Enhancing cerebral vasculature analysis with pathlength-corrected 2D angiographic parametric imaging: A feasibility study

BACKGROUND

2D angiographic parametric imaging (API) quantitatively extracts imaging biomarkers related to contrast flow and is conventionally applied to 2D digitally subtracted angiograms (DSA's). In the interventional suite, API is typically performed using 1-2 projection views and is limited by vessel overlap, foreshortening, and depth-integration of contrast motion.

PURPOSE

This work explores the use of a pathlength-correction metric to overcome the limitations of 2D-API: the primary objective was to study the effect of converting 3D contrast flow to projected contrast flow using a simulated angiographic framework created with computational fluid dynamics (CFD) simulations, thereby removing acquisition variability.

METHODS

The pathlength-correction framework was applied to in-silico angiograms, generating a reference (i.e., ground-truth) volumetric contrast distribution in four patient-specific intracranial aneurysm geometries. Biplane projections of contrast flow were created from the reference volumetric contrast distributions, assuming a cone-beam geometry. A Parker-weighted reconstruction was performed to obtain a binary representation of the vessel structure in 3D. Standard ray tracing techniques were then used to track the intersection of a ray from the focal spot with each voxel of the reconstructed vessel wall to a pixel in the detector plane. The lengths of each ray through the 3D vessel lumen were then projected along each ray-path to create a pathlength-correction map, where the pixel intensity in the detector plane corresponds to the vessel width along each source-detector ray. By dividing the projection sequences with this correction map, 2D pathlength-corrected in-silico angiograms were obtained. We then performed voxel-wise (3D) API on the ground-truth contrast distribution and compared it to pixel-wise (2D) API, both with and without pathlength correction for each biplane view. The percentage difference (PD) between the resultant API biomarkers in each dataset were calculated within the aneurysm region of interest (ROI).

RESULTS

Intensity-based API parameters, such as the area under the curve (AUC) and peak height (PH), exhibited notable changes in magnitude and spatial distribution following pathlength correction: these now accurately represent conservation of mass of injected contrast media within each arterial geometry and accurately reflect regions of stagnation and recirculation in each aneurysm ROI. Improved agreement was observed between these biomarkers in the pathlength-corrected biplane maps: the maximum PD within the aneurysm ROI is 3.3% with pathlength correction and 47.7% without pathlength correction. As expected, improved agreement with ROI-averaged ground-truth 3D counterparts was observed for all aneurysm geometries, particularly large aneurysms: the maximum PD for both AUC and PH was 5.8%. Temporal parameters (mean transit time, MTT, time-to-peak, TTP, time-to-arrival, TTA) remained unaffected after pathlength correction.

CONCLUSIONS

This study indicates that the values of intensity-based API parameters obtained with conventional 2D-API, without pathlength correction, are highly dependent on the projection orientation, and uncorrected API should be avoided for hemodynamic analysis. The proposed metric can standardize 2D API-derived biomarkers independent of projection orientation, potentially improving the diagnostic value of all acquired 2D-DSA's. Integration of a pathlength correction map into the imaging process can allow for improved interpretation of biomarkers in 2D space, which may lead to improved diagnostic accuracy during procedures involving the cerebral vasculature.

Interleaved x-ray imaging: A method for simultaneous acquisition of quantitative and diagnostic digital subtraction angiography

BACKGROUND

Flow altering angiographic procedures suffer from ill-defined, qualitative endpoints. Quantitative digital subtraction angiography (qDSA) is an emerging technology that aims to address this issue by providing intra-procedural blood velocity measurements from time-resolved, 2D angiograms. To date, qDSA has used 30 frame/s DSA imaging, which is associated with high radiation dose rate compared to clinical diagnostic DSA (up to 4 frame/s).

PURPOSE

The purpose of this study is to demonstrate an interleaved x-ray imaging method which decreases the radiation dose rate associated with high frame rate qDSA while simultaneously providing low frame rate diagnostic DSA images, enabling the acquisition of both datasets in a single image sequence with a single injection of contrast agent.

METHODS

Interleaved x-ray imaging combines low radiation dose image frames acquired at a high rate with high radiation dose image frames acquired at a low rate. The feasibility of this approach was evaluated on an x-ray system equipped with research prototype software for x-ray tube control. qDSA blood velocity quantification was evaluated in a flow phantom study for two lower dose interleaving protocols (LD1:3.7 ± 0.02 mGy / s $3.7 \pm 0.02\ {\mathrm{mGy}}/{\mathrm{s}}$ and LD2:1.7 ± 0.04 mGy / s $1.7 \pm 0.04{\mathrm{\ mGy}}/{\mathrm{s}}$ ) and one conventional (full dose) protocol (11.4 ± 0.04 mGy / s ) $11.4 \pm 0.04{\mathrm{\ mGy}}/{\mathrm{s}})$ . Dose was measured at the interventional reference point. Fluid velocities ranging from 24 to 45 cm/s were investigated. Gold standard velocities were measured using an ultrasound flow probe. Linear regression and Bland-Altman analysis were used to compare ultrasound and qDSA.

RESULTS

The LD1 and LD2 interleaved protocols resulted in dose rate reductions of -67.7% and -85.5%, compared to the full dose qDSA scan. For the full dose protocol, the Bland-Altman limits of agreement (LOA) between qDSA and ultrasound velocities were [0.7, 6.7] cm/s with a mean difference of 3.7 cm/s. The LD1 interleaved protocol results were similar (LOA: [0.3, 6.9] cm/s, bias: 3.6 cm/s). The LD2 interleaved protocol resulted in slightly larger LOA: [-2.5, 5.5] cm/s with a decrease in the bias: 1.5 cm/s. Linear regression analysis showed a strong correlation between ultrasound and qDSA derived velocities using the LD1 protocol, with aR 2 ${R}^2$ of0.96 $0.96$ , a slope of1.05 $1.05$ and an offset of1.9 $1.9$  cm/s. Similar values were also found for the LD2 protocol, with aR 2 ${R}^2$ of0.93 $0.93$ , a slope of0.98 $0.98$ and an offset of2.0 $2.0$  cm/s.

CONCLUSIONS

The interleaved method enables simultaneous acquisition of low-dose high-rate images for intra-procedural blood velocity quantification (qDSA) and high-dose low-rate images for vessel morphology evaluation (diagnostic DSA).

Culprit Lesion Vessel Size and Risk of Reperfusion Injury in ST-Segment Elevation Myocardial Infarction: A Cardiac Magnetic Resonance Imaging Study

BACKGROUND

Microvascular obstruction (MVO) and intramyocardial hemorrhage (IMH) are well-established imaging biomarkers of failed myocardial tissue reperfusion in patients with ST-segment elevation-myocardial infarction treated with percutaneous coronary intervention. MVO and IMH are associated with an increased risk of adverse outcome independent of infarct size, but whether the size of the culprit lesion vessel plays a role in the occurrence and severity of reperfusion injury is currently unknown. This study aimed to evaluate the association between culprit lesion vessel size and the occurrence and severity of reperfusion injury as determined by cardiac magnetic resonance imaging.

METHODS AND RESULTS

Patients (n=516) with first-time ST-segment-elevation myocardial infarction underwent evaluation with cardiac magnetic resonance at 4 (3-5) days after infarction. MVO was assessed with late gadolinium enhancement imaging and IMH with T2* mapping. Vessel dimensions were determined using catheter-based reference. Median culprit lesion vessel size was 3.1 (2.7-3.6) mm. MVO and IMH were found in 299 (58%) and 182 (35%) patients. Culprit lesion vessel size was associated with body surface area, diabetes, total ischemic time, postinterventional thrombolysis in myocardial infarction flow, and infarct size. There was no association between vessel size and MVO or IMH in univariable and multivariable analysis (P>0.05). These findings were consistent across patient subgroups with left anterior descending artery and non-left anterior descending artery infarctions and those with thrombolysis in myocardial infarction 3 flow post-percutaneous coronary intervention.

CONCLUSIONS

Comprehensive characterization of myocardial tissue reperfusion injury by cardiac magnetic resonance revealed no association between culprit lesion vessel size and the occurrence of MVO and IMH in patients treated with primary percutaneous coronary intervention for ST-segment-elevation myocardial infarction.

Recovery of complete time density curves from incomplete angiographic data using recurrent neural networks

Quantitative angiography is a 2D/3D x-ray imaging modality that summarizes hemodynamic information using time density curve (TDC) based parameters. Estimation of the TDC parameters are susceptible to errors due to various factors including, patient motion, incomplete temporal data, imaging trigger errors etc. In this study, we tested the feasibility of using recurrent neural networks (RNN) to recover complete TDC temporal information from incomplete sequences and evaluate quantitative parameters generated from the corrected TDCs. Digital subtraction angiograms (DSAs) were collected from patients undergoing endovascular treatments and angiographic parametric imaging (API) parameters were calculated from each DSA. Each set of API parameters was used to simulate a TDC resulting in a dataset of 760 TDCs. One-third of each TDC was continuously masked from pseudo-random points past the peak height (PH) point to simulate missing/artifact information. An RNN was developed, trained and tested to generate completed/corrected TDCs. The RNN recovered complete TDC temporal information with an average mean squared error of 0.0086±0.002. Average mean absolute errors were calculated between each API parameter generated from the ground truth TDCs and RNN corrected TDCs, these were 11.02%±0.91 for time to peak, 10.97%±0.69 for mean transit time, 5.65%±0.76 for PH, and 15.08%±0.98 for area under the TDC. The change in API parameters was not clinically significant and the predictive power of the API parameters was retained. This study proved the feasibility of using RNNs to mitigate motion artifacts and incomplete angiographic acquisitions to extract accurate quantitative parameters.

Quantifying the Intra-Operative Hemodynamic Effects of Glue Embolization in Vein of Galen Malformations

Vein of Galen malformation is a rare congenital pathological intracranial arteriovenous shunt which carries 30% risk of death before 28 days-of-age. Treatment is by high risk minimally invasive endovascular glue embolization of shunt feeding arteries under angiographic control. A tool to support intra-operative decision making would be useful. We present a novel method for visualizing angiography data to demonstrate the effect of the intervention based upon change the after embolization in the delay in time of peak contrast density relative to the injected artery and a novel method for quantifying the immediate effect of embolization on the hemodynamics of the shunt. The method is demonstrated on the angiograms of five neonates who underwent embolization. We show consistent results including a post-embolization increase in the delay in time of peak contrast density relative to the injected artery at the venous outflow in keeping with reduced shunting and redistribution of blood following embolization.

Comparison of carotid artery dimensions and lesion length measured by B-mode ultrasonography and quantitative angiography in patients with severe stenosis undergoing percutaneous revascularization

BACKGROUND

To evaluate the correlation and agreement of the carotid artery landmarks necessary for carotid artery stenting obtained by B-mode ultrasonography (BMU), and by quantitative angiography (QCA) in patients with severe carotid artery stenosis.

METHODS

In 75 patients undergoing carotid artery stenting, the distal common (CCA), proximal internal (ICA) carotid artery diameter, and lesion length were measured preoperatively by BMU, and intraoperatively by QCA.

RESULTS

In 96% of the subjects, BMU imaging was adequate for interpretation. BMU and QCA Pearson correlation and Lin concordance coefficients were 0.75 (p < 0.001) and 0.959 (95% CI: 0.930 - 0.996), respectively, for CCA diameter, 0.88 (p < 0.001) and 0.954 (95% CI: 0.928-0.983), respectively, for ICA diameter, and 0.62 (p < 0.001) and 0.734 (95% CI: 0.719-0.760), respectively, for lesion length, with a 0.765 bias correction factor and a wider data scatter by Bland Altman plots showing shorter lesion length by BMU than by QCA.

CONCLUSIONS

In patients with carotid artery stenosis, BMU can provide reliable distal CCA and proximal ICA diameters in comparison with QCA, whereas lesion length measured by BMU has an acceptable correlation, but a poor agreement with QCA.