Venous Output Function Research Articles

CT perfusion to measure venous outflow in acute ischemic stroke in patients with a large vessel occlusion

BackgroundRobust venous outflow (VO) profiles, measured by degree of venous opacification on pre-thrombectomy CT angiography (CTA) studies, are strongly correlated with favorable outcomes in patients with large vessel occlusion acute...

Arterial input function estimation compensating for inflow and partial voluming in dynamic contrast-enhanced MRI.

Both inflow and the partial volume effect (PVE) are sources of error when measuring the arterial input function (AIF) in dynamic contrast-enhanced (DCE) MRI. This is relevant, as errors in the AIF can propagate into pharmacokinetic parameter estimations from the DCE data. A method was introduced for flow correction by estimating and compensating the number of the perceived pulse of spins during inflow. We hypothesized that the PVE has an impact on concentration-time curves similar to inflow. Therefore, we aimed to study the efficiency of this method to compensate for both effects simultaneously. We first simulated an AIF with different levels of inflow and PVE contamination. The peak, full width at half-maximum (FWHM), and area under curve (AUC) of the reconstructed AIFs were compared with the true (simulated) AIF. In clinical data, the PVE was included in AIFs artificially by averaging the signal in voxels surrounding a manually selected point in an artery. Subsequently, the artificial partial volume AIFs were corrected and compared with the AIF from the selected point. Additionally, corrected AIFs from the internal carotid artery (ICA), the middle cerebral artery (MCA), and the venous output function (VOF) estimated from the superior sagittal sinus (SSS) were compared. As such, we aimed to investigate the effectiveness of the correction method with different levels of inflow and PVE in clinical data. The simulation data demonstrated that the corrected AIFs had only marginal bias in peak value, FWHM, and AUC. Also, the algorithm yielded highly correlated reconstructed curves over increasingly larger neighbourhoods surrounding selected arterial points in clinical data. Furthermore, AIFs measured from the ICA and MCA produced similar peak height and FWHM, whereas a significantly larger peak and lower FWHM was found compared with the VOF. Our findings indicate that the proposed method has high potential to compensate for PVE and inflow simultaneously. The corrected AIFs could thereby provide a stable input source for DCE analysis.

Computed Tomography Perfusion Imaging in Acute Ischemic Stroke: Accurate Interpretation Matters.

Computed Tomography Perfusion Imaging in Acute Ischemic Stroke: Accurate Interpretation Matters.

Abstract WMP51: Baseline Ct Perfusion Parameters Differ In Large Vessel Occlusion Stroke Patients With Reduced Left Ventricular Ejection Fraction

Introduction: Reduced left ventricular ejection fraction (EF) is associated with worse outcomes after stroke. CT perfusion (CTP) is widely used in acute stroke evaluation; its reliability in patients with reduced EF is unknown. We hypothesized that patients with reduced EF would have abnormal dose-time curves that may impair standard perfusion maps and reduce their quality. Methods: A retrospective database of all patients presenting to Stanford University for large vessel occlusion (LVO) stroke between 2010-2019 was used. CTPs were post-processed using RAPID software. Concentration-time curve metrics—width of the venous output function (VOF) at half maximum amplitude and VOF peak amplitude—were assessed automatically for each case. A reader blinded to EF visually evaluated the VOF curve for evidence of truncation (yes/no) and graded the quality of the time to maximum (Tmax) map on a scale from 0 (excellent) to 3 (poor), based on the degree of “speckling” present on the map. Continuous parameters were compared using the t-test or Mann-Whitney test and dichotomous parameters with the Fisher’s exact or Chi-square test, where appropriate. Results: 160 patients with sufficient quality baseline CTP and in-hospital echocardiogram were evaluated; 34 had EF <50%. Mean age, scan time, and cumulative contrast dose did not differ; men were more prevalent in the reduced EF group (68% vs 44%; p=0.016). Compared to patients with normal EF, patients with low EF more often had truncation of the VOF (11.8% vs 0.8%; p=0.007), had a lower VOF peak amplitude (mean 245.04 HU vs 299.07; p=0.013, 95% CI for difference 11.61 to 96.45), had wider VOF at half maximum amplitude (mean 17.84 seconds vs 14.56; p<0.0001, 1.9 to 4.65), and had worse Tmax quality (median 2 (IQR 1-3) vs 1 (0-2); p=0.002). Conclusion: Patients with reduced EF have dispersion of the contrast bolus as evidenced by a VOF curve that is wider and has a lower peak. Consequently, the quality of the Tmax map in patients with low EF is reduced, which may render automated CTP results unreliable. Additional studies are needed to determine to what extent poor CTP image quality in patients with low EF affects treatment decisions.

Real-time assessment of individual optimal CT perfusion acquisition time in patients with ischemic stroke.

This study aims to investigate the feasibility of a "real-time" estimate of the optimal CT perfusion (CTP) acquisition time (Top ) in ischemic stroke patients. The arterial input function, the venous output function (VOF), and the time-attenuation curves of ischemic core and ischemic penumbra of 51 patients with acute ischemic stroke in anterior circulation were obtained. The curves were analyzed to determine for each patient the Top value; additionally, several time parameters were derived from each waveform. The relationship between each of these parameters and Top was investigated. We found a strong linear correlation between each time parameter derived from VOF curve and Top , suggesting that the VOF waveform is rescaled from patient to patient without significant change in shape. The linear correlation between Top and the VOF time to peak is well suited to implement a new technique to automatically customize the patient's CTP acquisition time. The method does not require an additional dose of contrast medium and does not increase the overall study time, so its use would be desirable to decrease the average radiation dose.

AIFNet: Automatic vascular function estimation for perfusion analysis using deep learning.

Perfusion imaging is crucial in acute ischemic stroke for quantifying the salvageable penumbra and irreversibly damaged core lesions. As such, it helps clinicians to decide on the optimal reperfusion treatment. In perfusion CT imaging, deconvolution methods are used to obtain clinically interpretable perfusion parameters that allow identifying brain tissue abnormalities. Deconvolution methods require the selection of two reference vascular functions as inputs to the model: the arterial input function (AIF) and the venous output function, with the AIF as the most critical model input. When manually performed, the vascular function selection is time demanding, suffers from poor reproducibility and is subject to the professionals' experience. This leads to potentially unreliable quantification of the penumbra and core lesions and, hence, might harm the treatment decision process. In this work we automatize the perfusion analysis with AIFNet, a fully automatic and end-to-end trainable deep learning approach for estimating the vascular functions. Unlike previous methods using clustering or segmentation techniques to select vascular voxels, AIFNet is directly optimized at the vascular function estimation, which allows to better recognise the time-curve profiles. Validation on the public ISLES18 stroke database shows that AIFNet almost reaches inter-rater performance for the vascular function estimation and, subsequently, for the parameter maps and core lesion quantification obtained through deconvolution. We conclude that AIFNet has potential for clinical transfer and could be incorporated in perfusion deconvolution software.

Robust arterial input function surrogate measurement from the superior sagittal sinus complex signal for fast dynamic contrast-enhanced MRI in the brain.

Accurately estimating the arterial input function for dynamic contrast-enhanced MRI is challenging. An arterial input function is typically determined from signal magnitude changes related to a contrast agent, often leading to underestimation of peak concentrations. Alternatively, signal phase recovers the accurate peak concentration for straight vessels but suffers from high noise. A recent method proposed to fit the signal in the complex plane by combining the advantages of the previous 2 methods. The purpose of this work is to refine this complex-based method to determine the venous output function (VOF), an arterial input function surrogate, from the superior sagittal sinus. We propose a state-of-the-art complex-based method that includes direct compensation for blood inflow and signal phase correction accounting for the curvature of the superior sagittal sinus, generally assumed collinear with B0 . We compared the magnitude-, phase-, and complex-based VOF determination methods against various simulated biases as well as for 29 brain metastases patients. Angulation of the superior sagittal sinus relative to B0 varied widely within patients, and its effect on the signal phase caused an underestimation of peak concentrations of up to 65%. Correction significantly increased the VOF peak concentration for the phase- and complex-based VOFs in the cohort. The phase-based method recovered accurate peak concentrations but lacked precision in the tail of the VOF. Our complex-based VOF completely recovered the effect of inflow and resulted in a high-peak concentration with limited noise. The new complex-based method resulted in high-quality VOF robust against superior sagittal sinus curvature and variations in patient positioning.

Utilization of MR angiography in perfusion imaging for identifying arterial input function.

This research utilizes magnetic resonance angiography (MRA) to identify arterial locations during the parametric evaluation of concentration time curves (CTCs), and to prevent shape distortions in arterial input function (AIF). We carried out cluster analysis with the CTC parameters of voxels located within and around the middle cerebral artery (MCA). Through MRA, we located voxels that meet the AIF criteria and those with distorted CTCs. To minimize partial volume effect, we re-scaled the time integral of CTCs by the time integral of venous output function (VOF). We calculated the steady-state value to area under curve ratio (SS:AUC) of VOF and used it as a reference in selecting AIF. CTCs close to this reference value (selected AIF) and those far from it were used (eliminated AIF) to compute cerebral blood flow (CBF). Eliminated AIFs were found to be either on or anterior to MCA, whereas selected AIFs were located superior, inferior, posterior, or anterior to MCA. If the SS:AUC of AIF was far from the reference value, CBF was either under- or over-estimated by a maximum of 41.1±14.3 and 36.6±19.2%, respectively. MRA enables excluding voxels on the MCA during cluster analysis, and avoiding the risk of shape distortions.

Magnetic Resonance Perfusion in Brain Tumors: Comparison of Different Evaluation Approaches in Dual-Echo and Multi-Echo Techniques

Dynamic measurements of T1 shortening (dynamic contrast enhanced—DCE) as well as of T2* shortening (dynamic susceptibility contrast—DSC) as two separate measurement strategies are widely used to quantitatively describe tumor perfusion and vascularity. Dual-echo approaches allow for the simultaneous assessment of both effects. The extension to multi-echo sequences should inhere the advantage of improved signal-to-noise ratios and more precise sampling of the T2* decay. The aim of our study is to investigate, if an extension of the dual-echo approach to the multi-echo approach allows for more stable quantitative determination of pharmacokinetic parameters in brain tumors. This study applies a multi-echo approach to obtain different estimations of a vascular input function and analyzes various combinations of vascular input functions and pharmacokinetic models. Perfusion measurements were performed with 52 consecutive patients with different brain tumors using a 10-echo gradient echo sequence. Our findings show that the extension to multi-echo sequences leads to an 11%-improvement of the Contrast-to-Noise ratio. Compared to other combinations, an application of Extended Tofts model using the T2*-related venous output function or an output function estimated in the tumor tissue enables the most reliable determination of perfusion parameters, reducing the reproducibility range by a factor of 1.2 to 10 for Ktrans and of 1.2 to 5.5 in the case of rBV calculation. Determination of Ktrans within repeated measurements within about 3 days results as most stable, if AIF from tumor pixels is used as vascular input function, meaning that the scatter is reduced by a factor of 1.2 compared to the next best VIF and by a factor of 10 compared to the worst of the tested approaches. In addition, this study shows that signal decomposition into two components with different Larmor frequencies might provide additional information concerning tissue composition of brain tumors.

Optimal Computed Tomographic Perfusion Scan Duration for Assessment of Acute Stroke Lesion Volumes.

The minimal scan duration needed to obtain reliable lesion volumes with computed tomographic perfusion (CTP) has not been well established in the literature. We retrospectively assessed the impact of gradual truncation of the scan duration on acute ischemic lesion volume measurements. For each scan, we identified its optimal scan time, defined as the shortest scan duration that yields measurements of the ischemic lesion volumes similar to those obtained with longer scanning, and the relative height of the fitted venous output function at its optimal scan time. We analyzed 70 computed tomographic perfusion scans of acute stroke patients. An optimal scan time could not be determined in 11 scans (16%). For the other 59 scans, the median optimal scan time was 32.7 seconds (90th percentile 52.6 seconds; 100th percentile 68.9 seconds), and the median relative height of the fitted venous output function at the optimal scan times was 0.39 (90th percentile 0.02; 100th percentile 0.00). On the basis of a linear model, the optimal scan time was T0 plus 1.6 times the width of the venous output function (P<0.001; R2=0.49). This study shows how the optimal duration of a computed tomographic perfusion scan relates to the arrival time and width of the contrast bolus. This knowledge can be used to optimize computed tomographic perfusion scan protocols and to determine whether a scan is of sufficient duration. Provided a baseline (T0) of 10 seconds, a total scan duration of 60 to 70 seconds, which includes the entire downslope of the venous output function in most patients, is recommended.

Abstract TP38: Impact of CT Perfusion Scan Duration on Lesion Volume

Introduction: There is evidence that CTP volumes are affected by scan duration, but the minimal scan duration needed to obtain reliable volumes has not been established. Hypothesis: The minimal CTP scan duration for individual patients depends on the arrival time and width of the injected bolus. Methods: We included CTP scans of patients who presented with an acute ischemic stroke. To identify the optimal CTP scan duration, we assessed how truncation of the scan (from 90s to 20s) impacts lesion volumes. We included only patients for whom truncation of the final 10.8 sec (from 90 to 79.2 sec) did not affect lesion volumes to ensure that the subjects had been scanned with sufficient duration. For each included patient, further truncation was used to identify a “stability point”, defined as the minimal scan duration needed to achieve stable estimates of the perfusion lesion. For each patient we also determined the arrival time (t0) and width (full width half max - FWHM) of the Venous Output function (VOF). We used a linear model to test if these VOF features could predict the stability point. Results: We analyzed CTP scans of 70 patients with acute ischemic stroke. Of these, 59 (84.3 %) had sufficient scan duration and were included. Both t0 and VOF width were associated with the stability point (R2 = 0.49). Based on the regression model, minimum scan duration is predicted by “t0 + 1.61* FWHM” (p &lt; 0.001) with an interquartile prediction interval from -5 to 7s. Conclusions: The minimum CTP scan duration needed for accurate perfusion lesions are patient specific and depend on width and arrival of the injected bolus as expressed by the easy to measure VOF. This knowledge has important clinical implications. After acquisition, it can be used to rate if a scan is of sufficient duration for reliable lesion volume estimation. Prior to acquisition, a low contrast and radiation dose test bolus scan can be used to inform the optimal scan time, thereby limiting the radiation dose to the minimal necessary amount for accurate measurements. For example in our cohort, 52 of 70 patients could have been scanned for just 60 seconds (33% dose reduction) with the same results as a 90 second scan.

TH-CD-207-02: Quantification of Dynamic Susceptibility Contrast MRI: Comparison of Calibration Methods Using Venous Output Function and Arterial Spin Labeling

Purpose: To evaluate the validity of the calibration of arterial input function (AIF) by using a venous output function (VOF) for the quantification of cerebral blood flow (CBF) in dynamic susceptibility contrast MRI (DSC-MRI), by comparing to the arterial spin labeling (ASL) measurement. Method: Fourteen patients with unilateral ICA stenosis enrolled in the study on a 3T clinical MRI scanner. The ASL-CBF map was calculated by averaging 45 dynamic points acquired by using a Q2TIPS sequence. For DSC perfusion analysis, AIF was selected to derive the relative CBF (rCBF) map and the delay (Tmax) map; VOF with signal saturation was corrected by a small vein which was amplified and time-shifted to match the base and flanks of the original VOF. The DSC-CBF map was corrected by two different patient-specific correction factors (CF): (1) CF1: the ratio of the mean ASL-and DSC-CBF obtained from the mask based on Tmax < 3s, the gray matter mask and the exclusion of large vessels; (2) CF2: the ratio of the area under the first-pass phase in AIF and VOF concentration time curves. Result: Average whole-brain (WB) DSC-CBF calculated by CF1 and CF2 showed the same precision (coefficient of variation [COV] = 22%), while CBFCF2 (100.26 ± 23.36 ml/100g/min [mean ± SD]) was higher than CBFCF1 (37.4 ± 8.1 ml/100g/min). Significant correlation (R= 0.632; P<0.05) was found between the results obtained from these two calibration approaches. Conclusion: The degree of linear correlation between the WB DSC-CBF estimates obtained by AIF PV correction and ASL-based correction was significant. Comparing with CBF values obtained using ASL, the DSC-CBF estimates were high even after applying AIF PV correction. The over-estimation may come from the partial volume effect in VOF and other sources of errors in the DSC-MRI quantification.

Effect of extended CT perfusion acquisition time on ischemic core and penumbra volume estimation in patients with acute ischemic stroke due to a large vessel occlusion.

Background and PurposeIt has been suggested that CT Perfusion acquisition times <60 seconds are too short to capture the complete in and out-wash of contrast in the tissue, resulting in incomplete time attenuation curves. Yet, these short acquisitions times are not uncommon in clinical practice. The purpose of this study was to investigate the occurrence of time attenuation curve truncation in 48 seconds CT Perfusion acquisition and to quantify its effect on ischemic core and penumbra estimation in patients with acute ischemic stroke due to a proximal intracranial arterial occlusion of the anterior circulation.Materials and MethodsWe analyzed CT Perfusion data with 48 seconds and extended acquisition times, assuring full time attenuation curves, of 36 patients. Time attenuation curves were classified as complete or truncated. Ischemic core and penumbra volumes resulting from both data sets were compared by median paired differences and interquartile ranges. Controlled experiments were performed using a digital CT Perfusion phantom to investigate the effect of time attenuation curve truncation on ischemic core and penumbra estimation.ResultsIn 48 seconds acquisition data, truncation was observed in 24 (67%) cases for the time attenuation curves in the ischemic core, in 2 cases for the arterial input function and in 5 cases for the venous output function. Analysis of extended data resulted in smaller ischemic cores and larger penumbras with a median difference of 13.2 (IQR: 4.3–26.0)ml (P<0.001) and; 12.4 (IQR: 4.1–25.7)ml (P<0.001), respectively. The phantom data showed increasing ischemic core overestimation with increasing tissue time attenuation curve truncation.ConclusionsTruncation is common in patients with large vessel occlusion and results in repartitioning of the area of hypoperfusion into larger ischemic core and smaller penumbra estimations. Phantom experiments confirmed that truncation results in overestimation of the ischemic core.

Dynamic susceptibility contrast MRI with a prebolus contrast agent administration design for improved absolute quantification of perfusion

Arterial partial-volume effects (PVEs) often hamper reproducible absolute quantification of cerebral blood flow (CBF) and cerebral blood volume (CBV) obtained by dynamic susceptibility contrast MRI (DSC-MRI). The aim of this study was to examine whether arterial PVEs in DSC-MRI data can be minimized by rescaling the arterial input function (AIF) using a sagittal-sinus venous output function obtained following a prebolus administration of a low dose of contrast agent. The study was carried out as a test-retest experiment in 20 healthy volunteers to examine the repeatability of the CBF and CBV estimates. All subjects were scanned twice with 7-20 days between investigations. DSC-MRI returned an overestimated average whole-brain CBF of 220 ± 44 mL/100 g/min (mean ± SD) before correction and 44 ± 15 mL/100 g/min when applying the prebolus design, averaged over all scans. Average whole-brain CBV was 20 ± 2.0 mL/100 g before correction and 4.0 ± 1.0 mL/100 g after prebolus correction. Quantitative estimates of CBF and CBV, obtained with the proposed prebolus DSC-MRI technique, approached those typically obtained by other perfusion modalities. The CBF and CBV estimates showed good repeatability.

Effects of blood ΔR2* non-linearity on absolute perfusion quantification using DSC-MRI: Comparison with Xe-133 SPECT

PurposeTo evaluate whether a non-linear blood ΔR2*-versus-concentration relationship improves quantitative cerebral blood flow (CBF) estimates obtained by dynamic susceptibility contrast (DSC) MRI in a comparison with Xe-133 SPECT CBF in healthy volunteers. Material and MethodsLinear as well as non-linear relationships between ΔR2* and contrast agent concentration in blood were applied to the arterial input function (AIF) and the venous output function (VOF) from DSC-MRI. To reduce partial volume effects in the AIF, the arterial time integral was rescaled using a corrected VOF scheme. ResultsUnder the assumption of proportionality between the two modalities, the relationship CBF(MRI)=0.58CBF(SPECT) (r=0.64) was observed using the linear relationship and CBF(MRI)=0.51CBF(SPECT) (r=0.71) using the non-linear relationship. DiscussionA smaller ratio of the VOF time integral to the AIF time integral and a somewhat better correlation between global DSC-MRI and Xe-133 SPECT CBF estimates were observed using the non-linear relationship. The results did not, however, confirm the superiority of one model over the other, potentially because realistic AIF signal data may well originate from a combination of blood and surrounding tissue.

CT Brain Perfusion Protocol to Eliminate the Need for Selecting a Venous Output Function

In CTP, an arterial input function is used for cerebral blood volume measurement. AIFs are often influenced by partial volume effects resulting in overestimated CBV. A venous output function is manually selected to correct for partial volume. This can introduce variability. Our goal was to develop a CTP protocol that enables AIF selection unaffected by partial volume. First, the effects of partial volume on artery sizes/types including the MCA were estimated by using a CTP phantom with 9 protocols (section thicknesses of 1, 1.8, and 5 mm and image resolutions of 0.5, 1, and 1.5 mm). Next, these protocols were applied to clinical CTP studies from 6 patients. The influence of the partial volume effect was measured by comparison of the time-attenuation curves from different artery locations with reference veins. AIFs from MCAs were unaffected by partial volume effects when using high image resolution (1 mm) and medium section thickness (1.8 mm). For the clinical data, a total of 104 arteries and 60 veins was selected. The data confirmed that high image resolution and thin section thickness enable selection of MCAs for AIFs free of partial volume influences. In addition, we found that large veins were not insusceptible to partial volume effects relative to large arteries, questioning the use of veins for partial volume correction. A CTP protocol with 1.8-mm section thickness and 1-mm image resolution allows AIF selection unaffected by partial volume effects in MCAs.

Optimisation of vascular input and output functions in CT-perfusion imaging using 256(or more)-slice multidetector CT

To evaluate the accuracy and reproducibility of CT-perfusion (CTP) by finding the optimal artery for the arterial input function (AIF) and re-evaluating the necessity of the venous output function (VOF). Forty-four acute ischaemic stroke patients who underwent non-enhanced CT, CTP and CT-angiography using 256-slice multidetector computed tomography (MDCT) were evaluated. The anterior cerebral artery (ACA), middle cerebral artery (MCA), internal carotid artery (ICA) and basilar artery were selected as the AIF. Subsequently the resulting area under the time-enhancement curve of the AIF (AUCAIF) and quantitative perfusion measurements were analysed by repeated measures ANOVA and subsequently the paired t test. To evaluate reproducibility we examined if the VOF could be deleted by comparing the perfusion measurements using versus not using the VOF (paired t test). The AUCAIF and perfusion measurements resulting from the different AIFs showed significant group differences (all P < 0.0001). The ICA had the largest AUCAIF and resulted in the highest mean transient time (MTT) and lowest cerebral blood flow (CBF), whereas the basilar artery showed the lowest cerebral blood volume (CBV). Not using the VOF showed significantly higher CBV and CBF in 66 % of patients on the ipsilateral (P < 0.0001 and P = 0.007, respectively) and contralateral hemisphere (P < 0.0001 and P = 0.019, respectively). Selecting the ICA as the AIF and continuing the use of the VOF would improve the accuracy of CTP. • Perfusion imaging is an increasingly important aspect of multidetector computed tomography (MDCT). • Vascular input functions were evaluated for CT-perfusion using 256-slice MDCT. • Selecting different arterial input functions (AIFs) leads to variation in quantitative values. • Using the internal carotid artery for AIF provides optimal perfusion values. • Deleting the venous output function would be detrimental for validity.

Selection of Arterial Input Function for Postprocessing of Cerebral CT Perfusion in Chronic Unilateral High-grade Stenosis or Occlusion of the Carotid or Middle Cerebral Artery

We evaluated the effect of the arterial input function (AIF) on computed tomography perfusion (CTP) in patients with unilateral high-grade stenosis or occlusion in the carotid artery or middle cerebral artery without acute stroke. CTP datasets were retrospectively postprocessed using the same venous output function and different AIF selections: the second segment of the anterior cerebral artery (A2 AIF), the second segment of the middle cerebral artery (MCA) on the lesion side (affected M2 AIF), and M2 on the contralateral side (nonaffected M2 AIF). We measured CTP values in the region of interest (ROI) in the bilateral MCA territory and evaluated the lesion-to-contralateral ratios. The mean and standard deviations of cerebral blood flow (CBF) on the normal side were similar to previously reported data only when using "non-affected M2 AIF." Selecting an "affected M2 AIF" overestimated the CBF and shortened the mean transit time (MTT) in normal and lesion areas. Selecting an "A2 AIF" may cause overestimation of CBF in the normal side in patients with nonaffected-side A1 hypoplasia or occlusion. The sensitivity of the CBF ratio or MTT ratio to detect these unilateral cerebrovascular diseases was 100% using "nonaffected M2 AIF for bilateral MCA ROIs" and 70% (CBF ratio) and 90% (MTT ratio) using "respective AIF." The use of "nonaffected AIF for the bilateral MCA ROIs" was found to be the best of these AIF-ROI combinations in patients with chronic unilateral carotid or M1 severe stenosis or occlusion.

Poster — Thur Eve — 29: Determination of Contrast Agent Concentration in Tortuous Blood Vessels Using Measured MRI Phase Changes and Fourier‐Based Field Inhomogeneity Equations

There has been considerable research effort into obtaining quantitative measures of perfusion using dynamic contrast‐enhanced MRI. Absolute quantification of the arterial input function (AIF) and/or venous output function (VOF) in major blood vessels improves perfusion estimates, however reliable techniques for doing so are lacking. Using changes in phase (Δφ) in blood vessels is thought to be the best way to quantify the AIF or VOF. However, it is not yet clear how best to deal with the susceptibility physics when the blood vessels have a complex geometry. We propose a methodology for obtaining absolute quantification of the AIF or VOF using a Fourier‐based calculation of field inhomogeneities, in order to convert Δφ to absolute contrast agent concentration, regardless of the blood vessel geometry. This methodology was tested in an aqueous phantom system. The experimentally measured Δφ was divided by the expected Δφ on a pixel by pixel basis. Ideally, this ratio should be one. Considering all pixels in the phantom tubing, the measured Δφ divided by expected Δφ had a mean value of 1.02 and standard deviation of 0.172. The Fourier‐based calculation can therefore successfully predict Δφ and thus can be used to account for the effect of the vessel geometry and orientation in the conversion of MR phase to contrast agent concentration. The methodology is thus promising for making absolute measurements of the AIF or VOF.

Determination of the venous output function from MR signal phase: Feasibility for quantitative DCE-MRI in human brain

For dynamic contrast-enhanced MRI studies in the human brain, it is useful to measure the venous output function (VOF). The purpose of this work was to explore the feasibility of measuring the VOF using the MR signal phase (in absolute units of gadolinium concentration) in the superior sagittal sinus. Phantom experiments were performed to validate the technique for different superior sagittal sinus angles (theta = 0-48 degrees relative to the main magnetic field), different curvatures (straight or radius = 45 mm), and different spatial resolutions (2.2-5.5 mm, to study partial-volume effects). Additionally, the technique was tested on three patients. The phantom experimental results (echo time = 5.5 ms, theta <or= 21 degrees) agreed with theoretical predictions to within 10%. For the patient studies, the measured VOFs had reasonable amplitude and shape characteristics and the patients' superior sagittal sinus angles (<15 degrees) and curvatures (radii approximately 40 mm) were within the range explored with phantoms. Our results suggest that partial-volume contributions to the VOF will be <5% and that the VOF can be evaluated in vivo to within 10% error. In conclusion, it is highly feasible to use MR signal phase to measure the VOF in the superior sagittal sinus for human dynamic contrast-enhanced MRI.