Simultaneous multislice acquisition with arterial-flow tagging (SMART) using echo planar imaging (EPI)

Arterial spin tagging techniques have been used to image tissue perfusion in MR without contrast injection or ionizing radiation. Currently, spin tagging studies are performed primarily using single-slice imaging sequences, which are time consuming. This note reports a multislice echo-planar arterial spin tagging technique (Simultaneous Multislice Acquisition with aRterial-flow Tagging, or “SMART”). Multiband RF encoding (Hadamard) is used to provide simultaneous multislice acquisition capability for spin tagging techniques (such as echo planar imaging signal targeting with alternating radio frequency and flow-sensitive alternative inversion recovery). The method is illustrated with a two-slice pulse sequence that was implemented using the FAIR technique to generate two perfusion weighted images simultaneously. Compared with single-slice sequences, this two-slice sequence provided similar image quality, signal-to-noise ratio, and twice the spatial coverage compared with the single-slice technique within the same scan time.     

Whole-brain 3D perfusion MRI at 3.0 T using CASL with a separate labeling coil.

A variety of continuous and pulsed arterial spin labeling (ASL) perfusion MRI techniques have been demonstrated in recent years. One of the reasons these methods are still not routinely used is the limited extent of the imaging region. Of the ASL methods proposed to date, continuous ASL (CASL) with a separate labeling coil is particularly attractive for whole-brain studies at high fields. This approach can provide an increased signal-to-noise ratio (SNR) in perfusion images because there are no magnetization transfer (MT) effects, and lessen concerns regarding RF power deposition at high field because it uses a local labeling coil. In this work, we demonstrate CASL whole-brain quantitative perfusion imaging at 3.0 T using a combination of strategies: 3D volume acquisition, background tissue signal suppression, and a separate labeling coil. The results show that this approach can be used to acquire perfusion images in all brain regions with good sensitivity. Further, it is shown that the method can be performed safely on humans without exceeding the current RF power deposition limits. The current method can be extended to higher fields, and further improved by the use of multiple receiver coils and parallel imaging techniques to reduce scan time or provide increased resolution.     

A theoretical and experimental investigation of the tagging efficiency of pseudocontinuous arterial spin labeling.

Arterial spin labeling (ASL) is capable of noninvasively measuring blood flow by magnetically tagging the protons in arterial blood, which has been conventionally achieved using instantaneous (PASL) or continuous (CASL) RF pulses. As an intermediate method, pseudocontinuous ASL (pCASL) utilizes a train of discrete RF pulses to mimic continuous tagging that is often unavailable on imagers due to the requirement of continuous RF transmit capabilities. In the present study, we implemented two versions of pCASL (balanced and unbalanced gradient waveforms in tag and control scans) for both transmit/receive coils and array receivers. Experimental data show a 50% +/- 4% increase of signal-to-noise ratio (SNR) compared with PASL and a higher tagging efficiency than amplitude-modulated (AM) CASL (80% vs. 68%). Computer simulations predict an optimal tagging efficiency of 85% for flow velocities from 10 to 60 cm/s. It is theoretically and experimentally demonstrated that the tagging efficiency of pCASL is dependent upon the resonance offset and flip angle of the RF pulse train. We conclude that pCASL has the potential of combining the merits of PASL, including less hardware demand and higher tagging efficiency, and CASL, which includes a longer tagging bolus and thus higher SNR. These improvements provide a better balance between tagging efficiency and SNR.     

Technological advances in MRI measurement of brain perfusion

Measurement of brain perfusion using arterial spin labeling (ASL) or dynamic susceptibility contrast (DSC) based MRI has many potential important clinical applications. However, the clinical application of perfusion MRI has been limited by a number of factors, including a relatively poor spatial resolution, limited volume coverage, and low signal-to-noise ratio (SNR). It is difficult to improve any of these aspects because both ASL and DSC methods require rapid image acquisition. In this report, recent methodological developments are discussed that alleviate some of these limitations and make perfusion MRI more suitable for clinical application. In particular, the availability of high magnetic field strength systems, increased gradient performance, the use of RF coil arrays and parallel imaging, and increasing pulse sequence efficiency allow for increased image acquisition speed and improved SNR. The use of parallel imaging facilitates the trade-off of SNR for increases in spatial resolution. As a demonstration, we obtained DSC and ASL perfusion images at 3.0 T and 7.0 T with multichannel RF coils and parallel imaging, which allowed us to obtain high-quality images with in-plane voxel sizes of 1.5 x 1.5 mm(2).     

Spoiled gradient-echo as an arterial spin tagging technique for quick evaluation of local perfusion

PURPOSE: To introduce a simple gradient-echo arterial spin tagging (GREAST) technique available for most commercial magnetic resonance (MR) systems, for a quick evaluation of tissue perfusion. MATERIALS AND METHODS: The GREAST technique uses a combination of a short TR spoiled gradient-echo (SPGR) sequence with a selective presaturation radio frequency (RF) pulse that allows acquiring each tagged and control image within 10-20 seconds. The phantom and human studies were performed for evaluating the feasibility in measurement of local perfusion and the efficacy in alleviation of the asymmetric magnetization transfer (MT) and slice profile effects. RESULTS: Results show a good linear relationship between the signal attenuation caused by the presaturation pulse and flow rates in the phantom experiment and effective alleviation of the asymmetric MT and slice profile effects for various orientations of imaging slices. Human studies showed good perfusion results in brain imaging. Perfusion imaging on the liver and kidney were also conducted. The results could be significantly improved by effectively lessening motion-related artifacts. CONCLUSION: The GREAST technique is simple, easy to use, and applicable to examine local perfusion of the brain and other organs in the trunk.     

Application of variable‐rate selective excitation pulses for spin labeling in perfusion MRI

Arterial spin labeling offers great potential in clinical applications for noninvasive measurement of cerebral blood flow. Arterial spin labeling tagging methods such as the flow sensitive alternating inversion recovery technique require efficient spatial inversion pulses with high inversion accuracy and sharp transition zones between inverted and noninverted magnetization, i.e., require a high performance inversion pulse. This work presents a comprehensive comparison of the advantages offered by a variable‐rate selective excitation variant of the hyperbolic secant pulse against the widely used conventional hyperbolic secant pulse and the frequency offset corrected inversion pulses. Pulses were compared using simulation and experimental measurement in phantoms before being used in a flow sensitive alternating inversion recovery‐arterial spin labeling perfusion measurement in normal volunteers. Both the hyperbolic secant and frequency offset corrected inversion pulses have small variations in inversion profiles that may lead to unwanted subtraction errors in arterial spin labeling at a level where the residual signal is comparable to the desired perfusion contrast. The variable‐rate selective excitation pulse is shown to have improved inversion efficiency indicating its potential in perfusion MRI. The variable‐rate selective excitation pulse variant also showed greatest tolerance to radiofrequency variation and off‐resonance conditions, making it a robust choice for in vivo arterial spin labeling measurement. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Improved perfusion quantification in FAIR imaging by offset correction.

Perfusion quantification using pulsed arterial spin labeling has been shown to be sensitive to the RF pulse slice profiles. Therefore, in Flow-sensitive Alternating-Inversion Recovery (FAIR) imaging the slice selective (ss) inversion slab is usually three to four times thicker than the imaging slice. However, this reduces perfusion sensitivity due to the increased transit delay of the incoming blood with unperturbed spins. In the present article, the dependence of the magnetization on the RF pulse slice profiles is inspected both theoretically and experimentally. A perfusion quantification model is presented that allows the use of thinner ss inversion slabs by taking into account the offset of RF slice profiles between ss and nonselective inversion slabs. This model was tested in both phantom and human studies. Magn Reson Med 46:193-197, 2001.     

Improved perfusion-weighted MRI by a novel double inversion with proximal labeling of both tagged and control acquisitions.

A novel pulsed arterial spin labeling (PASL) technique for multislice perfusion-weighted imaging is proposed that compensates for magnetization transfer (MT) effects without sacrificing tag efficiency, and balances transient magnetic field effects (eddy currents) induced by pulsed field gradients. Improved compensation for MT is demonstrated using a phantom. Improvement in perfusion measurement was compared to other PASL techniques by acquiring perfusion images from 13 healthy volunteers (nine women and four men; age range 29-64 years; mean age 45 +/- 14 years) and second-order image texture analysis. The main improvements with the new method were significantly higher image contrast, higher mean signal intensity, and better signal uniformity across slices. In conclusion, this new PASL method should provide improved accuracy in measuring brain perfusion.     

High-resolution fast spin echo imaging of the human brain at 4.7 T: implementation and sequence characteristics.

In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162 degrees ) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B(1)) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B(1) inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness = 2 mm; in-plane resolution = 0.469 mm; acquisition time = 11 min 20 s) from normal volunteers are presented, which allow visualization of brain anatomy in fine detail. This study demonstrates that high-field FSE produces images of the human brain with high spatial resolution, SNR, and tissue contrast, within currently prescribed power deposition guidelines.     

Understanding and optimizing the amplitude modulated control for multiple-slice continuous arterial spin labeling.

Multiple-slice perfusion imaging by continuous arterial spin labeling (CASL) is made possible by amplitude modulation (AM) of the labeling RF pulse, but perfusion sensitivity is reduced relative to the single-slice technique. A computer model of the Bloch equations for velocity driven adiabatic fast passage was developed to elucidate the compromised sensitivity to perfusion of the AM control technique for CASL. Calculations were performed over ranges of RF pulse amplitude, B1; magnetic field gradient, G; phase, phi, and frequency, f, of the modulation function; velocity, v, and relaxation times, T1 and T2, of blood. It was found that unless f>2piB1, phi determines the performance of the AM control; excessively high B1 or v reduces the efficiency of the AM control; and T1 relaxation dominates if f is too great. In vivo, in rat brain (n=5) at 2.35 T, the sensitivity of the AM technique to perfusion was 70% of the sensitivity of single-slice CASL.     

Pseudocontinuous arterial spin labeling with optimized tagging efficiency

The adiabatic inversion of blood in pseudocontinuous arterial spin labeling (PCASL) is highly sensitive to off‐resonance effects and gradient imperfections and this sensitivity can lead to tagging efficiency loss and unpredictable variations in cerebral blood flow estimates. This efficiency loss is caused by a phase tracking error between the RF pulses and the flowing spins. This article introduces a new method, referred to as Optimized PCASL (OptPCASL), that minimizes the phase tracking error by applying an additional compensation RF phase term and in‐plane gradients to the PCASL pulse train. The optimal RF phase and gradient amplitudes are determined using a prescan procedure, which consists of a series of short scans interleaved with automated postprocessing routines integrated to the scanner console. The prescan procedure is shown to minimize the phase tracking error in a robust and time efficient manner. As an example of its application, the use of OptPCASL for the improved detection of functional activation in the visual cortex is demonstrated and temporal signal‐to‐noise ratio (SNR), image SNR, and baseline cerebral blood flow measures are compared to those acquired from conventional PCASL. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Comparison of first-pass Gd-DOTA and FAIRER MR perfusion imaging in a rabbit model of pulmonary embolism

PURPOSE: To compare the sensitivity of contrast-enhanced magnetic resonance imaging (MRI) and arterial spin labeling to perfusion deficits in the lung. MATERIALS AND METHODS: A rabbit model of pulmonary embolism was imaged with both flow-sensitive alternating inversion recovery with an extra radiofrequency pulse (FAIRER) arterial spin labeling and Gd-DOTA enhanced MRI. The signal-to-noise ratio (SNR) was measured in the area of the perfusion deficit and the normal lung for both techniques. RESULTS: The defect was readily visible in all images. The normal lung had an average of 3.8 +/- 1.2 times the SNR of the unperfused lung with the arterial spin labeling technique, and approximately 13.7 +/- 3.3 times the SNR with the contrast-enhanced technique. CONCLUSION: Gd-DOTA enhanced MRI provides higher SNR in pulmonary perfusion imaging; however, arterial spin labeling is also adequate and may be used when repeated studies are indicated.     

Multislice perfusion and perfusion territory imaging in humans with separate label and image coils.

An arterial spin labeling technique using separate RF labeling and imaging coils was used to obtain multislice perfusion images of the human brain at 3 T. Continuous RF irradiation at a peak power of 0.3 W was applied to the carotid arteries to adiabatically invert spins. Labeling was achieved without producing magnetization transfer effects since the B1 field of the labeling coil did not extend into the imaging region or couple significant power into the imaging coil. Eliminating magnetization transfer allowed the acquisition of multislice perfusion images of arbitrary orientation. Combining surface coil labeling with a reduced RF duty cycle permitted significantly lower SAR than single coil approaches. The technique was also found to allow selective labeling of blood in either carotid, providing an assessment of the artery's perfusion territory. In normal subjects, these territories were well-defined and localized to the ipsilateral hemisphere.     

The effect of B1 field inhomogeneity and the nonselective inversion profile on the kinetics of FAIR-based perfusion MRI.

Perfusion imaging with pulsed arterial spin labeling techniques, like flow-sensitive alternating inversion recovery (FAIR), may suffer from inflow of fresh, i.e., unlabeled, spins. Inflow of fresh spins is caused by the arrival of unlabeled spins in the image slice and can lead to underestimation of the perfusion if not taken into account. In this study it was shown that a decrease in B(1) field strength toward the edge of the transmit coil and the consequent reduction in the inversion efficiency leads to a narrowing of the arterial delivery function and a reduction in FAIR signal. Increasing the B(1) amplitude of the adiabatic inversion pulse from 2.3 to 5.7 times its minimum amplitude requirement resulted in an observed increase of 40 to 80% in the rat brain FAIR signal at inflow times longer than 0.65 s. For coils with limited dimensions and significant B(1) inhomogeneity over the perfusion labeling slab, the application of an excessively large B(1) amplitude in combination with adiabatic inversion is recommended to optimize the FAIR perfusion contrast.     

Multislice perfusion of the kidneys using parallel imaging: Image acquisition and analysis strategies

Flow‐sensitive alternating inversion recovery arterial spin labeling with parallel imaging acquisition is used to acquire single‐shot, multislice perfusion maps of the kidney. A considerable problem for arterial spin labeling methods, which are based on sequential subtraction, is the movement of the kidneys due to respiratory motion between acquisitions. The effects of breathing strategy (free, respiratory‐triggered and breath hold) are studied and the use of background suppression is investigated. The application of movement correction by image registration is assessed and perfusion rates are measured. Postacquisition image realignment is shown to improve visual quality and subsequent perfusion quantification. Using such correction, data can be collected from free breathing alone, without the need for a good respiratory trace and in the shortest overall acquisition time, advantageous for patient comfort. The addition of background suppression to arterial spin labeling data is shown to reduce the perfusion signal‐to‐noise ratio and underestimate perfusion. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Multi-Slice MRI of Rat Brain Perfusion During Amphetamine Stimulation Using Arterial Spin Labeling

When a single coil is used to measure perfusion by arterial spin labeling, saturation of macromolecular protons occurs during the labeling period. Induced magnetization transfer contrast (MTC) effects decrease tissue water signal intensity, reducing the sensitivity of the technique. In addition, MTC effects must be properly accounted for in acquiring a control image. This forces the image to a single slice centered between the labeling plane and the control plane. In this work, a two-coil system is presented as a way to avoid saturation of macromolecular spins during arterial spin labeling. The system consists of one small surface coil for labeling the arterial water spins, and a head coil for MRI, actively decoupled from the labeling coil by using PIN diodes. It is shown that no signal loss occurs due to MTC effects when the two-coil system is used for MRI of rat brain perfusion, enabling three-dimensional perfusion imaging. Using the two-coil system, a multi-slice MRI sequence was used to study the regional effects of amphetamine on brain perfusion. Amphetamine causes significant increases in perfusion in many areas of the brain including the cortex, cingulate, and caudate putamen, in agreement with previous results using deoxyglucose uptake to monitor brain activation.     

Efficiency of inversion pulses for background suppressed arterial spin labeling.

Background suppression strategies for arterial spin labeling (ASL) MRI offer reduced noise from motion and other system instabilities. However, the inversion pulses used for suppression can also attenuate the ASL signal, which may offset the advantages of background suppression. Numerical simulations were used to optimize the inversion efficiency of four candidate pulses over a range of radiofrequency (RF) and static magnetic field variations typical of in vivo imaging. Optimized pulses were then used within a pulsed ASL sequence to assess the pulses' in vivo inversion efficiencies for ASL. The measured in vivo inversion efficiency was significantly lower than theoretical predictions (e.g., 93% experimental compared to 99% theoretical) for the tangent hyperbolic pulse applied in a background suppression scheme. This inefficiency was supported by an in vitro study of human blood. These results suggest that slow magnetization transfer (MT) in blood, either with bound water or macromolecular protons, dominates the inversion inefficiency in blood. Despite the attenuated signal relative to unsuppressed ASL, the signal-to-noise ratio (SNR) with suppression was improved by 23-110% depending on the size of the region measured. Knowledge of efficiency will aid optimization of the number of suppression pulses and provide more accurate quantification of blood flow.     

Simple model of double adiabatic inversion (DAI) efficiency.

Multislice continuous arterial spin labeling (CASL) using alternating single adiabatic inversion (SAI) and double adiabatic inversion (DAI) (ASD) suffers from control inefficiencies that reduce the perfusion signal-to-noise ratio (SNR). A simple analytic model for DAI efficiency (chi) is presented, and compared with simulation and experimental data. The DAI efficiency (defined as the ratio of uninverted to equilibrium spins) depends primarily on the effective adiabaticity (beta') and the inversion time (both determined from the labeling RF field and gradient amplitudes, and flow velocities). In this study, chi behaved like a damped oscillator, starting from 1 at beta'=0 (no spin inversion), dropping to 0.5 at beta'=0.5 (spin saturation), rising again to a peak at beta' approximately 3, and subsequently oscillating at higher beta'. The simple model matched the flow phantom data for beta' < 2, with an accuracy of better than 10%, but it diverged from the flow phantom data as the data oscillated at higher beta'. The relative inversion efficiencies of ASD and simultaneous proximal and distal irradiation (SPDI) were measured in 28 elderly volunteers. The ratio of CASL difference (control-label) signals measured in gray matter (= 1.16 +/- 0.05), and the simple efficiency models (= 1.13 +/- 0.02) had a correlation coefficient of 0.974.     

Multiphase pseudocontinuous arterial spin labeling (MP‐PCASL) for robust quantification of cerebral blood flow

Pseudocontinuous arterial spin labeling (PCASL) has been demonstrated to provide the sensitivity of the continuous arterial spin labeling method while overcoming many of the limitations of that method. Because the specification of the phases in the radiofrequency pulse train in PCASL defines the tag and control conditions of the flowing arterial blood, its tagging efficiency is sensitive to factors, such as off‐resonance fields, that induce phase mismatches between the radiofrequency pulses and the flowing spins. As a result, the quantitative estimation of cerebral blood flow with PCASL can exhibit a significant amount of error when these factors are not taken into account. In this paper, the sources of the tagging efficiency loss are characterized and a novel PCASL method that utilizes multiple phase offsets is proposed to reduce the tagging efficiency loss in PCASL. Simulations are performed to evaluate the feasibility and the performance of the proposed method. Quantitative estimates of cerebral blood flow obtained with multiple phase offset PCASL are compared to estimates obtained with conventional PCASL and pulsed arterial spin labeling. Our results show that multiple phase offset PCASL provides robust cerebral blood flow quantification while retaining much of the sensitivity advantage of PCASL. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Breast tissue differentiation using arterial spin tagging.

An arterial spin tagging (AST) pulse sequence has been developed to measure T(1) and relative blood perfusion. This full sequence is composed of three sequences: selective tagging, nonselective tagging, and nontagging. Perfusion quantification error resulting from imperfect inversion and acquisition slice profiles has been addressed in the literature. In this work, the error is reduced through the application of optimized Shinnar-Le Roux (SLR) RF pulses and a semi-log linear regression data-processing technique. A threshold approach based on the breast tissue T(1) and relative blood perfusion is introduced to show that these two parameters can be applied to breast tissue differentiation and potentially to cancer detection.