Approaches And Clinical Application Of Pulmonary Parenchyma Magnetic Resonance Perfusion Imaging

【Background】Assessment of pulmonary perfusion is very important in clinical practice.Changes in regional pulmonary perfusion can be observed in a variety of lung diseases,such as pulmonary embolism,chronic obstructive pulmonary disease.Pulmonary parenchyma magnetic resonance imaging has been hampered by several factors,such as intrinsic low spin density,heterogeneous magnetic susceptibility,respiration and cardiac motion, pulmonary blood flow and molecular diffusion.All these factors attribute to the low quality of pulmonary parenchyma imaging.But with the fast development of MRI techniques, pulmonary parenchyma MRI has come into truth.Compared to nuclear medicine scintigraphy,MRI has several advantages,such as nonionization exposure and high spatial resolution that influence the quality of images.Pulmonary parenchyma MRI includes pulmonary ventilation imaging and pulmonary perfusion imaging.Methodologically,Perfusion imaging includes two basic approaches: arterial spin labeling(ASL) and dynamic contrast-enhanced(DCE) imaging.Arterial spin labeling is a relatively new technique for MRI perfusion imaging.It does not require injection of contrast agents,and uses magnetically labeled blood water as an endogenous, freely diffusible tracer;hence it is completely noninvasive.ASL can be subdivided into steady state and pulsed.In this study,a pulsed ASL technique:flow sensitive alternating inversion recovery(FAIR) was used.In this study,we emphasized on the evaluation of feasibility of pulmonary parenchyma perfusion imaging with FAIR,the optimization of TI value in order to acquire high quality perfusion images and the assessment the influence of gravity and lung inflation on pulmonary perfusion.Based above research,we evaluated the potential clinical application value of FAIR prospectively by comparing FAIR with 3D DCE-MRI.Part OnePulmonary Parenchyma Magnetic Resonance Perfusion Imaging with FAIR【Objective】To evaluate the feasibility of pulmonary parenchyma perfusion imaging with FAIR,optimize TI value of FAIR in order to acquire high quality perfusion images and evaluate two influence factors on pulmonary perfusion,such as gravity and lung inflation.【Materials and methods】A total of 32 healthy volunteers were undergone SSFSE-FAIR imaging.12 volunteered participated in optimization TI study.Five different TI values were 800ms,1000ms,1200ms,1400ms and 1600ms,respectively.The changes of signal intensity between control and tag images in lung and aorta were evaluated,signal to noise ratio(SNR) of perfusion images was also evaluated with different TI values.Ten volunteers were included in the gravity-dependent perfusion study.Five coronal slices at an interval of 3cm from dorsal to ventral(labeled as P3,P6,P9,P12,P15,sequently) were obtained when the volunteers performed breath holding on end expiration and the relative pulmonary blood flow(rPBF) was measured.Coronal perfusion-weighted images were obtained at different respiratory phases from the rest of 10 healthy volunteers using FAIR sequence.The change of tagging efficiency of pulmonary parenchyma(⊿SI%), pulmonary blood flow and area of the scanning slice of different respiratory phases were analyzed.【Results】1.Optimization TI study:①signal intensity change of lung:no statistic difference between 800 ms and 1600 ms(P＞0.05),there were significant statistic differences of any other two TI values(P＜0.05).the change was maximum at 1000ms;②signal intensity change of aorta:no statistic difference between 1400 ms and 1600 ms (P＞0.05),there were significant statistic differences of any other two TI values (P＜0.05).the change is maximum at 1000ms;③SNR of perfusion images:no statistic differences among 1000ms,1200ms,1400ms and 1600ms(P＞0.05),there were significant statistic differences of any other two TI values(P＜0.05).2.Gravity-dependent study: Along the direction of gravity,a gradient was visually perceived as a vertical increase in rPBF.There were significant statistic differences in rPBF between any two coronal planes except that between P 12 and P 15.In supine position,regression coefficients of right and left lung were—4.98 and—5.16,respectively.This means that rPBF decreased 4.98 (right) and 5.16(left) for each centimeter above the dorsal.No statistical difference was seen between ROIs placed along iso-gravitational plane.3.Lung inflation study:①Significant differences was found in⊿SI%between different respiratory phases(right lung P=0.0215,left lung P=0.0084,with that at end expiration greater than that at end inspiration.②Significant differences was also found in pulmonary blood flow at different respiratory phases(right lung P=8.92×10^-5,left lung P=0.0002),with that at end expiration higher than that at end inspiration.③The area of the scanning slice was also significantly different at different respiratory phases(right lung P=2.94×10^-5,left lung P=0.0005),with that at end inspiration larger than that at end expiration.【Conclusion】Pulmonary perfusion with FAIR is feasible.1000ms TI was the best choice,which generated the highest lung/aorta tagging efficiency and second highest PBF SNR.Both gravity and respiratory phase are important determinants of pulmonary perfusion heterogeneity.FAIR is sensitive to demonstrate gravity- and respiratory phase- dependent differences in lung perfusion.Positioning the patient so that the area of interest is down-gravity and asking patient to hold breath on end expiration may help in detection of perfusion defects.Part TwoPulmonary Parenchyma Perfusion Imaging with 3D-DCE MRI【Objective】To assess feasibility of pulmonary parenchyma perfusion with 3D dynamic contrast-enhanced(DCE) imaging in healthy volunteers and in patients.【Materials and methods】Ten healthy volunteers and 47 patients(32 with lung cancer,5 with inflammation,and 10 with pulmonary emboli) were included in this study.A time-resolution three dimensional gradient-echo pulse sequence with parallel acquisition was used for DCE-MRI.Firstly,mask images with LAVA in a coronal orientation were scanned before injection of contrast agent.Then,six phases of 3D coronal volume were acquired consecutively,starting 4 seconds after the injection.All injections were performed with an automatic power injector.30ml Gd-DTPA was injected followed by 30ml saline flush into the antecubital vein at a rate of 3ml/s.Mask images were subtracted from each phase for a pure perfusion image.The homogeneity of perfusion of volunteers was assessed.In case of perfusion abnormality,the contrast between normal lung and perfusion defects was quantified by calculating the signal intensity difference ratio(R_SI between the perfusion abnormalities and the opposite side normal lung.【Results】The signal intensity of perfusion images of healthy volunteers was homogeneous.Pulmonary parenchyma perfusion as well as pulmonary vasculature was depicted during a 24-s breath-hold with DCE-MRI.24 wedged-shaped or triangle perfusion defect or reduced regions were visualized in 22 patients including 10 pulmonary embolisms and 12 lung cancers infiltrating the pulmonary artery.There was significant difference in R_SI(t= -24.74,P 0.05).In the other 25 cases,all lesions showed hypo-intensity when pulmonary parenchyma was enhanced to peak during the first-pass of contrast agent through the pulmonary circulation.【Conclusion】3D parallel acquisition could obtain multiphases of dynamic volume scanning during a single breath-hold and minimize the respiratory artifaction.It could acquire not only perfusion imaging,but also the pulmonary angiography.The combination of perfusion imaging and angiography could be easier to assess the pulmonary blood abnormal diseases,such as pulmonary embolism. Semiquantitative analysis could improve the detection rate of perfusion abnormal. Pulmonary perfusion with DCE-MRI is feasible.Part ThreeAssessment of Pulmonary Parenchyma Perfusion with FAIR in Comparison with DCE-MRI【Objective】The aim of this study was to assess pulmonary parenchyma perfusion with flow-sensitive alternating inversion recovery(FAIR) in comparison with 3D dynamic contrast-enhanced(DCE) imaging in healthy volunteers and in patients with pulmonary embolism or lung cancer.【Materials and methods】FAIR imaging with the optimized TI was performed followed by DCE-MRI on the 10 healthy volunteers and 30 patients with pulmonary embolism(10 cases) or lung cancer(20 cases).The homogeneity of FAIR and DCE-MRI perfusion was assessed.In the cases of perfusion abnormality,the contrast between normal lung and perfusion defects was quantified by calculating a normalized signal intensity ratio.【Results】In the healthy volunteers,the signal intensity of perfusion images acquired with both FAIR and DCE-MRI was homogeneous.Wedged-shaped or triangle perfusion defects were visualized in 10 pulmonary embolisms and 12 lung cancer cases.There was no significant statistical difference in signal intensity ratio between FAIR and DCE-MRI(P＞0.05).In the rest of 8 lung cancers,all the lesions showed low perfusion against the higher perfused pulmonary parenchyma in both FAIR and DCE-MRI.【Conclusion】Pulmonary parenchyma perfusion imaging with FAIR and DCE-MRI was feasible and consistent,which has high diagnosis accuracy for the detection of perfusion abnormalities caused by pulmonary embolism or lung cancer.FAIR as a noninvasive technique could obtain the similar functional information,comparing with DCE-MRI.