Laser Doppler flowmetry for bone blood flow measurements: helium-neon laser light attenuation and depth of perfusion assessment

Laser Doppler flowmetry (LDF) has been successfully used in clinical and experimental settings to evaluate bone perfusion but unanswered questions regarding its capabilities and limitations still remain. This study was undertaken to determine absorption of He-Ne laser light (632.8 nm) and maximum depth for flow assessment (threshold thickness) under optimal conditions in bone. Light transmittance in bovine bone samples of femora and tibia was measured after each step of grinding and depth of penetration calculated. The threshold thickness was obtained by placing the same samples in a flow chamber where a solution of 2% latex circulated beneath; flow was detected by a laser Doppler probe resting on top of the sample. The results showed a significantly higher depth of penetration for trabecular than for cortical bone. A regression analysis showed a high correlation between the inorganic fraction of the bone and the depth of penetration. The maximum depth at which the laser Doppler probe can evaluate flow in bone conditions was found to be 2.9 +/- 0.2 mm in cortical bone, 3.5 +/- 0.3 mm in bone covered by 1 mm cartilage and 3.5 +/- 0.2 mm in trabecular bone. The study showed the limitations of LDF in bone and their correlations to various bone properties.     

Comparison of two laser Doppler flowmetry systems for bone blood flow analysis

Summary The development of laser Doppler flowmetry (LDF) has provided a real-time, reliable method for monitoring capillary perfusion in multiple tissues. LDF has potential for the experimental and clinical assessment of bone blood flow. To compare the accuracy and ease of use of two commercially available laser Doppler flowmeters with different mechanisms for processing the Doppler-shifted component of light, estimates of bone blood flow were obtained in a sheep model using the two systems, and the values derived then compared with estimates of bone blood flow also obtained in a sheep using the technique of injection of labeled microspheres. The single-channel laser Doppler flowmeter, the LD 5000, processes the reference and Doppler-shifted beams on the surface of a single photodetector using optical heterodyning for measurement. The dual-channel flowmeter, the Periflux 2, uses two optical fibers to transmit reference and Doppler-shifted light to two separate photodetectors. The differential amplification and detection system improves the signal-to-noise ratio. Measurement of both metaphyseal (cancellous) and diaphyseal (cortical) blood flow using both LDF systems was compared with values obtained with an injection of⁸⁵Sr-labeled microspheres in three sheep. The LDF measurements were repeated after occlusion of the left femoral artery, and a⁴⁶Sc microsphere injection was performed prior to animal sacrifice. Two of the animals developed vasomotor instability, resulting in poor correlation between the measurements obtained with the Periflux 2, which is motion sensitive, and the values obtained with the microsphere method. High correlation was apparent in two of the three animals for the LD 5000 and the microsphere values. Four additional sheep were evaluated using only the Periflux 2, and the data collected yielded R values of 0.6–0.98. Both the single- and dual-channel LDF systems offer accurate assessment of local cortical and cancellous bone blood flow.     

Assessment of the relationship between standard probe and implantable fiber measurements of cortical bone blood flow: A canine study

Laser Doppler flowmetry (LDF) has been used to assess cortical bone blood flow in various clinical situations, such as osteomyelitis and osteonecrosis. Standard metal-sheathed probes containing optical fibers, applied to cortical bone for perfusion measurements, require direct exposure of the bone surface for each measurement, making nonanesthetized assessments over time impractical. Implantable optical fibers offer a noninvasive method for evaluating cortical bone perfusion without repeated surgical exposure of the bone after initial surgical implantation of the fibers.In vitro studies have shown the reliability of laser Doppler (LD) fibers compared with those of the standard probe. This investigation studied the relationship between measurements of cortical bone perfusion obtained by implanted optical (LD) fibers and standard (LDF) probesin vivo. Midshaft tibial fractures were created in the right hindlimb of 11 adult, large (>25 kg) dogs and stabilized by low contact-dynamic compression plate fixation. Cortical bone blood flow was measured by LDF using standard probes and implantable fibers at five sites along the tibia prefracture, postfracture, immediately postplate application, and at 10 weeks postplating, immediately prior to euthanasia. The implantable fibers were secured onto the cortical bone via the plate and led through a percutaneous exit site. Histological examination of the inguinal and popliteal lymph nodes and soft tissue surrounding the fibers revealed mild inflammation. No significant correlation of blood flow assessed by the implantable fibers and standard probe occurred immediately postfracture (r < 0.13,P > 0.62). However, a statistically significant correlation was seen postplate application at one of the measurement sites in the distal fracture fragment (r = 0.78,P < 0.003). The fibers remained intact and functional until an average of 3 weeks at which time they either fractured or were removed by the animals. This is the firstin vivo study assessing the reliability of implantable fibers for the measurement of cortical bone blood flow. Further modification of the fibers will be necessary to improve their longevity and durability for assessment of cortical bone blood flow.     

Review of methodological developments in laser Doppler flowmetry

Laser Doppler flowmetry is a non-invasive method of measuring microcirculatory blood flow in tissue. In this review the technique is discussed in detail. The theoretical and experimental developments to improve the technique are reviewed. The limitations of the method are elaborated upon, and the research done so far to overcome these limitations is critically assessed.

The use of laser Doppler flowmetry to evaluate anterior cruciate blood flow

The purpose of this study was to quantitate the blood flow of the anterior cruciate ligament in vivo. Functional flow was evaluated using laser Doppler flowmetry (LDF), for which the output signal, blood cell flux (BCF), is expressed in terms of volts. Ten patients undergoing routine arthroscopic surgery with clinically intact anterior cruciate ligaments were selected at random for participation in the study. Under arthroscopic visualization, a 2.2-mm probe was placed through a trocar sleeve into the anterior cruciate ligament after the arthroscopic procedure. Pulsatile flow within the ligament was observed in all patients. The mean maximum BCF value ranged from 101 to 274 mV; SD range was +/- 3-9 mV. The mean minimum BCF ranged from 75 to 197 mV; SD range was +/- 0 to 9 mV. Laser Doppler flowmetry offers significant promise as a method for measurement of in vivo anterior cruciate and cruciate substitution blood flow.     

Acoustic Doppler sonography, color duplex ultrasound, and laser Doppler flowmetry as tools for successful autologous breast reconstruction

The sine qua non to best ensure viability of any autogenous tissues used for breast reconstruction is to maximize the appropriate circulatory pattern to that tissue. This overview of tools used in this regard, all based on the physical principles of the Doppler effect, compares the role today of acoustic Doppler sonography, color duplex ultrasound, and laser Doppler flowmetry for perforator identification and flap monitoring. The audible Doppler has recognized limitations, but remains the simplest and most universally available device to assist in this purpose. Laser Doppler flowmetry provides a reasonable system for both intraoperative and post-procedure objective monitoring of the chosen tissue transfer.     

A comparison of pulse oximetry and laser Doppler flowmetry in monitoring sequential vascular occlusion in a rabbit ear model

The ultimate success of any free flap transfer depends not only on the experience of the given surgical team, but also requires constant vigilance in the perioperative period to assure continued anastomotic patency. Clinical acumen remains the ‘industry’ standard, but adjunctive monitoring devices are important to reinforce these subjective evaluations. We routinely use laser Doppler flowmetry in spite of known drawbacks, including expense. On the other hand, we know that pulse oximetry is a possible and readily available alternative.The efficacy of these two monitoring systems was directly compared in a New Zealand white rabbit central artery and vein ear model. Arterial occlusion was immediately recognized by a precipitous drop in flow and oxygen saturation (SaO₂). A rapid drop in flow after venous occlusion with a trend to zero was also noted with laser Doppler flowmetry. The SaO₂ had a very slow but steady gradual decline, but values remained in the range of normoxemia for a prolonged time. Because a minimum threshold of SaO₂ for predicting venous occlusion was elusive, the usefulness of pulse oximetry for monitoring microsurgical composite tissue transfers is limited.     

The microcirculation in neodymion-YAG laser irradiated and in electro coagulated urinary bladder tumors evaluated with laser Doppler flowmetry

Summary Neodymion-YAG laser irradiation is a relatively new therapeutic modality for the treatment of urinary bladder tumors which is suggested to produce an instantaneous arrest of the blood circulation and a homogenous coagulation of the tumor tissue. Utilizing the laser Doppler flowmetry principle the microcirculatory changes in laser irradiated tumors were compared to the changes in tumors coagulated with diathermy. In laser treated tumors the blood cell flux decreased to near baseline levels throughout coagulated area. Vascular stasis was demonstrated in the erythematous zone encircling the necrosis. In tumors coagulated with diathermy a corresponding constant decrease in blood cell flux could not be demonstrated. Thus the Neodymion-YAG laser irradiation seems preferable to electro coagulation in the treatment of urinary bladder tumors as more complete and homogenous arest of the blood circulation in the treated area is achieved.     

Depth of tissue sampling in the optic nerve head using laser Doppler flowmetry

Laser Doppler flowmetry (LDF) was performed on a simulated blood vessel in a model eye through optic nerve tissue sections in order to ascertain the ability to detect flow through them. LDF was performed using either near-infra-red or green laser light. Tissue section thickness ranged from 50 μm to 1000 μm. As expected, we found that our ability to detect flow with LDF decreased as we increased the thickness of optic nerve sections interposed between the LDF apparatus and the simulated blood vessel. We also found that the sampled depth of LDF increased with increasing separation of the optical detection fibre from the centre of the illuminated tissue volume. With adequate separation, we were able to detect flow with LDF through tissue sections of 1000 μm thickness using either near-infra-red or green laser light.     

Laser Doppler flowmetry for bone blood flow measurement: correlation with microsphere estimates and evaluation of the effect of intracapsular pressure on femoral head blood flow

Laser Doppler flowmetry (LDF) was used to measure bone blood flow in the rabbit femoral head and femoral condyles. To correlate the LDF output signal blood cell flux to in vivo blood flow, simultaneous measurements using LDF and 85Sr-labeled microspheres were made in an adult rabbit model. There was no correlation between the two methods for blood flow in the femoral condyles and the correlation between the two methods for blood flow in the femoral head does not achieve statistical significance. An LDF signal of 0.4 V was approximately equal to a microsphere measured flow rate of 0.4 ml blood/g bone/min. The strength of the correlation in the latter case may have been affected by (a) large arteriovenous shunts, (b) inadequate mixing of the microspheres with a left ventricular injection, and (c) insufficient numbers of microspheres present in the bone samples with which to satisfy the mathematical requirements of the microsphere method. When LDF was used to evaluate the effect of elevated intracapsular pressure on femoral head blood flow in skeletally mature rabbits, femoral head subchondral bone blood flow declined with increasing intracapsular pressure from a baseline value of 0.343 +/- 0.036 to a value of 0.127 +/- 0.27 at 120 cm of water pressure. The decline in femoral head blood flow was statistically significant at pressures of 40 cm of water or higher (p less than 0.001), and evaluation of sections of the proximal femora made from preterminal disulphine blue injections confirmed these findings. Intracapsular tamponade has an adverse effect on femoral head blood flow beginning well below central venous pressure and should be considered in the pathophysiology of posttraumatic and nontraumatic necrosis of the femoral head. Laser Doppler flowmetry was easy to use and appears to be a reproducible technique for evaluating femoral head blood flow, offering distinct advantages over the microsphere technique for measuring bone blood flow. Further studies of the in vivo calibration of the LDF method for bone blood flow are necessary for the method to have potential for clinical application.     

Effect of periosteal stripping on cortical bone perfusion: A laser doppler study in sheep

The goals of internal fixation are an accurate reduction and stable fixation in the presence of adequate bony vascularity. This can be achieved by a variety of means including plate fixation. A certain amount of periosteal stripping is necessary for proper open reduction of a fracture and for proper plate application. With displaced diaphyseal fractures, cortical bone perfusion (CBP) is already compromised. Further damage, in terms of periosteal stripping for plate fixation, may not be acceptable. Little information is available as to what extent the periosteum contributes to cortical bone perfusion. The purpose of this study was to determine the acute effects of periosteal stripping on cortical bone perfusion in a sheep tibia model. Twenty-three sheep were operated on and had the medial aspect of their right tibia exposed. Cortical bone perfusion measurements were obtained using laser Doppler flowmetry prior to periosteal stripping and after periosteal stripping. The results of this study show that the cortical bone perfusion significantly decreased by 20% after periosteal stripping over the entire length of the tibia. We therefore conclude that the periosteum contributes to diaphyseal bone perfusion and that it is important to preserve this source with fractures where blood supply is already significantly compromised.     

In vitro evaluation of a laser Doppler flowmetry implantable fibre system: the effect of flow velocity and concentration on perfusion assessment

The utility of laser Doppler flowmetry (LDF) is limited by the requirement for placement of the LDF probe directly on the bone surface. A system of implantable and detachable fibres was developed so that repeated non-anaesthetized measurements could be made. Using a specially designed flow chamber and a flow medium of latex particles, we compared the in vitro properties of the implantable fibre and the standard probe, by determining the effect of flow velocity and concentration on perfusion assessment. For both probes, the LDF output increased in response to increasing flow velocities and concentrations of the flow medium. With increasing velocities of the flow medium, both probes responded similarly when assessing perfusion through cortical bone, but differed when measuring flow through cancellous bone. A difference between the probes also existed when determining the effect of concentration on perfusion assessment. With increasing concentrations of the latex particles, the rate of LDF output increased more rapidly when perfusion was measured end on to the direction of flow by the standard probe, compared to the implantable fibre. This situation was reversed when assessing perfusion tangential to the direction of flow. The differences in the properties of the two probe systems are related to the direction of flow assessment and the type of bone through which flow was measured. These factors should be considered when measuring blood flow with either the LDF implantable fibre system or the standard LDF probe.     

Assessment of microcirculatory changes of cold contact injuries in a swine model using laser Doppler flowmetry and tissue spectrophotometry

Background

Until now, the exact pathophysiology of frostbite injuries is poorly understood. The aim of the study is to evaluate the perfusion changes of frostbite injuries to get a better understanding of the exact mechanism underlying the resulting tissue damage. Particular attention has been given to the differentiation of changes between the various injury depths.

Methods

Cold contact injury ranging from superficial, superficial-partial, deep-partial to full thickness were generated using seven goettingen minipigs. The perfusion dynamics were assessed before and 3 h after the injuries were inflicted using the O2C-device, which combines a laser light, to determine blood flow, and white light to determine hemoglobin oxygenation and relative amount of hemoglobin. A total of 42 cold contact injuries were inflicted and 84 measurements were carried out.

Results

In superficial and superficial partial injuries there was an increase in the blood flow (mean, 20 ± 2.4%, 15 ± 5.1%, respectively) and in the relative amount of hemoglobin (mean, 29 ± 9.1%, 28 ± 7.2%, respectively), whereas the hemoglobin oxygenation did not alter (mean, −0.15 ± 2.4%, −0.8 ± 3.1%, respectively). In deep partial injuries there was a decrease in the blood flow (mean range, −4 ± 2.1 to −17 ± 4.7%) and an increase in the relative amount of hemoglobin (mean range, 79 ± 17.2 to 93 ± 17.7%), whereas the hemoglobin oxygenation did not alter (mean range, −0.7 ± 3.1 to −2.9 ± 3.3%). In full thickness injuries there was a drop in the blood flow (mean range, −24 ± 7.6 to −27 ± 11.8%) and in the hemoglobin oxygenation (mean range, −16 ± 3.5 to −19 ± 2.5%) and an increase in the relative amount of hemoglobin (mean range, +126 ± 19.8 to 145 ± 10.9%).

Conclusion

The results of this study form a more precise pattern of cold contact injury perfusion changes, which can be a valuable tool to assess the degree of cold contact injury and furthermore the efficacy of novel diagnostic aids, therapeutics and treatment modalities.     

Ultrasound assessed thickness of burn scars in association with laser Doppler imaging determined depth of burns in paediatric patients

This study describes the ultrasound assessment of burn scars in paediatric patients and the association of these scar thickness with laser Doppler imaging (LDI) determined burn depth. A total of 60 ultrasound scar assessments were conducted on 33 scars from 21 paediatric burn patients at 3, 6 and 9 months after-burn. The mean of peak scar thickness was 0.39 ± 0.032 cm, with the thickest at 6 months (0.40 ± 0.036 cm). There were 17 scald burn scars (0.34 ± 0.045 cm), 4 contact burn scars (0.61 ± 0.092 cm), and 10 flame burn scars (0.42 ± 0.058 cm). Each group of scars followed normal distributions. Twenty-three scars had original burns successfully scanned by LDI and various depths of burns were presented by different colours according to blood perfusion units (PU), with dark blue <125, light blue 125–250, and green 250–440 PU. The thickness of these scars was significantly different between the predominant colours of burns, with the thinnest scars for green coloured burns and the thickest for dark blue coloured burns. Within light blue burns, grafted burns healed with significantly thinner scars than non-grafted burns. This study indicates that LDI can be used for predicting the risk of hypertrophic scarring and for guiding burn care. To our knowledge, this is the first study to correlate the thickness of burns scars by ultrasound scan with burn depth determined by LDI.     

Evaluation of the Doppler technique for fat emboli detection in an experimental flow model

Pericardial suction blood (PSB) is known to be contaminated with fat droplets, which may cause embolic brain damage during cardiopulmonary bypass (CPB). This study aimed to investigate the possibility to detect fat emboli by a Doppler technique. An in vitro flow model was designed, with a main pump, a filter, a reservoir, and an injector. A Hatteland Doppler probe was attached to the circulation loop to monitor particle counts and their size distribution. Suspended soya oil or heat-extracted human wound fat was analyzed in the model. The concentrations of these fat emboli were calibrated to simulate clinical conditions with either a continuous return of PSB to the systemic circulation or when PSB was collected for rapid infusion at CPB weaning. For validation purpose, air and solid emboli were also analyzed. Digital image analysis was performed to characterize the nature of the tested emboli. With soya suspension, there was an apparent dose response between Doppler counts and the nominal fat concentration. This pattern was seen for computed Doppler output (p = .037) but not for Doppler raw counts (p = .434). No correlation was seen when human fat suspensions were tested. Conversely, the image analysis showed an obvious relationship between microscopy particle count and the nominal fat concentration (p < .001). However, the scatter plot between image analysis counting and Doppler recordings showed a random distribution (p = .873). It was evident that the Doppler heavily underestimated the true number of injected fat emboli. When the image analysis data were subdivided into diameter intervals, it was discovered that the few large-size droplets accounted for a majority of total fat volume compared with the numerous small-size particles (< 10 microm). Our findings strongly suggest that the echogenecity of fat droplets is insufficient for detection by means of the tested Doppler method.     

Evaluation of microcirculation and wound-closing tension after undermining the skin. A study in a porcine model using laser Doppler perfusion imaging

Undermining the skin can reduce wound tension when closing skin defects and thereby can provide a potentially better microcirculation. On the other hand, undermining causes damage to blood vessels. In this study the relationship between wound tension and microcirculation in undermined wounds was evaluated. A porcine model was used and in three pigs six undermined excisions and six control excisions were done. The tension needed to close the wound was noted and the postoperative microcirculation was measured with a laser Doppler perfusion imager. Undermining the skin gave significantly reduced (p<0.05) wound-closing tension with a mean decrease of 25% (95% CI: 17–33%). The postoperative microcirculation in the undermined wound was significantly reduced (p<0.05) with a mean of 28% (95% CI: 8–46%). Undermining the skin reduced the wound-closing tension. In our model the positive effect on the microcirculation by reducing the wound-closing tension was less than the negative effect of cutting the perforating blood vessels to the skin with a net decrease in blood flow.     

He-Ne laser effects on blood microcirculation during wound healing: a method of in vivo study through laser Doppler flowmetry

BACKGROUND AND OBJECTIVES: Low-intensity laser therapy (LILT) is widely used for wound healing promotion and its mechanism of action may be due to an enhancement of blood supply. The aim of this study was to evaluate blood flow alterations in a wound healing model, using laser Doppler flowmetry (LDF) associated with a normalized perfusion parameter. STUDY DESIGN/MATERIALS AND METHODS: An injury was provoked in 15 rats and blood flow was measured periodically over a period of 21 days. Control groups were established to evaluate LDF and He-Ne laser effects on microcirculation. A 1 J/cm(2) dose was utilized, with 6 mW/cm(2) irradiance. RESULTS: The results demonstrated flow alterations provoked by lesion, and inflammatory response (P < 0.05). There were no statistical differences between groups. CONCLUSIONS: The results did not show a significant sustained effect on microcirculation with this He-Ne dose.     

Comparison of Laser Doppler Imaging (LDI) and clinical assessment in differentiating between superficial and deep partial thickness burn wounds

Purpose of presentation/study

To compare the accuracy of Laser Doppler Imaging (LDI) and clinical assessment in differentiating between superficial and deep partial thickness burns to decide whether early tangential excision and grafting or conservative management should be employed to optimize burn and patient management.

Measurement of point velocity in the canine coronary artery by laser Doppler velocimeter with optical fiber

A laser Doppler velocimeter with an optical fiber was developed to investigate point blood velocity in the arteries and was applied to the measurement of blood velocity in the canine coronary artery Fundamental experiments revealed that the accuracy of this method was satisfactory, since an excellent linear relation was found between the known blood velocity and the Doppler shift frequency (r=0.998). The fiber tip (0.1 mm diam.) was inserted into the proximal portion of the left circumflex coronary artery and was traversed across the vascular lumen to measure the point velocity at each sampling point. Detailed velocity profiles were then reconstructed sequentially in three dimensional coordinates, i.e., velocity, diameter and time. In general the velocity waveform showed a diastolic dominant pattern which is characteristic of coronary arterial flow. One peak in early systole and two peaks in early and in late diastole were commonly observed in the velocity waveform near the center-line of the vessel. The volocity profiles were flat in the axial region and decined abruptly near the vessel wall. These results indicate that the laser Doppler velocimetry with an optical fiber proved to be an accurate and feasible method of evaluating the point velocity of coronary artery flow. Presented at the Post-congress Meeting on Noninvasive Flowmetry in Angiology, of the 21 st Congress of the Japanese College of Angiology, Fukuoka, Japan, October 30–November 1, 1980.