Detection of peripheral vascular stenosis by assessing skeletal muscle flow reserve

Alginate-Encapsulated Mesenchymal Stromal Cells Improve Hind Limb Ischemia in a Translational Swine Model

BACKGROUND

Cellular therapies have been investigated to improve blood flow and prevent amputation in peripheral artery disease with limited efficacy in clinical trials. Alginate-encapsulated mesenchymal stromal cells (eMSCs) demonstrated improved retention and survival and promoted vascular generation in murine hind limb ischemia through their secretome, but large animal evaluation is necessary for human applicability. We sought to determine the efficacy of eMSCs for peripheral artery disease-induced limb ischemia through assessment in our durable swine hind limb ischemia model.

METHODS AND RESULTS

Autologous bone marrow eMSCs or empty alginate capsules were intramuscularly injected 2 weeks post-hind limb ischemia establishment (N=4/group). Improvements were quantified for 4 weeks through walkway gait analysis, contrast angiography, blood pressures, fluorescent microsphere perfusion, and muscle morphology and histology. Capsules remained intact with mesenchymal stromal cells retained for 4 weeks. Adenosine-induced perfusion deficits and muscle atrophy in ischemic limbs were significantly improved by eMSCs versus empty capsules (mean±SD, 1.07±0.19 versus 0.41±0.16, P=0.002 for perfusion ratios and 2.79±0.12 versus 1.90±0.62 g/kg, P=0.029 for ischemic muscle mass). Force- and temporal-associated walkway parameters normalized (ratio, 0.63±0.35 at week 3 versus 1.02±0.19 preligation; P=0.17), and compensatory footfall patterning was diminished in eMSC-administered swine (12.58±8.46% versus 34.85±15.26%; P=0.043). Delivery of eMSCs was associated with trending benefits in collateralization, local neovascularization, and muscle fibrosis. Hypoxia-cultured porcine mesenchymal stromal cells secreted vascular endothelial growth factor and tissue inhibitor of metalloproteinase 2.

CONCLUSIONS

This study demonstrates the promise of the mesenchymal stromal cell secretome at improving peripheral artery disease outcomes and the potential for this novel swine model to serve as a component of the preclinical pipeline for advanced therapies.

Toward a Better Understanding of Muscle Microvascular Perfusion During Exercise in Patients With Peripheral Artery Disease: The Effect of Lower-Limb Revascularization

PURPOSE

Leg muscle microvascular blood flow (perfusion) is impaired in response to maximal exercise in patients with peripheral artery disease (PAD); however, during submaximal exercise, microvascular perfusion is maintained due to a greater increase in microvascular blood volume compared with that seen in healthy adults. It is unclear whether this submaximal exercise response reflects a microvascular impairment, or whether it is a compensatory response for the limited conduit artery flow in PAD. Therefore, to clarify the role of conduit artery blood flow, we compared whole-limb blood flow and skeletal muscle microvascular perfusion responses with exercise in patients with PAD (n=9; 60±7 years) prior to, and following, lower-limb endovascular revascularization.

MATERIALS AND METHODS

Microvascular perfusion (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after a 5 minute bout of submaximal intermittent isometric plantar-flexion exercise using contrast-enhanced ultrasound imaging. Exercise contraction-by-contraction whole-leg blood flow and vascular conductance were measured using strain-gauge plethysmography.

RESULTS

With revascularization there was a significant increase in whole-leg blood flow and conductance during exercise (p<0.05). Exercise-induced muscle microvascular perfusion response did not change with revascularization (pre-revascularization: 3.19±2.32; post-revascularization: 3.89±1.67 aU.s-1; p=0.38). However, the parameters that determine microvascular perfusion changed, with a reduction in the microvascular volume response to exercise (pre-revascularization: 6.76±3.56; post-revascularization: 2.42±0.69 aU; p<0.01) and an increase in microvascular flow velocity (pre-revascularization: 0.25±0.13; post-revascularization: 0.59±0.25 s-1; p=0.02).

CONCLUSION

These findings suggest that patients with PAD compensate for the conduit artery blood flow impairment with an increase in microvascular blood volume to maintain muscle perfusion during submaximal exercise.

CLINICAL IMPACT

The findings from this study support the notion that the impairment in conduit artery blood flow in patients with PAD leads to compensatory changes in microvascular blood volume and flow velocity to maintain muscle microvascular perfusion during submaximal leg exercise. Moreover, this study demonstrates that these microvascular changes are reversed and become normalized with successful lower-limb endovascular revascularization.

Radiomics of CTA is feasible in identifying muscle ischemia

BACKGROUND

Advanced models based on computed tomography angiography (CTA) radiomics features in discriminating muscle ischemia from normal condition are lacking.

PURPOSE

To investigate the feasibility of radiomics of CTA in discriminating ischemic muscle from normal muscle.

MATERIAL AND METHODS

A total of 102 patients (51 ischemia and 51 non-ischemia) were analyzed using a CTA radiomics method. The radiomics features of muscle were compared between ischemic and normal cases. The maximum relevance minimum redundancy (mRMR) algorithm and least absolute shrinkage and selection operator (LASSO) logistic regression model were used. The receiver operating characteristic (ROC) curve was used to determine the performance of radiomics signature.

RESULTS

Thirty-nine CTA radiomics features were significantly different between the two groups (P < 0.05). By LASSO, six features were used to construct a model. The signature area under the curve was 0.92 and 0.91 in the training and validation cohorts, respectively. The sensitivity and specificity of the signature were 92% and 86% for the training cohort, and 80% and 94% for the validation cohort, respectively.

CONCLUSION

CTA radiomics signature is useful in identifying ischemic muscle in selected patients.

Predictive Value of Contrast-Enhanced Ultrasound in Chemical Lumbar Sympathectomy for End-Stage Arteriosclerosis Obliterans of the Lower Extremities

Introduction

The therapeutic effect of chemical lumbar sympathectomy (CLS) on ischemic diseases of the lower limbs varies greatly among individuals. The time to peak (TTP) response in contrast-enhanced ultrasound (CEUS) can reflect the perfusion disorder of the calf skeletal muscle and the collateral circulation. In this study we evaluated the predictive value of CEUS in patients treated with CLS for end-stage atherosclerotic occlusive disease of the lower extremity (ASO-LE).

Methods

This was a prospective study that included patients with end-stage ASO-LE and moderate to severe pain who had undergone a CEUS examination and CLS procedure and who were observed for 12 months after surgery. The patients’ characteristics and prognostic factors, including lower limb pain score, skin temperature, walking distance, and ulcer and gangrene healing, were recorded.

Results

Fifty-eight patients with a mean age of 66.24 (range 58–78) years were included in the study, of whom 42 (71.41%) were men. Following the CLS procedure, the numerical rating scale (NRS)-measured pain decreased significantly, and the skin temperature of the affected limb increased significantly (P < 0.05). The satisfaction rate of lower limb pain relief 1 year after operation was 53.45%. Correlation analysis showed that preoperative TTP response was correlated with the NRS score and skin temperature of the affected limb at 6 months and 12 months post surgery (P < 0.05). The binary logistic regression analysis indicated that a longer preoperative TTP response was associated with a higher risk of poor pain relief after CLS (odds ratio 1.126, 95% confidence interval 1.058–1.205). The receiver operating characteristic curve showed that preoperative TTP response had a certain predictive value on CLS treatment effect, with a sensitivity and specificity of 81.5% and 83.9%, respectively. When the preoperative TTP response was > 77.5, the risk of poor response after CLS increased.

Conclusions

Preoperative TTP response was able to predict the therapeutic effect of CLS to a certain extent, and thus may aid physicians in determining the choice of CLS treatment for patients with ASO-LE.

Trial Registration

Chinese Clinical Trial Registry: ChicTR1900028424 (principal investigator: Yue Wu; date of registration: 21 December 2019).

Supplementary Information

The online version contains supplementary material available at 10.1007/s40122-021-00347-9.

The Role of Contrast-Enhanced Ultrasound in Managing Vascular Pathologies

INTRODUCTION

Ultrasound is a useful first-line imaging modality for diagnosing and monitoring vascular pathologies. Compared with other modalities, it is relatively low cost, requires no ionizing radiation, is often available at bedside, and is noninvasive. However, the modality can have limitations when differentiating normal from pathologic tissues. In this review, we discuss the role of contrast-enhanced ultrasound (CEUS) in carotid, aortic, and peripheral vascular conditions.

DISCUSSION

CEUS is a developing modality that supersedes standard vascular ultrasound imaging and complements other modalities such as computed topography and magnetic resonance angiograms. Administered intravenously, the contrast are microbubbles filled with gas, surrounded by a stabilizing shell. They have the ability to enhance the quality of images and quantify vascular pathologies by acting as intravascular tracers of ultrasound energy. Based on these properties, CEUS has the potential to play a pivotal role in the management of vascular pathologies through its utility in detection, diagnosis, risk-stratification, follow-up, and monitoring.

CONCLUSION

Studies have suggested that CEUS is superior compared with standard ultrasound and on-par with computed topography angiograms in the detection of vascular pathologies, concluding that CEUS should be part of standardized routine practice.

Sonographic real-time imaging of tissue perfusion in a porcine haemorrhagic shock model

Injection of fluorescence-labelled microspheres (FMs) in pigs allows only the postmortem determination of organ perfusion. Colour duplex ultrasound (CDU) and contrast-enhanced ultrasound were established as techniques for real-time imaging of tissue perfusion in a porcine haemorrhagic shock model. Haemorrhagic shock was provoked in nine domestic pigs by taking at least 15% of the calculated blood volume. Ultrasound examinations were performed with a Hitachi HI VISION Ascendus. SonoVue was injected for contrast-enhanced ultrasound. Monitoring of the resistive index and time-to-peak ratio enabled quantification of tissue perfusion in vivo during the entire study, allowing real-time differentiation of animals with systemic shock versus failing shock effect. Postmortem analyses of injected FMs confirmed the sonographic in vivo results. Determination of the resistive index and time-to-peak ratio by CDU and contrast-enhanced ultrasound allowed real-time monitoring of tissue perfusion. Effects of haemorrhagic shock and therapeutic approaches related to organ perfusion can be observed live and in vivo.

Limb Perfusion During Exercise Assessed by Contrast Ultrasound Varies According to Symptom Severity in Patients with Peripheral Artery Disease

BACKGROUND

In patients with peripheral artery disease (PAD), the severity of symptoms correlates poorly with ankle-brachial index (ABI). The aim of this study was to test the hypothesis that limb perfusion assessed using contrast-enhanced ultrasound (CEU) during contractile exercise varies according to functional class in patients with PAD, particularly those with ABIs in the 0.4 to 0.6 range whose symptoms vary widely.

METHODS

Bilateral quantitative CEU perfusion imaging of the calf was performed in normal control subjects (n = 10) and patients with PAD who had at least one limb with a moderately reduced ABI (0.4-0.6; n = 17). Imaging was performed at rest and immediately after 30 sec of modest periodic (0.3-Hz) plantar flexion (10 W).

RESULTS

In patients with PAD, Rutherford symptom classification for each limb varied widely, including in limbs with ABIs of 0.4 to 0.6 (n = 6 with mild or no symptoms, n = 14 with moderate to severe symptoms). CEU perfusion imaging parameters at rest were similar between control subjects and patients with PAD irrespective of ABI. In normal control subjects, limb flow increased on average by > 20-fold after only 30 sec of moderate exercise. In patients with PAD, muscle exercise perfusion for all limbs was reduced compared with control subjects and decreased according to the severity of ABI reduction, primarily from reduced microvascular flux rate. Even limbs with ABIs > 0.9 in patients with PAD had lower exercise perfusion than in control subjects (P = .03). In subjects with PAD, exercise perfusion was lower in those with moderate to severe versus mild symptoms when analyzed for all limbs (median, 30 IU/sec [interquartile range (IQR), 21-52 IU/sec] vs 84 IU/sec [IQR, 36-177 IU/sec]; P = .01) and limbs with ABIs of 0.4 to 0.6 (median, 26 IU/sec [IQR, 14-41 IU/sec] vs 54 IU/sec [IQR, 31-105 IU/sec]; P = .05).

CONCLUSIONS

In patients with PAD, CEU exercise perfusion imaging detects differences in limb muscle perfusion that are likely to be responsible for differences in symptom severity and can detect the flow abnormalities from microvascular dysfunction even in limbs with normal ABIs.

Contrast Enhanced Ultrasound Perfusion Imaging in Skeletal Muscle

The ability to accurately evaluate skeletal muscle microvascular blood flow has broad clinical applications for understanding the regulation of skeletal muscle perfusion in health and disease states. Contrast-enhanced ultrasound (CEU) perfusion imaging, a technique originally developed to evaluate myocardial perfusion, is one of many techniques that have been applied to evaluate skeletal muscle perfusion. Among the advantages of CEU perfusion imaging of skeletal muscle is that it is rapid, safe and performed with equipment already present in most vascular medicine laboratories. The aim of this review is to discuss the use of CEU perfusion imaging in skeletal muscle. This article provides details of the protocols for CEU imaging in skeletal muscle, including two predominant methods for bolus and continuous infusion destruction-replenishment techniques. The importance of stress perfusion imaging will be highlighted, including a discussion of the methods used to produce hyperemic skeletal muscle blood flow. A broad overview of the disease states that have been studied in humans using CEU perfusion imaging of skeletal muscle will be presented including: (1) peripheral arterial disease; (2) sickle cell disease; (3) diabetes; and (4) heart failure. Finally, future applications of CEU imaging in skeletal muscle including therapeutic CEU imaging will be discussed along with technological developments needed to advance the field.

Oxygen availability and skeletal muscle oxidative capacity in patients with peripheral artery disease: implications from in vivo and in vitro assessments

Evidence suggests that the peak skeletal muscle mitochondrial ATP synthesis rate ( V_max) in patients with peripheral artery disease (PAD) may be attenuated due to disease-related impairments in O₂ supply. However, in vitro assessments suggest intrinsic deficits in mitochondrial respiration despite ample O₂ availability. To address this conundrum, Doppler ultrasound, near-infrared spectroscopy, phosphorus magnetic resonance spectroscopy, and high-resolution respirometry were combined to assess convective O₂ delivery, tissue oxygenation, V_max, and skeletal muscle mitochondrial capacity (complex I + II, state 3 respiration), respectively, in the gastrocnemius muscle of 10 patients with early stage PAD and 11 physical activity-matched healthy control (HC) subjects. All participants were studied in free-flow control conditions (FF) and with reactive hyperemia (RH) induced by a period of brief ischemia during the last 30 s of submaximal plantar flexion exercise. Patients with PAD repeated the FF and RH trials under hyperoxic conditions (FF + 100% O₂ and RH + 100% O₂). Compared with HC subjects, patients with PAD exhibited attenuated O₂ delivery at the same absolute work rate and attenuated tissue reoxygenation and V_max after relative intensity-matched exercise. Compared with the FF condition, only RH + 100% O₂ significantly increased convective O₂ delivery (~44%), tissue reoxygenation (~54%), and V_max (~60%) in patients with PAD ( P < 0.05), such that V_max was now not different from HC subjects. Furthermore, there was no evidence of an intrinsic mitochondrial deficit in PAD, as assessed in vitro with adequate O₂. Thus, in combination, this comprehensive in vivo and in vitro investigation implicates O₂ supply as the predominant factor limiting mitochondrial oxidative capacity in early stage PAD. NEW & NOTEWORTHY Currently, there is little accord as to the role of O₂ availability and mitochondrial function in the skeletal muscle dysfunction associated with peripheral artery disease. This is the first study to comprehensively use both in vivo and in vitro approaches to document that the skeletal muscle dysfunction associated with early stage peripheral artery disease is predominantly a consequence of limited O₂ supply and not the impact of an intrinsic mitochondrial defect in this pathology.

Functional Assessment of Intermediate Vascular Disease

Interventional treatment in various vascular beds has advanced tremendously. However, there are several problems to be considered. We searched the literature and tried to analyze major parts of it. One is safety and applicability of coronary proven methods in other vascular beds. An unresolved problem is the functional assessment of intermediate lesions, as far as various target organs have quite different circulation from the coronary one and the functional tests should be modified in order to be applicable and meaningful. In the majority of the acute vascular syndromes, the culprit lesion is of intermediate size on visual assessment. On the other hand, a procedurally successfully managed high-degree stenosis is not always followed by clinical and prognostic benefit. In vascular beds, where collateral network naturally exists, the readings from the functional assessment are complicated and thus the decision for interventional treatment is even more difficult. Here come into help the functional assessment and imaging with IVUS, OCT, high-resolution MRI, and contrast enhanced CT or SPECT. The focus of the current review is on the functional assessment of intermediate stenosis in other vascular beds, unlike the coronary arteries.

Exercise versus vasodilator stress limb perfusion imaging for the assessment of peripheral artery disease

PURPOSE

Our aim was to determine whether pharmacologic vasodilation is an alternative to exercise stress during limb perfusion imaging for peripheral artery disease (PAD).

METHODS

Quantitative contrast-enhanced ultrasound (CEU) perfusion imaging of the bilateral anterior thigh and calf was performed in nine control subjects and nine patients with moderate to severe PAD at rest and during vasodilator stress with dipyridamole. For those who were able, CEU of the calf was then performed during modest plantar flexion exercise (20 watts). CEU time-intensity data were analyzed to quantify microvascular blood flow (MBF) and its parametric components of microvascular blood volume and flux rate.

RESULTS

Thigh and calf skeletal muscle MBF at rest was similar between control and PAD patients. During dipyridamole, MBF increased minimally (<twofold) for all groups and there were only nonsignificant trends for a reduction in calf MBF in those with PAD (13.5±6.9, 10.0±4.7, and 8.2±6.1 IU/s, for controls, moderate, and severe PAD, respectively; P=.11). In contrast, MBF during modest planar flexion exercise increased markedly in controls but not PAD patients (87.9±79.9 vs 15.2±12.9 IU/s, P<.05). In three moderate PAD patients restudied after undergoing surgical revascularization, MBF during dipyridamole did not change, whereas exercise MBF increased by an average of sevenfold.

CONCLUSIONS

Resting limb skeletal muscle MBF in patients with moderate to severe PAD is similar to that in normal subjects. However, differences in hyperemic flow during contractile exercise but not during dipyridamole allow evaluation of the degree of flow impairment from PAD and the degree of improvement with revascularization.

Rest-Stress Limb Perfusion Imaging in Humans with Contrast Ultrasound Using Intermediate-Power Imaging and Microbubbles Resistant to Inertial Cavitation

BACKGROUND

Contrast-enhanced ultrasound (CEU) limb perfusion imaging is a promising approach for evaluating peripheral artery disease (PAD). However, low signal enhancement in skeletal muscle has necessitated high-power intermittent imaging algorithms, which are not clinically feasible. We hypothesized that CEU using a combination of intermediate power and a contrast agent resistant to inertial cavitation would allow real-time limb stress perfusion imaging.

METHODS

In normal volunteers, CEU of the calf skeletal muscle was performed on separate days with Sonazoid, Optison, or Definity. Progressive reduction in the ultrasound pulsing interval was used to assess the balance between signal enhancement and agent destruction at escalating mechanical indices (MI, 0.1-0.4). Real-time perfusion imaging at MI 0.1-0.4 using postdestructive replenishment kinetics was performed at rest and during 25 W plantar flexion contractile exercise.

RESULTS

For Optison, limb perfusion imaging was unreliable at rest due to very low signal enhancement generated at all MIs and was possible during exercise-induced hyperemia only at MI 0.1 due to agent destruction at higher MIs. For Definity, signal intensity progressively increased with MI but was offset by microbubble destruction, which resulted in modest signal enhancement during CEU perfusion imaging and distortion of replenishment curves at MI ≥ 0.2. For Sonazoid, there strong signal enhancement at MI ≥ 0.2, with little destruction detected only at MI 0.4. Accordingly, high signal intensity and nondistorted perfusion imaging was possible at MI 0.2-0.3 and detected an 8.0- ± 5.7-fold flow reserve.

CONCLUSIONS

Rest-stress limb perfusion imaging in humans with real-time CEU, which requires only seconds to perform, is possible using microbubbles with viscoelastic properties that produce strong nonlinear signal generation without destruction at intermediate acoustic pressures.

Mitochondrial Bioenergetics in the Metabolic Myopathy Accompanying Peripheral Artery Disease

Peripheral artery disease (PAD) is a serious but relatively underdiagnosed and undertreated clinical condition associated with a marked reduction in functional capacity and a heightened risk of morbidity and mortality. The pathophysiology of lower extremity PAD is complex, and extends beyond the atherosclerotic arterial occlusion and subsequent mismatch between oxygen demand and delivery to skeletal muscle mitochondria. In this review, we evaluate and summarize the available evidence implicating mitochondria in the metabolic myopathy that accompanies PAD. Following a short discussion of the available in vivo and in vitro methodologies to quantitate indices of muscle mitochondrial function, we review the current evidence implicating skeletal muscle mitochondrial dysfunction in the pathophysiology of PAD myopathy, while attempting to highlight questions that remain unanswered. Given the rising prevalence of PAD, the detriment in quality of life for patients, and the associated significant healthcare resource utilization, new alternate therapies that ameliorate lower limb symptoms and the functional impairment associated with PAD are needed. A clear understanding of the role of mitochondria in the pathophysiology of PAD may contribute to the development of novel therapeutic interventions.

Validation of pressure gradient and peripheral fractional flow reserve measured by a pressure wire for diagnosis of iliofemoral artery disease with intermediate stenosis

Objective

To examine the pressure gradient and peripheral fractional flow reserve (pFFR) measured by a pressure wire as indicators of hemodynamic significance in iliofemoral angiographic intermediate stenosis.

Background

The utility of pressure measurements using a pressure wire with vasodilators is unclear in cases with intermediate iliofemoral stenosis.

Methods

The mean pressure gradient (MPG) and mean pressure ratio (MPR) were measured at baseline and after injection of isosorbide dinitrate in 23 lesions with angiographically intermediate iliofemoral stenosis. Patients with complex lesions, infrapopliteal artery lesions, chronic total occlusion, and surgical bypass grafts were excluded. Hyperemic MPR was considered equivalent to pFFR. Changes in parameters in response to vasodilators were assessed and correlations of peak systolic velocity ratio (PSVR) with hyperemic MPG and pFFR were examined using duplex ultrasound.

Results

After injection of isosorbide dinitrate, hyperemic MPG increased significantly (from 9.0±5.7 to 16.3±6.2 mmHg; P<0.05) and hyperemic MPR (pFFR) decreased significantly (from 0.92±0.06 to 0.81±0.07; P<0.05). PSVR was significantly correlated with hyperemic MPG (R=0.52; P<0.05) and pFFR (R=−0.50; P<0.05). The optimal cut-off value of pFFR as an indicator of significant hemodynamic stenosis (PSVR >2.5) was 0.85 (area under the curve 0.72; sensitivity 94%; specificity 50%, P<0.05).

Conclusion

pFFR measured using a pressure wire is reliable for prediction of hemodynamic significance in iliofemoral intermediate stenosis.

Pathogenesis of the limb manifestations and exercise limitations in peripheral artery disease

Patients with peripheral artery disease have a marked reduction in exercise performance and daily ambulatory activity irrespective of their limb symptoms of classic or atypical claudication. This review will evaluate the multiple pathophysiologic mechanisms underlying the exercise impairment in peripheral artery disease based on an evaluation of the current literature and research performed by the authors. Peripheral artery disease results in atherosclerotic obstructions in the major conduit arteries supplying the lower extremities. This arterial disease process impairs the supply of oxygen and metabolic substrates needed to match the metabolic demand generated by active skeletal muscle during walking exercise. However, the hemodynamic impairment associated with the occlusive disease process does not fully account for the reduced exercise impairment, indicating that additional pathophysiologic mechanisms contribute to the limb manifestations. These mechanisms include a cascade of pathophysiological responses during exercise-induced ischemia and reperfusion at rest that are associated with endothelial dysfunction, oxidant stress, inflammation, and muscle metabolic abnormalities that provide opportunities for targeted therapeutic interventions to address the complex pathophysiology of the exercise impairment in peripheral artery disease.

Real-time contrast ultrasound muscle perfusion imaging with intermediate-power imaging coupled with acoustically durable microbubbles

BACKGROUND

There is growing interest in limb contrast-enhanced ultrasound (CEU) perfusion imaging for the evaluation of peripheral artery disease. Because of low resting microvascular blood flow in skeletal muscle, signal enhancement during limb CEU is prohibitively low for real-time imaging. The aim of this study was to test the hypothesis that this obstacle can be overcome by intermediate- rather than low-power CEU when performed with an acoustically resilient microbubble agent.

METHODS

Viscoelastic properties of Definity and Sonazoid were assessed by measuring bulk modulus during incremental increases in ambient pressure to 200 mm Hg. Comparison of in vivo microbubble destruction and signal enhancement at a mechanical index (MI) of 0.1 to 0.4 was performed by sequential reduction in pulsing interval from 10 to 0.05 sec during limb CEU at 7 MHz in mice and 1.8 MHz in dogs. Destruction was also assessed by broadband signal generation during passive cavitation detection. Real-time CEU perfusion imaging with destruction-replenishment was then performed at 1.8 MHz in dogs using an MI of 0.1, 0.2, or 0.3.

RESULTS

Sonazoid had a higher bulk modulus than Definity (66 ± 12 vs 29 ± 2 kPa, P = .02) and exhibited less inertial cavitation (destruction) at MIs ≥ 0.2. On in vivo CEU, maximal signal intensity increased incrementally with MI for both agents and was equivalent between agents except at an MI of 0.1 (60% and 85% lower for Sonazoid at 7 and 1.8 MHz, respectively, P < .05). However, on progressive shortening of the pulsing interval, Definity was nearly completely destroyed at MIs ≥ 0.2 at 1.8 and 7 MHz, whereas Sonazoid was destroyed only at 1.8 MHz at MIs ≥ 0.3. As a result, real-time CEU perfusion imaging demonstrated approximately fourfold greater enhancement for Sonazoid at an MI of 0.3 to 0.4.

CONCLUSIONS

Robust signal enhancement during real-time CEU perfusion imaging of the limb is possible when using intermediate-power imaging coupled with a durable microbubble contrast agent.

Assessment of peripheral skeletal muscle microperfusion in a porcine model of peripheral arterial stenosis by steady-state contrast-enhanced ultrasound and Doppler flow measurement

OBJECTIVE

Noninvasive measurement of peripheral muscle microperfusion could potentially improve diagnosis, management, and treatment of peripheral arterial disease (PAD) and thus improve patient care. Contrast-enhanced ultrasound (CEUS) as a noninvasive diagnostic tool allows quantification of muscle perfusion. Increasing data on bolus technique CEUS reflecting microperfusion are becoming available, but only limited data on steady-state CEUS for assessment of muscle microperfusion are available. Therefore, the aim of this study was to evaluate steady-state CEUS for assessment of peripheral muscle microperfusion in a PAD animal model.

METHODS

In a porcine animal model, peripheral muscle microperfusion was quantified by steady-state CEUS replenishment kinetics (mean transit time [mTT] and wash-in rate [WiR]) of the biceps femoris muscle during intravenous steady-state infusion of INN-sulfur hexafluoride (SonoVue; Bracco, Geneva, Switzerland). In addition, macroperfusion was quantified at the external femoral artery with a Doppler flow probe. Peripheral muscle microperfusion and Doppler flow measurements were performed bilaterally at rest and under adenosine stress (70 μg/kg body weight) before and after unilateral creation of a moderate external iliac artery stenosis.

RESULTS

All measurements could be performed completely in 10 pigs. Compared with baseline measurements, peripheral muscle microperfusion decreased significantly during adenosine stress (rest vs adenosine stress: mTT, 7.8 ± 3.3 vs 21.2 ± 17.8 s, P = .0006; WiR, 58.4 ± 38.1 vs 25.3 ± 15.6 arbitrary units [a.u.]/s, P < .0001; Doppler flow, 122.3 ± 31.4 vs 83.6 ± 28.1 mL/min, P = .0067) and after stenosis creation (no stenosis vs stenosis: mTT, 8.1 ± 3.1 vs 29.2 ± 18.0 s, P = .0469; WiR, 53.0 ± 22.7 vs 13.6 ± 8.4 a.u./s, P = .0156; Doppler flow, 124.2 ± 41.8 vs 65.9 ± 40.0 mL/min, P = .0313). After stenosis creation, adenosine stress led to a further significant decrease of peripheral muscle microperfusion but had no effect on macroperfusion (mTT, 29.2 ± 18.0 vs 56.3 ± 38.7 s, P = .0078; WiR, 13.6 ± 8.4 vs 6.0 ± 4.1 a.u./s, P = .0078; Doppler flow, 65.9 ± 40.0 vs 79.2 ± 29.6 mL/min, P = .8125). Receiver operating characteristic curves for the presence of inflow stenosis showed an excellent area under the curve of 0.93 for mTT at rest and 0.86 for Doppler flow.

CONCLUSIONS

Peripheral muscle microperfusion measurement by steady-state CEUS with replenishment kinetics is feasible and allows detection of muscle microperfusion changes caused by vasodilative stress alone or in combination with a moderate inflow stenosis. Steady-state CEUS offers superior diagnostic performance compared with Doppler flow measurements. Therefore, steady-state CEUS may prove to be a useful tool in diagnosis of PAD and for evaluation of new therapies.

Effect of saline injection mixing on accuracy of conductance lumen sizing of peripheral vessels

Transient displacement of blood in vessel lumen with saline injection is necessary in the conductance method for measurement of arterial cross-sectional area (CSA). The displacement of blood is dictated by the interactions between arterial flow hemodynamics and saline injection dynamics. The objective of the present study is to understand how the accuracy of conductance measurements is affected by the saline injection. Computational simulations were performed to assess the error in predictions of arterial CSA using conductance measurements over a range of peripheral artery diameters (i.e., 4, 7, and 10 mm) with an introducing catheter (6 Fr.) for various blood flow and saline injection rates. The simulation results were validated using the conductance measurements of the phantoms with known diameters (i.e., 7 and 10 mm). The results demonstrated that a minimum ratio of saline injection rate to blood flow rate of 3 is needed to fully displace the blood and result in accurate measurement of CSA for the peripheral artery sizes considered. Furthermore, the error was shown to be minimized as the detection electrodes are positioned between the distal to the mixing zone induced by saline injection and far downstream (4–8 cm from the injection catheter tip). The present study shows that even for the large peripheral arteries (7–10 mm) where mixing can occur, an appropriate injection rate and detection position can produce accurate measurement of lumen size.

Combined measurement of perfusion, venous oxygen saturation, and skeletal muscle T2* during reactive hyperemia in the leg

BACKGROUND

The function of the peripheral microvascular may be interrogated by measuring perfusion, tissue oxygen concentration, or venous oxygen saturation (SvO2) recovery dynamics following induced ischemia. The purpose of this work is to develop and evaluate a magnetic resonance (MR) technique for simultaneous measurement of perfusion, SvO2, and skeletal muscle T2*.

METHODS

Perfusion, Intravascular Venous Oxygen saturation, and T2* (PIVOT) is comprised of interleaved pulsed arterial spin labeling (PASL) and multi-echo gradient-recalled echo (GRE) sequences. During the PASL post-labeling delay, images are acquired with a multi-echo GRE to quantify SvO2 and T2* at a downstream slice location. Thus time-courses of perfusion, SvO2, and T2* are quantified simultaneously within a single scan. The new sequence was compared to separately measured PASL or multi-echo GRE data during reactive hyperemia in five young healthy subjects. To explore the impairment present in peripheral artery disease patients, five patients were evaluated with PIVOT.

RESULTS

Comparison of PIVOT-derived data to the standard techniques shows that there was no significant bias in any of the time-course-derived metrics. Preliminary data show that PAD patients exhibited alterations in perfusion, SvO2, and T2* time-courses compared to young healthy subjects.

CONCLUSION

Simultaneous quantification of perfusion, SvO2, and T2* is possible with PIVOT. Kinetics of perfusion, SvO2, and T2* during reactive hyperemia may help to provide insight into the function of the peripheral microvasculature in patients with PAD.

Reproducibility of rest and exercise stress contrast-enhanced calf perfusion magnetic resonance imaging in peripheral arterial disease

BACKGROUND

The purpose was to determine the reproducibility and utility of rest, exercise, and perfusion reserve (PR) measures by contrast-enhanced (CE) calf perfusion magnetic resonance imaging (MRI) of the calf in normal subjects (NL) and patients with peripheral arterial disease (PAD).

METHODS

Eleven PAD patients with claudication (ankle-brachial index 0.67 ±0.14) and 16 age-matched NL underwent symptom-limited CE-MRI using a pedal ergometer. Tissue perfusion and arterial input were measured at rest and peak exercise after injection of 0.1 mM/kg of gadolinium-diethylnetriamine pentaacetic acid (Gd-DTPA). Tissue function (TF) and arterial input function (AIF) measurements were made from the slope of time-intensity curves in muscle and artery, respectively, and normalized to proton density signal to correct for coil inhomogeneity. Perfusion index (PI) = TF/AIF. Perfusion reserve (PR) = exercise TF/ rest TF. Intraclass correlation coefficient (ICC) was calculated from 11 NL and 10 PAD with repeated MRI on a different day.

RESULTS

Resting TF was low in NL and PAD (mean ± SD 0.25 ± 0.18 vs 0.35 ± 0.71, p = 0.59) but reproducible (ICC 0.76). Exercise TF was higher in NL than PAD (5.5 ± 3.2 vs. 3.4 ± 1.6, p = 0.04). Perfusion reserve was similar between groups and highly variable (28.6 ± 19.8 vs. 42.6 ± 41.0, p = 0.26). Exercise TF and PI were reproducible measures (ICC 0.63 and 0.60, respectively).

CONCLUSION

Although rest measures are reproducible, they are quite low, do not distinguish NL from PAD, and lead to variability in perfusion reserve measures. Exercise TF and PI are the most reproducible MRI perfusion measures in PAD for use in clinical trials.

MRI of arterial flow reserve in patients with intermittent claudication: feasibility and initial experience

Objectives

The aim of this work was to develop a MRI method to determine arterial flow reserve in patients with intermittent claudication and to investigate whether this method can discriminate between patients and healthy control subjects.

Methods

Ten consecutive patients with intermittent claudication and 10 healthy control subjects were included. All subjects underwent vector cardiography triggered quantitative 2D cine MR phase-contrast imaging to obtain flow waveforms of the popliteal artery at rest and during reactive hyperemia. Resting flow, maximum hyperemic flow and absolute flow reserve were determined and compared between the two groups by two independent MRI readers. Also, interreader reproducibility of flow measures was reported.

Results

Resting flow was lower in patients compared to controls (4.9±1.6 and 11.1±3.2 mL/s in patients and controls, respectively (p<0.01)). Maximum hyperemic flow was 7.3±2.9 and 16.4±3.2 mL/s (p<0.01) and the absolute flow reserve was 2.4±1.6 and 5.3±1.3 mL/s (p<0.01), respectively in patients and controls. The interreader coefficient of variation was below 10% for all measures in both patients and controls.

Conclusions

Quantitative 2D MR cine phase-contrast imaging is a promising method to determine flow reserve measures in patients with peripheral arterial disease and can be helpful to discriminate patients with intermittent claudication from healthy controls.

Ischemia-modified albumin in type 2 diabetic patients with and without peripheral arterial disease

OBJECTIVE:

To determine whether there is an association between serum ischemia-modified albumin and the risk factor profile in type 2 diabetic patients with peripheral arterial disease and to identify the risk markers for peripheral arterial disease.

METHODS:

Participants included 290 patients (35.2% women) with type 2 diabetes. The ankle-brachial pressure index was measured using a standard protocol, and peripheral arterial disease was defined as an ankle-brachial index <0.90 or ≥1.3. The basal ischemia-modified albumin levels and clinical parameters were measured and analyzed. The risk factors for peripheral arterial disease were examined by multiple logistic analyses.

RESULTS:

Age, systolic blood pressure, total cholesterol, low-density lipoprotein cholesterol, urine albumin, homocysteine, and ischemia-modified albumin were significantly higher in patients with peripheral arterial disease than in disease-free patients (p<0.05), while ankle-brachial index was lower in the former group (p<0.05). Ischemia-modified albumin was positively associated with HbA1c and homocysteine levels (r = 0.220, p = 0.030; r = 0.446, p = 0.044, respectively), while no correlation was found with ankle-brachial index. Multiple logistic analyses indicated that HbA1c, systolic blood pressure, homocysteine and ischemia-modified albumin were independent risk factors for peripheral arterial disease in the diabetic subjects.

CONCLUSION:

The baseline ischemia-modified albumin levels were significantly higher and positively associated with HbA1c and homocysteine levels in type 2 diabetic patients with peripheral arterial disease. Ischemia-modified albumin was a risk marker for peripheral arterial disease. Taken together, these results might be helpful for monitoring diabetic peripheral arterial disease.

Magnetic resonance imaging in peripheral arterial disease: reproducibility of the assessment of morphological and functional vascular status

OBJECTIVES

The aim of the current study was to test the reproducibility of different quantitative magnetic resonance imaging (MRI) methods to assess the morphologic and functional peripheral vascular status and vascular adaptations over time in patients with peripheral arterial disease (PAD).

MATERIALS AND METHODS

Ten patients with proven PAD (intermittent claudication) and arterial collateral formation within the upper leg and 10 healthy volunteers were included. All subjects underwent 2 identical MR examinations of the lower extremities on a clinical 1.5-T MR system, with a time interval of at least 3 days. The MR protocol consisted of 3D contrast-enhanced MR angiography to quantify the number of arteries and artery diameters of the upper leg, 2D cine MR phase contrast angiography flow measurements in the popliteal artery, dynamic contrast-enhanced (DCE) perfusion imaging to determine the influx constant and area under the curve, and dynamic blood oxygen level-dependent (BOLD) imaging in calf muscle to measure maximal relative T2* changes and time-to-peak. Data were analyzed by 2 independent MRI readers. Interscan and inter-reader reproducibility were determined as outcome measures and expressed as the coefficient of variation (CV).

RESULTS

Quantification of the number of arteries, artery diameter, and blood flow proved highly reproducible in patients (CV = 2.6%, 4.5%, and 15.8% at interscan level and 9.0%, 8.2%, and 7.0% at interreader level, respectively). Reproducibility of DCE and BOLD MRI was poor in patients with a CV up to 50.9%.

CONCLUSIONS

Quantification of the morphologic vascular status by contrast-enhanced MR angiography, as well as phase contrast angiography MRI to assess macrovascular blood flow proved highly reproducible in both PAD patients and healthy volunteers and might therefore be helpful in studying the development of collateral arteries in PAD patients and in unraveling the mechanisms underlying this process. Functional assessment of the microvascular status using DCE and BOLD, MRI did not prove reproducible at 1.5 T and is therefore currently not suitable for (clinical) application in PAD.

The assessment of microvascular flow and tissue perfusion using ultrasound imaging

Imaging microvascular flow is of diagnostic value for a wide range of diseases including cancer, inflammation, and cardiovascular disease. The introduction of microbubbles as ultrasound contrast agents offers significant signal enhancement to the otherwise weakly scattered signal from blood in the circulation. Microbubbles provide maximum impedance mismatch, but are not linear scatterers. Their complex response to ultrasound has generated research on both their behaviour and their scattered-signal processing. Nearly 20 years ago signal processing started with simple spectral filtering of harmonics showing contrast-enhanced images. More recent pulse encoding techniques have achieved good cancellation of tissue echoes. The good quality contrast-only images enabled ultrasound contrast-imaging applications to be established in microvascular measurements in the liver and the myocardium. The field promises to advance the quantification of microvascular flow kinetics.

Skeletal muscle microvascular flow in progressive peripheral artery disease: assessment with continuous arterial spin-labeling perfusion magnetic resonance imaging

OBJECTIVES

We present the novel application of continuous arterial spin-labeling (CASL) magnetic resonance imaging (MRI) for the measurement of calf muscle perfusion in subjects with progressive peripheral arterial disease (PAD).

BACKGROUND

Peripheral arterial disease is largely considered to be a disease of conduit vessels. The impact of PAD upon microvascular flow in the end-organ, muscle, remains unknown. Continuous arterial spin-labeling is a noninvasive MRI method capable of measuring microvascular flow and might assist in our understanding of the impact of PAD upon the microvasculature.

METHODS

Forty subjects with varying degrees of PAD and 17 age-matched PAD-free subjects were recruited and underwent measurement of the ankle-to-brachial index (ABI) and CASL. Peak hyperemic flow (PHF) and time-to-peak (TTP) were computed and assessed as a function of ABI and calf muscle group.

RESULTS

An ABI dependence was found in both PHF (p = 0.04) and TTP (p < 10(-4)). Whereas TTP increased almost immediately with increasing PAD severity, PHF was, in contrast, relatively well preserved until later stages of disease.

CONCLUSIONS

The CASL flow measurements correlate with disease state as measured by ABI and demonstrate preserved microvascular flow reserve in the presence of early to intermediate vascular disease.

Contrast sonography enables noninvasive and quantitative assessment of neovascularization after stem cell transplantation

Stem cell transplantation is one of the attractive therapeutic strategies for the treatment of hindlimb ischemia. However, few studies have quantitatively assessed perfusion noninvasively in deep tissues after cell transplantation. In this study, we examined the feasibility of contrast sonography for the assessment of perfusion after bone marrow-derived mesenchymal stem cell (MSC) transplantation by using a rat unilateral hindlimb ischemia model. The quantitative parameters derived from contrast sonography were compared with the colored microspheres-derived blood flow and the capillary density. Nine rats were assigned each to a control (saline injection) or a treated (MSC transplantation) group. Video intensity vs. pulsing interval plots were acquired with ultraharmonic imaging of SONOS5500 during IV infusion of Levovist. The left-to-right ratio of hindlimb blood volume (A-ratio), microbubble velocity (beta-ratio) and hindlimb blood flow (Abeta-ratio) were calculated. The MS-ratio, the ratio of the left to the right hindlimb blood flow determined using colored microspheres, was also calculated. Although A-ratio did not change, beta- and Abeta-ratio in the treated group were significantly higher than those in the control group. In addition, MS-ratio and capillary density in the treated group were significantly higher than those in the control group. Compared with A- and Abeta-ratio, beta-ratio had the highest correlation with MS-ratio and capillary density (vs. MS-ratio: r = 0.66, p < 0.01; vs. capillary density: r = 0.52, p < 0.05). The results of our study imply that the contrast sonography-derived beta-ratio is a useful parameter that reflects the perfusion after cell transplantation in ischemic hindlimb.

Simplified contrast ultrasound accurately reveals muscle perfusion deficits and reflects collateralization in PAD

BACKGROUND

Simplified contrast-enhanced ultrasound (CEUS) can be used to evaluate muscle perfusion in peripheral arterial disease (PAD). Here, we report its diagnostic accuracy for detecting symptomatic PAD. Additionally, we hypothesize that the extent of collateral formation is reflected by CEUS.

METHODS

Ultrasound contrast agent was injected into an antecubital vein of 58 control subjects and 52 symptomatic PAD patients and its appearance in the calf muscle was evaluated. Interreader variability was tested using 118 raw data films. Arterial collateralization of PAD patients was assessed by angiographic imaging.

RESULTS

PAD patients showed a significantly longer median time to peak intensity (TTP, 36.9s) than control subjects (19.4s, p<0.001) with longer TTPs in advanced PAD stages. The area under the receiver operating characteristic curve was 0.942 and the mean TTP difference between two blinded readers was 0.28s. A TTP cut off at 30.5s was associated with 91% positive predictive value. PAD patients with good collateralization showed a significantly shorter TTP (34.1s) than patients with poor collateralization (44.0 s, p=0.008) but not a higher ankle-brachial index (ABI).

CONCLUSIONS

CEUS accurately displays perfusion deficits of the calf muscle in symptomatic PAD patients. The degree of arterial collateralization is reflected by CEUS and not by ABI.

Recent advances in hemodynamics: noncoronary applications of a pressure sensor angioplasty guidewire

The use of the pressure sensor coronary guidewire is expanding into the peripheral circulation as well as into the realm of valvular heart disease. Small mechanistic studies and case reports have described the use of pressure wire technology in the renal and femoral arteries as well as in mechanical aortic valves. The use of this technology to measure hemodynamically significant stenoses in noncoronary locations will be discussed and a review of basic and more advanced hemodynamics in relation to problems encountered in clinical practice will be provided.

Limb stress-rest perfusion imaging with contrast ultrasound for the assessment of peripheral arterial disease severity

OBJECTIVES

We hypothesized that stress-rest perfusion imaging of skeletal muscle in the lower extremity with contrast-enhanced ultrasound (CEU) could evaluate the severity of peripheral arterial disease (PAD).

BACKGROUND

Perfusion imaging may provide valuable quantitative information on PAD, particularly in patients with diabetes in whom microvascular functional abnormalities are common.

METHODS

Study subjects included 26 control subjects and 39 patients with symptomatic PAD, 19 of whom had type 2 diabetes mellitus. A modified treadmill exercise test was performed to determine exercise time to development of claudication. Multilevel pulse-volume recordings and ankle-brachial index (ABI) at rest and post-exercise ABI were measured in both extremities. Microvascular blood flow in the gastrocnemius and soleus muscles was measured at rest and after 2 min of calibrated plantar-flexion exercise.

RESULTS

During exercise, claudication did not occur in normal subjects and occurred earlier in PAD patients with diabetes than without (median time 1.2 min [95% confidence interval (CI) 0.6 to 2.5] vs. 3.0 min [95% CI 2.1 to 6.0], p < 0.01). Compared to control subjects, patients with PAD had lower skeletal muscle blood flow during plantar-flexion exercise and lower flow reserve on CEU. After adjusting for diabetes, the only diagnostic tests that predicted severity of disease by claudication threshold were CEU exercise blood flow and flow reserve (odds ratios 0.67 [95% CI 0.51 to 0.88; p = 0.003] and 0.64 [95% CI 0.46 to 0.89, p = 0.008], respectively). A quasi-likelihood information analysis incorporating all non-invasive diagnostic tests indicated that the best models for predicting severity of disease were the combination of diabetes and either exercise blood flow or flow-reserve on CEU.

CONCLUSIONS

Perfusion imaging of limb skeletal during exercise and measurement of absolute flow reserve can provide valuable information on the severity PAD. This strategy may be useful for evaluating the total impact of disease in patients with complex disease or those with coexisting functional abnormalities of flow regulation.

Hyperemic flow heterogeneity within the calf, foot, and forearm measured with continuous arterial spin labeling MRI

Arterial spin labeling (ASL) is a noninvasive magnetic resonance imaging (MRI) technique for microvascular blood flow measurement. We used a continuous ASL scheme (CASL) to investigate the hyperemic flow difference between major muscle groups in human extremities. Twenty-four healthy subjects with no evidence of vascular disease were recruited. MRI was conducted on a 3.0 Tesla Siemens Trio whole body system with a transmit/receive knee coil. A nonmagnetic orthopedic tourniquet system was used to create a 5-min period of ischemia followed by a period of hyperemic flow (occlusion pressure = 250 mmHg). CASL imaging, lasting from 2 min before cuff inflation to 3 min after cuff deflation, was performed on the midcalf, midfoot, and midforearm in separate sessions from which blood flow was quantified with an effective temporal resolution of 16 s. When muscles in the same anatomic location were compared, hyperemic flow was found to be significantly higher in the compartments containing muscles known to have relatively higher slow-twitch type I fiber compositions, such as the soleus muscle in the calf and the extensors in the forearm. In the foot, the plantar flexors exhibited a slightly delayed hyperemic response relative to that of the dorsal compartment, but no between-group flow difference was observed. These results demonstrate that CASL is sensitive to flow heterogeneity between diverse muscle groups and that nonuniform hyperemic flow patterns following an ischemic paradigm correlate with relative fiber-type predominance.

Assessment of Metabolism and Microcirculation of Healthy Skeletal Muscles by Magnetic Resonance and Ultrasound Techniques

PURPOSE

To assess metabolism and microcirculation of healthy skeletal muscle by magnetic resonance (MR) and ultrasound techniques and to compare these data with muscle histology, and anthropometric and blood parameters.

METHODS

Thirty-four healthy volunteers were selected such that their measured aerobic capacity (VO2max) per body weight ranged between 23 and 66 mL/minute/kg to render a large variability of skeletal muscle capillarization as a result of their different physical activity. We analyzed body composition, blood parameters, and skeletal muscle fiber size and capillarization in biopsies of the vastus lateralis muscle. These data were compared with knee extensor cross-sectional area (CSA) obtained by MR imaging, microcirculation of the vastus lateralis muscle by contrast-enhanced ultrasound (CEUS), and its energy and lipid metabolism measured with 31P and 1H MR spectroscopy. Statistical analysis was performed using Pearson's correlation coefficient and significance was tested at a level of .5%.

RESULTS

The variable physical activity was reflected in a large variability of vastus lateralis muscle perfusion and metabolism at rest with highest histologic capillarization and CEUS-perfusion values observed in the best-trained volunteers. Levels of high-energy phosphates, such as phosphocreatine, were positively correlated with CSA (r= .5) and histologic fiber size (r= .6 for type IIA and IIX fibers), while phosphocreatine concentration was significantly negatively correlated to myocellular lipids (r=-.6) and trimethyl ammonium containing compounds (r=-.8). Local blood volume measured in vivo with CEUS was positively correlated with several histologic capillarization parameters.

CONCLUSIONS

Dedicated MR- and CEUS-methods deliver (patho-)physiologic information about capillarization and fiber characteristics of skeletal muscles in vivo and hence establish a useful diagnostic tool for muscular diseases.

Assessment of ischemia-induced microvascular remodeling using contrast-enhanced ultrasound vascular anatomic mapping

Our aim was to apply novel contrast-enhanced ultrasound (CEU) techniques to characterize remodeling in different vascular compartments during ischemia-mediated angiogenesis. Hind limb ischemia was produced by ligation of an external iliac artery in 60 rats, half of which were treated with intramuscular fibroblast growth factor (FGF)-2 (5 microg). The proximal adductor muscles of the ischemic and control hind limb were studied immediately after ligation and at days 4, 7, or 14. Low-power maximum intensity projection imaging was performed to assess large intramuscular vessels to the fourth branch order. CEU data were analyzed to measure capillary perfusion and functional noncapillary microvascular blood volume. Resting capillary perfusion was reduced by 30% after arterial ligation and recovered earlier in FGF-2-treated versus nontreated rats (day 4 vs. 14). Changes in perfusion were temporally related to expansion of noncapillary microvascular blood volume on CEU, which was associated with an arteriogenic response on histology. Expansion of and organization (fractal distribution) of large collateral vessels occurred gradually over 2 weeks and was slightly more rapid with FGF-2 treatment. We conclude that CEU can separately assess collateral development, more distal arteriogenesis, and secondary changes in capillary perfusion that occur differentially with ischemia and growth factor therapy.

Quantitative evaluation of muscle perfusion with CEUS and with MR

Functional imaging might increase the role of imaging in muscular diseases, since alterations of muscle morphology alone are not specific for a particular disease. Perfusion, i.e., the blood flow per tissue and time unit including capillary flow, is an important functional parameter. Pathological changes of skeletal muscle perfusion can be found in various clinical conditions, such as degenerative or inflammatory myopathies or peripheral arterial occlusive disease. This article reviews the theoretical basics of functional radiological techniques for assessing skeletal muscle perfusion and focuses on contrast-enhanced ultrasound (CEUS) and magnetic resonance imaging (MRI) techniques. Also, the applications of microvascular imaging, such as in detection of myositis and for discriminating myositis from other myopathies or evaluating peripheral arterial occlusive disease, are presented, and possible clinical indications are discussed. In conclusion, dedicated MR and CEUS methods are now available that visualize and quantify (patho-)physiologic information about microcirculation within skeletal muscles in vivo and hence establish a useful diagnostic tool for muscular diseases.

Contrast echocardiography

Myocardial contrast echocardiography (MCE) is a noninvasive imaging technique that relies on the ultrasound detection of microbubble contrast agents. These agents are confined to the intravascular space thereby producing signal enhancement from the blood pool. This review encompasses many of the key concepts regarding the clinical application of MCE. The first section focuses on the composition, safety, and biokinetics of ultrasound contrast agents. Then we discuss new ultrasound imaging methodology that has been developed to enhance detection of contrast agent and to assess perfusion at the tissue level. Next, the clinical applications of contrast ultrasound are reviewed. These include enhancement of the cardiac chambers for better assessment of cardiac function and masses, myocardial perfusion imaging for the detection of coronary artery disease, and the assessment of myocardial viability and microvascular reflow. Finally, we discuss some of the future applications for MCE, which include molecular imaging of disease and drug/gene delivery. The overall aim of the review is to update the clinician on state-of-the-art MCE and how it can be applied in patients with cardiovascular disease.

High-energy phosphate metabolism in the calf muscle of healthy humans during incremental calf exercise with and without moderate cuff stenosis

It is known that the relevance of a peripheral stenosis for muscle function increases with exercise. Our intention was to investigate the impact of a moderate cuff stenosis (CS) at 120 mmHg of the superficial femoral artery on high-energy phosphate (HEP) metabolism during isotonic, incremental calf exercise. Serial phosphorus 31 magnetic resonance spectroscopy (31P MRS) and velocity-encoded phase-contrast MR imaging (VEPC MRI) were carried out in each leg of ten healthy male volunteers. Each leg underwent four increments of calf exercise (2, 3, 4 and 5 W) followed by recovery during separate exercise sessions with and without a CS at 120 mmHg. The serial 31P MRS measurements had a time resolution of 10 s. VEPC MRI was performed at the end of each increment during separate sessions. During all increments, we detected significant differences (P < 0.05) in the phosphocreatine (PCr) time constants and the amount of PCr hydrolysis between the sessions without and with CS. Regarding the time courses of the PCr, inorganic phosphate (Pi) and pH level, we observed significant differences (P < 0.002) during exercise and recovery. During both conditions, the end-increment PCr levels as well as blood flow correlated significantly with the mechanical power. The PCr time constants during exercise significantly correlated with the intramuscular pH, but not with blood flow or mechanical power. However, the PCr recovery time constants correlated significantly with blood flow and end-exercise pH. Our study shows that reduction of blood flow due to a peripheral stenosis results in a prolongation of PCr time constants, decreased PCr and pH level as well as increased Pi level during exercise. We believe that 31P MRS during incremental exercise might provide additional information for assessing the relevance of a peripheral stenosis and its impact on muscle function.

High-Energy Phosphate Metabolism During Calf Ergometry in Patients With Isolated Aorto-Iliac Artery Stenoses

OBJECTIVES

Patients with peripheral arterial disease (PAD) and aorto-iliac atherosclerotic lesions suffer from a broad range of complaints, such as pain at the hip, the thigh, and calf claudication. The purpose of this study was to investigate the high-energy metabolism in the calf muscle of patients with PAD with isolated aorto-iliac stenoses during incremental plantar flexion exercise.

MATERIALS AND METHODS

Using a 1.5 T whole-body magnetic resonance (MR) scanner, 12 patients with PAD with uni- or bilateral aorto-iliac atherosclerotic lesions and 10 healthy male controls underwent serial phosphor-31 MR spectroscopy during incremental exercise at 2, 3, 4, and 5 W. The phosphocreatine (PCr) time constants were calculated for each increment and recovery using a monoexponential model. In the patient group, the run-off resistance was determined on MR angiograms. In both the patients and the controls, the ankle brachial pressure index was measured.

RESULTS

The diseased legs exhibited significantly increased PCr time constants during the second and the third workload increment at 3 and 4 W, but not during the first increment at 2 W and recovery compared with normal controls. Only 3 diseased legs succeeded the last increment at 5 W. We detected significant correlations between the ankle brachial pressure index scores and the PCr time constants when including both the diseased and the control legs. The diseased legs showed a significant correlation with the run-off resistance only during the first increment.

CONCLUSIONS

Our study shows that the impairment of muscle metabolism, expressed by prolonged PCr time constants, occurs with greater work intensities in patients with aorto-iliac disease compared with patients with multisegmental PAD, as recently published, whereas our patients collective exhibited normal PCr recovery time constants. Our findings may help to understand variability of clinical symptoms in aorto-iliac PAD.

High-energy phosphate metabolism during incremental calf exercise in patients with unilaterally symptomatic peripheral arterial disease measured by phosphor 31 magnetic resonance spectroscopy

BACKGROUND

The treadmill exercise test is the most important examination of the functional ability of patients with intermittent claudication or leg pain during exercise, but it does not provide any metabolic information in the calf muscle. The purpose of this study was to investigate the high-energy metabolism in the calf muscle during incremental progressive plantar flexion exercise of a selected peripheral arterial disease (PAD) patient group.

METHODS

Using a 1.5-T whole-body magnetic resonance scanner, 17 male patients with PAD who had 1 symptomatic and 1 asymptomatic leg and 9 healthy male controls underwent serial phosphor 31 (31P) magnetic resonance spectroscopy during incremental exercise at 2, 3, 4, and 5 W. Furthermore, magnetic resonance angiography was performed, and the ankle-brachial pressure index was determined in the patient group. The runoff resistance (ROR) was separately assessed in each patient's leg.

RESULTS

The symptomatic legs exhibited significantly increased phosphocreatine (PCr) time constants during the first three workload increments (2-4 W) and the recovery phase compared with the asymptomatic legs and the normal controls. Only two symptomatic legs reached the last increment at 5 W. Compared with the normal controls, the asymptomatic legs showed significantly increased PCr time constants only at 5 W. In the patient group, we detected significant correlations between the PCr time constants and the ROR, as well as the ankle-brachial pressure index. Moreover, the symptomatic legs presented significantly lower PCr levels and pH values at the end of exercise compared with the asymptomatic and control legs.

CONCLUSIONS

Our study shows that muscle function in PAD patients can be objectively quantified with the help of 31P magnetic resonance spectroscopy and correlates significantly with hemodynamic parameters such as ROR and ankle-brachial pressure index. Consequently, 31P magnetic resonance spectroscopy seems to be a useful method to monitor the muscle function of PAD patients for evaluation of established therapies or new therapeutic strategies during research trials.