Pulsatile coronary flow determination by digital angiography

Quantification of coronary microvascular resistance using angiographic images for volumetric blood flow measurement: in vivo validation

Structural coronary microcirculation abnormalities are important prognostic determinants in clinical settings. However, an assessment of microvascular resistance (MR) requires a velocity wire. A first-pass distribution analysis technique to measure volumetric blood flow has been previously validated. The aim of this study was the in vivo validation of the MR measurement technique using first-pass distribution analysis. Twelve anesthetized swine were instrumented with a transit-time ultrasound flow probe on the proximal segment of the left anterior descending coronary artery (LAD). Microspheres were injected into the LAD to create a model of microvascular dysfunction. Adenosine (400 μg·kg(-1)·min(-1)) was used to produce maximum hyperemia. A region of interest in the LAD arterial bed was drawn to generate time-density curves using angiographic images. Volumetric blood flow measurements (Q(a)) were made using a time-density curve and the assumption that blood was momentarily replaced with contrast agent during the injection. Blood flow from the flow probe (Q(p)), coronary pressure (P(a)), and right atrium pressure (P(v)) were continuously recorded. Flow probe-based normalized MR (NMR(p)) and angiography-based normalized MR (NMR(a)) were calculated using Q(p) and Q(a), respectively. In 258 measurements, Q(a) showed a strong correlation with the gold standard Q(p) (Q(a) = 0.90 Q(p) + 6.6 ml/min, r(2) = 0.91, P < 0.0001). NMR(a) correlated linearly with NMR(p) (NMR(a) = 0.90 NMR(p) + 0.02 mmHg·ml(-1)·min(-1), r(2) = 0.91, P < 0.0001). Additionally, the Bland-Altman analysis showed a close agreement between NMR(a) and NMR(p). In conclusion, a technique based on angiographic image data for quantifying NMR was validated using a swine model. This study provides a method to measure NMR without using a velocity wire, which can potentially be used to evaluate microvascular conditions during coronary arteriography.

Quantification of fractional flow reserve based on angiographic image data

Coronary angiography provides excellent visualization of coronary arteries, but has limitations in assessing the clinical significance of a coronary stenosis. Fractional flow reserve (FFR) has been shown to be reliable in discerning stenoses responsible for inducible ischemia. The purpose of this study is to validate a technique for FFR quantification using angiographic image data. The study was carried out on 10 anesthetized, closed-chest swine using angioplasty balloon catheters to produce partial occlusion. Angiography based FFR was calculated from an angiographically measured ratio of coronary blood flow to arterial lumen volume. Pressure-based FFR was measured from a ratio of distal coronary pressure to aortic pressure. Pressure-wire measurements of FFR (FFR_P) correlated linearly with angiographic volume-derived measurements of FFR (FFR_V) according to the equation: FFR_P = 0.41 FFR_V + 0.52 (P-value < 0.001). The correlation coefficient and standard error of estimate were 0.85 and 0.07, respectively. This is the first study to provide an angiographic method to quantify FFR in swine. Angiographic FFR can potentially provide an assessment of the physiological severity of a coronary stenosis during routine diagnostic cardiac catheterization without a need to cross a stenosis with a pressure-wire.

Estimation of coronary artery hyperemic blood flow based on arterial lumen volume using angiographic images

The purpose of this study is to develop a method to estimate the hyperemic blood flow in a coronary artery using the sum of the distal lumen volumes in a swine animal model. The limitations of visually assessing coronary artery disease are well known. These limitations are particularly important in intermediate coronary lesions where it is difficult to determine whether a particular lesion is the cause of ischemia. Therefore, a functional measure of stenosis severity is needed using angiographic image data. Coronary arteriography was performed in 10 swine (Yorkshire, 25-35 kg) after power injection of contrast material into the left main coronary artery. A densitometry technique was used to quantify regional flow and lumen volume in vivo after inducing hyperemia. Additionally, 3 swine hearts were casted and imaged post-mortem using cone-beam CT to obtain the lumen volume and the arterial length of corresponding coronary arteries. Using densitometry, the results showed that the stem hyperemic flow (Q) and the associated crown lumen volume (V) were related by Q = 159.08 V(3/4) (r = 0.98, SEE = 10.59 ml/min). The stem hyperemic flow and the associated crown length (L) using cone-beam CT were related by Q = 2.89 L (r = 0.99, SEE = 8.72 ml/min). These results indicate that measured arterial branch lengths or lumen volumes can potentially be used to predict the expected hyperemic flow in an arterial tree. This, in conjunction with measured hyperemic flow in the presence of a stenosis, could be used to predict fractional flow reserve based entirely on angiographic data.

Determination of fractional flow reserve (FFR) based on scaling laws: a simulation study

Fractional flow reserve (FFR) provides an objective physiological evaluation of stenosis severity. A technique that can measure FFR using only angiographic images would be a valuable tool in the cardiac catheterization laboratory. To perform this, the diseased blood flow can be measured with a first pass distribution analysis and the theoretical normal blood flow can be estimated from the total coronary arterial volume based on scaling laws. A computer simulation of the coronary arterial network was used to gain a better understanding of how hemodynamic conditions and coronary artery disease can affect blood flow, arterial volume and FFR estimation. Changes in coronary arterial flow and volume due to coronary stenosis, aortic pressure and venous pressure were examined to evaluate the potential use of flow and volume for FFR determination. This study showed that FFR can be estimated using arterial volume and a scaling coefficient corrected for aortic pressure. However, variations in venous pressure were found to introduce some error in FFR estimation. A relative form of FFR was introduced and was found to cancel out the influence of pressure on coronary flow, arterial volume and FFR estimation. The use of coronary flow and arterial volume for FFR determination appears promising.

Quantitative coronary angiography using image recovery techniques for background estimation in unsubtracted images

Densitometry measurements have been performed previously using subtracted images. However, digital subtraction angiography (DSA) in coronary angiography is highly susceptible to misregistration artifacts due to the temporal separation of background and target images. Misregistration artifacts due to respiration and patient motion occur frequently, and organ motion is unavoidable. Quantitative densitometric techniques would be more clinically feasible if they could be implemented using unsubtracted images. The goal of this study is to evaluate image recovery techniques for densitometry measurements using unsubtracted images. A humanoid phantom and eight swine (25-35 kg) were used to evaluate the accuracy and precision of the following image recovery techniques: Local averaging (LA), morphological filtering (MF), linear interpolation (LI), and curvature-driven diffusion image inpainting (CDD). Images of iodinated vessel phantoms placed over the heart of the humanoid phantom or swine were acquired. In addition, coronary angiograms were obtained after power injections of a nonionic iodinated contrast solution in an in vivo swine study. Background signals were estimated and removed with LA, MF, LI, and CDD. Iodine masses in the vessel phantoms were quantified and compared to known amounts. Moreover, the total iodine in left anterior descending arteries was measured and compared with DSA measurements. In the humanoid phantom study, the average root mean square errors associated with quantifying iodine mass using LA and MF were approximately 6% and 9%, respectively. The corresponding average root mean square errors associated with quantifying iodine mass using LI and CDD were both approximately 3%. In the in vivo swine study, the root mean square errors associated with quantifying iodine in the vessel phantoms with LA and MF were approximately 5% and 12%, respectively. The corresponding average root mean square errors using LI and CDD were both 3%. The standard deviations in the differences between measured iodine mass in left anterior descending arteries using DSA and LA, MF, LI, or CDD were calculated. The standard deviations in the DSA-LA and DSA-MF differences (both approximately 21 mg) were approximately a factor of 3 greater than that of the DSA-LI and DSA-CDD differences (both approximately 7 mg). Local averaging and morphological filtering were considered inadequate for use in quantitative densitometry. Linear interpolation and curvature-driven diffusion image inpainting were found to be effective techniques for use with densitometry in quantifying iodine mass in vitro and in vivo. They can be used with unsubtracted images to estimate background anatomical signals and obtain accurate densitometry results. The high level of accuracy and precision in quantification associated with using LI and CDD suggests the potential of these techniques in applications where background mask images are difficult to obtain, such as lumen volume and blood flow quantification using coronary arteriography.

Regional blood flow analysis and its relationship with arterial branch lengths and lumen volume in the coronary arterial tree

The limitations of visually assessing coronary artery disease are well known. These limitations are particularly important in intermediate coronary lesions (30-70% diameter stenosis) where it is difficult to determine whether a particular lesion is the cause of ischaemia. Therefore, a functional measure of stenosis severity is needed. The purpose of this study is to determine whether the expected maximum coronary blood flow in an arterial tree is predictable from its sum of arterial branch lengths or lumen volume. Using a computer model of a porcine coronary artery tree, an analysis of blood flow distribution was conducted through a network of millions of vessels that included the entire coronary artery tree down to the first capillary branch. The flow simulation results show that there is a linear relationship between coronary blood flow and the sum of its arterial branch lengths. This relationship holds over the entire arterial tree. The flow simulation results also indicate that there is a 3/4 power relation between coronary blood flow (Q) and the sum of its arterial lumen volume (V). Moreover, there is a linear relationship between normalized Q and normalized V raised to a power of 3/4 over the entire arterial tree. These results indicate that measured arterial branch lengths or lumen volumes can be used to predict the expected maximum blood flow in an arterial tree. This theoretical maximum blood flow, in conjunction with an angiographically measured blood flow, can potentially be used to calculate fractional flow reserve based entirely on angiographic data.

Automated technique for angiographic determination of coronary blood flow and lumen volume

RATIONALE AND OBJECTIVES

Visual interpretation of angiographic images has been shown to be inadequate for assessing the severity of intermediate coronary stenoses. An approach for evaluating both the anatomic and functional impact of a stenosis is needed. An automated technique for determining both coronary blood flow and lumen volume based on a first-pass analysis (FPA) of coronary angiograms and a template matching algorithm was evaluated.

MATERIALS AND METHODS

Coronary angiograms of a swine animal model were obtained during power injections of contrast material into the left coronary ostium. Background anatomy was subtracted with an automated phase matching program. A template matching algorithm and first-pass analysis were then used to quantify coronary blood flow and lumen volume. Coronary blood flow and lumen volume measurements were validated with a transit-time ultrasound flow probe and a polymer cast of the coronary arteries, respectively.

RESULTS

In 14 independent comparisons, the mean coronary blood flow measured with FPA showed strong correlation with the mean flow measured with the ultrasound flow probe (Q(FPA) = 0.88Q(probe) - 1.99; r = 0.977; standard error of estimate = 3.23 mL/minute). The lumen volumes determined with FPA and cast measurements demonstrated excellent correlation and can be related to each other by V(FPA) = 0.95V(C) - 0.01 (r = 0.997; standard error of estimate = 0.01 mL).

CONCLUSIONS

The proposed automated method for accurate determination of coronary blood flow and lumen volume can supplement visual evaluation of coronary anatomy with quantitative physiologic data. This automated technique potentially offers a clinically feasible method of quantifying coronary blood flow and lumen volume in conjunction with routine cardiac catheterization.

Novel approaches to the measurement of arterial blood flow from dynamic digital X-ray images

We have developed two new algorithms for the measurement of blood flow from dynamic X-ray angiographic images. Both algorithms aim to improve on existing techniques. First, a model-based (MB) algorithm is used to constrain the concentration-distance curve matching approach. Second, a weighted optical flow algorithm (OP) is used to improve on point-based optical flow methods by averaging velocity estimates along a vessel with weighting based on the magnitude of the spatial derivative. The OP algorithm was validated using a computer simulation of pulsatile blood flow. Both the OP and the MB algorithms were validated using a physiological blood flow circuit. Dynamic biplane digital X-ray images were acquired following injection of iodine contrast medium into a variety of simulated arterial vessels. The image data were analyzed using our integrated angiographic analysis software SARA to give blood flow waveforms using the MB and OP algorithms. These waveforms were compared to flow measured using an electromagnetic flow meter (EMF). In total 4935 instantaneous measurements of flow were made and compared to the EMF recordings. It was found that the new algorithms showed low measurement bias and narrow limits of agreement and also out-performed the concentration-distance curve matching algorithm (ORG) and a modification of this algorithm (PA) in all studies.

Regional volumetric coronary blood flow measurement by digital angiography: in vivo validation

RATIONALE AND OBJECTIVES

There are well-known limitations to the use of visual estimation to assess the severity of coronary artery disease and luminal stenosis. This is especially true in the case of an intermediate coronary lesion (30%-70% diameter stenosis), where coronary arteriography is very limited in distinguishing ischemia-producing intermediate coronary lesions from non-ischemia-producing ones. For this reason, a functional measure of stenosis severity is desirable. The goal of this study is to validate a video densitometry technique for quantitative assessment of regional volumetric coronary blood flow.

MATERIALS AND METHODS

Coronary arteriography was performed in eight swine (body weight, 25-50 kg) after power injection of contrast material into the left main coronary artery. Phase-matched subtracted images were used to quantify regional coronary blood flow using a video densitometry technique. The in vivo regional flow measurements were validated using a transit time ultrasound flow probe.

RESULTS

In 44 measurements, the ultrasound (Q(US)) and video densitometry (Q(VD)) regional flow measurements were related by Q(VD) = 0.98 Q(US) + 0.11 mL/min (r = 0.98). The results of mean regional coronary blood flow measurements for repeated coronary arteriograms of the first (Q(VD1)) and second (Q(VD2)) measured flows were related by Q(VD1) = 1.04 Q(VD2) + 0.05 mL/min (r = 0.97).

CONCLUSIONS

A video densitometry technique for quantification of regional coronary blood flow was validated using a swine animal model. The results demonstrated the feasibility and potential utility of the video densitometry technique for accurate measurement of regional coronary blood flow, in vivo. This study provides an angiographic method that can potentially be used to evaluate intermediate coronary lesions during routine coronary arteriography.

X-ray videodensitometric methods for blood flow and velocity measurement: a critical review of literature

Blood flow rate and velocity are important parameters for the study of vascular systems, and for the diagnosis, monitoring and evaluation of treatment of cerebro- and cardiovascular disease. For rapid imaging of cerebral and cardiac blood vessels, digital x-ray subtraction angiography has numerous advantages over other modalities. Roentgen-videodensitometric techniques measure blood flow and velocity from changes of contrast material density in x-ray angiograms. Many roentgen-videodensitometric flow measurement methods can also be applied to CT, MR and rotational angiography images. Hence, roentgen-videodensitometric blood flow and velocity measurement from digital x-ray angiograms represents an important research topic. This work contains a critical review and bibliography surveying current and old developments in the field. We present an extensive survey of English-language publications on the subject and a classification of published algorithms. We also present descriptions and critical reviews of these algorithms. The algorithms are reviewed with requirements imposed by neuro- and cardiovascular clinical environments in mind.

Comparison of methods for instantaneous angiographic blood flow measurement

Several different algorithms have been reported for measurement of blood flow rates and velocities from digital x-ray angiograms. We compare four videodensitometric methods: (1) distance-density curve matching (DDCM), (2) distance-density curve matching with curve-fitting (DDCM-F), (3) bolus mass tracking with curve-fitting (BMT-F) and (4) fluid continuity method (FCM). We tested the flow algorithms with simulated angiograms and with images obtained from a programmable flow phantom under clinically realistic flow and contrast injection conditions including imperfect mixing. All methods perform well for simulated angiograms. On phantom angiograms with constant flow, all methods tended to underestimate flow velocities by at least 7% and demonstrate high variability between consecutive measurements. The FCM demonstrated relatively low variability, but a large negative bias. The DDCM method was moderately biased and had the highest variability. The BMT-F method demonstrated the lowest bias (-7.1%) and the lowest variability both within (27%) and between (27%) studies. No method yields reliable measurements near the peak contrast opacification, when little or no gradient of contrast is present. The extrapolating version of the BMT-F method was also the most robust for estimation of interframe displacements longer than the field of view.

Absolute volumetric coronary blood flow measurement with digital subtraction angiography

The problems associated with visual interpretation of coronary arteriograms have been well-documented. There is a need for more physiologic means of assessing coronary artery stenosis during routine coronary arteriography. Volumetric coronary blood flow assessed as a function of time can be a valuable aid in the analysis of functional significance of arterial obstruction. A volumetric coronary blood flow measurement technique was investigated in a swine animal model using phase matched temporal subtraction images. The left anterior descending (LAD) coronary artery of swine animal models were instrumented with an ultrasound flow probe (US) and a vascular occluder producing stenosis. Contrast material injections (2-4 ml/sec for 3 sec) were made into the left coronary ostium during image acquisition. Phase-matched temporal subtraction (DSA) images were used to measure volumetric coronary blood flow in the LAD. In addition, a technique for automatic estimation of iodine calibration slope was also investigated. In 49 independent comparisons, the mean coronary blood flow (FPA) correlated extremely well with the mean US flow (FPA = 0.92US + 1.42 ml/min, r = 0.99, standard error of estimate (SEE) = 4.32 ml/min). Further more, the automatic iodine calibration technique was shown to be very accurate. In conclusion, accurate volumetric coronary blood flow measurements can be made before the onset of significant changes in coronary blood flow due to contrast injection.

Quantification of volumetric coronary blood flow with dual-energy digital subtraction angiography

BACKGROUND

As a solution to the well-documented problems associated with visual interpretation of coronary arteriograms, more physiological methods of assessing coronary artery stenosis are being investigated. Volumetric coronary blood flow (BF) can be a valuable aid in the analysis of functional significance of arterial obstruction.

METHODS AND RESULTS

The left anterior descending coronary artery (LAD) of 15 anesthetized pigs (40 to 50 kg) was dissected free from the epicardium in its proximal portion, and a transmit-time ultrasound flow probe of the appropriate size was applied. A vascular occluder was positioned distal to the flow probe for flow adjustments. Contrast injections (2 to 4 mL/s for 3 seconds) were made into the left main coronary artery during image acquisition with a motion-immune dual-energy digital subtraction angiography (DE DSA) system. Tissue-suppressed energy-subtracted images were used to generate time-density curves. BF measurements were made in the LAD vascular bed with use of the time-density curve, with consideration that blood was momentarily replaced with contrast during the injection. In 19 comparisons, the mean BF, measured with the use of DE DSA, correlated extremely well with the mean ultrasound flow (DE DSA=0.90 ultrasound+3.10 mL/min, r=.96). Also, contrast injection increased the BF by an average of only 15% during the image-acquisition time interval.

CONCLUSIONS

Accurate BF measurements can be made with motion-immune DE DSA. The BF measurements can be completed before the onset of significant changes in BF due to contrast injection. Furthermore, it is possible to make the BF measurements during routine coronary arteriography.