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

Abstract

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.

Figures

Figure 1

The Pulse sequence diagram for ASL measurements. One 6-sec TR interval is shown, with time-before-imaging indicated: a) showing pulses for initial saturation of the imaging region (axially-orientated slab), pulsed-continuous labeling, the post-labeling delay, and a SSFSE image acquisition; b) the same with background suppression inversion pulses added (slab-selective FOCI pulse before labeling and the non-selective adiabatic inversion pulses after labeling); c) the same with pulses added for saturation of superior inflowing arterial blood.

Figure 2

Summary of image assessment by radiologists. Average scores over all subjects are shown for each combined background suppression and breathing strategy (pair of bars), and for assessment of questions “Q1” pertaining to the appearance of artifacts, and “Q2” pertaining to overall image quality; scores are averages of both radiologists’ scores. (BH, F, and T refer to breathing strategy; ‘heavy’, ‘mod’, and ‘none’ refer to background suppression.) The most marked difference in scores is the low average scores for the scheme without any background suppression; a slight preference for timed breathing is also shown in its higher average scores.

Figure 3

Typical image quality in perfusion difference images and quantitative perfusion maps: a) the set of four images produced by the ASL experiment. Top left and right are the calculated T1 map and M0-image from the reference images. Bottom-left and right are the perfusion difference image (dM) and the quantitative perfusion map (f). Regions of higher and lower perfusion correspond to the cortical and medullary tissue structures in the kidney as also identified in the reference images. The dot-dash white line in the dM image is the axially-oriented imaging region, which is saturated prior to labeling as part of background suppression. The labeling plane, located at the superior edge of the imaging region, and orientated axially, is identified by the hatched section. All images were acquired with heavy background suppression. Breathing strategies were: in a) multiple breath-hold; in b) timed; and in c) free breathing. Good image quality is achieved in all images however (c) illustrates a case where bulk motion during free breathing introduces additional blurring.

Figure 4

Typical image quality in perfusion difference images for all imaging strategies. In the same volunteer, perfusion difference images are shown for each combination of background suppression and breathing strategy. Background suppression significantly improves image quality, removing artifacts from subtraction errors. In this volunteer, additional images without background suppression were acquired that were not repeated in all subjects for statistical analysis.

Figure 5

a) Correlation between measured perfusion and measured total renal flow in both kidneys of four subjects at test and retest. There is a significant correlation (5% level), with correlation coefficient R = 0.61, slope 29 ± 10 (mL/100g.min)/(mL/s) and intercept 28 ± 111 (mL/100g.min)/(mL/s). b) Correlation between changes in flow measurements at test and retest; flow measured by perfusion and phase-contrast angiography. No significant correlation of changes is observed with R = 0.11 for 8 samples.

Figure 6

Results from retrospective image sorting. In two subjects, perfusion difference images acquired during free breathing are shown (left), reconstructed from all 48 label-control pairs, and (right), after retrospectively sorting the images based on respiratory bellows position. During one quarter of the acquisition time, the subject was breathing deeply. The sorting strategy accepted the 50% of images closest to end-expiration. These images were acquired using the ‘heavy’ background suppression scheme.