Wanyong Shin

Northwestern University, Evanston, Illinois, United States

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Publications (16)51.3 Total impact

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    ABSTRACT: The singular value decomposition deconvolution of cerebral tissue concentration–time curves with the arterial input function is commonly used in dynamic susceptibility contrast cerebral perfusion MR imaging. However, it is sensitive to the time discrepancy between the arrival of the bolus in the tissue concentration–time curve and the arterial input function signal. This normally causes inaccuracy in the quantitative perfusion maps due to delay and dispersion effects. A comprehensive correction algorithm has been achieved through slice-dependent time-shifting of the arterial input function, and a delay-dependent dispersion correction model. The correction algorithm was tested in 11 healthy subjects and three ischemic stroke patients scanned with a quantitative perfusion pulse sequence at 1.5 T. A validation study was performed on five patients with confirmed cerebrovascular occlusive disease scanned with MRI and positron emission tomography at 3.0 T. A significant effect (P < 0.05) was reported on the quantitative cerebral blood flow and mean transit time measurements (up to 50%). There was no statistically significant effect on the quantitative cerebral blood volume values. The in vivo results were in agreement with the simulation results, as well as previous literature. This minimizes the bias in patient diagnosis due to the existing errors and artifacts in dynamic susceptibility contrast imaging. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 12/2011; · 3.27 Impact Factor
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    ABSTRACT: The Bookend technique is a magnetic resonance imaging (MRI) dynamic susceptibility contrast method that provides reliable quantitative measurement of cerebral blood flow (CBF) and cerebral blood volume (CBV). The quantification is patient specific, is derived from a steady-state measurement of CBV, and is obtained from T(1) changes in the white matter and the blood pool after contrast agent injection. In the current implementation, the Bookend technique consists of three scanning steps requiring a cumulative scan time of 3 minutes 47 seconds, a well-trained technologist, and extra time for offline image reconstruction. We present an automation and acceleration of the multiscan Bookend protocol through a self-calibrating pulse sequence, namely Self-Calibrated Epi Perfusion-Weighted Imaging (SCALE-PWI). The SCALE-PWI is a single-shot echo-planar imaging pulse sequence with three modules and a total scan time of under 2 minutes. It provides the possibility of performing online, quantitative perfusion image reconstruction, which reduces the latency to obtain quantitative maps. A validation study in healthy volunteers (N=19) showed excellent agreement between SCALE-PWI and the conventional Bookend protocol (P>0.05 with Student's t-test, r=0.95/slope=0.98 for quantitative CBF, and r=0.91/slope=0.94 for quantitative CBV). A single MRI pulse sequence for absolute quantification of cerebral perfusion has been developed.
    Journal of cerebral blood flow and metabolism: official journal of the International Society of Cerebral Blood Flow and Metabolism 12/2010; 31(5):1272-82. · 5.46 Impact Factor
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    ABSTRACT: To evaluate quantitative cerebral blood flow (qCBF) with traditional time-based measurements or metrics of cerebral perfusion: time to peak (Tmax) and mean transit time (MTT) in stroke patients. Nine ischemic stroke patients (four male, five female, 63 ± 16 years old) were included in the study which was Health Insurance Portability and Accountability Act compliant and institutional review board approved. Cerebral perfusion was quantified using the Bookend method. Mean values of qCBF, Tmax, and MTT were determined in regions of interest (ROIs). ROIs were drawn on diffusion weighted images in diffusion positive, critically ischemic (CI), in ipsilateral normal region immediately surrounding the critically ischemic region, the presumed penumbra (PP), and in contralateral diffusion negative control, presumed normal region (PN) of gray and white matter separately (GM and WM). In both GM and WM, qCBF measures distinguished the studied brain regions with the most markedly reduced values in regions corresponding to extent of likely ischemic injury. In planned comparisons, only qCBF measurements differed significantly between CI and PP tissues. ROC analysis supported the utility of qCBF for discriminating brain regions differing in the likely extent of ischemic injury (CI and PN regions - qCBF: area under the curve [AUC] = 0.96, Tmax: AUC = 0.96, MTT: AUC = 0.72). Importantly, qCBF afforded the best discrimination of CI and PP regions (qCBF: AUC = 0.82, Tmax: AUC = 0.65, MTT: AUC = 0.52). This initial evaluation indicates that quantitative MRI perfusion is feasible in ischemic stroke patients. qCBF derived with this strategy provide enhanced discrimination of CI and PP compared to time-based imaging metrics. This approach merits investigation in larger clinical studies.
    Journal of Magnetic Resonance Imaging 10/2010; 32(4):796-802. · 2.57 Impact Factor
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    ABSTRACT: To derive a magnetic resonance (MR)-based imaging metric that reflects local perfusion changes resulting from the administration of angiogenic-inhibiting chemotherapy in patients with recurrent glioblastoma multiforme (GBM). In this retrospective Institutional Review Board-approved HIPAA-compliant study, 16 patients (12 men, four women; mean age, 51.8 years + or - 15.1 [standard deviation]) with recurrent GBM received bevacizumab every 3 weeks (15 mg per kilogram of body weight) as part of a clinical trial. Baseline MR images were acquired, and follow-up images were acquired every 6 weeks thereafter until tumor progression or death. Imaging included perfusion and T1-weighted contrast material-enhanced MR imaging. Perfusion images were analyzed both with and without correction for contrast material leakage. The volumes of interest were selected as enhancing voxels on T1-weighted contrast-enhanced MR images. Relative cerebral blood volume (rCBV) maps were created from analysis of MR perfusion images. The volumes of interest were used to calculate the following parameters: size, mean rCBV, mean leakage coefficient K(2), and hyperperfusion volume (HPV), which is the fraction of the tumor with an rCBV higher than a predetermined threshold. Percent change in each parameter from baseline to first follow-up was compared with time to progression (TTP) by using a Cox proportional hazards model with calculation of hazard ratios. The most significant hazard ratio was seen with a DeltaHPV cutoff of rCBV greater than 1.00 (hazard ratio, 1.077; 95% confidence interval: 1.026, 1.130; P = .002). The only significant ratios greater than one were those that resulted from perfusion calculated as mean rCBV and DeltaHPV. The ratios were also higher after correction for leakage. This pilot study derived an imaging metric (HPV) that reflects local perfusion changes in GBMs. This metric was found to show a significantly improved correlation to TTP as compared with more commonly used metrics.
    Radiology 05/2010; 255(2):622-8. · 6.34 Impact Factor
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    ABSTRACT: To evaluate an algorithm based on algebraic estimation of T1 values (three-point estimation) in comparison with computational curve-fitting for the postprocessing of quantitative cerebral perfusion scans. Computer simulations were performed to quantify the magnitude of the expected error on T1 and consequently cerebral perfusion using the three-point estimation technique on a Look-Locker (LL) EPI scan. In 50 patients, quantitative cerebral perfusion was calculated using the bookend method with three-point estimation and curve-fitting. The bookend method, a novel approach for calculating quantitative cerebral perfusion based on changes in T1 values after a contrast injection, is currently being validated. The number of computations was used as a measure of computation speed for each method. Student's paired t-test, Bland-Altman, and correlation analyses were performed to evaluate the accuracy of estimation. There was a 99.65% reduction in the number of computations with three-point estimation. Student's t-test showed no significant difference in cerebral perfusion (P=0.80, 0.49, paired t-test N=50, quantitative cerebral blood flow-white matter [qCBF-WM], qCBF-gray matter [qCBF-GM]) when compared to curve-fitting. The results of the two techniques were strongly correlated in patients (slope=0.99, intercept=1.58 mL/(100 g/minute), r=0.86) with a small systemic bias of -0.97 mL/(100 g/minute) in Bland-Altman analysis. The three-point estimation technique is adequate for rapid calculation of qCBF. The estimation scheme drastically reduces processing time, thus making the method feasible for clinical use.
    Journal of Magnetic Resonance Imaging 12/2008; 28(5):1258-65. · 2.57 Impact Factor
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    ABSTRACT: We present a method of quantifying cerebral blood volume using dynamic susceptibility contrast. Our approach combines T(2)-weighted echo planar imaging (EPI) pulse sequences and reference scans that determine the parenchymal T(1) changes resulting from an injection of a gadolinium chelate. This combined T(2)- and T(1)-weighted approach (the "bookend" technique) has been shown to be effective in the quantification of gradient-echo (GRE) (T(2)*-weighted) perfusion images but has not been applied to spin-echo EPI (SE-EPI) (T(2)-weighted) images. The physics related to blood volume measurement based on T(2)- and T(2)*-weighted EPI sequences is known to be different, and there is a question as to whether the bookend approach is effective with SE-EPI. We have compared the quantitative SE-EPI with GRE-EPI in a series of patients with central nervous system (CNS) tumors. We found that quantitative cerebral blood volume (qCBV) values for SE-EPI and GRE-EPI are in agreement with each other and with historical reference values. A subjective evaluation of image quality showed that image quality in the SE-EPI scans was high and exhibited high interreader agreement. We conclude that measuring qCBV using the bookend technique with SE-EPI images is possible and may be a viable alternative to GRE-EPI in the evaluation of CNS tumors.
    Magnetic Resonance Imaging 07/2008; 26(10):1352-9. · 2.06 Impact Factor
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    ABSTRACT: A novel approach for quantifying cerebral blood flow (CBF) is proposed that combines the bookend technique of calculating cerebral perfusion with an automatic postprocessing algorithm. The reproducibility of the quantitative CBF (qCBF) measurement in healthy controls (N = 8) showed a higher intraclass correlation coefficient (ICC) and lower coefficient of variation (COV) when calculated with automatic analysis (ICC/COV = 0.90/0.09) than when compared to conventional manual analysis (ICC/COV = 0.58/0.19). Also, the reproducibility in patients (N = 25) was successfully evaluated with the automatic analysis (ICC/COV = 0.81/0.14). In 175 consecutive clinical scans, we found 3.0% and 7.4% of qCBF decrease per decade in white matter (WM) (21.5 +/- 6.66 ml/100 g-min) and gray matter (GM) (49.6 +/- 16.2 ml/100 g-min), respectively. Cerebral blood volume (CBV) showed a significant 3.7% decrease per decade in GM (3.00 +/- 0.94 ml/100 g) but not in WM (1.69 +/- 0.40 ml/100 g). Mean transit time (MTT) increased by 1.9% and 3.8% per decade in WM (5.04 +/- 0.88 s) and GM (4.14 +/- 0.80 s), respectively. qCBF and MTT values between males (N = 85) and females (N = 90) were significantly different in GM. Women showed 11% higher qCBF as well as a higher decrease in qCBF with increasing age than men in the whole brain (WB). Our results supported the notion that population average empirical quantification of cerebral perfusion is subject to individual variation as well as age- and gender-dependent variability.
    Magnetic Resonance in Medicine 01/2008; 58(6):1232-41. · 3.27 Impact Factor
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    ABSTRACT: A method is presented for high spatial and temporal resolution 3D contrast-enhanced magnetic resonance angiography. The overall technique involves a set of interrelated components suited to high-frame-rate angiography, including 3D cylindrical k-space sampling, angular undersampling, asymmetric sampling, sliding window reconstruction, pseudorandom view ordering, and a sliding subtraction mask. Computer simulations and volunteer studies demonstrated the utility of each component of the technique. Angiograms of one hemisphere of the intracranial vasculature were acquired with a pixel size of 1.1 x 1.1 x 2.8 mm and a frame rate of 0.35 sec based on a temporal resolution of 3.5 sec. Such a 3D time-resolved, or "4D," technique has the potential to noninvasively acquire diagnostic quality images of certain anatomic regions with a frame rate fast enough to not only ensure the capture of an uncontaminated arterial phase, but even demonstrate contrast bolus flow dynamics. Clinical applications include noninvasive imaging of arteriovenous shunting, which is demonstrated with a patient study.
    Magnetic Resonance in Medicine 12/2007; 58(5):962-72. · 3.27 Impact Factor
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    ABSTRACT: To prospectively test the hypothesis that magnetic resonance (MR) imaging can detect changes in renal function at the time of renal artery stent placement in a swine model of renal artery stenosis (RAS). In this animal care and use committee-approved study, hemodynamically significant (>50%) RAS was surgically induced in six pigs. MR imaging was employed for assessment of the anatomic and physiologic changes induced by fluoroscopically guided stent placement. With MR imaging, we assessed changes in renal blood flow (RBF), extraction fraction (EF), and single-kidney glomerular filtration rate (skGFR) during the procedure. Arterial diameter stenosis before and after stent placement was assessed with x-ray digital subtraction angiography (DSA). Mean changes in functional and anatomic parameters were compared with the Wilcoxon matched-pairs test, with an alpha level of 0.05. There was no significant change in mean RBF after stent deployment (P=.44). Mean EF increased from 0.19+/-0.08 before stent placement to 0.31+/-0.17 after stent placement (P=.16). Mean skGFR measurements were 25 mL/min+/-16 before stent placement and 41 mL/min+/-28 after stent placement (P<.05). According to x-ray DSA measurements, mean stenosis measurements were 60%+/-12% before stent placement and 24%+/-16% after stent placement (P<.02). In swine, MR imaging can detect immediate changes in renal function after radiographically guided stent placement for unilateral RAS. This functional MR technique may have applications in the setting of hybrid MR/x-ray DSA procedure suites.
    Journal of Vascular and Interventional Radiology 11/2007; 18(11):1409-16. · 2.00 Impact Factor
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    ABSTRACT: To prospectively test--in a swine model of renal artery stenosis (RAS)--the hypothesis that magnetic resonance (MR) imaging can reveal changes in renal function at the time of percutaneous transluminal angioplasty (PTA). In this animal care and use committee-approved study, high-grade unilateral RAS was surgically induced in six pigs. MR imaging at 3.0 T was used for intraprocedural assessment of the anatomic and physiologic changes induced by x-ray-guided PTA. With use of MR imaging, changes in single-kidney glomerular filtration rate, extraction fraction, and renal blood flow were assessed during PTA. The arterial diameter of stenosis before and after PTA was assessed by using conventional digital subtraction angiography. Mean changes in functional and anatomic parameters were compared by using the Wilcoxon signed rank test (alpha = .05). At digital subtraction angiography, the mean percentage of stenosis was 69% +/- 10 (standard deviation) before PTA and 26% +/- 10 after PTA (P<.03). Mean pre- and post-PTA extraction fraction values were 0.11 +/- 0.03 and 0.19 +/- 0.06, respectively (P<.03). The mean single-kidney glomerular filtration rate before PTA, 19 mL/min +/- 13, increased to 41 mL/min +/- 33 after PTA (P<.03). There was no significant change in mean renal blood flow after PTA (P=.44). In swine, MR imaging can reveal changes in renal function after x-ray-guided PTA for unilateral RAS.
    Radiology 07/2007; 244(1):144-50. · 6.34 Impact Factor
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    ABSTRACT: The purpose of this work was to develop an MR imaging-compatible animal model of reversible embolic stroke. We hypothesize that real-time MR imaging of the brain can be performed during stroke thrombolysis and can provide real-time feedback and guidance on the success of thrombolysis. Embolic strokes were induced in 5 adult dogs by the use of autologous blood clots, with a sixth dog serving as an experimental control. Serial MR anatomic and physiologic imaging was performed to track the evolution of the stroke. The apparent diffusion coefficient (ADC) and quantitative cerebral blood flow (qCBF) were compared in the normal and stroke regions. During and after the administration of a chemical thrombolytic agent, MR imaging was performed to assess the outcome of the treatment. Strokes were successfully created in 5 animals. No ADC or qCBF changes were observed in the control animal. Both ADC and qCBF values were found to be significantly different in the region affected by the stroke. Restoration of flow was observed in 1 case. We have successfully implemented an MR imaging-compatible canine model of reversible embolic stroke.
    American Journal of Neuroradiology 10/2006; 27(8):1788-93. · 3.17 Impact Factor
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    ABSTRACT: The steady-state (SS) approach has been proposed to measure quantitative cerebral blood volume (CBV). However, it is known that the CBV value in SS (CBVSS) is subject to error resulting from the effects of water diffusion from the intra- to extravascular space. CBVSS measurements were simulated in both fast- and no-water-exchange limits, and compared with measured CBVSS values to determine which limiting case is appropriate. Twenty-eight patients were scanned with a segmented Look-Locker echo-planar imaging (LL-EPI) sequence before and after the injection of 0.1 mmol/kg of a T1-shortening contrast agent. Signal changes and T1 values of brain parenchyma and the blood pool were measured pre- and postcontrast. These signal changes and T1 values, in combination with the simulated results, were used to estimate water-exchange rates. We found that the intra- to extravascular water-exchange rates in white matter (WM) and gray matter (GM) were 0.9 and 1.6 s-1, respectively. With these water-exchange rates, the fast-water-exchange limit of the CBV values showed good agreement with the simulation (r=0.86 in WM, and 0.78 in GM). The CBV values with the correction for water-exchange effects were recalculated as 2.73+/-0.44 and 5.81+/-1.12 of quantitative cerebral blood water volume (%) in WM and GM, respectively.
    Magnetic Resonance in Medicine 08/2006; 56(1):138-45. · 3.27 Impact Factor
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    ABSTRACT: To compare the accuracy of catheter-directed intraarterial (IA) magnetic resonance (MR) angiography at 3.0 T with that of x-ray digital subtraction angiography (DSA) for the measurement of renal artery stenosis (RAS) in swine. Unilateral hemodynamically significant RAS (>50%) was induced surgically in six pigs with use of reverse cable ties. One to two weeks after surgery, each pig underwent x-ray DSA and MR angiography before and after percutaneous transluminal balloon angioplasty (PTA). X-ray DSA was performed before and after PTA of RAS by injection of iodinated contrast agent through a 5-F multiple-side hole angiographic catheter placed in the abdominal aorta under fluoroscopic guidance. MR angiography of RAS was performed before and after PTA of RAS on a 3.0-T clinical MR imager with use of gadolinium-based contrast agent. MR angiography and DSA images were analyzed with the full width at half maximum method. Percent stenosis measurements between x-ray DSA and MR angiography were compared with a paired t test and were correlated with linear regression and Bland Altman analysis (alpha = 0.05). Six cases of RAS were induced and imaged successfully with DSA and MR angiography techniques before and after PTA. On x-ray DSA, median stenoses was 64% (95% CI 57%-80%) before PTA and 20% (95% CI 5%-32%) after PTA. Corresponding MR angiography median stenosis measurement was 69% (95% CI 58%-80%) before PTA and 26% (95% CI 16%-36%) after PTA. A paired t test comparison did not show a difference between DSA and MR angiography (P = .16). RAS measurements on MR angiography correlated closely (P < .01) with DSA measurements (r(2) = 0.92). In swine, the accuracy of catheter-directed IA MR angiography with use of a clinical 3.0-T MR imaging unit for the measurement of RAS was similar to that of conventional x-ray DSA.
    Journal of Vascular and Interventional Radiology 07/2006; 17(7):1131-7. · 2.00 Impact Factor
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    ABSTRACT: A method is presented for high-temporal-resolution MR angiography (MRA) using a combination of undersampling strategies and a high-field (3T) scanner. Currently, the evaluation of cerebrovascular disorders involving arteriovenous shunting or retrograde flow is accomplished with conventional radiographic digital subtraction angiography, because of its high spatial and temporal resolutions. Multiphase MRA could potentially provide the same diagnostic information noninvasively, though this is technically challenging because of the inherent trade-off between signal intensity-to-noise ratio (S/N), spatial resolution, and temporal resolution in MR imaging. Numerical simulations addressed the choice of imaging parameters at 3T to maximize S/N and the data acquisition rate while staying within specific absorption rate limits. The increase in S/N at 3T was verified in vivo. An imaging protocol was developed with S/N, spatial resolution, and temporal resolution suitable for intracranial angiography. Partial Fourier imaging, parallel imaging, and the time-resolved echo-shared acquisition technique (TREAT) were all used to achieve sufficient undersampling. In 40 volunteers and 10 patients exhibiting arteriovenous malformations or fistulas, intracranial time-resolved contrast-enhanced MRA with high acceleration at high field produced diagnostic-quality images suitable for assessment of pathologies involving arteriovenous shunting or retrograde flow. The technique provided spatial resolution of 1.1 x 1.1 x 2.5 mm and temporal resolution of 2.5 seconds/frame. The combination of several acceleration methods, each with modest acceleration, can provide a high overall acceleration without the artifacts of any one technique becoming too pronounced. By taking advantage of the increased S/N provided by 3T magnets over conventional 1.5T magnets and converting this additional S/N into higher temporal resolution through acceleration strategies, intracranial time-resolved MRA becomes feasible.
    American Journal of Neuroradiology 05/2006; 27(4):822-9. · 3.17 Impact Factor
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    ABSTRACT: To improve the accuracy of dynamic susceptibility contrast (DSC) measurements of cerebral blood flow (CBF) and volume (CBV). In eight volunteers, steady-state CBV (CBV(SS)) was measured using TrueFISP readout of inversion recovery (IR) before and after injection of a bolus of contrast. A standard DSC (STD) perfusion measurement was performed by echo-planar imaging (EPI) during passage of the bolus and subsequently used to calculate the CBF (CBF(DSC)) and CBV (CBV(DSC)). The ratio of CBV(SS) to CBV(DSC) was used to calibrate measurements of CBV and CBF on a subject-by-subject basis. Agreement of values of CBV (1.77 +/- 0.27 mL/100 g in white matter (WM), 3.65 +/- 1.04 mL/100 g in gray matter (GM)), and CBF (23.6 +/- 2.4 mL/(100 g min) in WM, 57.3 +/- 18.2 mL/(100 g min) in GM) with published gold-standard values shows improvement after calibration. An F-test comparison of the coefficients of variation of the CBV and CBF showed a significant reduction, with calibration, of the variability of CBV in WM (P < 0.001) and GM (P < 0.03), and of CBF in WM (P < 0.0001). The addition of a CBV(SS) measurement to an STD measurement of cerebral perfusion improves the accuracy of CBV and CBF measurements. The method may prove useful for assessing patients suffering from acute stroke.
    Journal of Magnetic Resonance Imaging 05/2005; 21(5):512-9. · 2.57 Impact Factor
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