Findings from quantitative measures of the images and from assessments by the radiologists are generally in agreement. There is a clear preference for background suppression, with little significant discrimination between other combinations of background suppression and breathing strategies. We surmise that the quantitative measurements of the mean and standard deviation of flow values within ROIs are unsuitable for detecting artifactual corruption of the image. Edge artifacts from misaligned label-control pairs or repetitions, or image ripples and other corruptions from k-space errors arising from unsuppressed physiologic noise tend to average toward the true value that would be obtained by that strategy if it were suffering no respiratory motion artifacts. The importance of background suppression is made apparent, qualitatively, by the low measurement variance and high reader-rating found for images acquired during free breathing when background suppression was used.
Physiologic variation of renal flow may have contributed to the test-retest variability of the results. The timing and content of recent meals, especially protein content (54
), can have a substantial effect on renal blood flow. No control for eating prior to imaging was used for this initial study. Variability in measured total flow may also have arisen from placement of the imaging slice through the kidneys leading to variable volumes of perfused parenchyma being included. Additionally, variability in measured total flow between test and retest may have been introduced through variable inversion efficiency as a function of changing blood flow speed or pulsatility in the aorta, or from off-resonance effects from residual field inhomogeneity differing due to patient positioning and/or shimming. Such sources of variability may have overshadowed variability due to the background suppression and breathing strategy acquisition schemes.
In our experience, when a subject follows a multiple breath-hold, or timed breathing scheme well, as monitored by the respiratory bellows position, the resulting images are most often of superior quality. However, no statistical preference for breathing strategies was found. Each breathing strategy has advantages and disadvantages, which may have compensated for one another under statistical analysis. Observation of breathing in our subjects indicates that multiple breath-hold acquisitions most often result in a consistent end-expiration position, which is favorable for restricting respiratory motion between repetitions. However, the breath-hold procedure requires additional time between acquisitions resulting in fewer averages being acquired in the same time compared to the timed and free breathing strategies, even accounting for image rejection. In addition, breath-holding may be impossible with some patients. The timed breathing strategy, when followed effectively, produces images of comparable quality to breath-held acquisition with additional benefit of a higher signal-to-noise ratio per imaging time. However, even with cooperative volunteers we found that the breathing scheme was not maintained well throughout all scans. When asked to breathe freely, subjects most often fall into a gentle shallow breathing pattern resulting in only small displacements between repeated acquisitions. In comparison, when conscious of following a breathing pattern (timed breathing) subjects are prone to taking sharper deep breaths. For a patient population where breath-held acquisition is not likely and timed breathing is unlikely to be followed well, our experience from clinical populations (not reported here) has indicated a suitable compromise: repeated, shallow, 6-s breath-holds for reference scans at the beginning of the scan followed by shallow free breathing. This is most likely to produce robust calculation of the T1 and M0 maps required for perfusion quantification and produce minimal disruption to the ASL difference image. Any periods of erratic breathing that induce large abdominal displacements are effectively dealt with using retrospective respiratory sorting. However, it is also useful to include additional reference image acquisitions during the remainder of the scan.
Retrospective sorting of image acquisitions based on respiratory position enables images without gross motion related artifact to be obtained with free breathing at the expense of lower signal-to-noise ratio resulting from image rejection. Typically, accepting half of the acquired images is offset by the possibility of acquiring data twice as long. The ease of preparation for imaging without breath-holding or timing breathing enables more time to be spent acquiring data. The acceptance criterion may be adjusted to trade off loss in signal-to-noise ratio for reduced image degradation due to motion. It is useful to reconstruct images both with sorting and using all available image acquisitions for comparison. Our initial study of retrospective sorting has used the respiratory bellows position to indicate the position of the kidneys. However, other techniques, including navigators, may prove more accurately correlated indicators of position (55
). More sophisticated motion-monitoring schemes may also prove useful. For example, each raw label and control image, or additional structural images, could be used in registration algorithms, although issues of image SNR, and acquisition time become important, as well as the possibility of non-rigid body motions.
Perfusion difference images were most often of high quality. The additional image-processing required to compute quantitative perfusion maps occasionally degraded image quality due to miss-registration between the perfusion difference image and the reference images used to calculate tissue-T1 and estimate relaxed blood magnetization in the quantification.
Whilst this study was carried out at 1.5 T, renal perfusion measurements at 3 T, where perfusion studies benefit from intrinsically higher SNR and longer T1 values, will also benefit from the respiratory motion artifact-reduction strategies described here. Background suppression pulse timings can be easily recalculated for different T1-relaxation rates, and their power deposition, despite being adiabatic in nature, is not a great concern over the course of the labeling and imaging cycle. pCASL labeling itself may be more problematic in the abdomen at increased field strength, where B1-inhomogeneity may cause inefficiencies in the labeling. Main-field inhomogeneity may also pose similar problems; however, studies in the brain have proven successful (28
Our focus in this study was more on reducing subtraction error due to motion of the background than on eliminating motion related degradation of the perfusion signal. In the absence of background subtraction error, the averaging of multiple images at slightly different positions is likely to cause blurring of the perfusion signal. If multi-shot imaging sequences are used, as might be desirable for 3-D imaging, then motion could also cause more complex artifacts linked to the phase acquisition order.
The successful application of abdominal ASL has merit in providing an alternative to gadolinium based contrast media for characterizing masses (56
), assessing organ perfusion (59
), and assessing tumor response to certain therapies, particularly those that are anti-angiogenic (61
). The techniques described herein, offer abdominal ASL studies of a quality to be considered reasonable for clinical use.