Analysis Of The Relationship Between The Degrees Of Cerebral Artery Stenosis And The Dynamic Changes On Arterial Spin Labeling Cerebral Perfusion Imaging

ObjectiveCerebral artery stenosis is a common etiology for ischemic cerebrovascular disease. The patients with cerebral artery stenosis have a high risk of stroke ipsilateral to the stenosis. Ischemic cerebrovascular diseases frequently manifest themselves first as disturbances in cerebral hemodynamics before cerebral infarction can be observed. We often found different cerebral ischemic strokes in the patients although the same degrees of stenoses in the same cerebral arteries were detected in clinical practice, which demonstrated that there were different cerebral hemodynamic changes in the territories supplied by the narrowed arteries. Arterial spin labeling (ASL) uses magnetically labeled water protons in arterial blood as the endogenous tracer. ASL is a nonionizing and completely noninvasive technique for measuring cerebral perfusion. Multiphase PASL imaging has high temporal resolution to acquire dynamic changes of cerebral perfusion. Mutiphase PASL cerebral perfusion imaging were performed on a 3.0 Tesla MR scanner to analyze the relationship between the degrees of cerebral artery stenosis and the dynamic changes on arterial spin labeling cerebral perfusion imaging and to assess the value of multiphase PASL in detecting dynamic changes of cerebral perfusion in the presence of a cerebral artery stenosis.Materials and Methods1. Seventeen patients with severe stenoses in unilateral M1 segment of MCA were involved in this study. All patients underwent multiphase PASL imaging. Bilateral basal ganglia were drawn by hand as regions of interest (ROIs) on eight-phase images of each patient. The signal intensities of ROIs were measured and the time-intensity curves (TICs) were acquired through postprocessing on a MR workstation. Whether there was a significant difference in the peak signal intensities of ROIs between the narrowed and normal sides was determined by the paired samples t test. A p value less than 0.05 was considered statistically significant.2.29 patients with less than 50% stenosis (mild stenosis group) and 22 patients with 50%-69% stenosis (moderate stenosis group) in Ml segment of unilateral middle cerebral artery (MCA) were included in this study. The degrees of MCA stenosis were measured using time of flight MR angiography (TOF MRA). Multiphase PASL imaging was performed to detect the signal changes in bilateral MCA. We selected and hand-drew bilateral symmetric branches of MCA as ROIs on eight-phase PASL images. The intensities of ROIs were measured, and the time-signal intensity curves were acquired through postprocessing on a MR workstation. SPSS 19.0 statistical software was used for statistics. The differences in the peak intensities and the times to peak intensities between the normal and narrowed sides of the mild and moderate stenosis groups were respectively examined by paired-samples t test. The differences in the changes of peak intensities and times to peak intensity of the two sides between the mild and moderate stenosis groups were examined by independent samples t test. A p value less than 0.05 was considered statistically significant.Results1. Three types of TICs were observed in severe stenosis group:eight cases with platform type, five cases with two-peak type, and four cases with single peak type. There was a significant difference in the peak signal intensities of ROIs between the narrowed and normal sides.2. There were significant difference in the peak intensities and no significant difference in the times to peak intensities between the normal and narrowed sides of the mild stenosis group. There were both significant differences in the peak intensities and times to peak intensities between the normal and narrowed side of the moderate stenosis group. There were both significant differences in the changes of peak intensities and times to peak intensity between the mild and moderate stenosis groups.Conclusions1. Different types of TICs represent different cerebral hemodynamic changes. Multiphase PASL can sensitively detect the dynamic characteristics of cerebral perfusion and provide important dynamic perfusion information for clinical treatment of arterial stenosis.2. The hemodynamic changes on multi-Td PASL images were different with the degrees of cerebral artery stenosis. Moderate stenosis means greater hemodynamic changes in the arteries than mild stenosis. Multi-Td PASL imaging is a promising means to evaluate the hemodynamic changes in cerebral arteries with stenosis.SignificanceMultiphase PASL sequence was used for cerebral perfusion imaging in this study. Signal-to-noise ratio (SNR) of PASL is inherently low, which can be influenced by many factors. The duration for the labeled tracer to travel from the labeling region to the imaging slices is known as arterial transit time (ATT). ASL is sensitive to ATT, which can either lead to overestimation of cerebral perfusion because of bright intravascular labels or underestimation of cerebral perfusion because of delayed arrival of labels. Longitudinal relaxation (T1) is another important factor. The labeled tracer decays with T1. T1 of labeled tracer is long on 3.0T MR, so labeled tracer can survive long enough to get into the imaging slices and high SNR can be obtained. It is advantageous to apply PASL at high-field MR system. The time between labeling and image acquisition, called delay time, is a key point for minimizing the influence of ATT and T1. A short delay time results in incomplete delivery of the labeled tracer to the imaging slices, whereas a long delay time results in increased T1 decay and low SNR. In this study, the starting delay time of 300 milliseconds and eight delay times were used to detect the dynamic perfusion signal changes in ROIs.High temporal resolution is needed to detect dynamic characteristics of cerebral perfusion. Methods for improving temporal resolution include single-shot ASL because the center of k space is acquired during each acquisition. Multiphase PASL imaging has the ability to improve temporal resolution using single-shot technique, which can accurately acquire cerebral perfusion signal changes. For the patients with cerebral arterial stenoses, PASL methods with a single delay time will obviously underestimate cerebral perfusion because of the delayed arrival of labels in the narrowed side. Multiphase PASL has the ability to detect the delayed perfusion signal changes regardless of the degree of stenosis.In the study of the patirnts with severe stenosis in M1 segments of MCA, three types of TICs were shown. The most common type was platform type which meant the prolonged travel time of labeled tracer in ROIs. The two-peak type meant partially delayed perfusion. The single-peak type with a reduced peak intensity was often associated with hypoperfusion in ROIs. Perfusion deficit in basal ganglia could be seen on multiphase PASL images. This area would derive little benefit from clinical treatment because no perfusion signal was seen. Delayed perfusion in basal ganglia could be seen on multiphase PASL images. The delayed perfusion area will probably be salvaged by thrombolytic therapy or endovascular recanalization, so the patients with delayed perfusion may receive a benefit from the therapy of arterial stenosis. These cases demonstrated that the patients with the same degree of stenosis could show large variability in perfusion and in the pattern of ischemic stroke. We also found that there was a significant difference in the peak signal intensities between the narrowed and normal sides. Hypoperfusion was detected in narrowed side although there was no obvious infarct. The quantitative measurement of the peak signal intensities on multiphase PASL images had high sensitivity in detecting cerebral abnormal perfusion. When no obvious infarct in the basal ganglia in the narrowed side was detected by T2WI and DWI, the dynamic signal changes on multiphase PASL images could provide complementary information to MRA, which was useful for identifying patients at high stroke risk or conversely those patients with adequate perfusion despite having a stenosis.The ischemic symptoms in patients with cerebral artery stenosis can be caused by atheromatous thrombosis, embolic disease, vasculitis or vasospasm. The hemodynamic changes in the arteries are correlated with the degrees of arterial stenosis. The results of our study showed that there were significant difference in the peak intensities and no significant difference in the times to peak intensities between normal and narrowed sides in mild stenosis group. Blood supply from MCA with mild stenosis weren’t delayed, but only decreased. There were both significant difference in the peak intensities and the times to peak intensities between normal and narrowed sides in moderate stenosis group. Blood supply from MCA with moderate stenosis were delayed and also decreased. The hemodynamic signal changes on multiphase PASL images represent the changes of blood supply from narrowed arteries. We also found that there were both significant differences in the changes of peak intensities and times to peak intensities between mild and moderate stenosis groups. Moderate stenosis means greater hemodynamic changes than mild stenosis.The hemodynamic changes were different with the degrees of cerebral artery stenosis. Moderate stenosis in M1 segment of MCA means greater dynamic changes than mild stenosis. Multiphase PASL imaging is a promising mean to demonstrate the hemodynamic changes of cerebral arteries with stenosis. The blood supply of brain from narrowed cerebral artery can be well displayed, which can provide useful information for clinical treatment. In conclusion, different types of TIC represent different cerebral hemodynamic properties. Multiphase PASL can sensitively identify important dynamic perfusion information of the patients with severe arterial stenoses.