Oxygen distribution in the vasculature of mouse tissue in vivo measured using a near infra red phosphor

Methods for noninvasive imaging of tissue hypoxia

The purpose of this review is to provide an overview of the methods available for imaging tissue oxygenation. The following imaging methods are reviewed: phosphorescence, near-infrared (NIR), positron emission tomography (PET), magnetic resonance imaging ((19)F MRI and BOLD MRI), and electron paramagnetic resonance (EPR). The methods are based on different principles and differ in their ability to accurately quantify tissue oxygenation, either the absolute value of a particular measure of oxygenation (partial pressure of oxygen, concentration), or a parameter related to it (oxygen saturation). Methods that can provide images of relative changes in oxygenation or visualization of hypoxia in a specific tissue of interest are also considered valuable tools for biomedical research and clinical applications.

Control of microvascular oxygen pressures during recovery in rat fast-twitch muscle of differing oxidative capacity

Whether the speed of recovery of microvascular O(2) pressures (Pmvo(2) ) differs within muscles composed primarily of type II fibres with contrasting oxidative capacity has not been determined. We tested the hypothesis that, following contractions, the recovery of Pmvo(2) would be slower in the white (WG; low oxidative capacity) versus the mixed gastrocnemius (MG; comparatively high oxidative capacity). Radiolabelled microsphere and phosphorescence quenching techniques were used to measure muscle blood flow ( Q, hence O(2) delivery, Q(O2)) and during contractions (1 Hz twitch) at low (LO, 2.5 V) and high intensities (HI, 4.5 V) in rat (n = 15) MG and WG muscle and during subsequent recovery. Following the LO protocol, end-contraction Pmvo(2) was lower in WG (11.6 +/- 0.5 mmHg) than in MG (16.2 +/- 0.6 mmHg; P < 0.05) while, contrary to our hypothesis, the initial rate of change in during recovery ( d P(O2)/dt; MG 0.11 +/- 0.01 mmHg s(-1) and WG 0.06 +/- 0.03 mmHg s(-1)) and mean response time (MRT; MG 110.3 +/- 5.1 s and WG 113.5 +/- 8.4 s, P > 0.05) were not different. In contrast, end-contraction baseline Pmvo(2) was not different following the HI protocol (MG 10.3 +/- 0.6 mmHg and WG 9.2 +/- 0.6 mmHg; P > 0.05) but, in agreement with our hypothesis, d P(O2)/dt was slower (MG 0.07 +/- 0.01 mmHg s(-1) and WG 0.03 +/- 0.003 mmHg s(-1); P < 0.05) and MRT longer (WG 180.8 +/- 4.5 s and MG 115.4 +/- 6.7 s; P < 0.05) in WG versus MG following the HI protocol. These data suggest that following high-intensity, though submaximal, muscle contractions, Pmvo(2) recovers much faster in the more oxidative mixed gastrocnemius than in the less oxidative white gastrocnemius.

Assessment of Endogenous and Therapeutic Arteriogenesis by Contrast Ultrasound Molecular Imaging of Integrin Expression

Background— We hypothesized that molecular imaging with contrast-enhanced ultrasound (CEU) and microbubbles targeted to endothelial integrins could be used to noninvasively assess early angiogenic responses to ischemia and growth factor therapy.

Methods and Results— Hindlimb ischemia was produced in 48 rats by ligation of an iliac artery. Half of the animals received intramuscular sustained-release fibroblast growth factor-2 (FGF-2). Immediately after ligation and at subsequent intervals from 4 to 28 days, blood flow and oxygen tension in the proximal adductor muscles were measured by CEU perfusion imaging and phosphor quenching, respectively. Targeted CEU imaging of α v - and α 5 β 1 -integrin expression was performed with microbubbles bearing the disintegrin echistatin. Iliac artery ligation produced a 65% to 70% reduction in blood flow and oxygen tension. In untreated ischemic muscle, muscle flow and oxygen tension partially recovered by days 14 to 28. In these animals, signal from integrin-targeted microbubbles was intense and peaked before flow increase (days 4 to 7). In comparison to untreated animals, FGF-2–treated muscle had a greater rate and extent of blood flow recovery and greater signal intensity from integrin-targeted microbubbles, which peaked before maximal recovery of flow. On immunohistology, arteriolar but not capillary density increased in the ischemic limb after ligation, the rate and degree of which were greater in FGF-2–treated rats. Immunofluorescence demonstrated intense staining for α v in arterioles, the temporal course of which correlated with targeted imaging.

Conclusions— Targeted CEU can be used to assess endogenous and therapeutic arteriogenesis before recovery of tissue perfusion. These results suggest that molecular imaging of integrin expression may be useful for evaluating proangiogenic therapies.

Control of microvascular oxygen pressures in rat muscles comprised of different fibre types

In response to an elevated metabolic rate ((.-)V(O(2)), increased microvascular blood-muscle O(2) flux is the product of both augmented O(2) delivery ((.-)Q(O(2)), and fractional O(2) extraction. Whole body and exercising limb measurements demonstrate that (.-)Q(O(2) and fractional O(2) extraction increase as linear and hyperbolic functions, respectively, of (.-)V(O(2). Given the presence of disparate vascular control mechanisms among different muscle fibre types, we tested the hypothesis that, in response to muscle contractions, (.-)Q(O(2) would be lower and fractional O(2) extraction (as assessed via microvascular O(2) pressure, P(mvO(2))) higher in fast- versus slow-twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were used to measure (.-)Q(O(2) and P(mvO(2)), respectively at rest and across the transition to 1 Hz twitch contractions at low (Lo, 2.5 V) and high intensities (Hi, 4.5 V) in rat (n = 20) soleus (Sol, slow-twitch, type I), mixed gastrocnemius (MG, fast-twitch, type IIa) and white gastrocnemius (WG, fast-twitch, type IIb) muscle. At rest and for Lo and Hi (steady-state values) transitions, P(mvO(2)) was lower (all P < 0.05) in MG (mmHg: rest, 22.5 +/- 1.0; Lo, 15.3 +/- 1.3; Hi, 10.2 +/- 1.6) and WG (mmHg: rest, 19.0 +/- 1.3; Lo, 12.2 +/- 1.1; Hi, 9.9 +/- 1.1) than in Sol (rest, 33.1 +/- 3.2 mmHg; Lo, 19.0 +/- 2.3 mmHg; Hi, 18.7 +/- 1.8 mmHg), despite lower (.-)V(O(2) and (.-)Q(O(2) in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship between (.-)Q(O(2) and O(2) extraction is dependent on fibre type (at least in the muscles studied herein), such that muscles comprised of fast-twitch fibres display a greater fractional O(2) extraction (i.e. lower P(mvO(2))) than their slow-twitch counterparts. These results also indicate that the greater sustained P(mvO(2)) in Sol may be important for ensuring high blood-myocyte O(2) flux and therefore a greater oxidative contribution to energetic requirements.

Effects of chronic heart failure in rats on the recovery of microvascular PO2 after contractions in muscles of opposing fibre type

Chronic heart failure (CHF) impairs muscle O2 delivery (QO2) and, at a given O2 uptake (VO2), lowers microvascular O2 pressures (PmvO2: determined by the QO2-to-VO2 ratio), which may impair recovery of high-energy phosphates following exercise. Because CHF preferentially decreases QO2 to slow-twitch muscles, we hypothesized that recovery PmvO2 kinetics would be slowed to a greater extent in soleus (SOL: approximately 84% type I fibres) than in peroneal (PER: approximately 14% type I) muscles of CHF rats. PmvO2 dynamics were determined in SOL and PER muscles of control (CON: n= 6; left ventricular end-diastolic pressure, LVEDP: approximately 3 mmHg), moderate CHF (MOD: n= 7; LVEDP: approximately 11 mmHg) and severe CHF (SEV: n= 4; LVEDP: approximately 25 mmHg) following cessation of electrical stimulation (180 s; 1 Hz). In PER, neither the recovery PmvO2 values nor the mean response time (MRT; a weighted average of the time to 63% of the overall response) were altered by CHF (CON: 66.8 +/- 8.0, MOD: 72.4 +/- 11.8, SEV: 69.1 +/- 9.5 s). In marked contrast, SOL PmvO2, at recovery onset, was reduced significantly in the SEV group ( approximately 6 Torr) and PmvO2 MRT was slowed with increased severity of CHF (CON: 45.1 +/- 5.3, MOD: 63.2 +/- 9.4, SEV: 82.6 +/- 12.3 s; P < 0.05 CON vs. MOD and SEV). These data indicate that CHF slows PmvO2 recovery following contractions and lowers capillary O2 driving pressure in slow-twitch SOL, but not in fast-twitch PER muscle. These results may explain, in part, the slowed recovery kinetics (phosphocreatine and VO2) and pronounced fatigue following muscular work in CHF patients.

Recovery of microvascular Po2 during the exercise off-transient in muscles of different fiber type

The speed with which muscle energetic status recovers after exercise is dependent on oxidative capacity and vascular O2 pressures. Because vascular control differs between muscles composed of fast- vs. slow-twitch fibers, we explored the possibility that microvascular O2 pressure (PmvO2; proportional to the O2 delivery-to-O2 uptake ratio) would differ during recovery in fast-twitch peroneal (Per: 86% type II) compared with slow-twitch soleus (Sol: 84% type I). Specifically, we hypothesized that, in Per, PmvO2 would be reduced immediately after contractions and would recover more slowly during the off-transient from contractions compared with Sol. The Per and Sol muscles of six female Sprague-Dawley rats (weight = ∼220 g) were studied after the cessation of electrical stimulation (120 s; 1 Hz) to compare the recovery profiles of PmvO2. As hypothesized, PmvO2 was lower throughout recovery in Per compared with Sol (end contraction: 13.4 ± 2.2 vs. 20.2 ± 0.9 Torr; end recovery: 24.0 ± 2.4 vs. 27.4 ± 1.2 Torr, Per vs. Sol; P ≤ 0.05). In addition, the mean response time for recovery was significantly faster for Sol compared with Per (45.1 ± 5.3 vs. 66.3 ± 8.1 s, Sol vs. Per; P < 0.05). Despite these findings, PmvO2 rose progressively in both muscles and at no time fell below end-exercise values. These data indicate that, during the recovery from contractions (which is prolonged in Per), capillary O2 driving pressure (i.e., PmvO2) is reduced in fast-compared with slow-twitch muscle. In conclusion, the results of the present investigation may partially explain the slowed recovery kinetics (phosphocreatine and O2 uptake) found previously in 1) fast- vs. slow-twitch muscle and 2) various patient populations, such as those with congestive heart failure and diabetes mellitus.

Effect of perfusion flow rate on tissue oxygenation in newborn piglets during cardiopulmonary bypass

BACKGROUND

Our knowledge of the best perfusion flow rate to use during cardiopulmonary bypass (CPB) in order to maintain tissue oxygenation remains incomplete. The present study examined the effects of perfusion flow rate and patent ductus arteriosus (PDA) during normothermic CPB on oxygenation in several organ tissues of newborn piglets.

METHODS

The experiments were performed on 12 newborn piglets: 6 with PDA ligation (PDA-L), and 6 without PDA ligation (PDA-NL). CPB was performed through the chest at 37 degrees C. During CPB, the flow rate was changed at 15-minute intervals, ranging from 100 to 250 ml/kg/min. Tissue oxygenation was measured by quenching of phosphorescence.

RESULTS

For the PDA-L group, oxygen in the brain did not change significantly with changes in flow rate. In contrast, for the PDA-NL group, oxygen was dependent upon the flow rate. Statistically significant decreases in cortical oxygen were observed with flow rates below 175 ml/kg/min. Within the myocardium, liver, and intestine, there were no significant differences in the oxygen levels between the PDA-L and PDA-NL groups. In these tissues, the oxygen decreased significantly as the flow rate decreased below 150 ml/kg/min, 125 ml/kg/min, and 175 ml/kg/min, respectively. Oxygen pressure in skeletal muscle was not dependent on either PDA ligation or flow rate.

CONCLUSIONS

In newborn piglets undergoing CPB, the presence of a PDA results in reduced tissue oxygenation to the brain but not to other organs. In general, perfusion flow rates of 175 ml/kg/min or greater are required in order to maintain normal oxygenation of all organs except muscle.

Effects of prior contractions on muscle microvascular oxygen pressure at onset of subsequent contractions

In humans, pulmonary oxygen uptake (.V(O2)) kinetics may be speeded by prior exercise in the heavy domain. This "speeding" arises potentially as the result of an increased muscle O(2) delivery (.Q(O2)) and/or a more rapid elevation of oxidative phosphorylation. We adapted phosphorescence quenching techniques to determine the.Q(O2)-to-O(2) utilization (.Q(O2)/.V(O2)) characteristics via microvascular O(2) pressure (P(O2,m)) measurements across sequential bouts of contractions in rat spinotrapezius muscle. Spinotrapezius muscles from female Sprague-Dawley rats (n = 6) were electrically stimulated (1 Hz twitch, 3-5 V) for two 3 min bouts (ST(1) and ST(2)) separated by 10 min rest. P(O2,m) responses were analysed using an exponential + time delay (TD) model. There was no significant difference in baseline and DeltaP(O2,m) between ST(1) and ST(2) (28.5 +/- 2.6 vs. 27.9 +/- 2.4 mmHg, and 13.9 +/- 1.8 vs. 14.1 +/- 1.3 mmHg, respectively). The TD was reduced significantly in the second contraction bout (ST(1), 12.2 +/- 1.9; ST(2), 5.7 +/- 2.2 s, P < 0.05), whereas the time constant of the exponential P(O2,m) decrease was unchanged (ST(1), 16.3 +/- 2.6; ST(2), 17.6 +/- 2.7 s, P > 0.1). The shortened TD found in ST(2) led to a reduced time to reach 63 % of the final response of ST(2) compared to ST(1) (ST(1), 28.3 +/- 3.0; ST(2), 20.2 +/- 1.8 s, P < 0.05). The speeding of the overall response in the absence of an elevated P(O2,m) baseline (which had it occurred would indicate an elevated.Q(O2)/.V(O2) or muscle blood flow suggests that some intracellular process(es) (e.g. more rapid increase in oxidative phosphorylation) may be responsible for the increased speed of P(O2,m) kinetics after prior contractions under these conditions.

Electron paramagnetic resonance for small animal imaging applications

Magnetic resonance imaging (MRI) provides high-resolution morphological images useful in diagnostic radiology to differentiate between normal and abnormal/pathological states in tissues. More recently, emerging developments in MRI have added a functional/physiological dimension to anatomical images. Electron paramagnetic resonance (EPR), a magnetic resonance technique similar to nuclear magnetic resonance, detects paramagnetic species such as free radicals. Like MRI, EPR can be implemented as an imaging technique for small animals and potentially human applications. Because of the low abundance of naturally occurring paramagnetic species, exogenous paramagnetic species are needed for in vivo EPR imaging (EPRI). The image data from EPRI contain both spatial distribution of paramagnetic species and spectral information. Hence, spatially encoded functional information such as tissue oxygen status and redox status can be extracted and coregistered with the spatial distribution of the spin probe, to the anatomy, or both by suitable means. Ultimately, the images obtained from EPRI may be used to overlay the functional information (containing spatial tissue physiology information) onto detailed anatomical maps. With its ability to enable whole animal imaging in mice, EPRI will be a useful imaging technique that complements other techniques such as MRI and positron emission tomography in obtaining valuable functional/physiological images.

Spatially variant regularization improves diffuse optical tomography

Diffuse tomography with near-infrared light has biomedical application for imaging hemoglobin, water, lipids, cytochromes, or exogenous contrast agents and is being investigated for breast cancer diagnosis. A Newton-Raphson inversion algorithm is used for image reconstruction of tissue optical absorption and transport scattering coefficients from frequency-domain measurements of modulated phase shift and light intensity. A variant of Tikhonov regularization is examined in which radial variation is allowed in the value of the regularization parameter. This method minimizes high-frequency noise in the reconstructed image near the source-detector locations and can produce constant image resolution and contrast across the image field.