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S N I L INTRODUCTION CONCLUSIONS REFERENCES & ACKNOWLEDGEMENTS RESULTS THEORY & METHODS Erin K Englund 1 , Michael C Langham 2 , Cheng Li 1 , Emile R Mohler 3 , Thomas F Floyd 4 , and Felix W Wehrli 2 1 Department of Bioengineering, 2 Department of Radiology, 3 Department of Cardiology, University of Pennsylvania, Philadelphia, PA, United States, 4 Department of Anesthesiology, Stony Brook University School of Medicine, Stony Brook, NY, United States Simultaneous quanficaon of perfusion, intravascular venous oxygen saturaon, and skeletal muscle T 2 * during reacve hyperemia in the leg using an interleaved PASL and mul-echo GRE sequence We propose to develop and validate an interleaved PASL/mul-echo GRE (PASL/Ox-BOLD) pulse sequence, capable of simultaneously measuring perfusion, venous oxygen saturaon, and skeletal muscle T 2 * during reacve hyperemia in the leg. • Peripheral arterial disease (PAD), generally a systemic manifestaon of atherosclerosis, is a debilitang disease affecng many people, parcularly the elderly [1]. • Risk factors include smoking, diabetes, older age, and high blood pressure. • Paents present with low ankle-brachial index (ABI) and claudicaon. • PAD is an independent risk factor for heart aack and stroke. • PAD results in structural as well as funconal impairments. • Structural - Flow-liming stenoses in large arteries and compensatory collateralizaon [2]. • Funconal - Endothelial dysfuncon, decreased perfusion, and blunted hyperemic response [3]. • Many current MRI-based methods for assessing PAD require an ischemia/reperfusion paradigm to invesgate the funconal reserve of the vasculature. • CASL [3] or PASL [4] to measure perfusion. • MR oximetry to assess dynamic parameters of venous oxygen saturaon (SvO 2 ) [5,6]. • Measurement of skeletal muscle T 2 * as a marker of muscle oxygenaon status [7,8]. • No previous study has been able to concurrently measure all funconal parameters. Image Analysis • Perfusion • Temporal matching of NS and SS images was achieved by averaging adjacent NS me points. • Regions of interest (ROIs) were drawn in the gastroc, soleus, peroneus, and bialis anterior (TA) muscles and perfusion was calculated from Eq 1. • Parameters extracted from the perfusion me course data include peak perfusion and me to peak perfusion (TTP). • Ox-BOLD • For each Ox-BOLD image, high spaal frequency data was filled in from the average of reference images acquired immediately aſter the scan. • Only data acquired following SS inversion was used for analysis, though the Ox-BOLD interleave was run every PLD to control for magnezaon transfer effects. • Oximetry • A phase map was generated for each of the images and the baseline phase accumulaon in ssue was subtracted [12]. • The larger of the peroneal veins was selected for SvO 2 analysis and an ROI was drawn in this vein. The reference ssue was selected in an ROI immediately surrounding the peroneal vein and SvO 2 was calculated from Eq 2. • Washout me (me to minimum SvO 2 ), and overshoot (peak SvO 2 - baseline SvO 2 ) were recorded from the SvO 2 me course. • T 2 * calculaon • Signal intensity in each of the 5 echoes in a homogeneous region of the soleus muscle was extracted and fied to a monoexponenal funcon. Perfusion - Saturaon Inversion Recovery • Perfusing blood is labeled by alternang between non-selecve (NS) and slice selecve (SS) adiabac inversion pulses [4]. • Perfusion is calculated as: • Where f is perfusion in classical units (mL·min -1 ·100g -1 ) • λ is the tissue-partition coefficient (0.9 mL/g) • T is the post labeling delay (PLD) + TE (T = 960.24 ms) • M SS and M NS are the magnitude signal intensities • T 1 =T 1ssue =T 1blood = 1420 ms • Paral-Fourier GRE-EPI readout following NS and SS inversions with the following parameters: • FOV = 250 × 250 mm 2 , ST = 10 mm • Matrix = 80 × 50, Reconstructed matrix = 80 × 80 • TR/TE = 1 s/8.05 ms, PLD = 952.19 ms Figure 2. Pulse sequence diagram of PASL/Ox-BOLD sequence. The PASL slice is located at mid calf. During the PLD of the PASL, a keyhole mul-echo GRE with 24 phase encoding steps acquires data at a slice located 3 cm inferiorly. [Eq 1] Oximetry and T 2 * - Mul-echo GRE Oximetry - MR Susceptometry • Magnec suscepbility induced differences in phase accumulaon between blood and surrounding ssue are used to calculate hemoglobin oxygen saturaon [6, 10-11] from phase images acquired at TE 1 and TE 2 as: [Eq 2] • Where Δχ do is the suscepbility difference between fully oxygenated and deoxygenated blood (4π·0.27 ppm) • Hematocrit (Hct) is taken to be 0.45 • θ is measured from axial scout images PASL/Ox-BOLD PASL Experimental Design • To validate PASL/Ox-BOLD, measured perfusion, SvO 2 , and T 2 * data were compared to an otherwise idencal PASL-only, or a keyhole Ox-BOLD-only version of the sequence. • 5 subjects were scanned on two separate days to assess accuracy and reproducibility. • For each subject, two PASL/Ox-BOLD, one PASL, and one Ox-BOLD sequence were run in a randomized order. The protocol was repeated in the same order on the second day. • All imaging was performed on a Siemens 3T scanner with the calf centered in an 8 ch tx/rx knee coil (Invivo), and a cuff (Hokanson) placed around the thigh (Figure 4). • Each scan lasted 10 min, with 1 min baseline, 3 min arterial occlusion with cuff inflated to >205 mmHg, and 6 min recovery. 1 min of rest was given before the next scan. Figure 4. Schemac of experimental setup (red slice =PASL, blue slice=Ox-BOLD), the blue cylinder around the thigh represents the cuff. The me line of each scan is shown under the leg. The period of arterial occlusion is always represented by a gray box. Baseline Arterial Occlusion (Cuff >205 mmHg) Recovery (Reacve Hyperemia) Rest t=0 60 240 600 660 s Skeletal muscle T 2 * • Magnitude SI from TE1 - TE5 are fied to a monoexponenal to calculate T 2 *. • Mul-echo GRE readout with keyhole acquision (central ¼ th of k-space acquired every PLD of PASL) and: • FOV = 96 × 96 mm 2 , ST = 10 mm, slice locaon located 3 cm in the foot direcon. • Matrix = 96 × 24, Reconstructed matrix = 96 × 96 using reference image acquired aſter dynamic scan. • TR/TE1/TE2/TE3/TE4/TE5 = 38.12/3.78/6.99/12.32/19.32/26.32 ms. Figure 3. Schemac of phase accumulaon in reference ssue (j t ) and in the vein (j v ). Difference in phase accumulaon (j v -j t =Dj) along with measurement of the angular deviaon of the vein (q) from the main magnec field (B 0 ) allows quanficaon of oxygen saturaon, given by Eq 2. Reference Tissue Vein Vein Reference Tissue Phase accumulaon over DTE Figure 5 a&b. Anatomical scout (a) and GRE-EPI (b) of the perfusion slice. Colored ROIs indicate muscles (Red = gastroc, Green = soleus, Purple = peroneus, Cyan = TA). Figure 5 c. Perfusion me course in gastroc, averaged over all subjects. Gray box indicates period of ischemia. Reacve hyperemia follows cuff release at 240 s. Error bars represent SD. Figure 5 d. Zoomed perfusion me course in an individual subject for each muscle. Dashed lines = PASL, Solid lines = PASL/Ox-BOLD. Colors correspond to muscle ROIs drawn in Figure 5 (b). PERFUSION OXIMETRY b. a. d. a. Baseline b. Figure 6 a. Anatomical image of the Ox-BOLD slice, indicang peroneal veins (yellow) and artery (red). Figure 6 b&c. Zoomed phase images of the medial (solid) and lateral (doed) peroneal veins during baseline (b) and reacve hyperemia (c). Figure 6 d. Oximetry me course in the larger peroneal vein averaged for all subjects. Figure 6 e. Comparison of average washout me. Average values are reported, error bars indicate SD. Average difference between methods is 9%. Figure 6 f. Comparison of average overshoot across all subjects. The average overshoot difference for PASL/Ox-BOLD compared to Ox-BOLD is 11%. Reacve Hyperemia c. h. T 2 * d. Ox-BOLD Ox-BOLD f. e. PASL/Ox-BOLD PASL c. f. T 2 * (ms) Figure 7 e&f. Comparison of average relave T 2 * min and T 2 * max (e), and t min and t max (f). Average values are reported, with error bars indicang SD. b. c. d. Figure 1. Flow-liming stenoses in the peripheral vasculature is the hallmark of PAD (NHLBI [9]). Figure 7 a. Anatomical image of the Ox-BOLD slice, with full Ox-BOLD FOV indicated by the green box. Figure 7 b&c. T 2 * images acquired during ischemia (b) and reacve hyperemia (c). Figure 7 d. Relave T 2 * me course in a ROI in the soleus muscle averaged for all subjects. Each subject’s T 2 * was normalized to his baseline value. T 2 * min T 2 * max t min t max e. f. • Perfusion • Good intra-scan reproducibility, but relavely poor inter-scan reproducibility of the perfusion parameters measured with PASL/Ox-BOLD. • Perfusion varies physiologically with me of day, hydraon, caffeine intake, hormonal fluctuaons, and exercise. • Oximetry and T 2 * • It is possible to measure venous oxygen saturaon in the peroneal vein with keyhole oximetry in less than one second. • T 2 * min ,t min , and t max are similar for PASL/Ox-BOLD and Ox-BOLD, though T 2 * max differs between PASL/Ox-BOLD and Ox-BOLD. • Although the order of the scans was randomized, one of the PASL/Ox-BOLD scans was first in all but one subject. This could explain the decreased T 2 * max seen in PASL/Ox-BOLD, however a larger sample size is necessary to fully invesgate this bias. • Ringing in Ox-BOLD images could be due to keyhole acquision; therefore exploraon of a BRISK acquision scheme will follow. • Conclusions • Simultaneous acquision of PASL, oximetry, and T 2 * has no affect on the quanficaon of perfusion, SvO 2 , or T 2 *. • The data demonstrate the feasibility of a combined PASL/Ox-BOLD method for simultaneous measurement of perfusion, venous oxygen saturaon, and skeletal muscle T 2 * during reacve hyperemia. • There are striking differences in the measured parameters between healthy subjects and PAD paents. • Further exploraon of these funconal parameters in PAD paents could: • Help to beer characterize pathophysiologic mechanisms underlying the funconal impairment in PAD. • Provide a new, noninvasive tool for the diagnosis, evaluaon, and monitoring of PAD disease progression and response to therapy. References. [1] Hirsch, et al. JAMA (2001); [2] Mohler. Arch Intern Medicine (2003); [3] Wu, et al. JACC (2009); [4] Raynaud, et al. MRM (2001); [5] Langham, et al. JACC (2010); [6] Langham, et al. ISMRM (2011); [7] Ledermann, et al. Circulaon (2006); [8] Pohast, et al. Fortschr Röntgenstr (2009); [9] NHLBI, What is Peripheral Arterial Disease?; [10] Haacke et al, Human Brain Mapping (1997); [11] Fernández-Seara et al,MRM (2006); [12] Langham, et al. MRM (2009). Acknowledgements. NIH Grants R01HL075649 and 5T32EB009384. g. e. Figure 5 e&f. Comparison of peak perfusion in the gastroc across all subjects (e) with 101 represenng subject 1 day 1, 102 represenng subject 1 day 2, etc. The average peak perfusion in each muscle is shown in (f) with error bars indicang SD. No significant difference was detected between PASL and PASL/Ox-BOLD. Figure 5 g&h. Similar comparison of TTP in the gastroc across all subjects (g) and the average TTP with PASL/Ox-BOLD and PASL (h) in each muscle. Figure 8. Comparison of perfusion (a), SvO 2 (b), and relave T 2 * (c) in a representave healthy subject (light lines) and PAD paent (dark lines). PAD paent exhibits decreased peak perfusion and increased TTP in (a), lower overshoot and increased washout me in (b), and decreased T 2 * max and increased t max in (c) compared to the healthy subject. Ischemia Reacve Hyperemia a. Relave T 2 * Time (s) a. b. c.
