A Study Of Rat Mesenteric Vasculature With Largefield-of-view Photoacoustic Microscopy

Mesenteric ischemia,especially acute mesenteric ischemia（AMI）,is a fatal disease with a mortality rate of up to 70%.Mesenteric venous thrombosis（MVT）,first reported by Elliot in 1895,is considered to be a major cause of mesenteric ischemia.MVT accounts for 5%to 15%of mesenteric ischemia according to the previous studies,resulting in impaired venous outflow,visceral edema,and abdominal pain.The clinical diagnosis of MVT and AMI mainly relies on computed tomography（CT）,magnetic resonance imaging（MRI）,and ultrasound（US）.However,in exploring the mechanism of MVT in animal experiments,all the clinical available imaging modalities have problems of low spatial resolution to resolve microvasculature,and insufficient sensitivity to hemoglobin.Due to the advantages of high spatial resolution,rich contrast source,and high sensitivity compared with conventional CT,MRI,and US,various optical imaging modalities have been widely used to study the mechanism of MVT with the development of laser techniques.For example,laser speckle imaging（LSI）,laser doppler velocimetry（LDV）,laser oblique scanning optical microscopy（LOSOM）et al.However,the pure optical imaging modalities have inherence limitations such as incapability of three-dimensional imaging,low sensitivity to small blood vessels,insufficient penetration depth,and limited FOV.Over the last decade,photoacoustic imaging,referred to as PAI,especially optical resolution photoacoustic microscopy（ORPAM）,is one of the most rapidly growing biomedical imaging modalities.In PAI,a sequence of laser pulses was used to illuminate the objective and induce photoacoustic signals:by selecting the wavelength of the laser beam,the light energy will be converted to heat by the absorbers due to the nonradiative relaxation,and resulting in a local temperature rise.The object vibrates mechanically due to thermal effect and emits ultrasonic waves,called photoacoustic signal.We can detect the photoacoustic signal using ultrasonic transducer,and the distance between the absorber and ultrasonic transducer can be calculated according to the sound velocity in the medium and the time of receiving the signal.To ensure the uniformity of excitation light,the distribution of light absorption coefficient inside the object can be derived by scanning and image reconstruction.PAI combines the advantages of conventional optical imaging and ultrasonic imaging.Compared with traditional optical imaging methods such as confocal imaging,confocal fluorescence imaging,two-photon imaging.PAI can realize three-dimensional imaging without depth scanning,and has a deeper penetration depth than most of traditional optical imaging methods.Compared with ultrasonic imaging,it can achieve higher spatial resolution and by selecting specific wavelength of excitation laser.In particular,ORPAM can improve the lateral resolution significantly by focusing photoacoustic excitation beam.In this study,we designed and realized a large-field-of-view ORPAM（L-ORPAM）combining optical scanning and rotation of ultrasonic transducer,which will remove the translation between the imaging interface and samples.An ultrafast pulsed laser with a high repetition rate of 600 k Hz was employed to excite the photoacoustic signals in L-ORPAM.An ultra-wide field of view of～40 mm in lateral and～10 mm in axial with a high lateral resolution～10μm and a moderate axial resolution of～150μm will be achieve by using the F-theta scan lens and a wide range ultrasonic transducer.Furthermore,we will establish two different types of MVT models by ligating the superior mesenteric vein and the inferior mesenteric vein of rats,respectively.The ligation method for simulating mesenteric venous thrombosis has been demonstrated by previous studies.The L-ORPAM system will be employed to the vascular responses of these two MVT models.We can derive some structural and functional parameters including the total vascular length（TVL）,relative concentration of total hemoglobin(C_{Hb T}),and vascular density（VD）over the ultra-large field of view to quantify and compare the vascular changes between these two different MVT models.