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PubMed2002

Improved efficiency in double-inversion fast spin-echo imaging.

Double-inversion fast spin-echo (FSE) pulse sequences can be designed to provide excellent suppression of blood signal in black-blood MRI. However, because a nonselective inversion is used, these sequences typically have been highly inefficient. In this work it is demonstrated that the efficiency of double-inversion sequences can be greatly improved by a form of interleaving in which all of the slices to be imaged in a single pass are reinverted each time a signal is obtained from any single slice. To date, several studies have demonstrated a high level of blood suppression with these more efficient techniques.

Aged↗
PubMed2002

T1-insensitive flow suppression using quadruple inversion-recovery.

A new flow suppression method has been proposed for the acquisition of blood-suppressed (black-blood) images in combination with administration of a positive contrast agent. The technique employs the quadruple inversion-recovery (QIR) preparative pulse sequence, which consists of two double-inversion modules followed by two delays. Within each double inversion, a nonselective RF pulse is immediately followed by a slice-selective one. The time intervals of the sequence can be calculated using an algorithm based on minimization of the variation of a signal equation over an entire range of T(1) occurring in blood before and after contrast administration. QIR is highly insensitive to variations of T(1), providing efficient suppression of a flow signal with T(1) in a range of 200-1200 ms. The technique utilizes identical scan parameters for pre- and postcontrast acquisition, and thus allows reliable quantitative interpretation of contrast enhancement (CE). The clinical application of QIR was demonstrated in high-resolution, contrast-enhanced, black-blood imaging of atherosclerotic plzzaque.

Contrast Media↗
PubMed2001

T(1) quantification with inversion recovery TrueFISP.

A snapshot FLASH sequence can be used to acquire the time course of longitudinal magnetization during its recovery after a single inversion pulse. However, excitation pulses disturb the exponential recovery of longitudinal magnetization and may produce systematic errors in T(1) estimations. In this context the possibility of using the TrueFISP sequence to detect the recovery of longitudinal magnetization for quantitative T(1) measurements was examined. Experiments were performed on different Gd-doped water phantoms and on humans. T(1) values derived from inversion recovery TrueFISP were in excellent agreement with the single-point method even for flip angles up to 50 degrees. In terms of T(1) accuracy and SNR, the proposed method seems to be superior to the conventional inversion recovery snapshot FLASH technique. Magn Reson Med 45:720-723, 2001.

Brain↗
PubMed2001

FLAIR imaging using nonselective inversion pulses combined with slice excitation order cycling and k-space reordering to reduce flow artifacts.

High-signal artifacts produced by cerebrospinal fluid (CSF) flow can adversely affect fluid-attenuated inversion recovery (FLAIR) imaging of the brain and spinal cord. This study explores the use of a nonslice-selective inversion pulse to eliminate CSF flow artifacts together with a technique called "K-space Reordered by Inversion-time for each Slice Position" (KRISP) to achieve constant contrast in a multislice acquisition. Theory shows that with this method the CSF point spread function (PSF) has a minimum at the center and attenuated side lobes, providing CSF suppression, but residual edge signals remain. The PSF for brain is only mildly attenuated and signals for extended regions are not attenuated. KRISP FLAIR sequences were assessed in 15 patients (10 brain and five spinal cord cases). The images showed reduced CSF and blood flow artifacts and higher conspicuity of the cortex, meninges, ventricular system, brainstem, and cerebellum when compared with conventional FLAIR sequences.

Adult↗
PubMed1987

Magnetic resonance imaging of midline brain tumors using inversion recovery sequences at 0.08 T (3.4 MHz).

The use of inversion recovery sequences to highlight intracranial tumors in children is illustrated. The effect of changing the inversion time (TI) to produce the best spatial resolution and to highlight the contrast resolution between different tumors and normal brain is analyzed. The normal appearances and clinical examples in the central nervous system are used to illustrate the options that are available using IR sequences. Variation of TI for providing a means of differentiating short T1 lesions from long T1 lesions is discussed, short TI sequences being best for demonstrating gliomas and astrocytomas whilst medium/long TI sequences are best for demonstrating vascular abnormalities and short T1 tumors. Inversion recovery imaging is considered to be an accurate alternative to spin-echo imaging as currently applied.

Adolescent↗
PubMed1988

Considerations of magnetic resonance angiography by selective inversion recovery.

In the selective inversion recovery method for projection angiography, upstream blood is tagged by an inversion excitation and then allowed to flow into the imaged region. The subtraction of this first image from a second image acquired without the tagging leaves a signal from only the selectively tagged blood. Pulse sequence design involves consideration of the duration of the blood transit interval, excitation timing and cardiac gating, static material suppression, inversion excitation pulses, and flow compensation. Each of these considerations must be viewed with respect to the particular application. The method has demonstrated potential application to areas such as the carotid arteries, aortic arch, and peripheral vessels.

Arteries↗
PubMed1990

Regional phase correction of inversion-recovery MR images.

Many MR imaging systems are limited in their ability to successfully display inversion-recovery images. The reason is that part of the contrast is encoded as phase differences between pixels, whereas in the more commonly used spin-echo pulse sequence all the information is contained in the pixel magnitude. Inversion-recovery images are often displayed in magnitude form, resulting in loss of potentially useful phase information contained in the data. Before this phase information can be used, phase errors which result from scanner imperfections must be removed. While most of the necessary correction can be accomplished using data obtained by scanning a uniform phantom, this approach has several disadvantages. An alternative method by which phase errors can be readily removed without phantom data is described. This method has been applied to images of the head, knee, and liver with good results. It is concluded that this technique is useful for producing phase corrected inversion-recovery MR images.

Algorithms↗
PubMed1991

Multiple-readout selective inversion recovery angiography.

We have developed a variation of selective inversion recovery (SIR) angiography that allows us to obtain a collection of several angiograms within the same acquisition time previously required to obtain a single image. In basic SIR, a single readout is performed after the tagging inversion pulse. In multiple-readout SIR, a succession of readout pulses is applied following the inversion pulse. By varying the gradients appropriately during the successive readouts, we can obtain a set of multiple projection-angle angiograms, or, by appropriately spacing the readouts throughout the cardiac cycle, we can obtain a set of time-resolved angiograms. This technique allows us to obtain additional spatial or temporal information without increasing total scan time. A sequence of increasing flip-angle read pulses is used to maintain a constant signal level across the images. A trade-off exists between SNR and the number of images acquired.

Blood Vessels↗
PubMed1993

Double inversion recovery improves water suppression in vivo.

It is shown that double inversion recovery used for water suppression can be made much less sensitive to pulse imperfections and to the variations in the relaxation times than a single inversion recovery. This insensitivity results in up to 10-fold improvement in the water suppression in vivo. The excellent water suppression by double inversion recovery is demonstrated experimentally by the in vivo proton spectra obtained from a rat brain.

Animals↗
PubMed1993

Continuous inversion angiography.

A subtractive time-of-flight technique for magnetic resonance angiography is described. In this approach, the arterial supply to an organ is inverted in a steady-state fashion by applying off-resonance irradiation in the presence of a linear magnetic field gradient. An angiogram is formed by subtracting an image acquired with arterial inversion from a control image acquired with no arterial inversion. A single coil is used to apply both the inversion and observation pulses. Intracranial angiograms obtained from normal volunteers using a two-dimensional projective implementation of this technique at 1.5 T illustrate excellent small vessel detail and background suppression.

Algorithms↗
PubMed1996

OIL FLAIR: optimized interleaved fluid-attenuated inversion recovery in 2D fast spin echo.

Inversion recovery may be used to suppress signal from cerebrospinal fluid, a technique which has been named "fluid attenuated with inversion recovery" (FLAIR). This report describes interleaving a slice selective inversion pulse within a rapid spin-echo sequence to obtain the desirable contrast characteristics of FLAIR in imaging times comparable to standard rapid spin echo. Additionally, the pulse repetition time is allowed to float above a defined minimum, which can further shorten scan times and dramatically ease the optimization process. The optimized interleaved sequence is referred to as OIL FLAIR.

Central Nervous System↗
PubMed1997

Perfusion imaging by a flow-sensitive alternating inversion recovery (FAIR) technique: application to functional brain imaging.

Perfusion is a crucial physiological parameter for tissue function. To obtain perfusion-weighted images and consequently to measure cerebral blood flow (CBF), a newly developed flow-sensitive alternating inversion recovery (FAIR) technique was used. Dependency of FAIR signal on inversion times (TI) was examined; signal is predominantly located in large vessels at short TI, whereas it is diffused into gray matter areas at longer TI. CBF of gray matter areas in the human brain is 71 +/- 15 SD ml/100 g/min (n = 6). In fMRI studies, micro- and macrovessel inflow contributions can be obtained by adjusting TIs. Signal changes in large vessel areas including the scalp were seen during finger opposition at a TI of 0.4 s; however, these were not observed at a longer TI of 1.4 s. To compare with commonly used BOLD and slice selective inversion recovery techniques, FAIR and BOLD images were acquired at the same time during unilateral finger opposition. Generally, activation sites determined by three techniques are consistent. However, activation of some areas can be detected only by FAIR, not by BOLD, suggesting that the oxygen consumption increase couples with the CBF change completely. Relative and absolute CBF changes in the contralateral motor cortex are 53 +/- 17% SD (n = 9) and 27 +/- 11 SD ml/100 g/min (n = 9), respectively.

Brain↗
PubMed1997

Two methods for peak RF power minimization of multiple inversion-band pulses.

Two novel methods to minimize peak RF power for high order longitudinal Hadamard encoding are described and demonstrated experimentally. The first method uses the fact that the choice of a reference phase in an inversion process does not affect the final frequency response. In this method, the different single inversion-band pulses are added together, each with a different reference phase. For a proper phase choice, minimization of the peak RF power is obtained. Scaling laws are defined allowing the use of a given phase-set in multiple cases. In the second method, single inversion-band pulses are added together, each partially shifted in time. This results in a significant reduction in peak power with only a moderate increase in pulse length. Theoretical conditions outlining the optimal addition order are defined. Experimental results verify the theoretical conditions and demonstrate that the frequency response is not affected by the peak power minimization process. With the new low peak RF power, longitudinal Hadamard encoding of 8TH (or 16TH) order can be performed in any clinical setting.

Magnetic Resonance Spectroscopy↗
PubMed2004

Modified Look-Locker inversion recovery (MOLLI) for high-resolution T1 mapping of the heart.

A novel pulse sequence scheme is presented that allows the measurement and mapping of myocardial T1 in vivo on a 1.5 Tesla MR system within a single breath-hold. Two major modifications of conventional Look-Locker (LL) imaging are introduced: 1) selective data acquisition, and 2) merging of data from multiple LL experiments into one data set. Each modified LL inversion recovery (MOLLI) study consisted of three successive LL inversion recovery (IR) experiments with different inversion times. We acquired images in late diastole using a single-shot steady-state free-precession (SSFP) technique, combined with sensitivity encoding to achieve a data acquisition window of < 200 ms duration. We calculated T1 using signal intensities from regions of interest and pixel by pixel. T1 accuracy at different heart rates derived from simulated ECG signals was tested in phantoms. T1 estimates showed small systematic error for T1 values from 191 to 1196 ms. In vivo T1 mapping was performed in two healthy volunteers and in one patient with acute myocardial infarction before and after administration of Gd-DTPA. T1 values for myocardium and noncardiac structures were in good agreement with values available from the literature. The region of infarction was clearly visualized. MOLLI provides high-resolution T1 maps of human myocardium in native and post-contrast situations within a single breath-hold.

Contrast Media↗
PubMed2004

Bandwidth-modulated adiabatic RF pulses for uniform selective saturation and inversion.

Radiofrequency (RF) inversion and saturation pulses with extremely high spatial selectivity and uniform profiles are a requirement for numerous MR techniques, such as pulsed arterial spin labeling and outer volume suppression. Adiabatic pulses used for inversion of longitudinal magnetization are ubiquitous, but the superior selectivity of adiabatic full passages has not been widely exploited for saturation because a simple way of calibrating the amplitude of these subadiabatic pulses is lacking. An analytically derived calibration equation is presented, applicable to a large class of pulses including the hyperbolic secant (HS) pulse and allowing the determination of the precise amplitude required to achieve any effective flip angle. The properties of this calibration are examined, and a highly selective and homogeneous HS saturation pulse is demonstrated. Based on this calibration a new class of RF pulses is developed. These bandwidth-modulated adiabatic selective saturation and inversion (BASSI) RF pulses afford optimal amplitude modulation, achieving uniform profiles at any effective flip angle. BASSI pulses are compared to existing gradient modulated adiabatic pulses in simulations and phantom experiments and shown to be superior in terms of selectivity and homogeneity, while requiring less RF energy. An application of BASSI pulses to pulsed arterial spin labeling is shown.

Brain↗
PubMed2005

Free-breathing renal magnetic resonance angiography with steady-state free-precession and slab-selective spin inversion combined with radial k-space sampling and water-selective excitation.

The impact of radial k-space sampling and water-selective excitation on a novel navigator-gated cardiac-triggered slab-selective inversion prepared 3D steady-state free-precession (SSFP) renal MR angiography (MRA) sequence was investigated. Renal MRA was performed on a 1.5-T MR system using three inversion prepared SSFP approaches: Cartesian (TR/TE: 5.7/2.8 ms, FA: 85 degrees), radial (TR/TE: 5.5/2.7 ms, FA: 85 degrees) SSFP, and radial SSFP combined with water-selective excitation (TR/TE: 9.9/4.9 ms, FA: 85 degrees). Radial data acquisition lead to significantly reduced motion artifacts (P < 0.05). SNR and CNR were best using Cartesian SSFP (P < 0.05). Vessel sharpness and vessel length were comparable in all sequences. The addition of a water-selective excitation could not improve image quality. In conclusion, radial k-space sampling reduces motion artifacts significantly in slab-selective inversion prepared renal MRA, while SNR and CNR are decreased. The addition of water-selective excitation could not improve the lower CNR in radial scanning.

Adult↗
PubMed2006

Inversion recovery radial MRI with interleaved projection sets.

The radial trajectory has found applications in cardiac imaging because of its resilience to undersampling and motion artifacts. Recent work has shown that interleaved and weighted radial imaging can produce images with multiple contrasts from a single data set. This feature was investigated for inversion recovery imaging of scar using a radial technique. The 2D radial imaging method was modified to acquire quadruply interleaved projection sets within each acquisition window of the cardiac cycle. These data were reconstructed using k-space weightings that used a smaller segment of the acquisition window for the central k-space data, the determinant of image contrast. This method generates four images with different T1 weightings. The novel approach was compared with noninterleaved radial imaging, interleaved radial without weightings, and Cartesian imaging in simulations, phantoms, and seven subjects with clinical myocardial infarction. The results show that during a typical acquisition window after an inversion pulse, magnetization changes rapidly. The interleaved acquisition provided better image quality than the noninterleaved radial acquisition. Interleaving with weighting provided better quality when the inversion time (TI) was shorter than optimal; otherwise, interleaving without weighting was superior. These methods enable a radial trajectory to be employed in conjunction with preparation pulses for viability imaging.

Algorithms↗
PubMedDate not supplied

Threshold voltages for hyperbolic secant inversion pulses.

Spin inversion produced by hyperbolic secant RF pulses is independent of pulse amplitude once a threshold value is exceeded. A semi-empirical formula for these pulses has been derived by application of analytical solutions to the Bloch equations. This predicts the required coil voltages for inversion as a function of inversion bandwidth and side-to-width parameter mu, based on knowledge of a reference voltage for the loaded coil. The voltage required is shown to increase linearly with bandwidth but to fall with the mu, approximately as (1/mu)0.4. Experimental confirmation is presented, and factors affecting choice of pulse parameters are discussed.

Algorithms↗