion pulses. The feasibility of this dual inversion recovery ultrashort echo time technique was demonstrated on phantoms, cadaveric specimens, and healthy volunteers, using a clinical 3‐T scanner. High image contrast was achieved for the deep radial and calcified layers of articular cartilage, cortical bone, and the Achilles tendon. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Optimizing conventional MR imaging of the spine

With the use of conventional spin-echo pulse sequences with a long repetition time (TR), the echo time (TE) and the number of echoes were varied to minimize cerebrospinal fluid (CSF) flow artifacts in a spine phantom and in cervical spines of three volunteers. The following echo trains were compared in both axial and sagittal planes with a TR of 2,000 msec: TE of 25, 80 msec ("asymmetric"); TE of 40, 80 msec ("symmetric long TE"); and TE of 20, 40, 60, and 80 msec ("symmetric short TE"). Variable degrees of even-echo rephasing of CSF flow artifacts were observed during sagittal but not axial imaging, depending on the echo train used. Even-echo rephasing was most complete with the symmetric short-TE echo train, less complete with the symmetric long-TE echo train, and absent with the asymmetric echo train. Switching the orientation of the phase and frequency encoding gradients and slightly modifying TR on the basis of the heart rate further improved image quality. The results suggest that a symmetric short-TE echo train may be used to provide velocity compensation (similar to that observed with rephasing gradients) on even echoes of conventional spin-echo pulse sequences during spine imaging.     

Mixed echo train acquisition displacement encoding with stimulated echoes: an optimized DENSE method for in vivo functional imaging of the human heart.

Mixed echo train acquisition displacement encoding with stimulated echoes (meta-DENSE) is a phase-based displacement mapping technique suitable for imaging myocardial function. This method has been optimized for use with patients who have a history of myocardial infarction. The total scan time is 12-14 heartbeats for an in-plane resolution of 2.8 x 2.8 mm2. Myocardial strain is mapped at this resolution with an accuracy of 2% strain in vivo. Compared to standard stimulated echo (STE) methods, both data acquisition speed and resolution are improved with inversion-recovery FID suppression and the meta-DENSE readout scheme. Data processing requires minimal user intervention and provides a rapid quantitative feedback on the MRI scanner for evaluating cardiac function. Published 2001 Wiley-Liss, Inc.     

Double inversion recovery imaging of the brain: deriving the most relevant sequence through real images

We propose a practical method for setting the optimal inversion times (TI) for double inversion recovery (DIR) sequences. Our method used the measurement of signal intensity (SI) from real images to set the optimal TI for white-matter (WM) and gray-matter (GM)-attenuated inversion recovery (WAIR and GAIR, respectively) images. 3D-DIR images of healthy volunteers were obtained on 1.5- and 3.0-T magnetic resonance (MR) scanners and the SIs of GM, WM, and cerebrospinal fluid (CSF) were evaluated on real images. We found TI₂s at which the SI of WM or GM was null. Then, we found TI₁₊₂ (=TI₁ + TI₂) at which the SI of CSF was null. We defined the two TIs as optimal TIs. We assessed the utility of these TIs with additional volunteers and patients, and similar images were obtained with the determined TIs. Optimal TIs for DIR images could be efficiently determined using this method.     

Optimization and validation of a fully-integrated pulse sequence for modified look-locker inversion-recovery (MOLLI) T1 mapping of the heart

PURPOSE: To optimize and validate a fully-integrated version of modified Look-Locker inversion-recovery (MOLLI) for clinical single-breathhold cardiac T1 mapping. MATERIALS AND METHODS: A MOLLI variant allowing direct access to all pulse sequence parameters was implemented on a 1.5T MR system. Varying four critical sequence parameters, MOLLI was performed in eight gadolinium-doped agarose gel phantoms at different simulated heart rates. T1 values were derived for each variant and compared to nominal T1 values. Based on the results, MOLLI was performed in midcavity short-axis views of 20 healthy volunteers pre- and post-Gd-DTPA. RESULTS: In phantoms, a readout flip angle of 35 degrees , minimum TI of 100 msec, TI increment of 80 msec, and use of three pausing heart cycles allowed for most accurate and least heart rate-dependent T1 measurements. Using this pulse sequence scheme in humans, T1 relaxation times in normal myocardium were comparable to data from previous studies, and showed narrow ranges both pre- and postcontrast without heart rate dependency. CONCLUSION: We present an optimized implementation of MOLLI for fast T1 mapping with high spatial resolution, which can be integrated into routine imaging protocols. T1 accuracy is superior to the original set of pulse sequence parameters and heart rate dependency is avoided.     

Contrast optimization of fluid-attenuated inversion-recovery (FLAIR) MR imaging in patients with high CSF blood or protein content.

After surgical resection of a brain tumor or infection of the cerebrospinal fluid (CSF), elevated levels of blood by-products or protein contaminations are seen in the patient's CSF spaces. In fast fluid-attenuated inversion-recovery (FLAIR) imaging CSF signal is nulled by an appropriate choice of the inversion recovery time TI to improve the contrast between tissue structures adjacent to CSF-filled volumes. With contaminated CSF, however, the longitudinal relaxation time T(1) may change significantly, which results in an incomplete suppression in the FLAIR images, if standard inversion times are used. In this work, a fast single-voxel T(1) measurement pulse sequence with integrated T(1) calculation that allows determination the optimal TI value in 15 sec is presented. The method was tested in five patients after surgical resection of a brain tumor, where FLAIR MRI with and without contrast agent was performed to identify remaining tumor fragments at the margin of the resection cavity.     

Enhanced viability imaging: improved contrast in myocardial delayed enhancement using dual inversion time subtraction.

In delayed contrast-enhanced MRI for the assessment of myocardial viability, the TI time in a gated inversion-recovery segmented gradient echo sequence is usually selected to null signal from normal myocardium. Although this TI time generates good contrast between the enhancing infarcted tissue and normal myocardium, there is usually less contrast between the infarct and the blood pool. A subtractive technique utilizing two acquisitions at a long and short TI time is proposed to improve the delineation between infarct-blood and infarct-myocardium. The concept was demonstrated in six mongrel dogs with reperfused myocardial infarction. Infarct-normal myocardium contrast (signal difference) using the proposed enhanced viability imaging (ENVI) technique was 142 +/- 50% (P < 0.001) that of standard magnitude inversion recovery (IR), while at the same TI time for the primary image, infarct-blood contrast, was 247 +/- 136% (P < 0.002) that of magnitude IR. Accounting for increased noise due to the subtraction, signal difference-to-noise ratios (SDNR) did not show a significant change for infarct-myocardium but infarct-blood SDNR for ENVI was 174 +/- 105% that of magnitude-IR (P < 0.03). Thus, marked improvement in the delineation of the infarcted zone was noted over a range of TI times.     

Optimization of TI values in inversion-recovery MR sequences for the depiction of fine structures within gray and white matter: separation of globus pallidus interna and externa

RATIONALE AND OBJECTIVES: The authors' purpose was to search the inversion time (TI) values that enable the best differentiation of fine structures in gray matter (gray-gray differentiation). MATERIALS AND METHODS: Seven healthy adult volunteers with no history of neurologic disease or head trauma were recruited and gave their informed consent. The subjects consisted of two men and five women ranging in age from 25 to 38 years, with a mean age of 28 years +/- 5. The subjects were imaged with a turbo spin-echo inversion-recovery sequence. This sequence was performed in the axial plane at the level of the basal ganglia with the following parameters: repetition time, 3,200 msec; echo time, 15 msec; three signals acquired; echo train, seven; section thickness, 3 mm; matrix size, 256 x 256; and field of view, 180 mm. The tested values were TI = 100, 200, 300, 400, and 500 msec. Region-of interest measurements were performed at the following anatomic structures and represent gray-gray and white-white differentiations, respectively: globus pallidus externa versus globus pallidus interna, and optic radiation versus surrounding white matter. RESULTS: The maximum contrast index value occurred at TI = 400 msec for globus pallidus externa versus globus pallidus interna (P < .05) With the contrast-to-noise ratio, no significant difference in gray-gray differentiation was observed among the various TIs. The minimum signal-to-noise ratio of the gray matter occurred at TI = 400 msec (P < .05). A subjective evaluation revealed an overall superiority of gray-matter differentiation with TI = 400 msec. CONCLUSION: A TI of 400 msec was the most suitable for this purpose.     

Fast short-tau inversion-recovery MR imaging

To enhance the versatility of the short-tau inversion-recovery (STIR) sequences, the authors determined a range of repetition time (TR) and inversion time (TI) combinations that suppress signal intensity from fat by study of both patient and phantom images. To make fast STIR images, variations in the following pulsing conditions were studied with use of an interactive computer program: decreasing the TR, limiting the number of excitations, and limiting the number of phase-encoding steps. The authors found that (a) STIR imaging need not be time consuming, (b) fat suppression can be accomplished at shorter TR by using shorter TI, and (c) short-TR fast STIR imaging is sensitive to enhancement with gadopentetate dimeglumine.     

Reduction of MRI signal distortion from titanium intracavitary brachytherapy applicator by optimizing pulse sequence parameters

PurposeTo demonstrate that optimized pulse sequence parameters for a T2-weighted (T2w) fast spin echo acquisition reduced artifacts from a titanium brachytherapy applicator compared to conventional sequence parameters.Methods and materialsFollowing Institutional Review Board approval and informed consent, seven patients were successfully imaged with both standard sagittal T2w fast spin echo parameters (voxel size of 0.98 × 0.78 × 4.0 mm³; readout bandwidth of 200 Hz/px; repetition time of 2800 ms; echo time of 91 ms; echo train length of 15; 36 slices; and imaging time of 3:16 min) and an additional optimized T2w sequence (voxel size of 0.98 × 0.98 × 4.0 mm³; readout bandwidth of 500 Hz/px; repetition time of 3610 ms; echo time of 91 ms; echo train length of 25; 18–36 slices; and imaging time of 1:15–2:30 min), which had demonstrated artifact reduction in prior phantom work. Visualized intracavitary tandem was hand-segmented by two of the authors. Three body imaging radiologists assessed image quality and intraobserver agreement scores were analyzed.ResultsThe average segmented volume of the intracavitary applicator significantly (p < 0.05) decreased with the experimental pulse sequence parameters as compared to the standard pulse sequence. Comparison of experimental and standard T2w sequence qualitative scores for each reviewer showed no significant differences between the two techniques.ConclusionsThis study demonstrated that pulse sequence parameter optimization can significantly reduce distortion artifact from titanium applicators while maintaining image quality and reasonable imaging times.     

MR pulse sequence selection for optimal display of acrylic polymer phantoms

PurposeAcrylic phantoms are being increasingly used as an alternative to cadaveric and animal derived tissue samples in pre-clinical magnetic resonance (MR) imaging research studies. Such phantoms have been imaged using a variety of MR sequences but little effort has been devoted to determining the most appropriate MR pulse sequence. In order to address this question, a prospective comparative study was performed to determine which MR sequence optimally demonstrates acrylic polymer phantoms.MethodsNine MR imaging sequences were selected and used to image an acrylic phantom placed in a water bath. The mid-sagittal slice of each sequence was used to determine signal-to-noise (S/N) and contrast-to-noise (C/N) ratios. The signal intensity maximum gradient at the phantom/fluid interface was used as a measure of edge delineation.ResultsOf the nine sequences reviewed the T2 driven equilibrium pulse sequence (DRIVE), a 3D turbo spin echo (TSE) sequence, provided the highest S/N (72.88) and C/N (69.62) ratios, while the proton density (PD) TSE or intermediate T2 TSE sequences provided the best edge delineation (definition of the acrylic-fluid interface).ConclusionThis study suggests that selection of MR pulse sequences when evaluating a submersed acrylic phantom is dependent on which information is most important to the researcher. If S/N and C/N are considered most important, of the nine MR sequences tested, the T2 DRIVE sequence may be best employed. If edge delineation is considered most important, then PD or intermediate T2 TSE sequences may be best employed.