Strategies for reducing respiratory motion artifacts in renal perfusion imaging with arterial spin labeling

Arterial spin labeling (ASL) perfusion measurements may have many applications outside the brain. In the abdomen, severe image artifacts can arise from motions between acquisitions of multiple signal averages in ASL, even with single‐shot image acquisition. Background suppression and respiratory motion synchronization techniques can be used to ameliorate these artifacts. Two separate in vivo studies of renal perfusion imaging using pulsed continuous ASL (pCASL) were performed. The first study assessed various combinations of background suppression and breathing strategies. The second investigated the retrospective sorting of images acquired during free breathing based on respiratory position. Quantitative assessments of the test‐retest repeatability of perfusion measurements and the image quality scored by two radiologists were made. Image quality was most significantly improved by using background suppression schemes and controlled breathing when compared to other combinations without background suppression or with free breathing, assessed by test‐retests (5% level, F‐test), and by radiologists' scores (5% level, Mann‐Whitney U‐test). Under free breathing, retrospectively sorting images based on respiratory position showed significant improvement. Both radiologists found 100% of the images had preferable image sharpness after sorting. High‐quality renal perfusion measurements with reduced respiratory motion artifacts have been demonstrated using ASL when appropriate background suppression and breathing strategies are applied. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Reproducibility of renal perfusion MR imaging in native and transplanted kidneys using non-contrast arterial spin labeling

Purpose:

To examine both inter‐visit and intra‐visit reproducibility of a MR arterial spin labeling (ASL) perfusion technique in native and transplanted kidneys over a broad range of renal function.

Materials and Methods:

Renal perfusion exams were performed at 1.5 T in a total of 24 subjects: 10 with native and 14 with transplanted kidneys. Using a flow‐sensitive alternating inversion recovery (FAIR) ASL scheme, 32 control/tag pairs were acquired and processed using a single‐compartment model. Two FAIR‐ASL MR exams were performed at least 24 h apart on all the subjects to assess inter‐visit reproducibility. ASL perfusion measurements were also repeated back‐to‐back within one scanning session in 8 native subjects and in 12 transplant subjects to assess intra‐visit reproducibility. Intra‐class correlations (ICCs) and coefficients of variation (CVs) were calculated as metrics of reproducibility.

Results:

Intra‐visit ICCs ranged from 0.96 to 0.98 while CVs ranged from 4.8 to 6.0%. Inter‐visit measurements demonstrated slightly more variation with ICCs from 0.89 to 0.94 and CVs from 7.6 to 13.1%. Medullary perfusion demonstrated greater variability compared with cortical blood flow: intra‐visit ICCs from 0.72 to 0.78 and CVs from 16.7 to 26.7%, inter‐visit ICCs from 0.13 to 0.63 and CVs from 19.8 to 37%.

Conclusion:

This study indicates that a FAIR‐ASL perfusion technique is reproducible in the cortex of native and transplanted kidneys over a broad range in renal function. In contrast, perfusion measurements in the medulla demonstrated moderate to poor reproducibility for intra‐visit and inter‐visit measures respectively. J. Magn. Reson. Imaging 2011;33:1414–1421. © 2011 Wiley‐Liss, Inc.     

Dual vessel arterial spin labeling scheme for regional perfusion imaging.

Regional perfusion imaging (RPI) based on pulsed arterial spin labeling and angulated inversion slabs has been recently proposed. The technique allows mapping of individual brain perfusion territories of the major feeding arteries and could become a valuable clinical tool for evaluation of patients with cerebrovascular diseases. Here we propose a new labeling scheme for RPI where lateral and posterior circulations are labeled simultaneously. Two scans instead of three are sufficient to obtain the same perfusion territories as in the original approach, allowing for a 33% reduction in the total RPI protocol time. Moreover, the position of the inversion slabs with respect to vascular anatomy facilitates the planning and allows potentially better labeling efficiency. The new approach was tested on seven healthy volunteers and compared to the original labeling scheme. The results showed that the same perfusion territories and regional CBF values can be obtained.     

MR perfusion imaging using the arterial spin labeling technique for breast cancer

Purpose:

To investigate the feasibility of perfusion imaging using an arterial spin labeling (ASL) technique for breast cancer.

Materials and Methods:

Thirteen female patients with primary breast cancers were included in this study. All examinations were performed on 1.5 Tesla MRI systems. Visual evaluations of the colored perfusion map and MRI perfusion values were assessed. MRI and computed tomography (CT) perfusion values were compared.

Results:

Thirteen of 14 tumor lesions could be visualized on the colored perfusion map. CT perfusion examinations were performed in eight breasts, and the relationship between the blood flow values of CT perfusion and of MR perfusion showed a significant correlation.

Conclusion:

Nonenhanced MR imaging by an ASL technique is valid for depicting breast cancer, and the MR perfusion value is thought to be helpful for quantitative diagnosis of breast cancer. J. Magn. Reson. Imaging 2012;436‐440. © 2011 Wiley Periodicals, Inc.     

Selective multivessel labeling approach for perfusion territory imaging in pseudo-continuous arterial spin labeling

Recently, a new method for perfusion territory imaging named superselective pseudo-continuous arterial spin labeling was introduced. The method uses additional time-varying gradients to create a circular labeling spot that can be adjusted in size and thus adapted to individual arteries. In this study, the additional gradients are adjusted in such a way that an elliptical labeling spot is formed, which can be applied to label the blood in multiple vessels simultaneously in conjunction with an increased labeling efficiency compared with the original superselective approach. When compared with other selective multivessel strategies, the proposed technique allows for an improved and flexible adaption of the labeling focus to different anatomical variations of the arteries in the neck so that a total of five perfusion territories from the data acquired in three measurements can be recalculated in a reduced scan time. These include not only the perfusion territories of the cerebrum but also the perfusion territories in the cerebellum fed by individual vertebral arteries.     

Quantification of renal perfusion: comparison of arterial spin labeling and dynamic contrast-enhanced MRI

Purpose:

To provide the first comparison of absolute renal perfusion obtained by arterial spin labeling (ASL) and separable compartment modeling of dynamic contrast‐enhanced (DCE) magnetic resonance imaging (MRI). Moreover, we provide the first application of the dual bolus approach to quantitative DCE‐MRI perfusion measurements in the kidney.

Materials and Methods:

Consecutive ASL and DCE‐MRI acquisitions were performed on six rabbits on a 1.5 T MRI system. Gadolinium (Gd)‐DTPA was administered in two separate injections to decouple measurement of the arterial input function and tissue uptake curves. For DCE perfusion, pixel‐wise and mean cortex region‐of‐interest tissue curves were fit to a separable compartment model.

Results:

Absolute renal cortex perfusion estimates obtained by DCE and ASL were in close agreement: 3.28 ± 0.59 mL/g/min (ASL), 2.98 ± 0.60 mL/g/min (DCE), and 3.57 ± 0.96 mL/g/min (pixel‐wise DCE). Renal medulla perfusion was 1.53 ± 0.35 mL/g/min (ASL) but was not adequately described by the separable compartment model.

Conclusion:

ASL and DCE‐MRI provided similar measures of absolute perfusion in the renal cortex, offering both noncontrast and contrast‐based alternatives to improve current renal MRI assessment of kidney function. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Functional perfusion imaging using continuous arterial spin labeling with separate labeling and imaging coils at 3 T.

Functional perfusion imaging with a separate labeling coil located above the common carotid artery was demonstrated in human volunteers at 3 T. A helmet resonator and a spin-echo echo-planar imaging (EPI) sequence were used for imaging, and a circular surface coil of 6 cm i.d. was employed for labeling. The subjects performed a finger-tapping task. Signal differences between the condition of finger tapping and the resting state were between -0.5% and -1.1 % among the subjects. The imaging protocol included a long post-label delay (PLD) to reduce transit time effects. Labeling was applied for all repetitions of the functional run to reduce the sampling interval.     

Multislice imaging of quantitative cerebral perfusion with pulsed arterial spin labeling

A method is presented for multislice measurements of quantitative cerebral perfusion based on magnetic labeling of arterial spins. The method combines a pulsed arterial inversion, known as the FAIR (Flow-sensitive Alternating Inversion Recovery) experiment, with a fast spiral scan image acquisition. The short duration (22 ms) of the spiral data collection allows simultaneous measurement of up to 10 slices per labeling period, thus dramatically increasing efficiency compared to current single slice acquisition protocols. Investigation of labeling efficiency, suppression of unwanted signals from stationary as well as intraarterial spins, and the FAIR signal change as a function of inversion delay are presented. The assessment of quantitative cerebral blood flow (CBF) with the new technique is demonstrated and shown to require measurement of arterial transit time as well as suppression of intraarterial spin signals. CBF values measured on normal volunteers are consistent with results obtained from H₂O¹⁵ positron emission tomography (PET) studies and other radioactive tracer approaches. In addition, the new method allows detection of activation-related perfusion changes in a finger-tapping experiment, with locations of activation corresponding well to those observed with blood oxygen level dependent (BOLD) fMRI.     

Evaluation of systematic quantification errors in velocity-selective arterial spin labeling of the brain.

Velocity-selective (VS) sequences potentially permit arterial spin labeling (ASL) perfusion imaging with labeling applied very close to the tissue. In this study the effects of cerebrospinal fluid (CSF) motion, radiofrequency (RF) field imperfections, and sequence timing parameters on the appearance and quantitative perfusion values obtained with VS-ASL were evaluated. Large artifacts related to CSF motion were observed with moderate velocity weighting, which were removed by inversion recovery preparation at the cost of increased imaging time. Imperfect refocusing and excitation pulses resulting from nonuniform RF fields produced systematic errors in the ASL subtraction images. A phase cycling scheme was introduced to eliminate these errors. Quantitative perfusion images were obtained with CSF suppression and phase cycling. Gray matter blood flow of 27.7 ml 100 g(-1) min(-1), approximately half the value reported in studies using spatially-selective ASL, was measured. Potential sources for this underestimation are discussed.     

A theoretical and experimental comparison of continuous and pulsed arterial spin labeling techniques for quantitative perfusion imaging

Under ideal conditions, continuous arterial spin labeling (ASL) techniques are higher in SNR than pulsed ASL techniques by a factor of e. Presented here is a direct theoretical and experimental comparison of continuous ASL and pulsed ASL, using versions of both that are amenable to multislice imaging and insensitive to variations in transit times (continuous ASL with a delay before imaging, and QUIPSS II (Quantitative Imaging of Perfusion Using a Single Subtraction–second version)). Perfusion image quality for comparable imaging time was nearly identical for both single-slice and multislice imaging. The measured raw signal was approximately 25% higher with continuous ASL, but the SNR per unit time was identical.     

Reduced resolution transit delay prescan for quantitative continuous arterial spin labeling perfusion imaging

Arterial spin labeling perfusion MRI can suffer from artifacts and quantification errors when the time delay between labeling and arrival of labeled blood in the tissue is uncertain. This transit delay is particularly uncertain in broad clinical populations, where reduced or collateral flow may occur. Measurement of transit delay by acquisition of the arterial spin labeling signal at many different time delays typically extends the imaging time and degrades the sensitivity of the resulting perfusion images. Acquisition of transit delay maps at the same spatial resolution as perfusion images may not be necessary, however, because transit delay maps tend to contain little high spatial resolution information. Here, we propose the use of a reduced spatial resolution arterial spin labeling prescan for the rapid measurement of transit delay. Approaches to using the derived transit delay information to optimize and quantify higher resolution continuous arterial spin labeling perfusion images are described. Results in normal volunteers demonstrate heterogeneity of transit delay across different brain regions that lead to quantification errors without the transit maps and demonstrate the feasibility of this approach to perfusion and transit delay quantification.