Time delay for arrival of MR contrast agent in collateral-dependent myocardium

Perfusion Collateral Index versus Hypoperfusion Intensity Ratio in Assessment of Collaterals in Patients with Acute Ischemic Stroke

BACKGROUND AND PURPOSE

Perfusion-based collateral indices such as the perfusion collateral index and the hypoperfusion intensity ratio have shown promise in the assessment of collaterals in patients with acute ischemic stroke. We aimed to compare the diagnostic performance of the perfusion collateral index and the hypoperfusion intensity ratio in collateral assessment compared with angiographic collaterals and outcome measures, including final infarct volume, infarct growth, and functional independence.

MATERIALS AND METHODS

Consecutive patients with acute ischemic stroke with anterior circulation proximal arterial occlusion who underwent endovascular thrombectomy and had pre- and posttreatment MRI were included. Using pretreatment MR perfusion, we calculated the perfusion collateral index and the hypoperfusion intensity ratio for each patient. The angiographic collaterals obtained from DSA were dichotomized to sufficient (American Society of Interventional and Therapeutic Neuroradiology [ASITN] scale 3-4) versus insufficient (ASITN scale 0-2). The association of collateral status determined by the perfusion collateral index and the hypoperfusion intensity ratio was assessed against angiographic collaterals and outcome measures.

RESULTS

A total of 98 patients met the inclusion criteria. Perfusion collateral index values were significantly higher in patients with sufficient angiographic collaterals (P < .001), while there was no significant (P = .46) difference in hypoperfusion intensity ratio values. Among patients with good (mRS 0-2) versus poor (mRS 3-6) functional outcome, the perfusion collateral index of ≥ 62 was present in 72% versus 31% (P = .003), while the hypoperfusion intensity ratio of ≤0.4 was present in 69% versus 56% (P = .52). The perfusion collateral index and the hypoperfusion intensity ratio were both significantly predictive of final infarct volume, but only the perfusion collateral index was significantly (P = .03) associated with infarct growth.

CONCLUSIONS

Results show that the perfusion collateral index outperforms the hypoperfusion intensity ratio in the assessment of collateral status, infarct growth, and determination of functional outcomes.

Direct Comparison of Bayesian and Fermi Deconvolution Approaches for Myocardial Blood Flow Quantification: In silico and Clinical Validations

Cardiac magnetic resonance myocardial perfusion imaging can detect coronary artery disease and is an alternative to single-photon emission computed tomography or positron emission tomography. However, the complex, non-linear MR signal and the lack of robust quantification of myocardial blood flow have hindered its widespread clinical application thus far. Recently, a new Bayesian approach was developed for brain imaging and evaluation of perfusion indexes (Kudo et al., 2014). In addition to providing accurate perfusion measurements, this probabilistic approach appears more robust than previous approaches, particularly due to its insensitivity to bolus arrival delays. We assessed the performance of this approach against a well-known and commonly deployed model-independent method based on the Fermi function for cardiac magnetic resonance myocardial perfusion imaging. The methods were first evaluated for accuracy and precision using a digital phantom to test them against the ground truth; next, they were applied in a group of coronary artery disease patients. The Bayesian method can be considered an appropriate model-independent method with which to estimate myocardial blood flow and delays. The digital phantom comprised a set of synthetic time-concentration curve combinations generated with a 2-compartment exchange model and a realistic combination of perfusion indexes, arterial input dynamics, noise and delays collected from the clinical dataset. The myocardial blood flow values estimated with the two methods showed an excellent correlation coefficient (r² > 0.9) under all noise and delay conditions. The Bayesian approach showed excellent robustness to bolus arrival delays, with a similar performance to Fermi modeling when delays were considered. Delays were better estimated with the Bayesian approach than with Fermi modeling. An in vivo analysis of coronary artery disease patients revealed that the Bayesian approach had an excellent ability to distinguish between abnormal and normal myocardium. The Bayesian approach was able to discriminate not only flows but also delays with increased sensitivity by offering a clearly enlarged range of distribution for the physiologic parameters.

Pixel-wise quantification of myocardial perfusion using spatial Tikhonov regularization

Quantification of myocardial perfusion by contrast-enhanced cardiovascular magnetic resonance imaging (CMR) aims for an observer independent and reproducible risk assessment of cardiovascular disease. Currently, the data used for the pixel-wise analysis of cardiac perfusion are either filtered prior to a fitting procedure, which inherently reduces the spatial resolution of data; or all pixels are considered without any regularization or prior filtering, which yields an unstable fit in the presence of low signal-to-noise ratio. Here, we propose a new pixel-wise analysis based on spatial Tikhonov regularization which exploits the spatial smoothness of the data and ensures accurate quantification even for images with low signal-to-noise ratio. The regularization parameter is determined automatically by an L-curve criterion. We study the performance of our method on a numerical phantom and demonstrate that the method reduces significantly the root-mean square error in the perfusion estimate compared to a non-regularized fit. In patient data our method allows us to recover the myocardial perfusion and to distinguish between healthy and ischemic regions.

An Open Benchmark Challenge for Motion Correction of Myocardial Perfusion MRI

Cardiac magnetic resonance perfusion examinations enable noninvasive quantification of myocardial blood flow. However, motion between frames due to breathing must be corrected for quantitative analysis. Although several methods have been proposed, there is a lack of widely available benchmarks to compare different algorithms. We sought to compare many algorithms from several groups in an open benchmark challenge. Nine clinical studies from two different centers comprising normal and diseased myocardium at both rest and stress were made available for this study. The primary validation measure was regional myocardial blood flow based on the transfer coefficient (Ktrans), which was computed using a compartment model and the myocardial perfusion reserve (MPR) index. The ground truth was calculated using contours drawn manually on all frames by a single observer, and visually inspected by a second observer. Six groups participated and 19 different motion correction algorithms were compared. Each method used one of three different motion models: rigid, global affine, or local deformation. The similarity metric also varied with methods employing either sum-of-squared differences, mutual information, or cross correlation. There were no significant differences in Ktrans or MPR compared across different motion models or similarity metrics. Compared with the ground truth, only Ktrans for the sum-of-squared differences metric, and for local deformation motion models, had significant bias. In conclusion, the open benchmark enabled evaluation of clinical perfusion indices over a wide range of methods. In particular, there was no benefit of nonrigid registration techniques over the other methods evaluated in this study. The benchmark data and results are available from the Cardiac Atlas Project ( www.cardiacatlas.org).

Perfusion dyssynchrony analysis

AIMS

We sought to describe perfusion dyssynchrony analysis specifically to exploit the high temporal resolution of stress perfusion CMR. This novel approach detects differences in the temporal distribution of the wash-in of contrast agent across the left ventricular wall.

METHODS AND RESULTS

Ninety-eight patients with suspected coronary artery disease (CAD) were retrospectively identified. All patients had undergone perfusion CMR at 3T and invasive angiography with fractional flow reserve (FFR) of lesions visually judged >50% stenosis. Stress images were analysed using four different perfusion dyssynchrony indices: the variance and coefficient of variation of the time to maximum signal upslope (V-TTMU and C-TTMU) and the variance and coefficient of variation of the time to peak myocardial signal enhancement (V-TTP and C-TTP). Patients were classified according to the number of vessels with haemodynamically significant CAD indicated by FFR <0.8. All indices of perfusion dyssynchrony were capable of identifying the presence of significant CAD. C-TTP >10% identified CAD with sensitivity 0.889, specificity 0.857 (P < 0.0001). All indices correlated with the number of diseased vessels. C-TTP >12% identified multi-vessel disease with sensitivity 0.806, specificity 0.657 (P < 0.0001). C-TTP was also the dyssynchrony index with the best inter- and intra-observer reproducibility. Perfusion dyssynchrony indices showed weak correlation with other invasive and non-invasive measurements of the severity of ischaemia, including FFR, visual ischaemic burden, and MPR.

CONCLUSION

These findings suggest that perfusion dyssynchrony analysis is a robust novel approach to the analysis of first-pass perfusion and has the potential to add complementary information to aid assessment of CAD.

Effects of tracer arrival time on the accuracy of high-resolution (voxel-wise) myocardial perfusion maps from contrast-enhanced first-pass perfusion magnetic resonance

First-pass perfusion cardiac magnetic resonance(CMR) allows the quantitative assessment of myocardial blood flow(MBF). However, flow estimates are sensitive to the delay between the arterial and myocardial tissue tracer arrival time (tOnset) and the accurate estimation of MBF relies on the precise identification of tOnset . The aim of this study is to assess the sensitivity of the quantification process to tOnset at voxel level. Perfusion data were obtained from series of simulated data, a hardware perfusion phantom, and patients. Fermi deconvolution has been used for analysis. A novel algorithm, based on sequential deconvolution,which minimizes the error between myocardial curves and fitted curves obtained after deconvolution, has been used to identify the optimal tOnset for each region. Voxel-wise analysis showed to be more sensitive to tOnset compared to segmental analysis. The automated detection of the tOnset allowed a net improvement of the accuracy of MBF quantification and in patients the identification of perfusion abnormalities in territories that were missed when a constant user-selected tOnset was used. Our results indicate that high-resolution MBF quantification should be performed with optimized tOnset values at voxel level.

Perfusion MRI at rest in subacute and chronic myocardial infarct

BACKGROUND

Perfusion magnetic resonance imaging (MRI) and delayed contrast-enhanced MRI (DE-MRI) serve as tools for tissue characterization.

PURPOSE

To assess and compare semi-quantitative parameters of myocardial infarct (MI) in the subacute and chronic phase, and to correlate these parameters with qualitative enhancement analysis.

MATERIAL AND METHODS

Perfusion MRI at rest and DE-MRI were performed in 63 patients with anterior wall MI at 2-3 weeks after revascularization and repeated after 6 months. Descriptive enhancement parameters of contrast arrival time, initial upslope, enhancement at normal tissue peak (TTPn) and wash-out slope, and kinetic tissue parameters rBF, K (trans), k ep and v e were calculated. Subacute infarct tissue was compared to normal myocardium and chronic infarct tissue. Patients were stratified at baseline according to a qualitative grading of hypoenhancement based on first-pass enhancement and presence of microvascular obstruction (MO) at perfusion MRI and on persistent MO at DE-MRI. The qualitative grade was correlated to semi-quantitative perfusion MRI parameters.

RESULTS

Initial upslope, enhancement at TTPn, rBF, and k ep were decreased and wash-out slope and v e were increased in infarct tissue (P < 0.001 for all analyses). Infarct tissue v e decreased from baseline to 6 months (P = 0.045). At baseline infarct tissue with persistent MO revealed decreased K (trans) and delayed contrast arrival, and more pronounced decrease of enhancement at TTPn, rBF and k ep compared to other enhancement groups (P < 0.008 for pairwise analyses).

CONCLUSION

Perfusion is decreased in subacute reperfused infarct tissue compared to normal tissue. K (trans) is not decreased, consistent with increased surface area of the vascular bed of the subacute infarct. Infarct tissue v e is increased, and decreases with scarring. The presence of persistent MO correlates to more pronounced perfusion reduction and results in delayed contrast arrival, indicating microvascular collateral circulation.

Myocardial blood flow quantification from MRI by deconvolution using an exponential approximation basis

We have evaluated the use of deconvolution using an exponential approximation basis for the quantification of myocardial blood flow from perfusion cardiovascular magnetic resonance. Our experiments, based on simulated signal intensity curves, phantom acquisitions, and clinical image data, indicate that exponential deconvolution allows for accurate quantification of myocardial blood flow. Together with automated respiratory motion correction myocardial contour delineation, the exponential deconvolution enables efficient and reproducible quantification of myocardial blood flow in clinical routine.

Comparison of myocardial perfusion estimates from dynamic contrast-enhanced magnetic resonance imaging with four quantitative analysis methods

Dynamic contrast-enhanced MRI has been used to quantify myocardial perfusion in recent years. Published results have varied widely, possibly depending on the method used to analyze the dynamic perfusion data. Here, four quantitative analysis methods (two-compartment modeling, Fermi function modeling, model-independent analysis, and Patlak plot analysis) were implemented and compared for quantifying myocardial perfusion. Dynamic contrast-enhanced MRI data were acquired in 20 human subjects at rest with low-dose (0.019 +/- 0.005 mmol/kg) bolus injections of gadolinium. Fourteen of these subjects were also imaged at adenosine stress (0.021 +/- 0.005 mmol/kg). Aggregate rest perfusion estimates were not significantly different between all four analysis methods. At stress, perfusion estimates were not significantly different between two-compartment modeling, model-independent analysis, and Patlak plot analysis. Stress estimates from the Fermi model were significantly higher (approximately 20%) than the other three methods. Myocardial perfusion reserve values were not significantly different between all four methods. Model-independent analysis resulted in the lowest model curve-fit errors. When more than just the first pass of data was analyzed, perfusion estimates from two-compartment modeling and model-independent analysis did not change significantly, unlike results from Fermi function modeling.

Assessment of tumor blood flow in breast tumors with T1-dynamic contrast-enhanced MR imaging: impact of dose reduction and the use of a prebolus technique on diagnostic efficacy

PURPOSE

To prospectively evaluate whether dose reduction and the application of a prebolus technique can effectively alleviate signal saturation effects in T1 dynamic contrast enhanced (T1-DCE) magnetic resonance imaging (MRI) data in breast tumors and lead to increased diagnostic efficacy of the regional tumor blood flow (TBF) values obtained with deconvolution of T1-DCE MRI data.

MATERIALS AND METHODS

After obtaining informed consent, 23 women (32-80 years) with histologically proven breast tumors underwent MR mammography that included a whole-breast T1 DCE sequence. In the slice where the tumor enhanced maximally, a prebolus protocol was applied. One mL of Gd-DTPA solution at 2 mL/s was injected at the beginning of a dynamic axial single slice inversion-prepared turbo field echo acquisition. At the 400th dynamic, a high dose of either 20 mL (15 patients) or 10 mL (8 patients) of contrast agent was injected at 2 mL/s and a further 400 dynamics were acquired. From the aortic prebolus curve an arterial input function (AIF) was reconstructed by time-shifting and adding the prebolus data. The relative enhancement time course from the tumor region of interest was deconvolved with the reconstructed AIF to generate the impulse response function, the maximum of which yielded the TBF. The institutional ethical committee approved the study.

RESULTS

Reducing the contrast dose by a factor of 2 led to an increase in diagnostic contrast for the TBF values of malignant and benign tumors by a factor of slightly more than 2. Addition of the prebolus technique improved this further by 45%. receiver operating characteristic analysis showed a significant increase of diagnostic yield related to the combined use of a prebolus and minimal dose.

CONCLUSION

Using a prebolus approach provides an estimate of the unsaturated AIF, while reduction of the high-dose bolus minimizes possible saturation effects in the tumor time course.

Quantification of absolute myocardial blood flow by magnetic resonance perfusion imaging

By serially imaging the myocardium during the initial transit of gadolinium contrast, magnetic resonance perfusion imaging can accurately assess relative reductions in regional myocardial blood flow and identify hemodynamically significant coronary artery disease. Models can be used to quantify myocardial blood flow (in milliliters/minute/gram) on the basis of dynamic signal changes within the myocardium and left ventricular cavity. Although the mathematical modeling involved in this type of analysis adds complexity, the benefits of absolute blood flow quantification might improve clinical diagnosis and have important implications for cardiovascular research.

Quantitative analysis of dynamic contrast-enhanced MR images based on Bayesian P-splines

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is an important tool for detecting subtle kinetic changes in cancerous tissue. Quantitative analysis of DCE-MRI typically involves the convolution of an arterial input function (AIF) with a nonlinear pharmacokinetic model of the contrast agent concentration. Parameters of the kinetic model are biologically meaningful, but the optimization of the nonlinear model has significant computational issues. In practice, convergence of the optimization algorithm is not guaranteed and the accuracy of the model fitting may be compromised. To overcome these problems, this paper proposes a semi-parametric penalized spline smoothing approach, where the AIF is convolved with a set of B-splines to produce a design matrix using locally adaptive smoothing parameters based on Bayesian penalized spline models (P-splines). It has been shown that kinetic parameter estimation can be obtained from the resulting deconvolved response function, which also includes the onset of contrast enhancement. Detailed validation of the method, both with simulated and in vivo data, is provided.

Vessel growth and function: depiction with contrast-enhanced MR imaging

Magnetic resonance (MR) imaging is a versatile noninvasive diagnostic tool that can be applied to the entire human body to revealing morphologic, functional, and metabolic information. The authors review how MR imaging can depict both the established and the developing vasculature with techniques involving intravenously administered contrast agents. In addition to macrovascular morphology and flow, MR imaging is able to exploit microvascular properties, including vessel size distribution, hyperpermeability, flow heterogeneity, and possibly also upregulation of endothelial biomarkers. For each MR method, the basic principles, potential acquisition and interpretation pitfalls, solutions, and applications are described. Furthermore, discussion includes current shortcomings and the impact of future developments (eg, higher magnetic field strength systems, targeted macromolecular contrast agents) on the visualization of blood vessel growth and function with contrast-enhanced MR imaging.

Percutaneous treatment of chronic total coronary occlusions improves regional hyperemic myocardial blood flow and contractility: insights from quantitative cardiovascular magnetic resonance imaging

OBJECTIVES

We sought to investigate temporal changes in contractility and hyperemic and resting myocardial blood flow (MBF) in dependent and remote myocardium after percutaneous coronary intervention (PCI) of chronic total occlusions (CTOs) by using cardiovascular magnetic resonance (CMR) imaging.

BACKGROUND

Data about the physiological consequences of revascularization of CTOs are limited. The use of CMR allows investigation of the regional effects of revascularization on MBF and left ventricular contractility.

METHODS

We prospectively recruited 3 patient groups: 17 patients scheduled for CTO PCI, 17 scheduled for PCI of a stenosed but nonoccluded coronary artery (non-CTO), and 6 patients with CTO who were not scheduled for revascularization. All patients undergoing PCI underwent CMR imaging <24 h before PCI, with repeat CMR imaging 24 h and 6 months after PCI. Each CMR scan consisted of cine, perfusion, and delayed enhancement imaging. Regional hyperemic and resting MBF, wall thickening, and transmural extent of infarction were calculated.

RESULTS

In both intervention groups, hyperemic MBF in treated segments increased 24 h after PCI compared with baseline: CTO group, 2.1 +/- 0.2 ml/min/g versus 1.4 +/- 0.2 ml/min/g (p < 0.01); non-CTO group, 2.5 +/- 0.2 ml/min/g versus 1.6 +/- 0.2 ml/min/g (p < 0.01). This improvement persisted 6 months after PCI (p < 0.01 for both groups). Contractility in treated segments was improved at 24 h and 6 months after CTO PCI but only at 6 months after non-CTO PCI. In both intervention groups, treated segments no longer had reduced MBF or contractility compared with remote segments. In patients with untreated CTO segments, MBF and wall thickening did not improve at follow-up.

CONCLUSIONS

Successful CTO PCI increases hyperemic MBF as early as 24 h after the procedure, with a greater and earlier improvement in regional contractility than after non-CTO PCI, despite a greater likelihood of irreversible injury in CTO segments.

Estimating myocardial perfusion from dynamic contrast-enhanced CMR with a model-independent deconvolution method

BACKGROUND

Model-independent analysis with B-spline regularization has been used to quantify myocardial blood flow (perfusion) in dynamic contrast-enhanced cardiovascular magnetic resonance (CMR) studies. However, the model-independent approach has not been extensively evaluated to determine how the contrast-to-noise ratio between blood and tissue enhancement affects estimates of myocardial perfusion and the degree to which the regularization is dependent on the noise in the measured enhancement data. We investigated these questions with a model-independent analysis method that uses iterative minimization and a temporal smoothness regularizer. Perfusion estimates using this method were compared to results from dynamic 13N-ammonia PET.

RESULTS

An iterative model-independent analysis method was developed and tested to estimate regional and pixelwise myocardial perfusion in five normal subjects imaged with a saturation recovery turboFLASH sequence at 3 T CMR. Estimates of myocardial perfusion using model-independent analysis are dependent on the choice of the regularization weight parameter, which increases nonlinearly to handle large decreases in the contrast-to-noise ratio of the measured tissue enhancement data. Quantitative perfusion estimates in five subjects imaged with 3 T CMR were 1.1 +/- 0.8 ml/min/g at rest and 3.1 +/- 1.7 ml/min/g at adenosine stress. The perfusion estimates correlated with dynamic 13N-ammonia PET (y = 0.90x + 0.24, r = 0.85) and were similar to results from other validated CMR studies.

CONCLUSION

This work shows that a model-independent analysis method that uses iterative minimization and temporal regularization can be used to quantify myocardial perfusion with dynamic contrast-enhanced perfusion CMR. Results from this method are robust to choices in the regularization weight parameter over relatively large ranges in the contrast-to-noise ratio of the tissue enhancement data.

Effect of Distal Embolization on Myocardial Perfusion Reserve After Percutaneous Coronary Intervention

Background— Studies have shown that a subset of patients demonstrate persistent impairment in microcirculatory function after percutaneous coronary intervention (PCI). Distal embolization of plaque contents has been postulated as the main mechanism for this. We sought to investigate this further by evaluating PCI-induced changes in myocardial perfusion reserve index (MPRI) over time in segments with “distal-type” procedure-related myonecrosis using high-resolution quantitative cardiovascular magnetic resonance imaging.

Methods and Results— Forty patients undergoing PCI were studied with pre-PCI and 24-hour post-PCI delayed-enhancement magnetic resonance imaging and first-pass perfusion magnetic resonance imaging at rest and stress. Twenty patients underwent a third magnetic resonance imaging scan at 6 months. For perfusion imaging, 3 short-axis images were acquired during every heartbeat with a T1-weighted turboFLASH sequence. MPRI was calculated as the ratio of hyperemic to resting myocardial blood flow and subdivided according to the presence and location of new delayed hyperenhancement. Twenty-one patients demonstrated new distal hyperenhancement after PCI. Mean MPRI in revascularized myocardial segments not demonstrating new HE was significantly increased after the procedure (2.06 [95% CI, 1.99 to 2.13] before PCI and 2.52 [95% CI, 2.42 to 2.62] after PCI; P <0.001). In contrast, MPRI in segments with distal hyperenhancement was reduced after PCI (2.16 [95% CI, 1.95 to 2.37] before PCI; 2.00 [95% CI, 1.82 to 2.19] after PCI; mixed-model z =−4.82; P <0.001). Changes in mean MPRI 24 hours after PCI in segments upstream to new injury were not significantly different compared with perfusion changes in remote myocardium ( z =−0.68; P =0.50). At 6 months after the procedure, mean MPRI in segments with new injury improved significantly compared with MPRI measured in these segments at 24 hours after PCI.

Conclusions— MPRI is reduced in myocardial segments that demonstrate new distal irreversible injury at 24 hours after PCI. These reductions are confined to the segments with injury and do not affect the entire supply territory of the culprit vessel.

Quantification of perfusion and permeability in breast tumors with a deconvolution-based analysis of second-bolus T1-DCE data

PURPOSE

To test the feasibility of using a second-bolus injection, added to a routine breast MRI examination, to measure regional perfusion and permeability in human breast tumors.

MATERIALS AND METHODS

In 30 patients with breast tumors, first a routine whole-breast T1-DCE sequence was applied, and the slice where the lesion enhanced maximally was located. At that slice position, T1-weighted MR images were acquired at 0.3-second resolution using a second-bolus dynamic inversion recovery (IR)-prepared turbo field echo (TFE) sequence. A pixel-by-pixel model-independent deconvolution of the relative signal enhancement was performed to estimate the tumor blood flow (TBF), tumor volume of distribution (TVD), mean transit time (MTT), extraction flow product (EF), and extraction fraction (E). In addition to this pilot study, a first appraisal of its sensitivity to tissue type was made on the basis of a small patient cohort.

RESULTS

In the malignant tumors, the parametric maps clearly delineated tumors from the breast tissue and enabled visualization of the heterogeneity. The deconvolution analysis provided objective parametric maps of tumor perfusion with a mean TBF (84 +/- 48 mL/100 mL/minute) in malignant tumors in the high range of literature values.

CONCLUSION

In terms of these perfusion values, our method appears promising to quantitatively characterize tumor pathophysiology. However, the number of patients was limited, and the separation between malignant and benign groups was not clear-cut. Additional parameters generated through compartment modeling may improve the tumor differentiation.

Evidence for microvascular dysfunction in hypertrophic cardiomyopathy: new insights from multiparametric magnetic resonance imaging

BACKGROUND

Microvascular dysfunction in hypertrophic cardiomyopathy (HCM) may create an ischemic substrate conducive to sudden death, but it remains unknown whether the extent of hypertrophy is associated with proportionally poorer perfusion reserve. Comparisons between magnitude of hypertrophy, impairment of perfusion reserve, and extent of fibrosis may offer new insights for future clinical risk stratification in HCM but require multiparametric imaging with high spatial and temporal resolution.

METHODS AND RESULTS

Degree of hypertrophy, myocardial blood flow at rest and during hyperemia (hMBF), and myocardial fibrosis were assessed with magnetic resonance imaging in 35 HCM patients (9 [26%] male/26 female) and 14 healthy controls (4 [29%] male/10 female), aged 18 to 78 years (mean+/-SD, 42+/-14 years) with the use of the American Heart Association left ventricular 16-segment model. Resting MBF was similar in HCM patients and controls. hMBF was lower in HCM patients (1.84+/-0.89 mL/min per gram) than in healthy controls (3.42+/-1.76 mL/min per gram, with a difference of -0.95+/-0.30 [SE] mL/min per gram; P<0.001) after adjustment for multiple variables, including end-diastolic segmental wall thickness (P<0.001). In HCM patients, hMBF decreased with increasing end-diastolic wall thickness (P<0.005) and preferentially in the endocardial layer. The frequency of endocardial hMBF falling below epicardial hMBF rose with wall thickness (P=0.045), as did the incidence of fibrosis (P<0.001).

CONCLUSIONS

In HCM the vasodilator response is reduced, particularly in the endocardium, and in proportion to the magnitude of hypertrophy. Microvascular dysfunction and subsequent ischemia may be important components of the risk attributable to HCM.

The delay of contrast arrival in magnetic resonance first-pass perfusion imaging: a novel non-invasive parameter detecting collateral-dependent myocardium

AIM

To establish the regional delay of contrast arrival in magnetic resonance perfusion imaging (MRPI) for the detection of collateral-dependent myocardium in patients with coronary artery disease.

DESIGN AND SETTING

Observational study, case series; single centre, university hospital.

PATIENTS

30 patients with coronary artery disease and collateral-dependent myocardium and 17 healthy volunteers.

METHODS

Resting and hyperaemic (adenosine) MRPI was used to determine the delay time (Deltat(d)) of contrast arrival between the left ventricle and collateral-dependent or antegradely perfused myocardium, and myocardial perfusion (MP, ml/min/g).

RESULTS

In healthy volunteers, mean (SD) Deltat(d) at rest and during hyperaemia were 0.8 (0.4) and 0.3 (0.3) s, and MP was 1.14 (0.21) and 4.23 (1.12) ml/min/g. In patients Deltat(d) in antegradely perfused vs collateral-dependent myocardium was 0.9 (0.7) vs 1.7 (1.0) s at rest (p<0.001), and 0.4 (0.3) vs 1.1 (0.6) s (p<0.001) during hyperaemia. MP was 1.12 (0.11) and 0.98 (0.28) ml/min/g (p = NS) at rest and 2.46 (0.85) vs 1.86 (0.91) ml/min/g (p<0.01) during hyperaemia. Receiver operating characteristics analysis showed the best sensitivity and specificity of 90% and 83% for hyperaemic Deltat(d) of >0.6 s (area under the curve (AUC) = 0.89) to detect collateral-dependent myocardium, while resting Deltat(d) (AUC = 0.77) and perfusion (AUC = 0.69 at rest or 0.70 during hyperaemia) were less accurate.

CONCLUSIONS

MRPI-derived hyperaemic delay of contrast arrival detects collateral-dependent myocardium with high sensitivity and specificity. Perfusion was less sensitive, emphasising the clinical role of Deltat(d) in non-invasive detection of collateral-dependent myocardium.

MRI of Myocardial Perfusion

An overwhelming number of myocardial perfusion studies are done by nuclear isotope imaging. Magnetic resonance imaging during the first pass of an injected, contrast bolus has some significant advantages for detection of blood flow deficits, namely higher spatial resolution, absence of ionizing radiation, and speed of the test. Previous clinical studies have demonstrated that excellent sensitivity and specificity can be achieved with MR myocardial perfusion imaging for detecting coronary artery disease, and assessment of patients with acute chest pain. Furthermore, an absolute quantification of myocardial blood flow is feasible, as was demonstrated by comparison of MR perfusion imaging, to measurements with isotope labeled microspheres in experimental models. An integrated assessment of perfusion, function, and viability, is thus feasible by MRI to answer important clinical challenges such as the identification of stunned or hibernating, but viable myocardium.

Resting myocardial blood flow is impaired in hibernating myocardium: a magnetic resonance study of quantitative perfusion assessment

BACKGROUND

Although impairment in perfusion reserve is well recognized in hibernating myocardium, there is substantial controversy as to whether resting myocardial blood flow (MBF) is reduced in such circumstances. Quantitative first-pass cardiovascular magnetic resonance (CMR) perfusion imaging allows absolute quantification of MBF. We hypothesized that MBF assessed at rest by quantitative CMR perfusion imaging is reduced in hibernating myocardium.

METHODS AND RESULTS

Twenty-seven patients with 1 or 2-vessel coronary disease and at least 1 dysfunctional myocardial segment undergoing PCI were studied with preprocedure, early (24 hours), and late (9 months) postprocedure CMR imaging. First-pass perfusion images at rest were acquired in 3 short-axis planes by use of a T1-weighted turboFLASH sequence. In each slice, MBF was determined for 8 myocardial segments in mL . min(-1) . g(-1) by deconvolution of signal intensity curves with an arterial input function measured in the left ventricular blood pool. Cine MRI for assessment of global and segmental function and delayed enhancement MRI for detection of viability were also obtained. All coronary lesions were 80% to 95% stenosis in severity. Over all segments, mean MBF normalized by rate-pressure product ("corrected MBF") was 1.2+/-0.3 mL . min(-1) . g(-1) . (mm Hg . bpm/10(4))(-1) in segments without significant coronary stenosis and 0.7+/-0.2 mL . min(-1) . g(-1) . (mm Hg . bpm/10(4))(-1) in segments with coronary stenosis before PCI (mixed model controlling for slice and segment z=-23.9, P<0.001). Early after the procedure, the MBF was 1.2+/-0.2 mL . min(-1) . g(-1) . (mm Hg . bpm/10(4))(-1) in revascularized segments and 1.3+/-0.2 mL . min(-1) . g(-1) . (mm Hg . bpm/10(4))(-1) in nondiseased segments (z=-6.1, P<0.001). Late after PCI, the systolic wall thickening and end-diastolic wall thickness both increased significantly more (both P<0.001) in the myocardial segments subtended by severe coronary stenosis (8+/-17% to 40+/-19% and 6.5+/-1.1 to 9.3+/-2 mm, respectively) than in the myocardial segments supplied by nondiseased vessels. Mean MBF in dysfunctional segments with significantly improved contraction after revascularization was 0.8+/-0.2 mL . min(-1) . g(-1) . (mm Hg . bpm/10(4))(-1) before PCI and 1.2+/-0.2 mL . min(-1) . g(-1) . (mm Hg . bpm/10(4))(-1) after PCI (z=2.0, P=0.04).

CONCLUSIONS

CMR perfusion imaging detects impaired resting MBF in hibernating myocardial segments.