Evaluation of 2D Imaging Schemes for Pulsed Arterial Spin Labeling of the Human Kidney Cortex

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1 diagnostics Article Evaluation 2D Imaging Schemes Pulsed Arterial Spin Labeling Human Kidney Cortex Charlotte E. Buchanan, Eleanor F. Cox Susan T. Francis * ID Sir Peter Mansfield Imaging Centre, School Physics Astronomy, University Nottingham, Nottingham NG7 2RD, UK; charlotte.buchanan@nottingham.ac.uk (C.E.B.); eleanor.cox@nottingham.ac.uk (E.F.C.) * Correspondence: susan.francis@nottingham.ac.uk; Tel.: +44-(0) Received: 25 May 2018; Accepted: 25 June 2018; Published: 28 June 2018 Abstract: A number imaging readout s are proposed renal arterial spin labeling (ASL) to quantify kidney cortex, including gradient echo-based methods balanced fast field echo (bffe) gradient-echo echo-planar imaging (GE-EPI), or spin echo-based s spin-echo echo-planar imaging (SE-EPI) turbo spin-echo (TSE). Here, we compare se two-dimensional (2D) imaging s to evaluate optimal imaging pulsed ASL (PASL) assessment human kidney cortex at 3 T. Ten healthy volunteers with normal renal function were scanned using 2D multi-slice imaging, in combination with a respiratory triggered flow-sensitive alternating inversion recovery (FAIR) ASL on a 3 T Philips Achieva scanner. All volunteers returned a second identical scan session within two weeks first scan session. Comparisons were made between imaging s in terms -weighted image (PWI) signal-to-noise ratio (SNR) quantification, temporal SNR (tsnr), spatial coverage, repeatability. For imaging, renal cortex was calculated (bffe: 276 ± 29 ml/100g/min, GE-EPI: 222 ± 18 ml/100g/min, SE-EPI: 201 ± 36 ml/100g/min, TSE: 200 ± 20 ml/100g/min). Perfusion was found to be higher GE-based readouts when compared with SE-based readouts, with significantly higher measured bffe readout when compared with all or s (p < 0.05), attributed to greater vascular signal present. Despite PWI-SNR being significantly lower SE-EPI when compared with all or s (p < 0.05), SE-EPI readout gave highest tsnr, was found to be most reproducible assessment kidney cortex, with a coefficient variation (CoV) 17.2%, whilst minimizing variability -weighted signal across slices whole-kidney assessment. For assessment kidney cortex using 2D readout s, SE-EPI provides optimal tsnr, minimal variability across slices, repeatable data acquired in a short scan time with low specific absorption rate. Keywords: magnetic resonance imaging (MRI); arterial spin labeling; renal MRI; ; renal ASL 1. Introduction In clinical practice, renal function is typically determined via serum creatinine measurements to estimate glomerular filtration rate (GFR); however, this method is not highly sensitive, changes in GFR may develop relatively late in progression chronic kidney disease (CKD). Renal inms on delivery nutrients oxygen to tissue, is a key measure by which to monitor renal function. A method providing reliable repeatable assessment kidney, in conjunction with precise morphological inmation, would significantly improve assessment monitoring renal health. Arterial spin labeling (ASL) is a magnetic resonance imaging (MRI) technique that allows non-invasive quantitative assessment tissue, with advantage Diagnostics 2018, 8, 43; doi: /diagnostics

2 Diagnostics 2018, 8, not requiring any exogenous contrast agent, instead using magnetization endogenous-labeled blood to provide contrast. The majority renal ASL studies in literature employed a pulsed ASL (PASL) technique using flow-sensitive alternating inversion recovery (FAIR) [1 8]. In FAIR, two images are collected, a selective image which contains non-inverted arterial blood a non-selective image in which inflowing blood is magnetically inverted. By subtracting non-selective image from selective image, a -weighted image (PWI) is med, which with appropriate modeling can be quantified using a map in units ml/100g/min. A number different two-dimensional (2D) imaging readout s were implemented in literature renal ASL studies. To determine optimal readout renal ASL, a number factors must be considered. The optimal readout should have a short echo time (TE) in order to provide highest image signal-to-noise ratio (SNR), to reduce amount signal dephasing distortion. The short intrinsic T 2 T 2 * in abdomen lead to rapid signal dropout loss signal at longer TEs. The ideal readout should be collected in a short shot length to enable multiple slices through kidney to be acquired prior to decay ASL label, thus enabling whole-kidney assessment. Furrmore, if acquisition is respiratorily triggered, it is important to acquire all images within a respiratory cycle, ideally within flat component respiratory cycle at end expiration where motion is minimal. Finally, ideal readout should have a low specific absorption rate (SAR) so that a short temporal spacing between 2D images can be achieved when collecting a multi-slice dataset. Echo planar imaging (EPI) is one most commonly used readout techniques ASL brain due to its relatively short acquisition time making whole-head coverage feasible, its applicability is used discussed in References [9 16]. However, in body, larger field view (FOV) means EPI readouts typically have a longer TE (>10 ms, dependent on parallel imaging acceleration factor), high-spatial-resolution images, acquisition time can become very long resulting in poor image quality due to susceptibility-induced signal inhomogeneities, particularly close to geometrically irregular tissue air boundaries. Eir gradient echo (GE)- or spin-echo (SE)-based EPI can be used. GE-EPI is more influenced by any B 0 field homogeneity than SE-EPI, since phase shifts from field inhomogeneities, static tissue susceptibility gradients, chemical shifts are not cancelled. Since EPI has a short acquisition time, order 30 ms, multiple slices at peak ASL signal curve can be imaged; thus, low variance is expected in -weighted ASL signal across a multi-slice dataset. Sokolska et al. used a GE-EPI readout combined with pseudo-continuous ASL (pcasl) labeling to assess feasibility within-subject repeatability renal measures [17], whilst Gardener et al. employed a SE-EPI readout to acquire multiple slices across kidney, assessed different breathing strategies to overcome respiratory motion [4]. A balanced fast field echo (bffe), is widely used as image readout renal ASL [3,5,6,18]. It has advantage providing a very short TE high image SNR. However, shot length a bffe is long, at approximately 300 ms a typical abdominal FOV with 3 mm voxel resolution. Thus, a multi-slice acquisition, not all slices are collected at peak ASL signal curve, potentially resulting in greater variance in image -weighted signal across slices in comparison to EPI. In addition, long shot length limits number slices which can be collected when respiratorily triggering data acquisition. The bffe s are also limited by ir sensitivity to field inhomogeneity, with bing artefacts apparent in areas f-resonance in image. Gillis et al. measured inter-study reproducibility ASL at 3 T using a FAIR combined with bffe readout, concluded that this provides a repeatable method measuring renal [5]. Turbo spin-echo (TSE) imaging, also known as fast spin-echo (FSE) imaging, is anor alternative spin-echo-based 2D imaging. Here, time saved by scanning multiple lines k-space at once, when compared with stard spin-echo imaging, means it is possible to lengn time between excitation pulse, thus allowing more time T 1 recovery within readout,

3 Diagnostics 2018, 8, thus, resulting in improved image SNR. A higher number phase-encoding steps can also be used improved spatial resolution, susceptibility-induced signal losses are low. However, TE a TSE readout is typically 50 ms, so some blurring image will occur due to T 2 decay, whilst shot length is long at approximately 160 ms, resulting in slices being acquired at different points in ASL signal curve respiratory cycle, ree, potentially increasing variance in -weighted signal across slices. A furr limitation TSE is high SAR due to multiple refocusing pulses which can result in a long temporal spacing between multi-slice images to keep within SAR limits. It should be noted that three-dimensional (3D) readout s are applied to renal ASL. In References [1,19], a 3D GRASE was applied to assess whole-kidney in healthy volunteers. In a more recent publication, a TSE acquisition was used to evaluate 3D volumetric, isotropic-resolution renal ASL [20]. To date, re are no direct comparisons se 2D imaging readout s ASL in kidney. In this work, we compared GE-EPI, SE-EPI, bffe (a balanced gradient echo also termed TrueFISP or FIESTA), single-shot TSE (SSTSE) (a fast spin-echo sequence also termed fast SE (FSE)) readout s used in combination with a FAIR labeling to assess optimal (s) renal ASL. 2. Materials Methods 2.1. Subjects The study was approved by local ethics committee, all participants gave inmed, written consent. Ten healthy volunteers (age 27 ± 10 years; five female) were scanned approximately one hour on a 3 T Philips Achieva MRI scanner using dual-transmit a 16-channel XLTorso receive coil. To assess repeatability readout, volunteers returned a second visit, which comprised an identical scan session within two weeks first visit scan session. The MRI was permed at same time day on all subjects to minimize potential diurnal variations in renal physiologic function. Subjects fasted previous evening from 8 pm to enable a controlled hydration status all subjects. To ensure that all volunteers had normal kidney function, blood urine samples were collected evaluated by a clinician. Urea, electrolytes, urine protein creatinine ratio were assessed MRI Acquisition Initially, bffe localizer scans were acquired in three orthogonal planes to plan placement imaging ASL-labeling slabs relative to kidneys vessels. The FAIR labeling used a frequency-fset corrected inversion (FOCI) pulse to achieve a 45 mm selective (S) inversion slab (10 mm wider than imaging volume) a 400 mm non-selective (NS) inversion slab. Using such a labeling slab means that trailing edge non-selective label does not arrive within label delay time. Coronal-oblique imaging slices were collected through kidneys in descending order (lateral medial) whilst taking care that selective inversion slab avoided aorta ( 1). Identical readout geometry was acquired on subject all imaging s, with a mm FOV, in-plane spatial resolution 3 mm 5 mm slice thickness. All readout s were acquired with parallel acceleration with a SENSE factor 2, thus reducing achievable GE-EPI SE-EPI TE, minimizing readout duration, reby limiting susceptibility-related distortions signal drop out, allowing multiple slices to be acquired to sample peak ASL signal curve.

4 Diagnostics 2018, 8, Diagnostics 2018, 8, x FOR PEER REVIEW Flow-sensitive Flow-sensitive alternating alternating inversion inversion recovery recovery (FAIR) (FAIR).. Positioning Positioning selective selective non-selective non-selective labeling labeling slabs slabs relative relative to to imaging imaging volume volume kidneys kidneys aorta. aorta. To suppress any static tissue signal in -weighted images, in-plane water To suppress any static tissue signal in -weighted images, in-plane water suppression suppression enhanced through T1 effects (WET) presaturation pulses were applied immediately prior enhanced through T to S/NS pulse 1 effects (WET) presaturation pulses were applied immediately prior to a sinc post-saturation pulse was applied immediately after. A post-label delay S/NS pulse a sinc post-saturation pulse was applied immediately after. A post-label delay (PLD (PLD equivalent to inversion time, TI), defined to be time between inversion pulse to equivalent to inversion time, TI), defined to be time between inversion pulse to central central k-space first slice, 1300 ms was used bffe TSE readouts, 1800 ms GEk-space first slice, 1300 ms was used bffe TSE readouts, 1800 ms GE-EPI EPI SE-EPI readouts. This accounted different readout duration s SE-EPI readouts. This accounted different readout duration s (see Table 1), (see Table 1), ensuring maximum -weighted signal was sampled in central slice ensuring maximum -weighted signal was sampled in central slice.. All datasets were acquired respiratorily triggered on S/NS RF pulse, with a All datasets were acquired respiratorily triggered on S/NS RF pulse, with a minimum repetition minimum repetition time (TR) 3 s between S/NS RF pulse. In total, 25 S/NS image pairs were time (TR) 3 s between S/NS RF pulse. In total, 25 S/NS image pairs were acquired acquired readout. readout. Table 1. Imaging parameters balanced fast field echo (bffe), gradient-echo echo-planar Table 1. Imaging parameters balanced fast field echo (bffe), gradient-echo echo-planar imaging (GE-EPI), spin-echo echo-planar imaging (SE-EPI), turbo spin-echo (TSE) readout imaging (GE-EPI), spin-echo echo-planar imaging (SE-EPI), turbo spin-echo (TSE) readout s. s. The post-label delay (PLD) is defined as time to central k-space first slice. The post-label delay (PLD) is defined as time to central k-space first slice. Post-Label Echo Number Slices Slice Readout Readout Scheme Post-Label Echo Time Flip Angle ( ) Delay (ms) Time (ms) Flip Angle ( (Slice Gap (mm)) Spacing (ms) Scheme Delay (ms) (ms) Number Slices Slice Spacing ) (Slice Gap (mm)) (ms) bffe (0) 280 bffe (0) 280 GE-EPI (0) (0) SE-EPI (0) (0) TSE (5) 480 TSE (5) 480 Base magnetization (M0) 0 ) T1 T 1 relaxation time images were acquired with geometry readout s matched to ASL to allow quantification. Base magnetization (M0) 0 ) images were acquired at same point in respiratory cycle as ASL data using a trigger delay matched to ASL PLD time. A modified respiratorily triggered inversion-recovery sequence was implemented to map TT1 1 relaxation time in renal cortex. Images were acquired at multiple inversion times (TI) 200 ms to 1500 ms in 100 ms steps, but with all TIs collected in respiratory cycle at at same same time time as as ASL ASL data by data introducing by introducing an additional an additional delay (Tv) delay following (Tv) following respiratory trigger, respiratory trigger, prior to prior inversion to pulse inversion [21]. pulse T 1 data [21]. were T1 data collected were collected with a minimum with a minimum TR 8 str to allow 8 s to full allow signal full recovery signal recovery using ausing 400 mm a 400 NS mm inversion NS inversion slab. slab. For GE-EPI For GE-EPI SE-EPI SE-EPI readouts, readouts, multi-slice multi-slice T 1 dataset T1 dataset was acquired was acquired in descending descending order, while order, while bffe bffe TSE readout TSE s, readout s, multi-slice multi-slice T 1 dataset T1 dataset was acquired was acquired both ascending both ascending descending descending orders orders to increase to increase dynamic dynamic range range TI TI [22]. [22] Quantification Renal Cortex Perfusion TT1 1 Analysis was was permed permedusing usingcustom-written MATLAB MATLAB programs programs (Matlab (Matlab version version 8.1, The 8.1, The MathWorks, MathWorks, Inc., Inc., Natick, Natick, MA, MA, USA). USA). ASL ASL -weighted -weighted (PW) (PW) difference differenceimages imageswere med by subtracting non-selective images from selective images [21]. PW were inspected motion, misaligned pairs discarded, remaining PW difference images averaged to m an average PW difference image (ΔM) slice. These were n normalized to base

5 Diagnostics 2018, 8, inspected motion, misaligned pairs discarded, remaining PW difference images averaged to m an average PW difference image ( M) slice. These were n normalized to base M 0 image. T 1 maps were med by fitting inversion recovery data to a two-parameter model. M, T 1, base M 0 maps were used to generate a renal (f ) map, in units ml/100g/min, by fitting data to a kinetic model [23]. Each slice was fitted taking into account exact post-label delay at which slice was collected following labeling pulse (see Table 1; i.e., 1300 ms, 1580 ms, 1860 ms, 2140 ms, 2420 ms slices 1 to 5 bffe ), assuming an arterial transit delay 400 ms (as defined from previous healthy volunteer data [21]). To segment renal cortex, kidneys were manually segmented bee a histogram kidney T 1 was produced along with a threshold to create a cortex mask. This analysis procedure is outlined in 3 Reference [21] from which mode distribution within renal cortex could be computed. The renal cortex masks were compared across readout s to ensure approximately same number voxels were assessed; average DICE similarity coefficient between visits was calculated as 0.53 ± The mean stard deviation in renal cortex were calculated both left right kidneys across subjects Image Quality Assessment The following quantitative metrics were computed in renal cortex to assess quality data readout : (i) -weighted image (PWI) SNR, defined as mean PW signal divided by stard deviation in background noise PW image; (ii) temporal SNR (tsnr) -weighted image, defined as mean PW signal divided by stard deviation across 25 ASL pairs (average across slices); (iii) variance in PW signal across slices (var M ), defined as stard deviation in PW signal across slices divided by mean PW signal Statistical Analysis Statistical analysis was permed using SPSS stware version 21(IBM ). Quantitative variables are expressed as mean ± stard deviation (SD) or median interquartile range (IQR) depending on normality, with a Shapiro Wilk test used to test normality data. In all analyses, a p-value < 0.05 was considered as statistically significant. To assess differences between readout s, a repeated measures ANOVA test was used. For readout, between- within-subject variability measurements was assessed by coefficient variation (bcv, wcv). In addition, coefficient variation (CoV) (stard deviation divided by mean) was calculated to assess repeatability between scan sessions. 3. Results All healthy volunteers were confirmed to have normal kidney function, with egfr > 60 ml/min/1.73 m 2, with a creatinine level 76 ± 15 µmol/l, a urea level 4.2 ± 1.1 mmol/l ASL Image Quality Base M 0 images 2D readout are in 2, with good data quality all readouts minimal distortions, even EPI acquisitions. Note that TSE suffered from blurring, whilst vessels appeared brighter in bffe image. The TSE ASL had highest SAR at approximately 70% whole-body averaged SAR, whilst SE-EPI, GE-EPI, bffe ASL s all had SAR less than 35%. To minimize SAR, TSE implements a longer temporal spacing between slice acquisitions (see Table 1), limiting TSE acquisition to three slices acquired at peak ASL signal curve.

6 Diagnostics 2018, 8, Diagnostics 2018, 8, x FOR PEER REVIEW Example base magnetization (M0) images balanced fast field echo (bffe), gradientecho echo-planar imaging (GE-EPI), spin-echo echo-planar imaging (SE-EPI), turbo spin-echo 2. Example base magnetization (M 0 ) images balanced fast field echo (bffe), gradient-echo echo-planar imaging (GE-EPI), spin-echo echo-planar imaging (SE-EPI), turbo (TSE) readout s. spin-echo (TSE) readout s. The TSE ASL had highest SAR at approximately 70% whole-body averaged 3 shows multi-slice average -weighted images readout. The tsnr, SAR, whilst SE-EPI, GE-EPI, bffe ASL s all had SAR less than 35%. To PWI-SNR, variability -weighted signal (var minimize SAR, TSE implements a longer temporal spacing M ) are provided in Table 2. The TSE between slice acquisitions (see had highest PWI-SNR, whilst SE-EPI had lowest PWI-SNR. However, tsnr was Table 1), limiting TSE acquisition to three slices acquired at peak ASL signal curve. optimal SE-EPI, whilst GE-EPI had lowest t-snr. The variability 3 shows multi-slice average -weighted images readout. The -weighted signal across slices (var tsnr, PWI-SNR, variability -weighted M ) was found to be smallest SE-EPI, signal (var M) are provided in Table 2. The reflecting this to be a good multi-slice whole kidney assessment, with highest variability TSE had highest PWI-SNR, whilst SE-EPI had lowest PWI-SNR. However, tsnr recorded was Diagnostics optimal 2018, 8, bffe x FOR. SE-EPI PEER, REVIEW 4 shows example maps readout. whilst GE-EPI had lowest t-snr. The variability weighted signal across slices (varδm) was found to be smallest SE-EPI, reflecting this to be a good multi-slice whole kidney assessment, with highest variability recorded bffe. 4 shows example maps readout Example Example -weighted -weighted images images (PWI) (PWI) (bffe, (bffe, GE-EPI, GE-EPI, SE-EPI, SE-EPI, TSE) TSE) from a single subject. The post-label delay slice is indicated on image. from a single subject. The post-label delay slice is indicated on image.

7 Diagnostics 2018, 8, Table 2. Perfusion-weighted image signal-to-noise ratio (PWI-SNR), temporal SNR (tsnr), variability -weighted signal (var M ). Readout Scheme PWI-SNR tsnr var M (%) bffe 6.2 ± ± ± 11 GE-EPI 6.3 ± ± ± 5 3. Example SE-EPI -weighted 4.9 images ± 1.5(PWI) 2.6 ± 1.6 (bffe, 11 GE-EPI, ± 3 SE-EPI, TSE 8.5 ± ± ± 4 TSE) from a single subject. The post-label delay slice is indicated on image Example Example maps maps readout readout (bffe, (bffe, GE-EPI, GE-EPI, SE-EPI, SE-EPI, single-shot single-shot TSE) TSE) from from a single single subject subject central central slice. slice. Note Note higher higher signal signal bffe bffe due due to to contributions contributions from from arcuate arcuate arteries arteries kidney. kidney. Table 2. Perfusion-weighted image signal-to-noise ratio (PWI-SNR), temporal SNR (tsnr), 3.2. Perfusion Quantification variability -weighted signal (varδm). Mean renal cortex across all readout s was 223 ± 11 ml/100g/min; error Readout Scheme PWI-SNR tsnr var M indicates stard error across readout s. (%) For readout, measured renal cortex bffe 6.2 ± ± 2.0 first visit 26 ± are 11 in 5, were found to be as follows: GE-EPI bffe 276 ± ± ml/100g/min, ± 0.8 GE-EPI ± 5 ± 18 ml/100g/min, SE-EPI 201 ± 36 ml/100g/min, SE-EPI TSE ± ml/100g/min. 2.6 ± A ± repeated 3 measures ANOVA showed that bffe readout produced TSE consistently 8.5 ± 4.1 higher 2.4 ± ± 4 when compared with or three s (p = 0.03), but also had largest variance between subjects. The SE-EPI gave 3.2. Perfusion Quantification Diagnostics lowest 2018, 8, within-subject x FOR PEER REVIEW variance Mean renal cortex across all readout s was 223 ± 11 ml/100g/min; error indicates stard error across readout s. For readout, measured renal cortex first visit are in 5, were found to be as follows: bffe 276 ± 29 ml/100g/min, GE-EPI 222 ± 18 ml/100g/min, SE-EPI 201 ± 36 ml/100g/min, TSE 200 ± 20 ml/100g/min. A repeated measures ANOVA showed that bffe readout produced consistently higher when compared with or three s (p = 0.03), but also had largest variance between subjects. The SE-EPI gave lowest within-subject variance. The between-subject variation (bcv) s was 53.3%, 23.7%, 26.2%, 35.9% bffe, GE-EPI, SE-EPI, TSE, respectively. The within-subject variation (wcv) was 18.8% bffe, 23.9% GE-EPI, 15.1% SE-EPI, 17.2% TSE Renal Renal cortical cortical measured from from readout readout first first visit. visit. Values Values are are mean mean with with error error bars bars showing showing stard stard error error mean. mean. The The bffe readout gave significantly higher than or readouts (repeated measures bffe readout gave significantly higher than or readouts (repeated measures ANOVA, p = 0.03). A significant difference was observed between SE-EPI GE-EPI., with p- ANOVA, p = 0.03). A significant difference was observed between SE-EPI GE-EPI., with p- post-hoc paired t-tests. post-hoc paired t-tests. For readout, T1 renal cortex was also assessed computed to be 1186 ± 147 ms, 1259 ± 214 ms, 1294 ± 168 ms, 1135 ± 149 ms bffe, GE-EPI, SE-EPI, TSE readout s, respectively Repeatability

8 Diagnostics 2018, 8, The between-subject variation (bcv) s was 53.3%, 23.7%, 26.2%, 35.9% bffe, GE-EPI, SE-EPI, TSE, respectively. The within-subject variation (wcv) was 18.8% bffe, 23.9% GE-EPI, 15.1% SE-EPI, 17.2% TSE. For readout, T 1 renal cortex was also assessed computed to be 1186 ± 147 ms, 1259 ± 214 ms, 1294 ± 168 ms, 1135 ± 149 ms bffe, GE-EPI, SE-EPI, TSE readout s, respectively Repeatability The most repeatable readout was SE-EPI, which had a CoV 17.2%; least repeatable was GE-EPI with a CoV 28.3%. For readout, across 10 subjects, re were no significant differences in renal cortex between first second visit (p > 0.05). 4. Discussion Previous renal ASL studies used a variety readout s in combination with FAIR labeling. In this work, a comparison balanced fast field echo (bffe), gradient-echo EPI (GE-EPI), spin-echo EPI (SE-EPI), turbo spin-echo (TSE) s was made renal ASL. For all s, multi-slice coverage could be achieved with five contiguous slices collected bffe, GE-EPI, SE-EPI, three slices with a 5 mm slice gap TSE. The higher SAR led to wider readout spacing TSE. GE-EPI SE-EPI could achieve whole-kidney coverage in shortest amount time. In this work, cortical was higher gradient-echo s (GE-EPI bffe) in comparison to spin-echo based s (SE-EPI TSE). In particular, calculated from bffe readout data was found to be significantly higher than that from all or s which could be attributed to presence vascular signal in se images. The bffe had highest intravascular signal contribution all readout s due to high signal intensity from blood, which has an intrinsically high T 2 /T 1 ratio, as well as continuous replenishment fresh blood fact that longitudinal magnetization flowing blood is minimally disturbed by alpha pulse train in bffe readout [24]. When comparing SNR, SE-EPI gave a significantly lower PWI-SNR when compared with or s; however, this could, in part, be attributed to lower echo time this sequence or lower contribution from vascular signal in SE-EPI images. The SE-EPI also gave highest temporal SNR, suggesting lowest fluctuations from pulsatile vessels. The SE-EPI gave lowest variance in -weighted signal (var M ) across slices all readout s. This is unsurprising as SE-EPI has a short shot length per slice, so, all slices are acquired at almost same point on both ASL signal curve in respiratory cycle, resulting in a small variance in signal slice. Conversely, TSE readouts have a long shot length; this yielded highest variance in signal across slices. All 2D readout s were determined to be repeatable with a CoV 28.2% or less. SE-EPI was found to be optimal with a CoV 17.2%. The ASL quantitative measurements normal showed good within-subject variability repeatability. Note that differences in distortions between readout s are likely partly reflected in DICE coefficients between masks. In addition, in this study, T 1 maps were generated readout, used in quantification, with mean T 1 renal cortex also computed. As expected, bffe TSE readout s led to a shorter measured longitudinal recovery time when compared with EPI methods, likely due to influence T 2 */T 2 bffe readout [25], contribution magnetization transfer effects TSE readout [26]. A limitation this study is that data were not collected with multiple post-label delay times to compute individual subject arterial transit delay time maps use in quantification; instead, a fixed arterial transit delay time was assumed. However, all readout s, same selective non-selective label positions widths were used, slice coverage was small

9 Diagnostics 2018, 8, at 2.5 cm; thus, arterial transit delay effects were minimal equally affected readout s in quantification. These estimates m basis interpreting optimal readout s guiding future study design in assessing ASL renal. 5. Conclusions FAIR ASL measures were collected bffe, GE-EPI, SE-EPI, TSE readout s. All s were found to be repeatable with coefficients variation less than 29% all techniques. When comparing all four techniques, we conclude that SE-EPI provides optimal temporal SNR, consistency across slices, repeatability between sessions, has lowest specific absorption rate. Author Contributions: S.T.F. conceived designed experiments; C.E.B., S.T.F., E.F.C. permed experiments; C.E.B. analyzed data; C.E.B., E.F.C., S.T.F. wrote paper. Funding: This work was funded by Dr Hadwen Trust. The Dr Hadwen Trust (DHT) is UK s leading non-animal biomedical research charity that exclusively funds promotes human-relevant research that replaces use animals whilst supporting progress medicine. Conflicts Interest: The authors declare no conflict interest. References 1. Cutajar, M.; Thomas, D.L.; Hales, P.W.; Banks, T.; Clark, C.A.; Gordon, I. Comparison ASL DCE MRI non-invasive measurement renal blood flow: Quantification reproducibility. Eur. Radiol. 2014, 24, [CrossRef] 2. Dong, J.; Yang, L.; Su, T.; Yang, X.; Chen, B.; Zhang, J.; Wang, X.; Jiang, X. Quantitative assessment acute kidney injury by noninvasive arterial spin labeling MRI: A pilot study. Sci. China Life Sci. 2013, 56, [CrossRef] 3. Gardener, A.; Francis, S. Multi-slice kidney using SE-EPI FAIR: Optimised acquisition analysis strategies. In Proceedings 17th Scientific Meeting, International Society Magnetic Resonance in Medicine, Honolulu, HI, USA, April 2009; p Gardener, A.G.; Francis, S.T. Multislice kidneys using parallel imaging: Image acquisition analysis strategies. Magn. Reson. Med. 2010, 63, [CrossRef] 5. Gillis, K.A.; McComb, C.; Foster, J.E.; Taylor, A.H.M.; Patel, R.K.; Morris, S.T.W.; Jardine, A.G.; Schneider, M.P.; Roditi, G.H.; Delles, C.; et al. Inter-study reproducibility arterial spin labelling magnetic resonance imaging measurement renal in healthy volunteers at 3 Tesla. BMC Nephrol. 2014, 15, 23. [CrossRef] 6. Martirosian, P.; Klose, U.; Mader, I.; Schick, F. FAIR True-FISP Perfusion Imaging Kidneys. Magn. Reson. Med. 2004, 51, [CrossRef] 7. Pedrosa, I.; Rafatz, K.; Robson, P.; Wagner, A.A.; Atkins, M.B.; Rsky, N.M.; Alsop, D.C. Arterial spin labeling MR imaging characterisation renal masses in patients with impaired renal function: Initial experience. Eur. Radiol. 2012, 22, [CrossRef] 8. Robson, P.M.; Madhuranthakam, A.J.; Dai, W.; Pedrosa, I.; Rsky, N.M.; Alsop, D.C. Strategies reducing respiratory motion artifacts in renal imaging with arterial spin labeling. Magn. Reson. Med. 2009, 61, [CrossRef] 9. Alsop, D.C.; Detre, J.A.; Golay, X.; Günr, M.; Hendrikse, J.; Hernez-Garcia, L.; Lu, H.; Macintosh, B.J.; Parkes, L.M.; Smits, M.; et al. Recommended implementation arterial spin-labeled MRI clinical applications: A consensus ISMRM study group european consortium ASL in dementia. Magn. Reson. Med [CrossRef] 10. Brookes, M.J.; Morris, P.G.; Gowl, P.A.; Francis, S.T. Noninvasive measurement arterial cerebral blood volume using Look-Locker EPI arterial spin labeling. Magn. Reson. Med. 2007, 58, [CrossRef] 11. Gunr, M.; Bock, M.; Schad, L.R. Arterial spin labeling in combination with a look-locker sampling strategy: Inflow turbo-sampling EPI-FAIR (ITS-FAIR). Magn. Reson. Med. 2001, 46, [CrossRef]

10 Diagnostics 2018, 8, Hall, E.; Hall, E.; Wesolowski, R.; Wesolowski, R.; Gowl, P.; Gowl, P.; Francis, S.; Francis, S. Improved detection estimation using high spatial resolution ASL at 7T. In Proceedings 17th Scientific Meeting, International Society Magnetic Resonance in Medicine, Honolulu, HI, USA, April 2009; p Kim, S.G. Quantification relative cerebral blood flow change by flow-sensitive alternating inversion recovery (FAIR) technique: Application to functional mapping. Magn. Reson. Med. 1995, 34, [CrossRef] 14. Wong, E.C. Quantifying CBF with pulsed ASL: Technical pulse sequence factors. J. Magn. Reson. Imaging 2005, 22, [CrossRef] 15. Wong, E.C. An introduction to ASL labeling techniques. J. Magn. Reson. Imaging 2014, 40, [CrossRef] 16. Wong, E.C.; Buxton, R.B.; Frank, L.R. A oretical experimental comparison continuous pulsed arterial spin labeling techniques quantitative imaging. Magn. Reson. Med. 1998, 40, [CrossRef] 17. Sokolska, M.; Thomas, D.; Bainbridge, A.; Golay, X.; Taylor, S.; Punwani, S.; Pendse, D.; Uk, L. Renal Pseudo-continuous Arterial Spin Labelling ( pcasl) MRI: A Repeatability Study. Eur. Congr. Radiol. 2014, [CrossRef] 18. Park, S.H.; Wang, D.J.J.; Duong, T.Q. Balanced steady state free precession arterial spin labeling MRI: Initial experience blood flow mapping in human brain, retina, kidney. Magn. Reson. Imaging 2013, 31, [CrossRef] 19. Cutajar, M.; Thomas, D.L.; Banks, T.; Clark, C.A.; Golay, X.; Gordon, I. Repeatability renal arterial spin labelling MRI in healthy subjects. Magn. Reson. Mater. Phys. Biol. Med. 2012, 25, [CrossRef] 20. Robson, P.M.; Madhuranthakam, A.J.; Smith, M.P.; Sun, M.R.M.; Dai, W.; Rsky, N.M.; Pedrosa, I.; Alsop, D.C. Volumetric Arterial Spin-labeled Perfusion Imaging Kidneys with a Three-dimensional Fast Spin Echo Acquisition. Acad. Radiol. 2016, 23, [CrossRef] 21. Cox, E.F.; Buchanan, C.E.; Bradley, C.R.; Prestwich, B.; Mahmoud, H.; Taal, M.; Selby, N.M.; Francis, S.T. Multiparametric renal magnetic resonance imaging: Validation, interventions, alterations in chronic kidney disease. Front. Physiol. 2017, 8, [CrossRef] 22. Hoad, C.L.; Palaniyappan, N.; Kaye, P.; Chernova, Y.; James, M.W.; Costigan, C.; Austin, A.; Marciani, L.; Gowl, P.A.; Guha, I.N.; et al. A study T1 relaxation time as a measure liver fibrosis influence confounding histological factors. NMR Biomed. 2015, 28, [CrossRef] 23. Buxton, R.B.; Frank, L.R.; Wong, E.C.; Siewert, B.; Warach, S.; Edelman, R.R. A general kinetic model quantitative imaging with arterial spin labeling. Magn. Reson. Med. 1998, 40, [CrossRef] 24. Yan, L.; Li, C.; Kilroy, E.; Wehrli, F.W.; Wang, D.J.J. Quantification arterial cerebral blood volume using multiphase-balanced SSFP-based ASL. Magn. Reson. Med. 2012, 68, [CrossRef] 25. Schmitt, P.; Griswold, M.A.; Jakob, P.M.; Kotas, M.; Gulani, V.; Flentje, M.; Haase, A. Inversion recovery TrueFISP: quantification T(1), T(2), spin density. Magn. Reson. Med. 2004, 51, [CrossRef] 26. Turner, R.; Oros-Peusquens, A.M.; Romanzetti, S.; Zilles, K.; Shah, N.J. Optimised in vivo visualisation cortical structures in human brain at 3 T using IR-TSE. Magn. Reson. Imaging 2008, 26, [CrossRef] 2018 by authors. Licensee MDPI, Basel, Switzerl. This article is an open access article distributed under terms conditions Creative Commons Attribution (CC BY) license (