[Technical Note]

1. Introduction

Human hemodynamics is affected by the alteration ofgravity ; in other words, gravity induces hemodynamicdifferences between supine and upright positions. Moreover,venous blood flow is easily influenced by postural changes[1] ; however, the effect of gravity on hemodynamics stillremains unclear. A study using Doppler ultrasound（US）reported the alteration of venous blood flow due to posturalchanges [2]. Although US system is highly versatile inmeasuring the changes of venous blood flow at differentpostures, including standing, sitting, and supine positions,absolute quantitative analysis of blood flow by US remains achallenge because of operator dependency and theoverestimation of blood flow [3]. In contrast, phase-contrastmagnetic resonance imaging（PC-MRI）is a noninvasivetechnique that can be used to measure blood flow velocity andvolume flow rate in the vascular system with higher reliabilityand reproducibility compared to that with US [4-6]. However,flow quantitation in standing or sitting position has beenlimited by the architecture of conventional MRI systems,which allow the subjects to be examined only in the supineposition. To address this situation, we developed a novel MRIsystem called “gravity MRI”, which was achieved byimproving an existing 0.4-T permanent magnet MRI systemand enables us to perform whole body magnetic resonanceexamination at different postures. If the reliability of bloodflow measurement using gravity MRI is guaranteed, it mayrender it possible to elucidate the effect of gravity on venousblood flow in humans. Although previous studies havevalidated the accuracy of flow measurement with PC-MRI athigher magnetic field strength（1.5-T and 3.0-T）[7, 8], a studyinvestigating the reliability of measuring blood flow at suchpermanent magnet MRI has yet to be performed, in which theachievable signal-to-noise ratio is limited due to the lowmagnetic field strength. Therefore, the purpose of this studywas to validate the quantitation of venous blood flow in

gravity MRI using a flow phantom.

2. Materials & Methods

2.1 Construction of a Venous Flow PhantomWe constructed a flow phantom as shown Fig. 1. A

pressure tube mimicking a venous vessel was suspended in aplastic container and connected to a programmable pump（mono-pump type-NE, HEISHIN, Kobe, Japan）locatedoutside of the MRI room. The plastic container was filled withwater solution containing 3.6 g/l NaCl and 7.1 g/l CuSO4 toadjust the electrical conductivity and relaxation time to beclose to those of the human body and placed in the center of amagnet. The pump produced a steady flow of 40% v/vglycerin-water solution, which has a relaxation time andviscosity similar to blood [9], through the simulated vessel.The flow velocity was set at 2.5, 5.0, 10, 20, or 40 cm/s,simulating representative venous flow velocities in humans [10,11]. These flow velocities set at the pump were regarded as thegold standard when validating the flow velocities with PC-MRI.A trigger output was generated by the pump control program（Pump, R-TEC, Tokyo, Japan）for electrocardiograph（ECG)-triggering of image acquisition. We defined the steady flowdirecting from the pump to the container as “inlet flow” andthe opposite flow, i.e., from the container to the pump, as“outlet flow.” We assessed the flow velocities and rates in both

Quantitation of Venous Blood Flow in Gravity MRI: A Phantom Study

Naoki OHNO, Tosiaki MIYATI, Yuki HIRAMATSU, Minami YAMASAKI

Faculty of Health Sciences, Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University,5-11-80 Kodatsuno, Kanazawa, Ishikawa, 920-0942, Japan（Received on June 6, 2017. In final form on July 25, 2017）

Abstract : This study aimed to validate the quantitation of venous blood flow in gravity magnetic resonance imaging（MRI）using a flow phantom. The phantom consisted of a pressure tube, which simulated a venous vessel, and aprogrammable pump. The pump produced a steady flow of a 40% v/v glycerin-water solution with different flow velocities（2.5, 5.0, 10, 20, or 40 cm/s）through the simulated vessel. Electrocardiograph-triggered phase-contrast（PC）MRI was

performed using a 0.4-T gravity MRI system, and the imaging plane was set to be perpendicular to the flow direction. Intotal, 17 pairs of magnitude and velocity-mapped phase images per simulated cardiac cycle were reconstructed for eachflow velocity. We placed a region of interest in the simulated vessel, determined the flow velocity for each cardiac phaseand mean flow rate in all phases, and compared them with the actual flow velocity and rate defined by the pump. Generally,the flow velocities were consistent with the actual ones. Moreover, the mean flow rate highly correlated with the actual rate.PC-MRI in gravity MRI makes it possible to quantify venous blood flow, thereby facilitating the investigations into theeffect of gravity on venous blood flow in humans.Keywords : magnetic resonance imaging, phase-contrast, gravity, venous flow, phantom

Fig. 1 Schematic representation of a venous flow phantom andan imaging plane for phase-contrast magnetic resonanceimaging.

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flow directions to determine the effect of gravity on thevalidation of flow quantitation with PC-MRI in the phantomstudy.

2.2 Image Acquisition and Analytical ProcedureECG-triggered PC cine MRI was performed in a 0.4-T

gravity MRI system（Hitachi Healthcare, Tokyo, Japan）equipped with a quadrature body coil. We set the imagingplane to be perpendicular to the flow direction. Triggering wasperformed prospectively with an ECG unit. In total, 17 pairs ofmagnitude and velocity-mapped phase images per simulatedcardiac cycle were reconstructed for each flow velocity. Todemonstrate the feasibility of a single breath-hold scan for themeasurement of flow in the upper abdomen, the scan time wasreduced to a breath-hold period of 30 s using the followingparameters : repetition time, 30 ms ; echo time, 12.6 ms ; flipangle, 20° ; slice thickness, 6 mm ; imaging matrix, 128 ×38 ; field of view, 240 × 70 mm ; and number of signalsaveraged, 1. Through-plane velocity encoding was set at least1.25 times higher than the actual velocity to avoid aliasing onthe phase images [12, 13]. We determined the mean velocity inregions of interest（ROIs）on the phase image using Image Jsoftware（NIH, Bethesda, MD, USA）. The circular ROIs weredrawn manually in the simulated venous vessels with the inletand outlet flow directions defined on the magnitude image andcopied onto the corresponding phase image. Annular ROIsurrounding the vessel was placed in the background regionwhich was assumed to be stationary area. The meanbackground velocity was subtracted from the mean velocity inthe vessel to correct artefactual phase offset errors due to localeddy currents. Thereafter, the corrected mean velocity at eachcardiac phase was compared with the actual velocity set at theprogrammable pump. Moreover, we calculated the flow rateusing the following equation and compared the mean flow ratefor all simulated cardiac phases with the actual flow rate usingPearson’s correlation coefficient.

F = V × A （1）

where F is the flow rate（L/min）, V is the velocity（cm/s）,and A is the cross-sectional area（cm2）.

3. Results and Discussion

Fig. 2 depicts the PC-MRI delivered mean velocity foreach cardiac phase. The velocities for both inlet and outletflows were generally consistent with the actual velocitiesdefined by the pump, although it appeared that there weresome fluctuations for a cardiac cycle. Fig. 3 demonstrates therelationship between the PC-MRI delivered mean flow rate forall cardiac phases and actual flow rate defined by the pump.The mean flow rates highly correlated with the actual flowrates（R2 = 0.99 and P < 0.001 for both inlet and outlet flows）.These results support the fact that venous blood flow can beevaluated accurately in both flow directions even with thelimited imaging conditions for a breath-hold scan in gravityMRI. Moreover, as shown in Figs. 2 and 3, there were nodirection-dependent differences in the flow velocity and ratebetween inlet and outlet flows, and the mean errors of the flowvelocity and rate for both flow directions were below 10%.

These results indicate that the effect of gravity on the flowmeasurements in this phantom study was small. Thefluctuation of velocity during a cardiac cycle is likely to becaused by the small number of pixels within the ROI [7]

because the spatial resolution was limited to reduce the scantime for a single breath-hold scan. Further investigations intothe optimal number of pixels within ROI for accurate flowquantitation should be pursued in future. It should be note thatthis study was designed only to validate the reliability of PC-based flow measurement in gravity MRI and not to simulatethe effect of gravity on human blood flow. Thus, it is difficultto elucidate the effect of gravity on blood flow in humansusing this phantom. Nonetheless, we believe that PC-basedflow measurements with gravity MRI in future studies canpromote further understanding of the effect of gravity onhuman blood flow.

4. Conclusion

PC-MRI in gravity MRI makes it possible to quantifyvenous blood flow, thereby facilitating the investigations intothe effect of gravity on venous blood flow in humans.

References

[ 1 ] Alperin N, Lee SH, Sivaramakrishnan A, et al.:Quantifying the effect of posture on intracranialphysiology in humans by MRI flow studies. J MagnReson Imaging, 22（5）, 591-596, 2005.

[ 2 ] Brown HS, Halliwell M, Qamar M, et al.: Measurementof normal portal venous blood flow by Dopplerultrasound. Gut, 30（4）, 503-509, 1989.

Fig. 2 The mean velocity at each cardiac phase in simulatedvenous vessels with（a）inlet and（b）outlet flowdirections. Broken lines indicate the actual velocities setat a programmable pump.

Fig. 3 Relationship between the delivered flow rate of phase-contrast magnetic resonance imaging for either the（a）inlet or（b）outlet flow direction and the actual flow rateset at a programmable pump.

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[ 3 ] Gill RW : Measurement of blood flow by ultrasound :accuracy and sources of error. Ultrasound Med Biol, 11（4）, 625-641, 1985.

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[ 9 ] Siegel JM, Jr., Oshinski JN, Pettigrew RI, et al.:Comparison of phantom and computer-simulated MRimages of flow in a convergent geometry : implicationsfor improved two-dimensional MR angiography. J MagnReson Imaging, 5（6）, 677-683, 1995.

[10] Sugano S, Yamamoto K, Sasao K, et al.: Portal venousblood flow while breath-holding after inspiration orexpiration and during normal respiration in controls andcirrhotics. J Gastroenterol, 34（5）, 613-618, 1999.

[11] Cheng CP, Herfkens RJ, Taylor CA : Inferior vena cavalhemodynamics quantified in vivo at rest and duringcycling exercise using magnetic resonance imaging. Am JPhysiol Heart Circ Physiol, 284（4）, H1161-1167, 2003.

[12] Evans AJ, Iwai F, Grist TA, et al.: Magnetic resonanceimaging of blood flow with a phase subtraction technique.In vitro and in vivo validation. Invest Radiol, 28（2）, 109-115, 1993.

[13] Lotz J, Meier C, Leppert A, et al.: Cardiovascular flowmeasurement with phase-contrast MR imaging : basicfacts and implementation. Radiographics, 22（3）, 651-671,2002.

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